WorldWideScience
 
 
1

Forced Convection Heat Transfer in Circular Pipes  

Science.gov (United States)

One of the pitfalls of engineering education is to lose the physical insight of the problem while tackling the mathematical part. Forced convection heat transfer (the Graetz-Nusselt problem) certainly falls into this category. The equation of energy together with the equation of motion leads to a partial differential equation subject to various…

Tosun, Ismail

2007-01-01

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)

1960-01-01

3

Theory of heat transfer with forced convection film flows  

CERN Document Server

Developing a new treatment of ""Free Convection Film Flows and Heat Transfer"" began in Shang's first monograph and is continued in this monograph. The current book displays the recent developments of laminar forced convection and forced film condensation. It is aimed at revealing the true features of heat and mass transfer with forced convection film flows to model the deposition of thin layers. The novel mathematical similarity theory model is developed to simulate temperature - and concentration - dependent physical processes. The following topics are covered in this book: Mathematical meth

Shang, Deyi

2010-01-01

4

Transient heat transfer for forced convection flow of helium gas  

Energy Technology Data Exchange (ETDEWEB)

The knowledge of forced convection transient heat transfer at various periods of exponential increase of heat input to a heater as a database for understanding the transient heat transfer process in a high temperature gas cooled reactor (HTGR) due to an accident in excess reactivity. The transient heat transfer coefficients for forced convection flow of helium gas over a horizontal cylinder were measured using a forced convection test loop. The platinum heater with a diameter of 1.0 mm was heated by electric current with an exponential increase of Q{sub 0} exp(t/ ). It was clarified that the heat transfer coefficient approaches the steady-state one for the period over 1 s, and it becomes higher for the period of shorter than 1 s. The transient heat transfer shows less dependent on the gas flowing velocity when the period becomes very shorter. Semi-empirical correlations for steady state and transient heat transfer were developed based on the experimental data. (authors)

Liu, Q.S.; Fukuda, K. [Kobe Univ. of Mercantile Marine, Dept. of Nuclear Eng. (Japan)

2001-07-01

5

Single phase channel flow forced convection heat transfer  

Energy Technology Data Exchange (ETDEWEB)

A review of the current knowledge of single phase forced convection channel flow of liquids (Pr > 5) is presented. Two basic channel geometries are considered, the circular tube and the rectangular duct. Both laminar flow and turbulent flow are covered. The review begins with a brief overview of the heat transfer behavior of Newtonian fluids followed by a more detailed presentation of the behavior of purely viscous and viscoelastic Non-Newtonian fluids. Recent developments dealing with aqueous solutions of high molecular weight polymers and aqueous solutions of surfactants are discussed. The review concludes by citing a number of challenging research opportunities.

Hartnett, J.P.

1999-04-01

6

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.

Michael Z. Podowski; Raf M. Podowski

2008-01-01

7

Forced convection heat transfer augmentation in a channel with a localized heat source using fibrous materials  

Energy Technology Data Exchange (ETDEWEB)

A numerical model is developed for high Reynolds number forced convection heat transfer in a channel filled with randomly oriented, thin fibrous materials of high porosity. A localized isothermal heat source, flush with one of the channel walls is considered to simulate an electronic component. The inertial coefficient, and the dispersion conductivity associated with high Reynolds number flows and convective heat transfer are empirically modeled from existing experimental and analytical studies. The resulting fluid flow and heat transfer relationships are presented for a relevant range of parameters, and the fundamental physical processes are explained.

Angirasa, D.; Peterson, G.P. [Texas A and M Univ., College Station, TX (United States). Dept. of Mechanical Engineering

1996-12-31

8

Analysis of free and forced convection in air flow windows using numerical simulation of heat transfer  

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

9

Combined forced and natural convection heat transfer for upward flow in a uniformly heated vertical pipe  

International Nuclear Information System (INIS)

[en] For predicting the fully developed upward flow in a uniformly heated vertical pipe by taking account of the buoyancy force, the k-? models of turbulence for low Reynolds number flows were adopted. The regime map for forced, mixed and natural convections as well as for laminar and turbulent flows was plotted from the numerical calculations. At the same time, an experiment was carried out at Reynolds numbers of 3000 and 5000 with the Grashof number varied over a wide range by using pressurized nitrogen gas as a test fluid. In agreement with the prediction, buoyancy-induced impairment of heat transfer was measured right in the mixed convection region. Further, from hot-wire measurement, complete laminarization was demonstrated in the mixed convection region at a Reynolds number of 3000. (author)

