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1

Forced convection heat transfer in narrow annulus with bilateral heating

International Nuclear Information System (INIS)

Experiments were conducted to study the single-phase forced convection heat transfer with water in vertical narrow annulus which is simultaneously heated from inside and outside by primary water. A comparative analysis on heat transfer performance between bilateral and unilateral heating was carried out. The results indicate that double side heating enhances the heat transfer on inside surface when Reynolds number is higher and reduces the heat transfer on outside surface when Reynolds number is small, compared with single-side heating. For both heating modes, the varying trend of Nusselt number is similar and the total heat transfer rate keeps almost equal in value

2

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)

3

Experimental investigation on forced convection heat transfer through narrow annulus

International Nuclear Information System (INIS)

Experiments have been carried out to investigate the forced convection heat transfer with water in narrow annulus which is heated from inside or outside by primary water. The results show that the narrow channel has a dual enhancement-suppression action on convection heat transfer. The performance of the heat transfer in narrow annulus is very similar to that in micro-channel, but is quite different from that in conventional channel. Heat transferring among laminar, transition and turbulent areas is with no obvious distinction

4

Topological design of heat dissipating structure with forced convective heat transfer

International Nuclear Information System (INIS)

This paper discusses the use of the topology optimization formulation for designing a heat dissipating structure that utilizes forced convective heat transfer. In addition to forced convection, there is also natural convection due to natural buoyancy forces induced by local heating inside fluid. In the present study, the temperature distribution due to forced convection, neglecting buoyancy and viscous dissipation inside fluid, was simulated and optimized. In order to analyze the heat transfer equation with forced convective heat loss and the Navier-Stokes equation, a common sequential computational procedure for this thermo/hydraulic characteristic was implemented. For topology optimization, four material properties were interpolated with respect to spatially defined density design variables: the inverse permeability in the Navier-Stokes equation, the conductivity, density, and the specific heat capacity of the heat transfer equation. From numerical examples, it was found that the balance between the conduction and convection of fluid is of central importance to the design of heat dissipating structures

5

Mechanistic Multidimensional Modeling of Forced Convection Boiling Heat Transfer

International Nuclear Information System (INIS)

Due to the importance of boiling heat transfer in general, and boiling crisis in particular, for the analysis of operation and safety of both nuclear reactors and conventional thermal power systems, extensive efforts have been made in the past to develop a variety of methods and tools to evaluate the boiling heat transfer coefficient and to assess the onset of temperature excursion and critical heat flux (CHF) at various operating conditions of boiling channels. The objective of this paper is to present mathematical modeling concepts behind the development of mechanistic multidimensional models of low-quality forced convection boiling, including the mechanisms leading to temperature excursion and the onset of CHF

6

Heat transfer in forced convection through fins

A novel structure for cooling silicon chips involving passage of water through fins etched in the back of the chips has recently been described in the literature. The properties of silicon and the nature of the structure are such that approximations that allow an analytic solution to the heat transfer problem in the structure can be introduced. Formulas for the fin and channel dimensions that provide optimum cooling under various conditions can then be derived. The results are also presented in graphical form and by means of examples.

Keyes, R. W.

1984-09-01

7

Forced convective post CHF heat transfer and quenching

International Nuclear Information System (INIS)

This paper discusses mechanisms in the post-CHF region which provide understanding and qualitative prediction capability for several current forced convective heat transfer problems. In the area of nuclear reactor safety, the mechanisms are important in the prediction of fuel rod quenches for the reflood phase, blowdown phase, and possibly some operational transients with dryout. Results using the mechanisms to investigate forced convective quenching are presented. Data reduction of quenching experiments is discussed, and the way in which the quenching transient may affect the results of different types of quenching experiments is investigated. This investigation provides an explanation of how minimum wall superheats greater than the homogeneous nucleation temperature result, as well as how these may appear to be either hydrodynamically or thermodynamically controlled. Finally, the results of a parametric study of the effects of the mechanisms upon the LOFT L2-3 hotpin calculation are presented

8

Heat transfer during forced convection condensation inside horizontal tube

Energy Technology Data Exchange (ETDEWEB)

This paper presents the results of an experimental investigation on heat transfer behaviour during forced convection condensation inside a horizontal tube for wavy, semi-annular and annular flows. A qualitative study was made of the effect of various parameters - refrigerant mass flux, vapour quality, condensate film temperature drop and average vapour mass velocity - on average condensing-heat transfer coefficient. Akers-Rosson correlations have been found to predict the heat transfer coefficients within {+-} 25% for the entire range of data. A closer examination of the data revealed that the nature of the relation for the heat transfer coefficient changes from annular and semi-annular flow to wavy flow. Akers-Rosson correlations with changed constant and power have been recommended for the two flow regimes. (author)

Tandon, T.N. [M.M.M. Engineering College, Gorakhpur, Uttar Pradesh (India). Dept. of Mechanical Engineering; Varma, H.K.; Gupta, C.P. [Roorkee Univ., Uttar Pradesh (India). Dept. of Mechanical and Industrial Engineering

1995-03-01

9

In this paper, convective heat transfer coefficient of Indian gooseberry (Emblica officinalis), in three different forms (shreds, slices and pieces), under forced convection mode has been determined. These forms were dried in laboratory drier. Values of constants C and n have been determined using experimental data and regression analysis for calculating values of convective heat transfer coefficient. It was found that the convective heat transfer coefficient varies with form of commodity bei...

Anwar, S. I.; Singh, R. D.

2012-01-01

10

Mechanistic Multidimensional Modeling of Forced Convection Boiling Heat Transfer

Directory of Open Access Journals (Sweden)

Full Text Available Due to the importance of boiling heat transfer in general, and boiling crisis in particular, for the analysis of operation and safety of both nuclear reactors and conventional thermal power systems, extensive efforts have been made in the past to develop a variety of methods and tools to evaluate the boiling heat transfer coefficient and to assess the onset of temperature excursion and critical heat flux (CHF at various operating conditions of boiling channels. The objective of this paper is to present mathematical modeling concepts behind the development of mechanistic multidimensional models of low-quality forced convection boiling, including the mechanisms leading to temperature excursion and the onset of CHF.

Raf M. Podowski

2008-12-01

11

Forced convection heat transfer to air/water vapor mixtures

International Nuclear Information System (INIS)

Heat transfer coefficients were measured using both dry air and air/water vapor mixtures in the same forced convection cooling test rig (jet array impingement configurations) with mass ratios of water vapor to air up to 0.23. The primary objective was to verify by direct experiment that selected existing methods for evaluation of viscosity and thermal conductivity of air/water vapor mixtures could be used with confidence to predict heat transfer coefficients for such mixtures using as a basis heat transfer data for dry air only. The property evaluation methods deemed most appropriate require as a basis a measured property value at one mixture composition in addition to the property values for the pure components. 20 references

12

Comparative analysis of heat transfer correlations for forced convection boiling

International Nuclear Information System (INIS)

A critical survey was conducted of the most relevant correlations of boiling heat transfer in forced convection flow. Most of the investigations carried out on partial nucleate boiling and fully developed nucleate boiling have led to the formulation of correlations that are not able to cover a wide range of operating conditions, due to the empirical approach of the problem. A comparative analysis is therefore required in order to delineate the relative accuracy of the proposed correlations, on the basis of the experimental data presently available. The survey performed allows the evaluation of the accuracy of the different calculating procedure; the results obtained, moreover, indicate the most reliable heat transfer correlations for the different operating conditions investigated. This survey was developed for five pressure range (up to 180bar) and for both saturation and subcooled boiling condition

13

Energy Technology Data Exchange (ETDEWEB)

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.)

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

14

Transient forces convection heat transfer to helium during a step in heat flux

International Nuclear Information System (INIS)

Transient forced convection heat transfer coefficients for both subcritical and supercritical helium in a rectangular flow channel heated on one side were measured during the application of a step in heat flow. Zero flow data were also obtained. The heater surface which served simultaneously as a thermometer was a fast response carbon film. Operating conditions covered the following range: Pressure, 1.0 x 105 PAia (1 bar) to 1.0 x 106 Pa (10 bar); Temperature, 4 K-10 K; Heat flux, 0.1 W/cm2-10 W/cm2; Reynolds number, 0-8 x 105. The experimental data and a predictive correlation are presented

15

Combined forced and natural convective heat transfer from a vertical circular cylinder

International Nuclear Information System (INIS)

An experimental study is conducted to investigate the characteristics of the combined forced and natural convection heat transfer on a vertical circular cylinder placed in a cross flow. Local heat transfer coefficients are measured precisely in the Reynolds number range from 760 to 3300 and the range of the modified Rayleigh number from 5.0 x 109 to 4.0 x 1014. The local heat transfer coefficients under combined convection increase both on the front and rear sides of the cylinder with an increase in heat flux. The reason is that the flow around the cylinder is sped by buoyancy force. The average Nusselt numbers for the combined convection become higher than those estimated for both of forced and natural convection. (author)

16

International Nuclear Information System (INIS)

In this paper, the case in which subcooled liquid enters stably into a vertical, concentric double-walled tube heated uniformly is treated. At present, the no-dimensional arrangement of critical heat flux has not been carried out in the true sense. Therefore, the result of determining the no-dimensionally arranged equations for critical heat flux is reported in this paper, extending the same analyzing method as that in case of forced convection boiling in a single circular tube to a double-walled tube. The equivalent diameter of any channel section for heating, the critical heat flux when only an internal tube is heated, the critical heat flux when only an outer tube is heated, and the critical heat flux when both tubes are heated simultaneously were determined. As for the critical heat flux of a double-walled tube, only the internal tube of which is heated, the existing experimental data were analyzed, and the no-dimensionally arranged equations for the critical heat flux under the condition of zero inlet subcooling enthalpy, the boundary curves between respective characteristic domains, and the no-dimensionally arranged equations for the effect of inlet subcooling were derived. In case of heating an outer tube only, the equations for the case of a single circular tube can be used by using the equivalent diameter for heating. (Kako, I.)

17

Transient forces convection heat transfer to helium during a step in heat flux

Energy Technology Data Exchange (ETDEWEB)

Transient forced convection heat transfer coefficients for both subcritical and supercritical helium in a rectangular flow channel heated on one side were measured during the application of a step in heat flow. Zero flow data were also obtained. The heater surface which served simultaneously as a thermometer was a fast response carbon film. Operating conditions covered the following range: Pressure, 1.0 x 10/sup 5/ PAia (1 bar) to 1.0 x 10/sup 6/ Pa (10 bar); Temperature, 4 K-10 K; Heat flux, 0.1 W/cm/sup 2/-10 W/cm/sup 2/; Reynolds number, 0-8 x 10/sup 5/. The experimental data and a predictive correlation are presented.

Giarratano, P.J.; Steward, W.G.

1983-05-01

18

NUMERICAL ANALYSIS OF FORCED CONVECTIVE HEAT TRANSFER THROUGH HELICAL CHANNELS

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 Dyna...

Reby Roy, Dr K. E.; Shenoy, Rohit N.; Bibin Prasad

2012-01-01

19

NUMERICAL ANALYSIS OF FORCED CONVECTIVE HEAT TRANSFER THROUGH HELICAL CHANNELS

Directory of Open Access Journals (Sweden)

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.

Dr. K. E. Reby Roy

2012-07-01

20

International Nuclear Information System (INIS)

Combined forced and free convective heat transfer characteristics were experimentally investigated for water flowing under about 1 atm in a narrow vertical rectangular (750 mm long, 50 mm wide and 18 mm in gap) channel heated from both sides. Experiments were carried out for both downward and upward forced convective flows for Reynolds number Re of 4 x 101 ? 5 x 105 and Grashof number Gr of 5 x 104 ? 4 x 1011, where the distance x from the inlet of the channel is adopted as the characteristic length in Re and Gr. As the results, the following were revealed : (1) Heat transfer coefficients for the combined convective flow region can be expressed in simple forms with a dimensionless parameter Gr/(Re21/8 Pr1/2) which is constituted by Gr, Re and Prandtl number Pr. (2) From the above-mentioned simple expressions it can be identified that the combined convective heat transfer is significant for the condition of 2.5 x 10-4 21/8 Pr2/1) -3. (3) In the present experiment, little significant differences were observed in heat transfer characteristics between the combined convective flow region with upward forced convective flow and that with downward forced convective flow. (author)

21

Study on forced convective heat transfer of non-newtonian nanofluids

This paper is concerned with the forced convective heat transfer of dilute liquid suspensions of nanoparticles (nanofluids) flowing through a straight pipe under laminar conditions. Stable nanofluids are formulated by using the high shear mixing and ultrasonication methods. They are then characterised for their size, surface charge, thermal and rheological properties and tested for their convective heat transfer behaviour. Mathematical modelling is performed to simulate the convective heat transfer of nanofluids using a single phase flow model and considering nanofluids as both Newtonian and non-Newtonian fluid. Both experiments and mathematical modelling show that nanofluids can substantially enhance the convective heat transfer. Analyses of the results suggest that the non-Newtonian character of nanofluids influences the overall enhancement, especially for nanofluids with an obvious non-Newtonian character.

He, Yurong; Men, Yubin; Liu, Xing; Lu, Huilin; Chen, Haisheng; Ding, Yulong

2009-03-01

22

Prediction of forced convection heat transfer to Lead-Bismuth-Eutectic

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...

Thiele, Roman

2013-01-01

23

Convective Heat Transfer Coefficients of the Human Body under Forced Convection from Ceiling

DEFF Research Database (Denmark)

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.

Kurazumi, Yoshihito; Rezgals, Lauris

2014-01-01

24

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

25

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

26

International Nuclear Information System (INIS)

An external forced-, free-, and mixed convection heat transfer, with opposing buoyant and inertial forces from heated vertical cylinders, was experimentally and numerically investigated. A Mach-Zehnder interferometer was used for the experiment, and a discretization method with Patankar's algorithm SIMPLER was used for the numerical analysis. The mixed-convection behavior which appears during the transition from downward forced flow to free convection was simulated by quasi-unsteady state conditions. Forced downward flow is commonly used in gas-cooled nuclear reactors. The results of this study have a bearing on gas-cooled reactor safety. The transition was characterized by five distinctive heat and flow regimes. They are free-, suppressed-, vortex-, unsteady-, and forced-convection regimes. Local and average heat transfer coefficients were determined at forced flow rates from 30 to 130 cm/sec., over a temperature difference of 85 to 130 K with heater diameters of 12.7, 19.05, and 25.4 mm

27

Forced convection heat exchange inside porous sintered metals

International Nuclear Information System (INIS)

Methods and results of investigating heat exchange in the process of liquid flow inside porous sintered metals have been analyzed. It has been shown that experimental data available include extremely conflicting correlations between heat transfer coefficient and Reynolds number, porosity, and relative wall thickness. Scattering of the data can attain one order of magnitude. The volume coefficient of heat transfer inside pores determined in papers does not correspond to its real value in the initial equations of the inner problem of porous cooling. Calculating and experimental method of determining the heat transfer coefficient has been developed and realized on the unit of radiation heating. More accurate experimental data on intraporous heat exchange have been obtained. It has been established that relative wall thickness does not affect the intensity of heat transfer inside pores

28

Mechanistic Multidimensional Modeling of Forced Convection Boiling Heat Transfer

Due to the importance of boiling heat transfer in general, and boiling crisis in particular, for the analysis of operation and safety of both nuclear reactors and conventional thermal power systems, extensive efforts have been made in the past to develop a variety of methods and tools to evaluate the boiling heat transfer coefficient and to assess the onset of temperature excursion and critical heat flux (CHF) at various operating conditions of boiling channels. The objective of this paper is...

Podowski, Raf M.; Podowski, Michael Z.

2009-01-01

29

International Nuclear Information System (INIS)

Forced convective, nonequilibrium post-CHF heat transfer data (766 points) have been obtained at high pressure and low mass flux ranges not previously tested. The data are unique in that the superheated vapor temperature was measured at multiple elevations allowing the direct calculation of vapor generation rates. The data are useful for verification or development of nonequilibrium heat and mass transfer models. Current film boiling heat transfer correlations do not predict the data well. The Dougall-Rohsenow equilibrium correlation is the only one investigated that reasonably predicts the measured wall heat fluxes. Current vapor generation rate models do not adequately predict the measured vapor generation rates

30

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)

31

Forced convective heat transfer across a pin fin micro heat sink

Energy Technology Data Exchange (ETDEWEB)

This paper investigates heat transfer and pressure drop phenomena over a bank of micro pin fins. A simplified expression for the total thermal resistance has been derived, discussed and experimentally validated. Geometrical and thermo-hydraulic parameters affecting the total thermal resistance have been discussed. It has been found that very low thermal resistances are achievable using a pin fin heat sink. The thermal resistance values are comparable with the data obtained in microchannel convective flows. In many cases, the increase in the flow temperature results in a convection thermal resistance, which is considerably smaller than the total thermal resistance. (author)

Peles, Y.; Kosar, A.; Mishra, C.; Chih-Jung Kuo; Schneider, B. [Rensselaer Polytechnic Institute, Troy, NY (United States). Dept. of Mechanical, Aerospace and Nuclear Engineering

2005-08-01

32

International Nuclear Information System (INIS)

For development of new reactor, supercritical water is expected to be used as coolant to improve thermal efficiency. However, the thermal characteristics of supercritical fluid is not revealed completely because its difficulty for experiment. Specific phenomena tend to occur near the pseudo-boiling point which is characterised by temperature corresponding to the saturation point in ordinary fluid. Around this point, the physic properties such as density, specific heat and thermal conductivity are drastically varying. Although there is no difference between gas and liquid phases in supercritical fluids, phenomena similar to boiling (with heat transfer deterioration) can be observed round the pseudo-boiling point. Experiments of heat transfer have been done for supercritical fluid in forced convective condition. However, these experiments were mainly realised inside stainless steel cylinder pipes, for which flow visualisation is difficult. Consequently, this work has been devoted to the development of method allowing the visualisation of supercritical flows. The experiment setup is composed of main loop and test section for the visualisation. Carbon dioxide is used as test fluid. Supercritical carbon dioxide flows upward in rectangular channel and heated by one-side wall to generate forced convection heat transfer. Through window at mid-height of the test section, shadowgraphy was applied to visualize density gradient distribution. The behavior of the density wave in the channel is visualized and examined through the variation of the heat transfer coefficient. (author)

33

In the present study, the fundamental experiments that investigate characteristics of local heat transfer in forced convective boiling on vertical flat plate with 2-mm channel height are taken to realize plate type compact evaporator for OTEC or STEC. The experiments are performed with ammonia as the working fluid. The experiments are also carried out with the following test conditions; saturated pressure = 0.7, 0.8, 0.9 MPa, mass flux = 7.5, 10, 15 kg/(m2•s), heat flux = 15, 20, 25 kW/m2 and inlet quality = 0.1 ~ 0.4 [-]. The result shows that the wall superheated temperature of forced convective boiling is lower than that of pool boiling. And the heat transfer coefficient increases with an increase in quality and the decrease in the local heat flux and saturated pressure for prescribed experimental conditions. However, local heat transfer coefficients are not affected by mass fluxes in the prescribed experimental conditions. An empirical correlation that can predict the local heat transfer coefficient on vertical flat plate within experimental conditions is also proposed.

Kim, Jeong-Hun; Arima, Hirofumi; Ikegami, Yasuyuki

34

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 aro...

Mohanraj, M.; Chandrasekar, P.

2009-01-01

35

Forced convection film boiling drag and heat transfer of a wedge

Energy Technology Data Exchange (ETDEWEB)

Laminar forced convection film boiling flow on a wedge is analyzed considering the streamwise pressure gradient imposed on the flow and the streamwise buoyancy force (important because of the large density difference between the vapor and liquid) acting on the vapor film. A two-phase boundary layer model is proposed, and the local similarity concept is applied to obtain an approximate solution of the governing equations. Parametric trends in this analysis show that, for a water-steam system at atmospheric pressure considered within this study, the density difference between the vapor and liquid is large enough. As a result, both the streamwise pressure gradient and the buoyancy force exert strong influence on the vapor flow dynamics. Wall skin friction results display a strong dependency on the streamwise buoyancy force driving the vapor film and the external pressure gradient. Previously observed skin friction bucket'' type phenomena with increased heating of the wedge are possible when the buoyancy force is small or negligible. Adverse streamwise buoyancy force action on the vapor film, which is the case on the lower surface of a horizontally aligned wedge, may cause vapor flow separation. In contrast to wall skin friction dependency, the wall heat-transfer parameter shows a secondary dependence on the streamwise pressure gradient and the buoyancy force. 16 refs., 9 figs., 3 tabs.

Chappidi, P.R.; Pasamehmetoglu, K.O. (Los Alamos National Lab., NM (USA)); Gunnerson, F.S. (University of Central Florida, Orlando, FL (USA). Dept. of Mechanical Engineering)

1990-01-01

36

International Nuclear Information System (INIS)

Combined forced and natural convection in a bundle of vertical cylinders has been studied numerically by use of Riemann geometry. The cylinders heated uniformly are arranged in an equilateral triangular pattern, and three bundle spacings, s/D=1.1, 1.5 and 1.9. Numerical calculations are made of the combined convection of air (Pr=0.7) in a subchannel of the bundle at the ratios of modified Grashof number to Reynolds number Gr*/Re from 1 to 105. The results show that reverse flows appear along the center line in the subchannel with increasing Gr*/Re. Local Nusselt numbers for the thermally fully-developed region are well correlated with Gr*/Re. (author)

37

International Nuclear Information System (INIS)

Experimental studies have been conducted to obtain local fluid velocity measurements within an electrically heated 2 x 6 rod bundle. Conditions have included both steady state and transient flows in forced, free, and combined convection regimes. The tests have presented challenging conditions for the application of conventional, forward scatter laser Doppler anemometry techniques. Severe thermally induced refractive index gradients have been found to limit measurement capabilities, but not to preclude them altogether so long as certain constraints are observed. Rather than rigorous analytical treatments of the complex optical effects involved, the paper is intended to present details of laboratory observations and experimental results

38

Forced convective boiling heat transfer in microtubes at low mass and heat fluxes

Energy Technology Data Exchange (ETDEWEB)

Convective boiling of HCFC123 and FC72 in 0.19, 0.3 and 0.51 mm ID tubes is investigated. The experimental setup as well as the data reduction procedure has carefully been designed, so that the relative uncertainty interval of the measured heat transfer coefficient in microtubes is kept within {+-} 10%. Up to 70 K liquid superheat over the saturation temperature is observed at low heat and mass fluxes. The onset of the superheat is found to be dependent on the mass flux and the boiling number of the refrigerant examined. In the saturated boiling regime, the heat transfer characteristics are much different from those in conventional-size tubes. The heat transfer coefficient is monotonically decreased with increasing the vapor quality, and becomes independent of the mass flux. Most empirical formulas are not in accordance with the present experimental data. Since the prediction using the nucleate boiling term of Kandlikar's empirical correlations coincides with the present results, the convection effect should be minor in microtubes. On the other hand, the pressure loss characteristics are qualitatively in accordance with the conventional correlation formula while quantitatively much lower. These phenomena can be explained by the fact that the annular flow prevails in microtubes. (Author)

Tzuhsiang Yen; Kasagi, Nobuhide; Suzuki, Yuji [Tokyo Univ., Dept. of Mechanical Engineering, Tokyo (Japan)

2003-12-01

39

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)

40

Performance of a forced convection solar drier integrated with gravel as heat storage material

Energy Technology Data Exchange (ETDEWEB)

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.

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

41

International Nuclear Information System (INIS)

Highlights: ? The cooling performance of water and n-heptane is compared during subcooled flow boiling. ? Although n-heptane leaves the heat exchanger warmer it has a lower heat transfer coefficient. ? Flow rate, heat flux and degree of subcooling have direct effect on heat transfer coefficient. ? The predictions of some correlations are evaluated against experimental data. - Abstract: In this research, subcooled flow boiling heat transfer coefficients of pure n-heptane and distilled water at different operating conditions have been experimentally measured and compared. The heat exchanger consisted of vertical annulus which is heated from the inner cylindrical heater with variable heat flux (less than 140 kW/m2). Heat flux is varied so that two different flow regimes from single phase forced convection to nucleate boiling condition are created. Meanwhile, liquid flow rate is changed in the range of 2.5 × 10?5–5.8 × 10?5 m3/s to create laminar up to transition flow regimes. Three subcooling levels including 10, 20 and 30 °C are also considered. Experimental results demonstrated that subcooled flow boiling heat transfer coefficient increases when higher heat flux, higher liquid flow rate and greater subcooling level are applied. Furthermore, influence of the operating conditions on the bubbles generation on the heat transfer surface is also discussed. It is also shown that water is better cooling fluid in comparison with n-heptane

42

Forced convection film boiling heat transfer on a vertical cylinder. 1

International Nuclear Information System (INIS)

Forced convection film boiling heat transfer on a 3 mm diameter vertical cylinder located in the center of the 40 mm inner diameter test channel was measured. Saturated water, and saturated and subcooled R113 were used as the test fluids that flowed upward along the cylinder. Flow velocities ranged from 0 to 3 m/s, pressures from 102 to 490 kPa, and liquid subcoolings for R113 from 0 to 60 K. The heat transfer coefficients are almost independent of flow velocity and of a vertical position on the cylinder for the velocities lower than about 1 m/s (the first range), and they become higher for the velocities higher than 1 m/s (the second range). (author)

43

Evaluating a tobacco-curing oven using a forced-convection heat exchanger USCO — MADR

Directory of Open Access Journals (Sweden)

Full Text Available A traditional oven for curing tobacco leaves was redesigned (based on existing infrastructure; a forced-convection heat exchan- ger 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.

Néstor Enrique Cerquera Peña

2010-05-01

44

International Nuclear Information System (INIS)

Forced convective, postcritical-heat-flux heat transfer experiments with water flowing upward in a vertical tube have been conducted at the Idaho National Engineering Laboratory. Thermodynamic nonequilibrium in the form of superheated vapor temperatures was measured at a maximum of three different axial levels. Steady-state experiments were conduced at pressures of 0.2 to 0.7 MPa, mass fluxes of 12 to 24 kg/m2 x s, heat fluxes of 7.7. to 27.5 kW/m2, and test section inlet qualities of 38 to 64%. Quasi-steady-state (slow moving quench front) experiments were conducted at pressures of 0.4 to 7 MPa, mass fluxes of 12 to 70 kg/m2 x s, heat fluxes of 8 to 225 kW/m2, and test section inlet qualities of -7 to 47%. The multiple probe data and the data taken above 0.4 MPa are new data in parameter ranges not previously obtained. Comparison of the data with current vapor generation models and wall heat transfer models yielded unsatisfactory results. This is attributed to the effects of nonequilibrium, quench front quality, and distance from the quench front, which are factors not included in the current models compared

45

Directory of Open Access Journals (Sweden)

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.

P. MOHAJERI KHAMENEH,

2010-11-01

46

Conceptual Design of Forced Convection Molten Salt Heat Transfer Testing Loop

Energy Technology Data Exchange (ETDEWEB)

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.

Manohar S. Sohal; Piyush Sabharwall; Pattrick Calderoni; Alan K. Wertsching; S. Brandon Grover

2010-09-01

47

Currently, the heat transfer equation used in the rotary combustion engine (RCE) simulation model is taken from piston engine studies. These relations have been empirically developed by the experimental input coming from piston engines whose geometry differs considerably from that of the RCE. The objective of this work was to derive equations to estimate heat transfer coefficients in the combustion chamber of an RCE. This was accomplished by making detailed temperature and pressure measurements in a direct injection stratified charge (DISC) RCE under a range of conditions. For each specific measurement point, the local gas velocity was assumed equal to the local rotor tip speed. Local physical properties of the fluids were then calculated. Two types of correlation equations were derived and are described in this paper. The first correlation expresses the Nusselt number as a function of the Prandtl number, Reynolds number, and characteristic temperature ratio; the second correlation expresses the forced convection heat transfer coefficient as a function of fluid temperature, pressure and velocity.

Lee, Chi M.; Schock, Harold J.

1988-01-01

48

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 differential equations using appropriate similarity variables and solved numerically. The numerical results are obtained by the shooting method using adaptive Runge Kutta method with Broyden’s method in the domain. The numerical results for the temperature field are found to be strongly dependent upon the power-law index, stretching parameter, wall temperature parameter, material parameter of the Sisko fluid and Prandtl number. In addition, the local Nusselt number versus wall temperature parameter is also graphed and tabulated for different values of pertaining parameters. Further, numerical results are validated by comparison with exact solutions as well as previously published results in the literature. PMID:24949738

Munir, Asif; Shahzad, Azeem; Khan, Masood

2014-01-01

49

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 2Dorced 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.

50

International Nuclear Information System (INIS)

In the present study, spray cooling heat transfer was experimentally investigated for the case in which water is sprayed onto the surfaces of micro-fins in forced convection and nucleate boiling regions. The experimental results show that an increase in the droplet flow rate improves heat transfer due to forced convection and nucleate boiling in the both case of smooth surface and surfaces of micro-fins. However, the effect of subcooling for fixed droplet flow rate is very weak. Micro-fins surfaces enhance the spray cooling heat transfer significantly. In the dilute spray region, the micro-fin structure has a significant effect on the spray cooling heat transfer. However, this effect is weak in the dense spray region. A previously determined correlation between the Nusselt number and Reynolds number shows good agreement with the present experimental data for a smooth surface

51

This paper reports the application of the adaptive neuro-fuzzy inference system to model the forced convection heat transfer from v-shaped plate internal surfaces exposed to an air impingement slot jet. The aim of the current study is to consider the effects of the angle of the v-shaped plate (Upphi ) , slot-to-plate spacing ratio (Z/W) and the Reynolds number (Re) variation on the average heat transfer from the v-shaped plate.

Karami, Alimohammad; Yousefi, Tooraj; Ebrahimi, Saeid; Rezaei, Ehsan; Mahmoudinezhad, Sajjad

2013-06-01

52

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.

Bouchenafa, Rachid; Saim, Rachid; Abboudi, Said

2015-01-01

53

Energy Technology Data Exchange (ETDEWEB)

In this study, numerical investigations are conducted for forced convective heat transfer in an annular helicoidal tube under uniform wall temperature condition for laminar flow including developing region. The numerical computations reveal the developments and distributions of heat transfer and flow fields in the annular helicoidal tube when the outer tube wall is heated and the inner tube wall is insulated. The effects of Reynolds number, curvature ratio, and coil pitch on the circumferential average friction factor and Nusselt number at different axial locations, and the non-dimensional entropy generation number of laminar convection in an annular helicoidal tube are investigated. In addition, the differences of flow and heat transfer characteristics between the annular helicoidal tube and circular helicoidal tube are also described.

Wu, Shuang Ying; Chen, Su Jun; Xiao, Lan; Li, You Rong [Chongqing University, Chongqing (China)

2011-06-15

54

International Nuclear Information System (INIS)

In this study, numerical investigations are conducted for forced convective heat transfer in an annular helicoidal tube under uniform wall temperature condition for laminar flow including developing region. The numerical computations reveal the developments and distributions of heat transfer and flow fields in the annular helicoidal tube when the outer tube wall is heated and the inner tube wall is insulated. The effects of Reynolds number, curvature ratio, and coil pitch on the circumferential average friction factor and Nusselt number at different axial locations, and the non-dimensional entropy generation number of laminar convection in an annular helicoidal tube are investigated. In addition, the differences of flow and heat transfer characteristics between the annular helicoidal tube and circular helicoidal tube are also described

55

Directory of Open Access Journals (Sweden)

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.

M. MOHANRAJ

2009-09-01

56

In this paper, copper and aluminum foams with different porosity, number of pores per inch (PPI) and foam core height, are experimentally studied during air forced convection. The experimental measurements permit to understand how each parameter (i.e. porosity, PPI, material, and foam thickness) affects the heat transfer and fluid flow behavior of the metal foams. The paper presents the experimental heat transfer coefficients, permeability and inertia coefficients; moreover, it reports the normalized mean wall temperature as a function of the pumping power per unit of heat transfer area: two meaningful parameters that allow quantitative comparisons of different enhanced surfaces, which can be considered suitable for electronic thermal management.

Mancin, Simone; Zilio, Claudio; Diani, Andrea; Rossetto, Luisa

2012-05-01

57

Design of Test Loops for Forced Convection Heat Transfer Studies at Supercritical State

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).

Balouch, Masih N.

58

Low Reynolds number forced convection steam cooling heat transfer in rod bundles

International Nuclear Information System (INIS)

A series of forced convection steam cooling tests at low Reynolds numbers were conducted in the rod bundle test facility of the FLECHT-SEASET program. The data was reduced using a rod-centered subchannel energy balance to obtain the vapor temperature and by modeling the bundle with the COBRA-IV-I computer code. The comparisons between the COBRA-IV-I vapor temperatures and subchannel energy balance vapor temperatures were quite good. 5 refs

59

Transient convective heat transfer

Directory of Open Access Journals (Sweden)

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 calculation 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.

J. Padet

2005-03-01

60

Transient convective heat transfer

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: English Abstract in english 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.

J., Padet.

