Forced Convection Heat Transfer in Circular Pipes
Tosun, Ismail
2007-01-01
One of the pitfalls of engineering education is to lose the physical insight of the problem while tackling the mathematical part. Forced convection heat transfer (the Graetz-Nusselt problem) certainly falls into this category. The equation of energy together with the equation of motion leads to a partial differential equation subject to various…
Forced heat convection in annular spaces
International Nuclear Information System (INIS)
This report deals with the experimental study of forced heat convection in annular spaces through which flow of air is passing when a uniform heat flux is dissipated across the inner wall. These observations took place chiefly in the region where thermal equilibrium are not yet established. Amongst other things it became apparent that, both in the region where thermal equilibrium conditions are on the way to establishment and where they are already established, the following relationship held good: the longitudinal temperature gradient, either on the wall or in the fluid stream, is proportional to the heat flux dissipated q, and inversely proportional to the average flow rate V: dT/dx = B (q/V). From this result the next step is to express the variations of the local convection coefficient ? (or of the Margoulis number M) in a relationship of the form: 1/M = ?(V) + F(x). If this relationship is compared with the classical empirical relationship ? = KVn (where n is close to 0.8), the relationship: 1/M = ?V1-n + F(x) is obtained (? is a constant for a given annular space); from this it was possible to coordinate the whole set of experimental results. (author)
Theory of heat transfer with forced convection film flows
Shang, Deyi
2010-01-01
Developing a new treatment of ""Free Convection Film Flows and Heat Transfer"" began in Shang's first monograph and is continued in this monograph. The current book displays the recent developments of laminar forced convection and forced film condensation. It is aimed at revealing the true features of heat and mass transfer with forced convection film flows to model the deposition of thin layers. The novel mathematical similarity theory model is developed to simulate temperature - and concentration - dependent physical processes. The following topics are covered in this book: Mathematical meth
NANOFLUID PROPERTIES FOR FORCED CONVECTION HEAT TRANSFER: AN OVERVIEW
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W.H.Azmi
2013-06-01
Full Text Available Nano?uids offer a significant advantage over conventional heat transfer ?uids and consequently, they have attracted much attention in recent years. The engineered suspension of nano-sized particles in a base liquid alters the properties of these nanofluids. Many researchers have measured and modeled the thermal conductivity and viscosity of nanofluids. The estimation of forced convective heat transfer coefficients is done through experiments with either metal or nonmetal solid particles dispersed in water. Regression equations are developed for the determination of the thermal conductivity and viscosity of nanofluids. The parameters influencing the decrease in convection heat transfer, observed by certain investigators, is explained.
Combined natural and forced convection between uniformly heated vertical plates
International Nuclear Information System (INIS)
Combined natural and forced convection between uniformly heated vertical plates was studied experimentally. The followings were revealed; the streak-line of visualizing dye was straight, when the buoyancy didn't have much effect. But the streak-line and the wall temperature began to fluctuate sinusoidally at halfway of channel and the local heat transfer coefficient increased abruptly, when the buoyancy became effective. (author)
NANOFLUID PROPERTIES FOR FORCED CONVECTION HEAT TRANSFER: AN OVERVIEW
W.H.Azmi; Sharma, K V; Rizalman Mamat; Shahrani Anuar
2013-01-01
Nano?uids offer a significant advantage over conventional heat transfer ?uids and consequently, they have attracted much attention in recent years. The engineered suspension of nano-sized particles in a base liquid alters the properties of these nanofluids. Many researchers have measured and modeled the thermal conductivity and viscosity of nanofluids. The estimation of forced convective heat transfer coefficients is done through experiments with either metal or nonmetal solid particles dispe...
Forced convection film boiling heat transfer from a vertical cylinder
International Nuclear Information System (INIS)
Forced convection film boiling heat transfer from a vertical 3mm-dia. platinum cylinder in R113 flowing upward along the cylinder was measured for the velocities ranging from 0 to 3 m/s, liquid subcoolings from 0 to 60 K at pressures from 102 to 490 kPa. The heat transfer coefficients, h, are almost independent of vertical position on the cylinder. The values of h are almost independent of velocities lower than about 1 m/s and become higher for the velocities higher than 1 m/s. The heat transfer coefficients for the former velocity range agree with those derived from pool film boiling heat transfer correlation of Sakurai et al. (author)
Forced convection film boiling heat transfer on spheres
International Nuclear Information System (INIS)
This paper presents an experimental study of forced convection film boiling on sphere. To measure the film boiling heat transfer coefficients, a heated sphere with a thermocouple embedded at the center is dropped in (1) a vertical tube filled with water, or (2) a rotating water pool. The rotating pool is for providing higher fluid velocity. The falling speed of the sphere in the vertical stainless steel tube is measured using a set of magnet pickup coils distributed along the tube. The rotating pool is made of Pyrex to make a visualization. The ranges of the experimental conditions are: sphere speed 0.1-1.6 m/s, sphere surface temperature 500 and 700degC. The results show that the forced convection film boiling heat transfer coefficients were 200?400 W/m2K in the present experimental conditions and also show that there are large differences between the data sets or correlations due mainly to experimental difficulties. (author)
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Ghadimi, Mohammad; Ghadamian, Hossein [Islamic Azad Univ., Tehran (Iran, Islamic Republic of). Dept. of Energy Engineering, Science and Research Branch; Hamidi, Aliasghar A. [Tehran Univ. (Iran, Islamic Republic of). Dept. of Chemical Engineering; Fazelpour, Farivar [Islamic Azad Univ. of South Tehran Branch, Tehran (Iran, Islamic Republic of). Dept. of Energy System Engineering; Behghadam, Mehdi [Islamic Azad Univ. of Roudehen Branch, Tehran (Iran, Islamic Republic of). Dept. of Mechanical Engineering
2012-11-01
The present paper describes a two-dimensional finite volume numerical simulation of flow and heat transfer in airflow windows by free and forced convection techniques. The governing equations are the fully elliptic, Reynolds-averaged Navier-Stokes equations. The simple algorithm is employed to correct the pressure term. The second-order upwind scheme is used to discretize the convection terms. The (k-{epsilon}/RNG) turbulence model is applied for the flow simulation. The mesh used is the body-fitted, multi-plane grid system. Results on the variations of velocity and temperature profiles with geometrical parameters, at different temperature and velocity, for heat transfer by free and forced convection techniques are presented. Comparisons of the present results on temperature distribution for forced convection and for free convection with the available experimental forced convection data indicate that the airflow-influenced forced convection methods are considerably enhanced. (orig.)
Effect of Buoyancy on Forced Convection Heat Transfer in Vertical Channels - a Literature Survey
International Nuclear Information System (INIS)
This report contains a short resume of the available information from various sources on the effect of free convection flow on forced convection heat transfer in vertical channels. Both theoretical and experimental investigations are included. Nearly all of the theoretical investigations are concerned with laminar flow with or without internal heat generation. More consistent data are available for upward flow than for downward flow. Curves are presented to determine whether free convection or forced convection mode of heat transfer is predominant for a particular Reynolds number and Rayleigh number. At Reb > 105 free convection effects are negligible. Downward flow through a heated channel at low Reynolds number is unstable. Under similar conditions the overall heat transfer coefficient for downward flow tends to be higher than that for upward flow
CFD simulation of forced convective boiling in heated channels
International Nuclear Information System (INIS)
In this paper a forced convective boiling of Refrigerant R-113 in a vertical annular channel has been simulated by a custom version of the CFX-5 code. The employed subcooled boiling model uses a special treatment of the wall boiling boundary, which assures the grid invariant solution. The simulation results have been validated against the published experimental data [1]. In general a good agreement with the experimental data has been achieved, which shows that the current model may be applied for the Refrigerant R-113 without significantly changing the model parameters. The influence of non-drag forces, bubble diameter size and interfacial drag model on the numerical results has been investigated as well. (authors)
Forced convection heat transfer from heated surface in a cavity formed by two high ribs
International Nuclear Information System (INIS)
The work under consideration is devoted to experimental study on forced convection heat transfer from heated surface between two high ribs. Great attention is paid to the flow visual picture. The experiments were carried out in the channel of a subsonic aerodynamic tube with 200x200 mm cross section. Ribs of material with low heat conductivity (organic glass) 3 mm thick were located at the channel lower wall. The ribs were of similar height (h=60 mm) and the distance between them was equal to L/h=1; 2; 3 calibers. The most interesting fact from practical point of view is that average heat transfer between the ribs appeared to be by 1.5-3 times lower as compared to streamlining smooth surface
Studies of Forced-Convection Heat Transfer Augmentation in Large Containment Enclosures
International Nuclear Information System (INIS)
Heat transfer enhancement due to jet mixing inside a cylindrical enclosure is discussed. This work addresses conservative heat transfer assumptions regarding mixing and condensation that have typically been incorporated into passive containment design analyses. This research presents the possibility for increasing decay heat removal of passive containment systems under combined natural and forced convection. Eliminating these conservative assumptions could result in a changed containment design and reduce the construction cost. It is found that the ratio of forced- and free-convection Nusselt numbers can be predicted as a function of the Archimedes number and a correlated factor accounting for jet orientation and enclosure geometry
NUMERICAL ANALYSIS OF FORCED CONVECTIVE HEAT TRANSFER THROUGH HELICAL CHANNELS
Directory of Open Access Journals (Sweden)
Dr. K. E. Reby Roy
2012-07-01
Full Text Available Helical ducts are used in a variety of applications including food processing, thermal processing plants and refrigeration. They are advantageous due to their high heat transfer coefficient and compactness compared to straight tubes. The curvature of the coil governs the centrifugal force resulting in development of secondaryflow i.e. the fluid stream in the outer side of the pipe moves faster than the fluid streams in the inner side of the pipe. In the present study, Computational Fluid Dynamics (CFD simulations using Fluent 6.3.26 are carried out for helical rectangular ducts wound over a cylindrical passage. The cylindrical passage is oriented horizontallyand acts as a counterflow heat exchanger. The analysis is done by changing the flow rates of four different fluids like Ethylene Glycol, Kerosene, Nano Fluid and Water. The fluid flow and heat transfer characteristics of the fluids are studied and Nusselt Number correlations with Dean Number are developed.
Analysis of Rectangular Microchannel under Forced convection heat transfer condition
Directory of Open Access Journals (Sweden)
Dr. B.S.Gawali,
2011-03-01
Full Text Available Micro-convection is a strategic area in transport phenomena, since it is the basis for a wide range of miniaturized high-performance pplications. Surface area is one of the important concepts for high flux heat transfer in Microchannel performance. Microchannel with hydraulic diameters 440?m, 476?m, 500?m and 550?m are analyzed for optimize microchannel hydraulic diameter. The microchannel having height of 400?m, 450?m, 500?m, 600?m with width of 500?m is analyzed numerically. Spacing between microchannel is also varied in range of 250?m, 300?m, 350?mand 400?m are considered for the analysis. Cu material microchannel having length of 30mm which carries 20 microchannels on top surface of the cu piece is to be considered. Flow rate also varied from 5lpm to 30 lpm for optimization with water as a medium. From numerical study it is observed that as hydraulic diameter increases from 444?m to 545 ?m the flow rate pressure drop also increases with decreases in diameter. Also heat in put to icrochannel as increases from 5 watt to 80 watt temperature drop is high at flow rate of 17lpm to 20 lpm. From analysis it is observed that as hydraulic diameter of microchannel is major concept of microchannel heat transfer which is dependent on flow rate of waterin microchannel. The microchannels with hydraulic diameter of 440?m to 600?m will follow temperature drop up-to 6 degree Kelvin with heat input of 5 watt to 80watt with flow rate of 5lpm to 25lpm.
Effect of vapor condensation on forced convection heat transfer of moistened gas
Liang, Yongbin; Che, Defu; Kang, Yanbin
2007-05-01
The forced convection heat transfer with water vapor condensation is studied both theoretically and experimentally when wet flue gas passes downwards through a bank of horizontal tubes. Extraordinarily, discussions are concentrated on the effect of water vapor condensation on forced convection heat transfer. In the experiments, the air steam mixture is used to simulate the flue gas of a natural gas fired boiler, and the vapor mass fraction ranges from 3.2 to 12.8%. By theoretical analysis, a new dimensionless number defined as augmentation factor is derived to account for the effect of condensation of relatively small amount of water vapor on convection heat transfer, and a consequent correlation is proposed based on the experimental data to describe the combined convection condensation heat transfer. Good agreement can be found between the values of the Nusselt number obtained from the experiments and calculated by the correlation. The maximum deviation is within ±6%. The experimental results also shows that the convection condensation heat transfer coefficient increases with Reynolds number and bulk vapor mass fraction, and is 1˜3.5 times that of the forced convection without condensation.
Heat-flux scaling for weakly forced turbulent convection in the atmosphere
Rao, Kusuma G.; Narasimha, R.
Observational data in the atmosphere indicate that conventionally defined drag and heat transfer coefficients increase rapidly as wind speed falls. It is shown here that, at sufficiently low wind speeds, the observed heat flux is nearly independent of wind speed but the drag increases linearly with it. These findings are not consistent with the free-convection limit of the Businger relations for Monin Obukhov theory, and lend support to the ideas of Ingersoll (1966) and Grachev (1990), till now checked only against laboratory experiments. We propose here that it is useful to define, within the regime of mixed convection, a sub-regime of ‘weakly forced convection’ in which, to a first approximation, the heat flux is determined by temperature differentials as in free convection and the momentum flux by a perturbation, linear in wind, on free convection. It is further proposed that this regime is governed by velocity scales determined by the heat flux (rather than by the friction velocity as in classical turbulent boundary layer theory). Three candidates for the heat-flux velocity scale are considered; novel definitions of the drag and heat exchange coefficients, based on the preferred scale, are found to show very weak dependence on wind speed up to values of about 5 10 m s^{-1}; but there is some evidence that the usefulness of heat-flux scaling may extend beyond the velocity limits where pure free-convection scaling for heat flux is valid.
Prediction of forced convection heat transfer to Lead-Bismuth-Eutectic
Thiele, Roman
2013-01-01
The goal of this work is to investigate the capabilities of two different commercial codes, OpenFOAM and ANSYS CFX, to predict forced convection heat transfer in low Prandtl number fluids and investigate the sensitivity of these predictions to the type of code and to several input parameters.The goal of the work is accomplished by predicting forced convection heat transfer in two different experimental setups with the codes OpenFOAM and ANSYS CFX using three different turbulence models and va...
Hirasawa, S.; Kawanami, T.; Kinoshita, T.; Watanabe, T.; Atarashi, T.
2012-11-01
In order to make clear the forced convection heat transfer phenomena around spherical particles packed in fluid flow, we numerically analyzed the heat transfer and flow pattern of the air using a single sphere and then the closest packed structure arrangement of spherical particles. We used 3-dimensional thermo fluid computation code "STAR-CCM+". We calculated the forced convection heat transfer coefficient for spheres of 10 mm diameter with Reynolds number 63 - 6340. Our calculation results of the average heat transfer coefficient for a single sphere agree with the correlation equation proposed by Ranz and Marshall. Local heat transfer coefficient is high at portions where local flow impinges to the surface of spheres for packed spherical particles. Our calculation results of the average heat transfer coefficient for packed spherical particles are close to the correlation equation proposed by Wakao et al.
International Nuclear Information System (INIS)
In order to make clear the forced convection heat transfer phenomena around spherical particles packed in fluid flow, we numerically analyzed the heat transfer and flow pattern of the air using a single sphere and then the closest packed structure arrangement of spherical particles. We used 3-dimensional thermo fluid computation code 'STAR-CCM+'. We calculated the forced convection heat transfer coefficient for spheres of 10 mm diameter with Reynolds number 63 – 6340. Our calculation results of the average heat transfer coefficient for a single sphere agree with the correlation equation proposed by Ranz and Marshall. Local heat transfer coefficient is high at portions where local flow impinges to the surface of spheres for packed spherical particles. Our calculation results of the average heat transfer coefficient for packed spherical particles are close to the correlation equation proposed by Wakao et al.
Nanofluid flow and forced convection heat transfer over a stretching surface considering heat source
Mohammadpour, M.; Valipour, P.; Shambooli, M.; Ayani, M.; Mirparizi, M.
2015-07-01
In this paper, magnetic field effects on the forced convection flow of a nanofluid over a stretching surface in the presence of heat generation/absorption are studied. The equations of continuity, momentum and energy are transformed into ordinary differential equations and solved numerically using the fourth-order Runge-Kutta integration scheme featuring the shooting technique. Different types of nanoparticles as copper (Cu), silver (Ag), alumina (Al2O3) and titania (TiO2) with water as their base fluid has been considered. The influence of significant parameters, such as magnetic parameter, volume fraction of the nanoparticles, heat generation/absorption parameter, velocity ratio parameter and temperature index parameter on the flow and heat transfer characteristics are discussed. The results show that the values of temperature profiles increase with increasing heat generation/absorption and volume fraction of the nanoparticles but they decrease with increasing velocity ratio parameter and temperature index parameter. Also, it can be found that selecting silver as nanoparticle leads to the highest heat transfer enhancement.
Forced-convection heat transfer in a spherical annulus heat exchanger
International Nuclear Information System (INIS)
Results are presented of a combined numerical and experimental study of steady, forced-convection heat transfer in a spherical annulus heat exchanger with 53 0C water flowing in an annulus formed by an insulated outer sphere and a 0 0C inner sphere. The inner sphere radius is 139.7 mm, the outer sphere radius is 168.3 mm. The transient laminar incompressible axisymmetric Navier-Stokes equations and energy equation in spherical coordinates are solved by an explicit finite-difference solution technique. Turbulence and buoyancy are neglected in the numerical analysis. Steady solutions are obtained by allowing the transient solution to achieve steady state. Numerically obtained temperature and heat-flux rate distributions are presented for gap Reynolds numbers from 41 to 465. Measurements of inner sphere heat-flux rate distribution, flow separation angle, annulus fluid temperatures, and total heat transfer are made for Reynolds numbers from 41 to 1086. The angle of separation along the inner sphere is found to vary as a function of Reynolds number. Measured total Nusselt numbers agree with results reported in the literature to within 2.0 percent at a Reynolds number of 974, and 26.0 percent at a Reynolds number of 66
Convective Heat Transfer Coefficients of the Human Body under Forced Convection from Ceiling
DEFF Research Database (Denmark)
Kurazumi, Yoshihito; Rezgals, Lauris
2014-01-01
The average convective heat transfer coefficient for a seated human body exposed to downward flow from above was determined. Thermal manikin with complex body shape and size of an average Scandinavian female was used. The surface temperature distribution of the manikin’s body was as the skin temperature distribution of an average person. The measurements were performed in a room with controlled thermal environment. Air temperature was set at 26ºC for cooling and at 20ºC for heating. The radiant temperature asymmetry in horizontal and vertical direction was close to zero, i.e. mean radiant temperature was equal to the air temperature. The air velocity of the isothermal downward flow from the ceiling at height of 1.5 m above the floor (above the top of the head) was set in a range between still air and 0.73 m/s. Based on the analyses of the results relationships for determination of the convective heat transfer coefficient of the whole body (hc [W/(m2•K)]) was proposed: hc=4.088+6.592V1.715 for a seated naked body at 20ºC and hc=2.874+7.427V1.345 for a seated naked body at 26ºC. Differences in the convective heat transfer coefficient of the whole body in low air velocity range, V<0.3 m/s, due to the natural convection were found. The results may be useful during design of air distribution in rooms, e.g. low impulse ventilation, diffuse ventilation, etc.
P. MOHAJERI KHAMENEH,; I. MIRZAIE,; N. POURMAHMOUD; M.RAHIMI EOSBOEE; S. MAJIDYFAR; M. MANSOOR
2010-01-01
Three dimensional simulations of the single-phase laminar flow and forced convective heat transfer of water in round tube and straight microchannel heat exchangers were investigated numerically. This numerical method was developed to measure heat transfer parameters of round tube and straight microchannel tube geometries. Then, similarities and differences were compared between different geometries. The geometries and operating conditions of those indicated heat exchangers were created using ...
International Nuclear Information System (INIS)
In fast reactors, for removing decay heat after the reactors are shut off by natural circulation, various heat exchangers have been devised, but because the flow rate in this case is very small, the state of flow becomes the coexistence of forced and natural convections. In this research, by using numerical calculation technique, investigation was carried out on the heat transfer by coexisting forced and natural convections around a circular cylinder which is inserted perpendicularly in the flow in the channel between parallel plates in low Reynolds number region, and the flow characteristics. As for the techniques of numerical analysis, calculation domain, basic equations, grid, finite difference method, algorithm, initial conditions, boundary conditions and calculation conditions are explained. As the results of calculation, Nusselt number distribution and velocity distribution are reported. The mean Nusselt number of coexisting convections takes the same value as that of pure forced convection in low Rayleigh number region, and as Rayleigh number becomes higher, it decreases to the minimum value, and thereafter, increases rapidly, and approaches to the value of pure natural convection. (K.I.)
Experimental and numerical study of steady forced-convection heat transfer in a spherical annulus
International Nuclear Information System (INIS)
The results of a combined experimental and numerical study of steady forced-convection heat transfer in a spherical annulus with 500C heated water flowing in the annulus, an insulated outer sphere, and a 00C cooled inner sphere are presented. The inner sphere radius is 139.7 mm, the outer sphere radius is 168.3 mm and the radius ratio is 1.2. Measurements of inner sphere heat- flux rate distribution, flow separation angle, annulus fluid temperatures and total heat transfer were made for gap Reynolds numbers from 41 to 1086. The angle of separation along the inner sphere was found to vary as a function of Reynolds number. Measured total Nusselt numbers agreed with results reported in the literature to within 2.0% at a gap Reynolds number of 974 and 26.0% at a gap Reynolds number of 66. Results of numerical calculations of laminar incompressible fluid flow and heat transfer in a spherical annulus are presented. Velocity, temperature, pressure and heat-flux rate distributions are presented for gap Reynolds numbers from 4.4 to 440. It is believed that this is the first experimental study of spherical annulus convective heat transfer in which inner sphere heat-fux distribution has been measured and in which sepration angle was measured by non-visual methods and found to vary as a function of Reynolds number. Also, the numerical analysis is the first solution of the full laminar Navier-Stokes equations for forced-convection heat transfer in a sphericaled-convection heat transfer in a spherical annulus. The computations predict well the experimental trends and qualitative aspects of the flow and heat transfer while underpredicting heat-flux rates by a factor of two or more. It is felt that a turbulence model is necessary to predict more accurately the experimental results
Munir, Asif; Azeem SHAHZAD; Khan, Masood
2014-01-01
The major focus of this article is to analyze the forced convective heat transfer in a steady boundary layer flow of Sisko fluid over a nonlinear stretching sheet. Two cases are studied, namely (i) the sheet with variable temperature (PST case) and (ii) the sheet with variable heat flux (PHF case). The heat transfer aspects are investigated for both integer and non-integer values of the power-law index. The governing partial differential equations are reduced to a system of nonlinear ordinary...
Heat transfer tests under forced convection conditions with high wettable heater surface
International Nuclear Information System (INIS)
Under forced convection and atmospheric pressure conditions, heat transfer tests were performed using the annulus channel of a heater rod with highly wettable surface. Improvement of boiling heat transfer requires that the cooling liquid can contact the heating surface, or a high-wettability heating surface, even if a vapor bubble layer is generated on the surface. From this point of view, high-wettable heating surface was studied. As oxide semiconductor-coated materials are highly-wettable, we made a TiO2 coated heater rod. TiO2 coated surface has a high-wettability, in terms of contact angle and Leidenfrost temperature. The boiling curve was measured with and without TiO coated surface. The results showed difference between with and without TiO2 coating. TiO2 coating rod showed lower boiling onset heat flux, wider nucleate boiling region and higher critical heat flux than without coating. In summary, high wettablity heater surface produced higher boiling heat transfer characteristics under forced convection conditions. (author)
Olakoyejo, O.T.; Bello-Ochende, Tunde; Meyer, Josua P.
2012-01-01
This paper presents a three-dimensional geometric optimisation of cooling channels in forced convection of a vascularised material with the localised self-cooling property subjected to a heat flux. A square configuration was studied with different porosities. Analytical and numerical solutions were provided. The geometrical configuration was optimised in such a way that the peak temperature was minimised at every point in the solid body. The optimisation was subject to the constra...
Forced Convection Heat Transfer Experiments of the Finned Plate in a Duct
International Nuclear Information System (INIS)
The studies have been focused on the optimization of fin geometries to maximize the heat transfer rate. The forced convection heat transfer rates were affected largely by the fin spacing, fin height, and tip clearance. As the fin spacing decreases and fin height increases, heat transfers from the fins to the ambient are enhanced as they are directly proportional to the surface area. For a large tip clearance, the fluid tends to escape from the inner fin region to the outer wall region resulting in the decrease of the overall heat removal capability. Thus, the parametric influences of these variables are to be investigated to develop a generalized heat transfer correlation for the geometry. This study is a preliminary experimental study for plate-fin geometries such as fin spacing, fin height and duct width. Mass transfer experiments were carried out based on the analogy concept, using a copper sulfate electroplating system. The work has the relevance with the Reactor Cavity Cooling System performance enhancement study in the VHTR. Forced convection heat transfer experiments were performed for the vertical plate-fins in a duct. Based on the analogy between heat and mass transfer systems, mass transfer rates were measured using the cupric acid copper sulfate electroplating system. The fin spacings were varied from 0.002m to 0.007m, fin heights 0.01m and 0.015m, ReDh from 10 to 6,500, and duct widths from 0.010m to 0.02m. The test results showed that the heat transfer rates enhanced with the increase of fin height and the decrease of fin spacing as they enlarge the heat transfer area. And the heat transfer rates were impaired with the increase of the duct width as the bypass flows increased to tip clearance region
Analysis of Forced Convection Heat Transfer for Axial Annular Flow of Giesekus Viscoelastic Fluid
Energy Technology Data Exchange (ETDEWEB)
Mohseni, Mehdi Moayed; Rashidi, Fariborz; Movagar, Mohammad Reza Khorsand [Amirkabir University of Technology, Tehran (Iran, Islamic Republic of)
2015-02-15
Analytical solutions for the forced convection heat transfer of viscoelastic fluids obeying the Giesekus model are obtained in a concentric annulus under laminar flow for both thermal and hydrodynamic fully developed conditions. Boundary conditions are assumed to be (a) constant fluxes at the walls and (b) constant temperature at the walls. Temperature profiles and Nusselt numbers are derived from dimensionless energy equation. Subsequently, effects of elasticity, mobility parameter and viscous dissipation are discussed. Results show that by increasing elasticity, Nusselt number increases. However, this trend is reversed for constant wall temperature when viscous dissipation is weak. By increasing viscous dissipation, the Nusselt number decreases for the constant flux and increases for the constant wall temperature. For the wall cooling case, when the viscous dissipation exceeds a critical value, the generated heat overcomes the heat which is removed at the walls, and fluid heats up longitudinally.
Analysis of Forced Convection Heat Transfer for Axial Annular Flow of Giesekus Viscoelastic Fluid
International Nuclear Information System (INIS)
Analytical solutions for the forced convection heat transfer of viscoelastic fluids obeying the Giesekus model are obtained in a concentric annulus under laminar flow for both thermal and hydrodynamic fully developed conditions. Boundary conditions are assumed to be (a) constant fluxes at the walls and (b) constant temperature at the walls. Temperature profiles and Nusselt numbers are derived from dimensionless energy equation. Subsequently, effects of elasticity, mobility parameter and viscous dissipation are discussed. Results show that by increasing elasticity, Nusselt number increases. However, this trend is reversed for constant wall temperature when viscous dissipation is weak. By increasing viscous dissipation, the Nusselt number decreases for the constant flux and increases for the constant wall temperature. For the wall cooling case, when the viscous dissipation exceeds a critical value, the generated heat overcomes the heat which is removed at the walls, and fluid heats up longitudinally
International Nuclear Information System (INIS)
In the first part, free-convection and nucleate pool boiling heat transfer (up to burn-out heat flux) between a platinum wire of 0.15 mm in diameter in neon, deuterium and hydrogen has been studied at atmospheric pressure. These measurements were continued in liquid neon up to 23 bars (Pc ? 26.8 b). Film boiling heat transfer coefficients have been measured in pool boiling liquid neon at atmospheric pressure with three heating wires (diameters 0.2, 0.5, 2 mm). All the results have been compared with existing correlations. The second part is devoted to measurements of the critical heat flux limiting heat transfer with small temperature differences between the wall and the liquid neon flowing inside a tube (diameters 3 x 3.5 mm) heated by joule effect on 30 cm of length. Influences of flow stability, nature of electrical current, pressure, mass flow rate and subcooling are shown. In conclusion, the similarity of the heat transfer characteristics in pool boiling as well as in forced convection of liquid neon and hydrogen is emphasized. (author)
Performance of a forced convection solar drier integrated with gravel as heat storage material
Energy Technology Data Exchange (ETDEWEB)
Mohanraj, M. [Dr Mahalingam College of Engineering and Technology, Pollachi (India). Dept. of Mechanical Engineering; Chandrasekar, P. [Swinburne Univ. of Technology, Sarawak (Malaysia). School of Engineering Sciences
2009-07-01
Sun drying is the most common method used in India to dry agricultural products such as grains, fruits and vegetables. The rate of drying depends on solar radiation, ambient temperature, wind velocity, relative humidity, initial moisture content, type of crops, crop absorptivity and mass product per unit exposed area. However, this method of spreading the crop in a thin layer on the ground has several disadvantages. This paper reported on a study that focused on developing a forced convection solar drier integrated with heat storage materials for drying various agricultural crops. The indirect forced convection solar drier, integrated with gravel as a sensible heat material, was used to dry pineapple slices under conditions similar to those found in Pollachi, India. The performance of the system was discussed along with the drying characteristics, drying rate, and specific moisture extraction rate. The results showed that the moisture content (wet basis) of pineapple was reduced from about 87.5 to 14.5 per cent (equilibrium moisture content) in about 29 hours in the bottom tray and 32 hours in the top tray. The thermal efficiency of the solar air heater was also reviewed. 9 refs., 5 figs.
Experiments on forced convection form a horizontal heated plate in a packed bed of glass spheres
Energy Technology Data Exchange (ETDEWEB)
Renken, K.J. (Univ. of Wisconsin, Milwaukee (USA)); Poulikakos, D. (Univ. of Illinois, Chicago (USA))
1989-02-01
This paper presents an experimental investigation of boundary-layer forced convective heat transfer from a flat isothermal plate in a packed bed of spheres. Extensive experimental results are reported for the thermal boundary-layer thickness, the temperature field, and the local wall heat flux (represented by the local Nusselt number). Theoretical findings of previous investigations using the Darcy flow model as well as a general model for themomentum equation accouting for flow inertia and macroscopic shear wtih and without variable porosity are used to evaluate the theoretical models. Several trends are revealed regarding the conditions of validity of these flow models. Overall the general flow model including variable porosity appears to perform better, even through the need for serious improvements in modeling becomes apparent.
Forced convection heat transfer in structure packed beds: packing form and particle shape
Energy Technology Data Exchange (ETDEWEB)
Jian, Yang; Min, Zeng; Qiuwang, Wang [Xi' an Jiaotong University, Xi' an (China); Xiao, Yan [Nuclear Power Institute of China, Chengdu (China)
2009-07-01
A forced convection heat transfer inside micro pores of structure packed beds with spherical or ellipsoidal particles are numerically studied in this paper. Three-dimensional Navier Stokes equations and RNG k-{epsilon} turbulent model with scalable wall function are adopted for present computations. The effects of packing form and particle shape are investigated in detail. The macroscopic hydrodynamic and heat transfer results are obtained from micro pore cells by using integrating method. The results show that, with the same physical parameters, the pressure drops in structure packed beds are much lower than those in randomly packed beds while the overall heat transfer efficiencies (except SC packing) are much higher. The traditional correlations of flow and heat transfer extracted from randomly packings are unavailable for structured packings, and some modified correlations are obtained. Furthermore, it finds that, with the same particle shape (sphere), the overall heat transfer performance of SC packing is the best and with the same packing form (FCC), the overall heat transfer performance of packing with long ellipsoidal particles is the best.
An assessment of correlations of forced convection heat transfer to water at supercritical pressure
International Nuclear Information System (INIS)
Highlights: • Evaluates 26 correlations of heat transfer to supercritical water. • Compiles 3220 experimental data points for the evaluation. • Evaluates based on both entire database and three heat transfer regimes. • Provides a guide to choosing the best correlation for engineering design. - Abstract: The heat transfer of supercritical water is essential for supercritical water-cooled nuclear reactors. Many empirical correlations for heat transfer to supercritical water were proposed over the past few decades. Some evaluations of the correlations were conducted, and inconsistent conclusions appeared owing to limited correlations or experimental data. This work presents an extensive survey of the literature of correlations and experiments of forced convection heat transfer to water flowing upward in vertical tubes at supercritical pressure. There are 26 correlations found, and an experimental database containing 3220 data points from vertical tubes are compiled from nine independent laboratories. All available correlations are assessed against the experimental database. The results show that the best correlation has a mean absolute deviation of 12.8%, predicting 82.3% of the database within ±20%. The entire database is divided into three categories, and the correlations which can give the most accurate predictions of the experimental data from different categories are also identified. The results provide a guide to choosing a proper correlation for engineering practice. Some topics worthy of attention for future studies are indicated
Directory of Open Access Journals (Sweden)
P. MOHAJERI KHAMENEH,
2010-11-01
Full Text Available Three dimensional simulations of the single-phase laminar flow and forced convective heat transfer of water in round tube and straight microchannel heat exchangers were investigated numerically. This numerical method was developed to measure heat transfer parameters of round tube and straight microchannel tube geometries. Then, similarities and differences were compared between different geometries. The geometries and operating conditions of those indicated heat exchangers were created using a finite volume-based computational fluid dynamics technique. In this article, at each Z-location variation of dimensionless local temperature, nondimensional local heat flux variation and dimensionless local Nusselt number distribution along the tube length were compared between round tube and straight microchannel heat exchangers. Consequently, averaged computational Nusselt number was obtained for those indicated models and then validation study was performed for round tube counter flow type heat exchanger model. Finally, all of these numerical results for both kind of geometries in counter flow heat exchangers were discussed in details.
Conceptual Design of Forced Convection Molten Salt Heat Transfer Testing Loop
Energy Technology Data Exchange (ETDEWEB)
Manohar S. Sohal; Piyush Sabharwall; Pattrick Calderoni; Alan K. Wertsching; S. Brandon Grover
2010-09-01
This report develops a proposal to design and construct a forced convection test loop. A detailed test plan will then be conducted to obtain data on heat transfer, thermodynamic, and corrosion characteristics of the molten salts and fluid-solid interaction. In particular, this report outlines an experimental research and development test plan. The most important initial requirement for heat transfer test of molten salt systems is the establishment of reference coolant materials to use in the experiments. An earlier report produced within the same project highlighted how thermophysical properties of the materials that directly impact the heat transfer behavior are strongly correlated to the composition and impurities concentration of the melt. It is therefore essential to establish laboratory techniques that can measure the melt composition, and to develop purification methods that would allow the production of large quantities of coolant with the desired purity. A companion report describes the options available to reach such objectives. In particular, that report outlines an experimental research and development test plan that would include following steps: •Molten Salts: The candidate molten salts for investigation will be selected. •Materials of Construction: Materials of construction for the test loop, heat exchangers, and fluid-solid corrosion tests in the test loop will also be selected. •Scaling Analysis: Scaling analysis to design the test loop will be performed. •Test Plan: A comprehensive test plan to include all the tests that are being planned in the short and long term time frame will be developed. •Design the Test Loop: The forced convection test loop will be designed including extensive mechanical design, instrument selection, data acquisition system, safety requirements, and related precautionary measures. •Fabricate the Test Loop. •Perform the Tests. •Uncertainty Analysis: As a part of the data collection, uncertainty analysis will be performed to develop probability of confidence in what is measured in the test loop. Overall, the testing loop will allow development of needed heat transfer related thermophysical parameters for all the salts, validate existing correlations, validate measuring instruments under harsh environment, and have extensive corrosion testing of materials of construction.
International Nuclear Information System (INIS)
Three-dimensional simulations of the single-phase laminar flow and forced convective heat transfer of water in microchannels with small rectangular sections having specific hydraulic diameters and distinct geometric configurations were investigated numerically. The numerical results indicated that the laminar heat transfer was to be dependent upon the aspect ratio and the ratio of the hydraulic diameter to the center to center distance of the microchannels. The geometries and operating conditions of that indicated microchannel were created using a finite volume-based computational fluid dynamics technique. The aim of this paper is to obtain computational Nusselt number in laminar flow using Fluent CFD Solver and to validate it with available experimental studies. Acquired numerical results have an admissible compatibility with available experimental studies. In addition, conceivable temperature profiles and pressure drops have been obtained at each Z-location in this model. Then, pressure drop values in the present model were explored for each Reynolds number. Finally, the effects of geometric parameters on the average Nusselt number in the laminar flow were illustrated numerically.
International Nuclear Information System (INIS)
Highlights: ? Superposition of forced and thermal convection is studied in a rectangular cavity. ? For pure forced convection the mean wind exhibits a solid body rotation. ? Four buoyancy induced convection rolls are formed for mixed convection at Ar ? 3.3. ? The enthalpy flux difference between out- and inflowing air has a maximum at Ar ? 0.6. - Abstract: Results of an experimental study of flow structure formation and heat transport in turbulent forced and mixed convection are presented. The experiments were conducted in a rectangular cavity with a square cross section, which has an aspect ratio between length and height of ?xz = 5. Air at atmospheric pressure was used as working fluid. The air inflow was supplied through a slot below the ceiling, while exhausting was provided by another slot, which is located directly above the floor. Both vents extend over the whole length of the cell. In order to induce thermal convection the bottom of the cell is heated while the ceiling is maintained at a constant temperature. This configuration allows to generate and study mixed convection under well defined conditions. Results of forced convection at Re = 1.07 x 104 as well as mixed convection at 1.01 x 104 ? Re ? 3.4 x 104 and Ra = 2.4 x 108 (3.3 ? Ar ? 0.3), which were obtained by means of Particle Image Velocimetry and local temperature measurements, are presented. For purely forced convection a 2D mean wind, which can be approximated by a solid body rotation, is found. With increasing Archimedes number this structure becomes unstable, leading to a transition of the solid body rotation into additional smaller convection rolls. Proper orthogonal decomposition of the instantaneous velocity fields has been performed for further analysis of these coherent large-scale structures. Their fingerprint is found in the spatial temperature distribution of the out flowing air at the end of the outlet channel, which reveals a temporally stable profile with two maxima over the length of the outlet. Moreover a maximum in the global enthalpy transport by the fluid is found at Ar ? 0.6.
Bouchenafa, Rachid; Saim, Rachid; Abboudi, Said
2015-09-01
Forced convection is a phenomenon associated with the heat transfer fluid flows. The presence of convection affects simultaneously the thermal and hydrodynamic fields, the problem is thus coupled. This form of heat transfer inside ducts occurs in many practical applications such as solar collectors, heat exchangers, cooling of electronic components as well as chemical and nuclear. In this work, we are interested primarily for a numerical study of thermo-hydraulic performances of an incompressible turbulent flow of air through a heat sink composed of several rows of bars of square section. Profiles and the axial velocity fields, as well as profiles and the distribution of the Nusselt number are plotted for all the geometry considered and chosen for different sections. The effects of geometrical parameters of the model and the operating parameters on the dynamic and thermal behavior of the air are analyzed.
Cherief, Wahid; Avenas, Yvan; Ferrouillat, Sébastien; Kedous-Lebouc, Afef; Jossic, Laurent; Berard, Jean; Petit, Mickael
2015-07-01
Applying a magnetic field on a ferrofluid flow induces a large increase of the convective heat transfer coefficient. In this paper, the thermal-hydraulic behaviors of two commercial ferrofluids are compared. The variations of both the pressure drop and the heat transfer coefficient due to the magnetic field are measured in the following conditions: square duct, laminar flow and uniform wall heat flux. The square section with two insulated walls allows for the characterization of the effect of the magnetic field direction. The experimental results show that the heat transfer is better enhanced when the magnetic field is perpendicular to the heat flux. In the best case, the local heat transfer coefficient increase is about 75%. On the contrary, another experimental setup shows no enhancement of thermal conductivity when the magnetic field is perpendicular to the heat flux. Contribution to the topical issue "Electrical Engineering Symposium (SGE 2014) - Elected submissions", edited by Adel Razek
Directory of Open Access Journals (Sweden)
M. MOHANRAJ
2009-09-01
Full Text Available An indirect forced convection solar drier integrated with different sensible heat storage maternal has been developed and tested its performance for drying chili under the metrological conditions of Pollachi, India. The system consists of a flat plate solar air heater with heat storage unit, a drying chamber and a centrifugal blower. Drying experiments have been performed at an air flow rate of 0.25 kg/s. Drying of chili in a forced convection solar drier reduces the moisture content from around 72.8% (wet basis to the final moisture content about 9.1% in 24 h. Average drier efficiency was estimated to be about 21%. The specific moisture extraction rate was estimated to be about 0.87 kg/kWh.
Forced convective heat transfer and pressure drop of a randomly packed HTGR core
International Nuclear Information System (INIS)
Heat and mass transfer have been measured for a randomly packed spherical pebble bed in the range of Reynolds number 30 4. The Prandtl number was Pr = 0.71. For the mass transfer measurements the method of naphthalene sublimation in air (Schmidt number Sc = 2.53) was applied. Effects of free convection, of temperature radiation, and of heat flux due to punctual contact of neighboring spheres are studied. Furthermore the wall effects on heat transfer are considered. For 3 x 102 4 the pressure drop coefficient has been measured. The importance of exact knowledge about the void fraction and its effect on the pressure drop is pointed out
Design of Test Loops for Forced Convection Heat Transfer Studies at Supercritical State
Balouch, Masih N.
Worldwide research is being conducted to improve the efficiency of nuclear power plants by using supercritical water (SCW) as the working fluid. One such SCW reactor considered for future development is the CANDU-Supercritical Water Reactor (CANDU-SCWR). For safe and accurate design of the CANDU-SCWR, a detailed knowledge of forced-convection heat transfer in SCW is required. For this purpose, two supercritical fluid loops, i.e. a SCW loop and an R-134a loop are developed at Carleton University. The SCW loop is designed to operate at pressures as high as 28 MPa, temperatures up to 600 °C and mass fluxes of up to 3000 kg/m2s. The R-134a loop is designed to operate at pressures as high as 6 MPa, temperatures up to 140 °C and mass fluxes in the range of 500-6000 kg/m2s. The test loops designs allow for up to 300 kW of heating power to be imparted to the fluid. Both test loops are of the closed-loop design, where flow circulation is achieved by a centrifugal pump in the SCW loop and three parallel-connected gear pumps in the R-134a loop, respectively. The test loops are pressurized using a high-pressure nitrogen cylinder and accumulator assembly, which allows independent control of the pressure, while simultaneously dampening pump induced pressure fluctuations. Heat exchangers located upstream of the pumps control the fluid temperature in the test loops. Strategically located measuring instrumentation provides information on the flow rate, pressure and temperature in the test loops. The test loops have been designed to accommodate a variety of test-section geometries, ranging from a straight circular tube to a seven-rod bundle, achieving heat fluxes up to 2.5 MW/m2 depending on the test-section geometry. The design of both test loops allows for easy reconfiguration of the test-section orientation relative to the gravitational direction. All the test sections are of the directly-heated design, where electric current passing through the pressure retaining walls of the test sections provides the Joule heating required to heat up the fluid to supercritical conditions. A high-temperature dielectric gasket isolates the current carrying parts of the test section from the rest of the assembly. Temperature and pressure drop data are collected at the inlet and outlet, and along the heated length of the test section. The test loops and test sections are designed according to American Society of Mechanical Engineers (ASME) Pressure Piping B31.1, and Boiler and Pressure Vessel Code, Section VIII-Division 1 rules. The final test loops and test sections assemblies are certified by Technical Standards and Safety Authority (TSSA). Every attempt is made to use off-the-shelf components where possible in order to streamline the design process and reduce costs. Following a rigorous selection process, stainless steel Types 316 and 316H are selected as the construction materials for the test loops, and Inconel 625 is selected as the construction material for the test sections. This thesis describes the design of the SCW and R-134a loops along with the three test-section geometries (i.e., tubular, annular and bundle designs).
Transient convective heat transfer
Scientific Electronic Library Online (English)
J., Padet.
2005-03-01
Full Text Available In nature, as well as within the human-made thermal systems, the time-variable regimes are more commonly encountered, if not always, than the permanent regimes. Nevertheless, studies in convection are still more frequent in the permanent regimes, undoubtedly due to the related difficulties in calcul [...] ation in terms of time and cost of computation. One may distinguish two categories of time-dependent transfers: those which are due to external causes (variable boundary conditions) and those that are due to internal causes (sources of variable power, instabilities, turbulence), and the combination of these two types may also be encountered. In this presentation, we shall analyze some situations which belong to the first category. These are concerned with: - a group of boundary layer flows in forced, natural or mixed convection, where the wall is subjected to time-variable conditions in temperature or flux. - another group of fluid flows within ducts, in laminar mixed convection regime, where the entry conditions (mass flow rate, temperature) are time-dependent. The techniques of analysis are mainly extensions to the differential method and to the integral method of Karman-Polhausen in boundary layer flows, and the finite differences solution of the vorticity and energy equations for internal flows. The results presented in the transient state are caused by steps of temperature, heat flux or velocity, and in particular show the time evolution of the dynamic and thermal boundary layers, as well of the heat transfer coefficients. Three examples of applications will then be treated: the active control of convective transfers, the measurement of heat transfer coefficients, and the analysis of heat exchangers. The main idea in the active control is that of managing the temperatures or heat fluxes by employing a variable regime. Under certain conditions, this procedure may reveal itself quite interesting. The measurement of transfer coefficients by the photothermal impulse method possesses a great interest since it is performed in a non-intrusive way without contact. However, in order to be precise, it needs to account for the thermal boundary layer perturbation due to the radiative flux sent over the surface, which means to know the evolution of the transfer coefficient during the measurement. Previous studies therefore provide essential information. Within the domain of heat exchangers, we shall present a different global method, which allows for the evaluation of the time constant of an equipment in response to sample variations of temperature or mass flow rates at the entrance. In conclusion, a brief balance of the ICHMT Symposium "Transient heat and mass transfer", Cesme, Turkey, August 2003, will be presented.
Kakac, Sadik; Pramuanjaroenkij, Anchasa
2014-01-01
Intended for readers who have taken a basic heat transfer course and have a basic knowledge of thermodynamics, heat transfer, fluid mechanics, and differential equations, Convective Heat Transfer, Third Edition provides an overview of phenomenological convective heat transfer. This book combines applications of engineering with the basic concepts of convection. It offers a clear and balanced presentation of essential topics using both traditional and numerical methods. The text addresses emerging science and technology matters, and highlights biomedical applications and energy technologies. What’s New in the Third Edition: Includes updated chapters and two new chapters on heat transfer in microchannels and heat transfer with nanofluids Expands problem sets and introduces new correlations and solved examples Provides more coverage of numerical/computer methods The third edition details the new research areas of heat transfer in microchannels and the enhancement of convective heat transfer with nanofluids....
Burnout in boiling heat transfer. Part II: subcooled and low quality forced-convection systems
International Nuclear Information System (INIS)
Recent experimental and analytical developments regrading burnout in subcooled and low quality forced-convection systems are reviewed. Much data have been accumulated which clarify the parametric trends and lead to new design correlations for water and a variety of other coolants in both simple and complex geometries. A number of critical experiments and models have been developed to attempt to clarify the burnout mechanism(s) in simpler geometries and power transients
Kahani, M.; Zeinali Heris, S.; Mousavi, S. M.
2014-05-01
Coiled tubes and nanofludics are two significant techniques to enhance the heat transfer ability of thermal equipments. The forced convective heat transfer and the pressure drop of nanofluid inside straight tube and helical coiled one with a constant wall heat flux were studied experimentally. Distilled water was used as a host fluid and Nanofluids of aqueous TiO2 nanoparticles (50 nm) suspensions were prepared in various volume concentrations of 0.25-2 %. The heat transfer coefficient of nanofluids is obtained for different nanoparticle concentrations as well as various Reynolds numbers. The experiments covered a range of Reynolds number of 500-4,500. The results show the considerable enhancement of heat transfer rate, which is due to the nanoparticles present in the fluid. Heat transfer coefficient increases by increasing the volume concentration of nanoparticles as well as Reynolds number. Moreover, due to the curvature of the tube when fluid flows inside helical coiled tube instead of straight one, both convective heat transfer coefficient and the pressure drop of fluid grow considerably. Also, the thermal performance factors for tested nanofluids are greater than unity and the maximum thermal performance factor of 3.72 is found with the use of 2.0 % volume concentration of nanofluid at Reynolds number of 1,750.
International Nuclear Information System (INIS)
Forced convection transient heat transfer for helium gas at various periods of exponential increase of heat input to a horizontal cylinder (heater) was theoretically and experimentally studied. In the theoretical study, transient heat transfer was numerically solved based on a turbulent flow model. It was clarified that the surface superheat and heat flux increase exponentially as the heat generation rate increases with the exponential function. The temperature distribution near the cylinder becomes larger as the surface temperature increases. The values of numerical solution for surface temperature and heat flux agree well with the experimental data for the cylinder diameter of 1 mm. However, the heat flux shows difference from the experimental values for the cylinder diameters of 0.7 mm and 2.0 mm. In the experimental studies, the authors measured heat flux, surface temperature, and transient heat transfer coefficients for forced convection flow of helium gas over horizontal cylinders under wide experimental conditions. The platinum cylinders with diameters of 1.0 mm, 0.7 mm, and 2.0 mm were used as test heaters and heated by electric current with an exponential increase of Q0exp(t/?) . The gas flow velocities ranged from 2 to 10 m/s, the gas temperatures ranged from 303 to 353 K, and the periods ranged from 50 ms to 20 s. It was clarified that the heat transfer coefficient approaches the quasi-steady-state one for the period ? longer than about 1 s, and it becomes higher for the period shorter than around 1 s. The transient heat transfer shows less dependence on the gas flowing velocity when the period becomes very shorter. The heat transfer shifts to the quasi-steady-state heat transfer for longer periods and shifts to the transient heat transfer for shorter periods at the same flow velocity. It also approaches the quasi-steady-state one for higher flow velocity at the same period. The transient heat transfer coefficients show significant dependence on cylinder diameters, there are higher for smaller cylinder diameters. The empirical correlations for quasi-steady-state heat transfer and transient heat transfer were obtained based on the experimental data. (authors)
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An experimental determination was made of heat transfer critical conditions in a circular channel, uniformly heated, and internally cooled by water in ascending forced convection, under a pressure slightly above atmospheric pressure. Measurements were made of water flow, pressure, electric power temperature and heating, and a systematic analysis was made of the system's parameters. The values obtained for the heat critical flux are circa 50% lower than those predicted by Becker and Biasi and this is accounted to flowing instabilities of thermo-hydrodynamic nature. It is suggested that the flowing channels of circuits aiming at the study of the boiling crisis phenomenon be expanded in its upper extremity, and that the coolant circulation be kept through a pump with a pressure X flow characteristic as vertical as possible
The influence of molten pool geometry on forced convective heat transfer
Wei, Cheng-hua; Fang, Bo-lang; Liu, Wei-ping; Wang, Li-jun; Ma, Zhi-liang
2015-05-01
An investigation was conducted to determine the relationship between heat transfer coefficient and molten pool's geometry. It was accomplished by performing an experimental and numerical investigation using a cylinder dimple with two different serials of geometry: (1) cylinder dimples with fixed print diameter D=50mm and different depth, and (2) cylinder dimples with fixed depth d=10mm and different print diameter. The airflow speed varies from 50m/s to 250m/s in the turbulent regime. The results consist of flow characteristics, mainly velocity profile and heat transfer characteristics, including heat transfer coefficient and Nusselt number along flow direction, were obtained. The comparison was held against the smooth surface. Results showed that a centrally-located vortex was formed due to the flow separation. For heat transfer coefficient, such augmentations are present near the downstream edges and diminutions are present near the upstream edges of dimple rims, both slightly within each depression. It was found that the convection heat transfer coefficients with different geometry parameters have similar distribution along flow direction. A uniform piecewise linear function was built to describe the heat transfer characterizes for different molten pool print diameter.
Energy Technology Data Exchange (ETDEWEB)
Vasic, S. [Centre de Recherche en Calcul Applique, Montreal, PQ (Canada); Baliga, B.R. [McGill Univ., Montreal, PQ (Canada). Dept. of Mechanical Engineering
1996-09-01
The performance of various turbulent flow prediction models were evaluated. In order to improve the accuracy of two-equation turbulence models, modifications have been made to account for streamline curvature, reduction of the eddy viscosity in reattachment regions, and better dynamical response of the equation governing the rate of dissipation of turbulent kinetic energy to sudden alterations of the velocity field. This paper reported on the computation of forced convection heat transfer in flow over a backward-facing step, using different eddy-viscosity low-Reynolds number turbulence models. Although the models showed different abilities, it was proposed that applying a simple damping function to the LSY (Launder and Sharma with Yap correction) model would yield accurate predictions of the turbulent heat transfer. 15 refs., 2 tabs., 7 figs.
International Nuclear Information System (INIS)
Highlights: ? Three different eddy viscosity turbulence models are validated. ? Two data sets and an extensive sensitivity study are employed for validation. ? Prediction errors for the velocity and temperature fields are analyzed. ? Turbulence Prandtl number has to be chosen in dependence of the turbulence model. - Abstract: This paper provides temperature and velocity distribution computations in heated annuli using RANS approach and employing three different turbulent viscosity models. In addition to comparison calculations an extensive sensitivity study was performed. The results show that the RANS approach and the turbulent viscosity models can be used for prediction of forced convection heat transfer to lead–bismuth-eutectic. However, the turbulent Prandtl number has to be carefully selected depending on the respective turbulence model.
Bejan, Adrian
2013-01-01
Written by an internationally recognized authority on heat transfer and thermodynamics, this second edition of Convection Heat Transfer contains new and updated problems and examples reflecting real-world research and applications, including heat exchanger design. Teaching not only structure but also technique, the book begins with the simplest problem solving method (scale analysis), and moves on to progressively more advanced and exact methods (integral method, self similarity, asymptotic behavior). A solutions manual is available for all problems and exercises.
Directory of Open Access Journals (Sweden)
Thamir K. Jassem
2013-05-01
Full Text Available An experimental forced laminar study was presented in this research for an air flowing through a circular channel for different angles ( ,30o,45o,60o, the channel was heated at constant heat flux , the channel also was packed with steel and glass spheres respectively . The tests were done for three values of Peclets number (2111.71,3945.42,4575.47 with changing the heat flux for each case and five times for each number.The results showed that the dimensionless temperature distribution will decrease with increasing the dimensionless channel length for all cases with changing Peclet number, heat flux and inclination angles, and its lowest value will be for glass spheres at highest flux, while at lower flux for , and the decreasing in dimensionless temperature was closed for both types of packed at other inclination angles.The study declared that the local Nusselt number decreases with increasing the dimensionless length of the channel for both packeds and for different applied heat flux, also through this study it was declared that the average Nusselt increases as Peclet number increases for both packed. Its value for the glass spheres is greater than the steel spheres with percentage (98.3% at small Peclet, and percentage (97.2% at large Peclet number for the horizontal tube, and (98.3% at small Peclet number and (97.8% at large Peclet number at .Through this study its was found that average Nusselt number increases along the channel as the heat flux increases, because the bulk temperature will increase as the flow proceeds toward the end of the channel , so the heat transfer coefficient will increase. It was declared from this study that in the case of the steel packed the heat transfer will occur mainly by conduction, while in the case of glass packed the heat transfer will occur mainly by laminar forced convection, where the lowest Nusselt number (Nu=3.8 was found when the pipe is horizontal and lowest heat flux and lowest Peclet number.