1986-01-01

10

CONVECTIVE HEAT TRANSFER COEFFICIENT OF INDIAN GOOSEBERRY (EMBLICA OFFICINALIS) DRIED IN THREE DIFFERENT FORMS UNDER FORCED CONVECTION MODE  

Directory of Open Access Journals (Sweden)

Full Text Available 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 being dried and decreases as the drying progresses. The value of convective heat transfer coefficient was highest for shredded form (30.39 W/m2oC) followed by slices (25.88 W/m2oC) and pieces (18.67 W/m2oC) when compared at certain final moisture content. The data were also analyzed for per cent uncertainty.

S. I. ANWAR; R.D. SINGH

2012-01-01

11

Effect of Buoyancy on Forced Convection Heat Transfer in Vertical Channels - a Literature Survey  

International Nuclear Information System (INIS)

This report contains a short resume of the available information from various sources on the effect of free convection flow on forced convection heat transfer in vertical channels. Both theoretical and experimental investigations are included. Nearly all of the theoretical investigations are concerned with laminar flow with or without internal heat generation. More consistent data are available for upward flow than for downward flow. Curves are presented to determine whether free convection or forced convection mode of heat transfer is predominant for a particular Reynolds number and Rayleigh number. At Reb > 105 free convection effects are negligible. Downward flow through a heated channel at low Reynolds number is unstable. Under similar conditions the overall heat transfer coefficient for downward flow tends to be higher than that for upward flow

1965-01-01

12

Analysis of Rectangular Microchannel under Forced convection heat transfer condition  

Directory of Open Access Journals (Sweden)

Full Text Available Micro-convection is a strategic area in transport phenomena, since it is the basis for a wide range of miniaturized high-performance pplications. Surface area is one of the important concepts for high flux heat transfer in Microchannel performance. Microchannel with hydraulic diameters 440?m, 476?m, 500?m and 550?m are analyzed for optimize microchannel hydraulic diameter. The microchannel having height of 400?m, 450?m, 500?m, 600?m with width of 500?m is analyzed numerically. Spacing between microchannel is also varied in range of 250?m, 300?m, 350?mand 400?m are considered for the analysis. Cu material microchannel having length of 30mm which carries 20 microchannels on top surface of the cu piece is to be considered. Flow rate also varied from 5lpm to 30 lpm for optimization with water as a medium. From numerical study it is observed that as hydraulic diameter increases from 444?m to 545 ?m the flow rate pressure drop also increases with decreases in diameter. Also heat in put to icrochannel as increases from 5 watt to 80 watt temperature drop is high at flow rate of 17lpm to 20 lpm. From analysis it is observed that as hydraulic diameter of microchannel is major concept of microchannel heat transfer which is dependent on flow rate of waterin microchannel. The microchannels with hydraulic diameter of 440?m to 600?m will follow temperature drop up-to 6 degree Kelvin with heat input of 5 watt to 80watt with flow rate of 5lpm to 25lpm.

Dr. B.S.Gawali,; D.A.Kamble,; V.B.Swami

2011-01-01

13

Numerical Analysis of Forced Convection Heat Transfer around Spherical Particles Packed in Fluid Flow  

International Nuclear Information System (INIS)

[en] In order to make clear the forced convection heat transfer phenomena around spherical particles packed in fluid flow, we numerically analyzed the heat transfer and flow pattern of the air using a single sphere and then the closest packed structure arrangement of spherical particles. We used 3-dimensional thermo fluid computation code 'STAR-CCM+'. We calculated the forced convection heat transfer coefficient for spheres of 10 mm diameter with Reynolds number 63 – 6340. Our calculation results of the average heat transfer coefficient for a single sphere agree with the correlation equation proposed by Ranz and Marshall. Local heat transfer coefficient is high at portions where local flow impinges to the surface of spheres for packed spherical particles. Our calculation results of the average heat transfer coefficient for packed spherical particles are close to the correlation equation proposed by Wakao et al.