2005-03-01

61

International Nuclear Information System (INIS)

Turbulent forced convection heat transfer in a circular tube with a nanofluid consisting of water and 1 vol.% Cu is studied numerically. Two phase mixture model has been implemented for the first time to study such a flow field. A single phase model formulation, which has been used frequently in the past for heat transfer with nanofluids, is also used for comparison with the mixture model. The comparison of calculated results with experimental values shows that the mixture model is more precise than the single phase model. The axial evolution of the flow field and fully developed velocity profiles at different Reynolds numbers are also presented and discussed

62

Energy Technology Data Exchange (ETDEWEB)

Turbulent forced convection heat transfer in a circular tube with a nanofluid consisting of water and 1 vol.% Cu is studied numerically. Two phase mixture model has been implemented for the first time to study such a flow field. A single phase model formulation, which has been used frequently in the past for heat transfer with nanofluids, is also used for comparison with the mixture model. The comparison of calculated results with experimental values shows that the mixture model is more precise than the single phase model. The axial evolution of the flow field and fully developed velocity profiles at different Reynolds numbers are also presented and discussed.

Behzadmehr, A. [Department of Mechanical Engineering, University of Sistan and Baluchestan, Zahedan (Iran, Islamic Republic of); Saffar-Avval, M. [Department of Mechanical Engineering, Amirkabir University of Technology, Hafez Avenue, P.O. Box 15875-4413, Tehran (Iran, Islamic Republic of)]. E-mail: mavval@aut.ac.ir; Galanis, N. [Department de genie mecanique, Universite de Sherbrooke, Que. (Canada)

2007-04-15

63

International Nuclear Information System (INIS)

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

64

Symmetry-breaking bifurcations of central forced and heated convection in a spherical fluid shell

We study convection in a spherical shell under a gravitational force designed to mimic the GeoFlow microgravity experiment, using a combination of time-dependent simulation and path-following methods. With an outer radius which is twice that of the inner radius, the critical modes are spherical harmonics with l=4, leading generically to transcritical bifurcations involving axisymmetric and octahedral branches, in agreement with predictions by Michel, Ihrig & Golubitsky, Chossat, Matthews, and Busse & Riahi. A secondary bifurcation involving the l=5 mode leads to an additional seven-cell branch. All three steady patterns are simultaneously stable for 7 150 18 710, simulations lead to time-dependent states, some periodic and some chaotic. The period varies greatly: some of the orbits belong to different branches and a global bifurcation is suspected of delimiting the lower limit of periodic states.

Tuckerman, L. S.; Feudel, F.; Bergemann, K.; Egbers, C.; Futterer, B.; Gellert, M.; Hollerbach, R.

2010-11-01

65

Effects of rolling on single-phase water forced convective heat transfer characteristics

International Nuclear Information System (INIS)

A series of single-phase forced circulation tests in a vertical tube with rolling motion were performed in order to investigate effects of rolling motion on thermal-hydraulic characteristics. The amplitudes of the rolling motion in the tests were 10 degree, 15 degree and 20 degree. The rolling periods were 7.5 s, 10 s, 15 s and 20 s. The Reynolds number was from 6000 to 15000. Heat transfer in the test tube is bated by the rolling motion. As the test-bed rolling more acutely, the heat transfer coefficient of the test tube becomes smaller when the mass flow rate in the test tube is a constant. The heat transfer coefficient calculated by the formula which is for stable state doesn't fit very well with that from experiments. At last a formula for calculating heat transfer in rolling motion was introduced. (authors)

66

The effect of an external magnetic field on the forced convection heat transfer and pressure drop of water based Fe3O4 nanofluid (ferrofluid) in a miniature heat sink is studied experimentally. The heat sink with the dimensions of 40 mm (L) × 40 mm (W) × 10 mm (H) consists of an array of five circular channels with diameter and length of 4 and 40 mm, respectively. It is heated from the bottom surface with a constant heat flux while the other surfaces are insulated. The heat sink is also influenced by an external magnetic field generated by an electromagnet. The local convective coefficients are measured at various flow rates (200 < Re < 900), magnetic field intensities (B < 1,400 G), and particle volume fractions (? = 0.5, 1, 2 and 3 %). Results show that using ferrofluid results in a maximum of 14 % improvement in heat transfer compared to the pure water, in the absence of magnetic field. This value grows up to 38 % when a magnetic field with the strength of 1,200 G is applied to the ferrofluid. On the other hand, it is observed that the significant heat transfer enhancement due to the magnetic field is always accompanied by a pressure drop penalty. The optimum operating condition is obtained based on the maximum heat transfer enhancement per pressure loss.

Ashjaee, Mehdi; Goharkhah, Mohammad; Khadem, Leila Azizi; Ahmadi, Reza

2014-12-01

67

Forced convection film boiling heat transfer on a vertical cylinder. 2

International Nuclear Information System (INIS)

It was clarified in Part (1) of this paper that the film boiling heat transfer coefficients on a vertical cylinder in forced flow of saturated water, and saturated and subcooled R113 at pressures were almost independent of flow velocity and of a vertical position on the cylinder for the velocities lower than about 1 m/s (the first range), and they become higher for the velocities higher than 1 m/s (the second range). In this paper, a generalized film boiling heat transfer correlation for the first and second ranges including radiation contribution was derived. It was confirmed that the experimental data of film boiling heat transfer coefficients in water and R113 were described by the correlation within ±20% difference. (author)

68

Energy Technology Data Exchange (ETDEWEB)

Highlights: Black-Right-Pointing-Pointer Three different eddy viscosity turbulence models are validated. Black-Right-Pointing-Pointer Two data sets and an extensive sensitivity study are employed for validation. Black-Right-Pointing-Pointer Prediction errors for the velocity and temperature fields are analyzed. Black-Right-Pointing-Pointer 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.

Thiele, Roman, E-mail: romant@kth.se [Nuclear Reactor Technology, School of Engineering Sciences, Royal Institute of Technology, SE-10691 Stockholm (Sweden); Anglart, Henryk, E-mail: henryk@kth.se [Nuclear Reactor Technology, School of Engineering Sciences, Royal Institute of Technology, SE-10691 Stockholm (Sweden)

2013-01-15

69

International Nuclear Information System (INIS)

A forced convective heat transfer correlation is proposed by determine the fractal dimension based on the self-avoiding random walk statistics. Nusselt number measuring the convective heat transfer area is correlated with Reynolds number measuring the line generated by the turbulent eddies. The fractal dimension , ?F = 3 over d+2 is derived from the self-avoiding random walking model. The proposed heat transfer correlation here is Nu = C Re?FPr1/3. The present model is well fitted with the Reynolds analogy between the friction factor and the heat transfer correlation

70

Directory of Open Access Journals (Sweden)

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.

Thamir K. Jassem

2013-05-01

71

Estimating the diffusive heat flux across a stable interface forced ewpage by convective motions

Directory of Open Access Journals (Sweden)

Full Text Available Entrainment at the top of the convectively-driven boundary layer (CBL is revisited using data from a high-resolution large-eddy simulation (LES. In the range of values of the bulk Richardson number *Ri*_{B} studied here (about 15–25, the entrainment process is mainly driven by the scouring of the interfacial layer (IL by convective cells. We estimate the length and time scales associated with these convective cells by computing one-dimensional wavenumber and frequency kinetic energy spectra. Using a Taylor assumption, based upon transport by the convective cells, we show that the frequency and wavenumber spectra follow the Kolmogorov law in the inertial range, with the multiplicative constant being in good agreement with previous measurements in the atmosphere. We next focus on the heat flux at the top of the CBL, , which is parameterized in classical closure models for the entrainment rate *w*_{e} at the interface. We show that can be computed exactly using the method proposed by Winters et al. (1995, from which the values of a turbulent diffusivity across the IL can be inferred. These values are recovered by tracking particles within the IL using a Lagrangian stochastic model coupled with the LES. The relative difference between the Eulerian and Lagrangian values of is found to be lower than 10%. A simple expression of *w*_{e} as a function of is also proposed. Our results are finally used to assess the validity of the classical "first-order'' model for *w*_{e}. We find that, when *Ri*_{B} is varied, the values for *w*_{e} derived from the "first-order'' model with the exact computation of agree to better than 10% with those computed directly from the LES (using its definition. The simple expression we propose appears to provide a reliable estimate of *w*_{e} for the largest values of *Ri*_{B} only.

C. Chemel

2010-04-01

72

Convection heat transfer is an important topic both for industrial applications and fundamental aspects. It combines the complexity of the flow dynamics and of the active or passive scalar transport process. It is part of many university courses such as Mechanical, Aeronautical, Chemical and Biomechanical Engineering. The literature on convective heat transfer is large, but the present manuscript differs in many aspects from the existing ones, particularly from the pedagogical point of view. Each chapter begins with a brief yet complete presentation of the related topic. This is followed by a

Favre-Marinet, Michel

2009-01-01

73

International Nuclear Information System (INIS)

An experimental model was developed for a nuclear waste repository storage room. Data were taken over a Reynolds number range of 6,000 to 180,000, covering both the forced and mixed (combined natural and forced) regimes of convection. Data are presented for several circumferential boundary conditions. Results indicate that the natural convection component is significant. Also presented is a plot of the tunnel air isotherms for Re = 10,000

74

Optimal Heating Strategies for a Convection Oven

In this study classical control theory is applied to a heat conduction model with convective boundary conditions. Optimal heating strategies are obtained through solution of an associated algebraic Riccati equation for a finite horizon linear quadratic regulator (LQR). The large dimensional system models, obtained after a Galerkin approximation of the original heat-conduction equations, describe the dynamics of the nodal temperatures driven by a forced convection boundary condition. The model...

Stigter, J. D.; Scheerlinck, N.; Nicolai, B. M.; Impe, J. F.

2001-01-01

75

International Nuclear Information System (INIS)

An investigation of the thermal hydraulic characteristics in the passive residual heat removal system of the System integrated Modular Advanced ReacTor-P (SMART-P) has been carried out using the MARS code, which is a best estimate system analysis code. The SMART-P is designed to cool the system during accidental conditions by a natural convection. The dominant heat transfer in the steam generator is a boiling mode under a forced convection condition, and it is a single-phase liquid and a boiling heat transfer under a natural convection condition. Most of the heat is removed in the heat exchanger of the passive residual heat removal system by a condensation heat transfer. The passive residual heat removal system can remove the energy from the primary side as long as the heat exchanger is submerged in the refueling water tank. The mass flow is stable under a natural circulation condition though it oscillates periodically with a small amplitude. The parameter study is performed by considering the effects of an effective height between the steam generator and the heat exchanger, a hydraulic resistance, an initial pressure, a non-condensable gas fraction in the compensating tank, and a valve actuation time, which are useful for the design of the passive residual heat removal system. The mass flow in the passive residual heat removal system has been affected by the height between the steam generator and the heat exchanger, and the hydraulic resistance of the loopydraulic resistance of the loop

76

Simulation of MHD CuO-water nanofluid flow and convective heat transfer considering Lorentz forces

Magnetic field effect on CuO-water nanofluid flow and heat transfer in an enclosure which is heated from below is investigated. Lattice Boltzmann method is applied to solve the governing equations. The effective thermal conductivity and viscosity of nanofluid are calculated by KKL (Koo-Kleinstreuer-Li) correlation. In this model effect of Brownian motion on the effective thermal conductivity is considered. Effect of active parameter such as: Hartmann number, heat source length, nanoparticle volume fraction and Rayleigh numbers on the flow and heat transfer characteristics have been examined. The results reveal that the enhancement in heat transfer increases as Hartmann number and heat source length increase but it decreases with increase of Rayleigh number. Also it can be found that effect of Hartmann number and heat source length is more pronounced at high Rayleigh number.

Sheikholeslami, Mohsen; Bandpy, Mofid Gorji; Ellahi, R.; Zeeshan, A.

2014-11-01

77

Convection heat transfer in electronic equipment cooling

To maintain the best possible thermal environment in electronic packages, the engineer must establish the most efficient path for heat transfer from the electronic devices to an external cooling agent. The path is typically subdivided into internal and external components, representing, respectively, heat transfer by conduction through different materials and interfaces separating the devices from the package surface and heat transfer by convection from the surface to the coolant. Depending on the scale and speed of the electronic circuits, as well as on constraints imposed by nonthermal considerations, the coolant may be a gas or a liquid and heat transfer may be by natural, forced, or mixed convection or, in the case of a liquid, by pool or forced convection boiling. In this paper a comprehensive review of convection cooling options is provided.

Incropera, F. P.

1988-11-01

78

Critical heat flux and associated phenomena in forced convective boiling in nuclear systems

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...

Ahmad, Masroor

2012-01-01

79

International Nuclear Information System (INIS)

Investigation of two-phase heat trnsfer in horizontal pipe flow have led to a new generalized correlation for the heat transfer coefficient. The proposed correlation equation is NUTP=3.04 [Pe]0.8 [Bo]0.6 This correlation was tested againts the existing and experimental data obtained on two-pahse heat transfer covering the entire possible flow regimes in a horizontal pipe flow. The correlation produces satisfactory result. (authors). 13 refs, 12 tabs, 2 figs

80

A review is made of some of the experimental data and analyses applicable to convective heat transfer in fully turbulent flow in smooth tubes with liquid metals and viscous Newtonian fluids. An empirical equation is evolved that closely approximates heat-transfer values obtained from selected analyses and experimental data for Prandtl numbers from 0.001 to 1000. The terms included in the equation are Reynolds number, Prandtl number, and an empirical diffusivity ratio between heat and momentum.

vonGlahn, Uwe H.

1960-01-01

81

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)

82

Energy Technology Data Exchange (ETDEWEB)

A model has been developed for the simulation of laminar hydrodynamic and heat transfer characteristics in a horizontal circular tube for a suspension flow with micro-size phase-change material (PCM) particles. The energy equation is formulated by taking into consideration the heat absorption (or release) due to the phase-change process. The heat source or heat generation term in the energy equation is derived from solutions for freezing or melting inside the particles. The supercooling breakdown induced by the stochastic character of the crystallization is taken into account in the modeling with nucleation laws. The solutions were compared to experimental results obtained by different authors. (author)

Kousksou, T.; El Rhafiki, T.; El Omari, K.; Zeraouli, Y.; Le Guer, Y. [Laboratoire de Thermique Energetique et Procedes, Universite de Pau et des Pays de l' Adour (UPPA), Campus Universitaire, 64000 Pau (France)

2010-12-15

83

Directory of Open Access Journals (Sweden)

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.

Sundus Hussein Abd

2012-01-01

84

Heterogeneous nanofluids: natural convection heat transfer enhancement

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 ...

Bennacer Rachid; Oueslati Fakhreddine

2011-01-01

85

International Nuclear Information System (INIS)

Highlights: ? The effect of holes placed on perforated finned heat exchangers on convective heat transfer experimentally investigated. ? Six millimeter-diameter holes were opened on each circular fin on a heating tube in order to increase convective heat transfer. ? These holes were placed on the circular fins in such a way as to follow each other at the same chosen angle. ? The holes created turbulence in a region near the heating tube surface on the bottom of the fin. - Abstract: In this study, the effect of holes placed on perforated finned heat exchangers on convective heat transfer experimentally investigated. Six millimeter-diameter holes were opened on each circular fin on a heating tube in order to increase convective heat transfer. These holes were placed on the circular fins in such a way as to follow each other at the same chosen angle. The holes created turbulence in a region near the heating tube surface on the bottom of the fin. Some experiments were then performed to analyze the effect of this turbulence on heat transfer and pressure drop. These experiments were carried out at six different angular locations in order to determine the best angular location. In addition, a perforated finned heater was compared with an imperforate finned heater to observe the differences. In the cases of the Re above the critical value, Nusselt numbers for the perforated finned positions are 12% higher than the Nusselt numbers for the imperforate state. Moreover, athe imperforate state. Moreover, a correlation has been obtained between the Re and Nu in the Re number above the critical value and the Re below the critical value. Meanwhile, correlations regarding pressure drops in the flow areas have been obtained.

86

Energy Technology Data Exchange (ETDEWEB)

Highlights: {yields} The effect of holes placed on perforated finned heat exchangers on convective heat transfer experimentally investigated. {yields} Six millimeter-diameter holes were opened on each circular fin on a heating tube in order to increase convective heat transfer. {yields} These holes were placed on the circular fins in such a way as to follow each other at the same chosen angle. {yields} The holes created turbulence in a region near the heating tube surface on the bottom of the fin. - Abstract: In this study, the effect of holes placed on perforated finned heat exchangers on convective heat transfer experimentally investigated. Six millimeter-diameter holes were opened on each circular fin on a heating tube in order to increase convective heat transfer. These holes were placed on the circular fins in such a way as to follow each other at the same chosen angle. The holes created turbulence in a region near the heating tube surface on the bottom of the fin. Some experiments were then performed to analyze the effect of this turbulence on heat transfer and pressure drop. These experiments were carried out at six different angular locations in order to determine the best angular location. In addition, a perforated finned heater was compared with an imperforate finned heater to observe the differences. In the cases of the Re above the critical value, Nusselt numbers for the perforated finned positions are 12% higher than the Nusselt numbers for the imperforate state. Moreover, a correlation has been obtained between the Re and Nu in the Re number above the critical value and the Re below the critical value. Meanwhile, correlations regarding pressure drops in the flow areas have been obtained.

Karabacak, Rasim [Department of Mechanical Engineering, Pamukkale University, 20070 Kinikli, Denizli (Turkey); Yakar, Guelay, E-mail: gyakar@pau.edu.tr [Department of Mechanical Engineering, Pamukkale University, 20070 Kinikli, Denizli (Turkey)

2011-08-15

87

Experiments on a forced convection heat transfer at supercritical pressures - 6.32 mm ID tube

International Nuclear Information System (INIS)

The size of a sub-channel of the conceptual SCWR core design studied at KAERI is 6.5 mm. In order to provide heat transfer information in such a narrow sub-channel at supercritical pressure, an experiment was performed with a test section made of Inconel 625 tube of 6.32 mm ID. The test pressures were 7.75 and 8.12 MPa corresponding to 1.05 and 1.1 times the critical pressure of CO2, respectively. The mass flux and heat flux, which were in the range of 285 ? 1200 kg/m2s and 30 ? 170 kW/m2, were changed at a given system pressure. The corresponding Reynolds numbers are 1.8 x 104 ? 7.5 x 104. The effect of mass flux and heat flux was dominant factor in the supercritical pressure heat transfer while the effect of pressure was negligible. The Bishop's correlation predicted the test result most closely and Bae and Kim's recent correlation was the next. The heat transfer deterioration occurred when GR)b/Reb2.7 > 2.0 x 10-5. As soon as the heat transfer was deteriorated, it entered a new regime and did not recover the normal heat transfer nevertheless Grb/Reb2.7 reduced below 2.0 x 10-5. It may mean that the correlation must be developed for the normal and deterioration regime separately

88

An Experimental Study on the Design of Miniature Heat Sinks for Forced Convection Air Cooling

An experimental study is performed on one of the smallest commercially available miniature fans, suitable for cooling portable electronic devices, used in conjunction with both finned and finless heat sinks of equal exterior dimensions. The maximum overall footprint area of the cooling solution is 534 mm2 with a profile height of 5 mm. Previous analysis has shown that due to fan exit angle, flow does not enter the heat sinks parallel to the fins or bounding walls. This results in a nonuniform...

Egan, V.; Stafford, J.; Walsh, P.; Walsh, E.

2009-01-01

89

An experimental study on the performance of miniature heat sinks for forced convection air cooling

In recent years the design of portable electronic devices must incorporate thermal analyses to ensure the device can be adequately cooled to acceptable temperatures. Consumer demand for smaller, more powerful devices has lead to an increase in the heat required to be dissipated and a reduction in the surface area both of which result in an increased heat flux. In this paper, an experimental study is performed on one of the smallest commercially available miniature fans, suitable for cooling p...

Egan, V.; Stafford, J.; Walsh, P.; Walsh, E.; Grimes, R.

2008-01-01

90

.Convective heat transfer intensification

International Nuclear Information System (INIS)

The effect of roughness on the process of convective heat transfer has been experimentally studied for intensification of heat transfer of tube fuel element wigh gas flow. Experiments were carried out at air flow in a ring channel formed by a ground case with interior diameter of 19.5 mm and a heated insertion 10 mm in diameter with artificially made roughness. The high of roughness juts is h=0.03; 0.05; 0.075; 0.1 mm; relative roughness pitch constitutes s/h=7; 10; 13; 16; 20 and 35; relative jut width constitutes f/h=0.8; 2.0 and 2.1. The Reynolds numbers calculated using the total flow changed in the course of the experiment from 25x103 up to 650x103; growth of air temperature from entry to exit constituted 30-40 K; thermal heat (Tsub(wall)-Tsub(gas)) changed from 60 up to 100 K; thermal factor (Tsub(wall)/Tsub(gas)) changed from 1.2 up to 1.3; the Mach number does not exceed 0.5; heat disbalance does not exceed 0.4%. The results of s/h and f/h, h and Re influence on heat-hydraulic characteristics and heat transfer intensity are presented. It has been found that the most effective of them are h=0.1 mm and f/h=0.8

91

Forced convective heat transfer in vertical air-water bubble flow

International Nuclear Information System (INIS)

The dependence of heat transfer on the cross-sectional distribution of void fractions in a diabatic two-phase bubble flow has been explored experimentally. A vertical tube of 16.9 mm ID was employed in the investigation. Two different types of the mixer were installed to realize the flows with different distributions of void fractions even at the same flow rates of both phases. It was found that these two types of flows present different heat transfer coefficients due to the difference in the void fraction distributions. Increasing the void fraction near the tube wall causes an increase in heat transfer coefficient. The effect of bubble agitation on the thermal diffusivity in the radial direction is discussed based on the distributions of both void fractions and liquid temperatures. (author)

92

International Nuclear Information System (INIS)

The formation of oxide scales on the structural components in a liquid metal system is considered as a viable measure in limiting the dissolution rates in the hot parts. A simple method has been devised to calculate heat and mass transfer in such systems. The method is based on the use of heat and mass transfer coefficients which determine the heat and mass flux from the wall into the fluid. These coefficients depend on characteristic thermo-hydraulic numbers like the Nusselt number and the Sherwood number. This is supplemented by the application of the mass and energy conservation laws to calculate the conditions in the bulk of the fluid. The dissolution and precipitation rates are then coupled to the oxidation kinetics of the structural components in order to calculate the evolution of the oxide scale thickness and the dimensional changes of the channel walls

93

Experimental investigation of forced convective heat transfer in rectangular micro-channels

Directory of Open Access Journals (Sweden)

Full Text Available This paper investigates the experimental program on the study of heat transfer characteristics in micro-channels. The two test sections used are of 47 and 50 micro-channels in rectangular cross-section of equivalent diameters 387 and 327 µm respectively. Each channel of length 192 mm is fabricated on a 304 stainless steel substrate (230 mm x 160 mm x 1.6 mm by photo chemical etching process. Covering the top with another plate of 0.5 mm thickness forms the channels by vacuum brazing. Experiments cover laminar region using the fluids ethanol, methanol and an ethanol-methanol mixture. The heat transfer coefficient is evaluated based on the heat carried away by the coolant and an average wall to mean fluid temperature difference. The Nusselt number is correlated through empirical correlations involving Reynolds number and Prandtl number with length parameter, the hydraulic diameter.

R. Kalaivanan

2010-05-01

94

Sensitivity studies of heat transfer: forced convection across a cylindrical pipe and duct flow

We consider two common heat transfer processes and perform a through sensitivity study of the variables involved. We derive and discuss analytical formulas for the heat transfer coefficient in function of film velocity, air temperature and pipe diameter. The according plots relate to a qualitative analysis of the multi-variable function $h$, according to functional optimization. For each process, we provide with graphs and tables of the parameters of interest, such as the Reynolds number. This method of study and the specific values can constitute a useful reference for didactic purposes.

Ferrantelli, Andrea; Viljanen, Martti

2013-01-01

95

International Nuclear Information System (INIS)

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

96

Two-phase pressure drop and heat transfer of sodium at forced convection

International Nuclear Information System (INIS)

Experiments with sodium for the two-phase pressure drop in vertical tubes with upward flow (internal diameters 6 and 9 mm) performed at the Joint Research Centre (JRC) of the European Communities in Ispra, Italy, and at the Nuclear Research Centre in Karlsruhe (KfK) were evaluated and analysed. Furthermore, experiments for the single-phase and two-phase heat transfer in the grid spaced twelve-rod bundle (pd/d =1.3, rod diameter 8 mm) with flow in axial direction performed at the JRC were evaluated and analysed. The pressure drop measurements were carried out at moderate to high mass flow rates (30 to 4500 kg/(m2s)) and at moderate pressures (50 to 300 kPa, density ratio ?f/?g = 950 to 5400). The measurements for the single-phase heat transfer at high heat fluxes (0.16 to 1.6 MW/m2) were carried out in the Reynolds number region (3100 2s)) and at high heat fluxes (0.46 to 1.6 MW/m2) within the temperature range from 870 to 9700C. For the subsequent calculation of the experiments relating to the two-phase pressure drop a computer program was developed, which is based on the so-called slip model. It requires a friction pressure loss correlation and a slip correlation. The tested correlations were not suitable for describing the experimental measurements. Accordingly, simplified equations of momentum were used to develop a new slip correlation for the case of annular flow together with the annular-mist flow, the most important two-phase flow regimes for sodium in the measurement range. After the inception of the entrainment - transition from the annular flow to the annular-mist flow - an even larger fraction of liquid enters the vapour core in the form of droplets, as the vapour quality increases. An equation was formulated for the slip in this region and adapted to the experiments via coefficients. (orig./GL)

97

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

98

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 ...

Sundus Hussein Abd

2012-01-01

99

International Nuclear Information System (INIS)

A model describing the thermal behaviour of a slurry of phase change material flow in a circular duct is presented. Reactors connected in series are considered for the representation of the circular duct with constant wall temperature. A phenomenological equation is formulated to take account of the heat generation due to phase change in the particles. Results of the simulation present a plateau of temperature along the longitudinal direction, characteristic of the phase change. The effect of different parameters such as the Reynolds number, the weight fraction and the temperature of the cold spring on the length of the plateau is analysed. A correlation resulting from numerical results is proposed for use in the determination of the characteristics of the exchanger for a phase change material slurry

100

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

101

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)

102

International Nuclear Information System (INIS)

Highlights: • We model MHD mixed convection in an inclined lid-driven cavity. • Increasing the Hartmann number leads to increase the heat transfer rate. • Increasing the inclination angle leads to the increase of the heat transfer rate. • Nusselt number at the left wall, for forced convection case, increases as the amplitude ratio increases. - Abstract: A numerical study of laminar magnetohydrodynamic mixed convection in an inclined lid-driven square cavity with opposing temperature gradients is presented. The vertical sidewalls are assumed to have non-uniform temperature variation while the top and bottom walls are kept insulated with the top surface moving at a constant speed. The transport equations are given in terms of the stream functions-vorticity formulation and are non-dimensionalized and then solved numerically by an accurate finite-volume method. The computation is carried out for wide ranges of the inclination angle (0 ? ? ? ?/2), the Richardson number (0.01 ? Ri ? 100), the Hartmann number (0 ? Ha ? 100), the amplitude ratio (0 ? ? ? 1) and the phase deviation (0 ? ? ? ?). The results indicate that the rate of heat transfer along the heated walls is enhanced on increasing either Hartmann number or inclination angle. Average Nusselt number is also, increased with increasing of the amplitude ratio for all values of the phase deviation. The non-uniform heating on both walls provides higher heat transfer rate than non-uniform

103

A study has been carried out on MHD boundary layer forced convection flow along a shrinking surface with variable heat flux in the presence of heat source. The flow is generated due to linear shrinking of the sheet and is influenced by uniform transverse magnetic field. The basic boundary layer momentum and heat transfer equations, which are nonlinear partial differential equations, are converted into nonlinear ordinary differential equations by means of similarity transformation. Numerical s...

Anjali Devi, S. P.; Wilfred Samuel Raj, J.

2014-01-01

104

Convective Flow under Rotating Force.

Directory of Open Access Journals (Sweden)

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.

Meenu Hooda

2013-09-01

105

Heat distribution by natural convection

Energy Technology Data Exchange (ETDEWEB)

Natural convection can provide adequate heat distribution in many situations that arise in buildings. This is appropriate, for example, in passive solar buildings where some rooms tend to be more strongly solar heated than others. Natural convection can also be used to reduce the number of auxiliary heating units required in a building. Natural airflow and heat transport through doorways and other internal building apertures are predictable and can be accounted for in the design. The nature of natural convection is described, and a design chart is presented appropriate to a simple, single-doorway situation. Experimental results are summarized based on the monitoring of 15 passive solar buildings which employ a wide variety of geometrical configurations including natural convective loops.

Balcomb, J.D.

1985-01-01

106

Laminar Mixed Convection Heat Transfer Correlation for Horizontal Pipes

Energy Technology Data Exchange (ETDEWEB)

This study aimed at producing experimental results and developing a new heat transfer correlation based upon a semi-empirical buoyancy coefficient. Mixed convection mass transfers inside horizontal pipe were investigated for the pipe of various length-to-diameters with varying Re. Forced convection correlation was developed using a very short cathode. With the length of cathode increase and Re decrease, the heat transfer rates were enhanced and becomes higher than that of forced convection. An empirical buoyancy coefficient was derived from correlation of natural convection and forced convection with the addition of L/D. And the heat transfer correlation for laminar mixed convection was developed using the buoyancy coefficient, it describes not only current results, but also results of other studies. Mixed convection occurs when the driving forces of both forced and natural convections are of comparable magnitude (Gr/Re{sup 2}?1). It is classical problem but is still an active area of research for various thermal applications such as flat plate solar collectors, nuclear reactors and heat exchangers. The effect of buoyancy on heat transfer in a forced flow is varied by the direction of the buoyancy force. In a horizontal pipe the direction of the forced and buoyancy forces are perpendicular. The studies on the mixed convections of the horizontal pipes were not investigated very much due to the lack of practical uses compared to those of vertical pipes. Even the definitions on the buoyancy coefficient that presents the relative influence of the forced and the natural convections, are different by scholars. And the proposed heat transfer correlations do not agree.

Chae, Myeong Seon; Chung, Bum Jin [Kyung Hee Univ., Yongin (Korea, Republic of)

2013-10-15

107

Performance of thermal adhesives in forced convection

Cooling is critical for the life and performance of electronic equipment. In most cases cooling may be achieved by natural convection but forced convection may be necessary for high wattage applications. Use of conventional type heat sinks may not be feasible from the viewpoint of specific applications and the costs involved. In a heat sink, fins can be attached to the well by ultrasonic welding, by soldering, or with a number of industrially available thermal adhesives. In this paper, the author investigates the heat transfer characteristics of several adhesives and compares them with ultrasonic welding and theoretically calculated values. This experiment was conducted in an air flow chamber. Heat was generated by using heaters mounted on the well. Thermstrate foil, Uniset A401, and Aremco 571 adhesives were tested along with an ultrasonically welded sample. Ultrasonic welding performed far better than the adhesives and Thermstrate foil. This type of experiment can be adapted for a laboratory exercise in an upper level heat transfer course. It gives students an exposure to industrial applications that help them appreciate the importance of the course material.

Kundu, Nikhil K.

1993-01-01

108

Forced convective heat transfer from a vertical circular tube conveying deionized (DI) water or very dilute Ag-DI water nanofluids (less than 0.02% volume fraction) in a cross flow of air has been investigated experimentally. Some experiments have been performed in a wind tunnel and heat transfer characteristics such as thermal conductance, effectiveness, and external Nusselt number has been measured at different air speeds, liquid flow rates, and nanoparticle concentrations. The cross flow of air over the tube and the liquid flow in the tube were turbulent in all cases. The experimental results have been compared and it has been found that suspending Ag nanoparticles in the base fluid increases thermal conductance, external Nusselt number, and effectiveness. Furthermore, by increasing the external Reynolds number, the external Nusselt number, effectiveness, and thermal conductance increase. Also, by increasing internal Reynolds number, the thermal conductance and external Nusselt number enhance while the effectiveness decreases.

Mohammadian, Shahabeddin Keshavarz; Layeghi, Mohammad; Hemmati, Mansor

2013-03-01

109

Convection driven by internal heating

Two-dimensional direct numerical simulations are conducted for convection sustained by uniform internal heating in a horizontal fluid layer. Top and bottom boundary temperatures are fixed and equal. Prandtl numbers range from 0.01 to 100, and Rayleigh numbers (R) are up to 5x10^5 times the critical R at the onset of convection. The asymmetry between upward and downward heat fluxes is non-monotonic in R. In a broad high-R regime, dimensionless mean temperature scales as R^{-1...

Goluskin, David; Spiegel, Edward A.

2012-01-01

110

Transient convective heat transfer

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 calculation 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 ...

Padet, J.