Influence of Tip Clearance on Forced Convection Heat Transfer of a Finned Plate in a Duct
International Nuclear Information System (INIS)
Optimizations are required for a proper enhancement of cooling capability. An important phenomenological consideration is to be reveals for a finned plate in a duct. Due to the high friction near the fin region and low friction near the wall region, the forced flow tends to bypass from fin region to wall region. The bypass flow increases the net flow and enhances the heat transfer for a moderate tip clearance which is defined by the distance from the tip of the fin and the wall. Meanwhile for a large tip clearance, most of the flow bypasses and does not contribute the heat transfer and impairs the heat transfer. This study is a preliminary numerical study on the influence of the tip clearance on the heat transfer of the finned plate in a duct. The study aimed at supporting an experimental research exploring the phenomena for a very small tip clearance. Thus material properties and test conditions were chosen to meet the experimental conditions. It investigated the phenomena at Pr of 2,014 and ReS of 58.3. In order to investigate the small tip clearance phenomena, a simple numerical scheme was developed using a commercial CFD code. A case with the same experimental condition was tested using the numerical scheme and the error was about 12%. The results show the clear evidence of the flow bypass from the fin region to wall region, which impair the heat removal capacity of the finned plate in a duct. The study has the relevance with the reactor cavity cooling system performance enhancement activities in the VHTR. The numerical scheme will be tested for narrower and wider tip clearances and find an optimal tip clearance
Theoretical and Computational Study of Forced-Convection Heat Transfer at Supercritical Pressures
Zhong, Jianguo
In the simulation of turbulent fluid flow and heat transfer at supercritical pressures, substantial difficulties have been encountered in the modeling of turbulence and bounda-ry layer. This is due to significant fluid property variations with respect to the local temperature and pressure, especially in the near-wall region of a heated wall, where large temperature differences occur. The classical turbulence models available in literature were typically developed for constant-property fluids, where an empirical wall function in the high-Re k-epsilon model, and a damping function in the low-Re k-epsilon model were derived based on the constant-property data to solve the boundary layer. As it can be found in the existing literature, large differences have been observed between the experimental and numerical simulation results of the heat transfer coefficient predictions in the en-hanced and deteriorated heat transfer situations for supercritical fluids. In this thesis, a novel near-wall treatment method is proposed to treat large property variations in the thermal and velocity sub-layers. In the near-wall region, the supercritical fluids can be considered thermal-conductive and viscous forces dominated. The thick-ness of the viscous sub-layer (VSL) and the conduction sub-layer (CSL) can be related to the wall shear stress and local Prandtl number information by using computational CFD models, such as that implemented in the NPHASE-CMFD code. The fluids' bulk and wall temperature information has been obtained from the literature review of experi-mental measurements. The wall temperature and heat transfer coefficient calculated from the k-epsilon model with the proposed wall treatment method have been found to be in good agreement with experimental data for both heat transfer enhancement and deterioration cases for two most widely used fluids: CO2 and water. The proposed model has been applied in the reactor-scale thermal-hydraulic analysis of different flow path designs in Gen-IV supercritical water nuclear reactors (SCWR). The main objective of this study has been to validate the performance of the current approach as a tool for the analysis of large-scale systems.
Aris, M.S.; McGlen, R.; Owen, I.; Sutcliffe, C.J.
2011-01-01
Forced air convection heat pipe cooling systems play an essential role in the thermal management of electronic and power electronic devices such as microprocessors and IGBT's (Integrated Gate Bipolar Transistors). With increasing heat dissipation from these devices, novel methods of improving the thermal performance of fin stacks attached to the heat pipe condenser section are required. The current work investigates the use of a wing type surface protrusions in the form of 3-D delta wing tabs...
Wen, M.-Y.
2005-08-01
An experiment was carried out to investigate the characteristics of the heat transfer and pressure drop for forced convection airflow over tube bundles that are inclined relative to the on-coming flow in a rectangular package with one outlet and two inlets. The experiments included a wide range of angles of attack and were extended over a Reynolds number range from about 250 to 12,500. Correlations for the Nusselt number and pressure drop factor are reported and discussed. As a result, it was found that at a fixed Re, for the tube bundles with attack angle of 45 ° has the best heat transfer coefficient, followed by 60, 75 and 90 °, respectively. This investigation also introduces the factors Eu_? = 90^ circ, which can be used for finding the heat transfer and the pressure drop factor on the tube bundles positioned at different angles to the flow direction. Moreover, no perceptible dependence of C?and C?? on Re was detected. In addition, flow visualizations were explored to broaden our fundamental understanding of the heat transfer for the present study.
Sivakumar, A.; Alagumurthi, N.; Senthilvelan, T.
2015-08-01
The microchannels are device used to remove high heat fluxes from smaller area. In this experimental research work the heat transfer performance of nanofluids of Al2O3/water and CuO/water were compared. The important character of such fluids is the enhanced thermal conductivity, in comparison with base fluid without considerable alteration in physical and chemical properties. The effect of forced convective heat transfer coefficient was calculated using serpentine shaped microchannel heat exchanger. Furthermore we calculated the forced convective heat transfer coefficient of the nanofluids using theoretical correlations in order to compare the results with the experimental data. The heat transfer coefficient for different particle concentration and temperature were analysed using forced convection heat transfer using nanofluids. The findings indicate considerable enhancement in convective heat transfer coefficient of the nanofluids as compared to the basefluid. The results also shows that CuO/water nanofluid has increased heat transfer coefficient compared with Al2O3/water and base fluids. Moreover the experimental results indicate there is increased forced convective heat transfer coefficient with the increase in nano particle concentration.
International Nuclear Information System (INIS)
Highlights: • Thermal conductivity is assumed a linear function of temperature. • It occurs due to diffusion-like radiation or a high temperature gradient. • Nusselt number and temperature profiles are obtained based on perturbation method. • Variable conductivity reveals enhancement obtained by porous media more clearly. • Nusselt number changes linearly with a linear change in conductivity. - Abstract: Effects of variation of the thermal conductivity on forced convection in a parallel-plates channel heat exchanger occupied by a fluid saturated porous medium are investigated analytically based on the perturbation methods. Walls of the channel are kept at a constant heat flux. Thermal conductivity of the medium is assumed to be a linear function of temperature (due to moderate radiation heat transfer in cellular foams or temperature dependent conductivity of the material). The Brinkman–Forchheimer–extended Darcy model for the flow field is used. Relations representing the temperature profile and Nusselt number as functions of porous medium shape parameter and thermal conductivity variation parameter are derived. Obtained Nusselt number and temperature profile are studied parametrically. No analytical investigation based on a variable conductivity approach for Brinkman–Forchheimer–extended Darcy model has been previously performed. Results show that a linear increase in the thermal conductivity of the medium results in a semi-linear increase in the Nusselt number
Ahmed, Mahmoud; Eslamian, Morteza
2015-12-01
Laminar natural convection in differentially heated (? = 0°, where ? is the inclination angle), inclined (? = 30° and 60°), and bottom-heated (? = 90°) square enclosures filled with a nanofluid is investigated, using a two-phase lattice Boltzmann simulation approach. The effects of the inclination angle on Nu number and convection heat transfer coefficient are studied. The effects of thermophoresis and Brownian forces which create a relative drift or slip velocity between the particles and the base fluid are included in the simulation. The effect of thermophoresis is considered using an accurate and quantitative formula proposed by the authors. Some of the existing results on natural convection are erroneous due to using wrong thermophoresis models or simply ignoring the effect. Here we show that thermophoresis has a considerable effect on heat transfer augmentation in laminar natural convection. Our non-homogenous modeling approach shows that heat transfer in nanofluids is a function of the inclination angle and Ra number. It also reveals some details of flow behavior which cannot be captured by single-phase models. The minimum heat transfer rate is associated with ? = 90° (bottom-heated) and the maximum heat transfer rate occurs in an inclination angle which varies with the Ra number. PMID:26183389
Ahmed, Mahmoud; Eslamian, Morteza
2015-07-01
Laminar natural convection in differentially heated ( ? = 0°, where ? is the inclination angle), inclined ( ? = 30° and 60°), and bottom-heated ( ? = 90°) square enclosures filled with a nanofluid is investigated, using a two-phase lattice Boltzmann simulation approach. The effects of the inclination angle on Nu number and convection heat transfer coefficient are studied. The effects of thermophoresis and Brownian forces which create a relative drift or slip velocity between the particles and the base fluid are included in the simulation. The effect of thermophoresis is considered using an accurate and quantitative formula proposed by the authors. Some of the existing results on natural convection are erroneous due to using wrong thermophoresis models or simply ignoring the effect. Here we show that thermophoresis has a considerable effect on heat transfer augmentation in laminar natural convection. Our non-homogenous modeling approach shows that heat transfer in nanofluids is a function of the inclination angle and Ra number. It also reveals some details of flow behavior which cannot be captured by single-phase models. The minimum heat transfer rate is associated with ? = 90° (bottom-heated) and the maximum heat transfer rate occurs in an inclination angle which varies with the Ra number.
Heterogeneous nanofluids: natural convection heat transfer enhancement
Bennacer Rachid; Oueslati Fakhreddine
2011-01-01
Abstract Convective heat transfer using different nanofluid types is investigated. The domain is differentially heated and nanofluids are treated as heterogeneous mixtures with weak solutal diffusivity and possible Soret separation. Owing to the pronounced Soret effect of these materials in combination with a considerable solutal expansion, the resulting solutal buoyancy forces could be significant and interact with the initial thermal convection. A modified formulation taking into account th...
Critical heat flux and associated phenomena in forced convective boiling in nuclear systems
Ahmad, Masroor
2012-01-01
In evaporation of a liquid flowing in a tube or nuclear fuel element, there exists a transition (known as "dryout", "burnout", "boiling crisis" or "critical heat flux", CHF) from a high heat transfer coefficient regime to one of greatly reduced heat transfer coefficient. The conditions leading to dryout or CHF and the behaviour of wall temperatures in the ("post dryout or post CHF") region beyond it are of immense importance in nuclear reactor safety. In a nuclear reactor, the clad temperatur...
Study of the equivalent diameter concept for heat transfer by forced convection in annular channels
International Nuclear Information System (INIS)
This work describes a comparative analysis between experimental values of heat transfer coefficients in fully developed turbulent flow for a concentric annular channel, and those calculated with the empirical correlations obtained for tubes by Dittus-Boelter, Sieder and Tate, a modified Colburn equation, and that proposed by Gnielinski which applies the analogy between friction and heat transfer. The coefficients were calculated by means of two different equivalent diameters: 1) The hydraulic equivalent diameter; and 2) The heated equivalent diameter. It was concluded that the hydraulic equivalent diameter gives much better results than the heated equivalent diameter. (Author)
Pallares, J.; Grau, F. X.; Davidson, L.
2005-07-01
This paper presents and discusses numerical simulations of forced convection heat transfer in a rotating square duct at high rotation rates. The mean pressure gradient has been kept constant in the simulations that were conducted with a second order finite volume code with a dynamical localized subgrid scale model. The rotation number based on the bulk velocity (Ro=2?D/U¯b) was varied from 0.12 to 6.6 and consequently the Reynolds number (Re=U¯bD/?) ranged from 3900 to 1810 according to the fact that rotation tends to increase the pressure drop in the duct. A model for estimating the velocities and the corresponding friction coefficient has been developed by analytically solving simplified versions of the momentum budgets within the Ekman layers occurring near the opposite two walls of the duct perpendicular to the rotation axis. The model reproduces accurately the velocity profiles of the numerical simulation at high rotation rates and predicts that the boundary layer quantities scale as Ek1/2 (Ek=?/?D2). At Ro >1 the Ekman layers are responsible for most of the pressure drop of the flow while the maximum heat transfer rates are found on the wall where the stratification of the x-momentum is unstable with respect to the Coriolis force. Rotation enhances the differences between the contributions of the local friction coefficients and local Nusselt numbers of the four walls of the duct and considerably increases, in comparison with the non-rotating case, the pressure drop of the flow and the Nusselt number. The overall friction coefficient of the measurements and the simulations existing in the literature, as well as the present numerical predictions, are well correlated with the equation 1.09(Cf/Ek1/2)1.25=Ro in the range Ro ?1 for Re ?104.
Directory of Open Access Journals (Sweden)
Sundus Hussein Abd
2012-01-01
Full Text Available In this research, an experimental study was conducted to high light the impact of the exterior shape of a cylindrical body on the forced and free convection heat transfer coefficients when the body is hold in the entrance of an air duct. The impact of changing the body location within the air duct and the air speed are also demonstrated. The cylinders were manufactured with circular, triangular and square sections of copper for its high thermal conductivity with appropriate dimensions, while maintaining the surface area of all shapes to be the same. Each cylinder was heated to a certain temperature and put inside the duct at certain locations. The temperature of the cylinder was then monitored. The heat transfer coefficient were then calculated for forced convection for several Reynolds number (4555-18222.The study covered free convection impact for values of Rayleigh number ranging between (1069-3321. Imperical relationships were obtained for all cases of forced and free convection and compared with equations of circular cylindrical shapes found in literature. These imperical equations were found to be in good comparison with that of other sources.
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Bu, S.S.; Yang, J.; Zhou, M.; Li, S.Y. [Key Laboratory of Thermo-Fluid Science and Engineering, Ministry of Education, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049 (China); Wang, Q.W., E-mail: wangqw@mail.xjtu.edu.cn [Key Laboratory of Thermo-Fluid Science and Engineering, Ministry of Education, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049 (China); Guo, Z.X. [Department of Mechanical and Aerospace Engineering, Rutgers, The State University of New Jersey, 98 Brett Road, Piscataway, NJ 08854 (United States)
2014-04-01
Highlights: • A systematical study on contact modifications is performed for structured packed beds. • The bridges modification is found to give the most reasonable macroscopic results. • The overlaps and bridges methods are suitable for predicting local heat transfer. • Reasonable bridge diameter is found in a range from 16% d{sub p} to 20% d{sub p}. - Abstract: The present paper systematically investigated the appropriateness of different contact point modification approaches for forced convective heat transfer analysis in structured packed beds of spheres. The three-dimensional Navier–Stokes equations and RNG k–? turbulence model with scalable wall function are adopted to model the turbulent flow inside the pores. Both macroscopic and local flow and heat transfer characteristics for different packing forms (simple cubic, body center cubic and face center cubic packing forms) and contact treatments (gaps, overlaps, bridges and caps modifications) are carefully examined. In particular, the effects caused by the bridge size for the bridges treatment are discussed, and the numerical results are compared with available experiments in literature. It is found that the effects of contact treatments on the pressure drops are remarkable for different structured packing forms, especially when the porosity is relatively low, while such effects on the Nusselt numbers are relatively small. Among the four different contact modifications, the bridges method would give the most reasonable pressure drops for all the structured packing forms studied and this method is also proved to be suitable for predicting the Nusselt numbers. The local flow and heat transfer characteristics in the structured packed bed are sensitive to the methodology of contact modifications. The gaps and caps treatments would distort the local flow and temperature distributions in the packed bed, especially near the contact zones. While the local flow and temperature distributions from the overlaps and bridges treatments would be more reasonable and close to those in the original packing with points contact. Based on both the macroscopic and local flow and heat transfer analyses, the bridges treatment is recommended. The effects caused by the bridge size in the bridges treatment are also remarkable. It is noted that too small or too large bridge size would lead to unreasonable results for both the macroscopic and local flow and heat transfer analyses. A reasonable range of bridge diameter is found to be from 16% d{sub p} to 20% d{sub p}.
International Nuclear Information System (INIS)
Mixed convection heat transfer in a ventilated cavity is numerically studied by solving the mixed convection equations with the Boussinesq approximation. Results are presented in terms of streamlines, isotherms and heat transfer for different combinations of the governing parameters namely, the Reynolds number (10 ? Re ? 5000), the Rayleigh number (104 ? Ra ? 106) and the relative height of the openings (B = h'/H' = 1/4). The numerical results show the presence of a maximum interaction between the effects of the forced and natural convection and the existence of different flow regimes. The latter are delineated in the Ra-Re plane and the values of Re separating the different regions are determined and correlated versus Ra
Heat transfer in transition domain on film boiling in case of forced convection
International Nuclear Information System (INIS)
The design of reactor safety installations is based on the assumption, that nucleate boiling changes immediately to film boiling or vice versa, if hypothetical emergencies like LOCA (loss of coolant accident) or ATWS (anticipated transient without scram) occur. Between nucleate and film boiling, however, a 'transition boiling' region exists with better heat transfer conditions than in the film boiling region. The report presents experimental and theoretical studies on transition boiling. Such data are suitable to improve computer codes which simulate LOCA and ATWS emergencies and prove the effectiveness of safety installations. The experiments are carried out under steady-state thermal and hydrodynamic conditions, to promote a better understanding of the basic physical mechanisms of heat transfer in the transition region. Measurements with temperature-controlled heat transfer surfaces are required to permit stabilized evaporation in the transition region. A vertical tube of 14 mm inner diameter is used as test section, to simulate approximately the size of flow channels in BWR and PWR reactors. The experiments are carried out with refrigerant R114. Methods for scaling of the results to transition boiling heat transfer of water are presented. (orig.)
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Turner, C.W.; Klimas, S.J.; Brideau, M.G
2000-02-01
Degradation of the thermal performance of steam generators(SGs) is a serious problem in nuclear power stations throughout the world (Lovett and Dow, 1991). In pressurized-heavy-water reactors (PWHRs), the reduced thermal performance of the SGs is manifested by an increase of the primary coolant reactor inlet header temperature (RIHT). In pressurized-light-water reactors(PWRs), which operate with fixed primary coolant temperature, the loss of thermal performance is manifested by a reduction of the steam pressure. Degradation mechanisms that may contribute to the loss of SG thermal performance include: fouling of the boiler tube inner surfaces (primary-side fouling); fouling of the boiler tube outer surfaces (secondary-side fouling); divider and thermal plate leakage that causes the coolant to bypass either the SG or the integral preheater and fouling of the steam separators. The relative contribution of these various degradation mechanisms to the overall loss of thermal performance is still under investigation. Soulard et al. (1990) examined the relative contributions of tube bundle fouling, divider plate leakage, and thermal plate leakage to the increase in RIHT at the Point Lepreau Generating Station, and concluded that tube fouling contributes to a significant fraction of the loss of thermal performance. Corrosion products deposit on both the inner and outer surfaces of the boiler tubes. Thus a complete understanding of the reasons fro the loss of thermal performance and the development of strategies to mitigate this loss requires a knowledge of the thermal resistance of tube deposits under primary and secondary side heat transfer conditions. We present here the results of measurements of the thermal resistance of primary-side and secondary-side boiler tube deposits performed under single-phase forced convection and flow-boiling conditions, respectively. The results are discussed in terms of the physical properties of the deposit and the mode of heat transfer.
International symposium on transient convective heat transfer: book of abstracts
International Nuclear Information System (INIS)
The international symposium on convective heat transfer was held on 19-23 August 1996, in Cesme, Izmir, Turkey. The spesialists discussed forced convection, heat exchangers, free convection and multiphase media and phase change at the meeting. Almost 53 papers were presented in the meeting
Yang, Yahui
2013-01-01
The last decade has witnessed very fast development in microfabrication technologies. The increasing industrial applications of microfluidic systems call for more intensive and systematic knowledge on this newly emerging field. Especially for gaseous flow and heat transfer at microscale, the applicability of conventional theories developed at macro scale is not yet completely validated; this is mainly due to scarce experimental data available in literature for gas flows. The objective of this...
International Nuclear Information System (INIS)
Some correlations of forced convection burn-out data are based on the approximate linearity of the relationship between burn-out heat flux and the channel-averaged quality at the burn-out point. These correlations perform satisfactorily on data obtained from uniformly heated configurations. Therefore the further inference is sometimes made that the burn-out heat flux is uniquely related to the quality, and that the burn-out in non-uniformly heated configurations can be calculated from measurements made with uniform heating. This report presents burn-out data for Freon 12 flowing vertically upwards through both uniformly and non-uniformly heated round tubes. This data shows that the quality at burn-out does depend on the heat flux profile, and that the inference mentioned above is not justified. (author)
Experimental study of micro-particle fouling under forced convective heat transfer
Scientific Electronic Library Online (English)
S. M., Peyghambarzadeh; A., Vatani; M., Jamialahmadi.
2012-12-01
Full Text Available Particulate fouling studies of a hydrocarbon based suspension containing 2 µm alumina particles were performed in an annular heat exchanger having a hydraulic diameter of 14.7 mm. During fouling experiments, the classical asymptotical behavior was observed. It is shown that particle concentration, f [...] luid velocity, and wall temperature have strong influences on the fouling curve and the asymptotic fouling resistance. Furthermore, a mathematical model is developed to formulate the asymptotic fouling resistance in terms of the mass transfer coefficient, thermophoresis velocity, and fluid shear rate. The results demonstrate that the prediction of the new model is in good agreement with the experimental observations.
International Nuclear Information System (INIS)
This paper explores the bearing that a non-uniform distribution of heat flux used as a wall boundary condition exerts on the heat transfer improvement in a round pipe. Because the overall heat load is considered fixed, the heat transfer improvement is viewed through a reduction in the maximum temperature (‘hot spot’) by imposing optimal distribution of heat flux. Two cases are studied in detail 1) fully developed and 2) developing flow. Peak temperatures in the heated pipe wall are calculated via an analytical approach for the fully developed case, while a numerical simulation based on CFD is employed for the developing case. By relaxing the heat flux distribution on the pipe wall, the numerical results imply that the optimum distribution of heat flux, which minimizes the peak temperatures corresponds with the ‘descending’ distribution. Given that the foregoing approach is quite different from the ‘ascending’ heat flux distribution recommended in the literature by means of the entropy generation minimization (EGM) method, it is inferred that the optimization of heat transfer and fluid flow, in comparison with the thermodynamic optimization, may bring forth quite different guidelines for the designs of thermal systems under the same constraints and circumstances. -- Highlights: • Considered the bearing of non-uniform distribution of heat flux on the hot spots. • Determined the optimal distribution of heat flux that minimizes the hot spots. • Results are compared with those obtained by EGM method
International Nuclear Information System (INIS)
In order to enhance the heat transfer performance of innovative decay heat removal system for FBRs, we have proposed to adopt a porous plate as a thermal radiation heat collector. Experimental studies are made on the convective heat transfer of three types of thin porous plates under natural convection. Heat transfer performances and temperature profiles near the porous plates are compared with each other. It is revealed that the porous plates have high heat transfer performances. The effect of pore size on the heat transfer characteristics under natural convection is smaller than that under forced convection. (author)
International Nuclear Information System (INIS)
In the heat transfer studies by forced convection, we have few data about behavior of the fluids in an annular channel heated by a concentric pipe, such date is necessary to know the heat transfer coefficient that establish the interchange of energy and the thermic properties of the fluid with the geometry of the flow. In this work the objective, was to compare some empirical correlations that we needed for determinate the heat transfer coefficient for annular channels, where we obtained similar at the theoretical results of an experiment made by Miller and Benforado. It is important to know such coefficients because we can determinate the heat quantity transmitted to a probe zone, in which we simulate a nuclear fuel element that developed huge heat quantity that must be dispersed in short time. We give theoretical data of the heat forced transfer convection and we analyzed the phenomena in annular channels given some empirical correlations employed by some investigators and we analyzed each one. (Author)
Convective Flow under Rotating Force.
Directory of Open Access Journals (Sweden)
Meenu Hooda
2013-09-01
Full Text Available A finite difference code using the primitive variables is used to simulate the mixed convection in air (Pr=0.7 and liquid metals (Pr=0.015. The present study involves numerical simulation of momentum and energy equations in order to analyze two dimensional mixed convection in air and liquid metals in a differentially heated square cavity subjected to rotation for a broad range of operating parameters i.e. Rayleigh number (Ra , Taylor number (Ta and rotational Raleigh number (Raw.
International Nuclear Information System (INIS)
Steady-state numerical results for the solution to the non-linear thermal problem of combined free and forced laminar convection in inclined rectangular channels with constant but unequal surface temperature are presented for an incompressible, viscous fluid whose Prandtl number, Pr = 0.73. Fluid properties are assumed constant, except for density variations with temperature. Maximum values exist for the mean friction factor, Nusselt and Stanton numbers when the inclination to the horizontal lies between 300 and 600 for a given Archimedes number, Ar. Also, for any given inclination a unique solution exists when Ar = 0,50. (Author)
Internally heated convection and Rayleigh-B\\'enard convection
Goluskin, David
2015-01-01
This work reviews basic features of both internally heated (IH) convection and Rayleigh-B\\'enard (RB) convection, along with findings on IH convection from laboratory experiments and numerical simulations. In the first chapter, six canonical models of convection are described: three configurations of IH convection driven by constant and uniform volumetric heating, and three configurations of RB convection driven by the boundary conditions. The IH models are distinguished by differing pairs of thermal boundary conditions: top and bottom boundaries of equal temperature, an insulating bottom with heat flux fixed at the top, and an insulating bottom with temperature fixed at the top. The RB models also are distinguished by whether temperatures or heat fluxes are fixed at the top and bottom boundaries. Integral quantities important to heat transport are discussed, including the mean fluid temperature, the mean temperature difference between the boundaries, and the mean convective heat transport. Integral relations...
Heat transfer in turbulent mixed convection
International Nuclear Information System (INIS)
The contents of this book are: Basic Equations of Convective Heat Transfer; Basic Information on the Theory of Turbulent Heat Transfer in Flow Near Walls; Heat Transfer in Laminar Mixed Convection; Turbulent Mixed Convection in Boundary Layers; Turbulent Flow and Heat Transfer in Horizontal Channels; Turbulent Flow and Heat Transfer in Vertical Channels; and Gravitational Effects on Heat Transfer in a Single-Phase Fluid Near the Critical Point
Natural convection with combined driving forces
Ostrach, S.
1980-01-01
The problem of free and natural convection with combined driving forces is considered in general and all possible configurations are identified. Dimensionless parameters are discussed in order to help categorize the various problems, and existing work is critically evaluated. Four distinct cases are considered for conventional convection and for the situation when the body force and the density gradient are parallel but opposed. Considerable emphasis is given to unstable convection in horizontal layers.
Terminal project heat convection in thin cylinders
International Nuclear Information System (INIS)
Heat convection in thin cylinders and analysis about natural convection for straight vertical plates, and straight vertical cylinders submersed in a fluid are presented some works carry out by different authors in the field of heat transfer. In the part of conduction, deduction of the equation of heat conduction in cylindrical coordinates by means of energy balance in a control volume is presented. Enthalpy and internal energy are used for the outlining of the equation and finally the equation in its vectorial form is obtained. In the convection part development to calculate the Nusselt number for a straight vertical plate by a forces analysis, an energy balance and mass conservation over a control volume is outlined. Several empiric correlations to calculate the Nusselt number and its relations with other dimensionless numbers are presented. In the experimental part the way in which a prototype rode is assembled is presented measurements of temperatures attained in steady state and in free convection for working fluids as air and water are showed in tables. Also graphs of Nusselt numbers obtained in the experimental way through some empiric correlations are showed (Author)
Convective heat transfer in rotating cylindrical cavity
Owen, J. M.; Onur, H. S.
1983-04-01
In order to gain an understanding of the conditions inside air-cooled, gas-turbine rotors, flow visualization, laser-doppler anemometry, and heat-transfer measurements have been made in a rotating cavity with either an axial throughflow or a radial outflow of coolant. For the axial throughflow tests, a correlation has been obtained for the mean Nusselt number in terms of the cavity gap ratio, the axial Reynolds number, and rotational Grashof number. For the radial outflow tests, velocity measurements are in good agreement with solutions of the linear (laminar and turbulent) Ekman layer equations, and flow visualization has revealed the destabilizing effect of buoyancy forces on the flow structure. The mean Nusselt numbers have been correlated, for the radial outflow case, over a wide range of gap ratios, coolant flow rates, rotational Reynolds numbers and Grashof numbers. As well as the three (forced convection) regimes established from previous experiments, a fourth (free convection) regime has been identified.
International Nuclear Information System (INIS)
This paper provides information on heat transfer enhancement due to jet mixing inside a cylindrical enclosure. The work addresses conservative heat transfer assumptions regarding mixing and condensation that have typically been incorporated into passive containment design analyses. The current research presents an interesting possibility for increasing decay heat removal of passive containment systems under combined natural and forced convection. Eliminating these conservative assumptions could provide the basis for a change of containment design and reduce the construction cost. It is found that the ratio of forced- and free convection Nusselt numbers can be predicted as a function of the Archimedes number and a correlated factor accounting for jet orientation and enclosure geometry. To use the small-scale tests for large containment design, scale-up methods and criteria are important for matching the key governing parameters and fluid properties. In the present experiment, a cylindrical enclosure was constructed with a vertical wall of 2.29-m diameter and 0.8-m height and a vertically adjustable ceiling. A horizontal copper plate was installed at the bottom to provide an isothermal heating surface. Cold air was injected at several positions with varying pipe diameters and injecting orientations and was removed from the top of the enclosure. The experiment was performed with an extensive set of tests to study the combined natural- and forced convection heat transfer ural- and forced convection heat transfer in a cylindrical enclosure mixed by an injected jet. The goals are to evaluate the key parameters governing the heat transfer augmentation by a forced jet and to investigate the effect of geometric factors, including jet diameter, jet injection orientation, and enclosure geometry (aspect ratio). Flow velocity measurement further provides a better understanding of the flow patterns developed inside the enclosure, which will determine the effectiveness of the whole volume mixing process. An additional experiment with a vertically cooled wall, similar to the condition of a passive containment cooling surface, is under construction. It will be used for further investigation of similar phenomena for steam condensation in the presence of non-condensable gases. From the current study with the cylindrical enclosure, it was found that the augmentation of natural-convection heat transfer by a forced jet is primarily determined by the jet Reynolds number, while the injecting nozzle diameter has a relatively weak effect. The jet orientation also plays an important role in determining the augmentation ratio, and of the four different jet orientations studied here, vertical downward injection at the center of the enclosure gives the highest augmentation, while azimuthal injection gives the lowest. The enclosure geometry and aspect ratio are also important factors, depending on the jet orientation. A combining rule is employed with a weighted relation to balance the contributions from separate heat-transfer correlations representing natural and forced dominated convections. It was found that under natural convection without the jet, the mean Nusselt number inside the large enclosure can be correlated by the enclosure Rayleigh number, RaD1/3, and under forced convection with a strong jet, the data are well represented as a function of the jet Reynolds number, Rej2/3. According to these relationships, a correlation form predicting mixed convection inside the enclosure was developed by a function that uses the ratio of forced- and free-convection heat transfer coefficients. This form can then be further reduced to predict the augmentation ratio as a function of the Archimedes number and a correcting factor accounting for jet orientation and enclosure geometry. Figure 1 presents the heat transfer augmentation as the Nusselt number ratio of mixed convection to natural convection (NuD/Nunc) versus the Archimedes number. The data, including four injecting orientations and three different jet diameters, are well correlated for each injecti
Scientific Electronic Library Online (English)
Philip O., Olanrewaju; Jacob A., Gbadeyan; Tasawar, Hayat; Awatif A., Hendi.
2011-10-01
Full Text Available In this paper we analyse the effects of internal heat generation, thermal radiation and buoyancy force on the laminar boundary layer about a vertical plate in a uniform stream of fluid under a convective surface boundary condition. In the analysis, we assumed that the left surface of the plate is in [...] contact with a hot fluid whilst a stream of cold fluid flows steadily over the right surface; the heat source decays exponentially outwards from the surface of the plate. The similarity variable method was applied to the steady state governing non-linear partial differential equations, which were transformed into a set of coupled non-linear ordinary differential equations and were solved numerically by applying a shooting iteration technique together with a sixth-order Runge-Kutta integration scheme for better accuracy. The effects of the Prandtl number, the local Biot number, the internal heat generation parameter, thermal radiation and the local Grashof number on the velocity and temperature profiles are illustrated and interpreted in physical terms. A comparison with previously published results on similar special cases showed excellent agreement.
International Nuclear Information System (INIS)
Forced air convection heat pipe cooling systems play an essential role in the thermal management of electronic and power electronic devices such as microprocessors and IGBT's (Integrated Gate Bipolar Transistors). With increasing heat dissipation from these devices, novel methods of improving the thermal performance of fin stacks attached to the heat pipe condenser section are required. The current work investigates the use of a wing type surface protrusions in the form of 3-D delta wing tabs adhered to the fin surface, thin wings punched-out of the fin material and TiNi shape memory alloy delta wings which changed their angles of attack based on the fin surface temperature. The longitudinal vortices generated from the wing designs induce secondary mixing of the cooler free stream air entering the fin stack with the warmer fluid close to the fin surfaces. The change in angle of the attack of the active delta wings provide heat transfer enhancement while managing flow pressure losses across the fin stack. A heat transfer enhancement of 37% compared to a plain fin stack was obtained from the 3-D tabs in a staggered arrangement. The punched-out delta wings in the staggered and inline arrangements provided enhancements of 30% and 26% respectively. Enhancements from the active delta wings were lower at 16%. However, as these devices reduce the pressure drop through the fin stack by approximately 19% in the de-activate position, over the activated position, a reduction in fan operating cost may be achieved for systems operating with inlet air temperatures below the maximum inlet temperature specification for the device. CFD analysis was also carried out to provide additional detail of the local heat transfer enhancement effects. The CFD results corresponded well with previously published reports and were consistent with the experimental findings. - Highlights: ? Heat transfer enhancements of heat pipe fin stacks was successfully achieved using fixed and active delta wing vortex generators. ? The active vortex generators, made from Ti-Ni, protruded into the flow stream at high temperatures and resume a low profile position at a low temperature set point. ? By considering wing spacing and the distance from the leading edge of the fin stack, heat transfer enhancements of up to 37%, compared to plane fin stacks, were achieved. ? By replacing the fixed delta wings with the active vortex generators, heat transfer enhancements of up to 16% was achieved and the pressure loss associated with flow obstructions was effectively managed.
Rajesh Khatri; Pankaj Agarwal
2012-01-01
In this paper heat transfer and fluid flow characteristics in a channel has been theoretically investigated. In this study, FEM is employed to analyze a fluid flow inside a channel and then solve for the heat flow transfer through the same channel. The fluid flow is expressed by partial differential equation (Poisson’s equation).While, heat transfer is analyzed using the energy equation. The Navier Stokes equations along with the energy equation have been solved by using simple technique. The...
Scientific Electronic Library Online (English)
Néstor Enrique, Cerquera Peña; Yaneth Liliana, Ruiz Osorio; Eduardo, Pastrana Bonilla.
2010-04-01
Full Text Available Teniendo como base la infraestructura existente de un horno tradicional de curado de tabaco, se rediseño e implementó en él un sistema de intercambio de calor por convección forzada que funciona con cisco de café como combustible. Este horno de curado de tabaco por convección forzada USCO-MADR fue e [...] valuado durante el periodo de cosecha, lográndose un manejo controlado de las variables de temperatura y humedad relativa dentro de él durante las tres etapas del curado de la hoja de tabaco; el equipo utilizado tuvo un excelente desempeño al emplear cisco de café como combustible con los siguientes consumos durante el proceso de curado: en la fase de “amarillamiento”, 8,92 kilogramos por hora; en la de “secado de paño y fijación de color”, 17,75 kilogramos por hora; y en la de “secado de vena”, 19,29 kilogramos por hora; el análisis comparativo de los costos operativos del horno evaluado, con los ajustes propuestos a éste, permiten presentarlo a la cadena de tabaco como una alternativa promisoria. Abstract in english A traditional oven for curing tobacco leaves was redesigned (based on existing infrastructure); a forced-convection heat exchanger system was implemented in it which worked with coffee hulls as fuel. This oven (called a forced-convection tobacco leaf curing oven) was evaluated during the harvesting [...] season. It was found that temperature and relative humidity inside the furnace could be controlled with this assembly during the three stages involved in curing tobacco leaves. The equipment used performed excellently when using coffee hulls as fuel, having the following approximate consumption during curing: 8.92 kilograms per hour during the yellowing stage, 17.75 kilograms per hour during the leaf drying and color fixation phase and 19.29 kilograms per hour during the stem drying stage. Comparative analysis of the oven´s operating costs along with the proposed adjustments to be made to it would allow its implementation as a promising alternative in the existing tobacco chain.
Effects of rib size and arrangement on forced convective heat transfer in a solar air heater channel
Skullong, Sompol; Thianpong, Chinaruk; Promvonge, Pongjet
2015-10-01
The article presents an experimental investigation on turbulent heat transfer and friction loss behaviors of airflow through a constant heat-fluxed solar air heater channel fitted with rib turbulators. The experiment was conducted for the airflow rate in terms of Reynolds numbers based on the hydraulic diameter of the channel in a range of 5000-24,000. In the present work, a comparative study between square and thin ribs (90°-rib) with three rib arrangements, namely, one ribbed wall (or single rib), in-line and staggered ribs on two opposite walls was first introduced. The study shows a significant effect of the presence of the ribs on the heat transfer rate and friction loss over the smooth wall channel. The comparison made at a single rib pitch and height also revealed that the thin rib performs better than the corresponding square one. Among the three arrangements, the in-line rib array provides higher heat transfer and friction loss than the staggered and the single one. However, the staggered thin rib provides the highest thermal performance. With this reason, only the staggered thin ribs at four different relative heights (BR = 0.1, 0.2, 0.3 and 0.4) and three relative pitches (PR = 0.5, 0.75 and 1.33) are investigated further. It is found that the staggered rib at BR = 0.4 and PR = 0.5 yields the highest heat transfer and friction factor but the maximum thermal performance is at BR = 0.2 and PR = 0.75.
Energy Technology Data Exchange (ETDEWEB)
Sridharan, Kumar; Anderson, Mark; Allen, Todd; Corradini, Michael
2012-01-30
The goal of this NERI project was to perform research on high temperature fluoride and chloride molten salts towards the long-term goal of using these salts for transferring process heat from high temperature nuclear reactor to operation of hydrogen production and chemical plants. Specifically, the research focuses on corrosion of materials in molten salts, which continues to be one of the most significant challenges in molten salts systems. Based on the earlier work performed at ORNL on salt properties for heat transfer applications, a eutectic fluoride salt FLiNaK (46.5% LiF-11.5%NaF-42.0%KF, mol.%) and a eutectic chloride salt (32%MgCl2-68%KCl, mole %) were selected for this study. Several high temperature candidate Fe-Ni-Cr and Ni-Cr alloys: Hastelloy-N, Hastelloy-X, Haynes-230, Inconel-617, and Incoloy-800H, were exposed to molten FLiNaK with the goal of understanding corrosion mechanisms and ranking these alloys for their suitability for molten fluoride salt heat exchanger and thermal storage applications. The tests were performed at 850Ã?Â?Ã?Â?Ã?Â?Ã?Â?C for 500 h in sealed graphite crucibles under an argon cover gas. Corrosion was noted to occur predominantly from dealloying of Cr from the alloys, an effect that was particularly pronounced at the grain boundaries Alloy weight-loss due to molten fluoride salt exposure correlated with the initial Cr-content of the alloys, and was consistent with the Cr-content measured in the salts after corrosion tests. The alloysÃ?Â?Ã?Â¢Ã?Â?Ã?Â?Ã?Â?Ã?Â? weight-loss was also found to correlate to the concentration of carbon present for the nominally 20% Cr containing alloys, due to the formation of chromium carbide phases at the grain boundaries. Experiments involving molten salt exposures of Incoloy-800H in Incoloy-800H crucibles under an argon cover gas showed a significantly lower corrosion for this alloy than when tested in a graphite crucible. Graphite significantly accelerated alloy corrosion due to the reduction of Cr from solution by graphite and formation on Cr-carbide on the graphite surface. Ni-electroplating dramatically reduced corrosion of alloys, although some diffusion of Fe and Cr were observed occur through the Ni plating. A pyrolytic carbon and SiC (PyC/SiC) CVD coating was also investigated and found to be effective in mitigating corrosion. The KCl-MgCl2 molten salt was less corrosive than FLiNaK fluoride salts for corrosion tests performed at 850oC. Cr dissolution in the molten chloride salt was still observed and consequently Ni-201 and Hastelloy N exhibited the least depth of attack. Grain-boundary engineering (GBE) of Incoloy 800H improved the corrosion resistance (as measured by weight loss and maximum depth of attack) by nearly 50% as compared to the as-received Incoloy 800H sample. Because Cr dissolution is an important mechanism of corrosion, molten salt electrochemistry experiments were initiated. These experiments were performed using anodic stripping voltammetry (ASV). Using this technique, the reduction potential of Cr was determined against a Pt quasi-reference electrode as well as against a Ni(II)-Ni reference electrode in molten FLiNaK at 650 oC. The integrated current increased linearly with Cr-content in the salt, providing for a direct assessment of the Cr concentration in a given salt of unknown Cr concentration. To study heat transfer mechanisms in these molten salts over the forced and mixed convection regimes, a forced convective loop was constructed to measure heat transfer coefficients, friction factors and corrosion rates in different diameter tubes in a vertical up flow configuration in the laminar flow regime. Equipment and instrumentation for the forced convective loop was designed, constructed, and tested. These include a high temperature centrifugal pump, mass flow meter, and differential pressure sensing capabilities to an uncertainty of < 2 Pa. The heat transfer coefficient for the KCl-MgCl2 salt was measured in two different diameter channels (0.083Ã?Â?Ã?Â¢Ã?Â?Ã?Â?Ã?Â
Comparison of Thermal Comfort by Radiant Heating and Convective Heating
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Shigeru Imai
2015-01-01
Full Text Available Currently, convective heating with a heat-pump system, which has high energy efficiency, is popular for room heating. However, it is possible that energy savings using convective heating can be further improved using heat pumps that service both occupied and unoccupied spaces. Moreover, convective heating increases vertical temperature gradients in a room; thus, it is hard to say whether occupants are being provided with sufficient thermal comfort. The purpose of this study is to compare the thermal comfort provided by both radiant and convective heating systems. In this study, a small office room was modeled, and then temperature and airflow distributions in the room were calculated by Computational Fluid Dynamics (CFD simulations using ESP-r (Environmental research simulation software. Furthermore, distributions of Standard Effective Temperatures (SET* were calculated using the air temperature distributions obtained from the CFD simulations, which allows us to compare the thermal comfort provided by convective heating with that provided by radiant heating. The results show that radiant heating can provide satisfactory thermal comfort, even when the room air temperature is low. However, thermal comfort also depends on the temperature of blowing air, and blowing air must reach occupied regions; thus, only radiant heating cannot circulate sufficient air. In contrast, convective heating increases vertical temperature gradients in a room. Therefore, rather than using only radiant or convective heating, it may be more effective to combine them efficiently.
Turbulent mixed convection in asymmetrically heated vertical channel
Directory of Open Access Journals (Sweden)
Mokni Ameni
2012-01-01
Full Text Available In this paper an investigation of mixed convection from vertical heated channel is undertaken. The aim is to explore the heat transfer obtained by adding a forced flow, issued from a flat nozzle located in the entry section of a channel, to the up-going fluid along its walls. Forced and free convection are combined studied in order to increase the cooling requirements. The study deals with both symmetrically and asymmetrically heated channel. The Reynolds number based on the nozzle width and the jet velocity is assumed to be 3 103 and 2.104; whereas, the Rayleigh number based on the channel length and the wall temperature difference varies from 2.57 1010 to 5.15 1012. The heating asymmetry effect on the flow development including the mean velocity and temperature the local Nusselt number, the mass flow rate and heat transfer are examined.
Winifred Nduku Mutuku-Njane; Oluwole Daniel Makinde
2013-01-01
We examine the effect of magnetic field on boundary layer flow of an incompressible electrically conducting water-based nanofluids past a convectively heated vertical porous plate with Navier slip boundary condition. A suitable similarity transformation is employed to reduce the governing partial differential equations into nonlinear ordinary differential equations, which are solved numerically by employing fourth-order Runge-Kutta with a shooting technique. Three different water-based nanofl...
Thermal interaction between free convection and forced convection along a vertical conducting wall
Shu, Jian-Jun
2015-01-01
A theoretical study is presented in this paper to investigate the conjugate heat transfer across a vertical finite wall separating two forced and free convection flows at different temperatures. The heat conduction in the wall is in the transversal direction and countercurrent boundary layers are formed on the both sides of the wall. The governing equations of this problem and their corresponding boundary conditions are all cast into a dimensionless form by using a non-similarity transformation. These resultant equations with multiple singular points are solved numerically using a very efficient singular perturbation method. The effects of the resistance parameters and Prandtl numbers on heat transfer characteristics are investigated.
Latent Heating Processes within Tropical Deep Convection
van den Heever, S. C.; Mcgee, C. J.
2013-12-01
It has been suggested that latent heating above the freezing level plays an important role in reconciling Riehl and Malkus' Hot Tower Hypothesis (HTH) with observational evidence of diluted tropical deep convective cores. In this study, recent modifications to the HTH have been evaluated through the use of Lagrangian trajectory analysis of deep convective cores simulated using the Regional Atmospheric Modeling System (RAMS), a cloud-resolving model (CRM) with sophisticated microphysical, surface and radiation parameterization schemes. Idealized, high-resolution simulations of a line of tropical convective cells have been conducted. A two-moment microphysical scheme was utilized, and the initial and lateral boundary grid conditions were obtained from a large-domain CRM simulation approaching radiative convective equilibrium. As the tropics are never too far from radiative convective equilibrium, such a framework is useful for investigating the relationships between radiation, thermodynamics and microphysics in tropical convection. Microphysical impacts on latent heating and equivalent potential temperature (?e) have been analyzed along trajectories ascending within convective regions. Changes in ?e along backward trajectories are partitioned into contributions from latent heating due to ice processes and a residual term that is shown to be an approximate representation of mixing. It is apparent from the CRM simulations that mixing with dry environmental air decreases ?e along ascending trajectories below the freezing level, while latent heating due to freezing and vapor deposition increase ?e above the freezing level. The along-trajectory contributions to latent heating from cloud nucleation, condensation, evaporation, freezing, deposition, and sublimation have also been quantified. Finally, the source regions of trajectories reaching the upper troposphere have been identified. The analysis indicates that while much of the air ascending within convective updrafts originates from above the lowest 2 km AGL, the strongest updrafts are composed of air from closer to the surface. Thus, both the boundary layer and mid-level inflow appears to play an important role in deep convection developing within moist environments.
Beam window convection cooling employing heat pipe mechanisms
International Nuclear Information System (INIS)
Since the yield strength of a metal foil decreases rapidly at high temperatures, the working pressure of a radiogas target is limited by the maximum temperature of the isolation foil. Forced gas convection cooling is used to augment the conduction and radiative heat transfer. This paper explores the heat transfer processes of a double window heat pipe, a non-mechanical and sealed device. The analysis is concerned with the effect of the heat pipe phenomena of capillary head, vapor velocity and entrainment on the foil cooling
Convective Heat Transfer and Infrared Thermography (IRTh
Directory of Open Access Journals (Sweden)
J.M. Buchlin
2010-01-01
Full Text Available The paper deals with the application of the infrared thermography to the determination of the convective heat transfer coefficient in complex flow configurations. The fundamental principles upon which the IRTh relies are reviewed. The different methods developed to evaluate the heat exchange are described and illustrated through applications to the aerospace and aeronautical field as well as to the industrial processes.
Convective Heat Transfer and Infrared Thermography (IRTh)
J.M. Buchlin
2010-01-01
The paper deals with the application of the infrared thermography to the determination of the convective heat transfer coefficient in complex flow configurations. The fundamental principles upon which the IRTh relies are reviewed. The different methods developed to evaluate the heat exchange are described and illustrated through applications to the aerospace and aeronautical field as well as to the industrial processes.
Energy Technology Data Exchange (ETDEWEB)
Kodama, Shigeo, E-mail: skodama@neltd.co.jp [Nuclear Engineering Ltd., 1-3-7, Tosabori, Nishi Ward, Osaka 550-0001 (Japan); Yoshida, Kenji; Kataoka, Isao [Department of Mechanical Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871 (Japan)
2014-03-15
Thermal stratification and mixing under single- and two-phase flow natural convection are studied experimentally in relation to the safety of nuclear reactors. Flow structure and temperature distribution were measured for a rod bundle with axially distributed heat flux wherein the upper parts of the heaters are heated and the lower parts are unheated. In this scenario, under conditions of single-phase flow, thermal stratification is quite appreciable. A drastic temperature change was observed at the interface of thermal stratification. In the heated region, upward flow occurred in the rod bundle and downward flow occurred at the peripheral region of the rod bundle. As the heat flux increased, radial mixing was observed between subchannels in the rod bundle. At the interface of thermal stratification, however, almost no mixing was observed. Under conditions of boiling two-phase flow, on the other hand, thermal stratification also occurred but mixing at the interface of thermal stratification was promoted due to agitation of flow induced by bubbles. As the heat flux increased and the void fraction in the heated section increased, the interface of thermal stratification gradually advanced toward the unheated section.
Transient conjugated forced convection in turbulent pipe flows
International Nuclear Information System (INIS)
This work deals with the problem of transient conjugated forced convection heat transfer in turbulent pipe flows. The external surface of the pipe over a finite heated section is subjected to either uniform heat flux or uniform wall temperature. The governing parameters identified in this work are the Reynolds number Re, the wall-to-fluid conductivity ratio K, the wall-to-fluid diffusivity ratio A, the dimensionless wall thickness ?, and the Prandtl number Pr. A modified low-Re ?-? turbulent model is adopted to solve for the fully developed velocity and eddy viscosity distributions. Predicted results show that effects of wall conduction and wall heat capacity have a significant impact on the unsteady heat transfer, especially in the early transient period
Infrared thermography for convective heat transfer measurements
Energy Technology Data Exchange (ETDEWEB)
Carlomagno, Giovanni Maria; Cardone, Gennaro [University of Naples Federico II, Department of Aerospace Engineering, Naples (Italy)
2010-12-15
This paper deals with the evolution of infrared (IR) thermography into a powerful optical tool that can be used in complex fluid flows to either evaluate wall convective heat fluxes or investigate the surface flow field behavior. Measurement of convective heat fluxes must be performed by means of a thermal sensor, where temperatures have to be measured with proper transducers. By correctly choosing the thermal sensor, IR thermography can be successfully exploited to resolve convective heat flux distributions with both steady and transient techniques. When comparing it to standard transducers, the IR camera appears very valuable because it is non-intrusive, it has a high sensitivity (down to 20 mK), it has a low response time (down to 20 {mu}s), it is fully two dimensional (from 80 k up to 1 M pixels, at 50 Hz) and, therefore, it allows for better evaluation of errors due to tangential conduction within the sensor. This paper analyses the capability of IR thermography to perform convective heat transfer measurements and surface visualizations in complex fluid flows. In particular, it includes the following: the necessary radiation theory background, a review of the main IR camera features, a description of the pertinent heat flux sensors, an analysis of the IR image processing methods and a report on some applications to complex fluid flows, ranging from natural convection to hypersonic regime. (orig.)
2013-01-01
We examine the effect of magnetic field on boundary layer flow of an incompressible electrically conducting water-based nanofluids past a convectively heated vertical porous plate with Navier slip boundary condition. A suitable similarity transformation is employed to reduce the governing partial differential equations into nonlinear ordinary differential equations, which are solved numerically by employing fourth-order Runge-Kutta with a shooting technique. Three different water-based nanofluids containing copper (Cu), aluminium oxide (Al2O3), and titanium dioxide (TiO2) are taken into consideration. Graphical results are presented and discussed quantitatively with respect to the influence of pertinent parameters, such as solid volume fraction of nanoparticles (?), magnetic field parameter (Ha), buoyancy effect (Gr), Eckert number (Ec), suction/injection parameter (fw), Biot number (Bi), and slip parameter (?), on the dimensionless velocity, temperature, skin friction coefficient, and heat transfer rate. PMID:24222749
Mutuku-Njane, Winifred Nduku; Makinde, Oluwole Daniel
2013-01-01
We examine the effect of magnetic field on boundary layer flow of an incompressible electrically conducting water-based nanofluids past a convectively heated vertical porous plate with Navier slip boundary condition. A suitable similarity transformation is employed to reduce the governing partial differential equations into nonlinear ordinary differential equations, which are solved numerically by employing fourth-order Runge-Kutta with a shooting technique. Three different water-based nanofluids containing copper (Cu), aluminium oxide (Al2O3), and titanium dioxide (TiO2) are taken into consideration. Graphical results are presented and discussed quantitatively with respect to the influence of pertinent parameters, such as solid volume fraction of nanoparticles (?), magnetic field parameter (Ha), buoyancy effect (Gr), Eckert number (Ec), suction/injection parameter (f w ), Biot number (Bi), and slip parameter ( ? ), on the dimensionless velocity, temperature, skin friction coefficient, and heat transfer rate. PMID:24222749
Natural convection in a horizontal fluid layer periodically heated from above and below
Hossain, M. Z.; Floryan, J. M.
2015-08-01
Natural convection in a horizontal slot heated from above and from below has been considered. Each heating has a certain spatial distribution. It has been demonstrated that a wide variety of convection patterns can be generated by changing the relative position of both heating patterns. A significant intensification of convection, compared to convection resulting from heating applied at one wall only, results if there is no phase shift between both patterns, while a significant reduction of convection results from the phase shift corresponding to half of the heating wavelength. The system generates a nonzero mean shear stress at each wall for all phase shifts except shifts corresponding to half of and one full heating wavelength. This effect, which is generated within one convection cell, gives rise to a global force which may lead to a thermally induced drift of the walls if such a drift was allowed.
Prandtl Number Dependent Natural Convection with Internal Heat Sources
International Nuclear Information System (INIS)
Natural convection plays an important role in determining the thermal load from debris accumulated in the reactor vessel lower head during a severe accident. Recently, attention is being paid to the feasibility of external vessel flooding as a severe accident management strategy and to the phenomena affecting the success path for retaining the molten core material inside the vessel. The heat transfer inside the molten core material can be characterized by the strong buoyancy-induced flows resulting from internal heating due to decay of fission products. The thermo-fluid dynamic characteristics of such flow depend strongly on the thermal boundary conditions. The spatial and temporal variation of heat flux on the pool wall boundaries and the pool superheat are mainly characterized by the natural convection flow inside the molten pool. In general, the natural convection heat transfer phenomena involving the internal heat generation are represented by the modified Rayleigh number (Ra'), which quantifies the internal heat source and hence the strength of the buoyancy force. In this study, tests were conducted in a rectangular section 250 mm high, 500 mm long and 160 mm wide. Twenty-four T-type thermocouples were installed in the test section to measure temperatures. Four T-type thermocouples were used to measure the boundary temperatures. The thermocouples were placed in designated locations after calibration. A direct heating method was adopted in this test to simulate the uniform heat generation. The experiments covered a range of Ra' between 1.5x106 and 7.42x1015 and the Prandtl number (Pr) between 0.7 and 6.5. Tests were conducted with water and air as simulant. The upper and lower boundary conditions were maintained uniform. The results demonstrated feasibility of the direct heating method to simulate uniform volumetric heat generation. Particular attentions were paid to the effect of Pr on natural convection heat transfer within the rectangular pool
An assessment on forced convection in metal foams
International Nuclear Information System (INIS)
Metal foams are a class of cellular structured materials with open cells randomly oriented and mostly homogeneous in size and shape. In the last decade, several authors have discussed the interesting heat transfer capabilities of these materials as enhanced surfaces for air conditioning, refrigeration, and electronic cooling applications. This paper reports an assessment on the forced convection through metal foams presenting experimental and analytical results carried out during air heat transfer through twelve aluminum foam samples and nine copper foam samples. The metal foam samples present different numbers of pores per linear inch (PPI), which vary between 5 and 40 with a porosity ranging between 0.896–0.956; samples of different heights have been studied. From the experimental measurements two correlations for the heat transfer coefficient and pressure drop calculations have been developed. These models can be successfully used to optimize different foam heat exchangers for any given application.
Directory of Open Access Journals (Sweden)
Kovalenko A. V.
2015-01-01
Full Text Available In the article, we have suggested a general mathematical model of non-stationary and non-isothermal process of a binary electrolyte transfer in dilute solutions in an electro-membrane system (EMS, taking into account the joint action of gravitational convection, forced convection and electro convection in potential dynamic mode. This model is a boundary problem for a system of two-dimensional quasi-linear Navier-Stokes equation and Nernst-Planck-Poisson in partial derivatives equation. We have developed a theory of similarity of the process of heat and mass transfer in electro-membrane systems, specifically, in a desalting channel of electro dialysis apparatus, taking into account joint actions of concentration polarization, space charge, gravity convection, forced convection and electro convection. It is shown that the criterion of electro convection does not directly depend on the initial concentration, and, therefore, electro convection occurs at any initial concentration. At the same time, the criterion of concentration convection linearly dependents on the initial concentration, and, therefore, at high concentrations, concentration convection prevails, while at lower concentrations, the role of gravitational convection begins to fall whereas the role of electro convection increases. The theory of similarity of the process of heat and mass transfer in the desalting channel of electro dialysis apparatus built in this work taking into account the joint action of concentration polarization, space charge, gravity convection, forced convection and electro convection is important for engineering calculations, for scaling the results of experiments in an electro-membrane cell for industrial electro dialysis water desalting apparatus
International Nuclear Information System (INIS)
The Muon Ionization Cooling Experiment (MICE) has three 350-mm long liquid hydrogen absorbers to reduce the momentum of 200 MeV muons in all directions. The muons are then re-accelerated in the longitudinal direction by 200 MHz RF cavities. The result is cooled muons with a reduced emittance. The energy from the muons is taken up by the liquid hydrogen in the absorber. The hydrogen in the MICE absorbers is cooled by natural convection to the walls of the absorber that are in turn cooled by helium gas that enters at 14 K. This report describes the MICE liquid hydrogen absorber and the heat exchanger between the liquid hydrogen and the helium gas that flows through passages in the absorber wall
Solar Hot Water Heating by Natural Convection.
Noble, Richard D.