2012-11-26

14

Numerical Analysis of Forced Convection Heat Transfer around Spherical Particles Packed in Fluid Flow  

Science.gov (United States)

In order to make clear the forced convection heat transfer phenomena around spherical particles packed in fluid flow, we numerically analyzed the heat transfer and flow pattern of the air using a single sphere and then the closest packed structure arrangement of spherical particles. We used 3-dimensional thermo fluid computation code "STAR-CCM+". We calculated the forced convection heat transfer coefficient for spheres of 10 mm diameter with Reynolds number 63 - 6340. Our calculation results of the average heat transfer coefficient for a single sphere agree with the correlation equation proposed by Ranz and Marshall. Local heat transfer coefficient is high at portions where local flow impinges to the surface of spheres for packed spherical particles. Our calculation results of the average heat transfer coefficient for packed spherical particles are close to the correlation equation proposed by Wakao et al.

Hirasawa, S.; Kawanami, T.; Kinoshita, T.; Watanabe, T.; Atarashi, T.

2012-11-01

15

Experiment of forced convection heat transfer using microencapsulated phase-change-material slurries  

Energy Technology Data Exchange (ETDEWEB)

The present study describes an experiment on forced convective heat transfer using a water slurry of Microencapsulated Phase-change-material. A normal paraffin hydrocarbon is microencapsulated by melamine resin, melting point of 28.1degC. The heat transfer coefficient and pressure drop in a circular tube were evaluated. The heat transfer coefficient using the slurry in case with and without phase change were compared to in case of using pure water. (author)

Kubo, Shinji; Akino, Norio; Tanaka, Amane [Japan Atomic Energy Research Inst., Tokai, Ibaraki (Japan). Tokai Research Establishment; Nagashima, Akira

1997-05-01

16

Forced-convection heat transfer in a spherical annulus heat exchanger  

International Nuclear Information System (INIS)

[en] 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

1982-01-01

17

Heat transfer in a sodium-to-sodium heat exchanger under conditions of combined force and free convection  

International Nuclear Information System (INIS)

A combined experimental and theoretical investigation of heat transfer in a vertical tube and annulus, countercurrent flow heat exchanger is reported. The working fluid was liquid sodium. Included in the range of conditions covered were those which are of interest in connection with the low flow rate operation of fast reactor intermediate heat exchanger systems. The heat transfer process ranged from that of pure forced convection to combined forced and free convection. By changing the direction of fluid flow or the direction of heat flow four different configurations were studied. In two cases the convection process was buoyancy aided and in the other two it was buoyancy opposed. Results are presented showing the influence of flow rate and temperature difference on overall heat transfer coefficient for each case. A theoretical model of turbulent flow and heat transfer incorporating influences of buoyancy was used to produce results for the range of conditions covered in the experiments. The predictions of overall heat transfer coefficient were found to be in reasonable general agreement with the measurements. It was clear from these calculations that the influence of buoyancy on heat transfer stemmed largely, under the conditions of the present experiment, from the modification of the convection process due to the distortion of the velocity field. This led to an enhancement of the heat transfer for the buoyancy-aided process and an impairment for the buoyancy-opposed process. The contribution of the turbulent diffusion of heat was relatively small. (author).

1987-01-01

18

Momentum and heat transfer by forced convection in fixed beds of granular active carbon  

Energy Technology Data Exchange (ETDEWEB)

Heat transfer and pressure loss characteristics of packed beds with active carbon 208C for low Reynolds numbers have been investigated using inert gases. It is intended that this granular material can be heated by forced convection of an adsorbant gas as part of a heat pump. This cycle will rely on the production of a thermal wave to give a high COP and thus requires good values of NTU or reduced length. These preliminary results suggest that NTU values of 100 may be easily obtained in this way. Heat transfer coefficients are related to the pressure-drop characteristic by a modified version of the Reynolds-Colburn analogy. (author)

Critoph, R.E.; Thorpe, R. [University of Warwick, Coventry (United Kingdom). Dept. of Engineering

1996-05-01

19

Conjugate forced convection and heat conduction in circular microchannels  

Energy Technology Data Exchange (ETDEWEB)

The effects of axial heat conduction in the solid walls of microchannels of circular cross-sections are analyzed here. A systematic approach is adopted, with the aim of pointing out the influence of geometrical parameters and of solid wall thermal conductivity on microchannel heat transfer. The reliability of a commonly adopted criterium, based on the so-called axial conduction number, to assess the relevance of axial heat conduction is also discussed. Numerical simulations concern the simultaneously developing laminar flow of a constant property fluid in microchannels of different length, wall thickness and wall material, heated with a uniform heat flux at the outer surface, for different values of the Reynolds number. Moreover, since often in experimental tests the two end sections of the microchannel wall are not perfectly insulated, the effects of heat losses through these sections are also considered. A hybrid finite element procedure, which implies the step-by-step solution of the parabolized momentum equations in the fluid domain, followed by the solution of the energy equation in the entire domain, corresponding to both the solid and the fluid parts, is used for the numerical simulations.