2005-01-01

111

International Nuclear Information System (INIS)

The flow and heat transfer characteristics of combined forced convection and radiation in the entrance region of internally finned tubes are investigated numerically in this paper. The uniform flow is considered for an inlet flow condition. A three dimensional parabolic problem is solved by a marching-type procedure envolving a series of two dimensional elliptic problems in the cross-stream plane. The SIMPLER-algorithm and Raithby's pressure-velocity coupling method are employed to analyze the flow and heat transfer characteristics. For the calculation of radiative heat transfer, the P1-approximation and the Weighted Sum of Gray Gases Method (WSGGM) are used. The effects of fin height, number of fins, optical thickness, reference temperature, and Planck number on the flow and heat transfer characteristics are examined. It was found that the effect of fin-height on the heat transfer characteristic is more dominant than that of number of fins. The present results show that the optimal non-dimensional fin height and number of fins are 0.4 and 16, respectively

112

Energy Technology Data Exchange (ETDEWEB)

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 {approx_equal} 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) [French] Dans la premiere partie, on a etudie les echanges thermiques en regime de convection libre et d'ebullition nucleee (jusqu'au flux critique de l'ebullition nucleee) entre un fil de platine (diametre 0,15 mm) chauffe electriquement et un bain de liquide (neon, deuterium, ou hydrogene) bouillant a la pression atmospherique. On a poursuivi ces mesures dans le neon jusqu'a une pression de 23 bars (Pc {approx_equal} 26,8 b). On a mesure en outre les coefficients d'echanges thermiques en regime de calefaction dans le neon liquide a pression atmospherique. Tous ces resultats sont compares a diverses correlations existantes. La deuxieme partie est axee sur la mesure des flux de chaleur critiques limitant les echanges thermiques avec faible ecart de temperatures entre la paroi et le neon liquide s'ecoulant dans un tube (diam. 3 x 3,5 mm) chauffe par effet joule sur 30 cm de longueur. On a etudie l'influence de la stabilite de l'ecoulement de la nature du courant electrique de chauffage, de la pression, du debit et du sous-refroidissement du liquide. On conclut en soulignant la ressemblance des caracteristiques d'echanges thermiques du neon et de l'hydrogene liquides tant en convection libre qu'en convection forcee. (auteur)

Astruc, J.M. [Commissariat a l' Energie Atomique, Grenoble (France). Centre d' Etudes Nucleaires

1967-12-01

113

Convection driven by internal heating

International Nuclear Information System (INIS)

Two-dimensional direct numerical simulations are conducted for convection sustained by uniform internal heating in a horizontal fluid layer. Top and bottom boundary temperatures are fixed and equal. Prandtl numbers range from 0.01 to 100, and Rayleigh numbers (R) are up to 5?105 times the critical R at the onset of convection. The asymmetry between upward and downward heat fluxes is non-monotonic in R. In a broad high-R regime, dimensionless mean temperature scales as R?1/5. We discuss the scaling of mean temperature and heat-flux-asymmetry, which we argue are better diagnostic quantities than the conventionally used top and bottom Nusselt numbers.

114

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)

115

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

116

Suppression of saturated nucleate boiling by forced convective flow

International Nuclear Information System (INIS)

Tube-side forced convective boiling nitrogen and oxygen and thin film shell-side forced convective boiling R-11 data demonstrate a reduction in the heat transfer coefficient associated with nucleate boiling as the two-phase friction pressure drop increases. Techniques proposed in the literature to account for nucleate boiling during forced convective boiling are discussed. The observed suppression of nucleate boiling for the tube-side data is compared against the Chen correlation. Although general agreement is exhibited, supporting the interactive heat transfer mechanism theory, better agreement is obtained by defining a bubble growth region within the thermal boundary layer. The data suggests that the size of the bubble growth region is independent of the friction drop, but is only a function of the physical properties of the boiling liquid. 15 refs

117

Convective heat transfer in planetary dynamo models

The magnetic fields of planets and stars are generated by the motions of electrically conducting fluids within them. These fluid motions are thought to be driven by convective processes, as internal heat is transported outward. The efficiency with which heat is transferred by convection is integral in understanding dynamo processes. Several heat transfer scaling laws have been proposed, but the range of parameter space to which they apply has not been firmly established. Following the plane layer convection study by King et al. (2009), we explore a broad range of buoyancy forcing (Ra) and rotation strength (E-1) to show that heat transfer (Nu) in spherical dynamo simulations occurs in two distinct regimes. We argue that heat transfer scales as Nu ˜ Ra6/5 in the rapidly rotating regime and Nu ˜ Ra2/7 in the weakly rotating regime. The transition between these two regimes is controlled by the competition between the thermal and viscous boundary layers. Boundary layer scaling theory allows us to predict that the transition between the regimes occurs at a transitional Rayleigh number, Rat = E-7/4. Furthermore, boundary layer control of heat transfer is shown to relate to the interior temperature profiles of the models. In the weakly rotating regime, the interior fluid is nearly adiabatic. In the rapidly rotating regime, adverse mean temperature gradients abide, irrespective of the Reynolds number (Re). Extrapolating our results to Earth's core, we estimate that core convection resides in the rapidly rotating regime, with Ra ? 2 × 1024 (Ra/Rat ? 0.02), corresponding to a superadiabatic density variation of ??/?o ? 10-7, which is significantly below the sensitivity of present seismic observations.

King, Eric M.; Soderlund, Krista M.; Christensen, Ulrich R.; Wicht, Johannes; Aurnou, Jonathan M.

2010-06-01

118

Experimental Measurements Of Air Forced Convection Through Copper Foams

This paper aims at investigating the air heat transfer and fluid flow through open-cells copper foam samples with different number of pores per unit of length (PPI) with constant porosity (?=0.93) and foam core height of 40 mm. The experimental heat transfer coefficient and pressure drop measurements were carried out during air forced convection through electrically heated copper foams; the data points are collected in a dedicated test rig. The experimental measurements permit to understand ...

Diani, Andrea; Mancin, Simone; Zilio, Claudio; Rossetto, Luisa

2012-01-01

119

Energy Technology Data Exchange (ETDEWEB)

A comprehensive theoretical and experimental study was carried out on the thermal performance of a pin-fin heat sink. A theoretical model was formulated that has the capability of predicting the influence of various geometrical, thermal, and flow parameters on the effective thermal resistance of the heat sink. An experimental technique was developed for measuring the thermal performance of the heat sink, and the overall convective heat transfer coefficient for the fin bundle. Experiments were carried out, and correlations obtained, for a wide range of parameters for pure natural convection and for combined forced and natural convection. The predictive capability of the theoretical model was verified by comparison with experimental data including the influence of various fin parameters and the existence of an optimum fin spacing. (Author)

Kobus, C.J.; Oshio, T. [Oakland Univ., School of Engineering and Computer Science, Rochester, MI (United States)

2005-03-01

120

Scientific Electronic Library Online (English)

Full Text Available SciELO South Africa | Language: English Abstract in english 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.

Philip O., Olanrewaju; Jacob A., Gbadeyan; Tasawar, Hayat; Awatif A., Hendi.

2011-10-01

121

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 fr 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.

122

Conventional versus upwind convective finite elements, and lumped versus consistent formulations for practical conduction/forced convection analysis are evaluated on the basis of numerical studies, with finite element and finite difference lumped-parameter temperatures compared to closed-form analytical solutions for convection problems. Attention is given to two practical combined conduction and forced convection applications, stressing that the finite element method, showing superior accuracy, is competitive with the finite difference lumped-parameter method. Also considered are the computational time savings offered by the zero capacitance nodes procedure and comparative finite element and finite difference lumped-parameter computer times. The present study has reference to the design of actively cooled engine and airframe structures for hypersonic flight.

Thornton, E. A.; Wieting, A. R.

1979-01-01

123

International Nuclear Information System (INIS)

To clarify thermohydrodynamic processes in a heat exchanger with a low relative length of heated sections (l/d < 50), convective heat transfer in a heated tube is analytically investigated. Equations of heat transfer in the wall and in liquid are considered along with motion equations, i.e. a conjugate problem is solved. Iterations over the joint of temperature fields of wall inner surface and the external current surface is carried out for boundary conditions of the 1st kind from the wall and the second kind from the current. The model adequacy to real physical processes is experimentally established. The information obtained permits to calculate practically any termohydrodynamic characteristic of the model considered. Results of investigations permit to recommend the above mathematical model to investigate analogous problems connected with the calculation of convection heat exchange in porced liquid currents with variable properties under conditions of considerable nonisothermality characteristic of small sections of active heating

124

Reynolds analogy for subcooled surface boiling under forced convection

International Nuclear Information System (INIS)

For the case of subcooled surface boiling under forced convection the analytic expression of analogy between the heat transfer and carry pulse (Reynolds analogy) is derived. It is concluded that the obtained dependence creates the basis for solution of a series of problems of surface boiling physics. On the basis of the performed analysis the method of coordinate calculation of the origin of intensive vapour generation is developed and the formula for calculation of the broken-off-bubble radius under forced convection is derived

125

Within the project "Convection in a Cylinder" (CiC) heat transfer enhancement is studied for the case of two concentric, vertically aligned cylinders. The cylindrical gap is filled with a dielectric liquid, which viscosity is just few times higher than that of water. The inner cylinder is heated and the outer one is cooled. This setup in a gravitational buoyancy field leads to a fluid movement in a single convective cell with hot fluid rising at the inner boundary and cold fluid sinking at the outer boundary. The top and bottom part of the system shows horizontal movement, again in boundary layers. The strengthening of temperature gradient induces instabilities of that convective motion. If we vary the buoyancy force by means of electro-hydrodynamic effects, the patterns of convection differ from those instabilities rising only from variation of the temperature gradient.

Dahley, N.; Futterer, B.; Egbers, C.; Crumeyrolle, O.; Mutabazi, I.

2011-12-01

126

International Nuclear Information System (INIS)

Within the project 'Convection in a Cylinder' (CiC) heat transfer enhancement is studied for the case of two concentric, vertically aligned cylinders. The cylindrical gap is filled with a dielectric liquid, which viscosity is just few times higher than that of water. The inner cylinder is heated and the outer one is cooled. This setup in a gravitational buoyancy field leads to a fluid movement in a single convective cell with hot fluid rising at the inner boundary and cold fluid sinking at the outer boundary. The top and bottom part of the system shows horizontal movement, again in boundary layers. The strengthening of temperature gradient induces instabilities of that convective motion. If we vary the buoyancy force by means of electro-hydrodynamic effects, the patterns of convection differ from those instabilities rising only from variation of the temperature gradient.

127

Modeling of laminar forced convection in spherical- pebble packed beds

International Nuclear Information System (INIS)

There are many parameters that have significant effects on forced convection heat transfer in packed beds, including Reynolds and Prandtl numbers of flow, porosity, pebble geometry, local flow conditions, wall and end effects. In addition, there have been many experimental investigations on forced convection heat transfer in packed beds and each have studied the effect of some of these parameters. Yet, there is not a reliable correlation that includes the effect of main parameters: at the same time, the prediction of precise correct limits for very low and high Reynolds numbers is off hand. In this article a general well-known model of convection heat transfer from isothermal bodies, next to some previous reliable experimental data has been used as a basis for a more comprehensive and accurate correlation to calculate the laminar constant temperature pebble-fluid forced convection heat transfer in a homogeneous saturated bed with spherical pebbles. Finally, for corroboration, the present results are compared with previous works and show a very good agreement for laminar flows at any Prandtl number and all porosities

128

Modeling of laminar forced convection in spherical- pebble packed beds

Energy Technology Data Exchange (ETDEWEB)

There are many parameters that have significant effects on forced convection heat transfer in packed beds, including Reynolds and Prandtl numbers of flow, porosity, pebble geometry, local flow conditions, wall and end effects. In addition, there have been many experimental investigations on forced convection heat transfer in packed beds and each have studied the effect of some of these parameters. Yet, there is not a reliable correlation that includes the effect of main parameters: at the same time, the prediction of precise correct limits for very low and high Reynolds numbers is off hand. In this article a general well-known model of convection heat transfer from isothermal bodies, next to some previous reliable experimental data has been used as a basis for a more comprehensive and accurate correlation to calculate the laminar constant temperature pebble-fluid forced convection heat transfer in a homogeneous saturated bed with spherical pebbles. Finally, for corroboration, the present results are compared with previous works and show a very good agreement for laminar flows at any Prandtl number and all porosities.

Hadad, Yaser; Jafarpur, Khosrow [Shiraz University, Shiraz (Iran, Islamic Republic of)

2012-02-15

129

Spatial forcing of Rayleigh-Benard Convection

We report experimental results on spatial forcing of thermally driven convection in a large aspect ratio fluid layer. In anisotropic systems, such as electroconvection in nematic liquid crystals (Lowe et. al, 1983), wavelength competition caused by spatially periodic forcing was found to generate novel pattern transitions. Thermally driven fluid convection offers a number of interesting possibilities arising from the isotropy of the system and from the presence of spatiotemporally chaotic states with multiple length scales of interest. We will discuss how broken translational symmetry caused by lithographically micromachining a forcing pattern onto the bottom plate of the convection cell affects (i) pattern selection at onset and (ii) the observed bistability of straight rolls and spiral defect chaos. This work is supported by the National Science Foundation grant DMR 0305151.

McCoy, Jonathan; Shaughnessy, Gabe; Hagedorn, Charlie; Bodenschatz, Eberhard; Lipson, Steve

2004-11-01

130

Directory of Open Access Journals (Sweden)

Full Text Available 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 domain is discretized using 2626 elements and that corresponds to a total number of nodes 2842. The channel has a constant heat flux at the two walls and the threedimensional numerical simulations. Numerical solutions were obtained using commercial software Ansys Fluent. The working fluid was air (Pr=0.7. The local Nusselt numbers are obtained, which can be used inestimation of flow and heat transfer performance in a channel In addition, local Nusselt numbers, velocity magnitude and temperature profiles, and pressure profiles are analyzed. Results showed that both fluid flow and temperature flow are influenced significantly with changing entrance velocity. The overall objective of thispaper is to study the flow characteristics and heat transfer analysis inside a channel while increasing entrance velocity.

Rajesh Khatri

2012-03-01

131

The aim of the hydraulic stimulations in the Soultz-sous-Forêts, France, Enhanced Geothermal System (EGS) project was to create, in crystalline rocks, a fractured reservoir 750 m high, 750 m long and 35 m thick interconnecting the injection and production wells. Increasing the permeability in a zone with a high geothermal gradient will trigger free convection, which will interact with the forced flow driven by pumping. A systematic numerical study of the coupling between forced and free conv...

Battaille?, Arnaud; Genthon, Pierre; Rabinowicz, Michel; Fritz, Bertrand

2006-01-01

132

Scientific Electronic Library Online (English)

Full Text Available SciELO Colombia | Language: Spanish Abstract in spanish 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.

Néstor Enrique, Cerquera Peña; Yaneth Liliana, Ruiz Osorio; Eduardo, Pastrana Bonilla.

2010-04-01

133

Energy Technology Data Exchange (ETDEWEB)

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

Sridharan, Kumar; Anderson, Mark; Allen, Todd; Corradini, Michael

2012-01-30

134

International Nuclear Information System (INIS)

Full text of publication follows: The prediction of the Critical Heat Flux (CHF) in a heat flux controlled boiling heat exchanger is important to assess the maximal thermal capability of the system. In the case of a nuclear reactor, CHF margin gain (using improved mixing vane grid design, for instance) can allow power up-rate and enhanced operating flexibility. In general, current nuclear core design procedures use quasi-1D approach to model the coolant thermal-hydraulic conditions within the fuel bundles coupled with fully empirical CHF prediction methods. In addition, several CHF mechanistic models have been developed in the past and coupled with 1D and quasi-1D thermal-hydraulic codes. These mechanistic models have demonstrated reasonable CHF prediction characteristics and, more remarkably, correct parametric trends over wide range of fluid conditions. However, since the phenomena leading to CHF are localized near the heater, models are needed to relate local quantities of interest to area-averaged quantities. As a consequence, large CHF prediction uncertainties may be introduced and 3D fluid characteristics (such as swirling flow) cannot be accounted properly. Therefore, a fully mechanistic approach to CHF prediction is, in general, not possible using the current approach. The development of CHF-enhanced fuel assembly designs requires the use of more advanced 3D coolant properties computations coupled with a CHF mechanistic modeling. In the present work, the commec modeling. In the present work, the commercial CFD code CFX-5 is used to compute 3D coolant conditions in a vertical heated tube with upward flow. Several CHF mechanistic models at low quality available in the literature are coupled with the CFD code by developing adequate models between local coolant properties and local parameters of interest to predict CHF. The prediction performances of these models are assessed using CHF databases available in the open literature and the 1995 CHF look-up table. Since CFD can reasonably capture 3D fluid flow characteristics in fuel rod bundles, this will eventually allow for numerical assessment of CHF performance of newly developed fuel assembly designs for scoping purposes before actual CHF testing. (authors)

135

Directory of Open Access Journals (Sweden)

Full Text Available A study has been carried out on MHD boundary layer forced convection flow along a shrinking surface with variable heat flux in the presence of heat source. The flow is generated due to linear shrinking of the sheet and is influenced by uniform transverse magnetic field. The basic boundary layer momentum and heat transfer equations, which are nonlinear partial differential equations, are converted into nonlinear ordinary differential equations by means of similarity transformation. Numerical solution of the resulting boundary value problem is obtained using Nachtsheim Swigert shooting iteration scheme for the satisfaction of asymptotic boundary conditions along with the Fourth Order Runge Kutta method. The effects of suction parameter, magnetic parameter, Prandtl number, heat source parameter, stretching/shrinking parameter and heat flux parameter on velocity and temperature are shown in several plots. The results are in good agreement with the earlier published works under some limiting cases. Skin friction coefficient and wall temperature are also explored for typical values of the parameter involved in the study.

S. P. Anjali Devi

2014-01-01

136

Comparison of Thermal Comfort by Radiant Heating and Convective Heating

Directory of Open Access Journals (Sweden)

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.

Shigeru Imai

2015-01-01

137

Two regimes of forced turbulent convection

We study experimentally a forced turbulent convection in the Rayleigh-B\\'{e}nard apparatus with an additional source of turbulence produced by the two oscillating grids located nearby the side walls of the chamber. Two different regimes have been observed in the forced turbulent convection. When the frequency of the grid oscillations f>2 Hz, the large-scale circulation (LSC) is totally destroyed, and the destruction of the LSC is accompanied by a strong change of the mean temperature distribution. For the very low frequency the thermal structure inside the LSC is inhomogeneous and anisotropic. The hot thermal plumes accumulate at one side of LSC, and cold plumes concentrate at the opposite side of LSC. The mean temperature gradient in the horizontal direction inside the LSC is significantly larger than in the vertical direction. For the high frequency (f > 10 Hz), LSC has not been observed and the mean temperature gradient in the central flow region in the vertical direction, \

Eidelman, A; Kleeorin, N; Rogachevskii, I; Sapir-Katiraie, I

2009-01-01

138

Experimental study on convective boiling heat transfer in narrow-gap annulus tubes

International Nuclear Information System (INIS)

Since convective boiling or highly subcooled single-phase forced convection in micro-channels is an effective cooling mechanism with a wide range of applications, more experimental and theoretical studies are required to explain and verify the forced convection heat transfer phenomenon in narrow channels. In this experimental study, authors model the convective boiling behavior of water with low latent heat substance Freon 113 (R-113), with the purpose of saving power consumption and visualizing experiments. Both heat transfer and pressure drop characteristics were measured in subcooled and saturated concentric narrow gap forced convection boiling. Data were obtained to qualitatively identify the effects of gap size, pressure, flow rate and wall superheat on boiling regimes and the transition between various regimes. Some significant differences from unconfined forced convection boiling were found, and also, the flow patterns in narrow vertical annulus tubes have been studied quantitatively. (authors)

139

International Nuclear Information System (INIS)

The shell side heat transfer and pressure drop to water flowing counter were experimentally investigated on the basis of the overall heat transfer coefficient. The investigation was intended to identify ways to get higher performance for the cooler in a BWR nuclear power plant. The following three conclusions were reached in the study. (1) From estimated performance of the heat exchanger using the overall heat transfer coefficient based on the outside area of the tube K0, performance of this heat exchanger was enhanced 92% as compared with the measured performance of the conventional segmental heat exchanger. Assuming that the fouling factor is Rf = 8.6 x 10-2 m2K/kW, the former was enhanced about 23%. (2) The tube side pressure drop ?Pt = 20 kPa and the shell side pressure drop ?Ps = 70 kPa were obtained, and they were within the allowable value ?Pa = 80 kPa. The shell side pressure drop of the standard spacer could be decreased 20% as compared with that of the low pressure drop spacer. (3) The enhancement constant of heat transfer of the low pressure drop spacer with thin plate-type supports was about 1.2 times as large as that of the standard spacer. The heat exchanger with the low pressure drop spacer was about 1.6 times more compact than that using the standard spacer. (author)

140

Latent Heating Processes within Tropical Deep Convection

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.

van den Heever, S. C.; Mcgee, C. J.

2013-12-01

141

Studies on forced convection nanofluid flow in circular conduits

Directory of Open Access Journals (Sweden)

Full Text Available An experimental system was developed and used to study the nanofluid flow and heat transfer in circular conduits. Experiments were performed for a variety of nanofluid flow features in the system. Results obtained from the study show that the heat transfer rate for flow of the base fluid is less than that of the nanofluid used in the study. It was also found that the observed relationship between molecular diffusivity of momentum and the molecular diffusivity of thermal energy at the macroscale may not necessarily be the same at the nanoscale. A heat transfer correlation for turbulent forced convection flow in circular pipes was developed from the results in terms of Nusselt number, Reynolds number and Prandtl number. The correlation developed was compared to related correlations in the literature. Important factors that affect nanofluid flow and heat transfer in circular conduits were also determined. This type of study is essential for heat exchanger applications.

Harikrishna Vishwanadula

2012-03-01

142

Double tube heat exchanger with novel enhancement: Part II - single phase convective heat transfer

Energy Technology Data Exchange (ETDEWEB)

The study is conducted to evaluate the heat transfer characteristics of two new and versatile enhancement configurations in a double tube heat exchanger annulus. The novelty is that they are usable in single phase forced convection, evaporation and condensation. Heat transfer coefficients are determined by the Wilson Plot technique in laminar and turbulent flow and correlations are proposed for Nusselt numbers. Comparisons are then made between heat transfer and flow friction. (orig.)

Tiruselvam, R.; Chin, W.M.; Raghavan, Vijay R. [OYL Sdn. Bhd., Research and Application Department, Kuala Lumpur (Malaysia)

2012-08-15

143

International Nuclear Information System (INIS)

Experiments of upflow- and downflow-forced turbulent and laminar convection, natural convection and buoyancy-assisted combined convection of water are performed in uniformly heated, square arrayed, nine-rod bundles having P/D ratios of 1.25, 1.38 and 1.5. The heat transfer data are correlated in the respective convective regimes, where the heated equivalent diameter is used as the characteristic length in all dimensionless quantities and water properties are evaluated at the mean bulk temperature. The forced convection data fall into two distinct regimes: forced turbulent and forced laminar; the Reynolds number at the transition between these regimes is a linear function of P/D. In the forced convection experiments the flow is hydrodynamically developing, but thermally fully developed. The natural convection data are correlated to within ± 10% in terms of Raq and the combined convection data are correlated to within ± 15% by superimposing the Nusselt numbers, raised to the fourth power, of forced laminar and natural convection. For all P/D values, the transition from forced laminar to combined convection occurs at Ri = 2.0. A comparison with triangularly arrayed rod bundles shows that for the same flow area per rod, the rod arrangement negligibly affects Nu in both forced and natural convection regimes. (author)

144

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

2013-01-01

145

Convective Heat Transfer and Infrared Thermography (IRTh

Directory of Open Access Journals (Sweden)

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.

J.M. Buchlin

2010-01-01

146

An assessment on forced convection in metal foams

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.

Mancin, S.; Rossetto, L.

2012-11-01

147

International Nuclear Information System (INIS)

The steady laminar flow and thermal characteristics of a continuously moving vertical sheet of extruded material are studied close to and far downstream from the extrusion slot. The velocity and temperature variations, obtained by a finite volume method, are used to map out the entire forced, mixed and natural convection regimes. The effects of the Prandtl number (Pr) and the buoyancy force parameter (B) on the friction and heat transfer coefficients are investigated. Comparisons with experimental measurements and solutions by others in the pure forced and pure natural convection regions are made. In the mixed convection region, the results are compared with available finite-difference solutions of the boundary layer equations showing excellent agreement. The region close to the extrusion slot is characterized as a non-similar forced-convection dominated region in which NuxRex-1/2 drops sharply with increasing Richardson number (Rix). This is followed by a self-similar forced-convection dominated region in which NuxRex-1/2 levels off with increasing Rix until the buoyancy effect sets in. The existence and extent of the latter region depend upon the value of B. A non-similar mixed convection region where increasing buoyancy effect enhances the heat transfer rate follows. Finally, this region is followed downstream by a self-similar natural-convection dominated region in which NuxRex-1/2 approaches the pure natural convection asymptote at large Rix. Critical values of Rix to distinguish the various convection regimes are determined for different Pr and B

148

Southern Ocean Deep Convection Forcing of North Atlantic Centennial Variability

We present an oceanic teleconnection associated with the Southern Ocean Centennial Variability (SOCV). The SOCV is driven by internal multi-centennial variability of open ocean deep convection in the Atlantic sector of the Southern Ocean, which forces significant variations in the strength of the Atlantic Meridional Overturning Circulation (AMOC) in control simulations with the Kiel Climate Model employing present climate greenhouse gas concentrations. The deep convection is stimulated by a strong built-up of heat at mid-depth and lasts until this heat reservoir is virtually depleted. The heat originates from relatively warm deep water formed in the North Atlantic. The several decades lasting recharge process sets a minimum time between convection events. Stochastically occurring, favorable sea ice conditions as well as coincidental strong surface freshening are further factors influencing the timing of convection onset and shutdown. The deep convection flip-flop has intriguing similarities to the Weddell Polynya observed during the 1970s. The state of Weddell Sea deep convection strongly influences the northward extent of Antarctic Bottom Water (AABW). The retreat of AABW results in an enhanced meridional density gradient that drives an increase in the strength and vertical extent of the North Atlantic Deep Water (NADW) cell. This shows, for instance, as a peak in AMOC strength at 30°N about a century after Weddell Sea deep convection has ceased. The stronger southward flow of NADW is compensated by an expansion of the North Atlantic subpolar gyre and an acceleration of the North Atlantic Current, indicating greater deep water formation. Contractions of the North Atlantic subpolar gyre enable anomalously warm water to penetrate farther to the north, eventually weakening the AMOC and closing a quasi-centennial cycle. In our simulation the SOCV is associated with sea level variations in the order of 10 to 30 cm/century in the North Atlantic and Southern Ocean, which is due to subpolar gyre extent and oceanic heat content changes, respectively. This suggests that internal variability on long time scales cannot be neglected a priori in assessments of 20th and 21st century AMOC and regional sea level change.

Martin, Torge; Park, Wonsun; Latif, Mojib

2014-05-01

149

In the second part of review, we have considered the problems related to momentum and heat transfer in nanofluids. Results on hydrodynamic friction, forced and free convection in the laminar and turbulent flows are analysed; heat transfer at boiling is considered. The available models describing heat transfer intensification and suppression in nanofluids are studied. It is shown that for some problems on convective heat transfer there is a contradiction in data of different authors; possible reasons for this contradiction are analysed.

Terekhov, V. I.; Kalinina, S. V.; Lemanov, V. V.

2010-06-01

150

Radial heat conduction in insulated pipes under external convection is usually dealt with great detail in several heat transfer publications. However, an analysis carried out through the graphical representation of the dimensionless characteristic parameters, will allow a better understanding of the phenomenon. Here the dependence of the convection coefficient on the external radius and external surface temperature, typical of forced and free convection, is taken into account. Assuming a powe...

Branco, J. F.; Pinho, C. T.; Figueiredo, R. A.

2001-01-01

151

A meshless method for modeling convective heat transfer

Energy Technology Data Exchange (ETDEWEB)

A meshless method is used in a projection-based approach to solve the primitive equations for fluid flow with heat transfer. The method is easy to implement in a MATLAB format. Radial basis functions are used to solve two benchmark test cases: natural convection in a square enclosure and flow with forced convection over a backward facing step. The results are compared with two popular and widely used commercial codes: COMSOL, a finite element model, and FLUENT, a finite volume-based model.

Carrington, David B [Los Alamos National Laboratory

2010-01-01

152

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 thod 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

153

Flow pattern at critical condition in forced convection boiling

International Nuclear Information System (INIS)

An experimental investigation on flow pattern at critical condition (burnout) in forced convection boiling was carried out using R-113 as a working fluid. The test section was an internally heated vertical annular channel with a stainless-steel heater tube of 10 mm O. D. and a glass shroud of 22 mm I. D.. The flow pattern was identified by means of photographic observation and statistical nature of void fraction. Measurements were performed at the pressure 0.3 MPa, mass flux of 500 to 2000 kg/m2s, inlet subcooling of 0 to 58 K. (author)

154

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

155

Natural and mixed convection heat transfer from a horizontal heated pipe buried in a porous medium

International Nuclear Information System (INIS)

The natural and mixed convection heat transfer from horizontal heated pipe (diam. 0.646'') embedded in a water saturated a porous medium (MIT Coarse Sand, porosity 30%) for three different depth of burial to pipe diameter ratios from 11.11, 9.69 and 4.55, have been studied experimentally. A galvanized steel box, 2.5 ft long x 1.25 ft wide x 0.5 ft deep, containing the coarse sand bed measuring 1.875 ft x 1.25 ft, was used for the experimental model. The pipe was heated by an electric heating element. In the mixed convection studies, water was allowed to flow in the sand bed and past the heated cylinder in a crosswise direction. The experimental results were correlated by using the normalized parameters - Nusselt number, Rayleigh number and Reynolds number, and the aspect ratios. In the case of crossflow past the pipe, the average Reynolds number at which forced convection becomes dominant over the natural convection mode of heat transfer has been experimentally determined. In order to check the accuracy of the experimental set up and instrumentations, natural convection heat transfer from the heated cylinder to water without the porous medium was conducted and the experimental results compared favorably with those of the previous investigators

156

Numerical simulation of forced convection film boiling on a sphere

International Nuclear Information System (INIS)

A CFD code using improved Volume of Fluid (VOF) method to track liquid-vapor interface is developed to simulate forced convection film boiling on a sphere. The simulation results are compared with the experimental correlation, and the result show that the numerical method could simulate the physical process of forced convection film boiling on a sphere successfully. (authors)

157

Viscous heating, adiabatic heating and energetic consistency in compressible mantle convection

Although it has been suggested that the total viscous heating, Qv, should be exactly balanced by the total adiabatic heating, Qa, for compressible mantle convection, previous numerical studies show a significant imbalance of up to several percent between Qv and Qa for simple isoviscous compressible convection. The cause of this imbalance and its potential effects on more complicated convective systems remain largely unknown. In this study, we present an analysis to show that total viscous heating and adiabatic heating for compressible mantle convection with anelastic liquid approximation (ALA) and the Adams-Williamson equation of state are balanced out at any instant in time, and that the previously reported imbalance between Qv and Qa for numerical models with a truncated anelastic liquid approximation (TALA) is caused by neglecting the effect of the pressure on the buoyancy force. Although we only consider the Adams-Williamson equation of state in our analysis, our method can be used to check the energetic consistency for other forms of equation of state. We formulate numerical models of compressible mantle convection under both TALA and ALA formulations by modifying the Uzawa algorithm in Citcom code. Our numerical results confirm our analysis on the balance between total viscous heating and total adiabatic heating.

Leng, Wei; Zhong, Shijie

2008-05-01

158

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

159

Heat Flux Sensors for Infrared Thermography in Convective Heat Transfer

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-...

Giovanni Maria Carlomagno; Luigi de Luca; Gennaro Cardone; Tommaso Astarita

2014-01-01

160

On the transfer of heat by free convection of air in heated vertical channels

International Nuclear Information System (INIS)

The heat transfer in upward flow in vertical channels is investigated in this work. The object of the study is to determine the dependence of the heat transfer on the channel dimensions and on the characteristic temperature difference in order to determine the connection between free and forced convection for through-flow channels and in order to be able to give generally applicable rules for optimum design from the technically interesting viewpoint. (orig.)

161

Natural convection heat transfer in a uniformly heated horizontal pipe

Natural convection heat transfers inside horizontal pipes were measured. The Rayleigh numbers were varied from 6.8 × 108 to 1.5 × 1012, while the Prandtl number was fixed at 2,094. Based on the analogy concept, a copper sulfate electroplating system was adopted to measure mass transfer rates in place of heat transfer rates. Test results using single-piece electrodes were in good agreement with the work of Sarac and Korkut. The angle-dependent mass transfer rates, measured using piecewise electrodes, were compared with the results of studies on natural convection in concentric annuli, and showed similar trends. The experiments were expanded to the turbulent region, and a transition criterion was proposed. Angle-dependent natural convection heat transfer correlations for the laminar and turbulent regions were derived.

Chae, Myeong-Seon; Chung, Bum-Jin

2014-01-01

162

Thermal Performance Of Convective-Radiative Heat Transfer In Porous Fins

Directory of Open Access Journals (Sweden)

Full Text Available 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 effects of porosity parameter ( , Radiation parameter (? and Temperature-Ratio parameter (µ on temperature distribution for both of the flows have been shown. The results shows that the effects of (? and (µ on temperature distribution in natural convection are based on porosity and in forced convection are uniform, approximately. Also, its shown that both VIM and HPM are capables to solving this nonlinear heat transfer equation.