1983-01-01
Presents an undergraduate laboratory experiment in which a solar collector is used to heat water for domestic use. The working fluid is moved by natural convection so no pumps are required. Experimental apparatus is simple in design and operation so that data can be collected quickly and easily. (Author/JN)
Free convection film flows and heat transfer
Shang, Deyi
2010-01-01
Presents development of systematic studies for hydrodynamics and heat and mass transfer in laminar free convection, accelerating film boiling and condensation of Newtonian fluids, and accelerating film flow of non-Newtonian power-law fluids. This book provides a system of analysis models with a developed velocity component method.
Heat Flux Sensors for Infrared Thermography in Convective Heat Transfer
Directory of Open Access Journals (Sweden)
Giovanni Maria Carlomagno
2014-11-01
Full Text Available This paper reviews the most dependable heat flux sensors, which can be used with InfraRed (IR thermography to measure convective heat transfer coefficient distributions, and some of their applications performed by the authors’ research group at the University of Naples Federico II. After recalling the basic principles that make IR thermography work, the various heat flux sensors to be used with it are presented and discussed, describing their capability to investigate complex thermo-fluid-dynamic flows. Several applications to streams, which range from natural convection to hypersonic flows, are also described.
Combined convective heat transfer of liquid sodium flowing across tube banks
International Nuclear Information System (INIS)
In order to clarify the heat transfer characteristics of combined convection of liquid sodium, a numerical analysis is performed for liquid sodium which flows through a single horizontal row of tubes in the direction of gravity. The correlation of heat transfer characteristics between liquid sodium and ordinary fluids is also discussed. The heat transfer characteristics at large Reynolds numbers are improved when the Richardson number is increased, and the improvement rate is enlarged with increase in p/d value, since convection effect is relatively large. However heat transfer coefficients do not differ from those of forced convection at small Reynolds numbers even when the Richardson number reaches a high value because of conduction effect. A good consistence of heat transfer characteristics of combined convection between liquid sodium and air is obtained at the same Peclet number and Richardson number. This means that the fundamental heat transfer characteristics of combined convection of liquid sodium can be investigated with ordinary fluids. (author)
Studies of heat source driven natural convection
Kulacki, F. A.; Nagle, M. E.; Cassen, P.
1974-01-01
Natural convection energy transport in a horizontal layer of internally heated fluid with a zero heat flux lower boundary, and an isothermal upper boundary, has been studied. Quantitative information on the time-mean temperature distribution and the fluctuating component of temperature about the mean temperature in steady turbulent convection are obtained from a small thermocouple inserted into the layer through the upper bounding plate. Data are also presented on the development of temperature at several vertical positions when the layer is subject to both a sudden increase and to a sudden decrease in power input. For changes of power input from zero to a value corresponding to a Rayleigh number much greater than the critical linear stability theory value, a slight hysteresis in temperature profiles near the upper boundary is observed between the heat-up and cool-down modes.
Natural convection in a uniformly heated pool
International Nuclear Information System (INIS)
In the event of a core meltdown accident, to prevent reactor vessel failure from molten corium relocation to the reactor vessel lower head, the establishment of a coolable configuration has been proposed by flooding with water the reactor cavity. In Reference 3, it was shown that for the heavy-water new production reactor (NPW-HWR) design, this strategy, e.g., the rejection of decay heat to a containment decay heat removal system by boiling of water in the reactor cavity, could keep the reactor vessel temperature below failure limits. The analysis of Ref. 3 was performed with the computer code COMMIX-1AR/P, and showed that natural convection in the molten-corium pool was the dominant mechanism of heat transfer from the pool to the wall of the reactor vessel lower head. To determine whether COMMIX adequately predicts natural convection in a pool heated by a uniform heat source, in Ref. 4, the experiments of free convection in a semicircular cavity of Jahn and Reineke were analyzed with COMMIX. It was found that the Nusselt (Nu) number predicted by COMMIX was within the spread of the experimental measurements. In the COMMIX analysis of Ref. 4, the semicircular cavity was treated as symmetric. The objective of the work presented in this paper was to extend the COMMIX validation analysis of Ref. 4 by removing the assumption of symmetry and expanding the analysis up to the highest Rayleigh (Ra) number that leads to a steady state. In conclusion, this work shows that the numerical predictions of natural convection in an internally heated pool bounded by a curved bottom are in reasonably good agreement with experimental measurements
Heat Flux Sensors for Infrared Thermography in Convective Heat Transfer
Giovanni Maria Carlomagno; Luigi de Luca; Gennaro Cardone; Tommaso Astarita
2014-01-01
This paper reviews the most dependable heat flux sensors, which can be used with InfraRed (IR) thermography to measure convective heat transfer coefficient distributions, and some of their applications performed by the authors’ research group at the University of Naples Federico II. After recalling the basic principles that make IR thermography work, the various heat flux sensors to be used with it are presented and discussed, describing their capability to investigate complex thermo-fluid-dy...
Mixed convective heat transfer in an inclined cavity with multiple heated elements on one wall
Oosthuizen, P. H.; de Champlain, A.
1989-06-01
A numerical investigation has been conducted into mixed forced and free convective heat transfer through a rectangular cavity having two or three identical rectangular elements on the walls, whose faces are heated to a uniform temperature, in the case where the forced flow enters and leaves through the same wall. The study employs the two-dimensional Navier-Stokes, energy, and continuity equations, under assumptions of steady and laminar flow characteristics. Solution parameters are Reynolds number, Grashof number, fluid Prandtl number, cavity aspect ratio, cavity inclination angle, inlet and outlet duct size, and position and aspect ratio of the heated elements.
Miniature convection cooled plug-type heat flux gauges
Liebert, Curt H.
1994-02-01
Tests and analysis of a new miniature plug-type heat flux gauge configuration are described. This gauge can simultaneously measure heat flux on two opposed active surfaces when heat flux levels are equal to or greater than about 0.2 MW/m(sup 2). The performance of this dual active surface gauge was investigated over a wide transient and steady heat flux and temperature range. The tests were performed by radiatively heating the front surface with an argon arc lamp while the back surface was convection cooled with air. Accuracy is about +20 percent. The gauge is responsive to fast heat flux transients and is designed to withstand the high temperature (1300 K), high pressure (15 MPa), erosive and corrosive environments in modern engines. This gauge can be used to measure heat flux on the surfaces of internally cooled apparatus such as turbine blades and combustors used in jet propulsion systems and on the surfaces of hypersonic vehicles. Heat flux measurement accuracy is not compromised when design considerations call for various size gauges to be fabricated into alloys of various shapes and properties. Significant gauge temperature reductions (120 K), which can lead to potential gauge durability improvement, were obtained when the gauges were air-cooled by forced convection.
Convective cooling of three discrete heat sources in channel flow
Scientific Electronic Library Online (English)
Thiago Antonini, Alves; Carlos A. C., Altemani.
2008-09-01
Full Text Available A numerical investigation was performed to evaluate distinct convective heat transfer coefficients for three discrete strip heat sources flush mounted to a wall of a parallel plates channel. Uniform heat flux was considered along each heat source, but the remaining channel surfaces were assumed adia [...] batic. A laminar airflow with constant properties was forced into the channel considering either developed flow or a uniform velocity at the channel entrance. The conservation equations were solved using the finite volumes method together with the SIMPLE algorithm. The convective coefficients were evaluated considering three possibilities for the reference temperature. The first was the fluid entrance temperature into the channel, the second was the flow mixed mean temperature just upstream any heat source, and the third option employed the adiabatic wall temperature concept. It is shown that the last alternative gives rise to an invariant descriptor, the adiabatic heat transfer coefficient, which depends solely on the flow and the geometry. This is very convenient for the thermal analysis of electronic equipment, where the components' heating is discrete and can be highly non-uniform.
Convective heat transfer in spent fuel canisters
International Nuclear Information System (INIS)
In this paper engineering correlations for convective heat transfer in enclosed vertical rod-bundles are reviewed. A criterion is proposed for determination of the heat transfer regime in the enclosure so that relevant correlations can be used. Moreover, a procedure for calculation of an approximate maximum center rod temperature in the canister is proposed. The procedure is applied to a PWR assembly, and the predicted values are compared with the measured data. Considering the approximate nature of the proposed procedure and the complexity of the problem, the predicted values are judged to be in good agreement with the measured values
Experimental studies on mixed convection heat transfer in laminar flow through a plain square duct
Patil, S. V.; Vijay Babu, P. V.
2012-12-01
This paper reports the findings of experimental studies on combined free and forced convection through a plain square duct in laminar region. The test fluid flows through an inner square duct, hot water at high flow rate circulated through a annular channel formed between square duct and circular tube, in counter current fashion to attain a nearly uniform wall temperature conditions. The importance of mixed convection is judged by the value of the Richardson number ( Ri). It was observed that at low Reynolds number, heat transfer was mainly governed by mixed convection. However at higher values of Reynolds number, heat transfer was significantly dominated by forced convection. It was found that Reynolds number higher than 1050 for water and 480 for ethylene glycol resulted in laminar forced convention heat transfer. The empirical correlation developed for Nusselt number in terms of Grashoff number and Graez number, was found to fit with experimental Nusselt number within ±11 and ±12 % for water and ethylene glycol respectively.
International Nuclear Information System (INIS)
Studies results on heat transfer by forced convection of highly sub-heated water in a smooth tube and with a worm feeder under asymmetric heating are presented. Experimental data file on stationary heat transfer during asymmetric tube heating by electron scanning beam with ultimately high density of heat fluxes, exceeding 100 MW/m2, are obtained. Brief description of an approximative model for temperature field calculation, simplifying experimental data processing, is given
Free surface deformation and heat transfer by thermocapillary convection
Fuhrmann, Eckart; Dreyer, Michael; Basting, Steffen; Bänsch, Eberhard
2015-06-01
Knowing the location of the free liquid/gas surface and the heat transfer from the wall towards the fluid is of paramount importance in the design and the optimization of cryogenic upper stage tanks for launchers with ballistic phases, where residual accelerations are smaller by up to four orders of magnitude compared to the gravity acceleration on earth. This changes the driving forces drastically: free surfaces become capillary dominated and natural or free convection is replaced by thermocapillary convection if a non-condensable gas is present. In this paper we report on a sounding rocket experiment that provided data of a liquid free surface with a nonisothermal boundary condition, i.e. a preheated test cell was filled with a cold but storable liquid in low gravity. The corresponding thermocapillary convection (driven by the temperature dependence of the surface tension) created a velocity field directed away from the hot wall towards the colder liquid and then in turn back at the bottom towards the wall. A deformation of the free surface resulting in an apparent contact angle rather different from the microscopic one could be observed. The thermocapillary flow convected the heat from the wall to the liquid and increased the heat transfer compared to pure conduction significantly. The paper presents results of the apparent contact angle as a function of the dimensionless numbers (Weber-Marangoni and Reynolds-Marangoni number) as well as heat transfer data in the form of a Nusselt number. Experimental results are complemented by corresponding numerical simulations with the commercial software Flow3D and the inhouse code Navier.
V.S.Daund*; D.D.Palande
2014-01-01
Experimental and CFD analysis is conducted in order to establish effect of geometrical fin parameters for natural convection heat transfer from vertical rectangular fin arrays.Natural convective heat transfer from rectangular vertical plates has been reviewed. Study revealed that most of the work was carried out considering various configurations. Experimental work carried on steady state natural convection heat transfer from vertical rectangular fins made of aluminum. Experim...
Development of a mechanistic model for forced convection subcooled boiling
Shaver, Dillon R.
The focus of this work is on the formulation, implementation, and testing of a mechanistic model of subcooled boiling. Subcooled boiling is the process of vapor generation on a heated wall when the bulk liquid temperature is still below saturation. This is part of a larger effort by the US DoE's CASL project to apply advanced computational tools to the simulation of light water reactors. To support this effort, the formulation of the dispersed field model is described and a complete model of interfacial forces is formulated. The model has been implemented in the NPHASE-CMFD computer code with a K-epsilon model of turbulence. The interfacial force models are built on extensive work by other authors, and include novel formulations of the turbulent dispersion and lift forces. The complete model of interfacial forces is compared to experiments for adiabatic bubbly flows, including both steady-state and unsteady conditions. The same model is then applied to a transient gas/liquid flow in a complex geometry of fuel channels in a sodium fast reactor. Building on the foundation of the interfacial force model, a mechanistic model of forced-convection subcooled boiling is proposed. This model uses the heat flux partitioning concept and accounts for condensation of bubbles attached to the wall. This allows the model to capture the enhanced heat transfer associated with boiling before the point of net generation of vapor, a phenomenon consistent with existing experimental observations. The model is compared to four different experiments encompassing flows of light water, heavy water, and R12 at different pressures, in cylindrical channels, an internally heated annulus, and a rectangular channel. The experimental data includes axial and radial profiles of both liquid temperature and vapor volume fraction, and the agreement can be considered quite good. The complete model is then applied to simulations of subcooled boiling in nuclear reactor subchannels consistent with the operating conditions of the AP1000 pressurized water reactor. The effects of both axial and lateral nonuniform power distributions inside reactor fuel elements are accounted for. Boiling flows are simulated for three different computational domains of increasing complexity: a quarter-subchannel bordering a single fuel pin, two subchannels surround by an array of 2 by 3 fuel pins, and in four subchannels surrounded by an array of 3 by 3 fuel pins. The predicted behavior is consistent with expectations. In the 3 by 3 array, the two-phase coolant is predicted to flow from the hot channels to the cold channels, enhancing heat exchange between subchannels. This, in turn, demonstrates that the new model is capable of capturing the turbulence- and buoyancy-induced coolant mixing across the neighboring channels.
Natural convection in a fluid layer periodically heated from above.
Hossain, M Z; Floryan, J M
2014-08-01
Natural convection in a horizontal layer subject to periodic heating from above has been studied. It is shown that the primary convection leads to the cooling of the bulk of the fluid below the mean temperature of the upper wall. The secondary convection may lead either to longitudinal rolls, transverse rolls, or oblique rolls. The global flow properties (e.g., the average Nusselt number for the primary convection and the critical conditions for the secondary convection) are identical to those of the layer heated from below. However, the flow and temperature patterns exhibit phase shifts in the horizontal directions. PMID:25215828
Heat transfer by natural convection in an internally heated reactor materials melt. Rev. 0
International Nuclear Information System (INIS)
The report is structured as follows: Heat flux calculations and estimates for In-Vessel Retention (IVR); Rayleigh-Benard convection and the Rayleigh number (criterion); Free convection in the horizontal layer of a liquid which is heated by internal heating sources, and a modified Rayleigh criterion; and Turbulent convection at the reactor vessel bottom during IVR. (P.A.)
Observation of dendritic growth under the influence of forced convection
Roshchupkina, O.; Shevchenko, N.; Eckert, S.
2015-06-01
The directional solidification of Ga-25wt%In alloys within a Hele-Shaw cell was visualized by X-ray radioscopy. The investigations are focused on the impact of melt convection on the dendritic growth. Natural convection occurs during a bottom up solidification because lighter solute is rejected during crystallization. Forced convection was produced by a specific electromagnetic pump. The direction of forced melt flow is almost horizontal at the solidification front. Melt flow induces various effects on grain morphology primarily caused by convective transport of solute, such as a facilitation of the growth of primary trunks or lateral branches, dendrite remelting, fragmentation or freckle formation depending on the dendrite orientation, the flow direction and intensity. Forced flow eliminates solutal plumes and damps local fluctuations of solute. A preferential growth of the secondary arms occurs at the upstream side of the dendrites, whereas high solute concentration at the downstream side inhibits the formation of secondary branches.
Computation of forced laminar convection in rotating cavities
Chew, J. W.
1985-05-01
Finite difference solutions are presented for forced laminar convection in a rotating cylindrical cavity with radial outflow. This forms a simple model of the cooling flow between two compressor disks in a gas turbine engine. If the fluid enters the cavity from a uniform radial source, it is shown that the local Nusselt number changes from that of a 'free disk' near the center of the cavity to that for Ekman layer flow at larger radii. With an axial inlet, the flow, and consequently, the heat transfer, is more complex. If vortex breakdown occurs, then the results are very similar to those for the radial inlet case, but otherwise a wall jet forms on the downstream disk, and the heat transfer from this disk may be several times that for the upstream disk. Variation of mean Nusselt number with rotational speed is qualitatively similar to previously published experimental measurements in turbulent flow. The effect of Prandtl number on heat transfer has also been demonstrated.
Investigation of the transition from forced to natural convection in the research reactor Munich II
International Nuclear Information System (INIS)
The new research reactor Munich II (FRM-II), which is under construction at the Technical University Munich, Germany, makes use of a newly developed compact reactor core consisting of a single fuel element, which is assembled of two concentric pipes. Between the fuel element's inner and outer pipe 113 involutely bent fuel plates are placed rotationally symmetric, forming 113 cooling channels of a constant width of 2.2 mm. After a shut down of the reactor, battery supported cooling pumps are started by the reactor safety system in order to remove the decay heat by a downwards directed forced flow. Three hours after they have been started, the cooling pumps are shut down and so-called 'natural convection flaps' are opened by their own weight. Through a flow path, which is provided by the opening of the natural convection flaps, the decay heat is given off to the water in the reactor pool after the direction of the flow has changed and an upwards directed natural convection flow has developed. At the Department for Nuclear and New Energy Systems of the Ruhr-University Bochum, Germany, a test facility has been built in order to confirm the concept of the decay heat removal in the FRM-II, to acquire data of single and two phase natural convection flows and to detect the dry out in a narrow channel. The thermohydraulics of the FRM-II are simulated by an electrically heated test section, which represents one cooling channel of the fuel element. At first experiments have been performed, which simulated the transition from forced to natural convection in the core of the FRM-II, both at normal operation and at a complete loss of the decay heat removal pumps. In case of normal operation, the transition from forced to natural convection takes place single phased. If a complete loss of the active decay heat removal system occurs, the decay heat removal is ensured by a quasi-steady two phase flow. In a second test series minimum heat flux densities leading to pressure pulsations up to limiting amplitudes of 0.1 bar, 0.2 bar and 0.3 bar at the transition from forced to natural convection have been determined. Further tests have been performed to determine minimum heat flux densities leading to boiling processes in the cooling channel and critical heat flux densities causing dry outs of the cooling channel at downwards directed forced flow. During the tests, flow reversals have been observed because of the buoyancy forces in the coolant causing a mixed convection flow. The last test series, which has been finished in March 1999, has been performed in order to determine critical heat flux densities during the transition from forced to natural convection and to measure the occurring pressure amplitudes. All results prove the possibility to remove the decay heat of the FRM-II by natural convection, even in case of a complete loss of the active decay heat removal system. Above this, large safety margins in the FRM-II concerning pressure pulsations, beginning of boiling and dry out could be verified. (author)
Numerical Simulations of Heat Explosion With Convection In Porous Media
Allali, Karam; Bikany, Fouad; Taik, Ahmed; Volpert, Vitaly
2013-01-01
In this paper we study the interaction between natural convection and heat explosion in porous media. The model consists of the heat equation with a nonlinear source term describing heat production due to an exothermic chemical reaction coupled with the Darcy law. Stationary and oscillating convection regimes and oscillating heat explosion are observed. The models with quasi-stationary and unstationary Darcy equation are compared.
International Nuclear Information System (INIS)
The vaporisation of an appreciable quantity of a liquid in a turbulent gas stream explains the increase in the heat capacity of the fluid and the improvement in the heat-transfer coefficient. The present study makes it clear that even with a very slight vaporisation, the transfer coefficient can be much increased, the pressure drop remaining nearly constant. (authors)
Performance of a convective, infrared and combined infrared- convective heated conveyor-belt dryer.
El-Mesery, Hany S; Mwithiga, Gikuru
2015-05-01
A conveyor-belt dryer was developed using a combined infrared and hot air heating system that can be used in the drying of fruits and vegetables. The drying system having two chambers was fitted with infrared radiation heaters and through-flow hot air was provided from a convective heating system. The system was designed to operate under either infrared radiation and cold air (IR-CA) settings of 2000 W/m(2) with forced ambient air at 30 °C and air flow of 0.6 m/s or combined infrared and hot air convection (IR-HA) dryer setting with infrared intensity set at 2000 W/m(2) and hot at 60 °C being blown through the dryer at a velocity of 0.6 m/s or hot air convection (HA) at an air temperature of 60 °C and air flow velocity 0.6 m/s but without infrared heating. Apple slices dried under the different dryer settings were evaluated for quality and energy requirements. It was found that drying of apple (Golden Delicious) slices took place in the falling rate drying period and no constant rate period of drying was observed under any of the test conditions. The IR-HA setting was 57.5 and 39.1 % faster than IR-CA and HA setting, respectively. Specific energy consumption was lower and thermal efficiency was higher for the IR-HA setting when compared to both IR-CA and HA settings. The rehydration ratio, shrinkage and colour properties of apples dried under IR-HA conditions were better than for either IR-CA or HA. PMID:25892769
Turbulent forced convective flow in an an-isothermal channel
International Nuclear Information System (INIS)
The influence of variable viscosity effects on momentum and heat transfer of a non-isothermal turbulent forced convective flow is studied using thermal large-eddy simulation (LES). LES of bi-periodic channel flow with significant heat transfer at a low Mach number was performed to study the modulation in the near-wall turbulence structure due to anisotropic viscosity. The temperature ratio (R? = Thot/Tcold) is varied from 1.01 to 5 to study the isolated effect of variable viscosity with (Thot) and (Tcold) as a wall temperature. It is shown that average and turbulent fields undergo significant changes in a broad range of Reynolds number, compared to isothermal flow with constant viscosity, we observe enhanced turbulence on the cold side of the channel, characterized by locally lower viscosity whereas a decrease of turbulent kinetic energy is found at the hot wall. The turbulent structures via H criteria of high vorticity shows very short and densely populated vortices near cold wall whereas long streaky structure or large elongated vortices at the hot wall. Q invariant totally eradicate all the streaky structure at the hot wall as a consequence of re-laminarization. To further clarify this issue spectral study is conducted that reveals complete suppression of turbulence at the hot side of the channel at large temperature ratio because no inertial zone (i.e. index of Kolmogorov scaling law is zero) is obtained on the spectra in these region. (authors)
Solution of heat removal from nuclear reactors by natural convection
Zitek Pavel; Valenta Vaclav
2014-01-01
This paper summarizes the basis for the solution of heat removal by natural convection from both conventional nuclear reactors and reactors with fuel flowing coolant (such as reactors with molten fluoride salts MSR).The possibility of intensification of heat removal through gas lift is focused on. It might be used in an MSR (Molten Salt Reactor) for cleaning the salt mixture of degassed fission products and therefore eliminating problems with iodine pitting. Heat removal by natural convection...
Analysis of natural convection in volumetrically-heated melt pools
Energy Technology Data Exchange (ETDEWEB)
Sehgal, B.R.; Dinh, T.N.; Nourgaliev, R.R. [Royal Inst. of Tech., Stockholm (Sweden). Div. of Nuclear Power Safety
1996-12-01
Results of series of studies on natural convection heat transfer in decay-heated core melt pools which form in a reactor lower plenum during the progression of a core meltdown accident are described. The emphasis is on modelling and prediction of turbulent heat transfer characteristics of natural convection in a liquid pool with an internal energy source. Methods of computational fluid dynamics, including direct numerical simulation, were applied for investigation. Refs, figs, tabs.
Analysis of natural convection in volumetrically-heated melt pools
International Nuclear Information System (INIS)
Results of series of studies on natural convection heat transfer in decay-heated core melt pools which form in a reactor lower plenum during the progression of a core meltdown accident are described. The emphasis is on modelling and prediction of turbulent heat transfer characteristics of natural convection in a liquid pool with an internal energy source. Methods of computational fluid dynamics, including direct numerical simulation, were applied for investigation
Endwall convective heat transfer for bluff bodies
DEFF Research Database (Denmark)
Wang, Lei; Salewski, Mirko
2012-01-01
The endwall heat transfer characteristics of forced flow past bluff bodies have been investigated using liquid crystal thermography (LCT). The bluff body is placed in a rectangular channel with both its ends attached to the endwalls. The Reynolds number varies from 50,000 to 100,000. In this study, a single bluff body and two bluff bodies arranged in tandem are considered. Due to the formation of horseshoe vortices, the heat transfer is enhanced appreciably for both cases. However, for the case of two bluff bodies in tandem, it is found that the presence of the second bluff body decreases the heat transfer as compared to the case of a single bluff body. In addition, the results show that the heat transfer exhibits Reynolds number similarity. For a single bluff body, the Nusselt number profiles collapse well when the data are scaled by Re0.55; for two bluff bodies arranged in tandem, the heat transfer scaling is changed to Re0.51, indicating that the power index of Reynolds number is flow dependent.
Enhancement of natural convection heat transfer rate by the improved heated surface
International Nuclear Information System (INIS)
The cooling system of water in spent fuel pit without electricity power is developed. The cooling system has a air radiator. The radiator size would be too large, because natural convection heat transfer is low. Therefore, enhancement of natural convection heat transfer is necessary. We applied thermal splay to heated surface, and found that thermal splay enhanced natural convection heat transfer performance. The material of thermal splay is aluminum bronze and polyester powder. Using this result, radiator size could be reduced. (author)
Time evolution simulation of heat removal in a small water tank by natural convection
International Nuclear Information System (INIS)
One of the cooling modes for any source of heat such as in a shutdown nuclear core is the natural convection. The design specifications of any cooling pool can only be done when the removal heat rate and the corresponding mass flow rate is reasonably established. In our simulation scheme, we assumed that the body forces acting in the cubic water cell are: the weight, the drag force and the integrated pressure forces on the horizontal surfaces, the viscosity shear forces on the vertical surfaces and also a special viscosity drag force due to the mass dislocation along a Bernoulli type current tube outside the motive region. For a suitable time step, the uprising convection velocity is determined by an implicit and also by an explicit solution algorithm. The resulting differential equation depends on updating specific mass, dynamic viscosity and constant pressure heat coefficient with the last known temperature in the cell that absorbed heat. Numerical calculation software was performed using MATLAB’s technical computing language and then applied for a heat generation plate simulating a spent fuel assembler from a shutdown nuclear core. The results show time evolution of convection, terminal velocity and water temperature distribution. Pool dimension as well as pool level decrement are also determined for various air exhausting system conditions and heat rate of the spent fuel plate being cooled. (author)
Time evolution simulation of heat removal in a small water tank by natural convection
Energy Technology Data Exchange (ETDEWEB)
Freitas, Carlos Alberto de, E-mail: carlos.freitas1950@hotmail.com [Instituto Federal do Rio de Janeiro (IFRJ), Nilopolis, RJ (Brazil); Jachic, Joao; Moreira, Maria de Lourdes, E-mail: jjachic@ien.gov.br, E-mail: malu@ien.gov.br [Instituto de Engenharia Nuclear (IEN/CNEN-RJ), Rio de Janeiro, RJ (Brazil)
2013-07-01
One of the cooling modes for any source of heat such as in a shutdown nuclear core is the natural convection. The design specifications of any cooling pool can only be done when the removal heat rate and the corresponding mass flow rate is reasonably established. In our simulation scheme, we assumed that the body forces acting in the cubic water cell are: the weight, the drag force and the integrated pressure forces on the horizontal surfaces, the viscosity shear forces on the vertical surfaces and also a special viscosity drag force due to the mass dislocation along a Bernoulli type current tube outside the motive region. For a suitable time step, the uprising convection velocity is determined by an implicit and also by an explicit solution algorithm. The resulting differential equation depends on updating specific mass, dynamic viscosity and constant pressure heat coefficient with the last known temperature in the cell that absorbed heat. Numerical calculation software was performed using MATLAB’s technical computing language and then applied for a heat generation plate simulating a spent fuel assembler from a shutdown nuclear core. The results show time evolution of convection, terminal velocity and water temperature distribution. Pool dimension as well as pool level decrement are also determined for various air exhausting system conditions and heat rate of the spent fuel plate being cooled. (author)
The Influence of Internal Heating on Nonlinear Compressible Convection
Hurlburt, N. E.; Weiss, N. O.
2000-05-01
In the bulk of the solar convection zone we expect convection to be efficient and therefore maintain an adiabatic temperature gradient. In most numerical simulations of solar convection the total energy flux within this region is due to the conduction down this gradient (which is small) and the various contributions due to the convective motions. What has often been neglected is the contribution that is transported by radiation. The contribution of this flux decreases across the layer and thereby deposits a significant amount of thermal energy in the midst of the convection zone. This is in contrast to most simulations of the convection where the input of energy is supplied exclusively by conduction from the boundaries. Mixing length models predict that approximately half of the total energy input to the solar convection zone is deposited, more-or-less uniformly over the convection zone, with the remaining half being conducted from the lower boundary. Thus the study of the behavior of internally-heated compressible convection is warranted. Previous studies of internally heated compressible convection have been inconclusive due to the shearing instabilities that arise in simple, periodic domains. Here we suppress these instabilities by considering flows in axisymmetric geometries. We conduct surveys of the structure and dynamics of the resulting flows and present possible applications to observed solar and stellar phenomena.
Nadia Potoceanu
2007-01-01
The paper presented the most aspects of convective circulate mode of heat transfer : heat transfer through the boundary layer formed at the surface of the heat generator; heat transfer in the heat carrier and heat transfer through the boundary layer formed at the heated surface
International Nuclear Information System (INIS)
A mechanistic model for forced convective transition boiling has been developed to investigate transition boiling mechanisms and to predict transition boiling heat flux realistically. This model is based on a postulated multi-stage boiling process occurring during the passage time of the elongated vapor blanket specified at a critical heat flux (CHF) condition. Between the departure from nucleate boiling (DNB) and the departure from film boiling (DFB) points, the boiling heat transfer is established through three boiling stages, namely, the macrolayer evaporation and dryout governed by nucleate boiling in a thin liquid film and the unstable film boiling characterized by the frequent touches of the interface and the heated wall. The total heat transfer rates after the DNB is weighted by the time fractions of each stage, which are defined as the ratio of each stage duration to the vapor blanket passage time. The model predictions are compared with some available experimental transition boiling data. The parametric effects of pressure, mass flux, inlet subcooling on the transition boiling heat transfer are also investigated. From these comparisons, it can be seen that this model can identify the crucial mechanisms of forced convective transition boiling, and that the transition boiling heat fluxes including the maximum heat flux and the minimum film boiling heat flux are well predicted at low qualities/high pressures near 10 bar. In future, this model will be improved in the unstable film boiling stage and generalized for high quality and low pressure situations
International Nuclear Information System (INIS)
The practical objective of research on 'burn-out' is a reliable method giving the maximum safe rating for any water cooled reactor. Experimental work, which began at numerous centres about 10 years ago, has been concerned principally with endeavouring to understand the phenomenon as it applies to simple geometries such as round and rectangular channels. Many millions of pounds have been spent on this work and several thousand separate experimental results obtained. This considerable effort has achieved little real success in providing an explanation of 'burn-out' however. Many conflicting views have arisen and correlations so far developed have been shown to give calculated 'burn-out' heat fluxes varying by a factor of the order of 5> when applied to a typical reactor situation. While some uncertainty may be due to experimental variations, inadequate analytical effort is considered to be the primary cause of the present confused situation. To overcome this various analytical studies are being initiated by the Reactor Development Division at Winfrith and a detailed plan is being evolved for bringing effort to bear on certain fundamental aspects of boiling which have been neglected and which in some oases will require the development of special experimental techniques. This report describes the result of some work already carried out. It concerns an initial examination made on a large group of 'burn-out' data and describes the development of a correlation which predicts 'burn-out' heat fluxes to within an R.M.S. error of less than 10% over a very wide range of operating conditions including pressure. (author)
Natural convective heat transfer from short inclined cylinders
Oosthuizen, Patrick H
2014-01-01
Natural Convective Heat Transfer from Short Inclined Cylinders examines a heat transfer situation of significant, practical importance not adequately dealt with in existing textbooks or in any widely available review papers. Specifically, the book introduces the reader to recent studies of natural convection from short cylinders mounted on a flat insulated base where there is an “exposed” upper surface. The authors considers the effects of the cylinder cross-sectional shape, the cylinder inclination angle, and the length-to-cross sectional size of the cylinder. Both numerical and experimental studies are discussed and correlation equations based on the results of these studies are reviewed. This book is ideal for professionals involved with thermal management and related systems, researchers, and graduate students in the field of natural convective heat transfer, instructors in graduate level courses in convective heat transfer.
Natural and forced convection film boiling over axisymmetric bodies at high temperatures
International Nuclear Information System (INIS)
Natural and forced convection film boiling over axisymmetric bodies is analysed in the case of high surface temperatures. In these conditions, the global heat transfer is no more driven by simple conduction through the vapour film, an assumption that is commonly made in film boiling modelling for simplicity reasons, but rather by convection through this film. Therefore a mathematical method was developed which enables a full description of the vapour flow by including the inertia and convection terms of the momentum and energy equations. It is based on classical two-phase boundary layer integral methods where polynomial functions of order 5 are used to describe the velocity and the temperature profiles in the vapour flow. Also, a simple scaling analysis is described in order to understand when inertial and convective effects in the vapour flow become important. It is shown for example that for any given fluid, these effects will become predominant even at a low surface superheat when the fluid pressure is increased near its critical pressure. The developed models are then compared with three simpler models: a similar model which uses order 3 polynomial functions, a model where the convective effects are modelled by just using an effective latent heat, and an even simpler model where convection and inertia in the vapour film are not considered at all. It is shown on some examples with very high surface temperatures that if convective and inertial effects are totally neglected, the global heat transfer is clearly underestimated compared to the ones calculated with the two other models, which are quite similar. However, if other important parameters such as vapour production or vapour film thickness at the front stagnation point are calculated, the results given by the model with the effective latent heat diverge from those given by the developed models. (author)
Dhara, Chirag; Renner, Maik; Kleidon, Axel
2015-04-01
The convective transport of heat and moisture plays a key role in the climate system, but the transport is typically parameterized in models. Here, we aim at the simplest possible physical representation and treat convective heat fluxes as the result of a heat engine. We combine the well-known Carnot limit of this heat engine with the energy balances of the surface-atmosphere system that describe how the temperature difference is affected by convective heat transport, yielding a maximum power limit of convection. This results in a simple analytic expression for convective strength that depends primarily on surface solar absorption. We compare this expression with an idealized grey atmosphere radiative-convective (RC) model as well as Global Circulation Model (GCM) simulations at the grid scale. We find that our simple expression as well as the RC model can explain much of the geographic variation of the GCM output, resulting in strong linear correlations among the three approaches. The RC model, however, shows a lower bias than our simple expression. We identify the use of the prescribed convective adjustment in RC-like models as the reason for the lower bias. The strength of our model lies in its ability to capture the geographic variation of convective strength with a parameter-free expression. On the other hand, the comparison with the RC model indicates a method for improving the formulation of radiative transfer in our simple approach. We also find that the latent heat fluxes compare very well among the approaches, as well as their sensitivity to surface warming. What our comparison suggests is that the strength of convection and their sensitivity in the climatic mean can be estimated relatively robustly by rather simple approaches.
Numerical solution of staggered circular tubes in two-dimensional laminar forced convection
Scientific Electronic Library Online (English)
Carlos Henrique, Marchi; Maykel Alexandre, Hobmeir.
2007-03-01
Full Text Available This paper aims to demonstrate the importance of adequately estimating the discretization error intrinsic in the result of any numerical simulation. The problem under consideration is forced convection in a staggered circular tube heat exchanger. The problem is solved to analyze the effect of the di [...] stance between the tubes, aiming to optimize the heat exchanger’s geometrical configuration by two Reynolds numbers (50 and 100). The present work did not confirm the existence of an optimal geometrical point for the operation of staggered circular tube heat exchangers, as claimed in a numerical study published in the literature.
Solution of heat removal from nuclear reactors by natural convection
Zitek, Pavel; Valenta, Vaclav
2014-03-01
This paper summarizes the basis for the solution of heat removal by natural convection from both conventional nuclear reactors and reactors with fuel flowing coolant (such as reactors with molten fluoride salts MSR).The possibility of intensification of heat removal through gas lift is focused on. It might be used in an MSR (Molten Salt Reactor) for cleaning the salt mixture of degassed fission products and therefore eliminating problems with iodine pitting. Heat removal by natural convection and its intensification increases significantly the safety of nuclear reactors. Simultaneously the heat removal also solves problems with lifetime of pumps in the primary circuit of high-temperature reactors.
Solution of heat removal from nuclear reactors by natural convection
Directory of Open Access Journals (Sweden)
Zitek Pavel
2014-03-01
Full Text Available This paper summarizes the basis for the solution of heat removal by natural convection from both conventional nuclear reactors and reactors with fuel flowing coolant (such as reactors with molten fluoride salts MSR.The possibility of intensification of heat removal through gas lift is focused on. It might be used in an MSR (Molten Salt Reactor for cleaning the salt mixture of degassed fission products and therefore eliminating problems with iodine pitting. Heat removal by natural convection and its intensification increases significantly the safety of nuclear reactors. Simultaneously the heat removal also solves problems with lifetime of pumps in the primary circuit of high-temperature reactors.
SIMULATION OF MIXED CONVECTIVE HEAT TRANSFER USING LATTICE BOLTZMANN METHOD
Directory of Open Access Journals (Sweden)
A. R. M. Rosdzimin
2010-12-01
Full Text Available In this paper, mixed (forced–natural convective heat transfer around a heated square cylinder located inside a lid driven cavity has been studied numerically using the lattice Boltzmann method in the range of 100? Re ? 1000 with the corresponding Richardson number 0.01?Ri?10. The double-population lattice Boltzmann formulation is used as the governing equation. Two dimensional nine-velocity models are used for the computation of the velocity field while a four-velocity model is used for the computation of the temperature field. We found that the combination of nine- and four-velocity models can be applied to the calculation without losing its accuracy. The results are presented in the form of streamline and isotherm plots as well as the variation of local Nusselt number at the top surface of the heated square. The computational results demonstrate that the flow pattern, formation of vortex and also the Nusselt number are influence by the Reynolds number and Richardson number.
Analytical Solution of Forced-Convective Boundary-Layer Flow over a Flat Plate
DEFF Research Database (Denmark)
Mirgolbabaei, H.; Barari, Amin
2010-01-01
In this letter, the problem of forced convection heat transfer over a horizontal flat plate is investigated by employing the Adomian Decomposition Method (ADM). The series solution of the nonlinear differential equations governing on the problem is developed. Comparison between results obtained and those of numerical solution shows excellent agreement, illustrating the effectiveness of the method. The solution obtained by ADM gives an explicit expression of temperature distribution and velocity distribution over a flat plate.
The optimization of longitudinal convective fins with internal heat generation
International Nuclear Information System (INIS)
The solution of the optimization problem for longitudinal convective fins of constant thickness, triangular or parabolic profile, and uniform internal heat generation, is presented. The cases considered are those of a given heat generation density, total heat generation and heat generation per unit width of the fin, when either the heat dissipation or the width of the fin is prescribed. The results are set forth in a nondimensional form, which are presented graphically. The effect of the fin's thermal conductivity upon the optimum dimensions is discussed, and limiting values for the heat generation and the heat dissipation, which may be imposed on the fin for a feasible optimization, are also obtained. (Auth.)
Thermal Performance of Convective-Radiative Heat Transfer in Porous Fins
Directory of Open Access Journals (Sweden)
Majid SHAHBABAEI
2014-01-01
Full Text Available Forced and natural convection in porous fins with convective coefficient at the tips under radiation and convection effects are investigated in this paper. Aluminum and copper as fin materials are investigated. In forced and natural convection, air and water are applied as working fluids, respectively. In order to solve this nonlinear equation, Homotopy Perturbation Method (HPM and Variational Iteration Method (VIM are used. To verify the accuracy of the methods, a comparison is made to the exact solution (BVP. In this work, the effects of porosity parameter (, Radiation parameter (? and Temperature-Ratio parameter (µ on non-dimensional temperature distribution for both of the flows are shown. The results show that the effects of (? and (µ on temperature distribution in natural convection are based on porosity and in forced convection are uniform, approximately. Also, it is shown that both VIM and HPM are capable of being used to solve this nonlinear heat transfer equation.doi:10.14456/WJST.2014.64
Validation of PARET for the modeling of heat transfer under natural convection core cooling
International Nuclear Information System (INIS)
The PARET code is a one-dimensional, coupled thermal-hydraulic and point-kinetics code, which was originally developed for the analysis of SPERT-I transients and later adapted for the analysis of transient behavior in research reactors. Due to its ease of transportability and relative simplicity of input preparation, it is widely used internationally and is particularly attractive for research reactors with limited computational facilities. The thermal-hydraulic modeling of the current version of PARET accounts for buoyancy forces in the core and external pressure gradients that may arise from density differences between the core inlet and outlet. This feature of PARET makes it a useful tool for the analysis of research reactors cooled by natural convection as well as those cooled by forced convection. Since PARET has been applied to the analysis of the International Atomic Energy Agency 10-MW benchmark cores for protected and unprotected transients and also for the analysis of SPERT-I transients, its forced convection heat-removal model is reliable. However, there has been little experience with the capability of PARET to model heat removal in cores cooled by natural convection. This paper reports the results of some experiments performed at the Malaysian PUSPATI reactor to compare PARET predictions for power increases under natural convection core cooling to measured data
Numerical simulations on natural convective heat transfer and active cooling of IFMIF Test Cell
International Nuclear Information System (INIS)
Highlights: •Thermo-hydraulic simulations of the IFMIF Test-Cell were carried out with Ansys-CFX V.14 on a reference case. •The current simulation model includes the natural convection inside the TC, several forced convective water flows in the pipelines attached on the steel liners and the helium-cooled HFTM (High Flux Test Module). •A kind of CFX Beta feature was used; in which multiple fluid domains associated with individual turbulence (laminar) models were defined and solved in a single simulation. •The simulations provide the key information on the flow and heat transfer of the Test-Cell components. -- Abstract: The International Fusion Materials Irradiation Facility (IFMIF) is designated to generate a materials irradiation database for the future fusion reactors. The Test Cell (TC) accommodates the Test Modules and the lithium target assembly. Due to the nuclear heat generation, all the Test Modules inside the TC will be actively cooled. Other components like supporting structures, pipelines, cables etc., will be passively cooled by natural convection. The heat will be removed from the steel liners surrounding the TC by active water cooling. This paper concerns the thermo-hydraulic simulations of the Test Cell using Ansys-CFX. The current simulation model includes the natural convection inside the TC, several forced convective water flows in the pipelines attached on the steel liners and the helium-cooled HFTM (High Flux Test Module). The simulations provide the only means for validating the design before the construction and operation
International Nuclear Information System (INIS)
In construction, the use of Phase Change Materials (PCM) allows the storage/release of energy from solar radiation and internal loads. The application of such materials for lightweight construction (e.g., a wood house) makes it possible to improve thermal comfort and reduce energy consumption. The heat transfer process between the wall and the indoor air is convection. In this paper, we have developed a numerical model to evaluate several convective heat transfer correlations from the literature for natural, mixed and forced convection flows. The results show that the convective heat transfer highly influences the storage/release process in case of PCM walls. For the natural convection, the numerical results are highly dependent on the correlation used and the results may vary up to 200%. In the case of mixed and forced convection flows, the higher is the velocity, the more important is the storage capacity. - Highlights: ? We study effect of inside convection correlation on energy stored in PCM wall. ? We developed a 1D conduction model for multilayer walls, with phase change material. ? Correlations have been constructed for mixed convection in all flow regimes. ? Up to 200% variation of energy stored in PCM layer, depending on convection correlation. ? Ventilation can increase the energy stored in the PCM layer.
International Nuclear Information System (INIS)
An experimental study was performed to obtain local fluid velocity and temperature measurements in the mixed (combined free and forced) convection regime for specific flow coastdown transients. A brief investigation of steady-state flows for the purely free-convection regime was also completed. The study was performed using an electrically heated 2 x 6 rod bundle contained in a flow housing. In addition a transient data base was obtained for evaluating the COBRA-WC thermal-hydraulic computer program
10,000 - A reason to study granular heat convection
Energy Technology Data Exchange (ETDEWEB)
Einav, I.; Rognon, P.; Gan, Y.; Miller, T.; Griffani, D. [Particles and Grains Laboratory, School of Civil Engineering, University of Sydney, Sydney, NSW 2006 (Australia)
2013-06-18
In sheared granular media, particle motion is characterized by vortex-like structures; here this is demonstrated experimentally for disks system undergoing indefinite deformation during simple shear, as often imposed by the rock masses hosting earthquake fault gouges. In traditional fluids it has been known for years that vortices represent a major factor of heat transfer enhancement via convective internal mixing, but in analyses of heat transfer through earthquake faults and base planes of landslides this has been continuously neglected. Can research proceed by neglecting heat convection by internal mixing? Our answer is astonishingly far from being yes.
Formulation of nano fluids for natural convective heat transfer applications
International Nuclear Information System (INIS)
The paper is concerned about formulation of aqueous based nanofluids and its application under natural convective heat transfer conditions. Titanium dioxide nanoparticles are dispersed in distilled water through electrostatic stabilization mechanisms and with the aid of a high shear mixing homogenizer. Nanofluids formulated in such a way are found very stable and are used to investigate their heat transfer behaviour under the natural convection conditions. The preliminary results are presented in this paper. Both transient and steady heat transfer coefficients are measured and the results show a systematic decrease in the natural convective heat transfer coefficient with increasing particle concentration. This is in contradiction to the initial expectation. Possible reasons for the observations are discussed
Kozlov, V. G.; Ivanova, A. A.; Vjatkin, A. A.; Sabirov, R. R.
2015-07-01
Thermal convection of fluid with internal heat sources in a rotating horizontal cylinder with isothermal boundary and adiabatic ends is investigated experimentally. Under the action of gravity nonisothermal liquid oscillates in the cavity frame. These tidal oscillations generate the average mass force and, as a result, excite convection. The steady convection developing by this mechanism is called thermal vibrational. Centrifugal force in the considered case plays a stabilizing role. The objects of studying are the excitation thresholds of the averaged convection, heat transfer and the structure of convective flows. The parameters varying in the experiments are heat release rate, relative length of the cylinder and rotation velocity. It is found that the inertial waves which are generated near the ends of the cavity by tidal oscillations of nonisothermal liquid effect the convection. The intensity of flows excited by these waves is relatively low, but significant especially below the threshold of thermal vibrational convection. It is shown that the influence of inertial waves on heat transfer and structure of convective flows strongly depends on the cavity aspect ratio.
Energy Technology Data Exchange (ETDEWEB)
Ghalambaz, M.; Noghrehabadi, A.; Ghanbarzadeh, A., E-mail: m.ghalambaz@gmail.com, E-mail: ghanbarzadeh.a@scu.ac.ir [Department of Mechanical Engineering, Shahid Chamran University of Ahvaz, Ahvaz (Iran, Islamic Republic of)
2014-04-15
In this paper, the natural convective flow of nanofluids over a convectively heated vertical plate in a saturated Darcy porous medium is studied numerically. The governing equations are transformed into a set of ordinary differential equations by using appropriate similarity variables, and they are numerically solved using the fourth-order Runge-Kutta method associated with the Gauss-Newton method. The effects of parametric variation of the Brownian motion parameter (Nb), thermophoresis parameter (Nt) and the convective heating parameter (Nc) on the boundary layer profiles are investigated. Furthermore, the variation of the reduced Nusselt number and reduced Sherwood number, as important parameters of heat and mass transfer, as a function of the Brownian motion, thermophoresis and convective heating parameters is discussed in detail. The results show that the thickness of the concentration profiles is much lower than the temperature and velocity profiles. For low values of the convective heating parameter (Nc), as the Brownian motion parameter increases, the non-dimensional wall temperature increases. However, for high values of Nc, the effect of the Brownian motion parameter on the non-dimensional wall temperature is not significant. As the Brownian motion parameter increases, the reduced Sherwood number increases and the reduced Nusselt number decreases. (author)
Scientific Electronic Library Online (English)
M., Ghalambaz; A., Noghrehabadi; A., Ghanbarzadeh.
2014-06-01
Full Text Available In this paper, the natural convective flow of nanofluids over a convectively heated vertical plate in a saturated Darcy porous medium is studied numerically. The governing equations are transformed into a set of ordinary differential equations by using appropriate similarity variables, and they are [...] numerically solved using the fourth-order Runge-Kutta method associated with the Gauss-Newton method. The effects of parametric variation of the Brownian motion parameter (Nb), thermophoresis parameter (Nt) and the convective heating parameter (Nc) on the boundary layer profiles are investigated. Furthermore, the variation of the reduced Nusselt number and reduced Sherwood number, as important parameters of heat and mass transfer, as a function of the Brownian motion, thermophoresis and convective heating parameters is discussed in detail. The results show that the thickness of the concentration profiles is much lower than the temperature and velocity profiles. For low values of the convective heating parameter (Nc), as the Brownian motion parameter increases, the non-dimensional wall temperature increases. However, for high values of Nc, the effect of the Brownian motion parameter on the non-dimensional wall temperature is not significant. As the Brownian motion parameter increases, the reduced Sherwood number increases and the reduced Nusselt number decreases.
International Nuclear Information System (INIS)
In this paper, the natural convective flow of nanofluids over a convectively heated vertical plate in a saturated Darcy porous medium is studied numerically. The governing equations are transformed into a set of ordinary differential equations by using appropriate similarity variables, and they are numerically solved using the fourth-order Runge-Kutta method associated with the Gauss-Newton method. The effects of parametric variation of the Brownian motion parameter (Nb), thermophoresis parameter (Nt) and the convective heating parameter (Nc) on the boundary layer profiles are investigated. Furthermore, the variation of the reduced Nusselt number and reduced Sherwood number, as important parameters of heat and mass transfer, as a function of the Brownian motion, thermophoresis and convective heating parameters is discussed in detail. The results show that the thickness of the concentration profiles is much lower than the temperature and velocity profiles. For low values of the convective heating parameter (Nc), as the Brownian motion parameter increases, the non-dimensional wall temperature increases. However, for high values of Nc, the effect of the Brownian motion parameter on the non-dimensional wall temperature is not significant. As the Brownian motion parameter increases, the reduced Sherwood number increases and the reduced Nusselt number decreases. (author)
Reynolds stress and heat flux in spherical shell convection
Käpylä, P J; Guerrero, G; Brandenburg, A; Chatterjee, P
2010-01-01
Context. Turbulent fluxes of angular momentum and heat due to rotationally affected convection play a key role in determining differential rotation of stars. Aims. We compute turbulent angular momentum and heat transport as functions of the rotation rate from stratified convection. We compare results from spherical and Cartesian models in the same parameter regime in order to study whether restricted geometry introduces artefacts into the results. Methods. We employ direct numerical simulations of turbulent convection in spherical and Cartesian geometries. In order to alleviate the computational cost in the spherical runs and to reach as high spatial resolution as possible, we model only parts of the latitude and longitude. The rotational influence, measured by the Coriolis number or inverse Rossby number, is varied from zero to roughly seven, which is the regime that is likely to be realised in the solar convection zone. Cartesian simulations are performed in overlapping parameter regimes. Results. For slow ...
Cao, Y.; Faghri, A.; Juhasz, A.
1991-01-01
Latent heat energy storage systems with both annular and countercurrent flows are modeled numerically. The change of phase of the phase-change material (PCM) and the transient forced convective heat transfer for the transfer fluid are solved simultaneously as a conjugate problem. A parametric study and a system optimization are conducted. It is found that the energy storage system with the countercurrent flow is an efficient way to absorb heat energy in a short period for pulsed power load space applications.
International Nuclear Information System (INIS)
Mixed forced and free convective heat transfer through a cavity with a rectangular element on one wall heated to a uniform temperature, and the opposite wall cooled to a uniform lower temperature, and with the remaining wall portions adiabatic, has been investigated. The forced flow, which is at the same temperature as the cold wall, enters through the cold vertical wall and also leaves through this wall. The study is based on the use of the two-dimensional Navier-Stokes, equations, the flow assumed steady and laminar. It is assumed that fluid properties are constant except for the density change with temperature which gives rise to the buoyancy force, this being treated using the Boussinesq approximation. These equations have been solved using the finite element method. The solution has, as parameters, the Reynolds number, the Grashof number, the fluid Prandtl number, the aspect ratio of the cavity, the angle of inclination of the cavity, the inlet and outlet duct size, and the aspect ratio of the heated element. Results have only been obtained for a Prandtl number of 0.7. 7 references
Oosthuizen, P. H.; de Champlain, A.
1988-06-01
Mixed forced and free convective heat transfer through a cavity with a rectangular element on one wall heated to a uniform temperature, and the opposite wall cooled to a uniform lower temperature, and with the remaining wall portions adiabatic, has been investigated. The forced flow, which is at the same temperature as the cold wall, enters through the cold vertical wall and also leaves through this wall. The study is based on the use of the two-dimensional Navier-Stokes, equations, the flow assumed steady and laminar. It is assumed that fluid properties are constant except for the density change with temperature which gives rise to the buoyancy force, this being treated using the Boussinesq approximation. These equations have been solved using the finite element method. The solution has, as parameters, the Reynolds number, the Grashof number, the fluid Prandtl number, the aspect ratio of the cavity, the angle of inclination of the cavity, the inlet and outlet duct size, and the aspect ratio of the heated element. Results have only been obtained for a Prandtl number of 0.7.
International Nuclear Information System (INIS)
A new experimental technique is presented that allows simultaneous measurement of convective and radiative heat flux in the underhood. The goal is to devise an easily implemented and accurate experimental method for application in the vehicle underhood compartment. The new method is based on a technique for heat-flux measurement developed by the authors (Heat flow (flux) sensors for measurement of convection, conduction and radiation heat flow 27036-2, © Rhopoint Components Ltd, Hurst Green, Oxted, RH8 9AX, UK) that uses several thermocouples in the thickness of a thermal resistive layer (foil heat-flux sensor). The method proposed here uses a pair of these thermocouples with different radiative properties. Measurements validating this novel technique are carried out on a flat plate with a prescribed constant temperature in both natural- and forced-convection flow regimes. The test flat plate is instrumented by this new technique, and also with a different technique that is intrusive but very accurate, used as reference here (Bardon J P and Jarny Y 1994 Procédé et dispositif de mesure transitoire de température et flux surfacique Brevet n°94.011996, 22 February). Discrepancies between the measurements by the two techniques are less than 10% for both convective and radiative heat flux. Error identification and sensitivity analysis of the new method are also presented
International Nuclear Information System (INIS)
The helical coiled tube of heat exchanger is used for the evaporator of prototype fast breeder reactor 'Monju'. This report aims at the grasp of two-phase flow phenomena of forced convective boiling of water inside helical coiled tube. A transparent double tube made of the glass is used as a heat exchanger, water flows up inside helical tube and the high temperature oil flows down in the outside tube. The oscillation of the dryout point was observed, that is mainly caused by intensive nucleate boiling near the dryout point and evaporation of thin liquid film flowing along the surface. Also, the oscillation characteristics were experimentally confirmed. (author)
An experimental investigation of forced convection flat plate solar air heater with storage material
Aissa Walid; El-Sallak Mostafa; Elhakem Ahmed
2012-01-01
Solar air heater (SAH) is a heating device that uses the heated air in the drying of agriculture products and many engineering applications. The purpose of the present work is to study a forced convection flat plate solar air heater with granite stone storage material bed under the climatic conditions of Egypt-Aswan. Experiments are performed at different air mass flow rates ; varying from 0.016 kg/s to 0.08 kg/s, for five hot summer days of July 2008. Hourly values of global solar radi...
Scientific Electronic Library Online (English)
Mariangela, Amendola; Saul, Dussán-Sarria; Anderson A., Rabello.
2009-04-01
Full Text Available Este trabalho busca, como função principal, determinar o valor do coeficiente convectivo de transferência de calor através da metodologia de modelagem matemática e simulação numérica associada ao processo experimental previamente realizado, de resfriamento rápido com ar forçado de figos "Roxo de Val [...] inhos". O objetivo foi comparar o resultado com o definido pelo ajuste entre os dados experimentais e o uso de equação empírica da literatura, já realizado. Para a predição da temperatura próximo ao centro das frutas, considerou-se o modelo da lei de Fourier em coordenadas esféricas e se implementaram algoritmos segundo os métodos de diferenças finitas e dos elementos finitos para os modelos unidimensional e tridimensional, respectivamente. O estabelecimento do referido valor se dá a partir da comparação desses dados simulados com os dados experimentais. Os valores resultantes dos dois modelos foram coincidentes e resultaram menores valores residuais que o obtido pela equação empírica; além disso, a representatividade da curva simulada mostra que a atual metodologia é mais precisa que a anteriormente realizada e, portanto, adequada para este e trabalhos futuros. Abstract in english The main purpose of this study is to determine the convective heat transfer coefficient value by using the methodology of mathematical modeling and numerical simulation associated with the experimental process previously performed, of the forced air precooling of the figs type 'Roxo de Valinhos'. Th [...] e objective was to compare the result with the one determined by fitting an empirical equation from the literature to the experimental data. For the prediction of the temperature, near to the center of fruits, the Fourier law model was considered. Algorithms employing the finite differences and finite elements methods were implemented for the one-dimensional and three-dimensional models. The establishment of the referred value was performed through the comparison of these simulated data with the experimental data. The resulting values of the two models were coincident and generated a smaller residual value than the one obtained by the empirical equation. Furthermore, the representation of the simulated curve shows that the methodology of the mathematical modeling and numerical simulation is more accurate than the previous one and suitable for this and future work.