Nonino, C. [Dipartimento di Energetica e Macchine, Universita degli Studi di Udine, Via delle Scienze 208, 33100 Udine (Italy)], E-mail: carlo.nonino@uniud.it; Savino, S.; Del Giudice, S.; Mansutti, L. [Dipartimento di Energetica e Macchine, Universita degli Studi di Udine, Via delle Scienze 208, 33100 Udine (Italy)

2009-10-15

20

Conjugate forced convection and heat conduction in circular microchannels  

International Nuclear Information System (INIS)

[en] The effects of axial heat conduction in the solid walls of microchannels of circular cross-sections are analyzed here. A systematic approach is adopted, with the aim of pointing out the influence of geometrical parameters and of solid wall thermal conductivity on microchannel heat transfer. The reliability of a commonly adopted criterium, based on the so-called axial conduction number, to assess the relevance of axial heat conduction is also discussed. Numerical simulations concern the simultaneously developing laminar flow of a constant property fluid in microchannels of different length, wall thickness and wall material, heated with a uniform heat flux at the outer surface, for different values of the Reynolds number. Moreover, since often in experimental tests the two end sections of the microchannel wall are not perfectly insulated, the effects of heat losses through these sections are also considered. A hybrid finite element procedure, which implies the step-by-step solution of the parabolized momentum equations in the fluid domain, followed by the solution of the energy equation in the entire domain, corresponding to both the solid and the fluid parts, is used for the numerical simulations.

2009-01-01

 
 
 
 
21

MHD forced convective laminar boundary layer flow from a convectively heated moving vertical plate with radiation and transpiration effect.  

Science.gov (United States)

A two-dimensional steady forced convective flow of a Newtonian fluid past a convectively heated permeable vertically moving plate in the presence of a variable magnetic field and radiation effect has been investigated numerically. The plate moves either in assisting or opposing direction to the free stream. The plate and free stream velocities are considered to be proportional to x(m) whilst the magnetic field and mass transfer velocity are taken to be proportional to x((m-1)/2) where x is the distance along the plate from the leading edge of the plate. Instead of using existing similarity transformations, we use a linear group of transformations to transform the governing equations into similarity equations with relevant boundary conditions. Numerical solutions of the similarity equations are presented to show the effects of the controlling parameters on the dimensionless velocity, temperature and concentration profiles as well as on the friction factor, rate of heat and mass transfer. It is found that the rate of heat transfer elevates with the mass transfer velocity, convective heat transfer, Prandtl number, velocity ratio and the magnetic field parameters. It is also found that the rate of mass transfer enhances with the mass transfer velocity, velocity ratio, power law index and the Schmidt number, whilst it suppresses with the magnetic field parameter. Our results are compared with the results existing in the open literature. The comparisons are satisfactory. PMID:23741295

Uddin, Md Jashim; Khan, Waqar A; Ismail, A I Md

2013-05-31

22

MHD forced convective laminar boundary layer flow from a convectively heated moving vertical plate with radiation and transpiration effect.  

UK PubMed Central (United Kingdom)

A two-dimensional steady forced convective flow of a Newtonian fluid past a convectively heated permeable vertically moving plate in the presence of a variable magnetic field and radiation effect has been investigated numerically. The plate moves either in assisting or opposing direction to the free stream. The plate and free stream velocities are considered to be proportional to x(m) whilst the magnetic field and mass transfer velocity are taken to be proportional to x((m-1)/2) where x is the distance along the plate from the leading edge of the plate. Instead of using existing similarity transformations, we use a linear group of transformations to transform the governing equations into similarity equations with relevant boundary conditions. Numerical solutions of the similarity equations are presented to show the effects of the controlling parameters on the dimensionless velocity, temperature and concentration profiles as well as on the friction factor, rate of heat and mass transfer. It is found that the rate of heat transfer elevates with the mass transfer velocity, convective heat transfer, Prandtl number, velocity ratio and the magnetic field parameters. It is also found that the rate of mass transfer enhances with the mass transfer velocity, velocity ratio, power law index and the Schmidt number, whilst it suppresses with the magnetic field parameter. Our results are compared with the results existing in the open literature. The comparisons are satisfactory.