Majid SHAHBABAEI

2014-11-01

163

Convectively driven shear and decreased heat flux

We report on direct numerical simulations of two-dimensional, horizontally periodic Rayleigh-B\\'enard convection, focusing on its ability to drive large-scale horizontal flow that is vertically sheared. For the Prandtl numbers ($Pr$) between 1 and 10 simulated here, this large-scale shear can be induced by raising the Rayleigh number ($Ra$) sufficiently, and we explore the resulting convection for $Ra$ up to $10^{10}$. When present in our simulations, the sheared mean flow accounts for a large fraction of the total kinetic energy, and this fraction tends towards unity as $Ra\\to\\infty$. The shear helps disperse convective structures, and it reduces vertical heat flux; in parameter regimes where one state with large-scale shear and one without are both stable, the Nusselt number of the state with shear is smaller and grows more slowly with $Ra$. When the large-scale shear is present with $Pr\\lesssim2$, the convection undergoes strong global oscillations on long timescales, and heat transport occurs in bursts. N...

Goluskin, David; Flierl, Glenn R; Spiegel, Edward A

2014-01-01

164

Convective cooling of three discrete heat sources in channel flow

Directory of Open Access Journals (Sweden)

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 adiabatic. 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.

Thiago Antonini Alves

2008-09-01

165

Convective cooling of three discrete heat sources in channel flow

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: English Abstract in english 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.

Thiago Antonini, Alves; Carlos A. C., Altemani.

2008-09-01

166

Combined free convection and forced convection from a flush-mounted uniform heat source on the bottom of a horizontal rectangular enclosure with side openings is studied numerically. The inlet opening allows an externally induced air stream at the ambient temperature to flow through the cavity and exits from another two openings placed top of the both side walls. Two-dimensional forms of Navier-Stokes equations are solved by using control volume based finite element technique. Three typical v...

Sumon Saha; Goutam Saha; Mohammad Ali; Md. Quamrul Islam

2006-01-01

167

Investigation of natural convective heat flow characteristics of heat sink

International Nuclear Information System (INIS)

To ensure proper functioning of electrical and mechanical systems, cooling devices are of great importance. A heat sink in the most common cooling device used in many industries such as the semiconductor, electronic instrument, LED lighting, and automotive industries. To design an optimal heat sink, the required surface area for heat radiation should be calculated based on an accurate expectation of the heat flow rate in the target environment. In this study, the convective heat flow characteristics were numerically investigated for a vertically installed typical heat sink and a horizontally installed one in free convection using ANSYS CFX. Comparative experiments were carried out to reveal the quantitative effect of the installation direction on the cooling performance. Moreover, the result was analyzed using the dimensionless correlation with the Nusselt number and Rayleigh number and compared with well known theories. Finally, it was observed that the cooling performance of the vertically installed heat sink is approximately 10?15% better than that of the one in natural convection

168

Investigation of natural convective heat flow characteristics of heat sink

Energy Technology Data Exchange (ETDEWEB)

To ensure proper functioning of electrical and mechanical systems, cooling devices are of great importance. A heat sink in the most common cooling device used in many industries such as the semiconductor, electronic instrument, LED lighting, and automotive industries. To design an optimal heat sink, the required surface area for heat radiation should be calculated based on an accurate expectation of the heat flow rate in the target environment. In this study, the convective heat flow characteristics were numerically investigated for a vertically installed typical heat sink and a horizontally installed one in free convection using ANSYS CFX. Comparative experiments were carried out to reveal the quantitative effect of the installation direction on the cooling performance. Moreover, the result was analyzed using the dimensionless correlation with the Nusselt number and Rayleigh number and compared with well known theories. Finally, it was observed that the cooling performance of the vertically installed heat sink is approximately 10{approx}15% better than that of the one in natural convection.

Jung, Tae Sung [Inha Technical College, Incheon (Korea, Republic of); Kang, Hwan Kook [Lutima Co. Ltd., Incheon (Korea, Republic of)

2013-01-15

169

nanofluids: the important factors impacting convective heat transfer

A high accuracy, counter flow double pipe heat exchanger system is designed for the measurement of convective heat transfer coefficients with different nanofluids. Both positive and negative enhancement of convective heat transfer of alumina nanofluids are found in the experiments. A modified equation was proposed to explain above phenomena through the physic properties of nanofluids such as thermal conductivity, special heat capacity and viscosity.

Cao, Jianguo; Ding, Yulong; Ma, Caiyun

2014-12-01

170

Development of a mechanistic model for forced convection subcooled boiling

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.

Shaver, Dillon R.

171

Laminar boundary layers in convective heat transport

We study Rayleigh-Benard convection in the high-Rayleigh-number and high-Prandtl-number regime, i.e., we consider a fluid in a container that is exposed to strong heating of the bottom and cooling of the top plate in the absence of inertia effects. While the dynamics in the bulk are characterized by a chaotic convective heat flow, the boundary layers at the horizontal container plates are essentially conducting and thus the fluid is motionless. Consequently, the average temperature exhibits a linear profile in the boundary layers. In this article, we rigorously investigate the average temperature and oscillations in the boundary layer via local bounds on the temperature field. Moreover, we deduce that the temperature profile is indeed essentially linear close to the horizontal container plates. Our results are uniform in the system parameters (e.g. the Rayleigh number) up to logarithmic correction terms. An important tool in our analysis is a new Hardy-type estimate for the convecting velocity field, which ca...

Seis, Christian

2012-01-01

172

Prediction of properties of arcs stabilized by forced convection

International Nuclear Information System (INIS)

A one-dimensional arc model has been used to calculate the central temperature, arc radius, and electric field as a function of axial position for steady-state arcs in forced convection in nozzles. Calculations indicate that the axial distributions of pressure, plasma velocity, and gas velocity are sensitive to the degree of gas heating by the arc of the surrounding gas, significant differences being obtained between assumptions of the gas being isothermal or expanding adiabatically. Despite the fact that turbulence effects are omitted in the model, good agreement is obtained with experimental results of arc temperature, radius, and electric field for a 2000-A arc in nitrogen. Calculations as a function of current indicate regimes of low, intermediate, and high current where arc voltage respectively decreases, is constant, and increases with increase of current. In the latter regime, the increase in voltage is caused by the arc ''clogging'' or restricting the gas flow in the nozzle

173

Mesospheric heating due to intense tropospheric convection

A series of rocket measurements made twice daily at Wallops Island, Va., revealed a rapid heating of the mesosphere on the order of 10 K on days when thunderstorms or squall lines were in the area. This heating is explained as the result of frictional dissipation of vertically propagating internal gravity waves generated by intense tropospheric convection. Ray-tracing theory is used to determine the spectrum of gravity wave groups that actually reach mesospheric heights. This knowledge is used in an equation describing the spectral energy density of a penetrative convective element to calculate the fraction of the total energy initially available to excite those waves that do reach the level of heating. This value, converted into a vertical velocity, is used as the lower boundary condition for a multilayer model used to determine the detailed structure of the vertically propagating waves. The amount of frictional dissipation produced by the waves is calculated from the solutions of the frictionless model by use of a vertically varying eddy viscosity coefficient. The heating produced by the dissipation is then calculated from the thermodynamic equation.

Taylor, L. L.

1979-01-01

174

Free Convection Flow with Constant Heat Sources in a Porous Channel

Directory of Open Access Journals (Sweden)

Full Text Available The effect of constant heat sources on fully developed free convection flow of a viscous fluid in a porous channel oriented in the direction of the body force has been studied when the walls are maintained at constant temperatures. It has been found that both the velocity and temperature depend on the heat source parameter alpha and the dimensional group Q representing the free convection effects.

S. S. Arora

2014-03-01

175

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...

Daund, V. S.; Palande, D. D.

2014-01-01

176

Evaluation of heat removal from vertical cylinder by natural convection

International Nuclear Information System (INIS)

We have studied a cooling system in a maintenance facility to store low level radioactive wastes by using natural draft without forced air ventilation. A fundamental study of natural convection around the vertical cylindrical heaters was carried out experimentally and numerically, and the ambient air was used as a cooling fluid. It was found that the velocity and temperature of air at the center of the flow channel surrounded by 4 heaters was high. And also it was shown that natural heat transfer formula for system design was conservative in the interior temperature of heater due to the increase of flow rate by chimney effect. (author)

177

Combined forced and free laminar convection in verticale rod bundles with longitudinal flow

International Nuclear Information System (INIS)

The heat-transfer problem of combined forced and free convection by fully developed laminar flow in a vertically mounted reactor fuel rod assembly has been solved analytically for both upflow and downflow. Results reveal the strong augmentation of buoyancy effects by radial gradients in the power generation across the assembly. 12 refs

178

Convective heat transfer in a closed two-phase thermosyphon

A numerical analysis of heat transfer processes and hydrodynamics in a two-phase closed thermosyphon in a fairly wide range of variation of governing parameters has been investigated. A mathematical model is formulated based on the laws of mass conservation, momentum and energy in dimensionless variables "stream function - vorticity vector velocity - temperature". The analysis of the modes of forced and mixed convection for different values of Reynolds number and heat flows in the evaporation zone, the possibility of using two-phase thermosyphon for cooling gas turbine blades, when the heat is coming from the turbine blades to the thermosyphon is recycled a secondary refrigerant has been studied with different values of the centrifugal velocity. Nusselet Number, streamlines, velocity, temperature fields and temperature profile has been calculated during the investigation.

Al-Ani, M. A.

2014-08-01

179

Forced convective cooling of a fin in a channel

International Nuclear Information System (INIS)

A numerical study of forced convection in a horizontal parallel-plate channel with a transverse fin located at lower channel wall is investigated. Through the use of a stream function vorticity transformation, solution of the transformed governing equations for the system is obtained using the control-volume method with non-uniform grid. The effects of the Reynolds number, thermal conductivity ratio of fin to fluid and fin profile area on heat transfer rate of the fin are presented. The results indicate that the optimum aspect ratio of a fin corresponding to the fin with maximum heat transfer rate increases with increasing Re but decreases with K for a fixed fin profile area. In addition, the optimum aspect ratio of a fin obtained from this study for smaller fin profile area and Re tends to approach that of analytical solution assuming constant heat-transfer coefficient. Finally, good agreements are found between the numerical predictions of this study and other experimental data.

180

Measurement of the convective heat-transfer coefficient

We propose an experiment for investigating how objects cool down toward the thermal equilibrium with its surrounding through convection. We describe the time dependence of the temperature difference of the cooling object and the environment with an exponential decay function. By measuring the thermal constant tau, we determine the convective heat-transfer coefficient, which is a characteristic constant of the convection system.

Conti, Rosaria; Gallitto, Aurelio Agliolo; Fiordilino, Emilio

2014-01-01

181

International Nuclear Information System (INIS)

Heat transfer coefficients were determined experimentally for two molten-fluoride salts [LiF-BeF2-ThF2-UF4 (72-16-12-0.3 mole %) and NaBF4-NaF (92-8 mole %] proposed as the fuel salt and coolant salt, respectively, for molten-salt breeder reactors. Information was obtained over a wide range of variables, with salt flowing through 12.7-mm-OD (0.5-in.) Hastelloy N tubing in a forced convection loop (FCL-2b). Satisfactory agreement with the empirical Sieder-Tate correlation was obtained in the fully developed turbulent region at Reynolds moduli above 15,000 and with a modified Hausen equation in the extended transition region (Re approx.2100-15,000). Insufficient data were obtained in the laminar region to allow any conclusions to be drawn. These results indicate that the proposed salts behave as normal heat transfer fluids with an extended transition region

182

Energy Technology Data Exchange (ETDEWEB)

Heat transfer coefficients were determined experimentally for two molten-fluoride salts (LiF-BeF/sub 2/-ThF/sub 2/-UF/sub 4/ (72-16-12-0.3 mole %) and NaBF/sub 4/-NaF (92-8 mole %) proposed as the fuel salt and coolant salt, respectively, for molten-salt breeder reactors. Information was obtained over a wide range of variables, with salt flowing through 12.7-mm-OD (0.5-in.) Hastelloy N tubing in a forced convection loop (FCL-2b). Satisfactory agreement with the empirical Sieder-Tate correlation was obtained in the fully developed turbulent region at Reynolds moduli above 15,000 and with a modified Hausen equation in the extended transition region (Re approx.2100-15,000). Insufficient data were obtained in the laminar region to allow any conclusions to be drawn. These results indicate that the proposed salts behave as normal heat transfer fluids with an extended transition region.

Silverman, M. D.; Huntley, W. R.; Robertson, H. E.

1976-10-01

183

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)

184

Natural convection heat transfer in Gambrel roofs

Energy Technology Data Exchange (ETDEWEB)

Buoyancy induced natural convection is investigated with a numerical technique in Gambrel roofs. The geometry adapted to both winter day conditions, the bottom is hot temperature while top is cold, and summer day conditions, bottom is cold and inclined top wall is hot temperature. Governing equations in stream function-vorticity form are solved with finite difference technique and algebraic equations are solved using successive under relaxation (SUR) method. Rayleigh number is taken as parameter which affects the flow and heat transfer. Its value changes between 10{sup 3} and 10{sup 7}. It is found that winterlike boundary conditions are more effective than summerlike boundary conditions on the flow field and heat transfer in the roof. (author)

Varol, Yasin; Koca, Ahmet [Department of Mechanical Education, Technical Education Faculty, Firat University, TR-23119 Elazig (Turkey); Oztop, Hakan F. [Department of Mechanical Engineering, Firat University,TR-23119 Elazig (Turkey)

2007-03-15

185

International Nuclear Information System (INIS)

The numerical solution of flow and heat transfer for steady and transient laminar mixed convection near a vertical uniformly heated surface exposed to a horizontal cross-flow is presented. The transients considered include the simultaneous initiation of flow and heating and the time-varying flow generated by starting or stopping forced convection with the surface heating condition unchanged. The partial differential equations describing the conservation of mass, momentum, and energy were solved in their time-dependent forms by an explicit finite-difference technique. Calculations were performed for fluids with Prandtl numbers of 0.733 and 6.7, nominally air and water. During both steady and transient circumstances, the effects of the horizontal forced flow were dominant near the vertical leading edge, whereas natural convection dictated the flow at large values of the horizontal coordinate. The heat transfer coefficient during mixed convection was significantly higher than that with either forced or free convection alone. During simultaneous starting of flow and heating, the time to reach steady state decreased with increase in cross-flow velocity. For the transients initiated by suddenly imposing a horizontal forced flow on an existing natural convection flow, the local temperature and vertical velocity were found to undershoot before reaching their respective steady-state values. The overshoot in heat transfer coefficient in such situations was significant for fluids with smaller Prandtl number. The transient initiated by stopping the horizontal flow during mixed convection was associated with overshoot in both vertical component of velocity and local fluid temperature

186

Transient natural convection in heated inclined tubes

Energy Technology Data Exchange (ETDEWEB)

To simulate natural convection flow patterns in directionally drilled wellbores, experiments and analyses were conducted for a circular tube with length-to-diameter (L/D) ratio of 36 at angles of 0{degree}, 20{degree}, and 35{degree} from the vertical. The tube was heated at the bottom and cooled at the top, and the insulation was adjusted so that approximately one- to two-thirds of the power dissipated was transferred through the tube wall to the surroundings. An aqueous solution of polyvinyl alcohol was employed as the working fluid in order to obtain low Rayleigh numbers corresponding to conditions in geothermal wellbores. Results were primarily qualitative but were useful in providing insight into the phenomena occurring. Steady-state temperature distributions were measured for the three orientations and for several heating rates to demonstrate the effects of tube angle and Rayleigh number. transient measurements of the temperature distribution were obtained during cooling from a higher temperature without a heat source to calibrate the heat losses. With the electrical heat source, temporal data were taken during heating to examine the approach to steady state. Quasi-steady flow conditions were approached rapidly, but the overall time constant of the apparatus was of the order of one-third of a day. Predictions with the three-dimensional TEMPEST code were first tested by comparison with simple conduction analyses. Comparison with actual data showed good agreement of the predicted temperature levels for the maximum inclination, 35{degree}, and slightly poorer agreement for the other limit, a vertical tube. Trends of temperature level and Nusselt number with heating rate or Rayleigh number were reasonable, but the predicted variation of the end Nusselt number versus inclination was in the opposite direction from the experiment. 75 refs., 20 figs., 8 tabs.

McEligot, D.M. (Westinghouse Electric Corp., Middletown, RI (USA). Oceanic Div.); Denbow, D.A. (Software AG of North America, Inc., Lakewood, CO (USA)); Murphy, H.D. (Los Alamos National Lab., NM (USA))

1990-05-01

187

International Nuclear Information System (INIS)

To continue with the equipment of the thermal hydraulics laboratory, it was designed thermal and mechanically an heat exchanger, to satisfy the requirements to have circuit that allows to carry out heat transfer experiments. The heat exchanger was manufactured and proven in the workshops of the Prototypes and Models Management, and it is expected that to obtain the foreseen results once completely installed the circuit, in the laboratory of thermal hydraulics of the Management of Nuclear Systems. (Author)

188

Dendritic solidification of binary alloys with free and forced convection

Dendritic solidification with forced convection and free convection driven by contraction and thermo- solutal buoyancy is simulated in two-dimensional space using a sharp-interface model. Both pure substances and alloys are considered. The model is formulated using the finite element method and works directly with primitive variables. The coupled energy- and solutal concentration-equations, along with the Navier-Stokes equations for incompressible flow, are solved using different meshes. Temperature is solved in a fixed mesh that covers the whole domain (solid + liquid) where the solid-liquid interface is explicitly tracked using marker points. The concentration and momentum equations are solved in the liquid region using an adaptive mesh of triangular elements that conforms to the interface. The velocity boundary conditions are applied directly on the interface. The model is validated using a series of problems that have analytical, experimental and numerical results. Four simulations are presented: (1) crystal growth of succinonitrile with thermal convection under two small undercoolings; (2) dendritic growth into an undercooled pure melt with a uniform forced flow; (3) equiaxial dendritic growth of a pure substance and an alloy with contraction-induced convection; and (4) directional solidification of Pb-0.2 wt% Sb alloy with convection driven by the combined action of contraction, thermal and solutal buoyancy. Some of the simulation results are compared to those reported using other methods including the phase-field method; others are new. In each case, the effects of convection on dendritic solidification are analysed.

Zhao, P.; Heinrich, J. C.; Poirier, D. R.

2005-09-01

189

Directory of Open Access Journals (Sweden)

Full Text Available 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. Experimental work carried investigates the effect of fin spacing, fin height, fin length on the performance of heat dissipation from the fin arrays. It is found that convection heat transfer rate depends on fin height and fin length. For a given fin spacing, the convection heat transfer rate from fins increases with fin height. For a given fin spacing, the convection heat transfer rate from fins increases with fin length .This trend is observed for every fin configuration. It is found that convection heat transfer rate is more for less aspect ratio fin array for same power input.

V.S.Daund*

2014-08-01

190

International Nuclear Information System (INIS)

This paper presents an extensive study of heat-transfer correlations applicable to supercritical-water flow in vertical bare tubes. A comprehensive dataset was collected from 33 papers by 27 authors, including more than 125 graphs and wide range of parameters. The parameters range was as follows: pressures 22.5 - 34.5 MPa, inlet temperatures 85 - 350oC, mass fluxes 250 - 3400 kg/m2s, heat fluxes 75 - 5,400 kW/m2, tube heated lengths 0.6 - 27.4 m, and tube inside diameters 2 - 36 mm. This combined dataset was then investigated and analyzed by calculating Heat Transfer Coefficients (HTCs) and wall temperatures using various correlations and comparing them with the corresponding experimental results. Three correlations were used in this comparison: original Bishop et al., Mokry et al. (modified Bishop et al.) and Gupta et al. (modified Swenson et al). The main objectives of this study were a selection of the best supercritical-water bare-tube correlation for HTC calculations in: 1) fuel bundles of SuperCritical Water-cooled Reactors (SCWRs) as a preliminary and conservative approach; 2) heat exchangers in case of indirect-cycle SCW Nuclear Power Plants (NPPs); and 3) heat exchangers in case of hydrogen co-generation at SCW NPPs from SCW side. The comparison showed that in most cases, the Bishop et al. correlation deviates significantly from the experimental data within the pseudocritical region and actually, underestimates the temperatured actually, underestimates the temperature in the most cases. On the other hand, the Mokry et al. and Gupta et al. correlations showed a relatively better fit within the most operating conditions. In general, the Gupta et al. correlation showed slightly better fit with the experimental data than the Mokry et al. correlation. (author)

191

Forced Convection and Sedimentation Past a Flat Plate

The steady laminar flow of a well-mixed suspension of monodisperse solid spheres, convected steadily past a horizontal flat plate and sedimenting under the action of gravity, is examined. It is shown that, in the limit as Re approaches infinity and epsilon approaches 0, where Re is the bulk Reynolds number and epsilon is the ratio of the particle radius a to the characteristic length scale L, the analysis for determining the particle concentration profile has several aspects in common with that of obtaining the temperature profile in forced-convection heat transfer from a wall to a fluid stream moving at high Reynolds and Prandtl numbers. Specifically, it is found that the particle concentration remains uniform throughout the O(Re(exp -1/2)) thick Blasius boundary layer except for two O(epsilon(exp 2/3)) thin regions on either side of the plate, where the concentration profile becomes non-uniform owing to the presence of shear-induced particle diffusion which balances the particle flux due to convection and sedimentation. The system of equations within this concentration boundary layer admits a similarity solution near the leading edge of the plate, according to which the particle concentration along the top surface of the plate increases from its value in the free stream by an amount proportional to X(exp 5/6), with X measuring the distance along the plate, and decreases in a similar fashion along the underside. But, unlike the case of gravity settling on an inclined plate in the absence of a bulk flow at infinity considered earlier, here the concentration profile remains continuous everywhere. For values of X beyond the region near the leading edge, the particle concentration profile is obtained through the numerical solution of the relevant equations. It is found that, as predicted from the similarity solution, there exists a value of X at which the particle concentration along the top side of the plate attains its maximum value phi(sub m) and that, beyond this point, a stagnant sediment layer will form that grows steadily in time. This critical value of X is computed as a function of phi(sub s), the particle volume fraction in the free stream. In contrast, but again in conformity with the similarity solution, for values of X sufficiently far removed from the leading edge along the underside of the plate, a particle-free region is predicted to form adjacent to the plate. This model, with minor modifications, can be used to describe particle migration in other shear flows, as, for example, in the case of crossflow microfiltration.

Pelekasis, Nikolaos A.; Acrivos, Andreas

1995-01-01

192

Due to the phase-out of all refrigerants with ozone depletion potential, a large void is left in the refrigeration market. This void was caused due to a lack of new, ozone friendly, pure refrigerants with similar thermodynamic properties to those of the banned refrigerants. As a result mixtures of refrigerants are used to create replacement refrigerants. These new mixtures have to be experimentally evaluated to derive correlations for the prediction of the heat transfer coeffic...

Vos, Wouter Philip

2006-01-01

193

International Nuclear Information System (INIS)

An experimental investigation has been conducted to determine the local condensation heat transfer coefficient (HTC) of steam in the presence of air or helium flowing downward inside a 46-mm-i.d. vertical tube. The gas-steam mixture flow rate was measured with a calibrated vortex flowmeter before it entered the 2.54-m-long test condenser. Cooling water flow rate in an annulus around the tube was measure with a calibrated rotameter. Temperatures of the cooling water, the gas-steam mixture, and the tube inside and outside surfaces were measured at 0.3-m intervals in the test condenser. Inlet and exit pressures and temperatures of the gas-steam mixture and of the cooling water were also measured. The local heat flux was obtained from the slope of the coolant axial temperature profile and the coolant mass flow rate. It was found that for the same mass fraction of the noncondensable gas, compared with air, helium has a more inhibiting effect on the heat transfer, but for the same molar ratio, air was found to be more inhibiting. An application where there is important is the proposed advanced passive boiling water reactor design (Simplified Boiling Water Reactor), which utilizes the isolation condenser as a main component of the passive containment cooling system (PCCS)

194

Dielectrophoretic force-driven thermal convection in annular geometry

The thermal convection driven by the dielectrophoretic force is investigated in an- nular geometry under microgravity conditions. A radial temperature gradient and a radial alternating electric field are imposed on a dielectric fluid that fills the gap of two concentric infinite-length cylinders. The resulting dielectric force is regarded as thermal buoyancy with a radial effective gravity. This electric gravity varies in space and may change its sign depending on the temperature gradient and...

Yoshikawa, Harunori; Crumeyrolle, Olivier; Mutabazi, Innocent

2013-01-01

195

Energy Technology Data Exchange (ETDEWEB)

Aerosol indirect effects, i.e., the interactions of aerosols with clouds by serving as cloud condensation nuclei (CCN) or ice nuclei (IN), constitute the largest uncertainty in climate forcing and projection. Previous IPCC reported aerosol indirect forcing is negative, which does not account for aerosol-convective cloud interactions because the complex processes involved are poorly understood and represented in climate models. Here we report that aerosol indirect effect on deep convective cloud systems can lead to enhanced regional convergence and a strong top-of atmosphere (TOA) warming. Aerosol invigoration effect on convection can result in a strong radiative warming in the atmosphere (+5.6 W m-2) due to strong night-time warming, a lofted latent heating, and a reduced diurnal temperature difference, all of which could remarkably impact regional circulation and modify weather systems. We further elucidated how aerosols change convective intensity, diabatic heating, and regional circulation under different environmental conditions and concluded that wind shear and cloud base temperature play key roles in determining the significance of aerosol invigoration effect for convective systems.

Fan, Jiwen; Rosenfeld, Daniel; Ding, Yanni; Leung, Lai-Yung R.; Li, Zhanqing

2012-05-10

196

Analysis of natural convection in volumetrically-heated melt pools

Energy Technology Data Exchange (ETDEWEB)

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.

Sehgal, B.R.; Dinh, T.N.; Nourgaliev, R.R. [Royal Inst. of Tech., Stockholm (Sweden). Div. of Nuclear Power Safety

1996-12-01

197

Laser Measurement Of Convective-Heat-Transfer Coefficient

Coefficient of convective transfer of heat at spot on surface of wind-tunnel model computed from measurements acquired by developmental laser-induced-heat-flux technique. Enables non-intrusive measurements of convective-heat-transfer coefficients at many points across surfaces of models in complicated, three-dimensional, high-speed flows. Measurement spot scanned across surface of model. Apparatus includes argon-ion laser, attenuator/beam splitter electronic shutter infrared camera, and subsystem.

Porro, A. Robert; Hingst, Warren R.; Chriss, Randall M.; Seablom, Kirk D.; Keith, Theo G., Jr.

1994-01-01

198

Modelling crystal growth: Convection in an asymmetrically heated ampoule

The objective was to develop and implement a numerical method capable of solving the nonlinear partial differential equations governing heat, mass, and momentum transfer in a 3-D cylindrical geometry in order to examine the character of convection in an asymmetrically heated cylindrical ampoule. The details of the numerical method, including verification tests involving comparison with results obtained from other methods, are presented. The results of the study of 3-D convection in an asymmetrically heated cylinder are described.

Alexander, J. Iwan D.; Rosenberger, Franz; Pulicani, J. P.; Krukowski, S.; Ouazzani, Jalil

1990-01-01

199

Solution of heat removal from nuclear reactors by natural convection

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...

Zitek Pavel; Valenta Vaclav

2014-01-01

200

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 internal 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 mode...

Andrade, Cla?udia R.; Zaparoli, Edson L.

2000-01-01

201

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)

202

Time evolution simulation of heat removal in a small water tank by natural convection

Energy Technology Data Exchange (ETDEWEB)

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)

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

203

Endwall convective heat transfer for bluff bodies

DEFF Research Database (Denmark)

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.

Wang, Lei; Salewski, Mirko

2012-01-01

204

Convection heat transfer from discrete heat sources in a liquid cooled rectangular channel

International Nuclear Information System (INIS)

Steady-state experiments are performed to study general convective heat transfer from an in-line four simulated electronic chips in a vertical rectangular channel using water as the working fluid. The experimental data covers a wide range for laminar flow under natural, mixed and forced convection conditions with Reynolds number based on channel hydraulic diameter ranging from 40 to 2220 and Reynolds number based on heat source length ranging from 50 to 2775, respectively. The heat flux ranges from 0.1 W/cm2 to 0.6 W/cm2. The inlet water temperature is at 24 deg. C. The effect of heat fluxes, flow rates and geometrical parameters such as chip number are investigated. The experimental results indicate that the heat transfer coefficient is strongly affected by Reynolds number and fully-developed values of heat transfer coefficient are reached before the first chip. Empirical correlations are developed for relations using Nusselt number, Reynolds number and Grashof number, based on channel hydraulic diameter

205

Numerical solution of staggered circular tubes in two-dimensional laminar forced convection

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: English Abstract in english 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.

Carlos Henrique, Marchi; Maykel Alexandre, Hobmeir.

2007-03-01

206

Forced-convection film condensation on a horizontal cylinder with wavy surface structure

Energy Technology Data Exchange (ETDEWEB)

The forced-convection film condensation on a horizontal cylinder with wavy surface structure was performed by boundary-layer-approximation. The local/mean heat fluxes were obtained for the effects of tube temperature, wave number, and wave amplitude. The mean heat flux increases with decreasing wavy amplitude and tube temperatures. Furthermore, when {beta}=20 and {alpha}=0.005, the mean heat flux slightly increases from 1.1 to 3.6% compared with that of smooth tube, depending on tube temperature. (orig.)

Chen, Cha' o-Kuang; Lin, Yan-Ting [National Cheng Kung University, Department of Mechanical Engineering, Tainan (China)

2009-04-15

207

Heat removal by natural convection in a RPR reactor

International Nuclear Information System (INIS)

In this paper natural convection in RPR reactor is analysed. The effect of natural convection valves size on cladding temperature is studied. The reactor channel heat transfer problem is solved using finite elements in a two-dimensional analysis. Results show that two valves with ? = 0.16 m are suited to keep coolant and cladding temperatures below 730C. (author)

208

Heat convection on cylinder at high Prandtl numbers

International Nuclear Information System (INIS)

Natural convection flow on a vertical cylinder is considered here when the Prandtl numbers is large. Little work has been done in this field apart form some experimental studies which are for lower Prandtl numbers. Here, the singular perturbation technique is used to solve this problem. The method adopted is to split the flow into a thin layer close to the surface of the cylinder, surrounded by a much thicker layer where the velocity is reduced to zero. It is shown that at high Prandtl numbers, the velocity boundary layer tends to be somewhat larger due to large kinematic viscosity relative to thermal diffusivity. The motion of the outer layer, however, seems to be caused by the drag force exerted by the inner layer, not due to the buoyancy itself. The basic properties of the flow are evaluated. The heat transfer coefficient is shown to give good prediction for all ranges of Prandtl numbers

209

Laminar forced convection inside externally finned tubes

International Nuclear Information System (INIS)

Analytical solutions are obtained for thermal entry region problems inside ducts with axially varying heat transFer coefficient, by making use of the ideas in the recently advanced generalized integral transform technique. The analysis is applied to stepwise variations of Biot number that simulate intermitent rows of external fins. Numerical results are obtained for different fin arrangements, in a systematic manner, so as to critically examine the relative marits of wider and/or more numerous fin rows. (author)

210

International Nuclear Information System (INIS)

Pressure drop is significantly affected by heat transfer in mixed convection situations. A semi-empirical correlation for pressure drop is developed from a theoretical base by first making use of the momentum integral solution to the heat and momentum equations for natural convection on a vertical surface. The boundary condition on the free-stream side of the boundary layer is changed to reflect the shear on that surface due to the forced convection, and empirical data are used to develop a formula useful in design and applications. The equation may be used with bulk fluid properties or film properties. The equation is valid for laminar, mixed convection conditions in vertical, internal, aiding flows with constant wall temperature boundary condition

211

Convection zone origins of solar atmospheric heating

International Nuclear Information System (INIS)

Spicules are examined as a means for supplying the corona with mass, energy, and magnetic field. It is suggested that spicules form from the supersonic upward expansion of material on nearly evacuated network flux tubes embedded within the sun's convection zone. This allows supersonic but subescape velocities to be attained by the material as it flows outward through the photosphere. Although supersonic, the kinetic energy (subescape) of the spicule material, as observed, is insufficient for coronal heating. It is suggested that, through buoyancy changes on evacuated flux tubes, the magnetic field first wicks material flow into the solar atmosphere. Subsequently, the magnetic field energizes the gaseous material to form the conventional hot, dynamically expanding, solar corona. This occurs through momentum and energy transport by Alfven waves and associated Maxwell stresses concurrently flowing upward on these geysers (spicules). The vertical momentum equation governing fluid flow is examined, and a particular equipartition solution is presented for the flow velocity along a simple field geometry. 39 references

212

A Study on the Mixed Convection Heat Transfer in a Vertical Cylinder Using Electroplating System

International Nuclear Information System (INIS)

Hydrogen economy has drawn public attentions as a promising future energy source. Hydrogen is a non-petroleum-based, non-toxic, renewable and clean burning energy source. Hydrogen is the secondary energy, which means that it is produced by consuming the first energy such as coal, gas, petroleum etc. This again means that it is clean so long as it is produced by clean methods. One of the promising production methods of hydrogen is to use the heat from an HTGR(High Temperature Gas-cooled Reactor), a next generation nuclear reactor for a safe and reliable operation as well as for efficient and economic generation of energy. The knowledge of detailed heat transfer phenomena in gaseous phase emerges as an important factor for HTGR, where buoyancy effect plays a significant role. Large and expensive test facilities are to be constructed to assess the detailed mixed convection phenomena. However, using analogy concept, heat transfer system can be transformed to mass transfer system and vice versa. If a simple mass transfer system could be devised, and the experimental solution from that system could be obtained, then this could theoretically lead to a solution for a similar heat transfer system. In this study, a copper electroplating system was selected as the mass transfer system. A copper electroplating system with limiting current technique has a good advantage to simulate heat transfer system as mass transfer coefficient, analogous with heat transfer coefficient, can be us with heat transfer coefficient, can be directly obtained from the information of the bulk concentration and electric current between electrodes. This study simulated the mixed convective heat transfer phenomena in a vertical cylinder using copper electroplating system. The mixed convection phenomenon is observed when the forced and natural convections are of comparable magnitudes in one system. The mixed convection is classified as laminar and turbulent flows depending on the exchange mechanism and also as buoyancy aided and buoyancy opposed flows depending on the directions of forced flows with respect to the buoyancy forces. For a laminar flow, the heat transfer rate of buoyancy aided flow is larger than the corresponding forced convection heat transfer due to the increased flow velocity while the heat transfer rate of buoyancy opposed flow is smaller. However for a turbulent flow, the buoyancy opposed flow shows higher heat transfer rates than corresponding forced convective flow due to the increased turbulent production and the buoyancy aided flow shower lower heat transfer rates due to laminarization. Mixed convection heat transfer in a vertical cylinder with aiding flow and opposing flow studied experimentally for Reynolds numbers ranging from 4,000 to 10,000 with a constant Grashof number 6.2x109 and Prandtl number about 2000. The experimental results reproduced the trend of mixed convection heat transfer phenomena in a turbulent situation and agree well with the study performed by Y. Parlatan(1996). The analogy experimental method successfully simulates the mixed convection heat transfer system and seems to be a useful tool for heat transfer studies for HTGR as well as the systems with high buoyancy condition and high Prandtl number fluid, as the electroplating method not only provides useful information regarding heat transfer but also has a cost-effective advantage over any other comparable experimental method

213

Abstract: The entropy generation due to steady laminar forced convection fluid flow through parallel plates microchannel is investigated numerically. The effect of Knudsen, Reynolds, Prandtl, Eckert numbers and the nondimensional temperature difference on entropy generation within the microchannel is discussed. The fraction of the entropy generation due to heat transfer to the total entropy generation within the microchannel is studied in terms of Bejan number. The entropy generation within t...