International Nuclear Information System (INIS)
Effect of buoyancy force in a laminar uniform forced convection flow past a semi-infinite vertical plate has been analyzed near the leading edge, taking into account the viscous dissipation. The coupled non-linear locally similar equations, which govern the flow, are solved by the method of parametric differentiation. Effects of the buoyancy force and the heat due to viscous dissipation on the flow and the temperature fields as well as on the wall shear-stress and the heat transfer at the surface of the plate are shown graphically for the values of the Prandtl number ? ranging from 10-1 to 1.0. (author). 20 refs, 3 figs, 2 tabs
Convective Heat Transfer In Porous Ceramic Materials
BÜYÜKALACA, Orhan
1999-01-01
In this study heat transfer in porous ceramic materials, which offer a potential as an alternative heat transfer medium in a number of systems in which heat transfer takes place, is investigated experimentally. Experiments were performed for five different specimens at various air flow rates and specimen temperatures. The volumetric heat transfer coefficient was determined using the results of the experiments. A characteristic length obtained from the pressure drop data was used in ...
Heating of matter by microwaves without convection
Draškovi?, Draško
2012-01-01
This thesis considers heating of matter by means of microwaves. The most common device that uses micro waves for the heating of matter is the microwave oven. Because of the microwave ovens ubiquity and accessibility there are many articles that address the way it works and in how it heats matter. However, it appears that articles contradict each other in their claims that the heating of the matter is either from the inside or from the outside. What actually means »heating the material in the...
Natural convection heat transfer within horizontal spent nuclear fuel assemblies
International Nuclear Information System (INIS)
Natural convection heat transfer is experimentally investigated in an enclosed horizontal rod bundle, which characterizes a spent nuclear fuel assembly during dry storage and/or transport conditions. The basic test section consists of a square array of sixty-four stainless steel tubular heaters enclosed within a water-cooled rectangular copper heat exchanger. The heaters are supplied with a uniform power generation per unit length while the surrounding enclosure is maintained at a uniform temperature. The test section resides within a vacuum/pressure chamber in order to subject the assembly to a range of pressure statepoints and various backfill gases. The objective of this experimental study is to obtain convection correlations which can be used in order to easily incorporate convective effects into analytical models of horizontal spent fuel systems, and also to investigate the physical nature of natural convection in enclosed horizontal rod bundles in general. The resulting data consist of: (1) measured temperatures within the assembly as a function of power, pressure, and backfill gas; (2) the relative radiative contribution for the range of observed temperatures; (3) correlations of convective Nusselt number and Rayleigh number for the rod bundle as a whole; and (4) correlations of convective Nusselt number as a function of Rayleigh number for individual rods within the array
Natural convection heat transfer within horizontal spent nuclear fuel assemblies
Energy Technology Data Exchange (ETDEWEB)
Canaan, R.E.
1995-12-01
Natural convection heat transfer is experimentally investigated in an enclosed horizontal rod bundle, which characterizes a spent nuclear fuel assembly during dry storage and/or transport conditions. The basic test section consists of a square array of sixty-four stainless steel tubular heaters enclosed within a water-cooled rectangular copper heat exchanger. The heaters are supplied with a uniform power generation per unit length while the surrounding enclosure is maintained at a uniform temperature. The test section resides within a vacuum/pressure chamber in order to subject the assembly to a range of pressure statepoints and various backfill gases. The objective of this experimental study is to obtain convection correlations which can be used in order to easily incorporate convective effects into analytical models of horizontal spent fuel systems, and also to investigate the physical nature of natural convection in enclosed horizontal rod bundles in general. The resulting data consist of: (1) measured temperatures within the assembly as a function of power, pressure, and backfill gas; (2) the relative radiative contribution for the range of observed temperatures; (3) correlations of convective Nusselt number and Rayleigh number for the rod bundle as a whole; and (4) correlations of convective Nusselt number as a function of Rayleigh number for individual rods within the array.
International Nuclear Information System (INIS)
This paper presents the results of the experimental investigation on heat transfer and fluid friction characteristics of a class of spiral spring coil used as a tube side forced convection heat transfer augmentation devices. Based on a lot of experimental data, the heat transfer correlation and fluid friction correlation revised by temperature were reached in terms of linear regression. At the same time, proper criteria were used to evaluate the economic performance of the spiral spring inserted tube according to the demand of practical application and some probing analysis were made
Heat transfer by natural convection into an horizontal cavity
International Nuclear Information System (INIS)
At this thesis it is studied the heat transfer by natural convection in an horizontal cavity, it is involved a boiling's part that is described the regimes and correlations differences for boiling's curve. It is designed a horizontal cavity for realize the experimental part and it's mention from equipment or instrumentation to succeed in a experimentation that permits to realize the analysis of heat transfer, handling as water fluid at atmospheric pressure and where it's present process from natural convection involving part boiling's subcooled. The system consists of heater zone submerged in a horizontal cavity with water. Once part finished experimental with information to obtained it's proceeded to obtain a correlation, realized starting from analysis dimensionless such as: Jakob, Bond and Grasoft (Boiling) besides of knows in natural convection: Prandtl and Nusselt. The mathematical model explains the behavior for natural convection continued part boiling's subcooled. It is realize analysis graphics too where it's show comparing with Globe Dropkin and Catton equations by natural convection with bottom heating. (Author)
Investigating Convective Heat Transfer with an Iron and a Hairdryer
Gonzalez, Manuel I.; Lucio, Jesus H.
2008-01-01
A simple experimental set-up to study free and forced convection in undergraduate physics laboratories is presented. The flat plate of a domestic iron has been chosen as the hot surface, and a hairdryer is used to generate an air stream around the plate. Several experiments are proposed and typical numerical results are reported. An analysis and…
International Nuclear Information System (INIS)
Heat transfer correlations are developed for forced turbulent and laminar, combined, and natural convections of water in a uniformly heated, square arranged, nine-rod bundle having a P/D ratio of 1.5. In all correlations, the heated equivalent diameter is used in all the dimensionless quantities, and the water physical properties are evaluated at the water bulk temperature. In the experiments, Re is varied from 300 to 2.5 X 104, Pr from 4 to 9, Raq from 3 x 106 to 3 x 108 for natural convection and from 5 x 107 to 7 , 108 for combined convection, and Ri from 0.04 to 100. In both upflow and downflow experiments, the transition from forced turbulent to forced laminar convection occurs at ReT = 6,700; while the transition from forced laminar to buoyancy assisted combined convection occurs at Ri = 2.0. Results show that the rod arrangement in the bundle has little effect on the values of Nu in the forced and natural convection regimes. In general, Nu values for the square arranged rod bundle are less than 8% higher and less than 10% lower than those for a triangularly arranged rod bundle in the forced and natural convection regimes, respectively. 16 refs., 7 figs
A multiple-relaxation-time lattice Boltzmann model for convection heat transfer in porous media
Liu, Qing; He, Ya-Ling; Li, Qing; Tao, Wen-Quan
2013-01-01
In this paper, a two-dimensional (2D) multiple-relaxation-time (MRT) lattice Boltzmann (LB) model is developed for simulating convection heat transfer in porous media at the representative elementary volume scale. In the model, a MRT-LB equation is used to simulate the flow field, while another MRT-LB equation is employed to simulate the temperature field. The effect of the porous media is considered by introducing the porosity into the equilibrium moments, and adding a forc...
On the Asymptotic Approach to Thermosolutal Convection in Heated Slow Reactive Boundary Layer Flows
Sandile S. Motsa; Precious Sibanda; Stanford Shateyi
2008-01-01
The study sought to investigate thermosolutal convection and stability of two dimensional disturbances imposed on a heated boundary layer flow over a semi-infinite horizontal plate composed of a chemical species using a self-consistent asymptotic method. The chemical species reacts as it diffuses into the nearby fluid causing density stratification and inducing a buoyancy force. The existence of significant temperature gradients near the plate surface results in additional buoyancy and decrea...
Energy Technology Data Exchange (ETDEWEB)
Kalteh, Mohammad [Department of Mechanical Engineering, Amirkabir University of Technology, Hafez Ave., P.O. Box 15916-34311, Tehran (Iran, Islamic Republic of); Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven (Netherlands); Abbassi, Abbas, E-mail: abbassi@aut.ac.i [Department of Mechanical Engineering, Amirkabir University of Technology, Hafez Ave., P.O. Box 15916-34311, Tehran (Iran, Islamic Republic of); Saffar-Avval, Majid [Department of Mechanical Engineering, Amirkabir University of Technology, Hafez Ave., P.O. Box 15916-34311, Tehran (Iran, Islamic Republic of); Harting, Jens [Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven (Netherlands); Institute for Computational Physics, University of Stuttgart, Pfaffenwaldring 27, 70569 Stuttgart (Germany)
2011-02-15
In this paper, laminar forced convection heat transfer of a copper-water nanofluid inside an isothermally heated microchannel is studied numerically. An Eulerian two-fluid model is considered to simulate the nanofluid flow inside the microchannel and the governing mass, momentum and energy equations for both phases are solved using the finite volume method. For the first time, the detailed study of the relative velocity and temperature of the phases are presented and it has been observed that the relative velocity and temperature between the phases is very small and negligible and the nanoparticle concentration distribution is uniform. However, the two-phase modeling results show higher heat transfer enhancement in comparison to the homogeneous single-phase model. Also, the heat transfer enhancement increases with increase in Reynolds number and nanoparticle volume concentration as well as with decrease in the nanoparticle diameter, while the pressure drop increases only slightly.
An investigation of natural convection-radiation combined heat transfer
International Nuclear Information System (INIS)
Heat transfer by natural convection and radiation in participating fluids enclosed in a square enclosure is numerically simulated. The continuity, momentum and energy equations are solved by the control volume method while the radiative heat transfer is analyzed by the Monte Carlo simulation. The Monte Carlo simulation is linked with the SIMPLE algorithm for solving the Navier-Stokes equations, therefore all variables such as U, V, and T are solved each time the radiative heat flux is updated. As the result, the influences of thermal radiation on the flow and temperature fields are found significant. The radiative heat transfer is dominant. The convection is also strengthened. These features become clear with higher optical thickness. (author)
Measurement of the Convective Heat-Transfer Coefficient
Conti, Rosaria; Gallitto, Aurelio Agliolo; Fiordilino, Emilio
2014-01-01
We propose an experiment for investigating how objects cool down toward the thermal equilibrium with their surroundings. We describe the time dependence of the temperature difference of the cooling objects and the environment with an exponential decay function. By measuring the thermal constant t, we determine the convective heat-transfer…
Heat convection in a set of three vertical cylinders
International Nuclear Information System (INIS)
Experimental results on temperature and heat flow in a set of three vertical cylinders with internal generation of heat, water submerged and in free convection are presented in this work . Temperature distribution, Nusselt number and convective coefficient (h) for each rod, developed for the distance between the axis of cylinders in vertical position, as a consequence of the application of power in its outside, are analyzed. Experimental information about heat transfer by free convection in vertical cylinders and surfaces is analyzed. Information of the several author who have carried out studies about the heat transfer on vertical cylinders was compiled, and the proposed equations with the experimental data obtained in the thermo fluids laboratory of National Institute of Nuclear Research (ININ) were tested. The way in which separation distance, s, distribution temperature array, Nusselt number, and convective coefficient calculated for the proposed channel with the Keyhani, Dutton and experimental equations are tabulated and they are plotted for each power value and for each separation between rods. The scheme of the used equipment and the experimentation description as well as the observations of tests and graphical results are included. (Author)
Relating Convective and Stratiform Rain to Latent Heating
Tao, Wei-Kuo; Lang, Stephen; Zeng, Xiping; Shige, Shoichi; Takayabu, Yukari
2010-01-01
The relationship among surface rainfall, its intensity, and its associated stratiform amount is established by examining observed precipitation data from the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR). The results show that for moderate-high stratiform fractions, rain probabilities are strongly skewed toward light rain intensities. For convective-type rain, the peak probability of occurrence shifts to higher intensities but is still significantly skewed toward weaker rain rates. The main differences between the distributions for oceanic and continental rain are for heavily convective rain. The peak occurrence, as well as the tail of the distribution containing the extreme events, is shifted to higher intensities for continental rain. For rainy areas sampled at 0.58 horizontal resolution, the occurrence of conditional rain rates over 100 mm/day is significantly higher over land. Distributions of rain intensity versus stratiform fraction for simulated precipitation data obtained from cloud-resolving model (CRM) simulations are quite similar to those from the satellite, providing a basis for mapping simulated cloud quantities to the satellite observations. An improved convective-stratiform heating (CSH) algorithm is developed based on two sources of information: gridded rainfall quantities (i.e., the conditional intensity and the stratiform fraction) observed from the TRMM PR and synthetic cloud process data (i.e., latent heating, eddy heat flux convergence, and radiative heating/cooling) obtained from CRM simulations of convective cloud systems. The new CSH algorithm-derived heating has a noticeably different heating structure over both ocean and land regions compared to the previous CSH algorithm. Major differences between the new and old algorithms include a significant increase in the amount of low- and midlevel heating, a downward emphasis in the level of maximum cloud heating by about 1 km, and a larger variance between land and ocean in the new CSH algorithm.
Optimisation of convective heat dissipation from ventilated brake discs
Galindo-Lopez, Carlos Hannover
2009-01-01
Fast heat dissipation from brake discs is sought in current vehicles, where high power braking duties demand harmonic combination of strength, (undamped) disc mass and cooling abilities for a wide speed range. This work analyses the convective heat dissipation from ventilated brake discs and proposes means for its optimisation. The focus of research is the ventilation geometry of a standard brake disc with an outer diameter of 434mm and radial channels of 101mm in length. After...
Second Law Analysis in Convective Heat and Mass Transfer
A. Ben Brahim; N. Hidouri; H. Abbassi; M. Magherbi
2006-01-01
This paper reports the numerical determination of the entropy generation due to heat transfer, mass transfer and fluid friction in steady state for laminar double diffusive convection, in an inclined enclosure with heat and mass diffusive walls, by solving numerically the mass, momentum, species conservation and energy balance equations, using a Control Volume Finite-Element Method. The influences of the inclination angle, the thermal Grashof number and the buoyancy ratio on total entropy gen...
Heat transport measurements in turbulent rotating Rayleigh-Benard convection
Liu, Yuanming; Robert E. Ecke
2008-01-01
We present experimental heat transport measurements of turbulent Rayleigh-B\\'{e}nard convection with rotation about a vertical axis. The fluid, water with Prandtl number ($\\sigma$) about 6, was confined in a cell which had a square cross section of 7.3 cm$\\times$7.3 cm and a height of 9.4 cm. Heat transport was measured for Rayleigh numbers $2\\times 10^5
Scaling of heat transport near onset in rapidly rotating convection
Ecke, Robert E.
2015-10-01
We consider the scaling of heat transport in the geostrophic regime of rotating Rayleigh-Bénard convection near onset for small Ekman number Ek from the perspective of weakly nonlinear theory. We show that available heat transport data from numerical simulation [1] for Ek law slope depends sensitively on the magnitude of the coefficients a and b. Similar arguments apply to Pr = 7 numerical data although the weakly nonlinear expansion appears valid for a smaller range of ? than in the Pr = 1 case.
Zhang, Nan; Park, Hyun Gyoon; Derby, Jeffrey J.
2013-03-01
Quasi-steady-state (QSS) and transient models, developed using the CrysMAS code, are employed to study the effects of transport mechanisms and cold finger design on the temperature distribution, melt flow field, and melt-crystal interface shape during the crystal growth of sapphire by a small-scale, modified heat exchanger method (HEM). QSS computations show the importance and effects of various heat transfer mechanisms in the crystal and melt, including conduction, internal radiation, and melt convection driven by buoyant and Marangoni forces. The design of the cold finger is demonstrated to have significant effects on growth states. Notably, transient computations on an idealized heat transfer model, supplemented with QSS calculations of a model with rigorous heat transfer representation, show that non-uniform growth conditions arise under uniform cooling of the system via a linear decrease in furnace set points. We suggest that more uniform HEM growth conditions may be achieved by using non-linear cool-down strategies.
International Nuclear Information System (INIS)
Natural convective heat transfer from a wide heated vertical isothermal plate with adiabatic surfaces above and below the heated surface has been considered. There are a series of equally spaced vertical thin, flat surfaces (termed 'slats') near the heated surface, these surfaces being, in general, inclined to the heated surface. There is, in general, a uniform heat generation in the slats. The slats are pivoted about their centre-point and thus as their angle is changed, the distance of the tip of the slat from the plate changes. The situation considered is an approximate model of a window with a vertical blind, the particular case where the window is hotter than the room air being considered. The heat generation in the slats in this situation is the result of solar radiation passing through the window and falling on and being absorbed by the slats of the blind. The flow has been assumed to be laminar and steady. Fluid properties have been assumed constant except for the density change with temperature that gives rise to the buoyancy forces. The governing equations have been written in dimensionless form and the resulting dimensionless equations have been solved using a commercial finite-element package. The solution has the following parameters: (1) the Rayleigh number (2) the Prandtl number (3) the dimensionless heat generation rate in the slats per unit frontal area (4) the dimensionless distance of the slat center point (the pivot point) from the surface (5) the dimensionless slat size (6) the dimensionless slat spacing (7) the angle of inclination of the slats. Because of the application that motivated the study, results have only been obtained for a Prandtl number of 0.7. The effect of the other dimensionless variables on the mean dimensionless heat transfer rate from the heated vertical surface has been examined. (author)
STAFFORD, J; Walsh, E; Egan, V.
2009-01-01
Convective heat transfer, due to axial flow fans impinging air onto a heated flat plate, is investigated with infrared thermography to assess the heated-thin-foil technique commonly used to quantify two-dimensional heat transfer performance. Flow conditions generating complex thermal profiles have been considered in the analysis to account for dominant sources of error in the technique. Uncertainties were obtained in the measured variables and the influences on the resultant heat transfer dat...
MHD forced and free convection boundary layer flow near the leading edge
International Nuclear Information System (INIS)
Magnetohydrodynamic forced and free convection flow of an electrically conducting viscous incompressible fluid past a vertical flat plate with uniform heat flux in the presence of a magnetic field acting normal to the plate that moves with the fluid has been studied near the leading edge of the plate. The coupled non-linear equations are solved by the method of superposition for the values of the Prandtl number ranges from 0.01 to 10.0. The velocity and the temperature profiles are presented graphically and the values of the wall shear-stress as well as the heat transfer rate are presented in tabular form showing the effect of the buoyancy force and the applied magnetic field. To show the accuracy of the present method some typical values are compared with the available one. (author). 17 refs, 3 figs, 2 tabs
Olson, Sandra
2011-01-01
To better evaluate the buoyant contributions to the convective cooling (or heating) inherent in normal-gravity material flammability test methods, we derive a convective heat transfer correlation that can be used to account for the forced convective stretch effects on the net radiant heat flux for both ignition delay time and burning rate. The Equivalent Low Stretch Apparatus (ELSA) uses an inverted cone heater to minimize buoyant effects while at the same time providing a forced stagnation flow on the sample, which ignites and burns as a ceiling fire. Ignition delay and burning rate data is correlated with incident heat flux and convective heat transfer and compared to results from other test methods and fuel geometries using similarity to determine the equivalent stretch rates and thus convective cooling (or heating) rates for those geometries. With this correlation methodology, buoyant effects inherent in normal gravity material flammability test methods can be estimated, to better apply the test results to low stretch environments relevant to spacecraft material selection.
International Nuclear Information System (INIS)
The report describes an experiment on natural convective heat transfer of silicone oil and water around a horizontal cylinder in a rectangular box and on the visualization of temperature distribution by the liquid-crystal suspension method. The natural convection around the heated cylinder in the box is effected by the thermal stratification and plume. Experimental results are summarized as follows. The Nusselt numbers on the heated horizontal cylinder are correlated with the Ra numbers. The reference fluid temperature on the heat transfer is determined from the thermal stratification in the box. The swaying motion of the thermal plume is effected by the stratification. (author)
A study on calibration of heat flux sensor by using convective heat transfer
International Nuclear Information System (INIS)
The objective of this work is to propose calibration facility in which a thin film type heat flux sensor can be calibrated under convective flow condition by using a small wind tunnel with the constant temperature plate condition. A small wind tunnel has been built to produce a boundary layer shear flow above a constant temperature copper plate. 12-independent copper blocks, thin film heaters, insulators and temperature controllers were used to keep the temperature of flat plate constant at a specified temperature. Three commercial thin film-type heat flux sensors were tested. Convective calibrations of these gages were performed over the available heat flux range of 1.4?2.5 kW/m2. The uncertainty in the heat flux measurements in the convective-type heat flux calibration facility was ±2.07%. Non-dimensional sensitivity is proposed to compare the sensitivity calibrated by manufacturer and that of experiment conducted in this study
Origin of Knudsen forces on heated microbeams
Zhu, Taishan
2010-09-09
The presented work probes the fundamentals of Knudsen forces. Using the direct simulation Monte Carlo (DSMC) method, the flows induced by temperature inhomogeneity within a representative configuration and the Knudsen force acting on a heated microbeam are captured as functions of Knudsen number in the entire flow regime. Both flow strength and Knudsen force peak in the transition regime and negative Knudsen force absent in experimental data is observed. The mechanisms of the thermally induced flows and Knudsen forces are studied. It has been found that thermal edge flow is the main driven source for the formation of the Knudsen force on microbeams and domain configuration plays an important role in the process.
A correlation for natural convection heat transfer from inclined plate-finned heat sinks
International Nuclear Information System (INIS)
Steady-state natural convection heat transfer from inclined plate-finned heat sinks to air is numerically investigated by using an experimentally validated model. The heat sinks with parallel arrangement of uniform rectangular cross section plate fins are inclined from the vertical in both forward and backward directions in order to investigate the effect of inclination on convection. Our previously validated numerical model for vertically oriented heat sinks is directly used without changing any model parameters, but only by varying the direction of the gravitational acceleration to create the effect of inclination. The flow and temperature fields are resolved using a finite volume computational fluid dynamics code. Performing a large number of simulations for the heat sink base inclination angles of ±4°, ±10°, ±20°, ±30°, ±45°, ±60°, ?65°, ?70°, ±75°, ±80°, ±85°, ±90° from the vertical, the dependence of the convective heat-transfer rate to the inclination angle and Rayleigh number is investigated. Scale analyses are performed in order to generalize estimates for the convection heat-transfer rates. A single correlation is suggested and shown to be valid for a very wide range of angles from ?60° (upward) to +80° (downward) in a wide range of Rayleigh numbers from 0 to 2 × 108. -- Highlights: ? Natural convection heat transfer from inclined plate-finned heat sinks is investigated. ? A correlation for estimating convection heat-transfer rates is suggested. ? The correlation is shown to be valid in a very wide range of angles, ?60° ? ? ? +80°. ? The correlation is verified with all available experimental data in literature. ? Flow separation and fin height play the most significant roles at high inclinations
International Nuclear Information System (INIS)
The electro-thermo-convective motion in a plane horizontal dielectric liquid layer subjected to simultaneous action of electric field and thermal gradient is numerically investigated. We consider the case of a strong unipolar charge injection C = 10 from above or below. Therefore in this context, we only take into account the Coulomb force, disregarding the dielectric one. The effect of the electric field on the heat transfer is analyzed through the characterization of the time history of the Nusselt number as well as its evolution according to the characteristic dimensionless electric parameter T. It is demonstrated that the electric effects dominate the buoyancy ones resulting in an electrically induced convection which significantly enhance the heat transfer.
Energy Technology Data Exchange (ETDEWEB)
Schumacher, Courtney
2012-12-13
Heating associated with tropical cloud systems drive the global circulation. The overall research objectives of this project were to i) further quantify and understand the importance of heating in tropical convective cloud systems with innovative observational techniques, and ii) use global models to determine the large-scale circulation response to variability in tropical heating profiles, including anvil and cirrus cloud radiative forcing. The innovative observational techniques used a diversity of radar systems to create a climatology of vertical velocities associated with the full tropical convective cloud spectrum along with a dissection of the of the total heating profile of tropical cloud systems into separate components (i.e., the latent, radiative, and eddy sensible heating). These properties were used to validate storm-scale and global climate models (GCMs) and were further used to force two different types of GCMs (one with and one without interactive physics). While radiative heating was shown to account for about 20% of the total heating and did not have a strong direct response on the global circulation, the indirect response was important via its impact on convection, esp. in how radiative heating impacts the tilt of heating associated with the Madden-Julian Oscillation (MJO), a phenomenon that accounts for most tropical intraseasonal variability. This work shows strong promise in determining the sensitivity of climate models and climate processes to heating variations associated with cloud systems.
Natural convection heat transfer from a vertical circular tube sheet
International Nuclear Information System (INIS)
Experiments were conducted to determine natural convection heat transfer coefficients (a) on a plain vertical circular plate, and (b) on a similar plate with a square array of non-conducting tubes fixed in it. The experiments were carried out using air as the heat transfer medium. The diameter of the brass plates used was 350 mm. The diameter of the bakelite tubes used was 19.2 mm. The range of Rayleigh numbers was from 1.06x108 to 1.66x108. The results show that the heat transfer coefficients in case (a) are very close to those obtained using standard correlations for vertical flat plates, whereas for case (b) the heat transfer coefficients are at least 50 percent higher than those predicted by the Churchill-Chu correlation. It is hence concluded that the disturbance to boundary layer caused by the presence of tubes enhances the heat transfer coefficient significantly. (author). 4 refs., 3 figs
Dendrite growth under forced convection: analysis methods and experimental tests
Alexandrov, D. V.; Galenko, P. K.
2014-08-01
An analysis is given of the nonisothermal growth of a dendrite crystal under forced fluid flow in a binary system. The theoretical model utilized employs a free moving crystal–liquid interface and makes use of the Oseen approximation for the equations of motion of the liquid. A criterion for the stable growth of two-dimensional and three-dimensional parabolic dendrites is derived under the assumption of an anisotropic surface tension at the crystal–liquid interface, which generalizes the previous known results for the stable growth of a dendrite with convection in a one-component fluid and for the growth of a dendrite in a two-component system at rest. The criterion obtained within the Oseen hydrodynamic approximation is extended to arbitrary Peclet numbers and dendrite growth with convection in a nonisothermal multicomponent system. Model predictions are compared with experimental data on crystal growth kinetics in droplets processed in electromagnetic and electrostatic levitation facilities. Theoretical and simulation methods currently being developed are applied to crystallization processes under earthly and reduced gravity conditions.
Dendrite growth under forced convection: analysis methods and experimental tests
International Nuclear Information System (INIS)
An analysis is given of the nonisothermal growth of a dendrite crystal under forced fluid flow in a binary system. The theoretical model utilized employs a free moving crystal–liquid interface and makes use of the Oseen approximation for the equations of motion of the liquid. A criterion for the stable growth of two-dimensional and three-dimensional parabolic dendrites is derived under the assumption of an anisotropic surface tension at the crystal–liquid interface, which generalizes the previous known results for the stable growth of a dendrite with convection in a one-component fluid and for the growth of a dendrite in a two-component system at rest. The criterion obtained within the Oseen hydrodynamic approximation is extended to arbitrary Peclet numbers and dendrite growth with convection in a nonisothermal multicomponent system. Model predictions are compared with experimental data on crystal growth kinetics in droplets processed in electromagnetic and electrostatic levitation facilities. Theoretical and simulation methods currently being developed are applied to crystallization processes under earthly and reduced gravity conditions. (reviews of topical problems)
Extinction transition in bacterial colonies under forced convection
Neicu, T; Larochelle, D A; Kudrolli, A
2000-01-01
We report the spatio-temporal response of {\\it Bacillus subtilis} growing on a nutrient-rich layer of agar to ultra-violet (UV) radiation. Below a crossover temperature, the bacteria are confined to regions that are shielded from UV radiation. A forced convection of the population is effected by rotating a UV radiation shield relative to the petri dish. The extinction speed at which the bacterial colony lags behind the shield is found to be qualitatively similar to the front velocity of the colony growing in the absence of the hostile environment as predicted by the model of Dahmen, Nelson and Shnerb. A quantitative comparison is not possible without considering the slow dynamics and the time-dependent interaction of the population with the hostile environment.
Measuring convective heat transfer coefficient around a heated fine wire in cross flow of nanofluids
International Nuclear Information System (INIS)
Recent researches on nanofluids have mainly focused on the increase of thermal conductivity of nanofluids under static condition. The ultimate goal of using nanofluids, however, is to enhance the heat transfer performance under fluid flow. So it has been highly necessary to devise a simple and accurate measuring apparatus which effectively compares the heat transfer capability between the base and nanofluids. Though the convective heat transfer coefficient is not the complete index for the heat transfer capability, it might be one of useful indications of heat transfer enhancement. In this article, the working principles of experimental system for convective heat transfer coefficient around a heated fine wire in cross flow of nanofluids and its application example to three samples of nano lubrication oils are explained in detail
Modeling of transient natural convection heat transfer in electric ovens
Energy Technology Data Exchange (ETDEWEB)
Mistry, Hitesh; Ganapathi-subbu,; Dey, Subhrajit; Bishnoi, Peeush [General Electric ACFD Lab, GE Global Research Centre, 122, EPIP, Whitefield Road, Bangalore 560 066, Karnataka (India); Castillo, Jose Luis [Mabe Mexico S de RL de CV, Acceso B 406, Parque Industrial Jurica, Queretaro 76120, Qro. (Mexico)
2006-12-15
Prediction of transient natural convection heat transfer in vented enclosures has multiple applications such as understanding of cooking environment in ovens and heat sink performance in electronic packaging industry. The thermal field within an oven has significant impact on quality of cooked food and reliable predictions are important for robust design and performance evaluation of an oven. The CFD modeling of electric oven involves three-dimensional, unsteady, natural convective flow-thermal field coupled with radiative heat transfer. However, numerical solution of natural convection in enclosures with openings at top and bottom (ovens) can often lead to non-physical solutions such as reverse flow at the top vent, partly a function of initialization and sometimes dependent on boundary conditions. In this paper, development of a physics based robust CFD methodology is discussed. This model has been developed with rigorous experimental support and transient validation of this model with experiments show less than 3% discrepancy for a bake cycle. There is greater challenge in simulating a broil cycle, where the fluid inside the cavity is stably stratified and is also highlighted. A comparative analyses of bake and broil cycle thermal fields inside the oven are also presented. (author)
AERIAL MEASUREMENTS OF CONVECTION CELL ELEMENTS IN HEATED LAKES
Energy Technology Data Exchange (ETDEWEB)
Villa-Aleman, E; Saleem Salaymeh, S; Timothy Brown, T; Alfred Garrett, A; Malcolm Pendergast, M; Linda Nichols, L
2007-12-19
Power plant-heated lakes are characterized by a temperature gradient in the thermal plume originating at the discharge of the power plant and terminating at the water intake. The maximum water temperature discharged by the power plant into the lake depends on the power generated at the facility and environmental regulations on the temperature of the lake. Besides the observed thermal plume, cloud-like thermal cells (convection cell elements) are also observed on the water surface. The size, shape and temperature of the convection cell elements depends on several parameters such as the lake water temperature, wind speed, surfactants and the depth of the thermocline. The Savannah River National Laboratory (SRNL) and Clemson University are collaborating to determine the applicability of laboratory empirical correlations between surface heat flux and thermal convection intensity. Laboratory experiments at Clemson University have demonstrated a simple relationship between the surface heat flux and the standard deviation of temperature fluctuations. Similar results were observed in the aerial thermal imagery SRNL collected at different locations along the thermal plume and at different elevations. SRNL will present evidence that the results at Clemson University are applicable to cooling lakes.
Convective Heat Transfer Analysis in Fluid Flow with Turbulence Promoters with Heat Pipes
Directory of Open Access Journals (Sweden)
Theodor Mateescu
2007-01-01
Full Text Available The present paper proposes the analysis and the simulation of the convection heat transfer into the fluid flow with turbulence promoters utilizing heat pipes. The study is based on the necesity of the unconventional energy forms capitalization, increasing of the energy efficiency and leads to the energy consumtion decrease in concordance with the sustainable development concept.
Convective Heat Transfer Analysis in Fluid Flow with Turbulence Promoters with Heat Pipes
Theodor Mateescu; Andrei Burlacu
2007-01-01
The present paper proposes the analysis and the simulation of the convection heat transfer into the fluid flow with turbulence promoters utilizing heat pipes. The study is based on the necesity of the unconventional energy forms capitalization, increasing of the energy efficiency and leads to the energy consumtion decrease in concordance with the sustainable development concept.
Convective Heat and Mass Transfer in Rotating Disk Systems
Shevchuk, Igor V
2009-01-01
The book describes results of investigations of a series of convective heat and mass transfer problems in rotating-disk systems, namely, over free rotating disks, under conditions of transient heat transfer, solid- body rotation of fluid, orthogonal flow impingement onto a disk, swirl radial flow between parallel co-rotating disks, in cone-disk systems and for Prandtl and Schmidt numbers larger than unity. Methodology used included integral methods, self-similar and approximate analytical solutions, as well as CFD. The book is aimed at the professional audience of academic researchers, industr
Optimal Heat Transport in Rayleigh-B\\'enard Convection
Sondak, David; Smith, Leslie M.; Waleffe, Fabian
2015-01-01
Steady flows that optimize heat transport are obtained for two-dimensional Rayleigh-B\\'enard convection with no-slip horizontal walls for a variety of Prandtl numbers $Pr$ and Rayleigh number up to $Ra\\sim 10^9$. Power law scalings of $Nu\\sim Ra^{\\gamma}$ are observed with $\\gamma\\approx 0.31$, where the Nusselt number $Nu$ is a non-dimensional measure of the vertical heat transport. Any dependence of the scaling exponent on $Pr$ is found to be extremely weak. On the other h...
Rayleigh-Benard convection heat transfer in nanoparticle suspensions
International Nuclear Information System (INIS)
Research highlights: ? The thermal instability is lower for the nanofluid than for the pure base liquid. ? The heat transfer enhancement is maximum at an optimal particle concentration. ? The maximum heat transfer enhancement increases as the average temperature increases. ? The maximum heat transfer enhancement increases as the particle size decreases. - Abstract: Natural convection heat transfer of nanofluids in horizontal enclosures heated from below is investigated theoretically. The main idea upon which the present work is based is that nanofluids behave more like a single-phase fluid rather than like a conventional solid-liquid mixture, which implies that all the convective heat transfer correlations available for single-phase flows can be extended to nanoparticle suspensions, provided that the thermophysical properties appearing in them are the nanofluid effective properties calculated at the reference temperature. In this connection, two empirical equations, based on a wide variety of experimental data reported in the literature, are developed for the evaluation of the nanofluid effective thermal conductivity and dynamic viscosity, whereas the other effective properties are evaluated by the traditional mixing theory. The heat transfer enhancement that derives from the dispersion of nano-sized solid particles into the base liquid is calculated for different operating conditions, nanoparticle diameters, and combinations of solid and liquid phases. One ofof solid and liquid phases. One of the fundamental results is the existence of an optimal particle loading for maximum heat transfer across the bottom-heated enclosure. In particular, for any assigned combination of suspended nanoparticles and base liquid, it is found that the optimal volume fraction increases as the nanofluid average temperature increases, and may either increase or decrease with increasing the nanoparticle size according as the flow is laminar or turbulent. Moreover, the optimal volume fraction has a peak at a definite value of the Rayleigh number of the base fluid, that depends on both the average temperature of the nanofluid and the diameter of the suspended nanoparticles.
Heat transport measurements in turbulent rotating Rayleigh-Benard convection
Energy Technology Data Exchange (ETDEWEB)
Ecke, Robert E [Los Alamos National Laboratory; Liu, Yuanming [Los Alamos National Laboratory
2008-01-01
We present experimental heat transport measurements of turbulent Rayleigh-Benard convection with rotation about a vertical axis. The fluid, water with Prandtl number ({sigma}) about 6, was confined in a cell which had a square cross section of 7.3 cm x 7.3 cm and a height of 9.4 cm. Heat transport was measured for Rayleigh numbers 2 x 10{sup 5} < Ra < 5 x 10{sup 8} and Taylor numbers 0 < Ta < 5 x 10{sup 9}. We show the variation of normalized heat transport, the Nusselt number, at fixed dimensional rotation rate {Omega}{sub D}, at fixed Ra varying Ta, at fixed Ta varying Ra, and at fixed Rossby number Ro. The scaling of heat transport in the range 10{sup 7} to about 10{sup 9} is roughly 0.29 with a Ro dependent coefficient or equivalently is also well fit by a combination of power laws of the form a Ra{sup 1/5} + b Ra{sup 1/3} . The range of Ra is not sufficient to differentiate single power law or combined power law scaling. The overall impact of rotation on heat transport in turbulent convection is assessed.
Natural Convection Heat Transfer Experiment in a Hemispherical Pool
International Nuclear Information System (INIS)
Natural convection plays an important role in determining the thermal load from molten core accumulated in the reactor vessel lower head during a severe accident. Several numerical and experimental programs were conducted to study the heat transfer in the molten pool. Previous investigations were mostly related to the rectangular and semicircular pools. Except for COPO, UCLA, ACOPO, and BALI, previous investigations suffer from inadequate representation of high modified Rayleigh number (Ra') in the hemispherical pool that may be formed in the reactor core and lower plenum. Thus, experimental work is conducted utilizing SIGMA SP (Simulant Internal Gravitated Material Apparatus Spherical Pool) producing high Ra' turbulent natural convection in a hemispherical pool up to 5.3 x 1011. The heating method has already been tested in SIGMA CP (Circular Pool). Six thin cable-type heaters, each with a diameter of 6 mm, are employed to simulate internal heating in the pool. They are uniformly distributed in the hemispherical pool to supply a maximum of 7.8 kW power to the pool. SIGMA SP has the inner and outer diameters of 500 mm and 520 mm, respectively. The upper flat plate and the curved wall of pool, with a 10 mm thick stainless steel plate, are cooled by a regulated water loop. A water-cooling system is used to maintain the temperature of water surrounding the test section nearly constant with time. This study focuses on quantifying the directional heat losses, angular heat flux distribution, and temperature distribution inside the molten pool
Heat transport measurements in turbulent rotating Rayleigh-Benard convection
Liu, Yuanming
2008-01-01
We present experimental heat transport measurements of turbulent Rayleigh-B\\'{e}nard convection with rotation about a vertical axis. The fluid, water with Prandtl number ($\\sigma$) about 6, was confined in a cell which had a square cross section of 7.3 cm$\\times$7.3 cm and a height of 9.4 cm. Heat transport was measured for Rayleigh numbers $2\\times 10^5 <$ Ra $ < 5\\times 10^8$ and Taylor numbers $0 <$ Ta $< 5\\times 10^{9}$. We show the variation of normalized heat transport, the Nusselt number, at fixed dimensional rotation rate $\\Omega_D$, at fixed Ra varying Ta, at fixed Ta varying Ra, and at fixed Rossby number Ro. The scaling of heat transport in the range $10^7$ to about $10^9$ is roughly 0.29 with a Ro dependent coefficient or equivalently is also well fit by a combination of power laws of the form $a Ra^{1/5} + b Ra^{1/3}$. The range of Ra is not sufficient to differentiate single power law or combined power law scaling. The overall impact of rotation on heat transport in turbulent convect...
Convective heat transfer analysis in aggregates rotary drum reactor
International Nuclear Information System (INIS)
Heat transport characterisation inside rotary drum dryer has a considerable importance linked to many industrial applications. The present paper deals with the heat transfer analysis from experimental apparatus installed in a large-scale rotary drum reactor applied to the asphalt materials production. The equipment including in-situ thermal probes and external visualization by mean of infrared thermography gives rise to the longitudinal evaluation of inner and external temperatures. The assessment of the heat transfer coefficients by an inverse methodology is resolved in order to accomplish a fin analysis of the convective mechanism inside baffled (or flights) rotary drum. The results are discussed and compared with major results of the literature. -- Highlights: ? A thermal and flow experimentation is performed on a large-scale rotary drum. ? Four working points is chosen in the frame of asphalt materials production. ? Evaluation of the convective transfer mechanisms is calculated by inverse method. ? The drying stage is performed in the combustion area. ? Wall/aggregates heat exchanges have a major contribution in the heating stage
Natural convection type after-heat removal device
International Nuclear Information System (INIS)
In a natural convection type after-heat device, a heat collector is disposed at the outside of a safety vessel to discharge the inflow external air to the outside passing through the space between the safety vessel and the heat collector by difference of density of air between the inner side and the outer side of the heat collector, to remove after heat. Then, the heat collector is constituted to have a substantially cylindrical shape prepared by laterally overlaying honeycome-like cylinders in a multi stage, as well as to have a flow blockage mechanism for preventing back flow of the air. Apparent radiation rate is increased by determining the shape of the honeycome-like cylinder appropriately, and since they are overlaied in the multi-stage, the area in contact with the air passing through the inside can be increased. As a result, the amount of after-heat removed can be remarkably increased compared with the existent ordinary flat heat collector. After-heat of a reactor core generated upon shut down of the reactor can be removed surely and statically by using natural ventilation. (N.H.)
Natural convection heat transfer from two horizontal cylinders
Energy Technology Data Exchange (ETDEWEB)
Reymond, Olivier; Murray, Darina B. [Department of Mechanical and Manufacturing Engineering, Trinity College Dublin (Ireland); O' Donovan, Tadhg S. [School of Engineering and Physical Sciences, Heriot-Watt University, Nasmyth Building, Edinburgh EH14 4AS (United Kingdom)
2008-09-15
Natural convection heat transfer from a single horizontal cylinder and a pair of vertically aligned horizontal cylinders is investigated. Surface heat transfer distributions around the circumference of the cylinders are presented for Rayleigh numbers of 2 x 10{sup 6}, 4 x 10{sup 6} and 6 x 10{sup 6} and a range of cylinder spacings of 1.5, 2 and 3 diameters. With a cylinder pairing the lower cylinder is unaffected by the presence of the second cylinder; the same is true of the upper cylinder if the lower one is not heated. However, when both cylinders are heated it has been found that a plume rising from the heated lower cylinder interacts with the upper cylinder and significantly affects the surface heat transfer distribution. Spectral analysis of surface heat transfer signals has established the influence of the plume oscillations on the heat transfer. Thus, when the plume from the lower cylinder oscillates out of phase with the flow around the upper cylinder it increases the mixing and results in enhanced heat transfer. (author)
Validating The Goddard Convective-Stratiform Heating Algorithm for PMM
Lang, S. E.; Tao, W. K.; Takayabu, Y. N.; Shige, S.; Johnson, R. H.; Ciesielski, P. E.
2014-12-01
RMS errors from 2007 through 2008 are computed between the equivalent rainfall obtained from integrating the cloud heating retrieved by the latest Goddard Convective-Stratiform Heating (CSH) algorithm (i.e., the TRMM 3G31 daily gridded product) and the observed surface rainfall obtained from the TRMM 2B31 Combined Algorithm at various time and space scales as well as for varying rain intensities both over land and ocean areas. RMS errors are significantly reduced by spatial and temporal averaging, noticeably lower over ocean than over land, and decrease with increasing rain intensity. There is a mean global positive heating bias of ~24%, which is much higher over land (~41%) than over ocean (~16%); these biases are reduced to (~5%) globally, ~23% over land and a negative bias of just ~2% over ocean when the residual background heating in far from rain regions is not included.
International Nuclear Information System (INIS)
The natural convection heat transfer coefficients on horizontal cylinders were derived numerically from the fundamental equations for laminar natural convection heat transfer by the finite difference method for a wide range of Rayleigh numbers. A new correlation for natural convection heat transfer from a horizontal cylinder, which can describe rigorous numerical solutions for uniform heat fluxes for Prandtl numbers ranging from 0.005 to 10 within ±4%, was derived. (author)
A generalized variational formulation for convective heat transfer
Keramidas, G. A.
1981-12-01
The scope of this paper is to develop the basic equations for a variational formulation which can be used to solve problems related to convection and/or diffusion dominated flows. The formulation is based on the introduction of a generalized quantity defined as the heat displacement. The governing equation is expressed in terms of this quantity and a variational formulation is developed which leads to a system of equations similar in form to Lagrange's equations of mechanics. These equations can be used for obtaining approximate solutions, though they are of particular interest for application of the finite element method. As an example of the formulation two finite element models are derived for solving convection-diffusion boundary value problems. The performance of the two models is investigated and numerical results are given for different cases of convection and diffusion with two types of boundary conditions. The applications of the developed formulations are not limited to convection-diffusion problems but can also be applied to other types of problems such as mass transfer, hydrodynamics and wave propagation.
Thermal Performance Of Convective-Radiative Heat Transfer In Porous Fins
Majid SHAHBABAEI; Seyfolah SAEDODIN
2014-01-01
Forced and natural convection in porous fin with convective coefficient at the tip under radiation and convection effects are investigated in this letter. Aluminum and copper as materials of fins are determined. In forced and natural convection, the air and water, are applied as working fluids, respectively. In order to solution of this nonlinear equation, HPM and VIM has been used . For verifying the accuracy of the solution methods, compare them with exact solutions (BVP). In this work the ...
Directory of Open Access Journals (Sweden)
S. Venkatachalapathy
2010-01-01
Full Text Available Natural convection cooling using air as a fluid is commonly used in the cooling of electronic equipment and many other devices. In this work, a three-dimensional numerical study of natural convection heat transfer from multiple protruding heat sources simulating electronic components is conducted. Computational fluid dynamics (CFD software, FLUENT is used in this analysis. A 4 by 5 array of heat sources are embedded in the bottom wall of an adiabatic square enclosure. The heat sources with a constant heat flux source at the bottom are of square cross-section and arranged in an in-line manner. Each heat source is attached with one thermocouple, which is connected to a data acquisition system and a computer. The steady state temperatures of heat sources, air inlet, outlet and enclosure walls are measured. The analysis is carried out by varying the heat fluxes and outlet areas. The heat transfer coefficient, Nusselt number and Grashof number are obtained. Results indicate that the heat sources inside the array are hotter and the heat transfer coefficient increases almost linearly with heat source surface temperatures. Grashof number and outlet opening areas strongly influence the Nusselt number. The heat transfer coefficient for the inner heat sources in a row is lower than those near the enclosure walls. The results of numerical analysis are compared with the experiments and there is a good agreement between the two.
Silk cocoon drying in forced convection type solar dryer
International Nuclear Information System (INIS)
The thin layer silk cocoon drying was studied in a forced convection type solar dryer. The drying chamber was provided with several trays on which the cocoons loaded in thin layer. The hot air generated in the solar air heater was forced into drying chamber to avoid the direct exposure of sunlight and UV radiation on cocoons. The drying air temperature varied from 50 to 75 oC. The cocoon was dried from the initial moisture content of about 60-12% (wb). The drying data was fitted to thin layer drying models. Drying behaviour of the silk cocoon was best fitted with the Wang and Singh drying model. Good agreement was obtained between predicted and experimental values. Quality of the cocoons dried in the solar dryer was at par with the cocoons dried in the conventional electrical oven dryer in term of the silk yield and strength of the silk. Saving of electrical energy was about 0.75 kWh/kg cocoons dried. Economic analysis indicated that the NPV of the solar dryer was higher and more stable (against escalation rate of electricity) as compare to the same for electrical oven dryer. Due to simplicity in design and construction and significant saving of operational electrical energy, solar cocoon dryer seems to be a viable option.
Wang, Liang-Bi; Zhang, Qiang; Li, Xiao-Xia
2009-01-01
This paper aims to contribute to a better understanding of convective heat transfer. For this purpose, the reason why thermal diffusivity should be placed before the Laplacian operator of the heat flux, and the role of the velocity gradient in convective heat transfer are analysed. The background to these analyses is that, when the energy…
DORE, VALENTINA
2010-01-01
The motion of buoyancy driven plumes is, on all scales, the most common heat and momentum transfer mechanism in geophysical flows, well known as Free Convection. Similarly, density stratification due to heating inequalities is also an ordinary scenario in nature. Free Convection phenomenon coupled with a density stratified fluid setting leads to the so-called Penetrative Free Convection (PFC). When a fluid, in static equilibrium, is stably stratified a thermal forcing can produce an unstable ...
Cherba?ski, Robert
2015-05-01
This paper presents a comparative study on heat transfer in a packed column. Two methods of heating are considered: microwave and convective. Transient one-dimensional mathematical models were proposed to describe the both alternatives. To account for significant differences in the temperatures between the gas and solid phase a heterogeneous model was applied in the modelling. The numerical simulations were carried out for different operating conditions. The effects of the gas inlet temperature and the microwave power, the bed porosity, the penetration depth of microwaves and the gas velocity were examined. The simulation results were compared on the basis of the time profiles of the average bed temperature and the outlet gas temperature. The same electric power utilized in the microwave heated packed column and the convective heated packed column was established as the key criterion for the comparisons. The compared profiles intersect indicating the time ranges in which the one or the other solution provides higher temperature of the bed. It was displayed that the microwave heated packed column should be preferred when longer heating times are required. In turn, the convective heated packed column is the better choice when shorter heating times are needed.
Oluwole Daniel Makinde
2011-01-01
Steady laminar natural convection flow over a semi-infinite moving vertical plate in the presence of internal heat generation and a convective surface boundary condition is examined in this paper. It is assumed that the left surface of the plate is in contact with a hot fluid while the cold fluid on the right surface of the plate contains a heat source that decays exponentially with the classical similarity variable. The governing non-linear partial differential equations have been transforme...
Experimental study of an upward sub-cooled forced convection in a rectangular channel
Kouidri, A.; Madani, B.; Roubi, B.; Hamadouche, A.
2015-08-01
The upward sub-cooled forced convection in a rectangular channel is investigated experimentally. The aim of the present work is the studying of the local heat transfer phenomena. Concerning the experimentation: the n-pentane is used as a working fluid, the independent variables are: the velocity in the range from 0.04 to 0.086 m/s and heat flux density with values between 1.8 and 7.36 W/cm2. The results show that the local Nusselt number distribution is not uniform along the channel; however, uniformity is observed in the mean Nusselt number for Reynolds under 1600. On the other hand, a new correlation to predict the local fluid temperature is established as a function of local wall temperature. The wall's heat is dissipated under the common effect of the sub-cooled regime; therefore, the local heat transfer coefficient is increased. The study of the thermal equilibrium showed that for Reynolds less than 1500; almost all of the heat flux generated by the heater cartridges is absorbed by the fluid.
International Nuclear Information System (INIS)
Convective heat transfer, due to axial flow fans impinging air onto a heated flat plate, is investigated with infrared thermography to assess the heated-thin-foil technique commonly used to quantify two-dimensional heat transfer performance. Flow conditions generating complex thermal profiles have been considered in the analysis to account for dominant sources of error in the technique. Uncertainties were obtained in the measured variables and the influences on the resultant heat transfer data are outlined. Correction methods to accurately account for secondary heat transfer mechanisms were developed and results show that as convective heat transfer coefficients and length scales decrease, the importance of accounting for errors increases. Combined with flow patterns that produce large temperature gradients, the influence of heat flow within the foil on the resultant heat transfer becomes significant. Substantial errors in the heat transfer coefficient are apparent by neglecting corrections to the measured data for the cases examined. Methods to account for these errors are presented here, and demonstrated to result in an accurate measurement of the local heat transfer map on the surface
Stafford, Jason; Walsh, Ed; Egan, Vanessa
2009-10-01
Convective heat transfer, due to axial flow fans impinging air onto a heated flat plate, is investigated with infrared thermography to assess the heated-thin-foil technique commonly used to quantify two-dimensional heat transfer performance. Flow conditions generating complex thermal profiles have been considered in the analysis to account for dominant sources of error in the technique. Uncertainties were obtained in the measured variables and the influences on the resultant heat transfer data are outlined. Correction methods to accurately account for secondary heat transfer mechanisms were developed and results show that as convective heat transfer coefficients and length scales decrease, the importance of accounting for errors increases. Combined with flow patterns that produce large temperature gradients, the influence of heat flow within the foil on the resultant heat transfer becomes significant. Substantial errors in the heat transfer coefficient are apparent by neglecting corrections to the measured data for the cases examined. Methods to account for these errors are presented here, and demonstrated to result in an accurate measurement of the local heat transfer map on the surface.
RELAP5/MOD3 simulation for steam condensation under forced convection conditions
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Experimental and theoretical investigations were conducted by a team in the Department of Nuclear Engineering at the Massachusetts Institute of Technology (MIT) to determine the effects of noncondensable gases on steam condensation under forced convection conditions. The main objective of this study was to determine the condensation heat transfer coefficient of the steam in the presence of noncondensable gases, such as air and helium. In particular, the work was aimed at predicting the in-tube steam condensation rate as applied to the analysis of the isolation condensers of the proposed simplified boiling water reactor. The RELAP5 code uses laminar (Nusselt correlation) and turbulent film condensation (Carpenter ampersand Colburn correlation) heat transfer correlations in the absence of noncondensable gases, whichever is maximum. A reduction factor that is a function of the noncondensable gas concentration is being used to take into account the effect of the noncondensable gas on the condensation heat transfer coefficient. The properties for the gaseous phase are calculated assuming a Gibbs-Dalton mixture of steam and an ideal noncondensable gas. Since the experimental data are limited in the open literature, the MIT experimental program gives us an opportunity to assess the RELAP5 code against the separate-effects test data. The MIT test facility was simulated using the RELAP5 code for steam condensation in the presence of air under forced convection conditions. This paper presents RELAP5 simulation results of the MIT test facility for various inlet air mass fractions with fixed mixture inlet temperature by comparing with the MIT experimental data
Liquid oil painting: Free and forced convection in an enclosure with mechanical and thermal forcing
Sheard, Gregory J; King, Martin P
2012-01-01
A fluid dynamics video is linked to this article, which have been submitted to the Gallery of Fluid Motion as part of the 65th American Physical Society meeting of the Division of Fluid Dynamics, held in San Diego, California, USA, over 17-20 November 2012. The video serves to visualize flows generated in a rectangular enclosure that are subjected to both mechanical and thermal forcing through a common horizontal boundary. This system exhibits features consistent with either horizontal convection or lid-driven cavity flows depending on the ratio between thermal and mechanical stirring, and three different cases are visualized in the linked videos.
Convective Heat Transfer in Impinging- Gas- Jet Arrangements
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J.M. Buchlin
2011-01-01
Full Text Available The paper deals with heat transfer by convection between impinging gas jets and solid surfaces. It considers both single and multiple jet systems. It emphasizes the flow and geometrical parameters as well as the environment conditions at which the jet emerges. In particular, it points out the effect of the jet tilting, thermal entrainment and jet confinement. ASN and ARN schemes are illustrated through industrial and aeronautical applications. Design correlations are proposed. Experimental data obtained from infrared thermography are compared to CFD simulations.
Natural convective heat transfer of lithium under magnetic field
International Nuclear Information System (INIS)
The magnetohydrodynamic (MHD) effect on a natural convection heat transfer of liquid metal lithium was experimentally studied. The test section consisted of a lithium pool and, a heater pin, which was settled in the center of the pool. The results are summarized as follows, (1) with increasing B (Ly?50), the Nusselt number decreased to 70 of the highest value at 0.05 T of the magnetic field; (2) large scale circulation flow profile and temperature profile were found to change drastically with the application of the weak field of 0.01 T
Mixed Convection Heat Transfer on the Outside of a Vertical Cylinder
International Nuclear Information System (INIS)
An experimental study was made of turbulent heat transfer from a vertical cylinder placed in a square channel. The flow medium was water flowing upwards. Basic differential equations governing the mixed flow heat transfer phenomena in a vertical annulus are presented. A dimensional analysis is done to find the dimensionless variables affecting the relative magnitude of the effect of buoyancy on forced convection heat transfer. Dimensionless equations correlating the experimental data ana incorporating a buoyancy parameter of the form Gr/Re2 are presented. Reynolds number range covered is 690 to 129,500 and the Rayleigh num- ber range covered is 109 to 4.2 x 1013 . Effect of different length parameters, like hydraulic diameter and distance of the measuring point from the inlet of the test section, on dimensionless equations are studied
Kao, A.; Shevchenko, N.; Roshchupinka, O.; Eckert, S.; Pericleous, K.
2015-06-01
Using a fully coupled transient 3-dimensional numerical model, the effects of convection on the microstructural evolution of a thin sample of Ga-In25%wt. was predicted. The effects of natural convection, forced convection and thermoelectric magnetohydrodynamics were investigated numerically. A comparison of the numerical results is made to experimental results for natural convection and forced convection. In the case of natural convection, density variations within the liquid cause plumes of solute to be ejected into the bulk. When forced convection is applied observed effects include the suppression of solute plumes, preferential secondary arm growth and an increase in primary arm spacing. These effects were observed both numerically and experimentally. By applying an external magnetic field inter-dendritic flow is generated by thermoelectrically induced Lorentz forces, while bulk flow experiences an electromagnetic damping force. The former causes preferential secondary growth, while the latter slows the formation of solute plumes. This work highlights that the application of external forces can be a valuable tool for tailoring the microstructure and ultimately the macroscopic material properties.