Uddin MJ; Khan WA; Ismail AI

2013-01-01

23

Film boiling heat transfer from a sphere in natural and forced convection of freon-113  

Energy Technology Data Exchange (ETDEWEB)

Boiling heat transfer fluxes were measured on a 3.84-cm hollow copper sphere, in both forced convection and pool boiling, as a function of angular position in Freon 113. This paper reports on forced-convection tests run at speeds of 0.5 to 1.9 m/s. These tests were conducted in the stable film boiling region of the boiling curve. Significant heat transfer rates were measured in the vapor wake region of the sphere for flow film boiling. Video observations of the boiling process revealed that the flow film boiling vapor removal mechanism consisted of periodic formation and detachment of a vapor wake in the rear of the sphere. For pool boiling it was found that the heated surface had a uniform rate of energy dissipation in the stable film boiling regime, whereas in forced convection the film boiling rate was dependent on angular position. Pool film boiling tests also showed multiple humps (more than one maximum heat flux) in the boiling curve when the liquid was subcooled.

Dix, D.; Orozco, J. (Illinois Univ., Chicago, IL (USA))

1990-01-01

24

The effect of internal ribbing on forced convective heat transfer in circular-sectioned tubes  

International Nuclear Information System (INIS)

This paper presents the results of an experimental examination of the effect of internal circumferential ribs on forced convection in circular-sectioned tubes. The work is relevant to the internal cooling of gas turbine rotor blades. The influence of rib geometry is investigated for three different rib configurations and simple design-type, empirical equations are developed for estimating heat transfer at rib and mid-rib locations. It is demonstrated that heat transfer may be improved by up to three fold in relation to fully developed forced convection in smooth-walled tubes. The geometric parameters which have been used for the experiments are typical of those currently applied to gas turbine blade cooling designs.

2003-01-01

25

Experimental and numerical study of steady forced-convection heat transfer in a spherical annulus  

International Nuclear Information System (INIS)

[en] 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 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

1980-01-01

26

Burnout in boiling heat transfer. Part III. High-quality forced-convection systems  

International Nuclear Information System (INIS)

[en] This is the final part of a review of burnout during boiling heat transfer. The status of burnout in high-quality forced-convection systems is reviewed, and recent developments are summarized in detail. A general guide to the considerable literature is given. Parametric effects and correlations for water in circular and noncircular ducts are presented. Other topics discussed include transients, steam-generator applications, correlations for other fluids, fouling, and augmentation

1979-01-01

27

The effect of fouling on thermodynamic performance of forced convective heat transfer through a duct  

International Nuclear Information System (INIS)

Based on the first and second thermodynamic laws, a new systematic approach to study in detail the effect of fouling on the thermodynamic performance of forced convective heat transfer through a duct with constant wall temperature and constant wall heat flux for thermally and hydrodynamically fully developed turbulent flow is investigated. When considering fouling exists inside the duct, the local and mean exergy variation coefficient, exergy variation flux, dimensionless exergy variation number and the equation of exergy variation rate of working fluids, etc. have been put forward and their generalized expressions derived. A criterion evaluating the effect of fouling on the exergy variation of working fluids of the forced convective heat transfer process, which is defined as the exergy variation degradation rate, has been put forward. By reference to a duct, the numerical results of the exergy variation of working fluids are obtained (considering fouling or not), the effects of Reynolds number, the thickness of the fouling layer, dimensionless inlet temperature difference and wall heat flux on the exergy variation of working fluids are discussed. The results show that the exergy variation degradation rate increases with the increase of Reynolds number and decreases with the increase of dimensionless inlet temperature and wall heat flux. The exergy variation caused by the heat conduction of fouling plays an important role in the total exergy variation of working fluids.