Mohammad Al-Nimr; Mohammad Abuzaid; Osamah Haddad

2004-01-01

214

Analytical Solution of Forced-Convective Boundary-Layer Flow over a Flat Plate

DEFF Research Database (Denmark)

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.

Mirgolbabaei, H.; Barari, Amin

2010-01-01

215

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

Nadia Potoceanu

2007-01-01

216

Directory of Open Access Journals (Sweden)

Full Text Available 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

Nadia Potoceanu

2007-10-01

217

A Study of Nucleate Boiling with Forced Convection in Microgravity

The ultimate objective of basic studies of flow boiling in microgravity is to improve the understanding of the processes involved, as manifested by the ability to predict its behavior. This is not yet the case for boiling heat transfer even in earth gravity, despite the considerable research activity over the past 30 years. The elements that constitute the nucleate boiling process - nucleation, growth, motion, and collapse of the vapor bubbles (if the bulk liquid is subcooled) - are common to both pool and flow boiling. It is well known that the imposition of bulk liquid motion affects the vapor bubble behavior relative to pool boiling, but does not appear to significantly influence the heat transfer. Indeed, it has been recommended in the past that empirical correlations or experimental data of pool boiling be used for design purposes with forced convection nucleate boiling. It is anticipated that such will most certainly not be possible for boiling in microgravity, based on observations made with pool boiling in microgravity. In earth gravity buoyancy will act to remove the vapor bubbles from the vicinity of the heater surface regardless of how much the imposed bulk velocity is reduced, depending, of course, on the geometry of the system. Vapor bubbles have been observed to dramatically increase in size in pool boiling in microgravity, and the heat flux at which dryout took place was reduced considerably below what is generally termed the critical heat flux (CHF) in earth gravity, depending on the bulk liquid subcooling. However, at heat flux levels below dryout, the nucleate pool boiling process was enhanced considerably over that in earth gravity, in spite of the large vapor bubbles formed in microgravity and perhaps as a consequence. These large vapor bubbles tended to remain in the vicinity of the heater surface, and the enhanced heat transfer appeared to be associated with the presence of what variously has been referred to as a liquid microlayer between the bubble and the heater surface. The enhancement of the boiling process with low velocities in earth gravity for those orientations producing the formation of a liquid macrolayer described above, accompanied by "sliding" vapor bubbles, has been demonstrated. The enhancement was presented as a function of orientation, subcooling, and heated length, while a criterion for the heat transfer for mixed natural/forced convection nucleate boiling was given previously. A major unknown in the prediction and application of flow boiling heat transfer in microgravity is the upper limit of the heat flux for the onset of dryout (or critical heat flux - CHF), for given conditions of fluid-heater surfaces, including geometry, system pressure and bulk liquid subcooling. It is clearly understood that the behavior in microgravity will be no different than on earth with sufficiently high flow velocities, and would require no space experimentation. However, the boundary at which this takes place is still an unknown. Previous results of CHF measurements were presented for low velocity flow boiling at various orientations in earth gravity as a function of flow velocity and bulk liquid subcooling, along with preliminary measurements of bubble residence times on a flat heater surface. This showed promise as a parameter to be used in modeling the CHF, both in earth gravity and in microgravity. The objective of the work here is to draw attention to and show results of current modeling efforts for the CHF, with low velocities in earth gravity at different orientations and subcoolings. Many geometrical possibilities for a heater surface exist in flowing boiling, with boiling on the inner and outer surfaces of tubes perhaps being the most common. If the vapor bubble residence time on and departure size from the heater surface bear a relationship to the CHF, as results to be given indicate, it is important that visualization of and access to vapor bubble growth be conveniently available for research purposes. In addition, it is desirable to reduce the number of variables as much as p

Merte, Herman, Jr.

1999-01-01

218

Sedimentation and convective boiling heat transfer of CuO-water/ethylene glycol nanofluids

The convective boiling characteristics of dilute dispersions of CuO nanoparticles in water/ethylene glycol as a base fluid were studied at different operating conditions of (heat fluxes up to 174 kW m-2, mass fluxes range of 353-1,059 kg m-2 s-1 and sub-cooling level of 343, 353 and 363 K) inside the annular duct. The convective boiling heat transfer coefficients of nanofluids in different concentrations (vol%) of nanoparticles (0.5, 1, and 1.5) were also experimentally quantified. Results demonstrated the significant augmentation of heat transfer coefficient inside the region with forced convection dominant mechanism and deterioration of heat transfer coefficient in region with nucleate boiling dominant heat transfer mechanism. Due to the scale formation around the heating section, fouling resistance was also experimentally measured. Experimental data showed that with increasing the heat and mass fluxes, the heat transfer coefficient and fouling resistance dramatically increase and rate of bubble formation clearly increases. Obtained results were then compared to some well-known correlations. Results of these comparisons demonstrated that experimental results represent the good agreement with those of obtained by the correlations. Consequently, Chen correlation is recommended for estimating the convective flow boiling heat transfer coefficient of dilute CuO-water/ethylene glycol based nanofluids.

Sarafraz, M. M.; Hormozi, F.; Kamalgharibi, M.

2014-09-01

219

Predictions of laminar natural convection in heated cavities

International Nuclear Information System (INIS)

Several examples of laminar, natural convection in heated cavities are discussed with illustrative calculations. These include convection in a square cavity at high Rayleigh number; in a narrow cavity at moderate aspect ratio; in a rectangular cavity heated from below; in a trapezoidal cavity, and in a rectangular cavity containing a conducting obstruction. The steady equations for the velocity, pressure and temperature are solved in the Boussinesq approximation, using a standard Galerkin formulation of the finite-element method. (author)

220

Experimental study on convective heat transfer with thin porous bodies

International Nuclear Information System (INIS)

Experimental studies are made on the convective heat transfer of three types of thin porous bodies. Heat transfer performances, flow patterns and temperature profiles near the porous bodies are compared with each other. The heat transfer performance of porous bodies with the largest pore diameter is large. It became clear that the high heat transfer performance depends on an excellent heat transportation ability inside the pore and near the surface of the porous bodies. (author)

221

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

222

Convective and radiative heat transfer coefficients for individual human body segments.

Human thermal physiological and comfort models will soon be able to simulate both transient and spatial inhomogeneities in the thermal environment. With this increasing detail comes the need for anatomically specific convective and radiative heat transfer coefficients for the human body. The present study used an articulated thermal manikin with 16 body segments (head, chest, back, upper arms, forearms, hands, pelvis, upper legs, lower legs, feet) to generate radiative heat transfer coefficients as well as natural- and forced-mode convective coefficients. The tests were conducted across a range of wind speeds from still air to 5.0 m/s, representing atmospheric conditions typical of both indoors and outdoors. Both standing and seated postures were investigated, as were eight different wind azimuth angles. The radiative heat transfer coefficient measured for the whole-body was 4.5 W/m2 per K for both the seated and standing cases, closely matching the generally accepted whole-body value of 4.7 W/m2 per K. Similarly, the whole-body natural convection coefficient for the manikin fell within the mid-range of previously published values at 3.4 and 3.3 W/m2 per K when standing and seated respectively. In the forced convective regime, heat transfer coefficients were higher for hands, feet and peripheral limbs compared to the central torso region. Wind direction had little effect on convective heat transfers from individual body segments. A general-purpose forced convection equation suitable for application to both seated and standing postures indoors was hc = 10.3v0.6 for the whole-body. Similar equations were generated for individual body segments in both seated and standing postures. PMID:9195861

de Dear, R J; Arens, E; Hui, Z; Oguro, M

1997-05-01

223

Solution of heat removal from nuclear reactors by natural convection

Directory of Open Access Journals (Sweden)

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.

Zitek Pavel

2014-03-01

224

SIMULATION OF MIXED CONVECTIVE HEAT TRANSFER USING LATTICE BOLTZMANN METHOD

Directory of Open Access Journals (Sweden)

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.

A. R. M. Rosdzimin

2010-12-01

225

SRS reactor control rod cooling without normal forced convection cooling

International Nuclear Information System (INIS)

This paper describes an analytical study of the coolability of the control rods in the Savannah River site (SRS) K production reactor under conditions of loss of normal forced convection cooling. The study was performed as part of the overall safety analysis of the reactor supporting its restart. The analysis addresses the buoyancy-driven boiling flow over the control rods that occurs when forced cooling is lost. The objective of the study was to demonstrate that the control rods will remain cooled (i.e., no melting) at powers representative of those anticipated for restart of the reactor

226

Directory of Open Access Journals (Sweden)

Full Text Available The magnetic field effect on laminar natural convection flow is investigated in a filled enclosure with internal heat generation using two-dimensional numerical simulation. The enclosure is heated by a uniform volumetric heat density and walls have constant temperature. A fixed magnetic field is applied to the enclosure. The dimensionless governing equations are solved numerically for the stream function, vorticity and temperature using finite difference method for various Rayleigh (Ra and Hartmann (Ha numbers in MATLAB software. The stream function equation is solved using fast Poisson's equation solver on a rectangular grid (POICALC function in MATLAB, voricity and temperature equations are solved using red-black Gauss-Seidel and bi-conjugate gradient stabilized (BiCGSTAB methods respectively. The results show that the strength of the magnetic field has significant effects on the flow and temperature fields. For the square cavity, the maximum temperature reduces with increasing Ra number. It is also observed that at low Ra number, location of the maximum temperature is at the centre of the cavity and it shifts upwards with increase in Ra number. Circulation inside the enclosure and therefore the convection becomes stronger as the Ra number increases while the magnetic field suppresses the convective flow and the heat transfer rate. The ratio of the Lorentz force to the buoyancy force (Ha2/Ra is as an index to compare the contribution of natural convection and magnetic field strength on heat transfer.

M.A. Taghikhani

2015-01-01

227

Laminar mixed convective heat transfer in two-dimensional rectangular inclined driven cavity is studied numerically by means of a double population thermal Lattice Boltzmann method. Through the top moving lid the heat flux enters the cavity whereas it leaves the system through the bottom wall; side walls are adiabatic. The counter-slip internal energy density boundary condition, able to simulate an imposed non zero heat flux at the wall, is applied, in order to demonstrate that it can be effectively used to simulate heat transfer phenomena also in case of moving walls. Results are analyzed over a range of the Richardson numbers and tilting angles of the enclosure, encompassing the dominating forced convection, mixed convection, and dominating natural convection flow regimes. As expected, heat transfer rate increases as increases the inclination angle, but this effect is significant for higher Richardson numbers, when buoyancy forces dominate the problem; for horizontal cavity, average Nusselt number decreases with the increase of Richardson number because of the stratified field configuration.

D'Orazio, A.; Karimipour, A.; Nezhad, A. H.; Shirani, E.

2014-11-01

228

Enhancement of external forced convection by ionic wind

An ionic wind is formed when air ions are accelerated by an electric field and exchange momentum with neutral air molecules, causing air flow. Because ionic winds can generate flow with no moving parts and have low power consumption, they offer an attractive method for enhancing convection heat transfer from a surface. In the present work, corona discharges are generated between a steel wire and coppertape electrode pair on a flat plate, perpendicular to the bulk flow direction such that the ...

Go, David B.; Maturana, Raul A.; Fisher, Timothy S.; Garimella, Suresh V.

2008-01-01

229

Due to scarcity of literature on forced-convection heat transfer in a solar collector with rhombic cross-section absorbing tubes, a series of experiments was arranged and conducted to determine heat transfer coefficient. In this study, a typical rhombic cross-section finned tube of flat-plate collectors used as the test section. Two correlations were proposed for the Nusselt number as a function of the Reynolds number and the Prandtl number based on hydraulic diameter for various heat fluxes....

Taherian, Hessam; Yazdanshenas, Eshagh

2006-01-01

230

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.

Cao, Y.; Faghri, A.; Juhasz, A.

1991-02-01

231

Heat transfer characteristics of forced circulation under inclined conditions

International Nuclear Information System (INIS)

Experimental researches and numerical analysis on the heat transfer characteristics of forced circulation flow in a circular tube under different inclined conditions were introduced in this paper. For the single-phase flow, the experimental results show that the heat transfer around the tube was unsymmetrical under the buoyancy effect, and the heat transfer was weakened at the upside of tube to increase the wall temperature, while the heat transfer was enhanced at downside to decrease the wall temperature. The numerical calculation results show the same phenomena. Based on the experiment, a modified factor for the inclined heat transfer coefficient was presented in this paper, to estimate more accurately the heat transfer variation in single phase convection at inclined condition. Stresses on the bubble in two-phase flow was analyzed, to illustrate the why the effect of inclined condition on the heat transfer is not obvious. (authors)

232

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)

233

International Nuclear Information System (INIS)

The void fraction in a forced convective flow boiling is very important information for understanding the characteristics of the boiling two-phase flow. Consequently, many experimental investigations have been carried out to obtain the local void fraction so far, but the detail data among the whole of the test-section has not been enough. Especially, the data under subcooled condition are quite limited. In this study, the void fraction distribution in a forced convective boiling was quantitatively measured by using the thermal neutron radiography. These results were compared with several existing void fraction correlations. Although these correlations show a good agreements with experimental results under low heat flux condition, there is no suitable correlation to estimate the void fraction under non-thermal equilibrium condition. (author)

234

Convective heat transfer around vertical jet fires: An experimental study

Energy Technology Data Exchange (ETDEWEB)

Highlights: Black-Right-Pointing-Pointer Experiments were carried out to analyze convection around a vertical jet fire. Black-Right-Pointing-Pointer Convection heat transfer is enhanced increasing the flame length. Black-Right-Pointing-Pointer Nusselt number grows with higher values of Rayleigh and Reynolds numbers. Black-Right-Pointing-Pointer In subsonic flames, Nusselt number increases with Froude number. Black-Right-Pointing-Pointer Convection and radiation are equally important in causing a domino effect. - Abstract: The convection heat transfer phenomenon in vertical jet fires was experimentally analyzed. In these experiments, turbulent propane flames were generated in subsonic as well as sonic regimes. The experimental data demonstrated that the rate of convection heat transfer increases by increasing the length of the flame. Assuming the solid flame model, the convection heat transfer coefficient was calculated. Two equations in terms of adimensional numbers were developed. It was found out that the Nusselt number attains greater values for higher values of the Rayleigh and Reynolds numbers. On the other hand, the Froude number was analyzed only for the subsonic flames where the Nusselt number grows by this number and the diameter of the orifice.

Kozanoglu, Bulent, E-mail: bulentu.kozanoglu@udlap.mx [Universidad de las Americas, Puebla (Mexico); Zarate, Luis [Universidad Popular Autonoma del Estado de Puebla (Mexico); Gomez-Mares, Mercedes [Universita di Bologna (Italy); Casal, Joaquim [Universitat Politecnica de Catalunya (Spain)

2011-12-15

235

International Nuclear Information System (INIS)

Highlights: • Convective–radiative radial fins with base convective heating were analyzed. • Homogeneous material and functionally graded material fins were investigated. • Fin efficiency and the effects of dimensionless parameters in fins were analyzed. - Abstract: This paper studies a radial fin of uniform thickness with convective heating at the base and convective–radiative cooling at the tip. The fin is assumed to experience uniform internal heat generation. The exposed surfaces of the fin lose heat by simultaneous convection and radiation to the surroundings. Two types of fin materials are investigated: homogeneous material and functionally graded material (FGM). For the homogeneous material, the thermal conductivity is assumed to be a linear function of temperature, while for the FGM fin the thermal conductivity is modeled as a linear function of the dimensionless radial coordinate. The analysis is conducted using the differential transformation method (DTM). The accuracy of DTM is verified by comparing the results for the simplified versions of the present model with an exact analytical solution derived here. Once the accuracy of DTM is authenticated, the method is used to generate results for the general problem formulated here. These results illustrate the effects of various dimensionless parameters on the thermal performance of homogeneous material fins and FGM fins

236

Transient convective heat transfer to laminar flow from a flat plate with constant heat capacity

International Nuclear Information System (INIS)

Most basic transient heat transfer problem is the transient response characteristics of forced convection heat transfer in the flow along a flat plate or in a tube. In case of the laminar flow along a flat plate, the profile method using steady temperature distribution has been mostly adopted, but its propriety has not been clarified yet. About the unsteady heat transfer in the laminar flow along a flat plate, the analysis or experiment evaluating the heat capacity of the flat plate exactly was never carried out. The purpose of this study is to determine by numerical calculation the unsteady characteristics of the boundary layer in laminar flow and to confirm them by experiment concerning the unsteady heat transfer when a flat plate with a certain heat capacity is placed in parallel in uniform flow and given a certain quantity of heat generation suddenly. The basic equation and the solution are given, and the method of numerical calculation and the result are explained. The experimental setup and method, and the experimental results are shown. Both results were in good agreement, and the response of wall temperature, the response of Nusselt number and the change of temperature distribution in course of time were able to be determined by applying Laplace transformation and numerical Laplace inverse transformation to the equation. (Kako, I.)

237

Turbulent mixed convection heat transfer to liquid sodium

International Nuclear Information System (INIS)

The influences of buoyancy on turbulent heat transfer to a liquid metal flowing in a vertical pipe are considered. A theoretical model is presented which provides a criterion for the conditions under which such influences become significant and which predicts the impairment of heat transfer for upward flow and enhancement for downward flow. The variation with Peclet number of the maximum impairment of heat transfer and conditions under which it occurs are established. A generalization of the model leading to an equation for the entire mixed convection region is proposed. From this an equation for turbulent free convection to liquid metals is obtained. (author)

238

10,000 - A reason to study granular heat convection

Energy Technology Data Exchange (ETDEWEB)

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.

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

239

10,000 – A reason to study granular heat convection

International Nuclear Information System (INIS)

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.

240

Energy Technology Data Exchange (ETDEWEB)

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)

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

241

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: English Abstract in english 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.

M., Ghalambaz; A., Noghrehabadi; A., Ghanbarzadeh.

2014-06-01

242

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

243

Reynolds stress and heat flux in spherical shell convection

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 ...

Käpylä, P J; Guerrero, G; Brandenburg, A; Chatterjee, P

2010-01-01

244

Convective heat transfer in building simulation; Convectieve warmteoverdracht in gebouwsimulatie

Energy Technology Data Exchange (ETDEWEB)

The sensitivity of the convective heat transfer correlations to air movements related to the normal use of a room is discussed. Also the parameters determining and the control variables directing the selection of the convection coefficient correlation during an energy calculation are discussed. The current approach for modelling convective heat transfer in common building simulation programs is discussed. As is the additional research required to further improve convection modelling. [Dutch] Er zijn drie vormen van warmteoverdracht: straling, geleiding en convectie. Terwijl de eerste twee accuraat en relatief eenvoudig analytisch kunnen worden uitgeschreven, is dat voor convectie minder evident. Een convectieve stroming is afhankelijk van een hele reeks parameters, die in min of meerdere mate relevant zijn voor de energieberekeningen in gebouwsimulatie. Het is dus van belang om te weten waar die sensitiviteiten liggen en hoe die worden gemodelleerd in gebouwsimulatie. Dit artikel geeft een analyse van de relevante parameters voor modellering van convectie in twee veel gebruikte simulatiecodes.

Peeters, L.F.R. [Departement Mechanica, Vrije Universiteit Brussel, Brussels (Belgium)

2012-05-15

245

Experimental turbulent mixed convection behind a heated cylinder

Turbulent mixed convection around a horizontal cylinder at constant temperature exposed to a cross-flow is experimentally investigated by means of flow visualizations, 2D particle image velocimetry and thermocouple measurements. This paper analyses the effect of buoyancy when the wake of the cylinder is 3D and fully turbulent. Within this investigation the Reynolds number was chosen to be 1000 and the Richardson number is taken as 2.77 and 0 for comparison with the isothermal case. Results show that buoyancy effects strongly disturb the wake which becomes asymmetric and undergoes an upward deflection. Flow visualizations show the presence of two kinds of structures in the wake with heating of the cylinder. At the rear of the cylinder, the ambient fluid is subject to a high buoyant force due to the great temperature gradients. These significant gradients allow typical mushroom-like structures to emerge in the near wake region but also several diameters downwards the cylinder. In the lower shear-layer, heating is responsible of the generation of Kelvin-Helmholtz instabilities that usually exist at higher Reynolds numbers under isothermal conditions. Moreover comparison of velocity and temperature fields exhibits an opposite behaviour between velocity and temperature fluctuations in the free shear-layers.

Boirlaud, Matthieu; Couton, Dominique; Plourde, Frédéric

2012-11-01

246

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...

Venkatachalapathy, S.; Udayakumar, M.

2010-01-01

247

International Nuclear Information System (INIS)

Results on numerical investigation of the local opposing mixed convection heat transfer in a vertical flat channel with symmetrical heating in laminar airflow are presented. A numerical two-dimensional simulation was performed using the FLUENT 6.1 code. Investigations were performed in airflow of 0.1, 0.2 and 0.4 MPa absolute pressure at Reynolds numbers from 1500 up to 4310 with Grq number variation from 1.65 * 105 to 3.1 * 109 in order to define the effect of the influence of buoyancy on heat transfer. Numerical calculations demonstrated that under the effect of small buoyancy there were only small transformations in the velocity profile, but the flow was oriented downward (direction of forced flow). With increasing the buoyancy forces, flow separation occurred at some distance from the beginning of the heated channel section. With a further increase of buoyancy, the position of flow separation point moved towards the beginning of the heated section. The channel wall temperature noticeably decreased at the flow separation point. Correlations for calculation of heat transfer in the laminar mixed convection region and for the determination of the position of flow separation from the wall are suggested. (author)

248

Convective Heat Transfer in the Reusable Solid Rocket Motor of the Space Transportation System

This simulation involved a two-dimensional axisymmetric model of a full motor initial grain of the Reusable Solid Rocket Motor (RSRM) of the Space Transportation System (STS). It was conducted with CFD (computational fluid dynamics) commercial code FLUENT. This analysis was performed to: a) maintain continuity with most related previous analyses, b) serve as a non-vectored baseline for any three-dimensional vectored nozzles, c) provide a relatively simple application and checkout for various CFD solution schemes, grid sensitivity studies, turbulence modeling and heat transfer, and d) calculate nozzle convective heat transfer coefficients. The accuracy of the present results and the selection of the numerical schemes and turbulence models were based on matching the rocket ballistic predictions of mass flow rate, head end pressure, vacuum thrust and specific impulse, and measured chamber pressure drop. Matching these ballistic predictions was found to be good. This study was limited to convective heat transfer and the results compared favorably with existing theory. On the other hand, qualitative comparison with backed-out data of the ratio of the convective heat transfer coefficient to the specific heat at constant pressure was made in a relative manner. This backed-out data was devised to match nozzle erosion that was a result of heat transfer (convective, radiative and conductive), chemical (transpirating), and mechanical (shear and particle impingement forces) effects combined.

Ahmad, Rashid A.; Cash, Stephen F. (Technical Monitor)

2002-01-01

249

Correlations for heat transfer coefficients in open gaps with respect to mixed convection

International Nuclear Information System (INIS)

Published results on mixed convection phenomena have been applied to determine the convective heat transfer between the cover gas and the open gaps in the roof of SNR 2. It has been reported in the literature that heat transfer coefficients for forced flow conditions in vertical heated or cooled pipes are modified by buoyancy effects. In the ''aiding'' condition, where buoyancy and flow act in the same direction, heat transfer is enhanced. In the ''opposing'' case heat transfer is reduced. This applies for laminar flow; the reverse is true for turbulent flow. The literature indicates furthermore that: the Reynolds number indicating turbulent flow can be as low as 30 for mixed convection; the criterion for the onset of mixed convection is given by the Richardson number Ri=Gr/Re2 > 0,002. The published results have been modified in consistency with the open gaps in the SNR 2 reactor roof. Several heat transfer correlations have been evaluated and their suitability examined. (author)

250

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.

Khaled, M.; Garnier, B.; Harambat, F.; Peerhossaini, H.

2010-02-01

251

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

252

Heat convection in a vertical channel : Plumes versus turbulent diffusion

Following a previous study [Gibert , Phys. Rev. Lett. 96, 084501 (2006)], convective heat transfer in a vertical channel of moderate dimensions follows purely inertial laws. It would be therefore a good model for convective flows of stars and ocean. Here we report new measurements on this system. We use an intrinsic length in the definition of the characteristic Rayleigh and Reynolds numbers. We explicit the relation between this intrinsic length and the thermal correlation length. Using part...

Gibert, Mathieu; Pabiou, Herve?; Tisserand, Jean-christophe; Gertjerenken, Betina; Castaing, Bernard; Chilla?, Francesca

2009-01-01

253

International Nuclear Information System (INIS)

The objective of this paper is to numerically investigate the cooling performance of electronic devices with an emphasis on the effects of the arrangement and number of electronic components. The analysis uses a two dimensional rectangular enclosure under combined natural and forced convection flow conditions and considers a range of Raleigh numbers. Heat sources in the enclosure generate the natural convection flow and an externally sourced air stream through the enclosure generates the forced convection flow. The results show that increasing the Raleigh number significantly improves the enclosure heat transfer process. At low Raleigh numbers, placing more heat sources within the enclosure reduces the heat transfer rate from the sources and consequently increases their overall maximum temperature. The arrangement and the number of heat sources have a considerable contribution to the cooling performance. However, increasing the Raleigh number reduces this contribution. (author)

254

We carried out three-dimensional computations of the magnetothermal convection of diamagnetic liquids (e.g., water) in a shallow cylindrical vessel of the Rayleigh–Benard model to investigate the effects of the radial components of the magnetic force (MFR) and the vertical components of the magnetic force (MFZ) on heat transfer and pattern formation. For the purpose of examining various magnitudes of radial and vertical components of the magnetic force, we used a 1800-turn coil instead of an actual conventional magnet, and a 2-turn coil as a new proposal. In the computation, the Prandtl number was constant at 6.0 (i.e., water at room temperature), and the Rayleigh number was 1.0 × 104. As a result, an axisymmetric pattern and a spokelike pattern appeared in the convection. Subsequently, we arranged all the results with a new parameter, which is the absolute ratio of the radial component of magnetic force at the vessel sidewall to the vertical resultant force at the vessel center. This parameter represents the relative contribution of MFR to the vertical resultant force (RFZ). The RFZ is the sum of the gravitational force and MFZ, and is associated with the magnetic Rayleigh number. By using this parameter, the spoke pattern convection appeared when the parameter was within 0.11 to 0.85. This result suggests that not only the MFR but also the MFZ plays an important role in determining whether the convection flow pattern transited to the axisymmetric flow or not. This parameter range is easily satisfied in a conventional superconducting magnet. We can say that the spoke pattern convection may be a common phenomenon in the magnet. On the other hand, the flow pattern had little effect on the heat transfer in convection. Thus, the effects of MFR and MFZ on the pattern formation in the magnet are much more complicated than those previously considered.

Maki, Syou; Sumitani, Mariko; Udagawa, Chikako; Morimoto, Shotaro; Tanimoto, Yoshifumi

2014-07-01

255

Forced air heat sink apparatus

A high efficiency forced air heat sink assembly employs a split feed transverse flow configuration to minimize the length of the air flow path through at least two separated fin structures. Different embodiments use different fin structure material configurations including honeycomb, corrugated and serpentine. Each such embodiment uses a thermally conductive plate having opposed exterior surfaces; one for receiving a component to be cooled and one for receiving the fin structures. The serpentine structured fin embodiment employs a plurality of fin supports extending from the plate and forming a plurality of channels for receiving the fin structures. A high thermal conductivity bondant, such as metal-filled epoxy, may be used to bond the fin structures to either the plate or the fin supports. Dip brazing and soldering may also be employed depending upon the materials selected.

Rippel, Wally E. (Inventor)

1989-01-01

256

Heat transfer mechanisms in bubbly Rayleigh-Benard convection

The heat transfer mechanism in Rayleigh-Benard convection in a liquid with a mean temperature close to its boiling point is studied through numerical simulations with point-like vapor bubbles, which are allowed to grow or shrink through evaporation and condensation and which act back on the flow both thermally and mechanically. It is shown that the effect of the bubbles is strongly dependent on the ratio of the sensible heat to the latent heat as embodied in the Jacob number...

Oresta, Paolo; Verzicco, Roberto; Lohse, Detlef; Prosperetti, Andrea

2008-01-01

257

Convective heat transfer around vertical jet fires: an experimental study.

The convection heat transfer phenomenon in vertical jet fires was experimentally analyzed. In these experiments, turbulent propane flames were generated in subsonic as well as sonic regimes. The experimental data demonstrated that the rate of convection heat transfer increases by increasing the length of the flame. Assuming the solid flame model, the convection heat transfer coefficient was calculated. Two equations in terms of adimensional numbers were developed. It was found out that the Nusselt number attains greater values for higher values of the Rayleigh and Reynolds numbers. On the other hand, the Froude number was analyzed only for the subsonic flames where the Nusselt number grows by this number and the diameter of the orifice. PMID:21962859

Kozanoglu, Bulent; Zárate, Luis; Gómez-Mares, Mercedes; Casal, Joaquim

2011-12-15

258

International Nuclear Information System (INIS)

A collection of papers is presented on mixed convection, pool boiling, and flow boiling and two-phase flow. A unified similarity analysis is presented for turbulent convection next to vertical surfaces. Topics of interest include free-forced convection from a heated cone, decay of vertical buoyant jets in uniform environment, minimum heat flux during film boiling, and the dynamics of two-phase flow in a duct

259

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

260

Simulation of Test Case B : isothermal forced convection

DEFF Research Database (Denmark)

This report shows the results of the simulations given in research item 1.19 (isothermal forced convection) within the work of International Energy Agency (lEA), Annex 20 subtask 1. The title of this work is "Air Flow within Buildings" and the working title for subtask 1 is "Room Air and Contaminant Flow'. The goal is to evaluate the performance of various computer codes and to test the computer codes applicability as design tools for complex flow patterns close to those seen in the-field of practical ventilation engineering.

Skovgaard, M.; Nielsen, Peter V.

1991-01-01

261

The experimental investigations were consisting of two parts. The first part was carried out to study the effect of corner geometry on the steady-state forced convection inside horizontal isosceles triangular ducts with sharp corners. The electrically-heated triangular duct was used to simulate the triangular passage of a plate-fin compact heat exchanger. The isosceles triangular ducts were manufactured with duralumin, and fabricated with the same length of 2.4m and hydraulic diameter of 0.44m, but five different apex angles (i.e. ?a=15?,30?, 40?,60?, and 90?) respectively. The investigation was performed under turbulent flow condition covering a wide range of Reynolds number (i.e. 7000<=ReD<=20000). It was found that the best thermal performance is achieved with the apex angle of 60?. The second part was performed to investigate the effect of surface roughness on the forced convection of the same system. Horizontal equilateral triangular ducts with an apex angle of 60? were fabricated with the same length and hydraulic diameter, but different average surface roughnesses of 1.2 m,3.0 m and 11.5 m respectively. It was concluded that the duct with a higher surface roughness will have a better heat transfer performance. Non-dimensional expressions for the determination of the heat transfer coefficient of the triangular ducts with different apex angles and surface roughnesses were also developed.