International Nuclear Information System (INIS)
One strategy for preventing the failure of lower head of a nuclear reactor vessel is to flood the concrete cavity with subcooled water in accidents in which relocation of core material into the vessel lower head occurs. After the core material relocates into the vessel, a crust of solid material forms on the inner wall of the vessel, however, most of the pool remains molten and natural convection exists in the pool. At present, uncertainty exists with respect to natural convection heat transfer coefficients between the pool of molten core material and the reactor vessel wall. In the present work, experiments were conducted to examine natural convection heat transfer in internally heated partially filled spherical pools with external cooling. In the experiments, Freon-113 contained in a Pyrex bell jar was used as a test liquid. The pool was bounded with a spherical segment at the bottom, and was heated with magnetrons taken from a conventional microwave oven. The vessel was cooled from the outside with natural convection of water or with nucleate boiling of liquid nitrogen
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Zeinali Heris Saeed
2011-01-01
Full Text Available Abstract In this article, laminar flow-forced convective heat transfer of Al2O3/water nanofluid in a triangular duct under constant wall temperature condition is investigated numerically. In this investigation, the effects of parameters, such as nanoparticles diameter, concentration, and Reynolds number on the enhancement of nanofluids heat transfer is studied. Besides, the comparison between nanofluid and pure fluid heat transfer is achieved in this article. Sometimes, because of pressure drop limitations, the need for non-circular ducts arises in many heat transfer applications. The low heat transfer rate of non-circular ducts is one the limitations of these systems, and utilization of nanofluid instead of pure fluid because of its potential to increase heat transfer of system can compensate this problem. In this article, for considering the presence of nanoparticl: es, the dispersion model is used. Numerical results represent an enhancement of heat transfer of fluid associated with changing to the suspension of nanometer-sized particles in the triangular duct. The results of the present model indicate that the nanofluid Nusselt number increases with increasing concentration of nanoparticles and decreasing diameter. Also, the enhancement of the fluid heat transfer becomes better at high Re in laminar flow with the addition of nanoparticles.
Marangoni mixed convection flow with Joule heating and nonlinear radiation
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Tasawar Hayat
2015-07-01
Full Text Available Marangoni mixed convective flow of Casson fluid in a thermally stratified medium is addressed. Flow analysis has been carried out in presence of inclined magnetic field. Heat transfer analysis is discussed in the presence of viscous dissipation, Joule heating and nonlinear thermal radiation. The governing nonlinear partial differential equations are first converted into ordinary differential systems and then developed the convergent series solutions. Flow pattern with the influence of pertinent parameters namely the magnetic parameter, Casson fluid parameter, temperature ratio parameter, stratification parameter, Prandtl number, Eckert number and radiation parameter is investigated. Expression of local Nusselt number is computed and analyzed. It is found that the Nusselt number decreases by increasing magnetic parameter, temperature ratio parameter, angle of inclination and stratification parameter. Moreover the effect of buoyancy parameter on the velocity distribution is opposite in both the opposing and assisting flow phenomena. Thermal field and associated layer thickness are enhanced for larger radiation parameter.
Marangoni mixed convection flow with Joule heating and nonlinear radiation
Hayat, Tasawar; Shaheen, Uzma; Shafiq, Anum; Alsaedi, Ahmed; Asghar, Saleem
2015-07-01
Marangoni mixed convective flow of Casson fluid in a thermally stratified medium is addressed. Flow analysis has been carried out in presence of inclined magnetic field. Heat transfer analysis is discussed in the presence of viscous dissipation, Joule heating and nonlinear thermal radiation. The governing nonlinear partial differential equations are first converted into ordinary differential systems and then developed the convergent series solutions. Flow pattern with the influence of pertinent parameters namely the magnetic parameter, Casson fluid parameter, temperature ratio parameter, stratification parameter, Prandtl number, Eckert number and radiation parameter is investigated. Expression of local Nusselt number is computed and analyzed. It is found that the Nusselt number decreases by increasing magnetic parameter, temperature ratio parameter, angle of inclination and stratification parameter. Moreover the effect of buoyancy parameter on the velocity distribution is opposite in both the opposing and assisting flow phenomena. Thermal field and associated layer thickness are enhanced for larger radiation parameter.
Experimental study of natural convection heat transfer in a volumetrically heated semicircular pool
International Nuclear Information System (INIS)
Highlights: • Natural convection tests were performed to study high modified Rayleigh number. • The relations between the Nu and Ra? were determined for boundary conditions. • The upward and downward natural convection heat transfer rates were affected by Pr. • From the test, the Rayleigh numbers was obtained from 5.7 × 106 up to 7.0 × 1011. • The resultant correlations can be applied to determine the reactor vessel failure. - Abstract: SIGMA CP (Simulant Internal Gravitated Material Apparatus Circular Pool) tests were performed to study natural convection characterized by a high modified Rayleigh number (Ra?) in a semicircular pool. The test apparatus had a 500 mm diameter, 250 mm height and 100 mm width. Two thin cartridge heaters, which had a sheath diameter of 6 mm and a length of 2000 mm, were used to simulate internal heating in the pool. They were uniformly distributed in the semicircular pool to supply a maximum heating power of 3 kW to the pool. The Ra? was varied from 106 to 1011 based on the power input. The working fluid Prandtl number (Pr) ranged between 4 and 8 for water and was 0.7 for air. Particular attention was paid to the influence of Pr on the natural convection heat transfer in the pool. A number of relations between the Nusselt number (Nu) and Ra? were determined for the boundary conditions in the semicircular pool. The boundary condition is that the upper and lower walls are cooled isothermally. The experimental results indicated that both the upward and downward natural convection heat transfer rates were affected by Pr. The Rayleigh numbers were determined up to 7 × 1011 from this test. The resultant engineering correlations can be applied to simulations of a spectrum of loss-of-coolant accidents to determine the reactor vessel failure time
Bhaskar Kalita
2012-01-01
This paper is devoted for the study of effects influences by heat source on unsteady free convection flow and heat transfer characteristic of a viscous incompressible and electrically conducting fluid between two heated vertical plates in the presence of a uniform magnetic field applied transversely to the flow. The leading momentum and energy equations are solved by the Laplace transform technique and solutions are presented through graphs for velocity and temperature distribution.
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Bhaskar Kalita
2012-12-01
Full Text Available This paper is devoted for the study of effects influences by heat source on unsteady free convection flow and heat transfer characteristic of a viscous incompressible and electrically conducting fluid between two heated vertical plates in the presence of a uniform magnetic field applied transversely to the flow. The leading momentum and energy equations are solved by the Laplace transform technique and solutions are presented through graphs for velocity and temperature distribution.
Heat transfer enhancement of turbulent natural convection adjacent to vertical heated plate
International Nuclear Information System (INIS)
Enhancement of heat transfer was investigated experimentally on natural convection adjacent to a vertical heated plate. In order to promote heat transfer from the heated plate, a V-shaped divertor plate of which the edge faced upstream was attached onto the surface of the vertical plate. The divertor plate redirects high-temperature fluids toward both sides of the plate and introduces low-temperature ambient fluids behind the plate instead. These two mechanisms enhance heat transfer, in particular, in the region behind the divertor plate. Local heat transfer coefficients around the divertor plate were measured using water as a test fluid. The coefficients behind the divertor plate reached 2.5 times of those without the divertor plate. The optimum heights and angles of the divertor plate that make the heat transfer maximum were also studied experimentally. (author)
Scientific Electronic Library Online (English)
Cláudia R., Andrade; Edson L., Zaparoli.
Full Text Available This work studies the forced convection problem in internal flow between concentric annular ducts, with radial fins at the internal tube surface. The finned surface heat transfer is analyzed by two different approaches. In the first one, it is assumed one-dimensional heat conduction along the intern [...] al tube wall and fins, with the convection heat transfer coefficient being a known parameter, determined by an uncoupled solution. In the other way, named conjugated approach, the mathematical model (continuity, momentum, energy and K-epsilon equations) applied to tube annuli problem was numerically solved using finite element technique in a coupled formulation. At first time, a comparison was made between results obtained for the conjugated problem and experimental data, showing good agreement. Then, the temperature profiles under these two approaches were compared to each other to analyze the validity of the one-dimensional classical formulation that has been utilized in the heat exchanger design.
Forced convection of ferro-fluids in a vented cavity with a rotating cylinder
International Nuclear Information System (INIS)
In this study, numerical investigation of the forced convection of ferro-fluid in a square cavity with ventilation ports in the presence of an adiabatic rotating cylinder is carried out. The governing equations are solved with a finite element based solver. The effects of Reynolds number (20 ? Re ? 400), angular rotational speed of the cylinder (-500 ? ? ? 500), strength and location of the magnetic dipole (0 ? ? ? 250), (0.2 ? a ? 0.8, -0.8 ? b ? -0.2) on the flow and thermal fields are numerically studied. It is observed that the length and size of the recirculation zones can be controlled with magnetic dipole strength and angular rotational speed of the cylinder. When the magnetic dipole is closer to the bottom wall of the cavity, flow is accelerated towards the bottom wall with larger influence area. The increasing values of the angular rotational speed of the cylinder in the clockwise direction enhance the heat transfer
Heat transfer enhancement and reduction by poylmer additives in turbulent Rayleigh Benard convection
Dubief, Yves
2010-01-01
This letter confirms the existence of heat transfer enhancement (HTE) and reduction (HTR) in turbulent natural convection with polymer additives. HTE and HTR were numerically predicted by Benzi et al.(PRL, 104, 024502, 2010) in homogenous turbulent convection, but experiments by Ahlers & Nikolaenko(PRL, 104, 034503, 2010) in turbulent natural convection observed HTR only. Using direct numerical simulation of natural convection, the present study reconciles earlier numerical ...
Development and performance evaluation of forced convection potato solar dryer
International Nuclear Information System (INIS)
This research paper deals with the design development and testing of a forced convection solar dryer, for drying and converting to flour of high moisture content vegetables like potatoes. The angle of solar collector was made adjustable for the absorption of maximum solar radiation by the absorber plate. The air flow rate was controlled by adjustable gate valve to find the optimum flow rate for dehydration of the product. The penetration of solar radiation raised the temperature of the absorber plate of the dryer to 110 deg. C during the operation under stagnation or no load conditions. The maximum air temperature attained in the solar air heater, under this condition was 80 deg. C. The dryer was loaded with 12 Kg of blanched potato chips having an initial moisture content of 89.75%, and the final desired moisture content of 6.95% was achieved within five hours without losing the color of potato chips, while the moisture contents reduction was from 89.75% to 33.75% for five hours in open sun drying under shade. The drying cost for 1 Kg of potatoes was calculated as Rs. 245 and it was Rs. 329 in the case of an electric dryer. The life span of the solar dryer was assumed to be 20 years. The cumulative present worth of annual savings over the life of the solar dryer was calculated for blanched potato chips drying, and it turned out be Rs.163177.67/- which was much higher than the capital cost of the dryer (Rs. 25000). The payback period was calculated as 0.89 years, which was also very small considering the life of the system (20 years). (author)
Experimental study of natural convective heat transfer in a vertical hexagonal sub channel
Energy Technology Data Exchange (ETDEWEB)
Tandian, Nathanael P.; Umar, Efrizon; Hardianto, Toto; Febriyanto, Catur [Faculty of Mechanical and Aerospace Engineering, Institut Teknologi Bandung, Jl. Ganesa 10, Bandung 40132 (Indonesia); Nuclear Technology Center for Materials and Radiometry, National Nuclear Energy Agency, Bandung (Indonesia); Faculty of Mechanical and Aerospace Engineering, Institut Teknologi Bandung, Jl. Ganesa 10, Bandung 40132 (Indonesia); Nuclear Energy Regulation Agency, Jakarta (Indonesia)
2012-06-06
The development of new practices in nuclear reactor safety aspects and optimization of recent nuclear reactors, including the APWR and the PHWR reactors, needs a knowledge on natural convective heat transfer within sub-channels formed among several nuclear fuel rods or heat exchanger tubes. Unfortunately, the currently available empirical correlation equations for such heat transfer modes are limited and researches on convective heat transfer within a bundle of vertical cylinders (especially within the natural convection modes) are scarcely done. Although boundary layers around the heat exchanger cylinders or fuel rods may be dominated by their entry regions, most of available convection correlation equations are for fully developed boundary layers. Recently, an experimental study on natural convective heat transfer in a subchannel formed by several heated parallel cylinders that arranged in a hexagonal configuration has been being done. The study seeks for a new convection correlation for the natural convective heat transfer in the sub-channel formed among the hexagonal vertical cylinders. A new convective heat transfer correlation equation has been obtained from the study and compared to several similar equations in literatures.
Ashraf, M.; Narahari, M.; Muthuvalu, Mohana Sundaram
2014-10-01
The series solution of the boundary layer flow over a permeable stretching wedge with convective boundary condition has been investigated in the presence of heat generation or absorption effects. The governing coupled non-linear partial differential equations are transformed to dimensionless system of coupled non-linear ordinary differential equations using the similarity variables and then solved by Homotopy Analysis Method (HAM). An analysis of the results shows that the velocity and temperature fields are significantly influenced by the velocity ratio parameter, wedge angle parameter, suction/injection parameter, heat generation/absorption parameter and convective heat transfer parameter.
Energy Technology Data Exchange (ETDEWEB)
Triplett, C.E.
1996-12-01
This thesis presents the results of an experimental investigation of natural convection heat transfer in a staggered array of heated cylinders, oriented horizontally within a rectangular enclosure. The main purpose of this research was to extend the knowledge of heat transfer within enclosed bundles of spent nuclear fuel rods sealed within a shipping or storage container. This research extends Canaan`s investigation of an aligned array of heated cylinders that thermally simulated a boiling water reactor (BWR) spent fuel assembly sealed within a shipping or storage cask. The results are presented in terms of piecewise Nusselt-Rayleigh number correlations of the form Nu = C(Ra){sup n}, where C and n are constants. Correlations are presented both for individual rods within the array and for the array as a whole. The correlations are based only on the convective component of the heat transfer. The radiative component was calculated with a finite-element code that used measured surface temperatures, rod array geometry, and measured surface emissivities as inputs. The correlation results are compared to Canaan`s aligned array results and to other studies of natural convection in horizontal tube arrays.
International Nuclear Information System (INIS)
Highlights: ? We study the effect of radiation on natural convection in a cavity with a heated plate. ? The heated plate is placed horizontally or vertically at the center of the cavity. ? The surface radiation makes the temperature distribution uniform inside the cavity. ? The average Nusselt number increases with the emissivity and the Rayleigh number. ? The heat transfer rate is higher when the plate is oriented vertically. -- Abstract: This paper reports a fundamental theoretical study made to understand the interaction of surface radiation and natural convection in an air filled cavity with a centrally placed thin heated plate. The vertical walls of the cavity are cooled while the horizontal ones are insulated. The thin plate is assumed to be isothermal and is placed horizontally or vertically. The governing equations were solved using a finite volume method on a uniformly staggered grid system. The effects of the pertinent parameters, viz., Rayleigh number (105 ? Ra ? 107), plate length (0.25 ? D ? 0.75) and emissivity (0 ? ? ? 1) are investigated in detail. In general the results indicate a better homogenization of temperature field within the cavity by radiation. It is also found that the contribution of the convective mechanism to the overall heat transfer increases with emissivity when the plate is horizontally placed whereas decreases when it is vertically placed. This study demonstrates that any model representing a situation of this kind that ignores surface radiation leads to erroneous predictions
International Nuclear Information System (INIS)
This thesis presents the results of an experimental investigation of natural convection heat transfer in a staggered array of heated cylinders, oriented horizontally within a rectangular enclosure. The main purpose of this research was to extend the knowledge of heat transfer within enclosed bundles of spent nuclear fuel rods sealed within a shipping or storage container. This research extends Canaan's investigation of an aligned array of heated cylinders that thermally simulated a boiling water reactor (BWR) spent fuel assembly sealed within a shipping or storage cask. The results are presented in terms of piecewise Nusselt-Rayleigh number correlations of the form Nu = C(Ra)n, where C and n are constants. Correlations are presented both for individual rods within the array and for the array as a whole. The correlations are based only on the convective component of the heat transfer. The radiative component was calculated with a finite-element code that used measured surface temperatures, rod array geometry, and measured surface emissivities as inputs. The correlation results are compared to Canaan's aligned array results and to other studies of natural convection in horizontal tube arrays
The effect of Coriolis force on nonlinear convection in a porous medium
D. H. Riahi
1994-01-01
Nonlinear convection in a porous medium and rotating about vertical axis is studied in this paper. An upper bound to the heat flux is calculated by the method initiated first by Howard [6] for the case of infinite Prandtl number.
Experimental investigation of turbulent mixed convection in the wake of a heated sphere
International Nuclear Information System (INIS)
The axisymmetric wake of a heated sphere under conditions of turbulent mixed convection is investigated in the water test section FLUTMIK. The sphere is located in a vertical channel with forced convective upward flow. The influence of buoyancy forces to the flow field is studied by comparison with the unheated wake. The theoretical fundamentals describing turbulent flows and different versions of the k-? turbulence model extended by buoyancy terms are described in detail. The quantities to be determined experimentally are derived. The temperature and the components of the velocity vector in axial and radial directions are measured simultaneously by means of a thermocouple probe and a two component, two color laser Doppler anemometer. The flow quantities are determined at axial distances between 5 and 106 sphere diameters. The functional principle and the basis of the laser Doppler anemometer are explained. The mean velocity, the mean temperature, the intensities of their fluctuations and the turbulent exchange quantities of momentum and heat transport are calculated. The decay laws of the quantities along the axis of the channel and the radial profiles are indicated and discussed. The applicability of the experimental results of the axisymmetric buoyancy influenced turbulent wake with respect to the turbulence models presented are shown. (orig.)
Convective Heating of the LIFE Engine Target During Injection
Energy Technology Data Exchange (ETDEWEB)
Holdener, D S; Tillack, M S; Wang, X R
2011-10-24
Target survival in the hostile, high temperature xenon environment of the proposed Laser Inertial Fusion Energy (LIFE) engine is critical. This work focuses on the flow properties and convective heat load imposed upon the surface of the indirect drive target while traveling through the xenon gas. While this rarefied flow is traditionally characterized as being within the continuum regime, it is approaching transition where conventional CFD codes reach their bounds of operation. Thus ANSYS, specifically the Navier-Stokes module CFX, will be used in parallel with direct simulation Monte Carlo code DS2V and analytically and empirically derived expressions for heat transfer to the hohlraum for validation. Comparison of the viscous and thermal boundary layers of ANSYS and DS2V were shown to be nearly identical, with the surface heat flux varying less than 8% on average. From the results herein, external baffles have been shown to reduce this heat transfer to the sensitive laser entrance hole (LEH) windows and optimize target survival independent of other reactor parameters.
Heat transport measurements in turbulent rotating Rayleigh-Bénard convection.
Liu, Yuanming; Ecke, Robert E
2009-09-01
We present experimental heat transport measurements of turbulent Rayleigh-Bénard convection with rotation about a vertical axis. The fluid, water with a Prandtl number (sigma) of about 6, was confined in a cell with a square cross section of 7.3 x 7.3 cm2 and a height of 9.4 cm. Heat transport was measured for Rayleigh numbers 2 x 10(5)fit by a combination of power laws of the form a Ra1/5+b Ra1/3. The range of Ra is not sufficient to differentiate single power law or combined power-law scaling. The data are roughly consistent with an assumption that the enhancement of heat transport owing to rotation is proportional to the number of vortical structures penetrating the boundary layer. We also compare indirect measures of thermal and Ekman boundary layer thicknesses to assess their potential role in controlling heat transport in different regimes of Ra and Ta. PMID:19905219
Numerical Simulations of Heat ExplosionWith Convection In Porous Media
Allali, Karam; Bikany, Fouad; Taik, Ahmed; Volpert, Vitaly
2013-01-01
In this paper we study the interaction between natural convection and heat explosion in porous media. The model consists of the heat equation with a nonlinear source term describing heat production due to an exothermic chemical reaction coupled with the Darcy law. Stationary and oscillating convection regimes and oscillating heat explosion are observed. The models with quasi-stationary and unstationary Darcy equation are compared.
Transient natural convection heat and mass transfer in crystal growth
Han, Samuel S.
1988-01-01
A numerical analysis of transient combined heat and mass transfer across a rectangular cavity is performed by a numerical method based on the SIMPLE algorithm. The physical parameters are selected to represent a range of possible crystal growth in solutions. Numerical results are compared with available experimental data to confirm the accuracy of the results. Good qualitative agreements are obtained for the average mass transfer rate across the cavity. Also, qualitative agreements are observed for the global development of thermal and solute fields. It is found that the thermal and solute fields become highly oscillatory when the thermal and solute Grashof numbers are large. Oscillations are probably caused by a number of different instability mechanisms. By reducing the gravity some of these instabilities were made to disappear at the lower Grashof numbers. Transient temperature and solute distribution near the crystal growing surface are highly non-uniform at the higher Grashof numbers. These non-uniformities are less severe in the reduced gravity environments but still exist. The effects of convection on the rate of average mass transfer are more than one order of magnitude higher than those of conduction in the range of Grashof numbers studied. Dependency of mass transfer rate on the Grashof number indicates that the convection effects many not be negligible even in the microgravity environments for the range of parameters investigated.
Lietzke, A F
1955-01-01
Results are presented of a theoretical and experimental investigation of heat transfer involving laminar natural convection of fluids enclosed between parallel walls oriented in the direction of the body force, where one wall is heated uniformly, and the other is cooled uniformly. For the experimental work, parallel walls were simulated by using an annulus with an inner-to-outer diameter ratio near 1. The results of the theoretical investigation are presented in the form of equations for the velocity and temperature profiles and the ratio of actual temperature drop across the fluid to the temperature drop for pure conduction. No experimental measurements were made of the velocity and temperature profiles, but the experimental results are compared with theory on the basis of the ratio of the actual temperature drop to the temperature drop for pure conduction. Good agreement was obtained between theory and experiment for axial temperature gradients of 10 degrees F. per foot or larger.
Heat transfer enhancement in a convective field by applying ionic wind
International Nuclear Information System (INIS)
This paper reports that this study has been conducted to pursue the heat transfer enhancement in a convective field by applying electric field. Firstly, aimed at thinning boundary layer, swirl motions were caused by utilizing the ionic wind in a channel flow with parallel wire-electrode arrangement. Secondly, ionic wind was induced at right angle to the primary flow at regular intervals by using cross wire-electrode arrangement. Thirdly, to utilize the dynamical effect of adding particles under the Coulomb force, electric field was applied to gas-solid suspensions flow field. On the basis of these results, fundamental characteristics of the combined flow structure and the heat transfer in the EHD field were clarified, and the possibility of the practical application will be insighted
Negative Knudsen force on heated microbeams
Zhu, Taishan
2011-11-18
Knudsen force acting on a heated microbeam adjacent to a cold substrate in a rarefied gas is a mechanical force created by unbalanced thermal gradients. The measured force has its direction pointing towards the side with a lower thermal gradient and its magnitude vanishes in both continuum and free-molecule limits. In our previous study, negative Knudsen forces were discovered at the high Knudsen regime before diminishing in the free-molecule limit. Such a phenomenon was, however, neither observed in experiment [A. Passian et al., Phys. Rev. Lett. 90, 124503 (2003)], nor captured in the latest numerical study [J. Nabeth et al., Phys. Rev. E 83, 066306 (2011)]. In this paper, the existence of such a negative Knudsen force is further confirmed using both numerical simulation and theoretical analysis. The asymptotic order of the Knudsen force near the collisionless limit is analyzed and the analytical expression of its leading term is provided, from which approaches for the enhancement of negative Knudsen forces are proposed. The discovered phenomenon could find its applications in novel mechanisms for pressure sensing and actuation.
Direct numerical simulation of liquid sodium droplet combustion in forced convection air flow
International Nuclear Information System (INIS)
In case of sodium leakage in liquid metal fast breeder reactor, the liquid sodium comes out in droplet form from a pipe accompanied with ignition and combustion Combustion heat and reaction products might affect integrity of steel liners in piping rooms. A direct numerical simulation code, COMET, is developed to simulate the combustion of a liquid sodium droplet. The extended MAC method coupled with a higher-order upwind scheme is used to calculate reacting compressible flow. Multicomponent counter diffusion of chemical species, mass and energy transfer by sodium evaporation, and heat transfer by radiation and thermal conductivity are calculated coupling with the flow. Chemical reaction of sodium, oxygen and water vapor is calculated by using the equation-solving methods of equilibrium constants. Thermodynamic properties of the mixed gas are evaluated based on the molecular transport theories. By using COMET, the single droplet combustion of liquid sodium in forced convection air flow is numerically simulated. Spatial distributions such as combustion heat, temperature, pressure, and chemical species behaviors such as formation, decomposition and transport are analyzed and discussed. The change of the droplet diameter agrees closely with the d-square law that has been experimentally observed and theoretically derived. (author)
Fast Prediction Method for Steady-State Heat Convection
Wáng, Yì
2012-03-14
A reduced model by proper orthogonal decomposition (POD) and Galerkin projection methods for steady-state heat convection is established on a nonuniform grid. It was verified by thousands of examples that the results are in good agreement with the results obtained from the finite volume method. This model can also predict the cases where model parameters far exceed the sample scope. Moreover, the calculation time needed by the model is much shorter than that needed for the finite volume method. Thus, the nonuniform POD-Galerkin projection method exhibits high accuracy, good suitability, and fast computation. It has universal significance for accurate and fast prediction. Also, the methodology can be applied to more complex modeling in chemical engineering and technology, such as reaction and turbulence. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Optimal Heat Transport in Rayleigh-B\\'enard Convection
Sondak, David; Waleffe, Fabian
2015-01-01
Steady flows that optimize heat transport are obtained for two-dimensional Rayleigh-B\\'enard convection with no-slip horizontal walls for a variety of Prandtl numbers $Pr$ and Rayleigh number up to $Ra\\sim 10^9$. Power law scalings of $Nu\\sim Ra^{\\gamma}$ are observed with $\\gamma\\approx 0.31$, where the Nusselt number $Nu$ is a non-dimensional measure of the vertical heat transport. Any dependence of the scaling exponent on $Pr$ is found to be extremely weak. On the other hand, the presence of two local maxima of $Nu$ with different horizontal wavenumbers at the same $Ra$ leads to the emergence of two different flow structures as candidates for optimizing the heat transport. For $Pr \\lesssim 7$, optimal transport is achieved at the smaller maximal wavenumber. In these fluids, the optimal structure is a plume of warm rising fluid which spawns left/right horizontal arms near the top of the channel, leading to downdrafts adjacent to the central updraft. For $Pr > 7$ at high-enough Ra, the optimal structure is a...
Mixed convection heat transfer in rotating vertical elliptic ducts
Scientific Electronic Library Online (English)
Olumuyiwa A., Lasode.
2007-06-01
Full Text Available This paper presents an investigation into the solution of laminar mixed convective heat transfer in vertical elliptic ducts containing an upward flowing fluid rotating about a parallel axis. The coupled system of normalized conservation equations are solved using a power series expansion in ascendin [...] g powers of rotational Rayleigh Number, Ratau - a measure of the rate of heating and rotation as the perturbation parameter. The results show the influence of rotational Rayleigh number, Ratau and modified Reynolds number, Re m on the temperature and axial velocity fields. The effect of Prandtl number, Pr, in the range 1 to 5, and eccentricity, e on the peripheral local Nusselt number are also reported. The mean Nusselt number is observed to be highest at duct eccentricity, e=0 for a given Prandtl number. However, results indicate insensitivity of peripheral local Nusselt number to Prandtl number at eccentricity, e=0.866, which is an important result to a designer of rotating vertical heat exchanger. The effect of eccentricity on the friction coefficient is also presented. The parameter space for the overall validity of the results presented is Ratau Re mPr
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The present study investigates natural convection heat transfer from a heated cylinder cooled by a water slurry of Microencapsulated Phase Change Material (MCPCM). A normal paraffin hydrocarbon with carbon number of 18 and melting point of 27.9degC, is microencapsulated by Melamine resin into particles of which average diameter is 9.5 ?m and specific weight is same as water. The slurry of the MCPCM and water is put into a rectangular enclosure with a heated horizontal cylinder. The heat transfer coefficients of the cylinder were evaluated. Changing the concentrations of PCM and temperature difference between cylinder surface and working fluid. Addition of MCPCM into water, the heat transfer is enhanced significantly comparison with pure water in cases with phase change and is reduced slightly in cases without phase change. (author)
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The stochastic Swift-Hohenberg equation is studied as a model of Rayleigh-Benard convection in a simple fluid. The equation has been solved numerically in two spatial dimensions to obtain the convective heat current and the roll pattern when either a bulk stochastic forcing field or different models of thermal-diffusivity mismatch at the sidewalls of the convective cell are considered. The parameters that enter the equation have been chosen to match the ramping experiments on Rayleigh-Benard convection by Meyer, Ahlers, and Cannell [Phys. Rev. Lett. 59, 1577 (1987)]. For any combination of forcing mechanisms, we are able to find values of their various amplitudes that lead to excellent fits to the experimentally measured convective current. In the case of a bulk random forcing field, we find an amplitude of F1=5x10-5, compared to Fth=1.92x10-9, the value obtained from fluctuation theory. A random, cellular pattern of rolls is observed, in agreement with experiments involving a gel sidewall designed to eliminate the influence of the sidewalls on the onset of convection. A thermal-conductivity mismatch at the sidewall has also been modeled by a variety of forcing fields. In all cases a roll-like pattern that reflects the geometry of the sidewalls is observed. Different combinations of both types of forcing fields have also been studied and found to yield patterns intermediate between cellular and roll-like, while yielding a very reasonable fit to the convective heat current measured experimentally
Heat Transfer Convection in The Cooking of Apple Using a Solar Cooker Box-Type
Terres, H.; Chávez, S.; Lizardi, A.; López, R.; Vaca, M.; Flores, J.; Salazar, A.
2015-01-01
In this work, experimental results to determine the convection heat transfer coefficient in the cooking process of apple using a solar cooker box-type are presented. Experimental data of temperatures for water, surface and central point of the apple were used. To determine the convection coefficient, the apple was modelled as a sphere. The temperatures evolution was defined using thermocouples located at water, surface and central point in the vegetables. Using heat transfer convection equations in transitory state and the temperatures measured, the Biot number and the convection coefficient were determined.
Scientific Electronic Library Online (English)
V. C., Mariani; L. S., Coelho.
2007-09-01
Full Text Available A numerical study was conducted to investigate steady heat transfer and flow phenomena of natural convection of air in enclosures, with three aspect ratios (H/W = 1, 2, and 4), within which there is a local heat source on the bottom wall at three different positions, Wh. This heat source occupies 1% [...] of the total volume of the enclosure. The vertical walls in the enclosures are insulated and there is an opening on the right wall. The natural convection is influenced by the difference in temperature between the left and right walls, represented by a Rayleigh number (Ra e), and by local heat source, represented by a Rayleigh number (Ra i). Numerical simulations were performed for several values of the Rayleigh number ranging between 10³ and 10(6), while the intensity of the two effects - the difference in temperature on the vertical walls and the local heat source - was evaluated based on the Ra i/Ra e ratio in the range between 0 and 2500. The analysis proceeds by observing variations in the streamlines and isotherms with respect to the different Ra e, R ratios, aspect ratios, of the radius and positions of the local heat source. The average Nusselt numbers on the hot and cold walls are influenced by different values of the parameters R, Ra e, Wh, and H/W. Results show the presence of different flow patterns in the enclosures studied. Thus, the flow and heat transfer can be controlled by external heating, and local heat source.
Energy Technology Data Exchange (ETDEWEB)
Hayat, T. [Department of Mathematics, Quaid-i-Azam University 45320, Islamabad 44000 (Pakistan); Department of Mathematics, Faculty of Science, King Abdulaziz University, P. O. Box 80257, Jeddah 21589 (Saudi Arabia); Iqbal, Z., E-mail: zahidiqbal_qau@yahoo.com [Department of Mathematics, Quaid-i-Azam University 45320, Islamabad 44000 (Pakistan); Mustafa, M. [Research Centre for Modeling and Simulation, National University of Sciences and Technology, Sector H-12, Islamabad 44000 (Pakistan); Alsaedi, A. [Department of Mathematics, Faculty of Science, King Abdulaziz University, P. O. Box 80257, Jeddah 21589 (Saudi Arabia)
2012-11-15
Highlights: Black-Right-Pointing-Pointer Boundary layer flow of an upper-convected Maxwell (UCM) fluid over a moving surface. Black-Right-Pointing-Pointer Convective boundary conditions have been used. Black-Right-Pointing-Pointer Series solutions are obtained by homotopy analysis method (HAM). Black-Right-Pointing-Pointer Graphical results for various interesting parametric values. - Abstract: This study discusses the flow and heat transfer in an upper-convected Maxwell (UCM) fluid over a moving surface in the presence of a free stream velocity. The convective boundary conditions have been handled. Similarly transformations are invoked to convert the partial differential equations governing the steady flow of a Maxwell fluid into an ordinary differential system. This system is solved by a homotopic approach. The effects of influential parameters such as Deborah number ({beta}), Prandtl number (Pr), Eckert number (Ec), suction parameter (S) and ratio ({lambda}) have been thoroughly examined.
Coupled heat and mass transfer in a convective tunnel dryer
International Nuclear Information System (INIS)
The mechanism of drying in a convective tunnel dryer with air heated in solar collectors was approached first experimentally with a pilot laboratory unit, then numerically taking into account the coupled heat and mass transfers. In the present study, several experimental essays were conducted followed by the adoption of a simulation tool describing the opening conditions of the tunnel dryer and a behavioural model that can be of great interest in the design and the automation of such industrial units. Indeed, behavioural models of thermodynamic system are characterised by the interactions of a large number of complex phenomenon, which call for various types of energy. This dynamic feature requires a modeling approach, using physical phenomenon such as energy storage. energy transformation and energy dissipation as data. The pseudo-bond graph methodology was used in modelling the drying system. This methodology was very suitable for thermo fluid process. It accepts the use of elements that do not exist in the traditional bond graph methods. An explicit pseudo-bond graph model who describes the process of water evaporation under the tray is studies in this paper and the governing equations are determined using bond graph properties.(Author)
Natural Convection in Parabolic Enclosure Heated from Below
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Ahmed W. Mustafa
2011-06-01
Full Text Available The effects of vertical parabolic walls on natural convection in a parabolic enclosure have investigated numerically in this paper. The bottom wall is heated isothermally, while the other vertical parabolic walls are maintained at constant cold temperature and the top wall is well insulated. The flow and temperature fields are studied numerically for three values (C = 0.1, 0.5, 1.0 of the parabolic equation constant. The laminar flow field is analyzed numerically by solving the steady, two-dimensional incompressible Navier-Stokes and energy equations. The Cartesian velocity components and pressure on a collocated (non-staggered grid are used as dependent variables in the momentum equations, which discretized by finite volume method, body fitted coordinates are used to represent the complex parabolic wall geometry accurately, and grid generation technique based on elliptic partial differential equations is employed. SIMPLE algorithm is used to adjust the velocity field to satisfy the conservation of mass. The range of Rayleigh number is (103? Ra ?105 and Prandtl number is 0.7. The results show that the heat transfer rates decrease with increase the parabolic equation constant.
HEAT TRANSFER BY NATURAL CONVECTION IN TWO VERTICAL AND ONE HORIZONTAL PLATE – AN OVERVIEW
MAHENDRA P NIMKAR; S. V. Prayagi
2011-01-01
Natural Convection flow in a vertical channel with internal objects is encountered in several technological applications of particular interest of heat dissipation from electronic circuits, refrigerators, heat exchangers, nuclear reactors fuel elements, dry cooling towers, and home ventilation etc.This study deals with the study of natural convection in horizontal plate with vertical channels. The parameters varied during the experimentation are heat input, aspect ratio (the ratio of gap of h...
Effects of Control on the Onset of Marangoni-Benard Convection with Uniform Internal Heat Generation
Norfifah Bachok; Norihan Md Arifin; Fadzilah Md Ali
2008-01-01
The effect of control on the onset of Marangoni-Benard convection in a horizontal layer of fluid with internal heat generation heated from below and cooled from above is investigated. The resulting eigenvalue problem is solved exactly. The effects of control are studied by examining the critical Marangoni numbers and wave numbers. It is found that the onset of Marangoni-Benard convection with internal heat generation can be delayed through the use of control.
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Ziyaddin RECEBL?
2008-01-01
Full Text Available In some studies, the effect of magnetic field on heat convection has been investigated given that physical properties are constant regardless of temperature. The effect of magnetic field on heat convection and fluids whose physical properties change by temperature has been investigated in this study as physical properties of fluids change by the effect of temperature. Momentum, continuity and energy equations including electromagnetic force affecting the fluid were used in the solution. Temperatures at axial and radial directions and Nusselt numbers were calculated depending on magnetic field intensity and other physical properties of fluid by solving the equation system written in cylindrical coordinates system by means of one of the numerical methods which is finite difference method. According to results, velocity and temperature of the cooled fluid decreased following an increase in the intensity of magnetic field placed vertically to flow direction. As determined in the previous one, this study also indicated that the increase in Reynolds number increases Nusselt number, and increasing the effect of magnetic field decreases Nusselt number. The theoretical results of the present study are in conformity with the results of our previous one.
Makinde, O. D.; Chinyoka, T.
2010-12-01
This present study consists of a numerical investigation of transient heat transfer in channel flow of an electrically conducting variable viscosity Boussinesq fluid in the presence of a magnetic field and thermal radiation. The temperature dependent nature of viscosity is assumed to follow an exponentially model and the system exchanges heat with the ambient following Newton's law of cooling. The governing nonlinear equations of momentum and energy transport are solved numerically using a semi-implicit finite difference method. Solutions are presented in graphical form and given in terms of fluid velocity, fluid temperature, skin friction and heat transfer rate for various parametric values. Our results reveal that combined effect of thermal radiation, magnetic field, viscosity variation and convective cooling have significant impact in controlling the rate of heat transfer in the boundary layer region.
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Graphical abstract: Photograph of the experimental set-up. - Highlights: • Thermal performance of an indirect-mode solar dryer is investigated. • Mathematical models are obtained for thin layer drying of thymus and mint. • Both thymus and mint show the constant and falling rate drying periods. - Abstract: An indirect-mode forced convection solar dryer was designed and fabricated. The thermal performance of the solar dryer under Tanta (latitude, 30° 47? N and longitude, 31° E) prevailing weather conditions was experimentally investigated. The system consists of a double pass v-corrugated plate solar air heater connected to a drying chamber. A blower was used to force the heated air to the drying chamber. Drying experiments were performed for thymus (initial moisture content 95% on wet basis) and mint (initial moisture content 85% on wet basis) at an initial temperature of 29 °C. The final moisture contents for thymus and mint were reached after 34 and 5 h, respectively. Fourteen mathematical models of thin layer drying were tested to specify the suitable model for describing the drying behavior of the studied products. It was found that, Midilli and Kucuk model is convenient to describe the thin layer solar drying of mint. However, the Page and modified Page models were found to be the best among others for describing the drying curves of thymus
An experimental investigation of forced convection flat plate solar air heater with storage material
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Aissa Walid
2012-01-01
Full Text Available Solar air heater (SAH is a heating device that uses the heated air in the drying of agriculture products and many engineering applications. The purpose of the present work is to study a forced convection flat plate solar air heater with granite stone storage material bed under the climatic conditions of Egypt-Aswan. Experiments are performed at different air mass flow rates ; varying from 0.016 kg/s to 0.08 kg/s, for five hot summer days of July 2008. Hourly values of global solar radiation and some meteorological data (temperature, pressure, relative humidities, etc. for measuring days are obtained from the Egyptian Meteorological Authority, Aswan station. Inlet and outlet temperatures of air from a SAH have been recorded. In this work, attempt has been made to present the temperature distribution in non dimensional form that makes it useable for any region and not restricted to local conditions. The variation of solar radiation, air heater efficiency, Nusselt number and temperature distribution along the air heater are discussed. Comparisons between the calculated values of outlet air temperatures, average air temperatures and storage material temperatures and the corresponding measured values showed good agreement. Comparison between current work and those in previous investigations showed fair agreement.
Mall, Gita; Hubig, Michael; Beier, Gundolf; Büttner, Andreas; Eisenmenger, Wolfgang
2002-06-01
The temperature-based determination of the time since death in the early post-mortem (pm) period plays an important role in medico-legal practice. In contrast to the common opinion according to which convection and conduction are mainly responsible for post-mortem heat loss, a considerable part of energy is emitted by thermal radiation. The present paper concentrates on the heat loss due to radiation and natural convection. Since both heat transfer mechanisms depend on the temperature gradient between skin and environment, the skin temperature was measured in corpses of different constitution (lean, medium and obese) and its decrease fitted by a single-exponential model. Heat loss due to radiation was calculated according to the non-linearized form of the law of Stefan and Boltzmann, heat loss due to natural convection according to the semi-empirical thermodynamic laws; the shape of the body in supine position was approximated to a semi-cylinder of finite length. The power due to radiation ranged between 386kJ/h (lean) and 550kJ/h (obese), that due to natural convection between 307kJ/h (lean) and 429kJ/h (obese) initially. Cumulative energy loss amounted to 2167kJ (lean) and 4239kJ (obese) by radiation and 1485kJ (lean) and 2922kJ (obese) by natural convection up to 20h pm. The energy loss due to radiation plus natural convection initially exceeded the energy loss due the decrease of the energy content of the body (mass x heat capacity x temperature decrease). This surplus can be explained only by exothermal processes in the phase of intermediary life and directly provides lower bounds for supravital energy production. Cumulative supravital energy ranges between 1139kJ up to 5h pm in the lean and 2516kJ up to 10h pm in the obese corpses. The courses of supravital energies and powers are presented as functions of time. Under standard conditions like still air (no forced convection) and insulating ground (little conductive heat transfer), the lower bounds represent estimates for total supravital energy production. PMID:12935672
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Natural convective heat transfer from a wide heated vertical isothermal plate with adiabatic surfaces above and below the heated surface has been considered. There are a series of equally spaced vertical thin, flat adiabatic surfaces (termed 'slats') near the heated surface, these surfaces being, in general, inclined to the heated surface. The slats are pivoted about their center-point and thus as their angle is changed, the distance of the tip of the slat from the plate changes. The situation considered is an approximate model of a window with a vertical blind, the particular case where the window is hotter than the room air, i.e. where air-conditioning is being used, being considered. The flow has been assumed to be laminar and steady. Fluid properties have been assumed constant except for the density change with temperature that gives rise to the buoyancy forces, this being treated by means of the Biuniqueness type approximation. Although the flow is in general three-dimensional, the flow over each slat is assumed to be the same and attention can therefore be restricted to flow over a single slat by using repeating boundary conditions. The governing equations have been written in dimensionless form and the resulting dimensionless equations have been solved using a commercial finite-element package. The solution has the following parameters: (1) the Rayleigh number (2) the Prandtl number (3) the dimensionless distance of the slat center point (the pivot point) from the surface (4) the dimensionless slat size (5) the dimensionless slat spacing (6) the angle of inclination of the slats. Because of the application that motivated the study, results have only been obtained for a Prandtl number of 0.7. The effect of the other dimensionless variables on the mean dimensionless heat transfer rate from the heated surface has been examined. (author)
Cebeci, Tuncer
1989-01-01
This book is designed to accompany Physical and Computational Aspects of Convective Heat Transfer by T Cebeci and P Bradshaw and contains solutions to the exercises and computer programs for the numerical methods contained in that book Physical and Computational Aspects of Convective Heat Transfer begins with a thorough discussion of the physical aspects of convective heat transfer and presents in some detail the partial differential equations governing the transport of thermal energy in various types of flows The book is intended for senior undergraduate and graduate students of aeronautical, chemical, civil and mechanical engineering It can also serve as a reference for the practitioner
International Nuclear Information System (INIS)
It is observed that the temperature distribution in the boundary layer changes qualitatively and the heat transfer in the upper part of a plate intensifies substantially in comparison with a surface with a continuous heat flux. For the case of substantial heat flux density in the initial stage of formation of a free convective flow a two-dimensional vortex is found to appear
Experimental transient natural convection heat transfer from a vertical cylindrical tank
International Nuclear Information System (INIS)
In this paper heat transfer experimental data is presented and compared to general correlations proposed in the literature for transient laminar free convection from a vertical cylindrical tank. The experimental data has been obtained from heating and cooling experiments carried out with a cylindrical full-scale hot water storage tank working under real operating conditions. The experimental device and the data acquisition system are described. The calculation procedures established to obtain the experimental values of the heat transfer coefficients, as well as the data reduction process, are detailed. The local convection and radiation heat transfer coefficients are obtained from different heating power conditions for local Rayleigh numbers within the range of 1x105-3x108. The great quantity of available experimental data allows a detailed analysis with a reliable empirical base. The experimental local convection heat transfer coefficients are correlated and compared to correlations proposed in open literature for engineering calculations. - Highlights: ? Experimental data of transient local convection heat transfer coefficients from a cylindrical tank for heating and cooling processes is obtained. ? The transient behaviour of the convection coefficients is dependent on temperature difference evolutions between the surface and the air. ? The Nu.Ra-1/4 ratio decreases proportionally in (Ts-T?)-0.9. ? A new correlation based on the semi-infinite region theory for laminar transient free convection is proposed.
Yano, Ryosuke
2015-01-01
We discuss the thermal conduction and convection of thermally relativistic fluids between two parallel walls under the gravitational force, both theoretically and numerically. In the theoretical discussion, we assume that the Lorentz contraction is ignored and spacetime is flat. For understanding of the thermal conduction and convection of thermally relativistic fluids between two parallel walls under the gravitational force, we solve the relativistic Boltzmann equation using the direct simulation Monte Carlo method. Numerical results indicate that strongly nonequilibrium states are formed in vicinities of two walls, which do not allow us to discuss the transition of the thermal conduction to the thermal convection of thermally relativistic fluids under the gravitational force in the framework of the relativistic Navier-Stokes-Fourier equation, when the flow-field is under the transition regime between the rarefied and continuum regimes, whereas such strongly nonequilibrium states are not formed in vicinities...
Bieniasz, Bogumi?
2014-09-01
A forced convective mass transfer coefficient was electrochemically measured for a cylindrical bundle of transverse needle-fins ?1 × 10.9, applied as the rotor porous matrix of a rotary heat regenerator. The baffle inside the rotor was present. The technique based on the ferricyanide-ferrocyanide redox reaction controlled at the cathode, in the presence of a sodium hydroxide based electrolyte, was used in this experiment. A set of the six neighbouring fins, connected in parallel, was the cathode. The distribution of the mass transfer coefficient according to different static rotor angle position and the mean mass transfer Chilton-Colburn coefficient correlation j M = j M ( Re) for rotation numbers, Ro: 0, 0.8, 1.6 and 2.0 were stated in the mean Reynolds number, Re, range 180-985. The comparison was made between the convective heat fluxes of the pin-fins and the sheet rotor, for Ro = 0.
Simulation of Convective Heat-Transfer Coefficient in a Buried Exchanger
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Taoufik Mnasri
2008-01-01
Full Text Available This study presents analytical models allowing to study a forced convection laminar flow in non-established dynamic and thermic regimes. We treated a flow in a bitubular exchanger in permanent thermal contact with a semi-infinite medium, such as the ground. The wall temperature as well as the wall heat flux evolve in the course of time until a quasi-steady mode. The theoretical method is original because it uses Green's functions method to determine the analytical solutions of the heat propagation equation on the wall during the heating phase. These analytical solutions allow to identify the temperature distribution versus time. The complexity of the system geometry as well as the infinity of the medium surrounding the exchanger make the traditional methods of numerical resolution unable to solve the problem. We used, to solve it, the finite volume method coupled with the finite element method at the boundary. We studied the effect of Reynolds number, the fluid entry temperature and the transfer duration on the axial evolution of the heat transfer coefficient. We illustrated also the profile of the temperature field in the fluid medium.
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Sourtiji Ehsan
2012-01-01
Full Text Available A numerical study of natural convection heat transfer through an alumina-water nanofluid inside L-shaped cavities in the presence of an external magnetic field is performed. The study has been carried out for a wide range of important parameters such as Rayleigh number, Hartmann number, aspect ratio of the cavity and solid volume fraction of the nanofluid. The influence of the nanoparticle, buoyancy force and the magnetic field on the flow and temperature fields have been plotted and discussed. The results show that after a critical Rayleigh number depending on the aspect ratio, the heat transfer in the cavity rises abruptly due to some significant changes in flow field. It is also found that the heat transfer enhances in the presence of the nanoparticles and increases with solid volume fraction of the nanofluid. In addition, the performance of the nanofluid utilization is more effective at high Rayleigh numbers. The influence of the magnetic field has been also studied and deduced that it has a remarkable effect on the heat transfer and flow field in the cavity that as the Hartmann number increases the overall Nusselt number is significantly decreased specially at high Rayleigh numbers.
International Nuclear Information System (INIS)
Two-phase flow heat transfer has been exhaustively studied over recent years. However, in this field several questions remain unanswered. Heat transfer coefficient prediction related to nucleate and convective boiling have been studied using different approaches, numerical, analytical and experimental. In this work, an experimental analysis, data representation and heat transfer coefficient prediction on two-phase heat transfer on nucleate and convective boiling are presented. An empirical correlation is obtained based on genetic algorithms search engine over a dimensional analysis of the two-phase flow heat transfer problem. (author)
On the Asymptotic Approach to Thermosolutal Convection in Heated Slow Reactive Boundary Layer Flows
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Sandile S. Motsa
2008-10-01
Full Text Available The study sought to investigate thermosolutal convection and stability of two dimensional disturbances imposed on a heated boundary layer flow over a semi-infinite horizontal plate composed of a chemical species using a self-consistent asymptotic method. The chemical species reacts as it diffuses into the nearby fluid causing density stratification and inducing a buoyancy force. The existence of significant temperature gradients near the plate surface results in additional buoyancy and decrease in viscosity. We derive the linear neutral results by analyzing asymptotically the multideck structure of the perturbed flow in the limit of large Reynolds numbers. The study shows that for small Damkohler numbers, increasing buoyancy has a destabilizing effect on the upper branch Tollmien-Schlichting (TS instability waves. Similarly, increasing the Damkohler numbers (which corresponds to increasing the reaction rate has a destabilizing effect on the TS wave modes. However, for small Damkohler numbers, negative buoyancy stabilizes the boundary layer flow.
International Nuclear Information System (INIS)
Numerical analysis of natural convection inside a heat generated fluid was performed for four different spherical geometries that match the experimental vessels used by Asfia et al. [5-7]. The transient calculations were performed with the CFX 5.7 fluid dynamic software. The simulations show that the highest heat flux is just below the rim of the cavity and it can be 50 times higher than at the bottom. Based on the numerical results, the local values of heat transfer coefficient and the distributions of global Nusselt number were calculated. The present, three-dimensional simulation results were compared with the numerical results of Mayinger et al. [3] and Reineke et al. [4], and with the experimental data of Asfia et al. [5-7]. The agreement between the results that is well inside the experimental scatter verifies the selected modeling approach. (author)
International Nuclear Information System (INIS)
This report is the user's manual for the computer code CONDIF that has been developed for solving natural and forced convection problems. It describes the preparation of input data and the solution of a test problem
Weight and water loss in the neonate in natural and forced convection.
Thompson, M. H.; Stothers, J K; McLellan, N J
1984-01-01
We describe a simple method of determining weight loss and hence water loss of infants in incubators. Unlike previously reported methods, it does not interfere with the microenvironment surrounding the infant. Weight loss of 16 term and 32 preterm infants was measured in both forced and natural convection. No significant increase in water loss was observed in the term infants but in the preterm infants the mean loss in natural convection was 0.85 g/kg/hour compared with 1.26 g/kg/hour in forc...
Development of the finite analytic method for turbulent forced and free convection
Bernatz, Richard Allen
The finite analytic (FA) method is used to solve turbulent forced and free convective flows. The governing partial differential equations are the primitive variable, ensemble averaged Navier-Stokes and energy equations for two dimensional, incompressible, and constant property fluids. Turbulent characteristics of the flow are simulated using the k-epsilon - Eddy turbulence model. Solutions are found on non-staggered grids using the momentum weighted interpolation method (MWIM). The turbulent flow near solid walls is modeled using a wall function. The square cavity laminar flow problem is solved for Reynolds (Re) numbers of 10(exp 2) and 10(exp 3) to verify the accuracy of the solution procedure incorporating the FA method and MWIM. Model results for both the velocity and thermal fields are consistent with previous experimental and numerical results. Turbulent flow within the square cavity is solved for Re = 200,000, extending the MWIM to turbulent flow problems for the first time. Results for the mean velocity field, and the turbulent fluctuations of the mean velocity field, compare favorably with experimental results. The thermal field predictions for this problem represent new findings for the high Reynolds number square cavity flow. The turbulent, free convective sea breeze flow is solved for a 24 hour heating and cooling cycle using the FA method and MWIM. The model predicts a spatial structure and magnitude for the mean circulation that is in agreement with experimental studies. In terms of turbulent quantities, the model results compare favorably with experimental findings that show the magnitudes of mean velocities and their turbulent fluctuations to be of the same order. Also, the model predicts the rate of turbulent kinetic energy dissipation to be on the order of 100 sq cm/cu sec, which is consistent with experimental measurements. The dynamics of the advancing sea breeze front is correctly simulated by the k-epsilon - Eddy turbulence model as its rate of inland penetration quickens as the land surface begins to cool in the late afternoon.
Ramachandran, N.; Ludovisis, D.; Cha, S. S.
2006-01-01
Heat transfer of a two-layer fluid system has been of great importance in a variety of industrial applications. For example, the phenomena of immiscible fluids can be found in materials processing and heat exchangers. Typically in solidification from a melt, the convective motion is the dominant factor that affects the uniformity of material properties. In the layered flow, thermocapillary forces can come into an important play, which was first emphasized by a previous investigator in 1958. Under extraterrestrial environments without gravity, thermocapillary effects can be a more dominant factor, which alters material properties in processing. Control and optimization of heat transfer in an immiscible fluid system need complete understanding of the flow phenomena that can be induced by surface tension at a fluid interface. The present work is focused on understanding of the magnetic field effects on thermocapillary convection, in order to optimize material processing. That is, it involves the study of the complicated phenomena to alter the flow motion in crystal growth. In this effort, the Marangoni convection in a cavity with differentially heated sidewalls is investigated with and without the influence of a magnetic field. As a first step, numerical analyses are performed, by thoroughly investigating influences of all pertinent physical parameters. Experiments are then conducted, with preliminary results, for comparison with the numerical analyses.
Accurate solution of the problem of natural convection in a heat releasing porous medium
International Nuclear Information System (INIS)
An analytical solution of the problem of hydrodynamics and heat transfer in a porous body with alternating heat release and natural convection is found. The obtained analytical formulae can be used for test verifications of numerical solutions of flow and heat transfer in complex real objects. Analysis of the solutons haspermitted to establish some complex criteria of similarity including the combination of source parameters
Janiga, M. A.; Li, X.; Hagos, S.; Feng, Z.; Wang, S.; Rowe, A.; Tao, W. K.; Zhang, C.
2014-12-01
This study compares simulations of convection during the AMIE/DYNAMO field campaign performed using three doubly-periodic cloud-resolving models (CRMs) and one regional CRM. A variety of microphysics parameterizations are used in these simulations. The target of these simulations is the second MJO event of the campaign, including suppressed periods before and after the passage of the convective envelope. The properties of convection in the CRM simulations are compared to observations of reflectivity and hydrometeor type from the dual-polarimetric SPOL radar. Contrasts in the properties of convection between the various simulations are related to their effect on the heat and moisture budgets.
Pierre, Charles; Bouyssier, Julien; De Gournay, Frédéric; Plouraboué, Franck
2014-01-01
We propose and develop a variational formulation dedicated to the simulation of parallel convective heat exchangers that handles possibly complex input/output conditions as well as connection between pipes. It is based on a spectral method that allows to re-cast three-dimensional heat exchangers into a two-dimensional eigenvalue problem, named the generalized Graetz problem. Our formulation handles either convective, adiabatic, or prescribed temperature at the entrance or at the exit of the e...
Predicting Turbulent Convective Heat Transfer in Fully Developed Duct Flows
Rokni, Masoud; Gatski, Thomas B.
2001-01-01
The performance of an explicit algebraic stress model (EASM) is assessed in predicting the turbulent flow and forced heat transfer in both straight and wavy ducts, with rectangular, trapezoidal and triangular cross-sections, under fully developed conditions. A comparison of secondary flow patterns. including velocity vectors and velocity and temperature contours, are shown in order to study the effect of waviness on flow dynamics, and comparisons between the hydraulic parameters. Fanning friction factor and Nusselt number, are also presented. In all cases. isothermal conditions are imposed on the duct walls, and the turbulent heat fluxes are modeled using gradient-diffusion type models. The formulation is valid for Reynolds numbers up to 10(exp 5) and this minimizes the need for wall functions that have been used with mixed success in previous studies of complex duct flows. In addition, the present formulation imposes minimal demand on the number of grid points without any convergence or stability problems. Criteria in terms of heat transfer and friction factor needed to choose the optimal wavy duct cross-section for industrial applications among the ones considered are discussed.