2007-01-01

28

Visualization study for forced convection heat transfer of supercritical carbon dioxide near pseudo-boiling point  

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)

2001-01-01

29

Visualization study for forced convection heat transfer of supercritical carbon dioxide near pseudo-boiling point  

Energy Technology Data Exchange (ETDEWEB)

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)

Sakurai, K.; Ko, H.S.; Okamoto, K.; Madarame, H. [Tokyo Univ., Tokai, Ibaraki (Japan). Nuclear Engineering Research Lab

2001-07-01

30

Convective heat transfer on Mars  

International Nuclear Information System (INIS)

An examination was made into the feasibility of using convective heat transfer on Mars to reject the waste heat from a Closed Brayton Cycle. Forced and natural convection were compared to thermal radiation. For the three radiator configurations studied, it was concluded that thermal radiation will yield the minimum mass and forced convection will result in the minimum area radiator. Other issues such as reliability of a fan motor were not addressed. Convective heat transfer on Mars warrants further investigation. However, the low density of the Martian atmosphere makes it difficult to utilize convective heat transfer without incurring a weight penalty.

1991-01-01

31

Wall heat transfer coefficient for condensation and boiling in forced convection of sodium  

International Nuclear Information System (INIS)

[en] The wall heat transfer coefficient for sodium boiling and condensation in forced convective flow is theoretically derived by using the momentum-heat transfer analogy and the logarithmic law for velocity distribution in the liquid film. Only one constant in the logarithmic form is empirically determined. The results are insensitive to some of the approximations used, namely in the evaluation of E and of /tau/ /SUB i/ //tau/ /SUB w/. The results from the suggested correlation are in good agreement with the Zeigarnick and Litvinov data over the range of parameters that were specified in their experiments, while the Chen and NATOF correlations predict lower heat transfer coefficients. The suggested correlation predicts well the data in the high-heat transfer coefficient region. It can be argued that the data in the low heat transfer coefficient region are affected by unstable flow conditions and the uncertainties in the saturation-temperature measurements

1982-01-01

32

Experimental study on forced convection boiling heat transfer on molten alloy  

Energy Technology Data Exchange (ETDEWEB)

In order to clarify the characteristics of forced convection boiling heat transfer on molten metal, basic experiments have been carried out with subcooled water flowing on molten Wood's alloy pool surface. In these experiments, water flows horizontally in a rectangular duct. A cavity filled with Wood's alloy is present in a portion of the bottom of the duct. Wood's alloy is heated by a copper conductor at the bottom of the cavity. The experiments have been carried out with various velocities and subcoolings of water, and temperature of Wood's alloy. Boiling curves on the molten alloy surface were obtained and compared with that on a solid heat transfer surface. It is observed that the boiling curve on molten alloy is in a lower superheat region than the boiling curve on a solid surface. This indicates that the heat transfer performance of forced convection boiling on molten alloy is enhanced by increase of the heat transfer area, due to oscillation of the surface and fragmentation of molten alloy.

Nishimura, Satoshi; Ueda, Nobuyuki; Nishi, Yoshihisa; Furuya, Masahiro; Kinoshita, Izumi

1999-07-01

33

The effects of buoyancy on the critical heat flux in forced convection  

Energy Technology Data Exchange (ETDEWEB)

The critical heat flux (CHF) in forced convection over a flat surface at relatively low flow velocities has been found, not unexpectedly, to depend upon the orientation of the buoyancy. The CHF for R-113 was measured at various heating surface orientations for test section Reynolds numbers ranging between 3000 and 6500. In this flow range, the buoyancy force acting on the vapor generally dominates over the flow inertia, yet the inertia would still be substantial were gravity to be reduced. In the experiments of this study, the CHF is determined for heating surface orientations ranging from 0 deg to 360 deg, for flow velocities between 4 cm/s and 35 cm/s, and for subcoolings between 2.8 C and 22.2 C. The results presented here demonstrate the strong influence of buoyancy at low flow velocities, which diminishes as the flow velocity and subcooling are increased. 15 refs.