Leung, C. W.; Wong, T. T.; Kang, H. J.

262

A contribution to incipient boiling in the case of subcooled boiling with forced convection

International Nuclear Information System (INIS)

The literature gives contradictory statements about incipient subcooled boiling. To clear up these contradictions it seems important to study the effect of different thermo- and hydrodynamic parameters, like heating surface load, system pressure, local supercooling, and flowrate. Further influencing quantities investigated here are the concentration dissolved gases and the surface condition of the heat surface. To carry out the experimental investigations a measuring method which has already been used by Mayinger applied. With this method, incipient boiling can be determined as the first measurable heat transfer improvement in comparison with single-phase forced convection. Besides, photographs sould make it possible to give statements on the quantity and size of the bubbles on the heating surface. (orig./GL)

263

Thermocapillary convection induced by laser surface heating

International Nuclear Information System (INIS)

Thermocapillary convection was excited by a laser source in experiments on molten paraffin. The parameters of the resultant flow were determined. The nature of the flow was demonstrated to correspond to shear-driven boundary-layer flow at high Reynolds numbers. Correlation dependences of the flow velocity of the melt and its temperature in the surface region were derived theoretically and were shown to agree with the experimental results. When the size of the laser spot was much less than the characteristic convection scales, three regions of flow of the melt could be distinguished: a viscous surface boundary layer, a stagnation zone under the laser spot, and a large-scale region of flow with a homogeneous temperature distribution. (interaction of laser radiation with matter. laser plasma)

264

Convective Heat Transport in Compressible Fluids

We present hydrodynamic equations of compressible fluids in gravity as a generalization of those in the Boussinesq approximation used for nearly incompressible fluids. They account for adiabatic processes taking place throughout the cell (the piston effect) and those taking place within plumes (the adiabatic temperature gradient effect). Performing two-dimensional numerical analysis, we reveal some unique features of plume generation and convection in transient and steady st...

Furukawa, Akira; Onuki, Akira

2002-01-01

265

Energy Technology Data Exchange (ETDEWEB)

An experimental study has been undertaken to investigate evaporatively driven convection underneath a meniscus (liquid-vapour interface) formed in a vertically oriented capillary tube. The evaporation process is found responsible for inducing a convection pattern in the liquid phase below the meniscus. The convective structure was revealed using a {mu}-PIV technique. When external heating is applied to the system, the convection pattern is altered and can be reversed depending on the relative position of the heating element with respect to the liquid-vapour interface. An IR camera was used to measure temperature gradients generated by the heater along the capillary wall and along the liquid-vapour interface. This allowed the investigation of the relation between the temperature gradients generated along the tube wall and the convection taking place in the liquid under the effect of thermocapillary stress thus generated. The present study has demonstrated that the meniscus interfacial temperature profile is key for the onset of thermocapillary convection which is observed experimentally. (author)

Buffone, C.; Sefiane, K. [School of Engineering and Electronics, The University of Edinburgh, Mayfield Road, Edinburgh EH9 3JL, Scotland (United Kingdom)

2008-05-15

266

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

267

Natural convection heat transfer within horizontal spent nuclear fuel assemblies

Energy Technology Data Exchange (ETDEWEB)

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.

Canaan, R.E.

1995-12-01

268

External Forced Convection Enhancement using a Corona Discharge

An ionic wind is formed when air ions generated by a corona discharge are accelerated by an electric field and exchange momentum with neutral air molecules, causing air flow. Because ionic winds can generate flow with no moving parts, they offer an attractive method for enhancing the heat transfer from a surface that would otherwise only be cooled by natural convection and/or radiation. In the presence of an external, flat plate flow, ionic winds distort the boundary layer such that local hea...

Go, David B.; Maturana, Raul A.; Fisher, Timothy S.; Garimella, Suresh V.

2007-01-01

269

Dielectrophoretic force-driven thermal convection in annular geometry

The thermal convection driven by the dielectrophoretic force is investigated in annular geometry under microgravity conditions. A radial temperature gradient and a radial alternating electric field are imposed on a dielectric fluid that fills the gap of two concentric infinite-length cylinders. The resulting dielectric force is regarded as thermal buoyancy with a radial effective gravity. This electric gravity varies in space and may change its sign depending on the temperature gradient and the cylinder radius ratio. The linear stability problem is solved by a spectral-collocation method. The critical mode is stationary and non-axisymmetric. The critical Rayleigh number and wavenumbers depend sensitively on the electric gravity and the radius ratio. The mechanism behind the instability is examined from an energetic viewpoint. The instability in wide gap annuli is an exact analogue to the gravity-driven thermal instability.

Yoshikawa, Harunori N.; Crumeyrolle, Olivier; Mutabazi, Innocent

2013-02-01

270

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)

271

Measurement of void behavior in NSRR forced convection tests, (1)

International Nuclear Information System (INIS)

Measurement of transient void generation behavior is being planned in the NSRR forced convection tests and in the loop tests. This report describes the ex-reactor test results of the void measurement conducted as the preliminary ones for the planned in pile measurement. The tests were preformed with a mock-up test rig fabricated in the same size as those to be used in the forced convection tests in the ambient pressure capsule. A newly designed vibrating-vane type densimeter and a drag-disc type flow meter were used for the two phase flow measurement at the test channel exit. The generation of voids at the test section was simulated by injecting air voids through tiny holes on the cladding of the dummy fuel rod, and the response of the densimeter and the flow meter was obtained as the functions of flow rates of inlet coolant and injected air under steady state condition. The results were, firstly, the densimeter needs minor modification for the quantitative measurement, although it is usable for the void detection, and secondly, the drag-disc flow meter gives fairly good results, although the indication becomes unstable under low coolant flow conditions. It was concluded that the tested sensors are well applicable in the future inpile measurement with minor modifications. (author)

272

Calculational method for combined natural circulation and forced-convection flow in a channel

International Nuclear Information System (INIS)

This paper presents a finite element solution for combined natural circulation and forced convection flow in a channel. Because the buoyancy force plays an important role in a mixed convection flow, an iteration scheme was used in solving the coupled energy-momentum equations. The momentum equations and the pressure equation are solved to calculate velocity profiles instead of solving the momentum equations with the continuity equation. Though the pressure equation is obtained by using the continuity equation, the continuity principle is reinforced into the momentum equations and the pressure equation at each iteration. Calculations are performed for the combined natural circulation and forced convection case and the forced convection only case

273

A multiple-relaxation-time lattice Boltzmann model for convection heat transfer in porous media

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...

Liu, Qing; He, Ya-ling; Li, Qing; Tao, Wen-quan

2013-01-01

274

Analysis of two-layered natural convection in a pool with heat generation by unstructured FVM

International Nuclear Information System (INIS)

The natural convection and heat transfer in a stratified pool with internal heat source are studied. Two dimensional semicircular or axi-symmetric hemispherical geometry is modeled to investigate convective heat transfer. The flow and heat transfer characteristics are compared between single-layered and double-layered pools. And local Nusselt number distributions on the outer walls are obtained to consider thermal loads on the vessel wall. The unstructured mesh is chosen for this study because of the non-orthogonality originated from the boundaries of double-layered pool. The interface between the layers is modeled to be fixed. With this assumption mass flux across the interface is neglected, but shear force and heat flux by conduction are considered by the boundary conditions. The colocated cell-centered finite volume method is used with the modified Rhie-Chow interpolation to include body force effect. The wall pressure is extrapolated by the way to include body force. The numerical solutions calculated by current method shows that averaged downward heat flux of the double-layered pool increases compared to single-layered pool

275

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

276

International Nuclear Information System (INIS)

In order to understand the influence of a semispherical crucible geometry combined with different convection modes as a thermocapillary convection, natural convection and forced convection, induced by crystal rotation, on melt flow pattern in silicon Czochralski crystal growth process, a set of numerical simulations are conducted using Fluent Software. We solve the system of equations of heat and momentum transfer in classical geometry as cylindrical and modified crystal growth process geometry as cylindro-spherical. In addition, we adopt hypothesis adapted to boundary conditions near the interface and calculations are executed to determine temperature, pressure and velocity fields versus Grashof and Reynolds numbers. The analysis of the obtained results led to conclude that there is advantage to modify geometry in comparison with the traditional one. The absence of the stagnation regions of fluid in the hemispherical crucible corner and the possibility to control the melt flow using the crystal rotation enhances the quality of the process comparatively to the cylindrical one. The pressure field is strongly related to the swirl velocity.

277

Energy Technology Data Exchange (ETDEWEB)

In order to understand the influence of a semispherical crucible geometry combined with different convection modes as a thermocapillary convection, natural convection and forced convection, induced by crystal rotation, on melt flow pattern in silicon Czochralski crystal growth process, a set of numerical simulations are conducted using Fluent Software. We solve the system of equations of heat and momentum transfer in classical geometry as cylindrical and modified crystal growth process geometry as cylindro-spherical. In addition, we adopt hypothesis adapted to boundary conditions near the interface and calculations are executed to determine temperature, pressure and velocity fields versus Grashof and Reynolds numbers. The analysis of the obtained results led to conclude that there is advantage to modify geometry in comparison with the traditional one. The absence of the stagnation regions of fluid in the hemispherical crucible corner and the possibility to control the melt flow using the crystal rotation enhances the quality of the process comparatively to the cylindrical one. The pressure field is strongly related to the swirl velocity.

Mokhtari, F [Physics Department, Faculty of Science, University of Mouloud Mammeri, Tizi Ouzou (Algeria); Bouabdallah, A; Zizi, M [LTSE Laboratory, University of Science and Technology USTHB. BP 32 Elalia, Babezzouar, Algiers (Algeria); Hanchi, S [UER Mecanique/ E.M.P B.P 17, Bordj El Bahri, Algiers (Algeria); Alemany, A, E-mail: abouab2002@yahoo.f [Laboratoire EPM, CNRS, Grenoble (France)

2010-03-01

278

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)

279

Periodically excited Marangoni convection in a locally heated liquid layer

The results of the numerical simulation of weakly-supercritical thermocapillary convection in a thin liquid layer with a free deformable upper boundary and an inhomogeneous nonstationary heat source (a pulsating thermal spot, localized in space and periodically modulated in time) are presented.

Wertgeim, Igor I.; Kumachkov, Marat A.; Mikishev, Alexander B.

2013-03-01

280

Coupled three-dimensional conduction and natural convection heat transfer

A numerical and experimental investigation of three-dimensional natural convection heat transfer coupled with conduction was performed. This general problem is of great importance because of its widespread applicability in areas such as compact natural convection heat exchangers, cooling of electronic equipment, and porous media flows. The determination of flow patterns and heat transfer coefficients in such situations is necessary because of its practical use in various industries. A vectorized finite difference code was developed for the Cray-2 supercomputer which has the capability of simulating a wide class of three-dimensional coupled conduction-convection problems. This program numerically solves the transient form of the complete laminar Navier-Stokes equations of motion using the vorticity-vector potential methods. Using this program, numerical solutions were obtained for 3-D natural convection from a horizontal isothermal heat exchanger tube with an attached circular cooling fin array. Experiments were performed to measure three-dimensional temperature fields using Mach-Zehnder interferometry. Software was developed to digitize and process fringe patterns and inversion algorithms used to compute the 3-D temperature field.

Tolpadi, Anil Kumar

1987-09-01

281

International Nuclear Information System (INIS)

The results of an experimental study of the forced convective heat transfer of flowing gas-solid suspensions at high temperature are presented. Results are reported for helium-graphite mixtures which are electrically heated with a constant wall temperature condition up to T sub(W) = 1173K, flowing upward through a vertical circular tube; entering gas Reynolds number R sub(ei) ranged from 1.0 x 104 to 2.0 x 104, particle loading ratio GAMMA reached about 4. The following conclusions are drawn from the current study. i) In the case of R sub(ei) = 1.0 x 104, the Nusselt numbers of the forced convective heat transfer of gas solid suspensions, N sub(ub), based on local bulk properties increase monotonously with increase in the loading ratio GAMMA. On the other hand, in the runs of R sub(ei) = 1.5 x 104 and 2.0 x 104, the Nusselt numbers of suspensions, N sub(ub), have a minimum value at low solid loadings. ii) The ratio of the Nusselt number of the suspension to that of gas alone increases considerably as the wall temperatures increase. (author)

282

Non intrusive measurement of the convective heat transfer coefficient

Energy Technology Data Exchange (ETDEWEB)

The efficiency of cooling methods in thermal systems such as radiators and heat exchangers must be improved in order to enhance performance. The evaluation of the heat transfer coefficients between a solid and a fluid is necessary for the control and the dimensioning of thermal systems. In this study, the pulsed photothermal method was used to measure the convective heat transfer coefficient on a solid-fluid interface, notably between an air flow and a heated slab mounted on a PVC flat plate. This configuration simulated the electronic air-cooling inside enclosures and racks. The influence of the deflector's inclination angle on the enhancement of heat transfer was investigated using 2 newly developed identification models. The first model was based on a constant heat transfer coefficient during the pulsed experiment, while the second, improved model was based on a variable heat transfer coefficient. The heat transfer coefficient was deduced from the evolution of the transient temperature induced by a sudden deposit of a luminous energy on the front face of the slab. Temperature evolutions were derived by infrared thermography, a camera for cartography and a detector for precise measurement in specific locations. The results show the improvement of measurement accuracies when using a model that considers the temporal evolution of the convective heat transfer coefficient. The deflection of air flow on the upper surface of the heated slab demonstrated better cooling of the slab by the deflection of air flow. 11 refs., 1 tab., 8 figs.

Rebay, M.; Mebarki, G.; Padet, J. [Reims Univ., Reims (France). Faculty of Science, GRESPI Thermomechanical Lab; Arfaoui, A. [Reims Univ., Reims (France). Faculty of Science, GRESPI Thermomechanical Lab; Tunis Univ., Tunis (Tunisia). Faculty of Science, EL MANAR, LETTM; Maad, B.R. [Tunis Univ., Tunis (Tunisia). Faculty of Science, EL MANAR, LETTM

2010-07-01

283

Natural convection inside enclosures partially heated at one side

Steady laminar natural convection heat transfer inside airfilled square cavities cooled at one side and partially heated at the opposite side, is studied numerically. A computational code based on SIMPLE-C algorithm is used for the solution of the mass, momentum, and energy transfer equations. Simulations are performed for different values of the dimensionless size and location of the heater, and the Rayleigh number, whose effects on the temperature and velocity fields, a...

Corcione, M.; Fontana, L.; Habib, E.

2008-01-01

284

Relating Convective and Stratiform Rain to Latent Heating

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.

Tao, Wei-Kuo; Lang, Stephen; Zeng, Xiping; Shige, Shoichi; Takayabu, Yukari

2010-01-01

285

Mixed convection heat transfer in rotating vertical elliptic ducts

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 ascending 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 ...

Lasode, Olumuyiwa A.

2007-01-01

286

Theoretical Convective Heat Transfer Model Developement of Cold Storage Using Taguchi Analysis.

Directory of Open Access Journals (Sweden)

Full Text Available Energy crisis is one of the most important problems the world is facing now-a-days. With the increase of cost of electrical energy operating cost of cold storage storing is increasing which forces the increased cost price of the commodities that are kept. In this situation if the maximum heat energy(Q is absorbed by the evaporator inside the cold room through convective heat transfer process in terms of –heat transfer due to convection and heat transfer due to condensation, more energy has to be wasted to maintain the evaporator space at the desired temperature range of 2- 8 degree centigrade. In this paper we have proposed a theoretical heat transfermodel of convective heat transfer incold storage using Taguchi L9 orthogonal array. Velocity of air(V, Temperature difference(dT, RelativeHumidity(RHare the basic variable and three ranges are taken each of them in the model development. Graphical interpretations from the model justifies the reality

Dr.N.Mukhopadhyay

2015-01-01

287

Laminar forced convection slip-flow in a micro-annulus between two concentric cylinders

Energy Technology Data Exchange (ETDEWEB)

Forced convection heat transfer in hydrodynamically and thermally fully developed flows of viscous dissipating gases in annular microducts between two concentric micro cylinders is analyzed analytically. The viscous dissipation effect, the velocity slip and the temperature jump at the wall are taken into consideration. Two different cases of the thermal boundary conditions are considered: uniform heat flux at the outer wall and adiabatic inner wall (Case A) and uniform heat flux at the inner wall and adiabatic outer wall (Case B). Solutions for the velocity and temperature distributions and the Nusselt number are obtained for different values of the aspect ratio, the Knudsen number and the Brinkman number. The analytical results obtained are compared with those available in the literature and an excellent agreement is observed. (author)

Avci, Mete; Aydin, Orhan [Karadeniz Technical University, Department of Mechanical Engineering, 61080 Trabzon (Turkey)

2008-07-01

288

Measurement of liquid-solid contact during forced convective transition boiling

International Nuclear Information System (INIS)

The technique for measuring liquid-solid contact, recently developed at Birmingham, has now been applied to forced convection. This paper is the first account of these preliminary measurements. The basis of the technique is that the heat transfer surface is covered with a thin electrically insulating layer and the impedance of the system measured during boiling. This is achieved by anodising an aluminium test section. Details of the experimental loop and the test section will be given in the full paper. Briefly the test section was a vertical thick walled commercially pure aluminium cylinder, 10 mm internal diameter. Measurements were made during quenching with upflow of water. Heat flux was deduced from a simple heat balance

289

Critical heat flux in natural convection cooled TRIGA reactors with hexagonal bundle

Energy Technology Data Exchange (ETDEWEB)

A three-rod bundle Critical Heat Flux (CHF) study at low flow, low pressure, and natural convection condition has been conducted, simulating TRIGA reactors with the hexagonally configured core. The test section is a custom-made trefoil shape tube with three identical fuel pin heater rods located symmetrically inside. The full scale fuel rod is electrically heated with a chopped-cosine axial power profile. CHF experiments were carried out with the following conditions: inlet water subcooling from 30 K to 95 K; pressure from 110 kPa to 230 kPa; mass flux up to 150 kg/m{sup 2}s. About 50 CHF data points were collected and compared with a few existing CHF correlations whose application ranges are close to the testing conditions. Some tests were performed with the forced convection to identify the potential difference between the CHF under the natural convection and forced convection. The relevance of the CHF to test parameters is investigated. (authors)

Yang, J.; Avery, M.; De Angelis, M.; Anderson, M.; Corradini, M. [Univ. of Wisconsin-Madison, 1500 Engineering Drive, Madison, WI 53706 (United States); Feldman, E. E.; Dunn, F. E.; Matos, J. E. [Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439 (United States)

2012-07-01

290

Critical heat flux in natural convection cooled TRIGA reactors with hexagonal bundle

International Nuclear Information System (INIS)

A three-rod bundle Critical Heat Flux (CHF) study at low flow, low pressure, and natural convection condition has been conducted, simulating TRIGA reactors with the hexagonally configured core. The test section is a custom-made trefoil shape tube with three identical fuel pin heater rods located symmetrically inside. The full scale fuel rod is electrically heated with a chopped-cosine axial power profile. CHF experiments were carried out with the following conditions: inlet water subcooling from 30 K to 95 K; pressure from 110 kPa to 230 kPa; mass flux up to 150 kg/m2s. About 50 CHF data points were collected and compared with a few existing CHF correlations whose application ranges are close to the testing conditions. Some tests were performed with the forced convection to identify the potential difference between the CHF under the natural convection and forced convection. The relevance of the CHF to test parameters is investigated. (authors)

291

Optimization of fin geometry in heat convection with entransy theory

International Nuclear Information System (INIS)

The entransy theory developed in recent years is used to optimize the aspect ratio of a plate fin in heat convection. Based on a two-dimensional model, the theoretical analysis shows that the minimum thermal resistance defined with the concept of entransy dissipation corresponds to the maximum heat transfer rate when the temperature of the heating surface is fixed. On the other hand, when the heat flux of the heating surface is fixed, the minimum thermal resistance corresponds to the minimum average temperature of the heating surface. The entropy optimization is also given for the heat transfer processes. It is observed that the minimum entropy generation, the minimum entropy generation number, and the minimum revised entropy generation number do not always correspond to the best heat transfer performance. In addition, the influence factors on the optimized aspect ratio of the plate fin are also discussed. The optimized ratio decreases with the enhancement of heat convection, while it increases with fin thermal conductivity increasing. (general)

292

Optimization of fin geometry in heat convection with entransy theory

The entransy theory developed in recent years is used to optimize the aspect ratio of a plate fin in heat convection. Based on a two-dimensional model, the theoretical analysis shows that the minimum thermal resistance defined with the concept of entransy dissipation corresponds to the maximum heat transfer rate when the temperature of the heating surface is fixed. On the other hand, when the heat flux of the heating surface is fixed, the minimum thermal resistance corresponds to the minimum average temperature of the heating surface. The entropy optimization is also given for the heat transfer processes. It is observed that the minimum entropy generation, the minimum entropy generation number, and the minimum revised entropy generation number do not always correspond to the best heat transfer performance. In addition, the influence factors on the optimized aspect ratio of the plate fin are also discussed. The optimized ratio decreases with the enhancement of heat convection, while it increases with fin thermal conductivity increasing.

Cheng, Xue-Tao; Zhang, Qin-Zhao; Xu, Xiang-Hua; Liang, Xin-Gang

2013-02-01

293

Experimental investigation of convective heat transfer of Al2O3/water nanofluid in circular tube

International Nuclear Information System (INIS)

Nanofluids are suspensions of metallic or nonmetallic nanopowders in base liquid and can be employed to increase heat transfer rate in various applications. In this work laminar flow forced convection heat transfer of Al2O3/water nanofluid inside a circular tube with constant wall temperature was investigated experimentally. The Nusselt numbers of nanofluids were obtained for different nanoparticle concentrations as well as various Peclet and Reynolds numbers. Experimental results emphasize the enhancement of heat transfer due to the nanoparticles presence in the fluid. Heat transfer coefficient increases by increasing the concentration of nanoparticles in nanofluid. The increase in heat transfer coefficient due to presence of nanoparticles is much higher than the prediction of single phase heat transfer correlation used with nanofluid properties

294

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 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)

295

Directory of Open Access Journals (Sweden)

Full Text Available Combined free convection and forced convection from a flush-mounted uniform heat source on the bottom of a horizontal rectangular enclosure with side openings is studied numerically. The inlet opening allows an externally induced air stream at the ambient temperature to flow through the cavity and exits from another two openings placed top of the both side walls. Two-dimensional forms of Navier-Stokes equations are solved by using control volume based finite element technique. Three typical values of the Reynolds numbers, based on the enclosure height, are chosen as Re = 50, 100 and 200, and steady, laminar results are obtained in the range of Richardson number as 0 = Ri = 10 and a fixed Prandtl number of 0.71. The parametric studies for a wide range of governing parameters show consistent performance of the present numerical approach to obtain as stream functions and temperature profiles. Heat transfer rates at the heated walls are presented in terms of average Nusselt numbers. The computational results indicate that the heat transfer coefficient is strongly affected by Reynolds number and Richardson number. An empirical correlation is developed by using Nusselt number, Reynolds number and Richardson number.

Sumon Saha

2006-10-01

296

Convective heat transfer with film cooling around a rotor blade

This paper deals with an experimental convective heat transfer investigation around a high pressure gas turbine film cooled rotor blade. The measurements were performed in the von Karman Institute short duration isentropic light piston compression tube facility allowing a correct simulation of Mach and Reynolds number as well as free stream to wall and free stream to coolant temperature ratios. The airfoil was mounted in a linear stationary cascade environment and heat transfer measurements were obtained by using platinum thin film gages painted on a blade made of machinable glass ceramic. The coolant flow was ejected simultaneously through the leading edge (3 rows of holes), the suction side (2 rows of holes), and the pressure side (1 row of holes). The coolant hydrodynamic behavior is described and the effects of overall coolant to free stream mass weight ratio, coolant to free stream temperature ratio, and free stream turbulence intensity on the convective heat transfer distribution are successively described.

Arts, T.

297

Heat transfer in natural convection in horisontal fuel element bundles

International Nuclear Information System (INIS)

The heat exchange in case of natural convection in horizontal fuel elements bundles enclosed in the can as applied to the conditions arising during transportation of spent fuel elements in horizontal transport containers is considered. The investigation is performed using horizontal bundles with different shape of the can and parameters of bundle spacing with different numbers of series and values of relative step of hundle location when filling the bundles space by air, carbon dioxide, water and MS-20 oils under pressure up to 1.8 MPa. The variation of distribution of rod temperatures and the form of coolant isotherms in the cross section of bundles with the development of natural convection are considered. The dependences which enable one at the designed heat release to calculate the medium temperature of rods and the temperature of the maximum heated rod in the bundle are suggested

298

Evaporation of a binary liquid film by forced convection

Directory of Open Access Journals (Sweden)

Full Text Available This paper deals with a numerical analysis of the evaporation of a thin binary liquid film by forced convection inside a channel constituted by two parallel plates. The first plate is externally insulated and wetted by a thin water ethylene glycol film while the second is dry and isothermal. The liquid mixture consists of water (the more volatile component and ethylene glycol while the gas mixture has three components: dry air, water vapour and ethylene-glycol vapour. The set of non linear and coupled equations expressing the conservation of mass, momentum, energy and species in the liquid and gas mixtures is solved numerically using a finite difference method. Results concerns with the effects of inlet ambience conditions and the inlet liquid concentration of ethylene glycol on the distribution of the temperature, concentrations profiles and the axial variation of the evaporation rate of species i.

Nasr Abdelaziz

2011-01-01

299

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.

Olson, Sandra

2011-01-01

300

Experimental analysis of the heat transfer induced by thermocapillary convection around a bubble

Energy Technology Data Exchange (ETDEWEB)

The surface tension driven flow in the liquid vicinity of gas bubbles on a heated wall and its contribution to the heat transfer are investigated experimentally in a configuration where surface tension force and buoyancy forces oppose one another. This liquid flow caused by the temperature gradient along the interface is called thermocapillary or thermal Marangoni convection. The studies were made with silicone oils of different viscosities so that a wide range of dimensionless numbers were encountered. The velocity fields are determined from the motion of carbon particles in the meridian plane of the bubble. The influence of the temperature gradient, the oil viscosity, and the bubble shape on the profiles along the interface and in the direction normal to the interface is analyzed. The temperature field is determined by holographic interferometry. For the axisymmetric problem, the interferograms are evaluated by solving the Abel-integral equation. From the isotherms, the temperature distribution along the bubble surface and in the liquid beneath the bubble is measured. To quantify the contribution of thermocapillarity to the heat transfer, the heat flux transferred by thermocapillarity is measured. A heat exchange law giving the increase in heat flux due to Marangoni convection in comparison to the conductive regime is proposed.

Arlabosse, P.; Tadrist, L.; Tadrist, H.; Pantaloni, J.

2000-02-01

301

Energy Technology Data Exchange (ETDEWEB)

Forced convection heat transfer in a phase change material (PCM) suspension in a rectangular duct was investigated. PCM particle sizes varied. Time-dependent moving-mesh computational fluid dynamics (CFD) models were used to simulate heat transfer processes for each particle and a series of melting and solidification processes. The effect of particle dimension on the heat transfer coefficient was also investigated. A simplified theoretical analysis was performed using a 3-D model of the heated channel. The liquid fraction was computed at each iteration in relation to an enthalpy balance. Enthalpy was computed as the sum of the sensible enthalpy and the latent heat. The finite volume method was used to derive the convection heat transfer coefficient. The model was validated by comparing it with results obtained during earlier experiments. Results of the study demonstrated that the size and fit of the particles plays a significant role in heat transfer efficiency. It was concluded that the heat transfer coefficient attains its maximum value for particles that are almost identical in size as the channel. 4 refs., 3 tabs., 7 figs.

Hassanipour, F.; Lage, J. [Southern Methodist Univ., Dallas, TX (United States)

2009-07-01

302

International Nuclear Information System (INIS)

Heat transfer coefficients and pressure drop of gaseous ammonia in forced convection are experimentally determined. The fluid flows (mass flow rate 0.6 to 2.4 g/s) in a long tungsten tube (di = 2.8 mm, de = 5.1 mm, L = 700 mm) electrically heated. The temperature of the wall reaches 3000 deg. K and the fluid 2500 deg. K; maximum heat flux 530 w/cm2. Ammonia is completely dissociated and the power necessary for dissociation reaches 30 per cent of the total power exchanged. Inlet pressure varies between 6 and 16 bars and the maximum pressure drop in the tube reaches 15 bars. Two regimes of dissociation have been shown: catalytic and homogeneous and the variation of dissociation along the length of the tube is studied. The measured heat transfer coefficients may be about 10 times these calculated by the means of classical formulae. A correlation of experimental results using enthalpy as a driving force for heat transmission is presented. Pressure drops may be calculated by the means of a classical friction factor. (authors)

303

International Nuclear Information System (INIS)

Magnetohydrodynamic (MHD) fully developed flow of a viscous incompressible electrically conducting fluid in a vertical channel during combined convection, with asymmetric heating of the wall, under the influence of a constant pressure gradient and in the presence of an uniform transverse magnetic field, is studied. Exact solution of the governing equation is obtained in a closed form. The solution in a dimensionless form contains two pertinent flow parameters, viz. M (the Hartmann number) and Gr (the Grashof number). The limiting cases of a MHD forced and free convection are analysed, what has not been done earlier in the literature. The occurrence of flow reversal indicates that there arises a flow reversal at the cold wall when rT=1 while, for rT<1, no flow reversal is possible in the absence of magnetic forces. (author)

304

Convective heat transfer in airflow through a duct with wall thermal radiation

International Nuclear Information System (INIS)

This paper presents a numerical investigation on airflow through a heated horizontal rectangular duct wherein the model considers the combined modes of natural and forced convection heat transfer and the thermal radiation from duct walls. The duct periphery is differentially heated with known temperature profiles imposed on the two opposite vertical sidewalls while the other two walls are treated as adiabatic. The air enters into the duct hydrodynamically fully developed and flows steadily under laminar conditions undergoing thermal development within the duct. Considering several temperature profiles on the two vertical sidewalls, the numerical simulation generates the heat transfer rates and associated fluid flow patterns in the duct for a range of airflow rates, duct aspect ratios and surface emissivity. The variation of local Nusselt number at duct walls and the fluid flow patterns are critically examined to identify thermal instabilities and the significance of wall thermal radiation effects on the overall heat transfer rates.

305

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

306

MHD mixed convective heat transfer flow about an inclined plate

Energy Technology Data Exchange (ETDEWEB)

Mixed convection heat transfer about a semi-infinite inclined plate in the presence of magneto and thermal radiation effects is studied. The fluid is assumed to be incompressible and dense. The nonlinear coupled parabolic partial differential equations governing the flow are transformed into the non-similar boundary layer equations, which are then solved numerically using the Keller box method. The effects of the mixed convection parameter R{sub i}, the angle of inclination {alpha}, the magnetic parameter M and the radiation-conduction parameter R{sub d} on the velocity and temperature profiles as well as on the local skin friction and local heat transfer parameters. For some specific values of the governing parameters, the results are compared with those available in the literature and a fairly good agreement is obtained. (orig.)