International Nuclear Information System (INIS)
This paper presents results of numerical investigation of turbulent natural convection in an internally-heated oxidic pool, and in a metallic layer heated from below and cooled from top and sidewalls. Emphasis is placed upon applicability of the existing heat transfer correlations (obtained from simulant-material experiments) in assessments of a prototypic severe reactor accident. The objectives of this study are (i) to improve the current understanding of the physics of unstably stratified flows, and (ii) to reduce uncertainties associated with modeling and assessment of natural convection heat transfer in the above configuration. Prediction capabilities of different turbulence modeling approaches are first examined and discussed, based on extensive results of numerical investigations performed by present authors. Findings from numerical modeling of turbulent natural convection flow and heat transfer in melt pools and metallic layers are then described. (authors)
Multi-crystalline silicon solidification under controlled forced convection
Cablea, M.; Zaidat, K.; Gagnoud, A.; Nouri, A.; Chichignoud, G.; Delannoy, Y.
2015-05-01
Multi-crystalline silicon wafers have a lower production cost compared to mono-crystalline wafers. This comes at the price of reduced quality in terms of electrical properties and as a result the solar cells made from such materials have a reduced efficiency. The presence of different impurities in the bulk material plays an important role during the solidification process. The impurities are related to different defects (dislocations, grain boundaries) encountered in multi-crystalline wafers. Applying an alternative magnetic field during the solidification process has various benefits. Impurities concentration in the final ingot could be reduced, especially metallic species, due to a convective term added in the liquid that reduces the concentration of impurities in the solute boundary layer. Another aspect is the solidification interface shape that is influenced by the electromagnetic stirring. A vertical Bridgman type furnace was used in order to study the solidification process of Si under the influence of a travelling magnetic field able to induce a convective flow in the liquid. The furnace was equipped with a Bitter type three-phase electromagnet that provides the required magnetic field. A numerical model of the furnace was developed in ANSYS Fluent commercial software. This paper presents experimental and numerical results of this approach, where interface markings were performed.
Efficiency of Heat Transfer in Turbulent Rayleigh-Benard Convection.
Czech Academy of Sciences Publication Activity Database
Urban, Pavel; Musilová, V?ra; Skrbek, L.
2011-01-01
Ro?. 107, ?. 1 (2011), 014302:1-4. ISSN 0031-9007 R&D Projects: GA AV ?R KJB200650902 Institutional research plan: CEZ:AV0Z20650511 Keywords : natural convection * thermal convection Subject RIV: BK - Fluid Dynamics Impact factor: 7.370, year: 2011
Fang, Pingping
1998-12-01
An extended numerical investigation of fully developed, forced convective laminar flows with heat transfer in eccentric annuli has been carried out. Both Newtonian and non-Newtonian (power-law or Ostwald-de Waele) fluids are studied, representing typical applications in petrochemical, bio-chemical, personal care products, polymer/plastic extrusion and food industries. For the heat transfer problem, with an insulated outer surface, two types of thermal boundary conditions have been considered: Constant wall temperature (T), and uniform axial heat flux with constant peripheral temperature (H1) on the inner surface of the annulus. The governing differential equations for momentum and energy conservation are solved by finite-difference methods. Velocity and temperature distributions in the flow cross section, the wall shear-stress distribution, and isothermal f Re, Nu i,T and Nu i,H1 values for different eccentric annuli (0/le?/*/le0.6,/ 0.2/le r/sp/*/le0.8) are presented. In Newtonian flows, the eccentricity is found to have a very strong influence on the flow and temperature fields. In an annulus with relatively large inner cylinder eccentricity, the flow tends to stagnate in the narrow section and has higher peak velocities in the wide section of the annulus. There is considerable flow maldistribution in the azimuthal direction, which in turn produces greater nonuniformity in the temperature field and a consequent degradation in the average heat transfer. Also, the H1 wall condition sustains higher heat transfer coefficients relative to the T boundary condition on the inner surface. For viscous, power-law type non-Newtonian flows, both shear thinning (n1) fluids are considered. Here, the non-linear shear behavior of the fluid is found to further aggravate the flow and temperature maldistribution, and once again the eccentricity is seen to exhibit a very strong influence on the friction and heat transfer behavior. Finally, the hydrodynamic characteristics of fully developed axial laminar flow of Newtonian fluids in eccentric annuli with a rotating inner cylinder are investigated. These are of significant importance to the design and operation of oil and gas drilling wells. Using finite-difference method to solve the governing flow equations in bipolar coordinates, computational results for a wide range of annulus geometry (0/le r/sp/*/le1,/ 0/le/varepsilon/sp/*/le0.8), and rotational Reynolds number (0/le Rer/le150) are presented, where the rotational speeds are restricted to the sub-critical Taylor number regime. The results delineate the effects of annuli r/sp/* and ?sp/*, and inner cylinder rotation speed on the flow structure and frictional losses.
Preliminary Numerical Analysis of Convective Heat Transfer Loop Using MARS Code
Energy Technology Data Exchange (ETDEWEB)
Lee, Yongjae; Seo, Gwang Hyeok; Jeun, Gyoodong; Kim, Sung Joong [Hanyang Univ., Seoul (Korea, Republic of)
2014-05-15
The MARS has been developed adopting two major modules: RELAP5/MOD3 (USA) for one-dimensional (1D) two-fluid model for two-phase flows and COBRA-TF code for a three-dimensional (3D), two-fluid, and three-field model. In addition to the MARS code, TRACE (USA) is a modernized thermal-hydraulics code designed to consolidate and extend the capabilities of NRC's 3 legacy safety code: TRAC-P, TRAC-B and RELAP. CATHARE (French) is also thermal-hydraulic system analysis code for Pressurized Water Reactor (PWR) safety. There are several researches on comparing experimental data with simulation results by the MARS code. Kang et al. conducted natural convection heat transfer experiments of liquid gallium loop, and the experimental data were compared to MARS simulations. Bang et al. examined the capability of the MARS code to predict condensation heat transfer experiments with a vertical tube containing a non-condensable gas. Moreover, Lee et al. adopted MELCOR, which is one of the severe accident analysis codes, to evaluate several strategies for the severe accident mitigation. The objective of this study is to conduct the preliminary numerical analysis for the experimental loop at HYU using the MARS code, especially in order to provide relevant information on upcoming experiments for the undergraduate students. In this study, the preliminary numerical analysis for the convective heat transfer loop was carried out using the MARS Code. The major findings from the numerical simulations can be summarized as follows. In the calculations of the outlet and surface temperatures, the several limitations were suggested for the upcoming single-phase flow experiments. The comparison work for the HTCs shows validity for the prepared input model. This input could give useful information on the experiments. Furthermore, the undergraduate students in department of nuclear engineering, who are going to be taken part in the experiments, could prepare the program with the input, and will be provided with expected results for the single-phase and forced convective phenomena. For the future study, different materials for the heating part are considered, such as other metals or silicon carbide (SiC) tube, which is a candidate material of fuel claddings for current and next-generation reactors.
Minder, J. R.; Smith, R. B.; Nugent, A. D.; Kirshbaum, D. J.
2011-12-01
Shallow convection is a pervasive feature of orographic precipitation, but its detailed role remains poorly understood. The mountainous Caribbean island of Dominica is a natural laboratory for isolating the role of shallow convection in orographic rainfall. It lies in a region of persistent easterly trade wind flow, and receives much of its rainfall from shallow convection that is forced mechanically by ascent of easterly flow over the Dominican terrain. The Dominica Experiment (DOMEX) has focused on convective orographic precipitation over the island from 2007-2011. The first phase of the project developed a climatology of rainfall and theories to explain the observed enhancement over the terrain. The second phase of the project (Apr-May 2011) has provided a detailed view of 20 individual rainfall events with data from: surface gauges, time-lapse photography, operational radar scans, a mountaintop weather station, and both in situ and remote observations from the University of Wyoming King Air research aircraft. Focusing on ascent--forced convection during DOMEX has revealed a number of the key processes that control the rainfall. Upwind of the island, clouds and water vapor anomalies exist that appear to play a crucial role in seeding the convection over the terrain and determining its vigor. Over the windward slopes the air is readily lifted with little flow deflection. Strong convective cells rapidly develop to produce large rainfall rates. Over the lee slopes of the terrain there is an abrupt transition to a deep and turbulent plunging flow that quickly eliminates convective clouds, but allows for the spillover of rainfall. The air that passes over the island is transformed such that low-levels are dried, warmed and decelerated, and the downwind wake becomes less hospitable to trade wind cumuli.
Influence of Pr on Natural Convection Heat Transfer of an Open Channel Finned Plate
International Nuclear Information System (INIS)
The finned plate provides the extended heat transfer area and improves the heat transfer. However when the fin spacing becomes small, the pressure drop increases due to frictional loss, heat transfer is impaired. Thus there is an optimal fin spacing. For the natural convection heat transfer, the heated thermal boundary layer drives the flow and the influence of the Prandtl number on the heat transfer will be very important as the thickness of the thermal boundary layer depends on it. This study aims at investigating the influence of the Prandtl number on the natural convection heat transfer of the finned plate. Numerical analyses were performed by varying the Pr from 2 to 2,014. Numerical analysis was performed for the natural convection heat transfer of a finned plate in an open channel. In order to investigate the influence of the Prandtl number on the heat transfer, four different values of Prandtl numbers were simulated and compared. As expected, the velocity profiles were almost similar except for the fact that the boundary layer develops earlier for smaller Prandtl number fluid. However the temperature profiles varied drastically depending on the values of the Prandtl number. As the Prandtl number increases, the thermal boundary layer reduces. The comparisons of the results with Le Fevre natural convection heat transfer correlation for vertical plate shows that as the Pr increases, the NuL of the finned plate becomes similar to that of the flat plate of the same heat transfer area
Jiao, Anjun; Zhang, Yuwen; Ma, Hongbin; Critser, John
2009-01-01
Heat and mass transfer in a circular tube subject to the boundary condition of the third kind is investigated. The closed form of temperature and concentration distributions, the local Nusselt number based on the total external heat transfer and convective heat transfer inside the tube, as well as the Sherwood number were obtained. The effects of Lewis number and Biot number on heat and mass transfer were investigated.
Directory of Open Access Journals (Sweden)
J. Zueco
2011-01-01
Full Text Available The steady, laminar axisymmetric convective heat and mass transfer in boundary layer flow over a vertical thin cylindrical configuration in the presence of significant surface heat and mass flux is studied theoretically and numerically. The governing boundary-layer equations for momentum, energy and species conservation are transformed from a set of partial differential equations in a (x,r coordinate system to a ( system using a group of similarity transformations. The resulting equations are solved using the Network Simulation Method (NSM for the buoyancy-assisted pure free convection and also the pure forced convection cases, wherein the effects of Schmidt number, Prandtl number and surface mass parameter on velocity, temperature and concentration distributions in the regime are presented graphically and discussed. For the buoyancy-assisted pure free convection case, nondimensional velocity (f/ is found to increase with a rise in surface mass transfer (S but decrease with increasing Prandtl number (Pr, particularly in the vicinity of the cylinder surface (small radial coordinate, . Dimensionless temperature ( decreases however with increasing S values from the cylinder surface into the free stream; with increasing Prandtl number, temperature is strongly reduced, with the most significant decrease at the cylinder surface. Dimensionless concentration ( is decreased continuously throughout the boundary layer regime with an increase in S; conversely is enhanced for all radial coordinate values with an increase in Prandtl number. For the pure forced convection case, velocity increases both with dimensionless axial coordinate ( and dimensionless radial coordinate ( but decays smoothly with increasing Prandtl number and increasing Schmidt number, from the cylinder surface to the edge of the boundary layer domain. The model finds applications in industrial metallurgical processes, thermal energy systems, polymer processing, etc.
A reassessment of the heat transport by variable viscosity convection with plates and lids
International Nuclear Information System (INIS)
The heat transport by a viscous fluid with temperature dependent viscosity has been studied numerically. As opposed to previous models, the top surface of the fluid clearly defines a tectonic plate with horizontally uniform velocity and subduction. Past studies failed to incorporate plates, the heat transport is as efficient as Rayleigh-Benard convection with constant viscosity; there is a strong buffering between internal temperature and heat loss. Past studies of parameterized convection which incorporated parameters indicative of strong buffering between internal temperature and total heat output still provide the most physically plausible representation of the Earth's thermal evolution
Transition to finger convection in double-diffusive convection
Kellner, M
2014-01-01
Finger convection is observed experimentally in an electrodeposition cell in which a destabilizing gradient of copper ions is maintained against a stabilizing temperature gradient. This double-diffusive system shows finger convection even if the total density stratification is unstable. Finger convection is replaced by an ordinary convection roll if convection is fast enough to prevent sufficient heat diffusion between neighboring fingers, or if the thermal buoyancy force is less than 1/30 of the compositional buoyancy force. At the transition, the ion transport is larger than without an opposing temperature gradient.
International Nuclear Information System (INIS)
As a result of the uncertainties in the understanding of the influence of thermal-buoyancy effects on the flow and heat transfer in Liquid Metal Fast Breeder Reactor heat exchangers and steam generators under off-normal operating conditions, an extensive experimental program is being conducted at Argonne National Laboratory to eliminate these uncertainties. Concurrently, a parallel analytical effort is also being pursued to develop a three-dimensional transient computer code (COMMIX-IHX) to study and predict heat exchanger performance under mixed, forced, and free convection conditions. This paper presents computational results from a heat exchanger simulation and compares them with the results from a test case exhibiting strong thermal buoyancy effects. Favorable agreement between experiment and code prediction is obtained
International Nuclear Information System (INIS)
The model laws for the initial film boiling at forced convection are realized in vertical tubes. The local conditions in the investigated area were regarded to be most effective and sufficient for the description. The theory was confirmed by experimental data. (orig.)
International Nuclear Information System (INIS)
External cooling of a light water reactor vessel by flooding of the concrete cavity with subcooled water is one of several management strategies currently being considered for accidents in which significant relocation of core material is predicted to occur. In the present work, experiments were conducted to examine natural convection heat transfer in internally heated partially filled spherical pools with external cooling. In the experiments, Freon-113 was contained in a Pyrex bell jar, which was cooled externally with subcooled water. The pool was heated using a 750 W magnetron taken from a conventional microwave. The pool had a nearly adiabatic free surface. The vessel wall temperature was not uniform and varied from the stagnation point to the free surface. A series of chromel-alumel thermocouples was used to measure temperatures in both steady-state and transient conditions. Each thermocouple was placed in a specific vertical and radial location in order to determine the temperature distribution throughout the pool and along the inner and outer walls of the vessel. In the experiment, pool depth and radius were varied parametrically. Both local and average heat transfer coefficients based on maximum pool temperature were obtained. Rayleigh numbers based on pool height were varied from 2 x 1010 to 1.1 x 1014. Correlations for the local heat transfer coefficient dependence on pool angle and for the dependence of average Nusselt number on Rayleigh number and pool depth have been developed. 11 refs., 7 figs., 2 tabs
Convective heat transfer from a heated elliptic cylinder at uniform wall temperature
Energy Technology Data Exchange (ETDEWEB)
Kaprawi, S.; Santoso, Dyos [Mechanical Department of Sriwijaya University, Jl. Raya Palembang-Prabumulih Km. 32 Inderalaya 50062 Ogan Ilir (Indonesia)
2013-07-01
This study is carried out to analyse the convective heat transfer from a circular and an elliptic cylinders to air. Both circular and elliptic cylinders have the same cross section. The aspect ratio of cylinders range 0-1 are studied. The implicit scheme of the finite difference is applied to obtain the discretized equations of hydrodynamic and thermal problem. The Choleski method is used to solve the discretized hydrodynamic equation and the iteration method is applied to solve the discretized thermal equation. The circular cylinder has the aspect ratio equal to unity while the elliptical cylinder has the aspect ratio less than unity by reducing the minor axis and increasing the major axis to obtain the same cross section as circular cylinder. The results of the calculations show that the skin friction change significantly, but in contrast with the elliptical cylinders have greater convection heat transfer than that of circular cylinder. Some results of calculations are compared to the analytical solutions given by the previous authors.
The effect of Coriolis force on nonlinear convection in a porous medium
Directory of Open Access Journals (Sweden)
D. H. Riahi
1994-09-01
Full Text Available Nonlinear convection in a porous medium and rotating about vertical axis is studied in this paper. An upper bound to the heat flux is calculated by the method initiated first by Howard [6] for the case of infinite Prandtl number.
Kumar, Varun; Kumar, Manoj; Shakher, Chandra
2014-09-20
In this paper, the local convective heat transfer coefficient (h) is measured along the surface of an electrically heated vertical wire using digital holographic interferometry (DHI). Experiments are conducted on wires of different diameters. The experimentally measured values are within the range as given in the literature. DHI is expected to provide a more accurate local convective heat transfer coefficient (h) as the value of the temperature gradient required for the calculation of "h" can be obtained more accurately than by other existing optical interferometric techniques without the use of a phase shifting technique. This is because in digital holography phase measurement accuracy is expected to be higher. PMID:25322139
The role of a convective surface in models of the radiative heat transfer in nanofluids
International Nuclear Information System (INIS)
Highlights: • The role of a convective surface in modelling with nanofluids is investigated over a wedge. • Surface convection significantly controls the rate of heat transfer in nanofluid. • Increased volume fraction of nanoparticles to the base-fluid may not always increase the rate of heat transfer. • Effect of nanoparticles solid volume fraction depends on the types of constitutive materials. • Higher heat transfer in nanofluids is found in a moving wedge rather than in a static wedge. - Abstract: Nanotechnology becomes the core of the 21st century. Nanofluids are important class of fluids which help advancing nanotechnology in various ways. Convection in nanofluids plays a key role in enhancing the rate of heat transfer either for heating or cooling nanodevices. In this paper, we investigate theoretically the role of a convective surface on the heat transfer characteristics of water-based nanofluids over a static or moving wedge in the presence of thermal radiation. Three different types of nanoparticles, namely copper Cu, alumina Al2O3 and titanium dioxide TiO2 are considered in preparation of nanofluids. The governing nonlinear partial differential equations are made dimensionless with the similarity transformations. Numerical simulations are carried out through the very robust computer algebra software MAPLE 13 to investigate the effects of various pertinent parameters on the flow field. The obtained results presented graphically as well as in tabular form and discussed from physical and engineering points of view. The results show that the rate of heat transfer in a nanofluid in the presence of thermal radiation significantly depends on the surface convection parameter. If the hot fluid side surface convection resistance is lower than the cold fluid side surface convection resistance, then increased volume fraction of the nanoparticles to the base fluid may reduces the heat transfer rate rather than increases from the surface of the wedge to the nanofluid. This finding is new and has not been reported in any open literature
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Oluwole Daniel Makinde
2011-01-01
Full Text Available Steady laminar natural convection flow over a semi-infinite moving vertical plate in the presence of internal heat generation and a convective surface boundary condition is examined in this paper. It is assumed that the left surface of the plate is in contact with a hot fluid while the cold fluid on the right surface of the plate contains a heat source that decays exponentially with the classical similarity variable. The governing non-linear partial differential equations have been transformed by a similarity transformation into a system of ordinary differential equations, which are solved numerically by applying shooting iteration technique together with fourth order Runge-Kutta integration scheme. The effects of physical parameters on the dimensionless velocity and temperature profiles are depicted graphically and analyzed in detail. Finally, numerical values of physical quantities, such as the local skin-friction coefficient and the local Nusselt number are presented in tabular form.
Extinction theorem, dispersion forces, and latent heat
International Nuclear Information System (INIS)
We show that multiple dipole scattering of zero-point radiation gives rise to the nonadditive contributions to the van der Waals dispersion interaction, and use the Ewald-Oseen extinction theorem to justify the Lifshitz theory of macroscopic (Casimir) effects of intermolecular van der Waals forces in the limit of continuous media. We derive the interaction energy between an atom and a dielectric medium of identical atoms and obtain, using standard ''cavity-QED'' methods, results identical to those of the source theory of Schwinger, DeRaad, and Milton [Ann. Phys. (N.Y.) 115, 1 (1978)]. Following Schwinger, DeRaad, and Milton, we apply the results to estimate the latent heat of liquid helium. We argue that the macroscopic theory does not unambiguously justify the conclusion that the dispersion interaction makes a significant contribution to the latent heat and surface tension. Simpler and well-established microscopic models, without the divergences incurred in the macroscopic theory, are invoked to support this conclusion
Open Channel Natural Convection Heat Transfer on a Vertical Finned Plate
International Nuclear Information System (INIS)
The natural convection heat transfer of vertical plate fin was investigated experimentally. Heat transfer systems were replaced by mass-transfer systems, based on the analogy concept. The experimental results lie within the predictions of the existing heat transfer correlations of plate-fin for the natural convections. An overlapped thermal boundary layers caused increasing heat transfer, and an overlapped momentum boundary layers caused decreasing heat transfer. As the fin height increases, heat transfer was enhanced due to increased inflow from the open side of the fin spacing. When fin spacing and fin height are large, heat transfer was unaffected by the fin spacing and fin height. Passive cooling by natural convection becomes more and more important for the nuclear systems as the station black out really happened at the Fukushima NPPs. In the RCCS (Reactor Cavity Cooling System) of a VHTR (Very High Temperature Reactor), natural convection cooling through duct system is adopted. In response to the stack failure event, extra cooling capacity adopting the fin array has to be investigated. The finned plate increases the surface area and the heat transfer increases. However, the plate of fin arrays may increase the pressure drop and the heat transfer decreases. Therefore, in order to enhance the passive cooling with fin arrays, the parameters for the fin arrays should be optimized. According to Welling and Wooldridge, a natural convection on vertical plate fin is function of Gr, Pr, L, t, S, and H. The present work investigated the natural convection heat transfer of a vertical finned plate with varying the fin height and the fin spacing. In order achieve high Rayleigh numbers, an electroplating system was employed and the mass transfer rates were measured using a copper sulfate electroplating system based on the analogy concept
Natural-convection heat transfer from horizontal circular cylinders in staggered arrangement to air
International Nuclear Information System (INIS)
Natural-convection heat transfer from uniformly heated horizontal cylinders to air was investigated experimentally for a bank of cylinders in staggered arrangement. The heat transfer coefficients for seven cylinders, arranged in a vertical array, were measured for varying cylinder spacings in the horizontal and vertical directions. A correlation equation for evaluating average heat transfer coefficients was deduced for banks of cylinders. It was shown that this equation was also applicable to cylinder banks in an in-line arrangement. (author)
International Nuclear Information System (INIS)
Convective heat transfer at exterior building surfaces has an impact on the design and performance of building components such as double-skin facades, solar collectors, solar chimneys and ventilated photovoltaic arrays, and also affects the thermal climate and cooling load in urban areas. In this study, an overview is given of existing correlations of the exterior convective heat transfer coefficient (CHTC) with the wind speed, indicating significant differences between these correlations. As an alternative to using existing correlations, the applicability of CFD to obtain forced CHTC correlations is evaluated, by considering a cubic building in an atmospheric boundary layer. Steady Reynolds-averaged Navier-Stokes simulations are performed and, instead of the commonly used wall functions, low-Reynolds number modelling (LRNM) is used to model the boundary-layer region for reasons of improved accuracy. The flow field is found to become quasi independent of the Reynolds number at Reynolds numbers of about 105. This allows limiting the wind speed at which the CHTC is evaluated and thus the grid resolution in the near-wall region, which significantly reduces the computational expense. The distribution of the power-law CHTC-U10 correlation over the windward and leeward surfaces is presented (U10 = reference wind speed at 10 m height). It is shown that these correlations can be accurately determined by simulations with relatively low wind speed values, which avoids the use of excessively fine grids for LRNM, and by using only two or three discrete wind speed values, which limits the required number of CFD simulations.
Lopez, Jose M; Avila, Marc
2015-01-01
The flow of fluid confined between a heated rotating cylinder and a cooled stationary cylinder is a canonical experiment for the study of heat transfer in engineering. The theoretical treatment of this system is greatly simplified if the cylinders are assumed to be of infinite length or periodic in the axial direction, in which cases heat transfer occurs only through conduction as in a solid. We here investigate numerically heat transfer and the onset of turbulence in such flows by using both periodic and no-slip boundary conditions in the axial direction. We obtain a simple linear criterion that determines whether the infinite-cylinder assumption can be employed. The curvature of the cylinders enters this linear relationship through the slope and additive constant. For a given length-to-gap aspect ratio there is a critical Rayleigh number beyond which the laminar flow in the finite system is convective and so the behaviour is entirely different from the periodic case. The criterion does not depend on the Pra...
GEOFLOW: simulation of convection in a spherical shell under central force field
Directory of Open Access Journals (Sweden)
P. Beltrame
2006-01-01
Full Text Available Time-dependent dynamical simulations related to convective motion in a spherical gap under a central force field due to the dielectrophoretic effect are discussed. This work is part of the preparation of the GEOFLOW-experiment which is planned to run in a microgravity environment. The goal of this experiment is the simulation of large-scale convective motion in a geophysical or astrophysical framework. This problem is new because of, on the one hand, the nature of the force field (dielectrophoretic effect and, on another hand, the high degree of symmetries of the system, e.g. the top-bottom reflection. Thus, the validation of this simulation with well-known results is not possible. The questions concerning the influence of the dielectrophoretic force and the possibility to reproduce the theoretically expected motions in the astrophysical framework, are open. In the first part, we study the system in terrestrial conditions: the unidirectional Earth's force is superimposed on the central dielectrophoretic force field to compare with the laboratory experiments during the development of the equipment. In the second part, the GEOFLOW-experiment simulations in weightless conditions are compared with theoretical studies in the astrophysical framework's, in the first instance a fluid under a self-gravitating force field. We present complex time-dependent dynamics, where the dielectrophoretic force field causes significant differences in the flow compared to the case that does not involve this force field.
Scientific Electronic Library Online (English)
I. K., Adegun; F. L., Bello-Ochende.
2004-09-01
Full Text Available A numerical study of steady state laminar forced and free convective and radiative heat transfer in an inclined rotating rectangular duct with a centered circular tube is reported for an hydrodynamically fully developed flow. The two heat transfer mechanisms of convection and radiation are treated i [...] ndependently and simultaneously. The coupled equations of momentum and energy transports are solved using Gauss-Seidel iteration technique subject to given boundary constraints. A thermal boundary condition of uniform wall temperature in the flow direction is considered. A special discritization method is employed to solve the problem associated with near boundary grid points. Results for mean and total mean Nusselt numbers for various values of Reynolds number ,Re; Rayleigh number , Ra ; Geometric ratio ,r g ; Aspect ratio, rA ; Radiation-Conduction parameter , . ; Optical thickness, J ; Rotational Reynolds number ,Ro and Emissivity, epsilon ; are presented. For the range of parameters considered, results show that radiation and rotation enhance heat transfer. It is also indicated in the results that heat transfer from the surface of the circle exceeds that of the rectangle. Optimum heat transfer and fluid bulk temperature are attained when the duct is vertically positioned. The Parameter ranges of 0.2 # r g # 0.84, 0 # PeRa # 7.3 x 10(5) and r g rA # 1 demarcate the extent of the validity of the numerical solution.
International Nuclear Information System (INIS)
Coupled buoyancy and thermo-capillary convection lead to a convective motion of the interface liquid/gas, which changes drastically the heat and mass transfer across the liquid layer. Two experiments are considered, depending on the fluid: oil or mercury. The liquid is set in a cooled cylindrical vessel, and heated by a heat flux across the center of the free surface. The basic flow, in the case of oil, is a torus. When the heat parameter increases, a stationary flow looking like petals or rays appears when the aspect ratio length/depth is small, and like concentric rings in the case of large values of the aspect ratio. The lateral confinement selects the azimuthal length wave. In the case of petals-like flow, a sub-critical Hopf bifurcation is underlined. The turbulence is found to be 'weak', even for the largest values of the Marangoni number (Ma ? 1.3 * 105). In the case of mercury, the thermo-capillary effect is reduced to zero, due to impurities at the surface, which have special trajectories we describe and compare to a simpler experiment. The only buoyancy forces induces an un-stationary, weakly turbulent flow as soon as the heating power exceeds 4 W (? 4.5 * 103, calculated with h = 1 mm). The last part concerns the analysis of the effect on the flow of the boundary conditions, the geometry, the Prandtl number, the buoyancy force, with the help of the literature. Results concerning heat transfer, especially the exponent of the law Nusselt number vs. heating power, are compared with available data. (author)
Directory of Open Access Journals (Sweden)
G. Pomalégni
2014-10-01
Full Text Available We investigate the combined effects of rotation, magnetic field and helical force on the onset of stationary and oscillatory convection in a horizontal electrically conducting fluid layer heated from below with free-free boundary conditions. For this investigation the linear stability analysis studied in detail by Chandrasekhar (1961 is used. We obtain the condition for the formation of a single large-scale structure. In (Pomalégni et al., 2014 it was shown the existence of a critical value of the intensity of the helical force for which the apparition of two cells at marginal stability for the oscillatory convection is obtained. Then, we have shown here how the increasing of parameter Ta influences this critical value of the helical force intensity.
International Nuclear Information System (INIS)
In this Letter, the problem of forced convection over a horizontal flat plate is presented and the homotopy perturbation method (HPM) is employed to compute an approximation to the solution of the system of nonlinear differential equations governing on the problem. It has been attempted to show the capabilities and wide-range applications of the homotopy perturbation method in comparison with the previous ones in solving heat transfer problems. The obtained solutions, in comparison with the exact solutions admit a remarkable accuracy. A clear conclusion can be drawn from the numerical results that the HPM provides highly accurate numerical solutions for nonlinear differential equations
Natural convection as the way of heat removal from fast reactor core at cooldown regimes
International Nuclear Information System (INIS)
The problems of thermohydraulics in fast reactors at cooldown regimes at heat removal by natural convection are considered The results of experiments and calculations obtained in various countries in this area are presented. The special attention is given to heat removal through inter-assembly space in the core and also to problems of thermohydraulics in the upper plenum. (author)
Convective Heat Transfer Augmentation by Flexible fins in Laminar Channel Pulsating flow
Joshi, Rakshitha U; Bhardwaj, Rajneesh
2015-01-01
Fluid-structure interaction (FSI) of thin flexible fins coupled with convective heat transfer has applications in energy harvesting and in understanding functioning of several biological systems. We numerically investigate FSI of the thin flexible fins involving large-scale flow-induced deformation as a potential heat transfer enhancement technique. An in-house, strongly-coupled fluid-structure interaction (FSI) solver is employed in which flow and structure solvers are based on sharp-interface immersed boundary and finite element method, respectively. We consider twin flexible fins in a heated channel with laminar pulsating cross flow. The vortex ring past the fin sweep higher sources of vorticity generated on the channel walls out into the downstream - promoting the mixing of the fluid. The moving fin assists in convective mixing, augmenting convection in bulk and at the walls; and thereby reducing thermal boundary layer thickness and improving heat transfer at the channel walls. The thermal augmentation is...
International Nuclear Information System (INIS)
Thermal histories have been calculated for simple models of the earth which assume that heat is transported by convection throughout the interior. The application of independent constraints to these solutions limits the acceptable range of the ratio of present radiogenic heat production in the earth to the present surface heat flux. The models use an empirical relation between the rate of convective heat transport and the temperature difference across a convecting fluid. This is combined with an approximate proportionality between effective mantle viscosity and T/sup -n/, where T is temperature and it is argued that n is about 30 throughout the mantle. The large value of n causes T to be strongly buffered against changes in the earth's energy budget and shortens by an order of magnitude the response time of surface heat flux to changes in energy budget as compared to less temperature-dependent heat transport mechanisms. Nevertheless, response times with n=30 are still as long as 1 or 2 b.y. Assuming that the present heat flux is entirely primordial (i.e., nonradiogenic) in a convective model leads back to unrealistically high temperatures about 1.7 b.y. ago. Inclusion of exponentially decaying (i.e., radiogenic) heat sources moves the high temperatures further into the past and leads to a transition from 'hot' to 'cool' calculated thermal histories for the case when the present rate of heat production is near 50% of the present rate of heat loss. Requiring the calculat rate of heat loss. Requiring the calculated histories to satisfy minimal geological constraints limits the present heat production/heat loss ratio to between about 0.3 and 0.85. Plausible stronger constraints narrow this range to between 0.45 and 0.65. These results are compatible with estimated radiogentic heat production rates in some meteorites and terrestrial rocks, with a whole-earth K/U ratio of 1--2 x 104 giving optimal agreement
The acoustic radiation force on a heated (or cooled) rigid sphere - Theory
Lee, C. P.; Wang, T. G.
1984-01-01
A finite amplitude sound wave can exert a radiation force on an object due to second-order effect of the wave field. The radiation force on a rigid small sphere (i.e., in the long wavelength limit), which has a temperature different from that of the environment, is presently studied. This investigation assumes no thermally induced convection and is relevant to material processing in the absence of gravity. Both isotropic and nonisotropic temperature profiles are considered. In this calculation, the acoustic effect and heat transfer process are essentially decoupled because of the long wavelength limit. The heat transfer information required for determining the force is contained in the parameters, which are integrals over the temperature distribution.
El-Amin, Mohamed
2013-01-01
In this paper, the effects of thermal dispersion and variable viscosity on the non-Darcy free, mixed, and forced convection heat transfer along a vertical flat plate embedded in a fluid-saturated porous medium are investigated. Forchheimer extension is employed in the flow equation to express the non-Darcy model. The fluid viscosity varies as an inverse linear function of temperature. The coefficient of thermal diffusivity has been assumed to be the sum of the molecular diffusivity and the dynamic diffusivity due to mechanical dispersion. Similarity solutions of the governing equations, for an isothermally heated plate, are obtained. Effects of the physical parameters, which govern the problem, on the rate of heat transfer in terms of Nusselt number, the slip velocity, and the boundary layer thickness, for the two cases Darcy and non-Darcy, are shown on graphs or entered in tables. © 2013 by Begell House, Inc.
Specialists' meeting on evaluation of decay heat removal by natural convection
International Nuclear Information System (INIS)
Decay heat removal by natural convection (DHRNC) is essential to enhancing the safety of liquid metal fast reactors (LMFRs). Various design concepts related to DHRNC have been proposed and experimental and analytical studies have been carried out in a number of countries. The purpose of this Specialists' Meeting on 'Decay Heat Removal by Natural Convection' organized by the International Working Group on Fast Reactors IAEA, is to exchange information about the state of the art related to methodologies on evaluation of DHRNC features (experimental studies and code developments) and to discuss problems which need to be solved in order to evaluate DHRNC properly and reasonably. The following main topical areas were discussed by delegates: Overview; Experimental studies and code validation; Design study. Two main DHR systems for LMFR are under consideration: (i) direct reactor auxiliary cooling system (DRACS) with immersed DFIX in main vessel, intermediate sodium loop and sodium-air heat exchanger; and (ii) auxiliary cooling system which removes heat from the outside surface of the reactor vessel by natural convection of air (RVACS). The practicality and economic viability of the use of RVACS is possible up to a modular type reactor or a middle size reactor based on current technology. For the large monolithic plant concepts DRACS is preferable. The existing experimental results and the codes show encouraging results so that the decay heat removal by pure natural convection is feasible. Concerning the objective, 'passive safety', the DHR by pure natural convection is essential feature to enhance the reliability of DHR
Heat transfers in a double-skin roof ventilated by natural convection in summer time
Biwole, Pascal; Woloszyn, Monika; Pompeo, C.
2008-01-01
The double-skin roofs investigated in this paper are formed by adding a metallic screen on an existing sheet metal roof. The system enhances passive cooling of dwellings and can help diminishing power costs for air conditioning in summer or in tropical and arid countries. In this work, radiation, convection and conduction heat transfers are investigated. Depending on its surface properties, the screen reflects a large amount of oncoming solar radiation. Natural convection in...
Lagrangian temperature, velocity and local heat flux measurement in Rayleigh-Benard convection
Gasteuil, Yoann; Shew, Woodrow; Gibert, Mathieu; Chillà, Francesca; Castaing, Bernard; Pinton, Jean-François
2007-01-01
We have developed a small, neutrally buoyant, wireless temperature sensor. Using a camera for optical tracking, we obtain simultaneous measurements of position and temperature of the sensor as it is carried along by the flow in Rayleigh-B\\'enard convection, at $Ra \\sim 10^{10}$. We report on statistics of temperature, velocity, and heat transport in turbulent thermal convection. The motion of the sensor particle exhibits dynamics close to that of Lagrangian tracers in hydrod...
Beukema, K.J.
1980-01-01
Three different models of bulk-stored agricultural products with air flow through the bulk, predicting the temperature profiles or the velocity of natural convection, are developed. The temperature distribution in a cylindrical container with insulated walls and open top and bottom, filled with heat-generating model material is studied experimentally, used to calculate the velocity of natural convection and compared with model predictions. A two-dimensional two-phase model of temperature and ...
Large scale three-dimensional topology optimisation of heat sinks cooled by natural convection
Alexandersen, Joe; Sigmund, Ole; Aage, Niels
2015-01-01
This work presents the application of density-based topology optimisation to the design of three-dimensional heat sinks cooled by natural convection. The governing equations are the steady-state incompressible Navier-Stokes equations coupled to the thermal convection-diffusion equation through the Bousinessq approximation. The fully coupled non-linear multiphysics system is solved using stabilised trilinear equal-order finite elements in a parallel framework allowing for the...
STUDY OF THE TURBULENT NATURAL CONVECTION INSIDE A DIFFERENTIALLY HEATED SQUARE CAVITY
Tcheukam-Toko D; Tientcheu-Nsiéwé M; Mouangue R; Djanna-Koffi F; Kuitche A
2013-01-01
The turbulent natural convection inside a differentially heated square cavity has been studied numerically inthis paper. Special attention has been paid for the wall effect on the convection. The turbulent model has beenapplied a standard ?-? two equations model and the two-dimensional Reynolds Averaged Navier–Stokes(RANS) equations are discredited with the second order upwind scheme. The SIMPLE algorithm, which isdeveloped using control volumes, is adopted as the numerical procedure. Calcula...
Kimball, J. T.; Hermanson, J. C.; Allen, J. S.
2012-05-01
The stability, convective structure, and heat transfer characteristics of upward-facing, evaporating, thin liquid films were studied experimentally. Dichloromethane, chloroform, methanol, and acetone films with initial thicknesses of 2-5 mm were subjected to constant levels of superheating until film rupture occurred (typically at a thickness of around 50 ?m). The films resided on a temperature controlled, polished copper plate incorporated into a closed pressure chamber free of non-condensable gasses. The dynamic film thickness was measured at multiple points using a non-intrusive ultrasound ranging system. Instability wavelength and convective structure information was obtained using double-pass schlieren imaging. The sequence of the convective structures as the film thins due to evaporation is observed to be as follows: (1) large, highly variable cells, (2) concentric rings and spirals, and (3) apparent end of convection. The transition from large, variable cells to concentric rings and spirals occurs at a Rayleigh number of 4800 ± 960. The apparent end of convection occurs at a Rayleigh number of 1580 ± 180. At the cessation of convection, the Nusselt number is nearly unity, indicating that there is little heat transfer in the film due to convection. In films where the Rayleigh number is above this transitional value, the Nusselt number increases with increasing Rayleigh number. The current results suggest that the equilibrium condition at the evaporating surface suppresses surface temperature variation, effectively eliminating thermocapillary-driven instability.
Saito, Kanichi; Raghavan, Vasudevan; Gogos, George
2007-07-01
Transient laminar natural convection over a sphere which is subjected to a constant heat flux has been studied numerically for high Grashof numbers (105 ? Gr ? 109) and a wide range of Prandtl numbers ( Pr = 0.02, 0.7, 7, and 100). A plume with a mushroom-shaped cap forms above the sphere and drifts upward continuously with time. The size and the level of temperature of the transient cap and plume stem decrease with increasing Gr and Pr. Flow separation and an associated vortex may appear in the wake of the sphere depending on the magnitude of Gr and Pr. A recirculation vortex which appears and grows until “steady state” is attained was found only for the very high Grashof numbers (105 ? Gr ? 109) and the lowest Prandtl number considered ( Pr = 0.02). The appearance and subsequent disappearance of a vortex was observed for Gr = 109 and Pr = 0.7. Over the lower hemisphere, the thickness of both the hydrodynamic (?H) and the thermal (?T) boundary layers remain nearly constant and the sphere surface is nearly isothermal. The surface temperature presents a local maximum in the wake of the sphere whenever a vortex is established in the wake of the sphere. The surface pressure recovery in the wake of the sphere increases with decreasing Pr and with increasing Gr. For very small Pr, unlike forced convection, the ratio ?T/?H remains close to unity. The results are in good agreement with experimental data and in excellent agreement with numerical results available in the literature. A correlation has also been presented for the overall Nusselt number as a function of Gr and Pr.
Scaling criteria for modeling natural- and forced-convection loops
International Nuclear Information System (INIS)
Nuclear reactor safety regulations have required extensive thermal-hydraulic testing of simulated reactor systems and components. In view of the inherent difficulties associated with full-scale testing, scale models for prototype systems have been extensively used to predict the behavior of nuclear reactor systems during normal and abnormal operations as well as under accident conditions. Several studies have been performed to establish similarity relations between a prototype and scale model. It is the purpose of the present study to develop scaling criteria for a forced and natural circulation loop under single- and/or two-phase flow conditions, and to apply the criteria to obtain the preliminary conceptual design parameters for the B and W 2 x 4 loop system. The 2 x 4 loop scaled system contains representative components of all thermal-hydraulic systems considered important in performing tests to obtain data representative of the response of the prototype plant
International Nuclear Information System (INIS)
While shape memory alloys (SMAs) have many actuation benefits, their frequencies are commonly restricted by slow cooling times caused by limitations in convective heat transfer. To increase the cooling speed and at the same time reduce excess power consumption from overheating, it is critical to understand the heat transfer from SMA wires. This requires accurate surface temperature measurement under a fixed input power, which is difficult to obtain using traditional methods because of the nature of SMAs (thin wires, large strains, heat activation, ambient environment, etc). This paper introduces a non-invasive technique for calculating the convective coefficient for SMAs by employing the temperature-induced transformation strain of SMAs to estimate the surface temperature. This method was experimentally validated for measurement of the convective coefficient in air where infrared cameras can operate, and then used to indirectly measure the convective coefficient across a range of commonly utilized SMA wire diameters and ambient media where traditional methods are limited. Formulated empirical correlations to the collected data provide a mathematical relationship to calculate the convective coefficient in material models which serve as better estimates of convection, and may be used for optimization of SMA actuators for increased frequency performance while ensuring that power draw is minimized
Anomalous heat transport and condensation in convection of cryogenic helium.
Czech Academy of Sciences Publication Activity Database
Urban, Pavel; Schmoranzer, D.; Hanzelka, Pavel; Sreenivasan, K. R.; Skrbek, L.
2013-01-01
Ro?. 110, ?. 20 (2013), s. 8036-8039. ISSN 0027-8424 R&D Projects: GA ?R GPP203/12/P897 Institutional support: RVO:68081731 Keywords : two-phase convection * temperature inversion * condensation * rain formation Subject RIV: BK - Fluid Dynamics Impact factor: 9.809, year: 2013
A new glass interim storage with heat removal by natural convection
International Nuclear Information System (INIS)
In order to meet COGEMA's strong operating and safety requirements, SGN proposed simple innovative solutions for the design of a new high level vitrified waste interim storage facility. The design is based on the use of steel jackets placed around vertical storage wells in vaults where the heat is removed by natural convection. Heat transfer in the annular space created by the steel jackets is enhanced. Efficient heat removal allows for a compact and cost-effective design. (author)
Local Convective Boiling Heat Transfer And Pressure Drop Of Nanofluid In Narrow Rectangular Channels
Boudouh, Mounir; Gualous, Hasna Louahlia; De Labachelerie, Michel
2010-01-01
Abstract This paper reports an experimental study on convective boiling heat transfer of nanofluids and de-ionized water flowing in a multichannels. The test copper plate contains 50 parallel rectangular minichannels of hydraulic diameter 800 ?m. Experiments were performed to characterize the local heat transfer coefficients and surface temperature using copper-water nanofluids with very small nanoparticles concentration. Axial distribution of local heat transfer is estimated using...
Abreu, Bruno; Lamas, Bruno; Fonseca, A.; Martins, N.; Oliveira, M. S. A.
2014-01-01
This study describes an investigation on the convective heat transfer performance of aqueous suspensions of multiwalled carbon nanotubes. The results suggested an increase on heat transfer coefficient of 47 % for 0.5 % volume fraction. Moreover, the enhancement observed during thermal conductivity assessment, cannot fully explain the heat transfer intensification. This could be associated to the random movements among the particles through a fluid, caused by the impact of the base fluid molecules.
Campbell, A N
2015-06-24
When any exothermic reaction proceeds in an unstirred vessel, natural convection may develop. This flow can significantly alter the heat transfer from the reacting fluid to the environment and hence alter the balance between heat generation and heat loss, which determines whether or not the system will explode. Previous studies of the effects of natural convection on thermal explosion have considered reactors where the temperature of the wall of the reactor is held constant. This implies that there is infinitely fast heat transfer between the wall of the vessel and the surrounding environment. In reality, there will be heat transfer resistances associated with conduction through the wall of the reactor and from the wall to the environment. The existence of these additional heat transfer resistances may alter the rate of heat transfer from the hot region of the reactor to the environment and hence the stability of the reaction. This work presents an initial numerical study of thermal explosion in a spherical reactor under the influence of natural convection and external heat transfer, which neglects the effects of consumption of reactant. Simulations were performed to examine the changing behaviour of the system as the intensity of convection and the importance of external heat transfer were varied. It was shown that the temporal development of the maximum temperature in the reactor was qualitatively similar as the Rayleigh and Biot numbers were varied. Importantly, the maximum temperature in a stable system was shown to vary with Biot number. This has important consequences for the definitions used for thermal explosion in systems with significant reactant consumption. Additionally, regions of parameter space where explosions occurred were identified. It was shown that reducing the Biot number increases the likelihood of explosion and reduces the stabilising effect of natural convection. Finally, the results of the simulations were shown to compare favourably with analytical predictions in the classical limits of Semenov and Frank-Kamenetskii. PMID:26059913
Forced convection in a square cavity with inlet and outlet ports
Energy Technology Data Exchange (ETDEWEB)
Saeidi, S.M.; Khodadadi, J.M. [Mechanical Engineering Department, Auburn University, 201 Ross Hall Auburn, AL 36849-5341 (United States)
2006-06-15
A finite-volume-based computational study of steady laminar forced convection inside a square cavity with inlet and outlet ports is presented. Given a fixed position of the inlet port, the location of the outlet port is varied along the four walls of the cavity. The widths of the ports are equal to 5%, 15% and 25% of the side. By positioning the outlet ports at nine locations on the walls for Re=10, 40, 100 and 500 and Pr=5, a total of 108 cases were studied. For the shortest distance between the inlet and outlet ports along the top wall, a primary clockwise (CW) rotating vortex that covers about 75-88% of the cavity is observed. As the outlet port is lowered along the right wall, the CW primary vortex diminishes in strength, however a counter-clockwise (CCW) vortex that is present next to the top right corner grows in size. With the outlet port moving left along the bottom wall, the CW primary vortex is weakened further and the CCW vortex occupies nearly the right half of the cavity. The pressure drop varies drastically depending on Re and the position of the outlet port. If the outlet port is on the opposite or the same wall as the inlet, the pressure drop is smaller in comparison to a case where it is located on the adjacent walls. The maximum pressure drop occurs when the outlet port is on the left side of the bottom wall and the minimum is achieved where the outlet is on the middle of the right wall. Regions of high temperature gradient are consistently observed at the interface of the throughflow and next to the solid walls on both sides of the outlet port. Local Nusselt numbers are low at three corners when no outlet port is present in their vicinity, whereas intense heat transfer rate is observed on the two sides of the outlet port. Between these minima and maxima, the local Nusselt number can vary drastically depending on the flow and temperature fields. By placing the outlet port with one end at three corners, maximum overall Nusselt number of the cavity can be achieved. Minimum overall heat transfer of the cavity is achieved with the outlet port located at the middle of the walls. The case exhibiting maximum heat transfer and minimum pressure drop is observed when the outlet port is located at dimensionless wall coordinate (2+0.5W). (author)
Energy Technology Data Exchange (ETDEWEB)
Kissick, M.W.; Efthimion, P.C.; Mansfield, D.K.; Callen, J.D.; Bush, C.E.; Park, H.K.; Schivell, J.; Synakowski, E.J.; Taylor, G.
1993-08-01
Using perturbations in electron density and temperature induced by small Helium gas puffs in TFTR (Tokamak Fusion Test Reactor) the dominance of convective heat transport in the core (r/a < 0.4) of supershot plasmas has been demonstrated in a new way. The TRANSP transport code was used to calculate the time-dependent particle and heat fluxes. Perturbations in the calculated convective and total electron heat fluxes were compared. They demonstrate that the conductive component decreases moving into the supershot core, and the convective component dominates in the supershot core. These results suggest a different transport drive in the supershot core compared to that in the rest of the supershot plasma.
International Nuclear Information System (INIS)
Using perturbations in electron density and temperature induced by small Helium gas puffs in TFTR (Tokamak Fusion Test Reactor) the dominance of convective heat transport in the core (r/a < 0.4) of supershot plasmas has been demonstrated in a new way. The TRANSP transport code was used to calculate the time-dependent particle and heat fluxes. Perturbations in the calculated convective and total electron heat fluxes were compared. They demonstrate that the conductive component decreases moving into the supershot core, and the convective component dominates in the supershot core. These results suggest a different transport drive in the supershot core compared to that in the rest of the supershot plasma
International Nuclear Information System (INIS)
For the long operation of secondary passive cooling system, however, water level goes down by evaporation in succession at emergency cooling tank. At the end there would be no place to dissipate heat from condensation heat exchanger. Therefore, steam cooling heat exchanger is put on the top of emergency cooling tank to maintain appropriate water level by collecting evaporating steam. Steam cooling heat exchanger is installed inside an air chimney and evaporated steam is cooled down by air natural convection. In this study, thermal sizing of steam cooling heat exchanger under air natural convection was conducted by TSCON program for the design of experimental setup as shown in Fig. 2. Thermal sizing of steam cooling heat exchanger tube under air natural convection was conducted by TSCON program for the design of experimental setup. 25 - 1' tubes which has a length 1687 mm was determined as steam cooling heat exchanger at 2 kW heat load and 100 liter water pool in emergency cooling tank (experimental limit condition). The corresponding width of two tubes is 50 mm and has 5 by 5 tube array for heat exchanger
Heat transport in the geostrophic regime of rotating Rayleigh-B{\\'e}nard convection
Ecke, Robert E
2013-01-01
We report experimental measurements of heat transport in rotating Rayleigh-B{\\'e}nard convection in a cylindrical convection cell with aspect ratio $\\Gamma = 1/2$. The fluid was helium gas with Prandtl number Pr = 0.7. The range of control parameters was Rayleigh number $4 \\times 10^9 < {\\rm Ra} < 4 \\times 10^{11}$ and Ekman number $2 \\times 10^{-7} < {\\rm Ek} < 3 \\times 10^{-5}$(corresponding to Taylor number $4 \\times 10^9 < {\\rm Ta} < 1 \\times 10^{14}$ and convective Rossby number $0.07 < {\\rm Ro} < 5$). We determine the crossover from weakly rotating turbulent convection to rotation dominated geostrophic convection through experimental measurements of the normalized heat transport Nu. The heat transport for the rotating state in the geostrophic regime, normalized by the zero-rotation heat transport, is consistent with scaling of $({\\rm RaEk}^{-7/4})^\\beta$ with $\\beta \\approx 1$. A phase diagram is presented that encapsulates measurements on the potential geostrophic turbulence reg...
Evaluation of convective heat transfer coefficient of various crops in cyclone type dryer
International Nuclear Information System (INIS)
In this paper, an attempt was made to evaluate the convective heat transfer coefficient during drying of various crops and to investigate the influences of drying air velocity and temperature on the convective heat transfer coefficient. Drying was conducted in a convective cyclone type dryer at drying air temperatures of 60, 70 and 80 deg. C and velocities of 1 and 1.5 m/s using rectangle shaped potato and apple slices (12.5 x 12.5 x 25 mm) and cylindrical shaped pumpkin slices (35 x 5 mm). The temperature changes of the dried crops and the temperature of the drying air were measured during the drying process. It was found that the values of convective heat transfer coefficient varied from crop to crop with a range 30.21406 and 20.65470 W/m2 C for the crops studied, and it was observed that the convective heat transfer coefficient increased in large amounts with the increase of the drying air velocity but increased in small amounts with the rise of the drying air temperature
Evaluation of convective heat transfer coefficient of various crops in cyclone type dryer
Energy Technology Data Exchange (ETDEWEB)
Akpinar, E. Kavak [Mechanical Engineering Department, Firat University, 23279 Elazig (Turkey)]. E-mail: eakpinar@firat.edu.tr
2005-09-15
In this paper, an attempt was made to evaluate the convective heat transfer coefficient during drying of various crops and to investigate the influences of drying air velocity and temperature on the convective heat transfer coefficient. Drying was conducted in a convective cyclone type dryer at drying air temperatures of 60, 70 and 80 deg. C and velocities of 1 and 1.5 m/s using rectangle shaped potato and apple slices (12.5 x 12.5 x 25 mm) and cylindrical shaped pumpkin slices (35 x 5 mm). The temperature changes of the dried crops and the temperature of the drying air were measured during the drying process. It was found that the values of convective heat transfer coefficient varied from crop to crop with a range 30.21406 and 20.65470 W/m{sup 2} C for the crops studied, and it was observed that the convective heat transfer coefficient increased in large amounts with the increase of the drying air velocity but increased in small amounts with the rise of the drying air temperature.
DEFF Research Database (Denmark)
Le Dreau, Jerome; Heiselberg, Per
2013-01-01
Night-time ventilation is a promising approach to reduce the energy needed for cooling buildings without reducing thermal comfort. Nevertheless actual building simulation tools have showed their limits in predicting accurately the efficiency of night-time ventilation, mainly due to inappropriate models for convection. In a full-scale test room, the heat transfer was investigated during 12 h of discharge by night-time ventilation. A total of 34 experiments have been performed, with different ventilation types (mixing and displacement), air change rates, temperature differences between the inlet air and the room, and floor emissivities. This extensive experimental study enabled a detailed analysis of the convective and radiative flow at the different surfaces of the room. The experimentally derived convective heat transfer coefficients (CHTC) have been compared to existing correlations. For mixing ventilation, existing correlations did not predict accurately the convective heat transfer at the ceiling due to differences in the experimental conditions. But the use of local parameters of the air flow showed interesting results to obtain more adaptive CHTC correlations. For displacement ventilation, the convective heat transfer was well predicted by existing correlations. Nevertheless the change of floor emissivity influenced the CHTC at the surface of interest.
NUMERICAL SIMULATION OF LAMINAR FORCED CONVECTION AIR FLOW IN A RECTANGULAR VENTURI CHANNEL
IGO SERGE WENDSIDA; KOKOU N'WUITCHA; ISSAKA OUEDRAOGO; DIEUDONNE JOSEPH BATHIEBO; BELKACEM ZEGHMATI
2014-01-01
In this work, a laminar forced convection air flow in a rectangular vertical venturi has been numerically simulated. Mathematical transformation has been used to transform the irregular profile of the venturi wall into straight line. Transfers equations are solved using finite volume method, Gauss and Thomas algorithms. A computing algorithm has been generated for the problem simulation. Hydrodynamics and thermals effects are investigated in detail. Results are presented as velocity and strea...
Numerical simulation of turbulent forced convection in liquid metals
Vodret, S.; Vitale Di Maio, D.; Caruso, G.
2014-11-01
In the frame of the future generation of nuclear reactors, liquid metals are foreseen to be used as a primary coolant. Liquid metals are characterized by a very low Prandtl number due to their very high heat diffusivity. As such, they do not meet the so-called Reynolds analogy which assumes a complete similarity between the momentum and the thermal boundary layers via the use of the turbulent Prandtl number. Particularly, in the case of industrial fluid-dynamic calculations where a resolved computation near walls could be extremely time consuming and could need very large computational resources, the use of the classical wall function approach could lead to an inaccurate description of the temperature profile close to the wall. The first aim of the present study is to investigate the ability of a well- established commercial code (ANSYS FLUENT v.14) to deal with this issue, validating a suitable expression for the turbulent Prandtl number. Moreover, a thermal wall-function developed at Universite Catholique de Louvain has been implemented in FLUENT and validated, overcoming the limits of the solver to define it directly. Both the resolved and unresolved approaches have been carried out for a channel flow case and assessed against available direct numerical and large eddy simulations. A comparison between the numerically evaluated Nusselt number and the main correlations available in the literature has been also carried out. Finally, an application of the proposed methodology to a typical sub-channel case has been performed, comparing the results with literature correlations for tube banks.