Brusstar, M.J.; Merte, H. Jr. (Michigan Univ., Ann Arbor (United States))

1993-01-01

34

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

35

Unsteady forced convection heat/mass transfer around two spheres in tandem at low Reynolds numbers  

Energy Technology Data Exchange (ETDEWEB)

This paper presents a computational study of the forced convection heat/mass transfer from two spheres placed in a uniform viscous flow parallel to their line of centers. The temperature/concentration inside the spheres are assumed spatially uniform but not constant in time. Axisymmetric, slow, viscous flow (Stokes flow) around the spheres was considered. The appropriate energy/chemical species balance equations were solved numerically in bispherical coordinates. The finite difference method was used to discretize the mathematical model equations. Various spheres spacing, sizes and physical properties were considered at moderate Pe numbers. (author)

Juncu, Gheorghe [Politehnica University Bucharest, Catedra Inginerie Chimica, Polizu 1, 78126 Bucharest (Romania)

2007-10-15

36

Effect of uncertainties in physical properties on forced convection heat transfer with nanofluids  

International Nuclear Information System (INIS)

Nanofluids are considered to offer important advantages over conventional heat transfer fluids. However, at this early stage of their development, their thermophysical properties are not known precisely. As a result, the assessment of their true potential is difficult. This fact is illustrated by analyzing their thermohydraulic performance for both laminar and turbulent fully developed forced convection in a tube with uniform wall heat flux. Two different models from the literature are used to express these properties in terms of particle loading and they lead to very different qualitative and quantitative results in two types of problems: replacement of a simple fluid by a nanofluid in a given installation and design of an elementary heat transfer installation for a simple fluid or a nanofluid

2007-01-01

37

Forced convection film boiling drag and heat transfer of a wedge.  

Science.gov (United States)

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 o...

P. R. Chappidi K. O. Pasamehmetoglu F. S. Gunnerson

1990-01-01

38

Transient forced convection heat transfer due to exponentially increasing heat input for helium gas flowing on a narrow plate  

International Nuclear Information System (INIS)

Forced convection transient heat transfer for helium gas at various periods of exponentially increasing heat input (Q0exp(t/?)) to a horizontal plate (ribbon) was experimentally and theoretically studied. In the experimental studies, the authors measured heat flux, surface temperature, and transient heat transfer coefficients for forced convection flow of helium gas over the horizontal plate under wide experimental conditions. The platinum plate with a length of 50 mm was used as a test heater. The gas flow velocities ranged from 4 to 10 m/s, the gas temperatures ranged from 313 to 353 K, and the periods of heat generation rate, ?, ranged from 46 ms to 17 s. The pressures were from 400 to 800 kPa. It was clarified that the heat transfer coefficient approaches the quasi-steady-state one for the period longer than about 1 s, and it becomes higher for the period shorter than around 1 s. Empirical correlations for quasi-steady state heat transfer and transient one were obtained based on the experimental data under various pressures. In the theoretical study, transient heat transfer was numerically solved based on a turbulent flow model. The values of numerical solutions for surface temperature and heat flux were compared and discussed with authors' experimental values. It was obtained that the surface temperature difference and heat flux increase exponentially as the heat generation rate increases with the exponential function. It is understood that the gradient of the temperature distribution near the heater surface is higher at a higher surface temperature difference. The values of numerical solutions for heat flux agree well with the experimental data, though surface temperatures show some differences. (author)

2009-01-01

39

Enhancement of forced convective heat transfer in narrow concentric annulus by turbulence promoters  

International Nuclear Information System (INIS)

[en] Enhancement of forced convective heat transfer in a narrow concentric annulus was performed by turbulence promoters in order to improve the heat removal from High Temperature Gas-cooled Reactor, Gas-cooled Nuclear Fusion Reactor and other narrow flow passages. Present experiments, which were different from the conventional research, were carried out to examine the effect of the turbulence promoters on the inner insulated wall opposite the outer smooth heated wall by changing the ratio of the pitch and the height P/epsilon (from 5 to 60), the ratio of the height and the space epsilon/epsilon' (from 0.2 to 2) and the type of turbulence promoters. Experimental results were arranged for the local heat transfer coefficient distribution on the smooth outer tube, the average heat transfer coefficient, the friction factor and the thermal performance. Five kinds of evaluation for thermal performance were presented: The ratio of heat transfer coefficient with and without turbulence promoters at three conditions, (1) constant Reynolds number, (2) constant pressure drop and (3) constant pumping power, additionally, (4) pumping power reduction at constant heat transfer coefficient and (5) working ratio. An example of a fuel element of HTGR with turbulence promoters was indicated as an application of present results. (author)

1984-01-01

40

Forced convection boiling heat transfer of CO{sub 2} in horizontal tubes  

Energy Technology Data Exchange (ETDEWEB)