Aydin, Orhan [Karadeniz Technical University, Department of Mechanical Engineering, Trabzon (Turkey); Kaya, Ahmet [Aksaray University, Department of Mechanical Engineering, Aksaray (Turkey)

2009-11-15

307

Natural Convection Heat Transfer of Two Vertically Staggered Cylinders

International Nuclear Information System (INIS)

This study measured the natural convection heat transfer of two vertically staggered cylinders varying RaD, the vertical pitch-to-diameter, Pv/D and the horizontal pitch-to-diameter, Ph/D. The geometry frequently appears in heat exchangers and other engineering applications. This study has the relevance with the design of the test facility for PDRC (Passive Decay Heat Removal System) in SFR (Sodium-cooled Fast Reactor). In the staggered arrangement, the heat transfer of the lower cylinder is unaffected by the presence of the upper cylinder. However, the heat transfer of the upper cylinder is affected by the plume developed from the lower cylinder on aligned of two vertically staggered cylinders. When the spacing between two cylinders was less than the critical distance, the upper cylinder was affected more by the preheating effect. As the distance increases, the preheating effect decreases and the velocity effect increases

308

Natural convection in wavy enclosures with volumetric heat sources

Energy Technology Data Exchange (ETDEWEB)

In this paper, the effects of volumetric heat sources on natural convection heat transfer and flow structures in a wavy-walled enclosure are studied numerically. The governing differential equations are solved by an accurate finite-volume method. The vertical walls of enclosure are assumed to be heated differentially whereas the two wavy walls (top and bottom) are kept adiabatic. The effective governing parameters for this problem are the internal and external Rayleigh numbers and the amplitude of wavy walls. It is found that both the function of wavy wall and the ratio of internal Rayleigh number (Ra{sub I}) to external Rayleigh number (Ra{sub E}) affect the heat transfer and fluid flow significantly. The heat transfer is predicted to be a decreasing function of waviness of the top and bottom walls in case of (IRa/ERa)>1 and (IRa/ERa)<1. (authors)

Oztop, H.F.; Varol, Y. [Department of Mechanical Engineering, Technology Faculty, Firat University, TR-23119 Elazig (Turkey); Abu-Nada, E. [Institut fur Technische Verbrennung, Leibniz Universitat, Hannover, Welfengarten 1a, 30167 Hannover (Germany); Department of Mechanical Engineering, King Faisal University, Al-Ahsa 31982 (Saudi Arabia); Chamkha, A. [Manufacturing Engineering Department, The Public Authority for Applied, Education and Training, Shuweikh 70654 (Kuwait)

2011-04-15

309

Natural convection in wavy enclosures with volumetric heat sources

International Nuclear Information System (INIS)

In this paper, the effects of volumetric heat sources on natural convection heat transfer and flow structures in a wavy-walled enclosure are studied numerically. The governing differential equations are solved by an accurate finite-volume method. The vertical walls of enclosure are assumed to be heated differentially whereas the two wavy walls (top and bottom) are kept adiabatic. The effective governing parameters for this problem are the internal and external Rayleigh numbers and the amplitude of wavy walls. It is found that both the function of wavy wall and the ratio of internal Rayleigh number (RaI) to external Rayleigh number (RaE) affect the heat transfer and fluid flow significantly. The heat transfer is predicted to be a decreasing function of waviness of the top and bottom walls in case of (IRa/ERa)>1 and (IRa/ERa)<1. (authors)

310

International Nuclear Information System (INIS)

Turbulent quantities a natural convection boundary layer were measured a newly using developed hot wire anemometer. These experimental results make clear the basic heat transfer characteristics and the problem of standard turbulence model. It is difficult to obtain reliable experimental results of the turbulent natural convection boundary layer, because this flow has large fluctuations of temperature at low velocity. In this study, two hot wire anemometers were developed for instantaneous measurement of both velocity and temperature in the turbulent natural convection boundary layer. Error of the measurements was evaluated by equations proposed in this study. These equations can estimate the error from time-response and space-resolution quantitatively. For turbulent natural convection around a horizontal cylinder, experiments were carried out using this measuring system to investigate turbulent quantities. From these experimental results it is found that the turbulent mechanism is different from that of forced convection, and this phenomenon can not to be predicted by standard k-? turbulence models. In order to attain high predictability, it is necessary to consider the effect of turbulent generation due to buoyancy at the transport equation of turbulent quantities. (author)

311

Numerical analysis of laminar forced convection in a spherical annulus

International Nuclear Information System (INIS)

Calculations of steady laminar incompressible fluid-flow and heat transfer in a spherical annulus are presented. Steady pressures, temperatures, velocities, and heat transfer coefficients are calculated for an insulated outer sphere and a 00C isothermal inner sphere with 500C heated water flowing in the annulus. The inner sphere radius is 13.97 cm, the outer sphere radius is 16.83 cm and the radius ratio is 1.2. The transient axisymmetric equations of heat, mass, and momentum conservation are solved numerically in spherical coordinates. The transient solution is carried out in time until steady state is achieved. A variable mesh is used to improve resolution near the inner sphere where temperature and velocity gradients are steep. It is believed that this is the first fully two-dimensional analysis of forced flow in a spherical annulus. Local and bulk Nusselt numbers are presented for Reynolds numbers from 4.4 to 440. Computed bulk Nusselt numbers ranged from 2 to 50 and are compared to experimental results from the literature. Inlet flow jetting off the inner sphere and flow separation are predicted by the analysis. The location of wall jet separation was found to be a function of Reynolds number, indicating the location of separation depends upon the ratio of inertia to viscous forces. Wall jet separation has a pronounced effect on the distribution of local heat flux. The area between inlet and separation was found to be the most significant area for heat transfer. Radial distributions of azimuthal velocity and temperature are presented for various angles beginning at the inlet. Inner sphere pressure distribution is presented and the effect on flow separation is discussed

312

A preliminary study on the mixed convection heat transfer in a horizontal pipe

Energy Technology Data Exchange (ETDEWEB)

Mixed convection phenomena have applications in heat exchangers designed for viscous liquids, pipelines used for transporting oil, and heat exchangers for gas flows and have been investigated for numerical study. The experimental studies on the topic are rare due to the limited practical uses. The definitions on the buoyancy coefficient that represent the relative influence of the forced and the natural convection are different by scholars. When the fluid passes through a horizontal heated pipe, the secondary flow patterns of the flows moving upward along inner wall and downward through the centerline of the pipe, appear. Two symmetric counter rotating, spiraling axial flows around the vertical centerline and the heat transfer of the bottom is higher due to the unstable stratification. This study is preliminary experiments for laminar and turbulent mixed convection for horizontal pipe geometries. The diameter of the pipe was 0.026m and lengths were ranging from 0.03m to 0.50m varying the Reynolds number. Experiments were carried out using a copper sulfate electroplating system was adopted for mass transfer based on the analogy concept.

Chae, Myeong Seon; Chung, Bum Jin [Jeju Nat' l Univ., Jeju (Korea, Republic of)

2012-10-15

313

Aerial measurements of convection cell elements in heated lakes

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.

Villa-Aleman, E.; Salaymeh, S. R.; Brown, T. B.; Garrett, A. J.; Nichols, L. S.; Pendergast, M. M.

2008-03-01

314

Thermal Performance Of Convective-Radiative Heat Transfer In Porous Fins

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 ...

Shahbabaei, Majid; Saedodin, Seyfolah

2014-01-01

315

Free-convective heat transfer in verticle bundles of heated rods

International Nuclear Information System (INIS)

The data of experimental studies of the character of transformation from radiation-conductive into free-convective heat transfer in bindles of heated rods depending on their height and configuration are given. It is shown that maximum temperatures in bundles with big height can be determined by radiation-conductive heat transfer mechanism under conditions when free convection changes noticeably the temperature field qualitative character. Quantitative recommendations for calculations of maximum temperature in casks used for storage and transportation of spent fuel assemblies from power reactors

316

Convective Heat Transfer Analysis in Fluid Flow with Turbulence Promoters with Heat Pipes

Directory of Open Access Journals (Sweden)

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.

Theodor Mateescu

2007-01-01

317

Convective Heat and Mass Transfer in Rotating Disk Systems

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

Shevchuk, Igor V

2009-01-01

318

Heat transfer by natural convection between enclosed concentric vertical cylinders

International Nuclear Information System (INIS)

The effect of natural convection upon the flow pattern was investigated in the closed annular cavity formed by two concentric vertical cylinders with a part of the inner cylinder heated. From observation of the streamline in the cavity by a smoke tracer, fundamental equations are solved numerically. Steady-state isothermals and streamlines are obtained for Grashof numbers up to 2 x 107 and for various lengths of heating zone. To check the calculated results, temperature distributions are measured in this cavity. The calculated temperatures are a good approximation of the experimental ones and the calculated Nusselt number is compatible with some previous theoretical and experimental results. (author)

319

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 ...

Dore, Valentina

2010-01-01

320

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.

321

Periodic heat transfer by forced laminar boundary layer flow over a semi-infinite flat plate

The paper reports a study of periodic convection in a steady forced laminar boundary layer flow over a semi-infiniteimpermeable .at plate due to periodical variation of the wall heat flux. The Fourier transform based approach allows to obtain a transfer function for the boundary layer that can be used to solve also transient (non-periodic) heating problems, and examples are reported comparing with available studies in the open literature. The effect of periodic heating on the value of the ave...

Cossali, Gianpietro

2005-01-01

322

Forced and combined convection of water in a vertical seven-rod bundle with P/D = 1.38

International Nuclear Information System (INIS)

Heat transfer experiments of forced turbulent and laminar, and combined laminar downflows of water are conducted in a uniformly heated, triangularly arranged, seven-rod bundle having a pitch-to-diameter ratio of 1.38. In the forced flow experiments Reynolds number (Re) ranged from 1200 to 24 800 and Prandtl number (Pr) from 6.8 to 9.0, while in the combined convection experiments Re varied from 148 to 3800, Grashof number (Grq) from 1.3 x 105 to 3 x 106, and Richardson number (Ri) from 0.01 to 9. The data in the forced turbulent and the laminar flow regimes are in good agreement with the upflow correlations (within ±10%). Also, the transition between these two regimes, occurring at Re = 3800, is the same as that for the upflow condition. In the laminar flow regime, the flow entering the heated section is hydrodynamically developing while the flow in the heated section is thermally developed. The transition from forced laminar to combined convection occurred at Ri = 0.1, which is an order of magnitude lower than that for upflow. The combined convection data are correlated by superimposing the correlations for forced laminar and natural laminar flows as: NuC,L =[NuF,L3 + NuN,L3]1/3, for upflow and NuC,L=[NuF,L2 -NuN,L2]1/2, for downflow, where NuC,L, NuF,L and NuN,L are the b>F,L and NuN,L are the Nusselt number for combined laminar flow, forced laminar flow and natural laminar flow respectively. These correlations are within ±11 and ±15% of the upflow and downflow data, respectively. (author)

323

Natural convective boundary layer flow of a nano-fluid past a convectively heated vertical plate

Energy Technology Data Exchange (ETDEWEB)

Natural convective flow of a nano-fluid over a convectively heated vertical plate is investigated using a similarity analysis of the transport equations followed by their numerical computations. The transport model employed includes the effect of Brownian motion and thermophoresis. The analysis shows that velocity, temperature and solid volume fraction of the nano-fluid profiles in the respective boundary layers depend, besides the Prandtl and Lewis numbers, on four additional dimensionless parameters, namely a Brownian motion parameter Nb, a thermophoresis parameter Nt, a buoyancy-ratio parameter Nr and convective parameter Nc. In addition to the study of these parameters on the boundary layer flow characteristics (velocity, temperature, solid volume fraction of the nano-fluid, skin friction, and heat transfer), correlations for the Nusselt and Sherwood numbers have been developed based on a regression analysis of the data. These linear regression models provide a highly accurate (with a maximum standard error of 0.004) representation of the numerical data and can be conveniently used in engineering practice. (authors)

Aziz, A. [Department of Mechanical Engineering, School of Engineering and Applied Science, Gonzaga University, Spokane, WA 99258 (United States); Khan, W.A. [Department of Engineering Sciences, PN Engineering College, National University of Sciences and Technology, Karachi 75350 (Pakistan)

2012-03-15

324

Natural convective boundary layer flow of a nano-fluid past a convectively heated vertical plate

International Nuclear Information System (INIS)

Natural convective flow of a nano-fluid over a convectively heated vertical plate is investigated using a similarity analysis of the transport equations followed by their numerical computations. The transport model employed includes the effect of Brownian motion and thermophoresis. The analysis shows that velocity, temperature and solid volume fraction of the nano-fluid profiles in the respective boundary layers depend, besides the Prandtl and Lewis numbers, on four additional dimensionless parameters, namely a Brownian motion parameter Nb, a thermophoresis parameter Nt, a buoyancy-ratio parameter Nr and convective parameter Nc. In addition to the study of these parameters on the boundary layer flow characteristics (velocity, temperature, solid volume fraction of the nano-fluid, skin friction, and heat transfer), correlations for the Nusselt and Sherwood numbers have been developed based on a regression analysis of the data. These linear regression models provide a highly accurate (with a maximum standard error of 0.004) representation of the numerical data and can be conveniently used in engineering practice. (authors)

325

Optimization of a Radial Flow Heat Sink Under Natural Convection

A steady-state three-dimensional numerical model is developed to predict natural convection heat transfer from a radial flow heat sink. The considered medium is air. The effect of several design parameters, such as the fin length and height, number of fins, and the heat sink base radius, on heat transfer is investigated. The Taguchi method, known to be a very useful tool for selecting the best levels of control factors, is employed. Five factors and four levels for each factor are chosen. Sixteen kinds of models are analyzed, and the total heat transfer for each model is obtained. The results are used to estimate the optimum design values of the parameters affecting the heat sink performance. The reliability of these values is verified. The average heat transfer rate of the optimum model is shown to increase by 60% as compared to the reference model. Finally, the heat transfer data at different outer radii of the radial flow heat sink are correlated.

Bhowmik, Himangshu

2014-01-01

326

International Nuclear Information System (INIS)

In this paper, a study of convective mass transfer coefficient and rate of moisture removal from cabbage and peas for open sun drying and inside greenhouse drying has been performed as a function of climatic parameters. The hourly data for the rate of moisture removal, crop temperature, relative humidity inside and outside the greenhouse and ambient air temperature for complete drying have been recorded. The experiments were conducted after the crop harvesting season from September to December 2001. These data were used for determination of the coefficient of convective mass transfer and then for development of the empirical relation of convective mass transfer coefficient with drying time under natural and forced modes. The empirical relations with convective mass transfer for open and greenhouse drying have been compared. The convective mass transfer coefficient was lower for drying inside the greenhouse with natural mode as compared to open sun drying. Its value was doubled under the forced mode inside the greenhouse drying compared to natural convection in the initial stage of drying

327

Freezing in forced convection flows inside ducts: A review

The present work gives an overview about the recent developments in research on freezing phenomena in forced convection flows inside ducts. Emphasis is given to the fundamental aspects of the phenomena observed in the solidification processes as well as on the analytical and numerical modelling aspects for this kind of problems. The paper deals with solidification problems inside tubes, parallel plate channels, curved rectangular channels and in diverging rectangular channels. Additionally, some new experimental and numerical results on solidification in duct flows are shown from the current research program in Darmstadt on freezing phenomena. Zusammenfassung Die vorliegende Arbeit gibt einen Überblick über aktuelle Forschungsentwicklungen auf dem Gebiet des Gefrierens in durchströmten Kanälen. Der Schwerpunkt wird auf die grundsätzlichen Aspekte gelegt, die sowohl bei den beobachteten Gefriervorgängen selbst, als auch bei deren analytischer oder numerischer Modellierung von Bedeutung sind. Der Artikel befaßt sich mit Gefriervorgängen in Rohren, zwischen parallelen Platten, gekrümmten, rechteckigen Kanälen und divergenten, rechteckigen Kanälen. Darüberhinaus werden einige aktuelle Ergebnisse beim Erstarren von Wasser in Kanalströmungen aus den laufenden Forschungsarbeiten in Darmstadt vorgestellt.

Weigand, B.; Braun, J.; Neumann, S. O.; Rinck, K. J.

328

Why convective heat transport in the solar nebula was inefficient

The radial distributions of the effective temperatures of circumstellar disks associated with pre-main sequence (T Tauri) stars are relatively well-constrained by ground-based and spacecraft infrared photometry and radio continuum observations. If the mechanisms by which energy is transported vertically in the disks are understood, these data can be used to constrain models of the thermal structure and evolution of solar nebula. Several studies of the evolution of the solar nebula have included the calculation of the vertical transport of heat by convection. Such calculations rely on a mixing length theory of transport and some assumption regarding the vertical distribution of internal dissipation. In all cases, the results of these calculations indicate that transport by radiation dominates that by convection, even when the nebula is convectively unstable. A simple argument that demonstrates the generality (and limits) of this result, regardless of the details of mixing length theory or the precise distribution of internal heating is presented. It is based on the idea that the radiative gradient in an optically thick nebula generally does not greatly exceed the adiabatic gradient.

Cassen, P.

1993-01-01

329

Energy Technology Data Exchange (ETDEWEB)

coupled buoyancy and thermo-capillary convection lead to a convective motion of the interface liquid/gas which drastically changes the heat and mass transfer across the liquid layer. Two experiments were 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 appears as petals or rays when the aspect ratio. The lateral confinement selects the azimuthal wavelength. 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 10{sup 5}). In the case of mercury, the thermo-capillary effect is reduced to zero to impurities at the surface which have special trajectories we describe and compare to a simpler experiment. Only the buoyancy forces induce a unstationary, weakly turbulent flow as soon as the heating power exceeds 4W (Ra = 4.5 10{sup 3}, calculated with h = 1 mm). The past part concerns the analysis of the effect on the flow of the boundary conditions, the geometry, the Prandtl number and the buoyancy force with the help of the literature. Results concerning heat transfer, in particular the exponent of the law Nusselt number vs. heating power, were compared with available data. (author) 115 refs.

Favre, E.

1997-09-26

330

Experimental Study of Forced Convection over Equilateral Triangle Helical Coiled Tubes

Directory of Open Access Journals (Sweden)

Full Text Available

This study presents an experimental investigation of an equilateral triangular cross-sectioned helical tube under uniform heat flux boundary condition. The experiments are carried out for nine helical coiled-tubes of different parameters. Different diameter ratio (D/a ranged from 6.77 to 15.43 and pitch ratio (P/a ranged from 1.127 to 3.062 are employed in the present study, The experiments covered a range of Reynolds number from 5.3X10^{2} to 2.2X10^{3}. Uniform heat flux is applied to the inside surface of the helical coil and air is selected as tested fluid. The experimental results obtained from the equilateral triangular cross-sectioned helical tube indicated that the parameters of the coil diameter and pitch of helical coil have important effects on the heat transfer coefficient. The Nusselt number increases with the increase of Reynolds number and coil diameter at constant pitch of the helical coil. Also, Nusselt number increases with the increase of Reynolds number and Pitch of helical coil at constant coil diameter tube. A comparison between the present experimental data with a previous work with circular cross-sectioned helical tubes have the same test conditions was achieved. From this comparison, it is clear that the average enhancement of Nusselt number for equilateral triangular cross-sectioned helical is about 1.12 ~ 1.25 times the circular cross-sectioned helical for all tested conditions. A general correlation of the average Nusselt number as a function in Re, D/a and P/a ratios is obtained to describe the forced convection from the equilateral triangle cross sectioned coiled tube.

** Key words: ** Forced convection; Helical coiled tubes; Coil diameter ratio; Pitch ratio

E. El-Kashif

2012-06-01

331

Directory of Open Access Journals (Sweden)

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.

S. Venkatachalapathy

2010-01-01

332

Numerical analysis of transient laminar forced convection of nanofluids in circular ducts

In this study, forced convection heat transfer characteristics of nanofluids are investigated by numerical analysis of incompressible transient laminar flow in a circular duct under step change in wall temperature and wall heat flux. The thermal responses of the system are obtained by solving energy equation under both transient and steady-state conditions for hydro-dynamically fully-developed flow. In the analyses, temperature dependent thermo-physical properties are also considered. In the numerical analysis, Al2O3/water nanofluid is assumed as a homogenous single-phase fluid. For the effective thermal conductivity of nanofluids, Hamilton-Crosser model is used together with a model for Brownian motion in the analysis which takes the effects of temperature and the particle diameter into account. Temperature distributions across the tube for a step jump of wall temperature and also wall heat flux are obtained for various times during the transient calculations at a given location for a constant value of Peclet number and a particle diameter. Variations of thermal conductivity in turn, heat transfer enhancement is obtained at various times as a function of nanoparticle volume fractions, at a given nanoparticle diameter and Peclet number. The results are given under transient and steady-state conditions; steady-state conditions are obtained at larger times and enhancements are found by comparison to the base fluid heat transfer coefficient under the same conditions.

Sert, ?smail Ozan; Sezer-Uzol, Nilay; Kakaç, Sad?k

2013-10-01

333

Laser heating of uncoated optics in a convective medium.

Powerful, long-pulse lasers have a variety of applications. In many applications, optical elements are employed to direct, focus, or collimate the beam. Typically the optic is suspended in a gaseous environment (e.g., air) and can cool by convection. The variation of the optic temperature with time is obtained by combining the effects of laser heating, thermal conduction, and convective loss. Characteristics of the solutions in terms of the properties of the optic material, laser beam parameters, and the environment are discussed and compared with measurements at the Naval Research Laboratory, employing kW-class, 1 µm wavelength, continuous wave lasers and optical elements made of fused silica or BK7 glass. The calculated results are in good agreement with the measurements, given the approximations in the analysis and the expected variation in the absorption coefficients of the glasses used in the experiments. PMID:22614476

Hafizi, B; Ting, A; Gordon, D F; Sprangle, P; Peñano, J R; Fischer, R F; DiComo, G P; Colombant, D C

2012-05-10

334

Gravity waves (GW) are responsible for driving large scale circulations like Brewer-Dobson circulation, contribute to the wave driving of the QBO in the tropics, and are also known as a coupling mechanism between tropospheric sources and the upper stratosphere to mesosphere region. Convection is a dominant source for tropical GWs, but also one of the most difficult and dynamic GW sources to understand. Therefore, we present the results of GW ray-tracing calculations from tropospheric (convective) sources up to the mesosphere. We used the Gravity wave Regional Or Global RAy-Tracer (GROGRAT) to perform the GW trajectory calculations and the convective GW source scheme from Yonsei University (South Korea) to quantify the excitation by convection. Heating rates, cloud data, and atmospheric background data were provided by the MERRA dataset for the calculation of convective forcing by deep convection and for the atmospheric background of the ray-tracing calculations afterwards. In order to validate our findings we compare our simulation results with satellite measurements of temperature amplitudes and momentum flux from the SABER instrument over a 10 years period. Simulation and measurements are in good agreement for the tropics throughout the whole simulated period and show similar seasonal behavior. Additionally, the observational filter of the instrument was taken into account and its influences are discussed. The modulation of GW momentum flux by the background winds and in particular the influence of the QBO is investigated. GW drag at various altitudes is calculated and compared to the drag required for the forcing of the QBO. Further, we show the results of a non-orographic background parameterization used as start conditions for the ray-tracer to emphasize the improvements of our coupled convective GW source model over non-orographic GW parameterizations.

Kalisch, S.; Trinh, T.; Chun, H.; Ern, M.; Preusse, P.; Kim, Y.; Eckermann, S. D.; Riese, M.

2013-12-01

335

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...

Oluwole Daniel Makinde

2011-01-01

336

Convective and radiative heat transfer in MHD radiant boilers

International Nuclear Information System (INIS)

A combined convection-gas radiation, two-zone flow model is formulated for study of the heat transfer characteristics of MHD radiant boilers. The radiative contributions of carbon dioxide, water vapor, potassium atoms, and slag particles are included in the formulation, and are determined by solving the radiation transport equation using the P1 approximation. The scattering and absorption cross section of slag particles are calculated from Mie theory. The model is used to analyze the scale-up of heat transfer in radiant boilers with refractory thickness, wall emissivity, and boiler size, under conditions of a gas composition and slag particle spectrum typical of coal-fired MHD combustion. A design procedure is suggested for sizing radiant boilers so as to achieve required heat extraction rate and to provide a flow residence time that is adequate for decomposition of NO/sub x/ to acceptable levels

337

Convective heat transfer for viscoelastic fluid in a curved pipe

In this paper, fully developed convective heat transfer of viscoelastic flow in a curved pipe under the constant heat flux at the wall is investigated analytically using a perturbation method. Here, the curvature ratio is used as the perturbation parameter and the Oldroyd-B model is applied as the constitutive equation. In the previous studies, the Dirichlet boundary condition for the temperature at the wall has been used to simplify the solution, but here exactly the non-homogenous Neumann boundary condition is considered to solve the problem. Based on this solution, the non-axisymmetric temperature distribution of Dean flow is obtained analytically and the effect of flow parameters on the flow field is investigated in detail. The current analytical results indicate that increasing the Weissenberg number, viscosity ratio, curvature ratio, and Prandtl number lead to the increase of the heat transfer in the Oldroyd-B fluid flow.

Norouzi, M.; Kayhani, M. H.; Nobari, M. R. H.; Joneidi, A. A.

2010-10-01

338

Heat flux scaling in turbulent Rayleigh-B\\'enard convection with an imposed longitudinal wind

We present a numerical study of Rayleigh-B\\'enard convection disturbed by a longitudinal wind. Our results show that under the action of the wind, the vertical heat flux through the cell initially decreases, due to the mechanism of plumes-sweeping, and then increases again when turbulent forced convection dominates over the buoyancy. As a result, the Nusselt number is a non-monotonic function of the shear Reynolds number. We provide a simple model that captures with good accuracy all the dynamical regimes observed. We expect that our findings can lead the way to a more fundamental understanding of the of the complex interplay between mean-wind and plumes ejection in the Rayleigh-B\\'enard phenomenology.

Scagliarini, Andrea; Toschi, Federico

2013-01-01

339

International Nuclear Information System (INIS)

The natural convection heat transfer from a heated vertical plate with wall temperature decreasing linearly from the lower end and embedded in a water saturated porous medium (coarse sand of 23% porosity) has been investigated experimentally. The steady state isothermal lines in the porous medium were developed with the aid of a computer program from the measured temperature data for several rates of heat input. The computed values of the local Nusselt number and modified Rayleigh number were used to obtain the empirical correlations. In order to check the accuracy of the measuring instruments and experimental procedures in simulating the thermal field, experimental investigations of heat transfer by natural convection from a vertical heated plate with constant wall temperature and embedded in the water saturated sand were conducted and the experimental results were compared with those previous investigations. The agreement was very good indicating that the instrumentation and experimental set-up used in the present study were reasonably satisfactory

340

Directory of Open Access Journals (Sweden)

Full Text Available The present work investigates the laminar forced convection flow of a radiating gas over an inclined backward facing step (BFS in a horizontal duct. The momentum and energy equations are solved numerically by the CFD techniques to obtain the velocity and temperature fields. Since, the twodimensional Cartesian coordinate system is used to solve the governing equations; the flow over inclined surface is simulated by considering the blocked-off region in regular grid. Discretized forms of the governing equations in the (x,y plane are obtained by the control volume method and solved using the SIMPLE algorithm. The fluid is treated as a gray, absorbing, emitting and scattering medium. Therefore, all of the convection, conduction and radiation heat transfer mechanisms take place simultaneously in the gas flow. For computation of the radiative term in the gas energy equation, the radiative transfer equation (RTE is solved numerically by the discrete ordinates method (DOM to find the radiative heat flux distribution inside the radiating medium. In the numerical results, effects of inclination angle, optical thickness, scattering albedo and the radiation-conduction parameter on the heat transfer behavior of the convection flow are investigated. This research work is a new one in which a combined convection-radiation thermal system with a complex flow geometry is simulate by efficient numerical techniques.

Ansaria Amir Babak

2013-01-01

341

GEOFLOW: simulation of convection in a spherical shell under central force field

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 (dielectroph...

Beltrame, P.; Travnikov, V.; Gellert, M.; Egbers, C.

2006-01-01

342

Bifurcation and stability of low-order steady flows in horizontally and vertically forced convection

A nonlinear spectral model of two-dimensional, shallow Boussinesq convection which responds to heating in both the horizontal and vertical directions is examined. The governing partial differential system is converted to an infinite set of ordinary differential equations and truncated to a small set to permit detailed study of the number and types of transitions from one flow configuration to another. The Hadley number and the Rayleigh number are defined as the horizontal and vertical thermal forcing mechanisms, respectively, for inclusion in the nonlinear spectral model, which is composed of three equations. The model is then used to describe steady states, linearly stable solutions, and balancing factors in unstable stratification. The number and the distribution of the steady states are found to be qualitatively independent of the aspect ratio and the Prandtl number.

Yost, D. A.; Shirer, H. N.

1982-01-01

343

Study of turbulent natural-circulation flow and low-Prandtl-number forced-convection flow

International Nuclear Information System (INIS)

Calculational methods and results are discussed for the coupled energy and momentum equations of turbulent natural circulation flow and low Prandtl number forced convection flow. The objective of this paper is to develop a calculational method for the study of the thermal-hydraulic behavior of coolant flowing in a liquid metal fast breeder reactor channel under natural circulation conditions. The two-equation turbulence model is used to evaluate the turbulent momentum transport property. Because the analogy between momentum transfer and heat transfer does not generally hold for low Prandtl number fluid and natural circulation flow conditions, the turbulent thermal conductivity is calculated independently using equations similar to the two-equation turbulence model. The numerical technique used in the calculation is the finite element method

344

Numerical simulation of combined natural and forced convection during thermal-hydraulic transients

International Nuclear Information System (INIS)

The single-phase COMMIX (COMponent MIXing) computer code performs fully three-dimensional, transient, thermal-hydraulic analyses of liquid-sodium LMFBR components. It solves the conservation equations of mass, momentum, and energy as a boundary-value problem in space and as an initial-value problem in time. The concepts of volume porosity, surface permeability and distributed resistance, and heat source have been employed in quasi-continuum (rod-bundle) applications. Results from three transient simulations involving forced and natural convection are presented: (1) a sodium-filled horizontal pipe initially of uniform temperature undergoing an inlet velocity rundown transient, as well as an inlet temperature transient; (2) a 19-pin LMFBR rod bundle undergoing a velocity transient; and, (3) a simulation of a water test of a 1/10-scale outlet plenum undergoing both velocity and temperature transients

345

Energy Technology Data Exchange (ETDEWEB)

The current study presents a numerical computation of combined gas radiation and forced convection through two parallel plates. A laminar flow of a temperature-dependent and non-grey gas in the entrance region of the channel was investigated. Over-heated water vapor was chosen as a gas because of its large absorption bands. Some special attention was given to entropy generation and its dependence on geometrical and thermodynamic parameters. The radiative part of the study was solved using the ''Ray Tracing'' method through S{sub 4} directions, associated with the ''statistical narrow band correlated-k'' (SNBCK) model. The temperature fields were used to calculate the distributions of local and global entropy generation. (author)

Ben Nejma, F.; Mazgar, A.; Abdallah, N.; Charrada, K. [Unite de recherche: Etudes des Milieux Ionises et Reactifs (EMIR), Institut Preparatoire aux Etudes d' Ingenieur de Monastir, Avenue Ibn Eljazar, Monastir 5019 (Tunisia)

2008-07-15

346

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, whic period was calculated as 0.89 years, which was also very small considering the life of the system (20 years). (author)

347

Modeling of Rayleigh–Bénard natural convection heat transfer in nanofluids

Heat transfer in Rayleigh-Bénard convection is investigated for three types of nanofluid. Instead of using the expressions commonly found in the literature for specific heat capacity and thermal expansion coefficient, we used two relations that are in agreement with the laws of thermodynamics. The influence of nanoparticles on conductive and convective heat transfer is studied. It is shown that adding nanoparticles in a fluid delays the onset of convection. Contrary to what is argued by many...

Elhajjar, Bilal; Bachir, Glades; Mojtabi, Abdelkader; Fakih, Chakib; Charrier-mojtabi, Marie-catherine

2010-01-01

348

Convective Heat Transfer in Impinging- Gas- Jet Arrangements

Directory of Open Access Journals (Sweden)

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.

J.M. Buchlin

2011-01-01

349

A survey on infrared thermography for convective heat transfer measurements

Energy Technology Data Exchange (ETDEWEB)

During the past several years infrared thermography has evolved into a powerful investigative means, of thermo-fluid-dynamic analysis to measure convective heat fluxes as well as to investigate the surface flow field behaviour over complicated body shapes. The basic concepts that govern this innovative measurement technique together with some particular aspects linked to its use are herein reviewed. Different operating methods together with their implementations are also discussed. Finally, the capability of infrared thermography to deal with several simple, or complex, fluid flow configurations is analysed. (author)

Astarita, T.; Cardone, G.; Carlomagno, G.M.; Meola, C. [Universita degli Studi di Napoli ' ' Federico II' ' (Italy). Dipartimento di Energetica

2000-11-01

350

Transient forced convection in the entrance region of a porous tube

Energy Technology Data Exchange (ETDEWEB)

Study of the flow of energy-carrying fluids through porous media is a rapidly growing branch of fluid mechanics and heat transfer since its applications include thermal insulation, direct contact heat exchangers, nuclear waste repositories, and the petroleum and geothermal industries. The problem of transient forced convection in the entrance region of a cylindrical porous medium with developing thermal boundary layer is investigated. The hydrodynamic behavior of the flow is assumed to be steady and fully developed, and both Darcian and non-Darcian effects on the flow are considered. The numerical solution uses finite-difference discretization. Thermal transients are created by a step change in the temperature of the tube wall. The effect of different fluid and solid-matrix parameters on Nusselt number, thermal entrance length, total heat absorbed and mixing cup temperature are described. Numerical solutions are obtained for the velocity and temperature fields. It is found that increasing the inertia parameter increases the heat absorbed by the fluid. 15 refs., 11 figs.

Al-Nimr, M.A.; Aldoss, T.; Naji, M.I. (Jordan Univ. of Science and Technology, Irbid (Jordan))

1994-04-01

351

The thermal response of electronic assemblies during forced convection-infrared reflow soldering is studied numerically. Soldering for attaching electronic components to printed circuit boards is performed in a process oven that is equipped with porous panel heaters, through which air is injected in order to dampen temperature fluctuations in the oven. Forced convection-infrared reflow soldering process with air injection is simulated using a two-dimensional numerical model. The multimode heat transfer within the reflow oven as well as within the electronic assembly is simulated. Parametric study is also performed to study the effects of various conditions such as conveyor speed, air injection velocity, and electronic assembly emissivity on the thermal response of electronic assemblies. The results of this study can be used in the process oven design and selecting the oven operating conditions to ensure proper solder melting and solidification.