Transient Convection Due to Imposed Heat Flux: Application to Liquid-Acquisition Devices
Duval, Walter M. B.; Chato, David J.; Doherty, Michael P.
2014-01-01
A model problem is considered that addresses the effect of heat load from an ambient laboratory environment on the temperature rise of liquid nitrogen inside an enclosure. This model has applications to liquid acquisition devices inside the cryogenic storage tanks used to transport vapor-free propellant to the main engine. We show that heat loads from Q = 0.001 to 10 W, with corresponding Rayleigh numbers from Ra = 109 to 1013, yield a range of unsteady convective states and temperature rise in the liquid. The results show that Q = 1 to 10 W (Ra = 1012 to 1013) yield temperature distributions along the enclosure height that are similar in trend to experimental measurements. Unsteady convection, which shows selfsimilarity in its planforms, is predicted for the range of heat-load conditions. The onset of convection occurs from a free-convection-dominated base flow that becomes unstable against convective instability generated at the bottom of the enclosure while the top of the enclosure is convectively stable. A number of modes are generated with small-scale thermals at the bottom of the enclosure in which the flow selforganizes into two symmetric modes prior to the onset of the propagation of the instability. These symmetric vertical modes transition to asymmetric modes that propagate as a traveling-wave-type motion of convective modes and are representative of the asymptotic convective state of the flow field. Intense vorticity production is created in the core of the flow field due to the fact that there is shear instability between the vertical and horizontal modes. For the higher Rayleigh numbers, 1012 to 1013, there is a transition from a stationary to a nonstationary response time signal of the flow and temperature fields with a mean value that increases with time over various time bands and regions of the enclosure.
Experimental study of Cu-water nanofluid forced convective flow inside a louvered channel
Khoshvaght-Aliabadi, M.; Hormozi, F.; Zamzamian, A.
2015-03-01
Heat transfer enhancement plays a very important role for energy saving in plate-fin heat exchangers. In the present study, the influences of simultaneous utilization of a louvered plate-fin channel and copper-base deionized water nanofluid on performance of these exchangers are experimentally explored. The effects of flow rate (2-5 l/min) and nanoparticles weight fraction (0-0.4 %) on heat transfer and pressure drop characteristics are determined. Experimental results indicate that the use of louvered channel instead of the plain one can improve the heat transfer performance. Likewise, addition of small amounts of copper nanoparticles to the base fluid augments the convective heat transfer coefficient remarkably. The maximum rise of 21.7 % in the convective heat transfer coefficient is observed for the 0.4 % wt nanofluid compared to the base fluid. Also, pumping power for the base fluid and nanofluids are calculated based on the measured pressure drop in the louvered channel. The average increase in pumping power is 11.8 % for the nanofluid with 0.4 % wt compared to the base fluid. Applied performance criterion shows a maximum performance index of 1.167 for the nanofluid with 0.1 % wt Finally, two correlations are proposed for Nusselt number and friction factor which fit the experimental data with in ±10 %.
Latent heating and mixing due to entrainment in tropical deep convection
McGee, Clayton J.
Recent studies have noted the role of latent heating above the freezing level in reconciling Riehl and Malkus' Hot Tower Hypothesis (HTH) with evidence of diluted tropical deep convective cores. This study evaluates recent modifications to the HTH through Lagrangian trajectory analysis of deep convective cores in an idealized, high-resolution cloud-resolving model (CRM) simulation. A line of tropical convective cells develops within a high-resolution nested grid whose boundary conditions are obtained from a large-domain CRM simulation approaching radiative-convective equilibrium (RCE). Microphysical impacts on latent heating and equivalent potential temperature are analyzed along trajectories ascending within convective regions of the high-resolution nested grid. Changes in equivalent potential temperature along backward trajectories are partitioned into contributions from latent heating due to ice processes and a residual term. This residual term is composed of radiation and mixing. Due to the small magnitude of radiative heating rates in the convective inflow regions and updrafts examined here, the residual term is treated as an approximate representation of mixing within these regions. The simulations demonstrate that mixing with dry air decreases equivalent potential temperature along ascending trajectories below the freezing level, while latent heating due to freezing and vapor deposition increase equivalent potential temperature above the freezing level. The latent heating contributions along trajectories from cloud nucleation, condensation, evaporation, freezing, deposition, and sublimation are also quantified. Finally, the source regions of trajectories reaching the upper troposphere are identified; it is found that two-thirds of backward trajectories with starting points within strong updrafts or downdrafts above 10 km have their origin at levels higher than 2 km AGL. The importance of both boundary layer and mid-level inflow in moist environments is underscored in this study.
Modeling on Oxygen Transfer in the Forced Convection Lead-bismuth Eutectic Flow
International Nuclear Information System (INIS)
Lead-bismuth eutectic (LBE) technology is being developed as a primary candidate for a nuclear coolant in accelerator-driven systems and advanced reactors. However, the corrosion of containment and structural materials remains a major issue. Properly controlling the low oxygen level in LBE to mitigate corrosion proves effective under certain conditions. To mix the oxygen uniformly and quickly, the forced convection is proposed to enhance the oxygen transport using cover gas scheme. A lattice Boltzmann simulation of LBE flow and oxygen transport in a simplified container was carried out to explore characteristics of the oxygen transport. The oxygen control technique with cover gas scheme is formulated. To find more efficient way to mix the oxygen, three different forced convection flow patterns on the oxygen transport are investigated. The simulation results show that the force convections induced by all the boundary conditions greatly enhance the oxygen transport in the liquid metal of our system. Both of transient oxygen distribution, oxygen diffusion time, and average Sherwood number at the interface are investigated and some useful information are also provided to calibrate low concentration level oxygen sensors. (authors)
Numerical Simulation of Convective Heat and Mass Transfer in a Two-Layer System
Myznikova, B. I.; Kazaryan, V. A.; Tarunin, E. L.; Wertgeim, I. I.
The results are presented of mathematical and computer modeling of natural convection in the “liquid-gas” two-layer system, filling a vertical cylinder surrounded by solid heat conductive tract. The model describes approximately the conjugate heat and mass transfer in the underground oil product storage, filled partially by a hydrocarbon liquid, with natural gas layer above the liquid surface. The geothermal gradient in a rock mass gives rise to the intensive convection in the liquid-gas system. The consideration is worked out for laminar flows, laminar-turbulent transitional regimes, and developed turbulent flows.
Combined convection heat transfer of liquid sodium in cross flow through horizontal tube banks
International Nuclear Information System (INIS)
The objective of the present study is to clarify the heat transfer characteristics of combined convection of liquid sodium flowing through horizontal tube banks in the direction of gravity. The inviscid flow model is applied to analysis, since liquid sodium has a low Prandtl number. A boundary-fitted coordinate transformation technique is adopted to the numerical analysis. It is found that the heat transfer characteristics of combined convection largely depend upon the change of the wall temperature of tubes in the flow direction. (author)
Scaling analysis: Equivalence of convective and radiative heating of levitated droplet
Saha, Abhishek; Basu, Saptarshi; Kumar, Ranganathan
2012-05-01
This letter develops theoretical relationships for equilibrium timescale and temperature scale of a vaporizing droplet in a convective and a radiative environment. The transient temperature normalized by the respective scales exhibits a unified profile for both modes of heating. The analysis allows for the prediction of the required laser flux to show its equivalence in a corresponding heated gas stream. The theoretical equivalence shows good agreement with experiments across a range of droplet sizes. Simple experiments can be conducted in a levitator to extrapolate information in realistic convective environments like combustion and spray drying.
Lagrangian temperature, velocity, and local heat flux measurement in Rayleigh-Bénard convection.
Gasteuil, Y; Shew, W L; Gibert, M; Chillá, F; Castaing, B; Pinton, J-F
2007-12-01
We have developed a small, neutrally buoyant, wireless temperature sensor. Using a camera for optical tracking, we obtain simultaneous measurements of position and temperature of the sensor as it is carried along by the flow in Rayleigh-Bénard convection, at Ra approximately 10;{10}. We report on statistics of temperature, velocity, and heat transport in turbulent thermal convection. The motion of the sensor particle exhibits dynamics close to that of Lagrangian tracers in hydrodynamic turbulence. We also quantify heat transport in plumes, revealing self-similarity and extreme variations from plume to plume. PMID:18233369
International Nuclear Information System (INIS)
Natural convection is a physical phenomenon that has been investigated in nuclear engineering so as to provide information about heat transfer in severe accident conditions involving nuclear reactors. This research reported transient natural convection of fluids with uniformly distributed volumetrically heat generation in square cavity with isothermal side walls and adiabatic top/bottom walls. Two Prandtl numbers were considered, 0:0321 and 0:71. Direct numerical simulations were applied in order to obtain results about the velocities of the fluid in directions x and y. These results were used in Fast Fourier Transform, which showed the periodic, quasi-chaotic and chaotic behavior of transient laminar flow. (author)
HEAT TRANSFER ENHANCEMENT IN PLATES BY NATURAL CONVECTION WITH AND WITHOUT VERTICAL CONFINING WALLS
Directory of Open Access Journals (Sweden)
Mahendra P Nimkar*
2015-01-01
Full Text Available Natural Convection flow in a vertical channel with internal objects is encountered in several technological applications of particular interest of heat dissipation from electronic circuits, refrigerators, heat exchangers, nuclear reactors fuel elements, dry cooling towers, and home ventilation etc.This study deals with the study of natural convection in horizontal plate with and without vertical confining walls. The parameters varied during the experimentation are heat input, aspect ratio (the ratio of gap of horizontal plate with respect to vertical plate and gap of horizontal plate from bottom to top. The present study aims to determine the heat transfer characteristics, along the plate for the selection of optimum dimension for design purpose. Further, the influences of aspect ratio on the performance characteristics of heat transfer will be studied.
Experimental and numerical investigation on natural convection heat transfer in nanofluids
International Nuclear Information System (INIS)
Currently, a lot of research is being carried out on the potential application of nanofluids as a coolant in nuclear reactors owing to their enhanced heat transfer characteristics as compared to base fluid. In this regards, an experimental study has been undertaken concerning natural convection heat transfer of nanofluids over a cylindrical heater with a constant wall heat flux condition. The heat flux was varied from 0-50000 W/m2 and Rayleigh number range is 30000 to 1.65 X 105. Results show that there was a reduction in natural convection heat transfer coefficient of nanofluids as compared to water. Experimental results were compared with existing models for similar geometry. However, the available correlation was found to be unable to predict experimental data. A new empirical model was developed based on the experimental data including the effect of nanoparticles concentration which predicts the experimental data satisfactorily. (author)
Energy Technology Data Exchange (ETDEWEB)
Yousefi, T. [Mechanical Engineering Department, Razi University, Kermanshah (Iran); Ashjaee, M. [Mechanical Engineering Department, University of Tehran, Tehran 11365-4563 (Iran)
2007-10-15
Laminar free convection heat transfer from vertical array of horizontal isothermal elliptic cylinders with vertical major axis has been experimentally investigated. Experiments were carried out using Mach-Zehnder interferometer with the cylinder spacing from two to five cylinder major axis and at Rayleigh numbers in the range between 10{sup 3} and 2.5 x 10{sup 3}. The effects of cylinder spacing and Rayleigh number on the heat transfer from the individual cylinder and the array were investigated. It is found that the free convection heat transfer from any individual cylinder in the array depends on the Rayleigh number, cylinders separation distance and cylinder position in the array. Heat transfer correlations have been developed for both the individual cylinder in the array and the array. Also, a heat transfer correlation has been proposed for a single elliptic cylinder with vertical major axis and is compared with earlier works. (author)
Azih, C.; Brinkerhoff, J. R.; Yaras, M. I.
2012-02-01
Experimental research has long shown that forced-convective heat transfer in wall-bounded turbulent flows of fluids in the supercritical thermodynamic state is not accurately predicted by correlations that have been developed for single-phase fluids in the subcritical thermodynamic state. In the present computational study, the statistical properties of turbulent flow as well as the development of coherent flow structures in a zero-pressure-gradient flat-plate boundary layer are investigated in the absence of body forces, where the working fluid is in the supercritical thermodynamic state. The simulated boundary layers are developed to a friction Reynolds number of 250 for two heat-flux to mass-flux ratios corresponding to cases where normal heat transfer and improved heat transfer are observed. In the case where improved heat transfer is observed, spanwise spacing of the near-wall coherent flow structures is reduced due to a relatively less stable flow environment resulting from the lower magnitudes of the wall-normal viscosity-gradient profile.
Directory of Open Access Journals (Sweden)
Moh'd A. Al-Nimr
2004-06-01
Full Text Available Magnetic field effect on local entropy generation due to steady two-dimensional laminar forced convection flow past a horizontal plate was numerically investigated. This study was focused on the entropy generation characteristics and its dependency on various dimensionless parameters. The effect of various dimensionless parameters, such as Hartmann number (Ha, Eckert number (Ec, Prandtl number (Pr, Joule heating parameter (R and the free stream temperature parameter (ÃŽÂ¸Ã¢ÂˆÂž on the entropy generation characteristics is analyzed. The dimensionless governing equations in Cartesian coordinate were solved by an implicit finite difference technique. The solutions were carried out for Ha2=0.5-3, Ec=0.01-0.05, Pr=1-5 and ÃŽÂ¸Ã¢ÂˆÂž=1.1-2.5. It was found that, the entropy generation increased with increasing Ha, Ec and R. While, increasing the free stream temperature parameter, and Prandtl number tend to decrease the local entropy generation.
Chakraborty, Swastika; Saha, Upal; Maitra, Animesh
2015-07-01
The response of atmospheric heat fluxes and sea surface temperatures on the convective precipitation over the tropics has been an important area of research in recent decades. A long-term observation (1979-2008) of the increase in convective precipitation in relation to the latent and sensible heat fluxes on a tropical location, Kolkata, has been investigated in the present study. Invigoration of convective precipitation has been caused by vertically integrated divergence of moisture flux, rise in sea surface temperatures, convective cloud cover and surface evaporation rate over the tropical region. A convective precipitation estimation (CPE) index is proposed, considering the Bowen ratio, surface evaporation rate, sea surface temperature and temperatures at 500 hpa pressure level during the pre-monsoon season (March-May), to estimate the amount of convective precipitation over the tropics using multiple linear regression technique is also another aim of this study. A good agreement is obtained between the results from the proposed model and the MERRA observations during the years 2009-2013.
S.KARUNAKAR REDDY; D Chenna Kesavaiah; M N Raja Shekar
2013-01-01
he effects of heat and mass transfer on MHD mixed convection flow of a vertical surface with radiation, heat source/absorption and chemical reaction has been is discussed. The resulting set of coupled non-linear ordinary differential equations is solved by perturbation technique and for graphs we used MATLAB software. Approximate solutions have been derived for the velocity, temperature, concentration profiles, skin friction and Nusselt number. The obtained res...
Vidyasagar, G.
2013-01-01
This paper deals with the combined effect of thermal diffusion and heat absorption on the MHD free convection heat and mass transfer flow of a viscous incompressible fluid past a continuously moving infinite plate. Closed form of solution for the velocity, temperature and concentration field are obtained and discussed graphically for various values of the physical parameters present. In addition, expressions for the skin friction is also derived and finally discussed with the help of table an...
International Nuclear Information System (INIS)
The main objectives of this thesis are the direct numerical simulation of natural convection in a vertical differentially heated slot and the improvements of second-order turbulence modelling. A three-dimensional direct numerical simulation code has been developed in order to gain a better understanding of turbulence properties in natural convection flows. This code has been validated in several physical configurations: non-stratified natural convection flows (conduction solution), stratified natural convection flows (double boundary layer solution), transitional and turbulent Poiseuille flows. For the conduction solution, the turbulent regime was reached at a Rayleigh number of 1*105 and 5.4*105. A detailed analysis of these results has revealed the principal qualities of the available models but has also pointed our their shortcomings. This data base has been used in order to improve the triple correlations transport models and to select the turbulent time scales suitable for such flows. (author). 122 refs., figs., tabs., 4 appends
Experimental study of cooling BIPV modules by forced convection in the air channel
International Nuclear Information System (INIS)
Highlights: • An experimental setup for studying the effects of forced convection on cell temperature. • The induced velocity within the forced convection channel significantly affects the PV cooling. • Correlations for the Ross coefficient, module temperature, efficiency, and power output. • Prediction of the thermal behavior of the PV module in BIPV configurations. - Abstract: The efficiency of photovoltaic systems depends mainly on the cell temperature. Frequently, the PV collectors are installed on the top of the building. One cost effective method to regulate the temperature of rooftop integrated photovoltaic panels is to provide an open air channel beneath the panel. The cell temperature of these PV modules is very much influenced by the capability of ventilating this channel. The ventilation may be modified by different factors such as the wind velocity, the air gap size, and the forced convection induced by a fan or by a conventional air conditioning system. This paper describes an experimental setup to study the influence of the air gap size and the forced ventilation on the cell temperature (and consequently on the electrical efficiency of the PV module) of a BIPV configuration, for different values of the incident solar radiation, ambient temperatures, and aspect ratios, as well as for several forced ventilation conditions. Semi empirical correlations for the Ross coefficient, module temperature, electrical efficiency, and power output are proposed, showing a good agreement with respect to experimental measurements. A critical channel aspect ratio close to 0.11 can be considered to minimize overheating of PV devices. For a duct velocity Vv = 6 m/s, a power output increase of 19% is observed over the natural ventilation case (Vv = 0.5 m/s)
The effect of natural and forced melt convection on dendritic solidification in Ga-In alloys
Shevchenko, N.; Roshchupkina, O.; Sokolova, O.; Eckert, S.
2015-05-01
The directional solidification of Ga-25 wt%In alloys within a Hele-Shaw cell was visualized by means of X-ray radioscopy. The experimental investigations are especially focused on the impact of melt convection on the dendritic growth. Natural convection occurs during a bottom up solidification because lighter solute is rejected at the solid-liquid interface leading to an unstable density stratification. Forced convection was produced by a rotating wheel with two parallel disks containing at their inner sides a set of permanent NdFeB magnets with alternating polarization. The direction of forced melt flow is almost horizontal at the solidification front whereas local flow velocities in the range between 0.1 and 1.0 mm/s were achieved by controlling the rotation speed of the magnetic wheel. Melt flow induces various effects on the grain morphology primarily caused by the convective transport of solute. Our observations show a facilitation of the growth of primary trunks or lateral branches, suppression of side branching, dendrite remelting and fragmentation. The manifestation of all phenomena depends on the dendrite orientation, local direction and intensity of the flow. The forced flow eliminates the solutal plumes and damps the local fluctuations of solute concentration. It provokes a preferential growth of the secondary arms at the upstream side of the primary dendrite arms, whereas the high solute concentration at the downstream side of the dendrites can inhibit the formation of secondary branches completely. Moreover, the flow changes the inclination angle of the dendrites and the angle between primary trunks and secondary arms.
Forced convection to laminar flow of liquid egg yolk in circular and annular ducts
Scientific Electronic Library Online (English)
M., Bernardi; V., Silveira Jr.; V. R. N., Telis; A. L., Gabas; J., Telis-Romero.
2009-06-01
Full Text Available The steady-state heat transfer in laminar flow of liquid egg yolk - an important pseudoplastic fluid food - in circular and concentric annular ducts was experimentally investigated. The average convection heat transfer coefficients, determined by measuring temperatures before and after heating secti [...] ons with constant temperatures at the tube wall, were used to obtain simple new empirical expressions to estimate the Nusselt numbers for fully established flows at the thermal entrance of the considered geometries. The comparisons with existing correlations for Newtonian and non-Newtonian fluids resulted in excellent agreement. The main contribution of this work is to supply practical and easily applicable correlations, which are, especially for the case of annulus, rather scarce and extensively required in the design of heat transfer operations dealing with similar shear-thinning products. In addition, the experimental results may support existing theoretical analyses.
DEFF Research Database (Denmark)
Bhattacharyya, S.; Singh, Ashok
2010-01-01
The influence of surface heating of a circular cylinder on the wake structure and heat transfer in the range of Reynolds number (Re) for which parallel vortex shedding occurs, is investigated numerically for different values of the buoyancy parameter, Gr. The role of buoyancy induced baroclinic vorticity on the wake formation is addressed in the present study. The variation of Strouhal number and Nusselt number with the 'effective Reynolds number', is analyzed for different values of cylinder to free stream temperature ratio. Both Strouhal number and the rate of heat transfer increases monotonically with the increase of the effective Reynolds number. The validity of the correlations, which have been established by several authors, between the effective Reynolds number and Strouhal/ Nusselt number for forced convection, is examined in the mixed convection regime. The curves between the effective Reynolds number and the computed data for Strouhal number and Nusselt number do not collapse for the range of temperature ratio considered here. The flow field is found to be asymmetric and the cylinder experiences a negative lift. The drag coefficient increases steadily with the rise of surface temperature. © 2010 Elsevier Ltd. All rights reserved.
Determining convective heat transfer coefficient using phoenics software package
Energy Technology Data Exchange (ETDEWEB)
Kostikov, A.; Matsevity, Y. [Institute of Mechanical Engineering Problems of National Academy of Sciences of Ukraine, Kharkov (Ukraine)
1997-12-31
The two methods of determination of such important quantity of heat exchange on a body surface using PHOENICS are suggested in the presentation. The first method consists in a post-processing of results of conjugate heat transfer problem solved by PHOENICS. The second one is solving an inverse heat conduction problem for solid body using PHOENICS. Comparative characteristic of these two methods is represented. (author) 4 refs.
Ben Shabat, Yael; Shitzer, Avraham
2012-07-01
Facial heat exchange convection coefficients were estimated from experimental data in cold and windy ambient conditions applicable to wind chill calculations. Measured facial temperature datasets, that were made available to this study, originated from 3 separate studies involving 18 male and 6 female subjects. Most of these data were for a -10°C ambient environment and wind speeds in the range of 0.2 to 6 m s(-1). Additional single experiments were for -5°C, 0°C and 10°C environments and wind speeds in the same range. Convection coefficients were estimated for all these conditions by means of a numerical facial heat exchange model, applying properties of biological tissues and a typical facial diameter of 0.18 m. Estimation was performed by adjusting the guessed convection coefficients in the computed facial temperatures, while comparing them to measured data, to obtain a satisfactory fit (r(2) > 0.98, in most cases). In one of the studies, heat flux meters were additionally used. Convection coefficients derived from these meters closely approached the estimated values for only the male subjects. They differed significantly, by about 50%, when compared to the estimated female subjects' data. Regression analysis was performed for just the -10°C ambient temperature, and the range of experimental wind speeds, due to the limited availability of data for other ambient temperatures. The regressed equation was assumed in the form of the equation underlying the "new" wind chill chart. Regressed convection coefficients, which closely duplicated the measured data, were consistently higher than those calculated by this equation, except for one single case. The estimated and currently used convection coefficients are shown to diverge exponentially from each other, as wind speed increases. This finding casts considerable doubts on the validity of the convection coefficients that are used in the computation of the "new" wind chill chart and their applicability to humans in cold and windy environments. PMID:21725871
International Nuclear Information System (INIS)
This report describes the governing equations and the finite element modelling used in the computer code CONDIF. The code has been designed for solving transient natural or forced convection problems in two dimensions. Applications are described to illustrate the code capabilities
Solar drying of whole mint plant under natural and forced convection.
Sallam, Y I; Aly, M H; Nassar, A F; Mohamed, E A
2015-03-01
Two identical prototype solar dryers (direct and indirect) having the same dimensions were used to dry whole mint. Both prototypes were operated under natural and forced convection modes. In the case of the later one the ambient air was entered the dryer with the velocity of 4.2 m s(-1). The effect of flow mode and the type of solar dryers on the drying kinetics of whole mint were investigated. Ten empirical models were used to fit the drying curves; nine of them represented well the solar drying behavior of mint. The results indicated that drying of mint under different operating conditions occurred in the falling rate period, where no constant rate period of drying was observed. Also, the obtained data revealed that the drying rate of mint under forced convection was higher than that of mint under natural convection, especially during first hours of drying (first day). The values of the effective diffusivity coefficient for the mint drying ranged between 1.2 × 10(-11) and 1.33 × 10(-11) m(2) s(-1). PMID:25750751
Heat Transfer of Forced Fluid Flow in a Channel with Parallel Fillisters
Hai-Ping Hu
2013-01-01
This study analyzes heat transfer and fluid dynamics in a forced convection laminar flow in a channel with parallel fillisters. The problem is solved by the point-matching method. The influence of the height and width of the fillisters on the thermal-fluid characteristics of a channel flow is discussed in the present research. The local dimensionless velocity, f Re values, local dimensionless temperature and mean Nusselt number of the fluid flow are all obtained for a channel flow under the i...
Heat pipe applications for future Air Force spacecraft
International Nuclear Information System (INIS)
This paper summarizes the envisioned, future usage of high and low temperature heat pipes in advanced Air Force spacecraft. Thermal control requirements for a variety of communications, surveillance, and space defense missions are forecast. Thermal design constraints implied by survivability to potential weapons effects are outlined. Applications of heat pipes to meet potential low and high power spacecraft mission requirements and envisioned design constraints are suggested. A brief summary of past Air Force sponsored heat pipe development efforts is presented and directions for future development outlined, including those applicable to advanced photovoltaic and nuclear power subsystem applications of heat pipes
International Nuclear Information System (INIS)
In this study, we aimed to realize a parametric determination of the convective heat transfer coefficient of a gasoline engine on both the in-cylinder and jacket sides. The combustion products have been determined as a function of excess air coefficient. Cylinder temperature and pressure have been calculated with a simplistic model based on the First Law of Thermodynamics. The in-cylinder heat transfer coefficient is evaluated by using two different expressions; one a specific form of Annand's equation and the other the Woschni equation. Newton's convective heat transfer equation has been utilized to obtain the heat transfer coefficient between the engine block and the cooling water. This process is based on some temperature measurements at various locations in the cooling space, as the distance between the tip of the temperature sensor and the inner cylinder surface is varied for each location. We determined values of both coefficients for various engine parameters
International Nuclear Information System (INIS)
Unsteady natural convection of a heat-generating fluid with phase transitions in the enclosures of a square section with isothermal rigid walls is investigated numerically for a wide range of dimensionless parameters. The quasisteady state solutions of conjugate heat and mass transfer problem are compared with available experimental results. Correlation relations for heat flux distributions at the domain boundaries depending on Rayleigh and Ostrogradskii numbers are obtained. It is shown that generally heat transfer is governed both by natural circulation and crust formation phenomena. Results of this paper may be used for analysis of experiments with prototypic core materials
Henkes, R. A. W. M.; Hoogendoorn, C. J.
1989-01-01
A numerical code for solving the boundary-layer equations is used to evaluate the performance of various turbulence models for the natural convection boundary layer along a heated vertical plate. The Cebeci-Smith (1974) model yields wall-heat transfer and turbulent viscosity values that are lower than the experimental values, while the standard k-epsilon model with wall functions for k and epsilon yields high wall-heat transfer values and resonable velocity and temperature profiles. Low-Reynolds-number k-epsilon models provide accurate wall-heat transfer results.
DEFF Research Database (Denmark)
Steskens, Paul Wilhelmus Maria Hermanus; Janssen, Hans
2009-01-01
Current models to predict heat, air and moisture (HAM) conditions in buildings assume constant boundary conditions for the temperature and relative humidity of the neighbouring air and for the surface heat and moisture transfer coefficients. These assumptions may introduce errors in the predicted HAM conditions. The paper focuses on the influence of the interior surface heat and moisture transfer coefficients, and investigates its effect on the hygrothermal performance. The parameter study showed that the magnitude of the convective surface transfer coefficients have a relatively large influence on the predicted hygrothermal conditions at the surface of a building component and on the heat and vapour exchange with the indoor environment.
Preliminary experiments on natural convection heat transfer of the vertical plate with fin arrays
International Nuclear Information System (INIS)
Passive cooling by natural convection becomes more and more important for the nuclear systems as the station black out really happened at the Fukushima NPPs. In the RCCS (Reactor Cavity Cooling System) of a VHTR (Very High Temperature Reactor), natural convection cooling through duct system is adopted. In response to the stack failure event, extra cooling capacity adopting the fin array has to be investigated. The finned plate increases the surface area and the heat transfer increases. However, the plate of fin arrays may increase the pressure drop and the heat transfer decreases. Therefore, in order to enhance the passive cooling with fin arrays, the parameters of the fin arrays should be optimized. According to Welling and Woodridge, a natural convection on vertical plate fin is function of Nu = f(Gr, Pr, L, t, S, H). Present study aimed at the determination of the effects of geometric parameters, L(fin length) and S(fin spacing), and H(fin height) on the heat transfer, in order to find optimum parameters on the natural convection heat transfer
Comments on heat trasfer efficiency in cryogenic helium turbulent Rayleigh-Bénard convection.
Czech Academy of Sciences Publication Activity Database
Urban, Pavel; Musilová, V?ra; Králík, Tomáš; Skrbek, L.
2011-01-01
Ro?. 318, ?. 8 (2011), s. 082012. ISSN 1742-6588 R&D Projects: GA AV ?R KJB200650902 Institutional research plan: CEZ:AV0Z20650511 Keywords : Rayleigh-Benard convection * heat transfer efficiency Subject RIV: BK - Fluid Dynamics
Mendez, Sergio; AungYong, Lisa
2014-01-01
To help students make the connection between the concepts of heat conduction and convection to real-world phenomenon, we developed a combined experimental and computational module that can be incorporated into lecture or lab courses. The experimental system we present requires materials and apparatus that are readily accessible, and the procedure…
Directory of Open Access Journals (Sweden)
Vohmin V. S.
2011-06-01
Full Text Available The influence of convective induction heating on methane producing bacteria vital functions at various temperature modes is studied. The intensive way of recycling of animal farms waste in a unified process at the same time with obtaining alternative energy is offered
Modeling the Effect of Internal Convection Currents on Heat Transfer Coefficient of Liquid Foods
Directory of Open Access Journals (Sweden)
SAJID ALI
2013-04-01
Full Text Available The internal convection currents generated during the cooling process affect convective heat transfer coefficient from the surface of the container, these convection currents may increase the effective value of the surface film conductance (h. Therefore, in such situation the Nu-Re correlations, which are generally used to predict h-values, may not yield realistic results. In the present work, this effect has been investigated by using the empirical correlation developed through Temperature-Time measurements at the centre of liquid food containers during cooling .The main concern of the present work is about considering the heat transfer behaviour for liquid foods for which a cylindrical shape container of brass metal have taken, in this work the transient Time-Temperature relation is utilized to calculate the value of convection heat transfer coefficient (h for each measured temperature at the centre of the cylinder (r¬¬o=0. Then after plotting the graph between ‘h’ and ‘T’ an expression between h and T is obtained, which is fed back in the programme developed with the help of finite difference method by which Time-Temperature variation is obtained. Experimental procedure was used to determine surface film conductance of cylindrical Apple and Orange juice container, calculated temperatures have been compared with the experimental results when the measured surface film conductance were used to solve the transient heat conduction equation in cylindrical coordinates. A consistently excellent agreement was observed.
Thermal performance of a porus radial fin with natural convection and radiative heat losses
Directory of Open Access Journals (Sweden)
Darvishi M.T.
2015-01-01
Full Text Available An analytic (series solution is developed to describe the thermal performance of a porous radial fin with natural convection in the fluid saturating the fin and radiation heat loss from the top and bottom surfaces of the fin. The HAM results for the temperature distribution and base heat flux are compared with the direct numerical results and found to be very accurate.
MHD unsteady free convective Walter’s memory flow with constant suction and heat sink
M.V Ramana Murthy; G. Noushima Humera; Rafiuddin; M. Chenna Krishan Reddy
2007-01-01
The study of unsteady hydromagnetic free convective memory flow of incompressible and electrically conducting fluids past an infinite vertical porous plate in the presence of constant suction and heat absorbing sinks have been made. Approximate solutions have been derived for the mean velocity, mean temperature, mean skin-friction and mean rate of heat transfer using multi-parameter perturbation technique. It is observed that magnetic field strength decreases the mean velocity of the fluid. A...
Transition to turbulence in strongly heated vertical natural convection boundary layers
De Larochelambert, Thierry
2003-01-01
The mechanisms governing the transition to turbulence in natural convection boundary layers along strongly heated vertical walls remain neither very clear nor well understood, because of the lack of experiments and the difficulties of physical modelling. Our measurements bring experimental data focusing on this transition in quiescent air along radiating and conducting plates in the whole range of 2000 to 8000 W/m\\^2 heating rate. The analysis of the time series obtained by ...
Single phase laminar convective heat transfer of nanofluids in a micro-tube
Lumbreras Basagoiti, Itziar
2011-01-01
Nanofluids are homogeneous mixture of dispersed solid particles in base fluids. These solid particles are usually smaller than 100nm. Suspended nanoparticles modify the properties of based fluids. It is claimed, in some literature, for nanofluids to have greater than expected heat transfer performance. Due to this, nanofluids have gained great attention from both research and development and industries active in cooling systems. This thesis reports several measurements of convective heat tran...
Convective Heat Transfer Enhancement in Nanofluids: Real Anomaly or Analysis Artifact?
Prabhat, Naveen; Buongiorno, Jacopo; Hu, Lin-wen
2012-01-01
The nanofluid literature contains many claims of anomalous convective heat transfer enhancement in both turbulent and laminar flow. To put such claims to the test, we have performed a critical detailed analysis of the database reported in 12 nanofluid papers (8 on laminar flow and 4 on turbulent flow). The methodology accounted for both modeling and experimental uncertainties in the following way. The heat transfer coefficient for any given data set was calculated according to the established...
CFD numerical simulation of air natural convection over a heated cylindrical surface
Flori, M.; Vîlceanu, L.
2015-06-01
In this study a CFD numerical simulation is used to describe the fluid flow and heat transfer in air surrounding a heated horizontal cylinder. The model is created in 2D space dimension involving a finite element solver of Navier-Stokes equations. As natural convection phenomenon is induced by a variable fluid density field with temperature rising, the Boussinesq approximation was coupled to the model.
A Magneto-convection Over a Semi -infinite Porous Plate with Heat Generation
T. Raja; Karthikeyan Shailendhra; B Senthilnathan
2013-01-01
Convective flow through porous media is a branch of research undergoing rapid growth in fluid mechanics and heat transfer. This is quite natural because of its important applications in environmental, geophysical and energy related engineering problems. Prominent applications are the utilization of geothermal energy, the control of pollutant spread in ground water, the design of nuclear reactors, solar power collectors and the heat transfer associated with the deep storage of nuclear waste. T...
Longwave Marangoni convection in a binary liquid layer heated from above: weakly nonlinear analysis
Energy Technology Data Exchange (ETDEWEB)
Shklyaev, Sergey [Institute of Continuous Media Mechanics, Ural Branch of the Russian Academy of Sciences, Perm 614013 (Russian Federation); Nepomnyashchy, Alexander A, E-mail: nepom@math.technion.ac.il, E-mail: shklyaev@yandex.ru [Department of Mathematics, Technion—Israel Institute of Technology, Haifa 32000 (Israel)
2014-08-01
Nonlinear regime of longwave surface-tension-driven convection in a layer of binary mixture heated from above is considered. Under the assumption of the small Biot number, which corresponds to the almost heat insulated free surface, we derive the nonlocal amplitude equation. Analysis of pattern selection demonstrates that hexagons emerge subcritically and up-hexagons are stable within the entire domain of their existence. Squares become stable if the absolute value of the Marangoni number exceeds a certain value. (paper)
Two-phase numerical model for thermal conductivity and convective heat transfer in nanofluids
Kondaraju Sasidhar; Lee Joon Sang
2011-01-01
Abstract Due to the numerous applications of nanofluids, investigating and understanding of thermophysical properties of nanofluids has currently become one of the core issues. Although numerous theoretical and numerical models have been developed by previous researchers to understand the mechanism of enhanced heat transfer in nanofluids; to the best of our knowledge these models were limited to the study of either thermal conductivity or convective heat transfer of nanofluids. We have develo...
Pantokratoras, Asterios
2014-01-01
In the above paper the authors treat the boundary layer flow along a stationary, vertical, permeable, flat plate within a vertical free stream. Fluid is sucked or injected through the vertical plate. The fluid species concentration at the plate is constant and different from that of the ambient fluid. It is also assumed that the plate is heated by convection from another fluid with constant temperature with a constant heat transfer coefficient. The temperature and species concentration difference between the plate and the ambient fluid creates buoyancy forces and the flow is characterized as mixed convection. The partial differential equations of the boundary layer flow (Eqs. 1-4 in their paper) are transformed and subsequently are solved numerically using an implicit finite difference scheme in combination with a quasi-linearization technique. The quasi-linearization technique is a Newton-Raphson method. The results are presented in 12 figures.
Micro-channel convective boiling heat transfer with flow instabilities
Energy Technology Data Exchange (ETDEWEB)
Consolini, L.; Thome, J.R. [Ecole Polytechnique Federale de Lausanne (Switzerland). Lab. de Transfert de Chaleur et de Masse], e-mail: lorenzo.consolini@epfl.ch, e-mail: john.thome@epfl.ch
2009-07-01
Flow boiling heat transfer in micro-channels has attracted much interest in the past decade, and is currently a strong candidate for high performance compact heat sinks, such as those required in electronics systems, automobile air conditioning units, micro-reactors, fuel cells, etc. Currently the literature presents numerous experimental studies on two-phase heat transfer in micro-channels, providing an extensive database that covers many different fluids and operating conditions. Among the noteworthy elements that have been reported in previous studies, is the sensitivity of micro-channel evaporators to oscillatory two-phase instabilities. These periodic fluctuations in flow and pressure drop either result from the presence of upstream compressibility, or are simply due to the interaction among parallel channels in multi-port systems. An oscillating flow presents singular characteristics that are expected to produce an effect on the local heat transfer mechanisms, and thus on the estimation of the two-phase heat transfer coefficients. The present investigation illustrates results for flow boiling of refrigerants R-134a, R-236fa, and R-245fa in a 510 {mu}m circular micro-channel, exposed to various degrees of oscillatory compressible volume instabilities. The data describe the main features of the fluctuations in the temperatures of the heated wall and fluid, and draw attention to the differences in the measured unstable time-averaged heat transfer coefficients with respect to those for stable flow boiling. (author)
Micro-channel convective boiling heat transfer with flow instabilities
International Nuclear Information System (INIS)
Flow boiling heat transfer in micro-channels has attracted much interest in the past decade, and is currently a strong candidate for high performance compact heat sinks, such as those required in electronics systems, automobile air conditioning units, micro-reactors, fuel cells, etc. Currently the literature presents numerous experimental studies on two-phase heat transfer in micro-channels, providing an extensive database that covers many different fluids and operating conditions. Among the noteworthy elements that have been reported in previous studies, is the sensitivity of micro-channel evaporators to oscillatory two-phase instabilities. These periodic fluctuations in flow and pressure drop either result from the presence of upstream compressibility, or are simply due to the interaction among parallel channels in multi-port systems. An oscillating flow presents singular characteristics that are expected to produce an effect on the local heat transfer mechanisms, and thus on the estimation of the two-phase heat transfer coefficients. The present investigation illustrates results for flow boiling of refrigerants R-134a, R-236fa, and R-245fa in a 510 ?m circular micro-channel, exposed to various degrees of oscillatory compressible volume instabilities. The data describe the main features of the fluctuations in the temperatures of the heated wall and fluid, and draw attention to the differences in the measured unstable time-averaged heat transfer coefficients with respect to those for stable flow boiling. (author)
International Nuclear Information System (INIS)
A non-similar steady laminar boundary layer model is described for the hydromagnetic convection flow of a Newtonian, electrically-conducting liquid metal past a translating, non-conducting plate with a magnetic field aligned with the plate direction. The non-dimensional boundary layer equations are solved with the Sparrow-Quack-Boerner local non-similarity method (LNM). An increase in magnetic Prandtl number (Prm) is found to strongly enhance wall heat transfer rate (Nux Rex-1/2), velocity (f') and induced magnetic field function (g), but exerts negligible influence on the temperature (?) in the boundary layer. A rise in magnetic force number (?) increases velocity, f', shear stress function, f'', and wall heat transfer gradient, i.e. Nux Rex-1/2, but reduces magnetic field function, g and temperature, ?. Increasing ordinary Prandtl number (Pr), decreases temperature, ?, but increases wall heat transfer rate (Nux Rex-1/2). An increase in wall to free stream velocity ratio parameter, ?, increases flow velocity, f', and induced magnetic field gradient, g' for small ? but reduces g' for larger ?, and also boosts the wall temperature gradient, Nux Rex-1/2. The model has potential applications in astronautic magneto-thermo-aerodynamics, nuclear reactor channel flow control with magnetic fields and MHD (magnetohydrodynamic) energy generators. (authors)
Directory of Open Access Journals (Sweden)
S Karunakar Reddy
2013-05-01
Full Text Available he effects of heat and mass transfer on MHD mixed convection flow of a vertical surface with radiation, heat source/absorption and chemical reaction has been is discussed. The resulting set of coupled non-linear ordinary differential equations is solved by perturbation technique and for graphs we used MATLAB software. Approximate solutions have been derived for the velocity, temperature, concentration profiles, skin friction and Nusselt number. The obtained results are discussed with the help of the graphs to observe the effect of various parameters like Grashof number, modified Grashof number, Schmidt number, Prandtl number, Magnetic parameter, Radiation parameter, Chemical reaction, Heat source parameter, Radiation absorption parameter.
Scientific Electronic Library Online (English)
A., Mahdy; R.A., Mohamed; F.M., Hady.
2009-12-01
Full Text Available The problem of combined heat and mass transfer in buoyancy-induced MHD natural convection flow of an electrically conducting fluid along a vertical wavy plate with power-law variation of both heat and mass flux was investigated. The resulting transformed governing equations are solved numerically by [...] an implicit finite-difference scheme. The results are presented for the major parameters including the wave amplitude a, the magnetic parameter Mn, the buoyancy ratio between species and thermal diffusion Br, the Lewis number Le, and the power-law parameter ?. A systematic study on the effects of the various parameters on flow, heat and mass transfer characteristics is carried out.
Directory of Open Access Journals (Sweden)
Lenhard Richard
2012-04-01
Full Text Available In the call OPVaV-2008/2.2/01-SORO Operational Programme Research and Development - knowledge and technology transfer from research and development into practice (ITMS-26220220057, whose strategic goal is "Device to use low-potential geothermal heat without forced circulation of heat carrier deep in the well "in the Department of Energy laboratory techniques to construct a simulator of transport low potential of geothermal energy in comparative test-drilling in the laboratory. The article describes a device that was designed as a scale model of two deep boreholes each of which withdraws the earth's heat by heat transfer technology and heat carrier. Device using forced circulation of heat carrier will respond in the construction of equipment currently used to transport heat from deep borehole. As the heat carrier will be used CO2. Facilities without using forced circulation of heat carrier, the new technology, which will be used as heat carrier ammonia (NH3.
International Nuclear Information System (INIS)
Previous studies showed that, under free or forced convection, the application of a non-uniform electric field with a sufficient intensity at the neighbourhood of the heating elements resulted in a significant improvement of the thermal exchange coefficient under some flow rate and pressure conditions. But these improvements first required the creation of ions in the gas (shock ionization and existence of a corona effect above a sufficient voltage). This document therefore report further studies performed at the neighbourhood of the Melusine atomic pile which would provide ionizing radiations of much greater intensity. The objectives were, on the one hand, a global verification of work hypotheses imagined for the exchange improvement mechanism, and, on the other hand, to obtain data on the conditions under which this exchange improvement mechanism could be used in channels of a reactor. The authors present the instrumentation (measurement cells, gas circuit, heating circuit, high voltage circuit), describe how physical values are measured (pressure, gas flow rate, temperature of the heating element, gas temperature, high voltage). They describe how they compare powers exchanged under forced convection between dioxide carbon and a heated wire with or without external ionizing radiations, and report the various performed calculations (power supplied to the gas, temperature shift, Nusselt and Reynolds numbers). They report and discuss experimental results obtained outside the pile without ionizing radiations, and within the pile in presence of a ionizing radiation
Convection in a nematic liquid crystal with homeotropic alignment and heated from below
Energy Technology Data Exchange (ETDEWEB)
Ahlers, G. [Univ. of California, Santa Barbara, CA (United States)
1995-12-31
Experimental results for convection in a thin horizontal layer of a homeotropically aligned nematic liquid crystal heated from below and in a vertical magnetic field are presented. A subcritical Hopf bifurcation leads to the convecting state. There is quantitative agreement between the measured and the predicted bifurcation line as a function of magnetic field. The nonlinear state near the bifurcation is one of spatio-temporal chaos which seems to be the result of a zig-zag instability of the straight-roll state.
Dynamics of a secondary instability in Benard-Marangoni convection with unidimensional heating
International Nuclear Information System (INIS)
The dynamics of Benard-Marangoni convection with unidimensional heating in a pure fluid is studied experimentally. Convection begins with rolls parallel to the heater. The characteristics of these primary rolls have been determined. When the temperature difference across the liquid layer is increased beyond a critical value a secondary instability appears. Motions transverse to the heater with a definite wavelength can be seen. Moreover, for small angles between the heater and the fluid surface, the pattern drifts along the heater with a velocity that depends almost linearly on the inclination. A phenomenological phase equation is proposed to interpret this observation. (orig.)
Critical heat flux for free convection boiling in thin rectangular channels
International Nuclear Information System (INIS)
A review of the experimental data on free convection boiling critical heat flux (CHF) in vertical rectangular channels reveals three mechanisms of burnout. They are the pool boiling limit, the circulation limit, and the flooding limit associated with a transition in flow regime from churn to annular flow. The dominance of a particular mechanism depends on the dimensions of the channel. Analytical models were developed for each free convection boiling limit. Limited agreement with data is observed. A CHF correlation, which is valid for a wide range of gap sizes, was constructed from the CHFs calculated according to the three mechanisms of burnout. 17 refs., 7 figs
Turbulent convective heat transfer in an inclined tube filled with sodium
Vasil'ev, A. Yu.; Kolesnichenko, I. V.; Mamykin, A. D.; Frick, P. G.; Khalilov, R. I.; Rogozhkin, S. A.; Pakholkov, V. V.
2015-09-01
Turbulent free convection of liquid sodium in a straight thermally insulated tube with a length equal to 20 diameters and with end heat exchangers ensuring a fixed temperature drop is investigated experimentally. The experiments are performed for a fixed Rayleigh number Ra = 2.4 × 106 and various angles of inclination of the tube relative to the vertical. A strong dependence of the power transferred along the tube on the angle of inclination is revealed: the Nusselt number in the angular range under investigation changes by an order of magnitude with a maximum at the angle of 65° with the vertical. The characteristics of large-scale circulation and turbulent temperature pulsations show that convective heat transfer is mainly determined by the velocity of large-scale circulation of sodium. Turbulent pulsations are maximal for small angles of inclination (? = 20°-30°) and reduce the heat flux along the channel, although in the limit of small angles (vertical tube), there is no large-scale circulation, and the convective heat flux, which is an order of magnitude larger than the molecular heat flux, is ensured only by small scale (turbulent) flow.
Study on the natural convection heat transfer characteristics in the air duct
Energy Technology Data Exchange (ETDEWEB)
Kim, Y. K.; Lee, Y. B.; Choi, S. K.; Hwang, J. S.; Nam, H. Y. [Korea Atomic Energy Research Institute, Taejon (Korea, Republic of)
1997-12-31
Temperature distribution measurements in the mockup apparatus of reactor vessel were performed to determine the effective thermal conductivity of porous media with different geometry and to obtain the experimental data for the heat transfer processes by natural convection occurring in the air duct. The temperature distributions at four separated sections with different arrangements of porous media have different slopes according to the geometrical configuration. From the measured temperature distribution, effective thermal conductivity have been derived using the least square fitting method. The test at air duct was performed to the high heat removal at 3.4kW/m{sup 2} by the natural convection from the outer wall to the air. And also the temperature distributions in the sir duct agree well with the 1/7th power-law turbulent temperature distribution. The obtained heat transfer data have been compared with the Shin`s and Sieger`s correlations. 10 refs., 6 figs. (Author)
Heat transfers in a double-skin roof ventilated by natural convection in summer time
Biwole, Pascal; Pompeo, C
2013-01-01
The double-skin roofs investigated in this paper are formed by adding a metallic screen on an existing sheet metal roof. The system enhances passive cooling of dwellings and can help diminishing power costs for air conditioning in summer or in tropical and arid countries. In this work, radiation, convection and conduction heat transfers are investigated. Depending on its surface properties, the screen reflects a large amount of oncoming solar radiation. Natural convection in the channel underneath drives off the residual heat. The bi-dimensional numerical simulation of the heat transfers through the double skin reveals the most important parameters for the system's efficiency. They are, by order of importance, the sheet metal surface emissivity, the screen internal and external surface emissivity, the insulation thickness and the inclination angle for a channel width over 6 cm. The influence of those parameters on Rayleigh and Nusselt numbers is also investigated. Temperature and air velocity profiles on seve...
Thermal modeling of the forced convection Sandwich Greenhouse drying system for rubber sheets
International Nuclear Information System (INIS)
Highlights: • Sandwich Greenhouse is designed for better quality and efficiency of rubber sheet drying. • Thermal models are developed to predict the convection heat transfer coefficient. • The models are validated and show good agreement with the actual experimental data. • The proposed greenhouse can maintain 40–60 °C, suitable for rubber sheet drying. • This greenhouse can bring down the moisture content to 2.8% in fewer than 2 days. - Abstract: In this paper, a novel “Sandwich Greenhouse” for rubber sheet drying is proposed. Using solar energy as the only heat source instead of traditional smoke house that requires firewood, it eliminates shortcomings such as skilled labor monitoring requirement, possible fire hazard, and darken-color rubber sheets due to soot particle contamination. Our greenhouse is specially designed to retain solar energy within, while minimizing the heat loss to the outside environment. The mathematical models are developed to predict the convection mass transfer coefficient and to study the thermal behavior during the drying of rubber sheets under our proposed greenhouse design. Validated with experimental observations, the models show good agreement with the actual experimental data. The experiment demonstrates an effectiveness of our proposed Sandwich Greenhouse, as the temperature of the rubber sheet is 15 °C and 5 °C higher than the ambient temperature during the daytime and nighttime, respectively. As a result, the moisture content of the rubber sheets can decrease from 36.4% to 2.8% in fewer than 2 days
International Nuclear Information System (INIS)
This book contains the short papers from the International Symposium on convective heat and Mass Transfer in sustainable Energy ( conv-09), organized on behalf of the International Centre for Heat and Mass Transfer, it was held on April 26- 1st May, In Hammamet, Tunisia. The objective of this conference is to bring together researchers in a forum to exchange innovative ideas, methods and results, and visions of the future related to the general theme of convective heat and mass transfer
International Nuclear Information System (INIS)
This book contains the short papers from the International Symposium on Convective heat and Mass Transfer in sustainable Energy ( Conv-09), organized on behalf of the International Centre for Heat and Mass Transfer, it was held on April 26- 1st May, In Hammamet, Tunisia. The objective of this conference is to bring together researchers in a forum to exchange innovative ideas, methods and results, and visions of the future related to the general theme of convective heat and mass transfer
Hall effects on unsteady MHD free and forced convection flow in a porous rotating channel
International Nuclear Information System (INIS)
An initial value investigation of an unsteady combined free and forced convection flows in porous rotating channel has been made. Both the porous walls execute non-torsional oscillations in their own plane and in addition they are maintained at a constant temperature gradient. The solutions for the velocity, temperature distributions and the shear stresses are obtained in closed form by using Laplace transform technique. The structure of the associated boundary layers are discussed for small (? much less than 1), intermediate (?=O (1)) and high frequency (? much greater than 1) of oscillations. (author). 7 refs
Bibliography on augmentation of convective heat and mass transfer
Energy Technology Data Exchange (ETDEWEB)
Bergles, A.E.; Webb, R.L.; Junkhan, G.H.; Jensen, M.K.
1979-05-01
Heat transfer augmentation has developed into a major specialty area in heat transfer research and development. A bibliography of world literature on augmentation is presented. The literature is classified into passive augmentation techniques, which require no external power, and active techniques, which do require external power. The fourteen techniques are grouped in terms of their application to the various modes of heat transfer. Mass transfer is included for completeness. Key words are included with each citation for technique/mode identification. The total number of publications cited is 1,967, including 75 surveys of various techniques and 42 papers on performance evaluation of passive techniques. Patents are not included as they will be the subject of a future topical report.
Bibliography on augmentation of convective heat and mass transfer
International Nuclear Information System (INIS)
Heat transfer augmentation has developed into a major specialty area in heat transfer research and development. A bibliography of world literature on augmentation is presented. The literature is classified into passive augmentation techniques, which require no external power, and active techniques, which do require external power. The fourteen techniques are grouped in terms of their application to the various modes of heat transfer. Mass transfer is included for completeness. Key words are included with each citation for technique/mode identification. The total number of publications cited is 1,967, including 75 surveys of various techniques and 42 papers on performance evaluation of passive techniques. Patents are not included as they will be the subject of a future topical report
Directory of Open Access Journals (Sweden)
S. S. Das, S. Mishra, P. Tripathy
2014-01-01
Full Text Available This paper analyzes the effect of mass transfer on natural convection hydromagnetic flow of a viscous incompressible fluid through a porous medium past an oscillating porous plate in a porous medium with heat source. The governing equations of the flow field are solved analytically and the expressions for velocity and temperature of the flow field, skin friction t and the heat flux in terms of Nusselts number Nu are obtained. The effects of the important flow parameters such as magnetic parameter M, permeability parameter Kp, Grashof number for heat and mass transfer Gr, Gc, Schmidt number Sc, heat source parameter S and the Prandtl number Pr on the velocity and temperature of the flow field are to be discussed with the help of figures. It is observed that a growing magnetic parameter M retards the magnitude of the velocity of the flow field at all points due to the action of the Lorentz force on the flow field. The heat source parameter S has an accelerating effect on the magnitude of the velocity of the flow field at all points. The effect of growing Grashof number for mass transfer Gc and the permeability parameter Kp is to enhance the velocity (absolute value of the flow field at all points. An increase in Schmidt number Sc is to increase the magnitude of the velocity of the flow field at all points. A growing rarefaction parameter R enhances the magnitude of the velocity of the flow field at all points.
Modeling of heat transfer in cooling towers with natural convection.
Czech Academy of Sciences Publication Activity Database
Zuniga-Gonzalez, Israel; Maršík, František
Gdansk : IFFM Publishers, 2005 - (Mikielewicz, J.; Butrymowicz, D.; Trela, M.; Cie?li?ski, J.), s. 585-592 ISBN 83-88237-90-X. [ HEAT 2005 : International Conference on Transport Phenomena in Multiphase Systems. Gdansk (PL), 26.06.2005-30.06.2005] R&D Projects: GA ?R GA101/02/0364; GA ?R(CZ) GA101/05/2536 Institutional research plan: CEZ:AV0Z20760514 Keywords : cooling tower * heat transfer * evaporative cooling Subject RIV: BK - Fluid Dynamics
Natural convective performance of perforated heat sinks with circular pin fins
Wen, Mao-Yu; Yeh, Cheng-Hsiung
2015-10-01
This study examines heat transfer performance under natural convection for two different types (Type A and Type B) of pin-fin heat sinks with/without a hollow in the heated base. The effects of the rate of heat transfer, the height of fin and base plate, the heat sink porosity and the perforated base plate on the heat-transfer coefficient and enhancement factor were investigated and evaluated. In order to obtain insight into the fluid flow phenomena, flow visualization was also made to observe the detailed fluid flow characteristics of the present pin-fin heat sinks. Experimental results show that heat transfer, using pin-fin heat sinks, is significantly different for Type A and Type B. Comparisons of the heat transfer coefficients, with respect to the unfinned base plate under the same experimental conditions, for the pin-fin heat sinks were about 1.48-fold-1.64-fold (Type A) and 1.81-fold-1.94-fold (Type B), respectively. In addition, results also show that a Type B-heat sink has a higher enhancement factor than a Type A-heat sink.