Son, Young-Seok; Shin, Jee-Young

352

International Nuclear Information System (INIS)

This work was undertaken as part of an investigation into the heat transfer mechanism related to thermal explosion. The theoretical model of forced convection film boiling in the stagnation point region of an axi-symmetrical molten drop is presented. The model is particularly relevant to the coarse pre-mixing stage of vapour explosion: it takes account of the fact that, at this stage, the hot substance is molten. Stagnation point momentum and energy equations, for cold and hot liquids and for the vapour layer, were solved numerically, with a wide range of boundary-matching conditions at the interfaces. The model presented, by covering a wide range of the conditions and parameters, is more general than the cases discussed in other publications. For this reason the results can be applied not only to the coarse pre-mixing stage of vapour explosion, but also to other situations, where three-phase forced convection occurs on an axi-symmetrical body. (author)

353

Unsteady natural convection heat and mass transfer in a saturated porous enclosure

International Nuclear Information System (INIS)

A detailed numerical study has been performed to investigate transient natural convection heat and mass transfer in a porous enclosure. Major dimensionless groups governing the present problem are Ra, N, Le, ? and Ar. Results are particular presented to illustrate the effects of the combined thermal and solutal buoyancy forces on the temporal evolution of local/average Nusselt and Sherwood numbers. The results show that with the increase in the Rayleigh number, the heat and mass transfer is enhanced as a result of greater buoyancy effect. Additionally, the increase in buoyancy ratio N results in an improvement in the heat and mass transfer rates and in the mean time causes a short time duration for the flow to approach the steady-state condition. (orig.)

354

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

355

Double-layer thermocapillary convection in a differentially heated cavity.

Many materials-processing applications such as crystal growth from the melt involve thermocapillary flows that can affect the quality of the final product, particularly under microgravity conditions where the influence of buoyancy-driven convection is minimized. When the melt contains volatile components, as in the production of III-V semiconductor crystals, it is often encapsulated in a low-melting point amorphous molten glass phase such as boron oxide or pyrolytic boron nitride in order to prevent evaporation of the volatile components. The addition of the encapsulant layer and the melt-encapsulant interface in such cases can alter the thermocapillary flow in the melt. In this study, thermocapillary convection within a differentially heated rectangular cavity containing two immiscible liquid layers is considered in the absence of gravity. Domain mapping is used in conjunction with a finite difference scheme on a staggered grid to solve for the temperature and flow fields. The melt-encapsulant and the air-encapsulant interfaces are allowed to deform, with the contact lines pinned on the solid boundaries. The computed flow fields are compared to the corresponding results for a cavity with a rigid top surface. The presence of a free surface at the top leads to increased convection in the encapsulant phase while suppressing the thermocapillary flow in the melt phase. The flow pattern in the encapsulated layer is strongly dependent on the viscosity of the encapsulant layer. The intensity of the thermocapillary flow within the melt is significantly reduced as the viscosity of the encapsulant layer is increased. However, for a higher encapsulant viscosity, the retarding effect of the free top surface on thermocapillary convection in the melt is weakened. PMID:17124137

Gupta, Nivedita R; Haj-Hariri, Hossein; Borhan, Ali

2006-09-01

356

International Nuclear Information System (INIS)

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 conservation equation of convective heat transfer is used to explain convective heat transfer there are two points that are difficult for teachers to explain and for undergraduates to understand: thermal diffusivity is placed before the Laplacian operator of temperature; on the wall surface (the fluid side) the velocity is zero, a diffusion equation of temperature is gained from energy conservation equation, however, temperature cannot be transported. Consequently, the real physical meaning of thermal diffusivity is not clearly reflected in the energy conservation equation, and whether heat transfer occurs through a diffusion process or a convection process on the wall surface is not clear. Through a simple convective heat transfer case: laminar convective heat transfer in a tube with a uniform wall heat flux on the tube wall, this paper explains these points more clearly. The results declare that it is easier for teachers to explain and for undergraduates to understand these points when a description of heat transfer in terms of the heat flux is used. In this description, thermal diffusivity is placed before the Laplacian operator o is placed before the Laplacian operator of the heat flux; the role of the velocity gradient in convective heat transfer appears, on the wall surface, the fact whether heat transfer occurs through a diffusion process or a convection process can be explained and understood easily. The results are not only essential for teachers to improve the efficiency of university-level physics education regarding heat transfer, but they also enrich the theories for understanding heat transfer

357

Transition to finger convection in double-diffusive convection

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...

Kellner, M.; Tilgner, A.

2014-01-01

358

The effect of Coriolis force on nonlinear convection in a porous medium

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.

Riahi, D. H.

1994-01-01

359

The convective flux in the unstable boundary layer has a prominent role in inducing mixing in PBL and regulating the intensity of convection. Using satellite, ground-based and re-analyzed data sets, the characteristics of convective heat flux in the PBL and the possible role of aerosols on deep convective events are examined during the Ganges Valley Aerosol Experiment (GVAX) over the Indo-Gangetic Plain (IGP) in 2011. Convective cases were identified based on the ISCCP classification. Being home to nearly one billion people, the IGP is a rapidly developing industrial section and is the greatest source of both natural and anthropogenic aerosols over the Indian subcontinent. We have made a preliminary analysis on the aerosol indirect influence in effective radii and optical depth of ice-clouds in association with the cloud and ice water amount. In addition, we investigate the variations in the PBL height and the direct effect of aerosols in modifying the thermodynamic structure of the lower troposphere during deep convection events. Comparison of the convective heat flux and PBL heights during dirty and clean conditions is expected to unravel the direct role of aerosols in inducing changes in the PBL thermal structure during the transition from shallow to deep convection.

MG, M.; Li, Z.

2013-12-01

360

Hydromagnetic convective heat transfer at a stretching surface with heat generation or absorption

International Nuclear Information System (INIS)

The problem of steady, laminar , hydromagnetic convective heat transfer at a stretching surface with heat generation or absorption is considered. the sheet is linearly stretched in the presence of a uniform free stream of constant velocity and temperature. the effects of free convection and heat generation or absorption are also considered. the governing partial differential equations for this investigation are transformed into ordinary differential equations . the resulting equations are solved numerically by a perturbation technique and the obtained results are compared favorably with previously published work. A parametric study is performed to illustrate the influence of the Prandtl number, Hartmann number, heat generation/absorption coefficient, and the surface mass transfer coefficient on the profiles of the velocity components and temperature. numerical results for the skin friction coefficient and the Nusselt number functions have been studied for various parametric conditions

361

The non-hydrostatic, quasigeostrophic approximation for rapidly rotating Rayleigh-B\\'enard convection admits a class of exact `single mode' solutions. These solutions correspond to steady laminar convection with a separable structure consisting of a horizontal planform characterized by a single wavenumber multiplied by a vertical amplitude profile, with the latter given as the solution of a nonlinear boundary value problem. The heat transport associated with these solutions is studied in the regime of strong thermal forcing (large reduced Rayleigh number $\\widetilde{Ra}$). It is shown that the Nusselt number $Nu$, a nondimensional measure of the efficiency of heat transport by convection, for this class of solutions is bounded below by $Nu\\gtrsim \\widetilde{Ra}^{3/2}$, independent of the Prandtl number, in the limit of large reduced Rayleigh number. Matching upper bounds include only logarithmic corrections, showing the accuracy of the estimate. Numerical solutions of the nonlinear boundary value problem for ...

Grooms, Ian

2014-01-01

362

Mixed-convective, conjugate heat transfer during molten salt quenching of small parts

Energy Technology Data Exchange (ETDEWEB)

It is common in free quenching immersion heat treatment calculations to locally apply constant or surface-averaged heat-transfer coefficients obtained from either free or forced steady convection over simple shapes with small temperature differences from the ambient fluid. This procedure avoids the solution of highly transient, non-Boussinesq conjugate heat transfer problems which often involve mixed convection, but it leaves great uncertainty about the general adequacy of the results. In this paper we demonstrate for small parts (dimensions of the order of inches rather than feet) quenched in molten salt, that it is feasible to calculate such nonuniform surface heat transfer from first principles without adjustable empirical parameters. We use literature physical property salt data from the separate publications of Kirst et al., Nissen, Carling, and Teja, et al. for T<1000 F, and then extrapolate it to the initial part temperature. The reported thermal/chemical breakdown of NaNO{sub 2} for T>800 F is not considered to be important due to the short time the surface temperature exceeds that value for small parts. Similarly, for small parts, the local Reynolds and Rayleigh numbers are below the corresponding critical values for most if not all of the quench, so that we see no evidence of the existence of significant turbulence effects, only some large scale unsteadiness for brief periods. The experimental data comparisons from the open literature include some probe cooling-rate results of Foreman, as well as some cylinder thermal histories of Howes.

Chenoweth, D.R.

1997-02-01

363

Turbulence convective heat transfer for cooling the photovoltaic cells

Solar PV (photovoltaic) is a rapidly advancing renewable energy technology which converts sunlight directly into electricity. One of the outstanding challenges of the current PV technology is the reduction in its conversion efficiency with increasing PV panel temperature, which is closely associated with the increase in solar intensity and the ambient temperature surrounding the PV panels. To more effectively capture the available energy when the sun is most intense, significant efforts have been invested in active and passive cooling research over the last few years. While integrated cooling systems can lead to the highest total efficiencies, they are usually neither the most feasible nor the most cost effective solutions. This work examines some simple passive means of manipulating the prevailing wind turbulence to enhance convective heat transfer over a heated plate in a wind tunnel.

Arianmehr, Iman

364

A method of adaptive nodes for convective heat transfer problems

International Nuclear Information System (INIS)

A simple method of adaptive (time-dependent) spatial node intervals was developed that gives, with a small number of nodes, accurate solutions to convective heat transfer problems. In this method, the nodes intervals change with time such that the maximum absolute value of error estimates arising from numerical spatial integration are the same in all intervals. At this stage the method has been applied only to one-dimensional problems. Numerical results from its application to three transients in an intermediate heat exchanger of a nuclear liquid metal reactor are presented. In these transients, the temperatures predicted with 20 adaptive node intervals differed by less than 2 K from those predicted with 140 intervals of equal size. For the same transients, if intervals of fixed size are used to avoid either failure of convergency or large errors, more than 100 nodes are needed

365

Experimental Study of Free Convection in Coiled Tube Heat Exchanger

Directory of Open Access Journals (Sweden)

Full Text Available An experimental study has been conducted on steady-state natural convection heat transfer from helical coil tubes in vertical orientation. Water was used as a bath liquid without any mixing and cold water was used as a coolant fluid. A straight copper tube of 6 mm ID, 8 mm OD and 3 m length was bend to fabricate the helical coil. Four coils are used in this experiment has different curvature ratios and pitches. The data were correlated using tube diameter as the characteristic length. The results show that the overall heat transfer coefficient and Nusselt number increase when the flow rate of coolant and curvature ratio increase. The effect of coil pitch was investigated and the results show that when of the coil pitch (angle of inclination increases Nusselt number increase. A correlation was presented to calculate the outside average Nusselt number of coil.

Harith Mohammed

2013-05-01

366

Directory of Open Access Journals (Sweden)

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.

Zeinali Heris Saeed

2011-01-01

367

A highly stable microchannel heat sink for convective boiling

International Nuclear Information System (INIS)

To develop a highly stable two-phase microchannel heat sink, we experimented with convective boiling in diverging, parallel microchannels with different distributions of laser-etched artificial nucleation sites. Each microchannel had a mean hydraulic diameter of 120 µm. The two-phase flow visualization and the magnitudes of pressure drop and inlet temperature oscillations under boiling conditions demonstrated clearly the merits of using artificial nucleation sites to further stabilize the flow boiling in diverging, parallel microchannels. The stability map showed the plane of subcooling number versus phase change number. It illustrated that diverging, parallel microchannels with artificial nucleation cavities have a much wider stable region than parallel microchannels with uniform cross-sections or diverging, parallel microchannels without artificial nucleation cavities. In addition, the results revealed that the design with cavities distributed uniformly along the downstream half of the channel presented the best stability performance among the three distributions of nucleation sites. This particular design can be regarded as a highly stable microchannel heat sink for convective boiling

368

Heat transport measurements in turbulent rotating Rayleigh-Bénard convection.

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)numbers 0number, at fixed dimensional rotation rate OmegaD, at fixed Ra varying Ta, at fixed Ta varying Ra, and at fixed Rossby number Ro. The scaling of heat transport in the range of 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 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

Liu, Yuanming; Ecke, Robert E

2009-09-01

369

Directory of Open Access Journals (Sweden)

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 internal 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.

Andrade Cláudia R.

2000-01-01

370

Heat transfer enhancement and reduction by poylmer additives in turbulent Rayleigh Benard convection

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 ...

Dubief, Yves

2010-01-01

371

Directory of Open Access Journals (Sweden)

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.

Bhaskar Kalita

2012-12-01

372

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.)

373

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.

Ashraf, M.; Narahari, M.; Muthuvalu, Mohana Sundaram

2014-10-01

374

Forced Convection Flow of Nanofluids Past Power Law Stretching Horizontal Plates

Directory of Open Access Journals (Sweden)

Full Text Available In the present work, we studied a nonsimilar solution of steady forced convection boundary layer flow and heat transfer of a nanofluid past a stretching horizontal plate. One-phase model has been used for this study. The nonsimilarity equations are solved numerically. We considered a nanofluid consists of AL_{2}O_{3} as a nanoparticles and water as a base fluid. The volume fraction of nanoparticles is considered in the range 0 ? * ø* ? 0.2. with prandtl number *pr* = 6.2 for the water working as a regular fluid. The parameters which governing the solution are volume fraction of nanoparticles , stretching plate parameter *?* and power law index *N*. We investigated the effect of these parameters on the skin friction coefficient, Nusselt number, velocity and temperature profiles. We found that heat transfer rate and skin fraction increased when * ø* increased. On the other hand, we concluded that the increase in *?* and *N* made heat transfer rate increases and skin fraction decreases.

Ahmed Mostafa Abdelhady

2012-02-01

375

Surface curvature effect on MHD forced convection with aligned magnetic field

International Nuclear Information System (INIS)

Matched asymptotic expansions technique is used for the formulation and analysis of the singular perturbation problem arising out of the consideration of two dimensional forced convection of an electrically conducting incompressible and viscous fluid past a smooth curved surface with an impressed aligned magnetic field and assuming negligible effect of viscous dissipation and Joule heating. The partial differential equations formulated taking into account only the longitudinal surface curvature have been transformed into ordinary differential equations by similarity analysis technique and then solved numerically on a computer. The results clearly show that the first order co-efficient of heat transfer at the convex surfaces decreases with the magnetic field but the second order correction to the co-efficient increases for different values of Falkner-Skan constant. Thus the aligned magnetic field helps the curvature of the surface in decreasing the heat transfer at the wall. The numerical evaluation of the formulated expressions for enthalpy thickness gives clearer insight into the structure of the thermal boundary layer. (author)

376

Experimental study of natural convective heat transfer in a vertical hexagonal sub channel

Energy Technology Data Exchange (ETDEWEB)

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.

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

377

Experimental study of natural convective heat transfer in a vertical hexagonal sub channel

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.

Tandian, Nathanael P.; Umar, Efrizon; Hardianto, Toto; Febriyanto, Catur

2012-06-01

378

Experimental study of natural convective heat transfer in a vertical hexagonal sub channel

International Nuclear Information System (INIS)

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.

379

Enhanced heat transfer performances of molten salt mixed convection in a vertical annular duct

The mixed convection heat transfer of upward molten salt flow in a vertical annular duct is experimentally and numerically studied. The heat transfer performances of mixed convection are measured under Reynolds number 2,500-12,000 and inlet temperature 300-400 °C, and Nusselt number of molten salt flow with cooled inner wall monotonically increases with buoyancy number. The mixed convection is further simulated by low-Reynolds number k- ? model and variable properties, and the heat transfer tendency from numerical results agrees with that from experiments. At low Reynolds number, the natural convection plays more important role in the mixed convection. As the buoyancy number rises, the thickness of flow boundary layer near the inner wall increases, while the effective thermal conductivity remarkably rises, so the enhanced heat transfer of mixed convection is mainly affected by the effective thermal conductivity due to turbulent diffusion.

He, Shiquan; Ding, Jing; Lu, Jianfeng; Wang, Weilong; Yang, Jianping

2014-07-01

380

This book presents recent developments in our systematic studies of hydrodynamics and heat and mass transfer in laminar free convection, accelerating film boiling and condensation of Newtonian fluids, as well as accelerating film flow of non-Newtonian power-law fluids (FFNF). These new developments provided in this book are (i) novel system of analysis models based on the developed New Similarity Analysis Method; (ii) a system of advanced methods for treatment of gas temperature- dependent physical properties, and liquid temperature- dependent physical properties; (iii) the organically combined models of the governing mathematical models with those on treatment model of variable physical properties; (iv) rigorous approach of overcoming a challenge on accurate solution of three-point boundary value problem related to two-phase film boiling and condensation; and (v) A pseudo-similarity method of dealing with thermal boundary layer of FFNF for greatly simplifies the heat-transfer analysis and numerical calculati...

Shang, De-Yi

2012-01-01

381

The Arctic Mediterranean Sea - Deep convection, oceanic heat transport and freshwater

The speculations about the driving forces behind the oceanic meridional circulation and the importance of the northward transports of oceanic heat for the ice conditions in the Arctic Ocean have a long history, but only after the Fram expedition 1893-1896 and from the studies by Nansen, Helland-Hansen and Sandström in the early 1900s did these speculations attain observational substance. In the late 1970s and onward these questions have again risen to prominence. A study of deep convection in the Greenland Sea, then assumed to drive the global thermohaline circulation, started with the Greenland Sea Project (GSP), while the investigation of the exchanges of volume and heat through Fram Strait had a more hesitant start in the Fram Strait Project (FSP). Not until 1997 with the EC project VEINS (Variation of Exchanges in the Northern Seas) was a mooring array deployed across Fram Strait. This array has been maintained and has measured the exchanges ever since. Eberhard Fahrbach was closely involved in these studies, as a secretary for the GSP and as the major driving force behind the Fram Strait array. Here we shall examine the legacy of these projects; How our understanding of these themes has evolved in recent years. After the 1980s no convective bottom water renewal has been observed in the Greenland Sea, and the Greenland Sea deep waters have gradually been replaced by warmer, more saline deep water from the Arctic Ocean passing through Fram Strait. Small-scale convective events penetrating deeper than 2500m but there less dense than their surroundings were, however, observed in the early 2000s. The Fram Strait exchanges have proven difficult to estimate due to strong variability, high barotropic and baroclinic eddy activity and short lateral coherence scales. The fact that the mass transports through Fram Strait do not balance complicates the assessment of the heat transport through Fram Strait into the Arctic Ocean and mass (volume) and salt (freshwater) balances for the entire Arctic Ocean are needed. The waters exiting the Arctic Ocean through Fram Strait are colder than those entering and with reasonable assumptions about the origin of the waters providing the net outflow it is possible to deduce the amount of the entering oceanic heat going to the atmosphere (>50%), to ice melt (20%). Almost all of this heat loss occurs in the Nansen Basin. The rest of the heat is used for heating the net outflow. It also becomes clear that freshwater, with its phase changes and its multiple transport pathways, plays a crucial role in the climate, not just of the Arctic Ocean but of the Arctic as a whole.

Rudels, Bert

2014-05-01

382

Energy Technology Data Exchange (ETDEWEB)

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.

Triplett, C.E.

1996-12-01

383

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

384

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

385

Convective Heating of the LIFE Engine Target During Injection

Energy Technology Data Exchange (ETDEWEB)

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.

Holdener, D S; Tillack, M S; Wang, X R

2011-10-24

386

International Nuclear Information System (INIS)

In this study, the problem of conjugated mixed convection-conduction heat transfer along the vertical circular pin-fin around which gases flow in the direction of fin axes is analysed by taking the radiative effect into account and the general algorithm that can be applied to both circular fin and flat-plate fin is suggested. The governing equations of cylindrical coordinates are nondimensionalized by introducing the radiation-conduction parameter M, convection-condution parameter Nc, buoyancy force parameter Ri and transverse curvature parameter ? and solved numerically by varying these parameters properly. The radiative heat flux in the energy equation is presented by using the Rosseland approximation and is calculated by the modified Killer's Box method together with the momentum equation and the finite control volume method is used to solve the heat conduction equation of the fin. The calculated results for the cases of neglecting radiative effect(M=0), forced convection alone(Ri=0) and flat-plate fin (?=0) were compared fairly well with those of previous papers. When radiation effect was not considered, the local heat transfer coefficient was almost constant from the fin root to the fin tip. But when the radiative effect was considered, in the all over the fin surface the local modified heat transfer coefficient became higher and increased as it goes from the fin tip to the fin root. (Author)

387

International Nuclear Information System (INIS)

Highlights: ? Transport processes in isothermal hexagonal sheath with 19 heat generating rods is studied. ? Correlation is given to predict the maximum temperature considering all transport processes. ? Effective thermal conductivity of rod bundle can be obtained using max temperature. ? Data on the critical Rayleigh numbers for p/d ratios of 1.1-2.0 is presented. ? Radiative heat transfer contributes to heat dissipation of 38-65% of total heat. - Abstract: A numerical study of conjugate natural convection and surface radiation in a horizontal hexagonal sheath housing 19 solid heat generating rods with cladding and argon as the fill gas, is performed. The natural convection in the sheath is driven by the volumetric heat generation in the solid rods. The problem is solved using the FLUENT CFD code. A correlation is obtained to predict the maximum temperature in the rod bundle for different pitch-to-diameter ratios and heat generating rates. The effective thermal conductivity is related to the heat generation rate, maximum temperature and the sheath temperature. Results are presented for the dimensionless maximum temperature, Rayleigh number and the contribution of radiation with changing emissivity, total wattage and the pitch-to-diameter ratio. In the simulation of a larger system that contains a rod bundle, the effective thermal conductivity facilitates simplified modelling of the rod bundle by treating it as a solid of effective thermal conductivity. The ptive thermal conductivity. The parametric studies revealed that the contribution of radiation can be 38-65% of the total heat generation, for the parameter ranges chosen. Data for critical Rayleigh number above which natural convection comes into effect is also presented.

388

Abstract In this article, developing turbulent forced convection flow of a water-Al2O3 nanofluid in a square tube, subjected to constant and uniform wall heat flux, is numerically investigated. The mixture model is employed to simulate the nanofluid flow and the investigation is accomplished for particles size equal to 38 nm. An entropy generation analysis is also proposed in order to find the optimal working condition for the given geometry under given ...

Bianco Vincenzo; Nardini Sergio; Manca Oronzio

2011-01-01

389

Joint action of buoyancy and thermocapillary forces can destabilize the motionless state in a liquid layer heated from above due to the coupling of internal and surface waves. The nonlinear evolution of this oscillatory instability is studied using three-dimensional direct numerical simulations with a pseudospectral Fourier-Chebyshev code. Alternating rolls and standing, oscillating squares are observed as final convective patterns. The flow is strongly localized near the free surface. Buoyancy plays a negligible role in kinetic energy production. PMID:12636867

Boeck, Thomas; Jurgk, Matthias; Bahr, Ute

2003-02-01

390

Numerical Simulations of Heat ExplosionWith Convection In Porous Media

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.

Allali, Karam; Bikany, Fouad; Taik, Ahmed; Volpert, Vitaly

2013-01-01

391

Evaluation of finite element formulations for transient conduction forced-convection analysis

Numerical studies clarifying the advantages and disavantages of conventional versus upwind convective finite elements are presented along with lumped versus consistent formulations for practical conduction forced-convection analysis. A finite-element procedure for treatment of negligible capacitance fluid nodes is presented. The procedure is based on procedures used in finite-element structural dynamics to treat nodes with negligible structural mass. Two finite-element programs and a finite-difference lumped-parameter program used in the studies are discussed. Evaluation studies utilizing three convection and two combined conduction-convection problems are then presented and discussed. Additionally, the computational time saving offered by the finite element procedure is considered for a practical combined conduction-convection problem.

Thornton, E. A.; Wieting, A. R.

1979-01-01

392

International Nuclear Information System (INIS)

The air-side forced convective heat transfer of a plate fin-tube heat exchanger is investigated by experimental measurement and numerical computation. The heat exchanger consists of a staggered arrangement of refrigerant pipes with a diameter of 10.2 mm and a fin pitch of 3.5 mm. In the experimental study, the forced convective heat transfer was measured at Reynolds numbers of 1082, 1397, 1486, 1591 and 1649 based on the diameter of the refrigerant piping and on the maximum velocity. The average Nusselt number for the convective heat transfer coefficient was also computed for the same Reynolds number by using the commercial software STAR-CD with the standard k .? turbulent model. It was found that the relative errors of the average Nusselt numbers between the experimental and numerical data were less than 6 percent in a Reynolds number range of 1082?1649. The errors between the experiment and other correlations from literature ranged from 7% to 32.4%. However, the literature correlation of Kim et al. is closest to the experimental data within a relative error of 7%

393

We report the design, construction, and performances of a microwave (MW) heating device for laboratory experiments with non-contact, homogeneous internal heating. The device generates MW radiation at 2.47 GHz from a commercial magnetron supplied by a pulsed current inverter using proprietary, feedback based command and control hardware and software. Specially designed MW launchers direct the MW radiation into the sample through a MW homogenizer, devised to even the MW power distribution into the sample's volume. An adjustable MW circuit adapts the MW generator to the load (i.e., the sample) placed in the experiment chamber. Dedicated heatsinks maintain the MW circuits at constant temperature throughout the experiment. Openings for laser scanning for image acquisition with a CCD camera and for the cooling circuits are protected by special MW filters. The performances of the device are analyzed in terms of heating uniformity, long term output power stability, and load matching. The device is used for small scale experiments simulating Earth's mantle convection. The 30 × 30 × 5 cm3 convection tank is filled with a water-based viscous fluid. A uniform and constant temperature is maintained at the upper boundary by an aluminum heat exchanger and adiabatic conditions apply at the tank base. We characterize the geometry of the convective regime as well as its bulk thermal evolution by measuring the velocity field by Particle Image Velocimetry and the temperature field by using Thermochromic Liquid Crystals.

Surducan, E.; Surducan, V.; Limare, A.; Neamtu, C.; Di Giuseppe, E.

2014-12-01

394

We report the design, construction, and performances of a microwave (MW) heating device for laboratory experiments with non-contact, homogeneous internal heating. The device generates MW radiation at 2.47 GHz from a commercial magnetron supplied by a pulsed current inverter using proprietary, feedback based command and control hardware and software. Specially designed MW launchers direct the MW radiation into the sample through a MW homogenizer, devised to even the MW power distribution into the sample's volume. An adjustable MW circuit adapts the MW generator to the load (i.e., the sample) placed in the experiment chamber. Dedicated heatsinks maintain the MW circuits at constant temperature throughout the experiment. Openings for laser scanning for image acquisition with a CCD camera and for the cooling circuits are protected by special MW filters. The performances of the device are analyzed in terms of heating uniformity, long term output power stability, and load matching. The device is used for small scale experiments simulating Earth's mantle convection. The 30 × 30 × 5 cm(3) convection tank is filled with a water?based viscous fluid. A uniform and constant temperature is maintained at the upper boundary by an aluminum heat exchanger and adiabatic conditions apply at the tank base. We characterize the geometry of the convective regime as well as its bulk thermal evolution by measuring the velocity field by Particle Image Velocimetry and the temperature field by using Thermochromic Liquid Crystals. PMID:25554309

Surducan, E; Surducan, V; Limare, A; Neamtu, C; Di Giuseppe, E

2014-12-01

395

Energy stability bounds on convective heat transport: Numerical study

International Nuclear Information System (INIS)

The concept of nonlinear energy stability has recently been extended to deduce bounds on energy dissipation and transport in incompressible flows, even for turbulent flows. In this approach an effective stability condition on open-quotes backgroundclose quotes flow or temperature profiles is derived, which when satisfied ensures that the profile produces a rigorous upper estimate to the bulk dissipation. Optimization of the test background profiles in search of the lowest upper bounds leads to nonlinear Euler-Lagrange equations for the extremal profile. In this paper, in the context of convective heat transport in the Boussinesq equations, we describe numerical solutions of the Euler-Lagrange equations for the optimal background temperature and present the numerical computation of the implied bounds. copyright 1997 The American Physical Society

396

Fully developed laminar mixed convection in uniformly heated pipes

Energy Technology Data Exchange (ETDEWEB)

Approximate solutions are carried out for fully developed laminar mixed convection in uniformly heated horizontal pipes. The problem considered is steady state and validity of the Boussinesq approximation is assumed. The analysis reveals that the relevant parameters are the Prandtl number and the product of the Rayleigh number based on temperature gradient along the pipe wall and of the Reynolds number based on pressure gradient along the pipe axis. Solutions for the velocity and temperature fields are obtained by a spectral Galerkin method, which proves capable of giving rapid convergence and, by implication, good accuracy, as demonstrated by the comparison with other numerical predictions. The results are presented in terms of fluid flow patterns, isotherms, and graphs of mass flow rate, local and average Nusselt number for various values of the parameters. (orig.)

Muzzio, A. (Dipt. di Energetica, Politecnico di Milano (Italy)); Parolini, P. (Dipt. di Energetica, Politecnico di Milano (Italy))

1994-09-01

397

New force or thermal convection in the differential-accelerometer experiment

International Nuclear Information System (INIS)

Recently Thieberger reported that the result of his differential-accelerometer experiment is consistent with the existence of a substance-dependent medium-range repulsive force. It is shown that the effect due to thermal convection is not negligible and can be large enough to account for his data. Methods of measuring and minimizing the thermal convection effect, as well as several ways of improving the differential-accelerometer experiment, are proposed. (orig.)

398

Mixed convection heat transfer in rotating vertical elliptic ducts

Scientific Electronic Library Online (English)

Full Text Available SciELO Brazil | Language: English Abstract in english 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

Olumuyiwa A., Lasode.

2007-06-01

399

Heat Transfer Characteristics on Toroidal Convection Loop with Nanofluids

International Nuclear Information System (INIS)

Experimental studies on single-phase toroidal circulation loop(thermosyphon) have been performed in the present study with Ag-nanofluids as a working fluids. The present paper deals with an experimental study on the heat transfer behavior of single-phase toroidal loop. Toroidal loop charged with nanofluid has been constructed and a number of tests have been carried out. Different geometric parameter, e.g., orientation has been investigated. The tests were conducted employing two fluids: distilled water and Ag-nanofluid of various volume concentrations. The experiments at Rayleigh number from 105 to 106 showed a systematic and slight deterioration in natural convective heat transfer. It was observed that the deterioration due to the particle concentration was in the range of 5-10%. At a given particle concentration of 0.05%, abrupt decrease in the Nusselt number and the Raleigh number was observed. The present study with toroidal loop shows that the application of nanofluids for heat transfer intensification should not be decided only by the effective thermal conductivity with increasing particle concentration

400

Mixed convection heat transfer in rotating vertical elliptic ducts

Directory of Open Access Journals (Sweden)

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 ascending 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<820.

Olumuyiwa A. Lasode

2007-06-01

401

International Nuclear Information System (INIS)

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

402

Analysis of the heat transfer from horizontal pipes at natural convection

These article deals with heat transfer from "n" horizontal pipes one above another at natural convection. On the bases of theoretical models have been developed for calculating the thermal performance of natural convection by Churilla and Morgan, for various pipe diameters and temperatures. These models were compared with models created in CFD-Fluent Ansys the same boundary conditions. The aim of the analyze of heat and fluxional pipe fields "n" pipes one about another at natural convection is the creation of criterion equation on the basis of which the heat output of heat transfer from pipe oriented areas one above another with given spacing could be quantified.

Kapjor, Andrej; Huzvar, Jozef; Ftorek, Branislav; Smatanova, Helena

2014-08-01

403

Flow characteristics of NSRR forced convection testing rigs at void generation

International Nuclear Information System (INIS)

This report describes the out-of-pile test results to study the characteristics of two phase flow that occurs in the testing rigs to be used in the NSRR forced convection tests. In the tests a void generation was simulated by blowing air into the testing rigs assembled equivalently to the ones used in the in-pile forced convection test. Behavior of flow velocity was measured by drag-disc type flowmeters as the functions of initial flow velocity at the testing section and air flow rate blew into the section. Through the tests and the analyses, the following conclusions are obtained; the large flow depressions observed in the previous forced convection tests were caused by the pump cavitation; the prevention of the pump cavitation was achieved by lowering the pump position; the by-pass flow gave the slightest effect on the flow behavior around the fuel. (author)

404

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...

Yano, Ryosuke

2015-01-01

405

Convective heat transfer from a heated elliptic cylinder at uniform wall temperature

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 discretiz...

Kaprawi S, Dyos Santoso

2013-01-01

406

Heat Transfer Convection in The Cooking of Apple Using a Solar Cooker Box-Type

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.

Terres, H.; Chávez, S.; Lizardi, A.; López, R.; Vaca, M.; Flores, J.; Salazar, A.

2015-01-01

407

Mixed turbulent convective heat transfer in vertical ducts: Final report

International Nuclear Information System (INIS)

A recently developed heat transfer correlation can help determine acceptable water temperatures during a severe overcooling of the reactor vessel downcomer. This study contributes to an ongoing effort to prevent thermally induced cracking in reactor vessels. Thermally induced cracking develops when cold water suddenly contacts hot metal walls. This phenomenon could occur during certain PWR transients. Following an extensive literature survey, investigators built an ex