Convection Heat Transfer in Three-Dimensional Turbulent Separated/Reattached Flow
Energy Technology Data Exchange (ETDEWEB)
Bassem F. Armaly
2007-10-31
The measurements and the simulation of convective heat transfer in separated flow have been a challenge to researchers for many years. Measurements have been limited to two-dimensional flow and simulations failed to predict accurately turbulent heat transfer in the separated and reattached flow region (prediction are higher than measurements by more than 50%). A coordinated experimental and numerical effort has been initiated under this grant for examining the momentum and thermal transport in three-dimensional separated and reattached flow in an effort to provide new measurements that can be used for benchmarking and for improving the simulation capabilities of 3-D convection in separated/reattached flow regime. High-resolution and non-invasive measurements techniques are developed and employed in this study to quantify the magnitude and the behavior of the three velocity components and the resulting convective heat transfer. In addition, simulation capabilities are developed and employed for improving the simulation of 3-D convective separated/reattached flow. Such basic measurements and simulation capabilities are needed for improving the design and performance evaluation of complex (3-D) heat exchanging equipment. Three-dimensional (3-D) convective air flow adjacent to backward-facing step in rectangular channel is selected for the experimental component of this study. This geometry is simple but it exhibits all the complexities that appear in any other separated/reattached flow, thus making the results generated in this study applicable to any other separated and reattached flow. Boundary conditions, inflow, outflow, and wall thermal treatment in this geometry can be well measured and controlled. The geometry can be constructed with optical access for non-intrusive measurements of the flow and thermal fields. A three-component laser Doppler velocimeter (LDV) is employed to measure simultaneously the three-velocity components and their turbulent fluctuations. Infrared thermography is utilized to measure the wall temperature and that information is used to determine the local convective heat transfer coefficient. FLUENT – CFD code is used as the platform in the simulation effort and User Defined Functions are developed for incorporating advanced turbulence models into this simulation code. Predictions of 3-D turbulent convection in separated flow, using the developed simulation capabilities under this grant, compared well with measured results. Results from the above research can be found in the seventeen refereed journal articles, and thirteen refereed publications and presentations in conference proceedings that have been published by the PI during the this grant period. The research effort is still going on and several publications are being prepared for reporting recent results.
International Nuclear Information System (INIS)
A model of both equiaxed and columnar dendritic growth was developed that incorporates thermal, solutal and fluid flow effects in either two or three dimensions. The model solves the momentum, mass and energy transport equations, including phase change. An imposed anisotropy algorithm, combined with a modified projection method solution of the Navier–Stokes equations, allows a relative coarse mesh and hence excellent computational efficiency. The model was used to study the effect of dimensionality (2D versus 3D) on dendritic growth with and without convection. The influence of forced convection on unconstrained equiaxed growth was studied first. In 3D, the upstream boundary layer is much thinner with a lower concentration than in 2D. This increases tip undercooling, accelerating upstream tip growth and promoting secondary branching. The influence of natural convection on constrained, columnar dendritic, growth was then studied. The 2D flow is blocked by the primary dendrite arms (which are effectively plates), while the 3D flow can wrap around the primaries. This change in flow strongly alters solute distribution and consequently the developing dendritic microstructure. 3D simulations are required to correctly predict unconstrained solidification microstructures
MHD unsteady free convective Walter’s memory flow with constant suction and heat sink
Directory of Open Access Journals (Sweden)
M.V Ramana Murthy
2007-10-01
Full Text Available The study of unsteady hydromagnetic free convective memory flow of incompressible and electrically conducting fluids past an infinite vertical porous plate in the presence of constant suction and heat absorbing sinks have been made. Approximate solutions have been derived for the mean velocity, mean temperature, mean skin-friction and mean rate of heat transfer using multi-parameter perturbation technique. It is observed that magnetic field strength decreases the mean velocity of the fluid. Also the mean skin-friction and mean rate of heat transfer of the conducting fluid decreases with the increase in magnetic field strength.
Khan, Masood; Malik, Rabia; Munir, Asif
2015-08-01
In this article, the mixed convective heat transfer to Sisko fluid over a radially stretching surface in the presence of convective boundary conditions is investigated. The viscous dissipation and thermal radiation effects are also taken into account. The suitable transformations are applied to convert the governing partial differential equations into a set of nonlinear coupled ordinary differential equations. The analytical solution of the governing problem is obtained by using the homotopy analysis method (HAM). Additionally, these analytical results are compared with the numerical results obtained by the shooting technique. The obtained results for the velocity and temperature are analyzed graphically for several physical parameters for the assisting and opposing flows. It is found that the effect of buoyancy parameter is more prominent in case of the assisting flow as compared to the opposing flow. Further, in tabular form the numerical values are given for the local skin friction coefficient and local Nusselt number. A remarkable agreement is noticed by comparing the present results with the results reported in the literature as a special case.
Transient natural convection heat and mass transfer in crystal growth
Han, Samuel S.
1990-01-01
A numerical analysis of transient combined heat and mass transfer across a rectangular cavity is performed. The physical parameters are selected to represent a range of possible crystal growth in solutions. Good agreements with measurement data are observed. It is found that the thermal and solute fields become highly oscillatory when the thermal and solute Grashof numbers are large.
Models for fluid flows with heat transfer in mixed convection
International Nuclear Information System (INIS)
Second order models were studied in order to predict turbulent flows with heat transfer. The equations used correspond to the characteristic scale of turbulent flows. The order of magnitude of the terms of the equation is analyzed by using Reynolds and Peclet numbers. The two-equation model (K-?) is applied in the hydrodynamic study. Two models are developed for the heat transfer analysis: the Prt + teta 2 and the complete model. In the first model, the turbulent thermal diffusivity is calculated by using the Prandtl number for turbulent flow and an equation for the variance of the temperature fluctuation. The second model consists of three equations concerning: the turbulent heat flow, the variance of the temperature fluctuation and its dissipation ratio. The equations were validated by four experiments, which were characterized by the analysis of: the air flow after passing through a grid of constant average temperature and with temperature gradient, an axysymmetric air jet submitted to high and low heating temperature, the mixing (cold-hot) of two coaxial jets of sodium at high Peclet number. The complete model is shown to be the most suitable for the investigations presented
Avallone, F.; Greco, C. S.; Schrijer, F. F. J.; Cardone, G.
2015-04-01
The measurement of the convective wall heat flux in hypersonic flows may be particularly challenging in the presence of high-temperature gradients and when using high-thermal-conductivity materials. In this case, the solution of multidimensional problems is necessary, but it considerably increases the computational cost. In this paper, a low-computational-cost inverse data reduction technique is presented. It uses a recursive least-squares approach in combination with the trust-region-reflective algorithm as optimization procedure. The computational cost is reduced by performing the discrete Fourier transform on the discrete convective heat flux function and by identifying the most relevant coefficients as objects of the optimization algorithm. In the paper, the technique is validated by means of both synthetic data, built in order to reproduce physical conditions, and experimental data, carried out in the Hypersonic Test Facility Delft at Mach 7.5 on two wind tunnel models having different thermal properties.
Mixed convection boundary layer flow over a horizontal elliptic cylinder with constant heat flux
Javed, Tariq; Ahmad, Hussain; Ghaffari, Abuzar
2015-09-01
Mixed convection boundary layer flow of a viscous fluid over a horizontal elliptic cylinder with a constant heat flux is investigated numerically. The governing partial differential equations are transformed to non-dimensional form and then are solved by an efficient implicit finite different scheme known as Keller-box method. The solutions are expressed in the form of skin friction and Nusselt number, which are plotted against the eccentric angle. The effect of pertinent parameters such as mixed convection parameter, aspect ratio (ratio of lengths of minor axis to major axis), and Prandtl number on skin friction and Nusselt number are illustrated through graphs for both blunt and slender orientations. The increase in the value of mixed convection parameter results in increase in skin friction coefficient and Nusselt number for blunt as well as slender orientations.
Influence of Chimney Width in Natural Convection Heat Transfer on a Vertical Finned Plate
International Nuclear Information System (INIS)
The RCCS adopts the chimney system to increase the flow rate. Also the fins installed in the inner walls of the chimney will provide an additional cooling performance by increasing the heat transfer area. On the other hand, the fins also increase the friction loss i. e. the pressure drop. Thus, in order to improve the heat transfer performance of the RCCS, an optimization among the fin parameters is necessary. Many experimental and numerical studies regarding the fin and the chimney are available. In this study, the natural convection heat transfer of the fin system located inside the chimney was measured. Based on the analogy concept, heat transfer experiments were replaced by mass transfer experiments using a sulfuric acid-copper sulfate (H2SO4-CuSO4) electroplating system. The experiments were conducted by varying fin spacing, fin height, chimney width, and chimney height. This study experimentally investigated the natural convection heat transfer of the vertical finned plate in the chimney. Using an analogy, the heat transfer systems were replaced by mass transfer systems. The measured mass transfer coefficients was the difference with the existing heat transfer correlations due to the large value of the Pr, but exhibited similar trends with the existing heat transfer correlations. The heat transfer rate is increased by the decrease of the fin spacing and the increased fin height due to increased heat transfer area and chimney flow pattern. The chimney effect enhances heat transfer rate of vertical finned plate and the chimney effect on the Nus In this study, the heat transfer rate on vertical finned plate in the chimney was confirmed by experiment for high values of Ra was stronger for the narrow chimney width, and became stronger by the higher chimney. The chimney effect was not observed when the chimney width becomes larger than a certain value depending on the Ras
Conjugate Heat Transfer of Mixed Convection for Viscoelastic Fluid Past a Stretching Sheet
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Guan-Bang Chen
2007-03-01
Full Text Available A conjugate heat transfer problem of a second-grade viscoelastic fluid past a stretching sheet has been studied. Governing equations include heat conduction equation of a stretching sheet, continuity equation, momentum equation, and energy equation of a second-grade fluid, analyzed by a combination of a series expansion method, the similarity transformation, and a second-order accurate finite-difference method. These solutions are used to iterate with the heat conduction equation of the stretching sheet to obtain distributions of the local convective heat transfer coefficient and the stretching sheet temperature. Ranges of dimensionless parameters, the Prandtl number Pr, the elastic number E and the conduction-convection coefficient Ncc are from 0.001 to 10, 0.0001 to 0.01, and 0.5 to 2.0, respectively. A parameter G, which is used to represent the dominance of the buoyant effect, is present in governing equations. Results indicated that elastic effect in the flow could increase the local heat transfer coefficient and enhance the heat transfer of a stretching sheet. In addition, same as the results from Newtonian fluid flow and conduction analysis of a stretching sheet, a better heat transfer is obtained with a larger Ncc, G, and E.
Asymptotic solution for heat convection-radiation equation
Energy Technology Data Exchange (ETDEWEB)
Mabood, Fazle; Ismail, Ahmad Izani Md [School of Mathematical Sciences, Universiti Sains Malaysia, 11800 USM, Penang (Malaysia); Khan, Waqar A. [Department of Engineering Sciences, National University of Sciences and Technology, PN Engineering College, Karachi, 75350 (Pakistan)
2014-07-10
In this paper, we employ a new approximate analytical method called the optimal homotopy asymptotic method (OHAM) to solve steady state heat transfer problem in slabs. The heat transfer problem is modeled using nonlinear two-point boundary value problem. Using OHAM, we obtained the approximate analytical solution for dimensionless temperature with different values of a parameter ?. Further, the OHAM results for dimensionless temperature have been presented graphically and in tabular form. Comparison has been provided with existing results from the use of homotopy perturbation method, perturbation method and numerical method. For numerical results, we used Runge-Kutta Fehlberg fourth-fifth order method. It was found that OHAM produces better approximate analytical solutions than those which are obtained by homotopy perturbation and perturbation methods, in the sense of closer agreement with results obtained from the use of Runge-Kutta Fehlberg fourth-fifth order method.
Investigation of combined free and forced convection in a 2 x 6 rod bundle
International Nuclear Information System (INIS)
The report documents a study designed to provide velocity and temperature data in a rod bundle geometry with low Reynolds number flows and severe radial power gradients. The data were then used to verify predictions by the COBRA computer program in the combined free and forced convection flow regime. Comparable flow conditions may occur in the radial blanket of a Liquid Metal Fast Breeder Reactor (LMFBR), in which radial power gradients up to 4 : 1 may exist across an assembly. Under low flow rate conditions the power gradient may cause significant buoyancy forces potentially resulting in flow redistribution within the radial assemblies. Consequently, there is a need to evaluate whether available thermal-hydraulic design codes can accurately predict the corresponding temperature and velocity profiles
Convective heat transfer fouling of aqueous solutions on modified surfaces
Janabi, Abdullah K. O. Al-
2011-01-01
The present research study was part of the European project "MEDESOL" entitled "Seawater desalination by innovative solar-powered membrane-distillation system". The project aimed at developing a stand-alone desalination unit to produce fresh water with a maximum of 50 m3/day. Several components such as suitable membrane and efficient solar collectors had to be developed as well as a plate heat exchanger for a maximum life expectancy with least deposition occurrence on its surfaces. The contri...
Enhanced convective heat transfer using graphene dispersed nanofluids
Baby Tessy; Ramaprabhu Sundara
2011-01-01
Abstract Nanofluids are having wide area of application in electronic and cooling industry. In the present work, hydrogen exfoliated graphene (HEG) dispersed deionized (DI) water, and ethylene glycol (EG) based nanofluids were developed. Further, thermal conductivity and heat transfer properties of these nanofluids were systematically investigated. HEG was synthesized by exfoliating graphite oxide in H2 atmosphere at 200°C. The nanofluids were prepared by dispersing functionalized HEG (f...
International Nuclear Information System (INIS)
In heated tunnels such as those designated for emplacement of radioactive waste at Yucca Mountain, axial temperature gradients may cause natural convection processes that can significantly influence the moisture conditions in the tunnels and in the surrounding fractured rock. Large-scale convection cells would provide an effective mechanism for axial vapor transport, driving moisture out of the formation away from the heated tunnel section into cool end sections (where no waste is emplaced). To study such processes, we have developed and applied an enhanced version of TOUGH2 (Pruess et al., 1999) adding a new module that solves for natural convection in open cavities. The new TOUGH2 simulator simultaneously handles (1) the flow and energy transport processes in the fractured rock; (2) the flow and energy transport processes in the cavity; and (3) the heat and mass exchange at the rock-cavity interface. The new module is applied to simulate the future thermal-hydrological (TH) conditions within and near a representative waste emplacement tunnel at Yucca Mountain. Particular focus is on the potential for condensation along the emplacement section, a possible result of heat output differences between individual waste packages
Mixed convection heat transfer from confined tandem square cylinders in a horizontal channel
Huang, Zhu
2013-11-01
This paper presents a numerical study on the two-dimensional laminar mixed convective flow and heat transfer around two identical isothermal square cylinders arranged in tandem and confined in a channel. The spacing between the cylinders is fixed with four widths of the cylinder and the blockage ratio and the Prandtl number are fixed at 0.1 and 0.7 respectively. The mixed convective flow and heat transfer is simulated by high accuracy multidomain pseudospectral method. The Reynolds number (Re) is studied in the range 80 ? Re ? 150, the Richardson number (Ri) demonstrating the influence of thermal buoyancy ranges from 0 to 1. Numerical results reveal that, with the thermal buoyancy effect, the mixed convective flow sheds vortex behind the cylinders and keeps periodic oscillating. The variations of characteristic quantities related to flow and heat transfer processes, such as the overall drag and lift coefficients and the Nusselt numbers, are presented and discussed. Furthermore, the influence of thermal buoyancy on the fluid flow and heat transfer are discussed and analysed. © 2013 Elsevier Ltd. All rights reserved.
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Fox, E.; Visser, A.; Bridges, N.
2011-07-18
This paper presents an experimental study of natural convection heat transfer for an Ionic Liquid. The experiments were performed for 1-butyl-2, 3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide, ([C{sub 4}mmim][NTf{sub 2}]) at a Raleigh number range of 1.26 x 10{sup 7} to 8.3 x 10{sup 7}. In addition to determining the convective heat transfer coefficients, this study also included experimental determination of thermophysical properties of [C{sub 4}mmim][NTf{sub 2}] such as, density, viscosity, heat capacity, and thermal conductivity. The results show that the density of [C{sub 4}mmim][NTf{sub 2}] varies from 1.437-1.396 g/cm{sup 3} within the temperature range of 10-50 C, the thermal conductivity varies from 0.105-0.116 W/m.K between a temperature of 10 to 60 C, the heat capacity varies from 1.015 J/g.K - 1.760 J/g.K within temperature range of 25-340 C and the viscosity varies from 18cp-243cp within temperature range 10-75 C. The results for density, thermal conductivity, heat capacity, and viscosity were in close agreement with the values in the literature. Measured dimensionless Nusselt number was observed to be higher for the ionic liquid than that of DI water. This is expected as Nusselt number is the ratio of heat transfer by convection to conduction and the ionic liquid has lower thermal conductivity (approximately 18%) than DI water.
Kenjeres; Hanjalic
2000-12-01
A two-dimensional (2D) numerical study using a single-point algebraic k-straight theta;(2)-varepsilon-varepsilon(straight theta) turbulence closure was performed to detect the existence, origin, creation and behavior of convective rolls and associated wall Nusselt (Nu) number variation in thermal convection in 2D horizontal slender enclosures heated from below. The study covered the Rayleigh (Ra) numbers from 10(5) to 10(12) and aspect ratios from 4:1 to 32:1. The time evolution of the convective rolls and the formation of the corner vortices were analyzed using numerical flow visualization, and the correlation between roll structures and heat transfer established. A major consequence of the imposed two dimensionality appeared in the persistence of regular roll structures at higher Ra numbers that approach a steady state for all configurations considered. This finding contradicts the full three-dimensional direct numerical simulations (DNS), large eddy simulations (LES), and three-dimensional transient Reynolds-averaged Navier-Stokes (TRANS) computations, which all show continuously changing unsteady patterns. However, the final-stage roll structures, long-term averaged mean temperature and turbulence moments, and the Nusselt number (both local and integral), are all reproduced in good agreement with the ensemble-averaged 3D DNS, TRANS, and several recent experimental results. These findings justified the 2D approach as an acceptable method for ensemble average analysis of fully 3D flows with at least one homogeneous direction. Based on our 2D computations and adopting the low and high Ra number asymptotic power laws of Grossmann and Lohse [J. Fluid Mech. 407, 27 (2000)], new prefactors in the Nu-Ra correlation for Pr=O(1) were proposed that fit better several sets of data over a wide range of Ra numbers and aspect ratios: Nu=0.1Ra(1/4)+0.05Ra(1/3). Even better agreement of our computations was achieved with the new correlation Nu=0.124 Ra0.309 proposed recently by Niemela et al. [Nature (London) 404, 837 (2000)] for 10(6)
Kwon, Younghwan; Lee, Kwangho; Park, Minchan; Koo, Kyoungmin; Lee, Jaekeun; Doh, Youngjin; Lee, Soowon; Kim, Doohyun; Jung, Yoongho
2013-12-01
An experimental investigation on the characteristics of the convective heat transfer in the fully developing region of a circular straight tube with a constant heat flux was carried out with Al2O3 nanofluids. Stable nanofluids, which were water-based suspensions of Al2O3 nanoparticles, were prepared by two-step method. The effects of the thermal conductivity, viscosity, and heat capacity of the nanofluids on convective heat transfer were investigated. The result showed that the coefficient enhancement of the convective heat transfer in the Al2O3 nanofluids was increased with increasing fluid temperature compared to that of water at a volume fraction of 3.0% in the turbulent flow region. Thermal conductivity was increased from 8% to 20%, and the increment of convective heat transfer coefficient was enhanced from 14% to 30% with fluid temperature from 22 degrees C to 75 degrees C, respectively. We observed that the increment of convective heat transfer coefficient in nanofluids was much higher than that of the thermal conductivity at a given temperature condition. The enhancement of Brownian motion due to the decreasing kinematic viscosity led to a higher convective heat transfer coefficient at a higher temperature condition. PMID:24266161
DEFF Research Database (Denmark)
Steskens, Paul W. M. H.; Janssen, Hans
2013-01-01
Currently, researchers are striving to advance the possibilities to calculate the integrated phenomena of heat, air and moisture flows in buildings, with specific focus on the interactions between the building zones and building components. This paper presents an investigation of the capability and applicability of the sub-zonal airflow model to predict the local indoor environmental conditions, as well as the local surface transfer coefficients near building components. Two test cases were analyzed for, respectively, natural and forced convection in a room. The simulation results predicted from the sub-zonal airflow models are compared to experimental data and numerical computational fluid dynamics (CFD) results. The study shows that sub-zonal models combined with an appropriate surface transfer coefficient model are able to give reliable predictions of the local indoor environmental conditions and surface transfer coefficients near the building component for the analyzed cases. The relatively short computation time and flexibility of the sub-zonal model makes the application attractive for transient simulation of heat, air and moisture transport in buildings. However, the availability of appropriate reference conditions, for example experimental or numerical results, is a prerequisite for the development of a reliable sub-zonal model.
Antonini Alves, Thiago; Santos, Paulo H. D.; Barbur, Murilo A.
2015-09-01
In this research, the temperatures of threedimensional (3D) protruding heaters mounted on a conductive substrate in a horizontal rectangular channel with laminar airflow are related to the independent power dissipation in each heater by using a matrix G + with invariant coefficients, which are dimensionless. These coefficients are defined in this study as the conjugate influence coefficients (g +) caused by the forced convection- conduction nature of the heaters' cooling process. The temperature increase of each heater in the channel is quantified to clearly identify the contributions attributed to the self-heating and power dissipation in the other heaters (both upstream and downstream). The conjugate coefficients are invariant with the heat generation rate in the array of heaters when assuming a defined geometry, invariable fluid and flow rate, and constant substrate and heater conductivities. The results are numerically obtained by considering three 3D protruding heaters on a twodimensional (2D) array by ANSYS/Fluent™ 15.0 software. The conservation equations are solved by a coupled procedure within a single calculation domain comprising of solid and fluid regions and by considering a steady state laminar airflow with constant properties. Some examples are shown, indicating the effects of substrate thermal conductivity and Reynolds number on conjugate influence coefficients.
Atashafrooz, M.; Gandjalikhan Nassab, S. A.; Lari, K.
2015-04-01
In the present work, the interaction between non-gray radiation and forced convection in a laminar radiating gas flow over a recess including two backward and forward facing steps in a duct is investigated numerically. Distributions of absorption coefficients across the spectrum (50 cm-1 transfer equation is solved by the discrete ordinates method. The effects of radiation-conduction parameter, wall emissivity, scattering coefficient and recess length on heat transfer behaviors of the convection-radiation system are investigated for both gray and non-gray mediums. In addition, the results of gray medium are compared with non-gray results in order to judge if the differences between these two approaches are significant enough to justify the usage of non-gray models. Results show that for air mixture with 10 % CO2 and 20 % H2O, use of gray model for the radiative properties may cause significant errors and should be avoided.
Sarvari Seyed; Allahyari Shahriar; Behzadmehr Amin
2011-01-01
Abstract Laminar mixed convection of a nanofluid consisting of water and Al2O3 in an inclined tube with heating at the top half surface of a copper tube has been studied numerically. The bottom half of the tube wall is assumed to be adiabatic (presenting a tube of a solar collector). Heat conduction mechanism through the tube wall is considered. Three-dimensional governing equations with using two-phase mixture model have been solved to investigate hydrodynamic and thermal behaviours of the n...
Heat and mass transfer bibliography - Soviet works
International Nuclear Information System (INIS)
A bibliography of over 350 references of Soviet works in heat and mass transfer is presented. The topics covered are; thermodynamics, thermophysical (transport) properties, heat conduction, hydromechanics, natural convection, forced convection, phase changes, radiation, combined heat and mass transfer, high temperature thermophysics, rheophysics and heat and mass transfer in technological process (drying, dispersed systems, and heat and mass exchangers). (U.K.)
Entropy generation due to natural convection in discretely heated porous square cavities
International Nuclear Information System (INIS)
Optimization of industrial processes for higher energy efficiency may be effectively carried out based on the thermodynamic approach of entropy generation minimization (EGM). This approach provides the key insights on how the available energy (exergy) is being destroyed during the process and the ways to minimize its destruction. In this study, EGM approach is implemented for the analysis of optimal thermal mixing and temperature uniformity due to natural convection in square cavities filled with porous medium for the material processing applications. Effect of the permeability of the porous medium and the role of distributed heating in enhancing the thermal mixing, temperature uniformity and minimization of entropy generation is analyzed. It is found that at lower Darcy number (Da), the thermal mixing is low and the heat transfer irreversibility dominates the total entropy generation. In contrast, thermal mixing is improved due to enhanced convection at higher Da. Friction irreversibility is found to dominate the total entropy generation for higher Prandtl number (Pr) fluids at higher Da, whereas the heat transfer irreversibility dominates the total entropy generation for lower Pr fluids. Based on EGM analysis, it is established that larger thermal mixing at high Darcy number may not be always recommended as the total entropy production is quite large at high Darcy number. Overall, it is found that the distributed heating methodology with multiple heat sources may be an efficient strategy for the optimal thermal processing of materials. -- Highlights: ? Analysis on entropy generation is carried out for uniform and distributed heating systems. ? Natural convection for fluids within porous cavities are considered. ? Detailed analysis on flow, temperature and entropy generation characteristics has been done. ? Optimal thermal mixing vs entropy generation is compared for all types of distributed heating systems.? Effects of Darcy number and Prandtl numbers are also investigated.
International Nuclear Information System (INIS)
In the present analysis, the forced convective heat transfer in smooth and corrugated helical coiled tubes was experimentally studied in the Reynolds and Dean number ranges 50÷1200 and 12÷295 respectively, by adopting Ethylene Glycol as working fluid. The primary aim of the investigation is to study the combined effect of the wall curvature and of the wall corrugation in the thermal entrance region for highly viscous fluids. Two coiled tubes with a curvature ratio of about 0.06, one with smooth wall and the other with spirally corrugated wall, were investigated under the uniform heat flux boundary condition. The main conclusion is that in the Reynolds number range analyzed, both curvature and corrugation enhance the heat transfer. For Dean number values lower than about 120 the wall curvature effect prevails, and the heat transfer enhancement reflects Nusselt numbers that are approximately 2–3 times higher than the straight smooth section. For greater Dean number values, the wall corrugation instead prevails. In fact the corrugated coiled tube reaches Nusselt number values which are up to 8 times higher than the ones expected for the smooth straight tube. The smooth coiled tube shows instead thermal performances at maximum 3.6 times over the straight section.
Nonlinear thermal convection in a layer of nanofluid under G-jitter and internal heating effects
Directory of Open Access Journals (Sweden)
Bhadauria B. S.
2014-01-01
Full Text Available This paper deals with a mathematical model of controlling heat transfer in nanofluids. The time-periodic vertical vibrations of the system are considered to effect an external control of heat transport along with internal heating effects. A weakly non-linear stability analysis is based on the five-mode Lorenz model using which the Nusselt number is obtained as a function of the thermal Rayleigh number, nano-particle concentration based Rayleigh number, Prandtl number, Lewis number, modified diffusivity ratio, amplitude and frequency of modulation. It is shown that modulation can be effectively used to control convection and thereby heat transport. Further, it is found that the effect of internal Rayleigh number is to enhance the heat and nano-particles transport.
International Nuclear Information System (INIS)
An experimental test to study natural convection heat transfer to air within a trapezoidal channel area was carried out, heating one of its faces. The temperature in different points along the heated face, inlet and outlet air temperature, environment temperature, the current and voltage supplied to the heater were measured. From the measures, the power applied and the average heat transfer coefficient in the channel were determined. During the experimental test the power applied, channel inclination and the air entrance and exit position, were changed. The values obtained from the different test modes show that the heat transfer coefficient is independent of the power and strongly dependent of the channel inclination and channel position. (author)
Similarity analysis of laminar free convection in square cavity containing heat-generating Kr-85 gas
International Nuclear Information System (INIS)
The Nusselt number, or a dimensionless temperature gradient averaged over a heat transfer surface, of a cavity containing heat-generating fluid is correlated to non-dimensional parameters for free convection of heat generating fluid: Y = (d5?3gE0N)/ (?2kT) and Pr = (Cp?/k), where d is a characteristic length of the system (here, the length of sides of square), ? is a density, g a gravitational acceleration, E0 a heat liberated per second per unit mass of Kr-85, N a mole fraction of Kr-85 in Kr gas, ? a viscosity, k a thermal conductivity, T0 a characteristic temperature and Cp a specific heat at constant pressure, based on a similarity analysis of equations of change as well as numerical solution of these equations
Unsteady laminar flow and convective heat transfer in a sharp 180 deg. bend
International Nuclear Information System (INIS)
Unsteady laminar flow and heat transfer in a sharp 180 deg. bend is studied numerically to investigate a convective heat transfer regime of especial relevance to electronic systems. Due to the high geometrical aspect ratios occurring in the practical application, two-dimensional unsteady simulations are considered. The two-dimensionality assumption adopted is validated by three-dimensional test simulations. Unsteady heat transfer simulations are performed for 50?Re?1000. Results show that the flow remains steady until Re?600. In this steady regime, the re-attachment length increases gradually with the Reynolds number. For Re>600, the flow becomes unsteady with large-scale vortices emanating from the sharp edge dominating the flow field. Flow oscillation causes a substantial reduction in the re-attachment length and a dramatic heat transfer enhancement. As the vortices move downstream, the Nusselt number along the wall oscillates significantly. The correlation between the flow structure and the heat transfer is found to be strong
Directory of Open Access Journals (Sweden)
Mehran Khaki JAMEI
2014-04-01
Full Text Available This work presents a boundary-layer analysis of an incompressible viscous steady flow and forced convection over a horizontal flat plate. The solution for velocity and temperature are calculated by applying the Homotopy perturbation method (HPM. A special technique is attempted by which one is able to obtain solutions that are close to the exact solution of the equation. The obtained results are compared to the exact solution and another results provided by previous works so that the high accuracy of the obtained results is clear. Also, the results reveal that this method is effective, simple, and can be applied for other nonlinear problems in different fields of science and engineering, especially some fluid mechanics and heat transfer equations.doi:10.14456/WJST.2014.47
Heat Transport by Coherent Rayleigh-B\\'enard Convection
Waleffe, Fabian; Smith, Leslie M
2015-01-01
Steady but generally unstable solutions of the 2D Boussinesq equations are obtained for no-slip boundary conditions and Prandtl number 7. The primary solution that bifurcates from the conduction state at Rayleigh number $Ra \\approx 1708$ has been calculated up to $Ra\\approx 5. 10^6$ and shows heat flux $Nu \\sim 0.143\\, Ra^{0.28}$ with a delicate spiral structure in the temperature field. Another solution that maximizes $Nu$ over the horizontal wavenumber has been calculated up to $Ra=10^9$ and its heat flux scales as $Nu \\sim 0.115\\, Ra^{0.31}$ for $10^7 < Ra \\le 10^9$, quite similar to 3D turbulent data. The latter is a simple yet multi-scale coherent solution whose horizontal wavenumber scales as $0.133 \\, Ra^{0.217}$ in that range. That optimum solution is unstable to larger scale perturbations and in particular to mean shear flows, yet it appears to be relevant as a backbone for turbulent solutions, possibly setting the scale, strength and spacing of elemental plumes.
International Nuclear Information System (INIS)
In this paper a Computer Code COSINAC (Computer Simulation of Natural Convection from Assembly of vertical Cylinders) has been developed to simulate the natural convection heat transfer from an assembly of vertical cylinders of Pakistan Research Reactor-2 (PARR-2), under the steady state reactor operation. The momentum and energy equations in cylindrical co-ordinates, representing the thermal hydraulic behavior of a typical fuel pin in Pakistan Research Reactor-2, have been solved numerically for a two dimensional axisymmetric domain. The temperature and velocity profiles and Nusselt number variations have been studied and results have been presented. The computer code COSINAC has been validated against experimental results carried out in previous studies at different occasions. Average outlet coolant temperature simulated by computer code, at different wall heat fluxes, has been found in good agreement with experimental results
Energy Technology Data Exchange (ETDEWEB)
Basit, M. Abdul [Department of Chemical and Material Engineering, Pakistan Institute of Engineering and Applied Science Nilore, Islamabad 45650 (Pakistan); Rafique, Muhammad [Department of Physics, University of Azad Jammu and Kashmir Muzaffarabad 13100, Azad Kashmir (Pakistan)]. E-mail: rafi_722002@yahoo.com; Chughtai, Imran R. [Department of Chemical and Material Engineering, Pakistan Institute of Engineering and Applied Science Nilore, Islamabad 45650 (Pakistan); Inayat, Mansoor H. [Department of Chemical and Material Engineering, Pakistan Institute of Engineering and Applied Science Nilore, Islamabad 45650 (Pakistan)
2007-01-15
In this paper a Computer Code COSINAC (Computer Simulation of Natural Convection from Assembly of vertical Cylinders) has been developed to simulate the natural convection heat transfer from an assembly of vertical cylinders of Pakistan Research Reactor-2 (PARR-2), under the steady state reactor operation. The momentum and energy equations in cylindrical co-ordinates, representing the thermal hydraulic behavior of a typical fuel pin in Pakistan Research Reactor-2, have been solved numerically for a two dimensional axisymmetric domain. The temperature and velocity profiles and Nusselt number variations have been studied and results have been presented. The computer code COSINAC has been validated against experimental results carried out in previous studies at different occasions. Average outlet coolant temperature simulated by computer code, at different wall heat fluxes, has been found in good agreement with experimental results.
Natural convection and boiling heat transfer of a liquid metal in a magnetic field
International Nuclear Information System (INIS)
A liquid metal is often assumed as a coolant and a breeding material of a Tokamak fusion reactor. However, many problems on the thermo-hydraulics of a liquid metal in a magnetic field are still remained to be studied. In the present report, natural convection and boiling of a liquid metal in a strong magnetic field are studied to examine a fundamental feasibility of a fusion reactor cooled by a liquid metal. In the experimental study of the natural convection, the circulation of a liquid metal was found to be surpressed even by a magnetic field parallel to the gravity. A numerical study has confirmed the conclusion drawn by the experiment. In the study of boiling heat transfer, stable boiling of a liquid metal has been found also in a strong magnetic field. The burnout heat flux hardly affected by the magnetic field. These indicate a fundamental feasibility of the liquid-metal cooling for a Tokamak fusion reactor. (author)
Review of fluid flow and convective heat transfer within rotating disk cavities with impinging jet
Harmand, Souad; Poncet, Sébastien; Shevchuk, Igor V; 10.1016/j.ijthermalsci.2012.11.009
2013-01-01
Fluid flow and convective heat transfer in rotor-stator configurations, which are of great importance in different engineering applications, are treated in details in this review. The review focuses on convective heat transfer in predominantly outward air flow in the rotor-stator geometries with and without impinging jets and incorporates two main parts, namely, experimental/theoretical methodologies and geometries/results. Experimental methodologies include naphthalene sublimation techniques, steady state (thin layer) and transient (thermochromic liquid crystals) thermal measurements, thermocouples and infra-red cameras, hot-wire anemometry, laser Doppler and particle image velocimetry, laser plane and smoke generator. Theoretical approaches incorporate modern CFD computational tools (DNS, LES, RANS etc). Geometries and results part being mentioned starting from simple to complex elucidates cases of a free rotating disk, a single disk in the crossflow, single jets impinging onto stationary and rotating disk,...
International Nuclear Information System (INIS)
Highlights: • Analytical model for thermal analysis of moving porous fins. • Heat transfer from the fin surface due to convection and radiation. • For practical design aspects, optimization analysis was carried out. • Comparative study was made between the solid and porous moving fins. • Porous moving fin has more heat transfer ability than the stationary fin. - Abstract: In the present article, an exercise has been devoted to establish an analytical model for the determination of temperature distribution, fin efficiency and optimum design parameters of a porous moving fin which is losing heat by simultaneous convection and radiation to its surroundings. For the adaptation of this consideration, the governing equation becomes highly nonlinear. An analytical technique called Adomian decomposition method (ADM) is proposed for the solution methodology. The accuracy of the analytic solution is validated by using a numeric scheme called finite difference method. The results indicate that the numerical data and analytical approach are in agreement with each other. As the present study is an analytic, it is extended to the analysis for determination of optimum dimensions of said fin by satisfying either the maximization of rate of heat transfer for a given fin volume or by the minimization of fin volume for a desired heat transfer rate. The study is further extended to the porous fin in stationary condition and it is found that porous fin in moving condition transfers more heat than stationary condition. Investigation has also been made on solid moving fin to compare the outcomes of these parameters
DEFF Research Database (Denmark)
Hosseini, R.; Kolaei, Alireza Rezania
2012-01-01
In this work, the natural convection heat transfer from a long vertical electrically heated cylinder to an adjacent air gap is experimentally studied. The aspect and diameter ratios of the cylinder are 55.56 and 6.33, respectively. The experimental measurements were obtained for a concentric condition and six eccentricities from 0.1 to 0.92 at five different heat fluxes. The surface temperature of the heated rod is measured at different heights, and the Nusselt number is calculated at the temperature measurement locations. A correlation is suggested to determine the Nusselt number based on the variation of the eccentric ratio values. The experimental results show a good agreement with other studies.
Fluid flow and heat convection studies for actively cooled airframes
Mills, A. F.
1993-01-01
This report details progress made on the jet impingement - liquid crystal - digital imaging experiment. With the design phase complete, the experiment is currently in the construction phase. In order to reach this phase two design related issues were resolved. The first issue was to determine NASP leading edge active cooling design parameters. Meetings were arranged with personnel at SAIC International, Torrance, CA in order to obtain recent publications that characterized expected leading edge heat fluxes as well as other details of NASP operating conditions. The information in these publications was used to estimate minimum and maximum jet Reynolds numbers needed to accomplish the required leading edge cooling, and to determine the parameters of the experiment. The details of this analysis are shown in Appendix A. One of the concerns for the NASP design is that of thermal stress due to large surface temperature gradients. Using a series of circular jets to cool the leading edge will cause a non-uniform temperature distribution and potentially large thermal stresses. Therefore it was decided to explore the feasibility of using a slot jet to cool the leading edge. The literature contains many investigations into circular jet heat transfer but few investigations of slot jet heat transfer. The first experiments will be done on circular jets impinging on a fiat plate and results compared to previously published data to establish the accuracy of the method. Subsequent experiments will be slot jets impinging on full scale models of the NASP leading edge. Table 1 shows the range of parameters to be explored. Next a preliminary design of the experiment was done. Previous papers which used a similar experimental technique were studied and elements of those experiments adapted to the jet impingement study. Trade-off studies were conducted to determine which design was the least expensive, easy to construct, and easy to use. Once the final design was settled, vendors were contacted to verify that equipment could be obtained to meet our specifications. Much of the equipment required to complete the construction of the experiment has been ordered or received. The material status list is shown in Appendix B.
Gopal Kumar Deshmukh1 , Rajesh Gupta
2013-01-01
Simulation of combustion phenomenon in a vertical diffuser in the presence of buoyancy-induced airflow and radial fuel inflow is presented. This combustion problem is similar to the combustion of pyrolysis gases in an annular vertical diffuser which is widely used in rural areas for cooking. The analysis of the problem is complicated owing to the coupling amongst natural convection flow, heat transfer and combustion. A simple finite reaction rate model for a single component fuel is presented...
Explicit solutions of a gravity-induced film flow along a convectively heated vertical wall.
Raees, Ammarah; Xu, Hang
2013-01-01
The gravity-driven film flow has been analyzed along a vertical wall subjected to a convective boundary condition. The Boussinesq approximation is applied to simplify the buoyancy term, and similarity transformations are used on the mathematical model of the problem under consideration, to obtain a set of coupled ordinary differential equations. Then the reduced equations are solved explicitly by using homotopy analysis method (HAM). The resulting solutions are investigated for heat transfer effects on velocity and temperature profiles. PMID:24453862
The Soret Effect On Free Convective Unsteady Mhd Flow Over A Vertical Plate With Heat Source
M BHAVANA, D CHENNA KESAVAIAH, A SUDHAKARAIAH
2013-01-01
The work is focused on free convective an unsteady MHD flow in a vertical plate with heat source, thermo diffusion (Soret effect) and the influence of the thermal radiation on hydromagnetic for a viscous fluid past a semi-infinite vertical moving porous plate embedded in a porous medium. The plate moves with constant velocity in the direction of fluid flow while the free stream velocity is assumed to follow the exponentially increasing small perturbation law. The external flow field is assume...
International Nuclear Information System (INIS)
The effects of natural turbulent convection with the interaction of surface radiation in a rectangular enclosure have previously been numerically and theoretically studied. The analyses were carried out over a wide range of enclosure aspect ratios ranging from 0.0625 to 16, different enclosure sizes, with cold wall temperatures ranging from 283 to 373 K, and temperature ratios ranging from 1.02 to 2.61. The work was carried out using four fluids (Argon, Air, Helium and Hydrogen; whose properties vary with temperature). These can be used to calculate Nusselt number for pure natural convection and also to calculate the ratio between convection to radiation heat transfer for both square and rectangular enclosures. This work extends these results by providing an empirical solution for the case of radiation and natural convection in square and rectangular enclosures and also provides a correlation equation to calculate the total Nusselt number for these cases. This method allows the simple calculation of either the total heat transfer rate, given the fluid, the geometry and the temperatures of the hot and cold walls, or via a straightforward iterative technique, the temperature of one wall given the other wall temperature and the total heat transfer. -- Highlights: ? Previous work has non-dimensionalised flow in enclosures with and without radiation. ? This extends the work by enabling a simple iterative technique to work out temperatures for total heat transfer rate. ? The provided solution has a maximum deviation of 7.7%. ? The method works for a variety of enclosures sizes, aspect ratios, temperatures and gases
Improved convective heat transfer and air infiltration models for building thermal simulation
Melo, C.
1985-01-01
10 Intermediate-level'o computer codes are advocated as being the most appropriate for meeting the requirements of dynamic building thermal models. Such codes may be developed via the .4 computer-generalizationA Of analytical solutions and data correlations, which are then verified using higher-level ccoputational procedures and/or experimental measurements. Two intermediate-level ccniputer codes are described: one to model the convective heat exchange at the external fac...
Calculation of Post-Closure Natural Convection Heat and Mass Transfer in Yucca Mountain Drifts
International Nuclear Information System (INIS)
Natural convection heat and mass transfer under post-closure conditions has been calculated for Yucca Mountain drifts using the computational fluid dynamics (CFD) code FLUENT. Calculations have been performed for 300, 1000, 3000, and 10,000 years after repository closure. Effective dispersion coefficients that can be used to calculate mass transfer in the drift have been evaluated as a function of time and boundary temperature tilt
Calculation of Post-Closure Natural Convection Heat and Mass Transfer in Yucca Mountain Drifts
Energy Technology Data Exchange (ETDEWEB)
S. Webb; M. Itamura
2004-03-16
Natural convection heat and mass transfer under post-closure conditions has been calculated for Yucca Mountain drifts using the computational fluid dynamics (CFD) code FLUENT. Calculations have been performed for 300, 1000, 3000, and 10,000 years after repository closure. Effective dispersion coefficients that can be used to calculate mass transfer in the drift have been evaluated as a function of time and boundary temperature tilt.
A wavelet regularization method for an inverse heat conduction problem with convection term
Wei Cheng; Ying-Qi Zhang; Chu-Li Fu
2013-01-01
In this article, we consider an inverse heat conduction problem with convection, which is ill-posed; i.e., the solution does not depend continuously on the given data. A special projection dual least squares method generated by the family of Shannon wavelets is applied to formulate an approximate solution. Also an optimal-order estimate for the error between the approximate solution and exact solution is obtained.
A wavelet regularization method for an inverse heat conduction problem with convection term
Directory of Open Access Journals (Sweden)
Wei Cheng
2013-05-01
Full Text Available In this article, we consider an inverse heat conduction problem with convection, which is ill-posed; i.e., the solution does not depend continuously on the given data. A special projection dual least squares method generated by the family of Shannon wavelets is applied to formulate an approximate solution. Also an optimal-order estimate for the error between the approximate solution and exact solution is obtained.
Effects of Convective Heating on Entropy Generation Rate in a Channel with Permeable Walls
Adetayo Samuel Eegunjobi; Oluwole Daniel Makinde
2013-01-01
This study deals with the combined effects of convective heating and suction/injection on the entropy generation rate in a steady flow of an incompressible viscous fluid through a channel with permeable walls. The model equations for momentum and energy balance are solved numerically using shooting quadrature. Both the velocity and temperature profiles are obtained and utilized to compute the entropy generation number. The effects of the key parameters on the fluid velocity, temperature, entr...
A p-version finite element method for steady incompressible fluid flow and convective heat transfer
Winterscheidt, Daniel L.
1993-01-01
A new p-version finite element formulation for steady, incompressible fluid flow and convective heat transfer problems is presented. The steady-state residual equations are obtained by considering a limiting case of the least-squares formulation for the transient problem. The method circumvents the Babuska-Brezzi condition, permitting the use of equal-order interpolation for velocity and pressure, without requiring the use of arbitrary parameters. Numerical results are presented to demonstrate the accuracy and generality of the method.
Natural convection in an adiabatic vertical channel due to a dissipated heat element
International Nuclear Information System (INIS)
An experimental study was perfomed on natural convection heat transfer to air in a vertical channel due to an isothermal heated element attached in one of the walls of the channel. The heated element dissipates heat due to the Joule effect. To determine the heat transfer coefficient, it is necessary to evaluate the heat transferred to air by natural convenction alone. Hence, the heat lost by the element due to conduction and radiation is evaluated in order to correct the measured heat transfer. The natural-convenction heat transfer coefficient is a function of the following parameters: the temperature difference between the element and the ambient air, the position of the element in the channel, and the channel spacing. An optimal value of the channel spacing, when the heat transfer coefficient attains its maximum value, was observed for each of the temperature difference investigated. These maximum values may be up to 25% higher than the value for the case of infinite spacing. Comparisons are made with results available in the literature for similar configurations, and the values found in this work are higher. (author)
Examination of heat transfer characteristics by natural convection of liquid sodium
International Nuclear Information System (INIS)
The reports on the experiment and analysis of the heat transfer by natural convection of liquid metals, particularly sodium, are very rare. Also in the experiment on liquid metals, large errors are often included. When these data with the possibility of errors are reflected to thermal design, conservative treatment becomes necessary. In this research, by the numerical analysis which can avoid the problem like this in the experiment on liquid sodium, the accuracy of forecasting the heat transfer characteristics was investigated. First, the validity of the Boussinesq approximation which is usually used for the analysis of natural convection was examined. And it was clarified that this is the desirable approximation for liquid sodium as compared with ordinary fluids. Thereafter, the heat transfer coefficient by natural convection of the liquid metals around a single horizontal cylinder was determined, and the accuracy of the experimental values obtained before was investigated. As for the analysis method, the investigation of the basic equations, the basic equations for the numerical analysis and the method of the numerical analysis are described. As the results of calculation, the results of the analysis of air and sodium are reported. (K.I.)
Integral transform solution of natural convection in a square cavity with volumetric heat generation
Scientific Electronic Library Online (English)
C., An; C. B., Vieira; J., Su.
2013-12-01
Full Text Available The generalized integral transform technique (GITT) is employed to obtain a hybrid numerical-analytical solution of natural convection in a cavity with volumetric heat generation. The hybrid nature of this approach allows for the establishment of benchmark results in the solution of non-linear parti [...] al differential equation systems, including the coupled set of heat and fluid flow equations that govern the steady natural convection problem under consideration. Through performing the GITT, the resulting transformed ODE system is then numerically solved by making use of the subroutine DBVPFD from the IMSL Library. Therefore, numerical results under user prescribed accuracy are obtained for different values of Rayleigh numbers, and the convergence behavior of the proposed eigenfunction expansions is illustrated. Critical comparisons against solutions produced by ANSYS CFX 12.0 are then conducted, which demonstrate excellent agreement. Several sets of reference results for natural convection with volumetric heat generation in a bi-dimensional square cavity are also provided for future verification of numerical results obtained by other researchers.
A Magneto-convection Over a Semi -infinite Porous Plate with Heat Generation
Directory of Open Access Journals (Sweden)
T. Raja
2013-01-01
Full Text Available Convective flow through porous media is a branch of research undergoing rapid growth in fluid mechanics and heat transfer. This is quite natural because of its important applications in environmental, geophysical and energy related engineering problems. Prominent applications are the utilization of geothermal energy, the control of pollutant spread in ground water, the design of nuclear reactors, solar power collectors and the heat transfer associated with the deep storage of nuclear waste. The study of heat generation in moving fluids is important in problems dealing with chemical reactions and those concerned with dissociating ?uids. Heat generation effects may alter the temperature distribution and this in turn can affect the particle deposition rate in nuclear reactors, electronic chips and semi conductor wafers. Although exact modeling of internal heat generation is quite difficult, some simple mathematical models can be used to express its general behaviour for most physical situations. The objective of this work is to investigate the effects of internal heat generation on an unsteady two-dimensional magnetohydrodynamic free convection flow of a viscous, incompressible fluid free convection flow past a semi-infinite vertical porous plate embedded in a porous medium, in the presence of variable suction. The equations of continuity, linear momentum and energy, which govern the flow field, are transformed to a system of ordinary differential equations by perturbation technique. The resulting equations are solved analytically to obtain the solutions for the velocity and temperature fields. The behavior of the velocity, temperature, skin-friction and Nusselt number have been discussed for variations in the physical parameters.
Virtual Study of Natural Convection Heat Transfer in an Inclined Square Cavity
Directory of Open Access Journals (Sweden)
C.S. Nor Azwadi
2010-01-01
Full Text Available This study presented numerical prediction of natural convection heat transfer inside an inclined square cavity with perfectly conducting boundary conditions for the top and bottom walls. The modified Navier Stokes equations were solved using finite difference approach with uniform mesh resolution. The inclination angels were varied from 0° to 90° with 20° intervals. The results were presented in terms of streamlines and isotherms plots. The detailed heat transfer mechanism based on the average Nusselt number and inclination angles are presented. The effects of the boundary conditions on the sidewalls on the flow behavior are also demonstrated numerically.
CFD and experimental study of convectional heat transfer in free falling particle curtains
Afshar, Sepideh; Sheehan, Madoc
2013-10-01
The convection heat transfer in particle curtains was studied using a combination of experiments and CFD simulations. The Eulerian-Eulerian approach of CFD has been used to simulate particle curtains. Experiments were conducted with glass beads (290 and 400 ?m in mean diameter) flowing through a 20×150mm slot at different mass flow rates (0.06-0.135kg/s). The centreline temperature of particle curtains was determined experimentally using infrared photography. Effects of mass flow rate and particle size on heat transfer have been investigated.
Heat transfer analysis in the flow of Walters' B fluid with a convective boundary condition
Hayat, T.; Sadia, Asad; Mustafa, M.; Hamed, H. Alsulami
2014-08-01
Radiative heat transfer in the steady two-dimensional flow of Walters' B fluid with a non-uniform heat source/sink is investigated. An incompressible fluid is bounded by a stretching porous surface. The convective boundary condition is used for the thermal boundary layer problem. The relevant equations are first simplified under usual boundary layer assumptions and then transformed into a similar form by suitable transformations. Explicit series solutions of velocity and temperature are derived by the homotopy analysis method (HAM). The dimensionless velocity and temperature gradients at the wall are calculated and discussed.
Aspect ratio dependence of heat transfer and large-scale flow in turbulent convection
Bailon-Cuba, Jorge; Emran, Mohammad S.; Schumacher, Joerg
2010-01-01
The heat transport and corresponding changes in the large-scale circulation (LSC) in turbulent Rayleigh-B\\'{e}nard convection are studied by means of three-dimensional direct numerical simulations as a function of the aspect ratio $\\Gamma$ of a closed cylindrical cell and the Rayleigh number $Ra$. For small and moderate aspect ratios, the global heat transfer law $Nu=A\\times Ra^{\\beta}$ shows a power law dependence of both fit coefficients $A$ and $\\beta$ on the aspect ratio...
Energy Technology Data Exchange (ETDEWEB)
Davidson, J.H.
1998-06-01
The goals of this project are: (1) to develop guidelines for the design and use of thermosyphon side-arm heat exchangers in solar domestic water heating systems, and (2) to establish appropriate modeling and testing criteria for evaluating the performance of systems using this type of heat exchanger. The tasks for the project are as follows: (1) Develop a model of the thermal performance of thermosyphon heat exchangers in solar water heating applications. A test protocol will be developed which minimizes the number of tests required to adequately account for mixed convection effects. The TRNSYS component model will be fully integrated in a system component model and will use data acquired with the specified test protocol. (2) Conduct a fundamental study to establish friction and heat transfer correlations for conditions and geometries typical of thermosyphon heat exchangers in solar systems. Data will be obtained as a function of a buoyancy parameter based on Grashof and Reynolds numbers. The experimental domain will encompass the ranges expected in solar water heating systems.
Enhanced convective heat transfer using graphene dispersed nanofluids
Directory of Open Access Journals (Sweden)
Baby Tessy
2011-01-01
Full Text Available Abstract Nanofluids are having wide area of application in electronic and cooling industry. In the present work, hydrogen exfoliated graphene (HEG dispersed deionized (DI water, and ethylene glycol (EG based nanofluids were developed. Further, thermal conductivity and heat transfer properties of these nanofluids were systematically investigated. HEG was synthesized by exfoliating graphite oxide in H2 atmosphere at 200°C. The nanofluids were prepared by dispersing functionalized HEG (f-HEG in DI water and EG without the use of any surfactant. HEG and f-HEG were characterized by powder X-ray diffractometry, electron microscopy, Raman and FTIR spectroscopy. Thermal and electrical conductivities of f-HEG dispersed DI water and EG based nanofluids were measured for different volume fractions and at different temperatures. A 0.05% volume fraction of f-HEG dispersed DI water based nanofluid shows an enhancement in thermal conductivity of about 16% at 25°C and 75% at 50°C. The enhancement in Nusselts number for these nanofluids is more than that of thermal conductivity.
HEAT TRANSFER BY NATURAL CONVECTION IN TWO VERTICAL AND ONE HORIZONTAL PLATE – AN OVERVIEW
Directory of Open Access Journals (Sweden)
MAHENDRA P NIMKAR
2011-02-01
Full Text Available Natural Convection flow in a vertical channel with internal objects is encountered in several technological applications of particular interest of heat dissipation from electronic circuits, refrigerators, heat exchangers, nuclear reactors fuel elements, dry cooling towers, and home ventilation etc.This study deals with the study of natural convection in horizontal plate with vertical channels. The parameters varied during the experimentation are heat input, aspect ratio (the ratio of gap of horizontal plate with respect to vertical plate and gap of horizontal plate from bottom to top, and horizontal plate with and without Vslot. The present study aims to determine the heat transfer characteristics, temperature distribution along the plate to develop a correlation in the form of Nu = c (Ra n for different values of aspect ratio, for the selection of optimum dimension for design purpose. Further, the influences of aspect ratio on theperformance characteristics of heat transfer will be studied and also, the experimental results will be validated with CFD simulation (FLUENT SOFTWARE.
Energy Technology Data Exchange (ETDEWEB)
Castell, Albert; Sole, Cristian; Medrano, Marc; Roca, Joan; Cabeza, Luisa F. [Departament d' Informatica i Enginyeria Industrial, Universitat de Lleida, Pere de Cabrera s/n, 25001 Lleida (Spain); Garcia, Daniel [Departament Projectes d' Enginyeria, Universitat Politecnica de Catalunya, Colom 11, 08222 Terrassa (Spain)
2008-09-15
To determine the heat transfer coefficient by natural convection for specific geometries, experimental correlations are used. No correlations were found in the literature for the geometries studied in this work. These geometries consisted of a cylindrical module of 88 mm of diameter and 315 mm height with external vertical fins of 310 mm height and 20 and 40 mm length. To determine the heat transfer coefficient by natural convection, experimental work was done. This module, containing PCM (sodium acetate trihydrate), was situated in the middle upper part of a cylindrical water tank of 440 mm of diameter and 450 mm height. The calculated heat transfer coefficient changed by using external fins, as the heat transfer surface was increased. The temperature variation of the PCM and the water are presented as a function of time, and the heat transfer coefficient for different fins is presented as a function of the temperature difference. Experimental correlations were obtained, presenting the Nusselt number as a function of different dimensionless numbers. Different correlations were analysed to find which one fit better to the experimental data. (author)
Optimal Prandtl number for heat transfer in rotating Rayleigh-Benard convection
International Nuclear Information System (INIS)
Numerical data for the heat transfer as a function of the Prandtl (Pr) and Rossby (Ro) numbers in turbulent rotating Rayleigh-Benard convection are presented for Rayleigh number Ra=108. When Ro is fixed, the heat transfer enhancement with respect to the non-rotating value shows a maximum as a function of Pr. This maximum is due to the reduced effect of Ekman pumping when Pr becomes too small or too large. When Pr becomes small, i.e. for large thermal diffusivity, the heat that is carried by the vertical vortices spreads out in the middle of the cell and Ekman pumping thus becomes less effective. For higher Pr the thermal boundary layers (BLs) are thinner than the kinetic BLs and therefore the Ekman vortices do not reach the thermal BL. This means that the fluid that is sucked into the vertical vortices is colder than that for lower Pr, which limits the upwards heat transfer.
Energy Technology Data Exchange (ETDEWEB)
Lee, Shinpyo [Kyonggi Univ., Suwon (Korea, Republic of)
2012-01-15
This paper describes a measuring apparatus that can be used to appraise the effectiveness of nanofluids as new heat transfer enhancing fluids. A couple of apparatuses using fine hot wires as sensors have been proposed for this purpose; however, they have a technical weakness related to the uncertain working conditions of the sensor. The present method used the convective heat transfer coefficient from a hot wire as an indication of the heat transfer effectiveness of the nanofluid, where the temperature of the wire remains constant during the experiment. The operating principle and experimental procedure are explained in detail, and the validity of the system is tested with pure base fluids. The effects of particle concentration, velocity, and temperature on the heat transfer coefficients of the nanofluids are discussed comprehensively using the experimental data for graphite nanolubrication oil.