From ozone depletion and global warming (ODP and GWP) viewpoints, natural refrigerants offer a clear advantage over CFCs and their substitutes. Here, an experimental investigation is performed to study the forced convection boiling heat transfer of CO{sub 2} in a smooth tube. Tests are conducted in a horizontal stainless steel tube. The tube has an inner diameter of 6.00 mm, wall thickness of 1.00 mm, and length of 1.78 m. Heat to the test section is provided by a heating wire wrapped around the tube. Experiments are conducted with lubricant-free CO{sub 2} at saturation temperature of 283 K, mass fluxes of 160 to 320 kg/m{sup 2}s, heat fluxes of 10 to 20 kW/m{sup 2}, and inlet quality of 5% to 60%. The heat transfer coefficients of CO{sub 2} increase with increasing the heat and/or mass flux. For the test conditions considered, the vapor quality had an adverse effect on the heat transfer coefficients. The present experimental data are found to be close to the results predicted by Gungor and Winterton (1987) with the mean deviation of less than 10%. The experimental data are correlated and the results are discussed.

Zhao, Y.; Ohadi, M.M.; Dessiatoun, S.V.; Molki, M.; Darabi, J.

1999-07-01

 
 
 
 
41

Non-Newtonian fluid laminar flow and forced convection heat transfer in rectangular ducts  

International Nuclear Information System (INIS)

Numerical solutions for fully developed laminar flow forced convection heat transfer of a power law non-Newtonian fluid in rectangular ducts are presented in this paper. Finite difference methods are developed for the governing equations to obtain the velocity and temperature distributions. Friction factor results are given for flow through rectangular ducts of aspect ratios of 0.2, 0.5 and 1.0 with power law index n values of 0.5 to 1.0. For the same flow conditions the Nusselt values, maximum wall temperatures, and minimum wall temperatures for the H2 thermal boundary condition for different combinations of heated and adiabatic walls are obtained. Also the Nusselt values for slug flow (n = 0) are presented for the H2 boundary condition.

1992-01-01

42

Non-Newtonian fluid laminar flow and forced convection heat transfer in rectangular ducts  

Energy Technology Data Exchange (ETDEWEB)

Numerical solutions for fully developed laminar flow forced convection heat transfer of a power law non-Newtonian fluid in rectangular ducts are presented in this paper. Finite difference methods are developed for the governing equations to obtain the velocity and temperature distributions. Friction factor results are given for flow through rectangular ducts of aspect ratios of 0.2, 0.5 and 1.0 with power law index n values of 0.5 to 1.0. For the same flow conditions the Nusselt values, maximum wall temperatures, and minimum wall temperatures for the H2 thermal boundary condition for different combinations of heated and adiabatic walls are obtained. Also the Nusselt values for slug flow (n = 0) are presented for the H2 boundary condition.

Gao, S.X.; Hartnett, J.P. (Illinois Univ., Chicago, IL (United States). Energy Resources Center)

1992-09-01

43

A dry-spot model of critical heat flux applicable to both pool boiling and subcooled forced convection boiling  

Energy Technology Data Exchange (ETDEWEB)

A study has been performed to predict CHF in pool boiling and subcooled forced convection boiling using the dry-spot model presented by the authors and existing correlations for heat transfer coefficient, active site density and bubble departure diameter in nucleate boiling. Comparisons of the model predictions with experimental data for pool boiling of water and subcooled upward forced convection boiling of water in vertical, uniformly-heated round tubes have been performed and the parametric trends of CHF have been investigated. The results of the present study strongly support the validity of physical feature of the present model on the CHF mechanism in pool boiling and subcooled forced convection boiling. To improve the prediction capability of the present model, further works on active site density, bubble departure diameter and suppression factor in subcooled boiling are needed.

Ha, S.J.; No, H.C.

1999-07-01

44

Numerical simulation of forced convective cooling within a massive array of heated tubes  

Energy Technology Data Exchange (ETDEWEB)

A multidimensional finite element model has been developed to calculate forced convective flow of coolant within a 600 tube array consisting of 2 cm and 10 cm diameter heated tubes over 5 m in length. The 10 cm tubes are hollow and thin shelled, with coolant flowing vertically downward within the tubes. The 2 cm tubes are sealed and used for support. The time dependent primitive equations of fluid motion and energy are solved using equal order, isoparametric linear elements for all variables. Petrov-Galerkin weighting is used for the advection terms, along with mass lumping, reduced integration, and explicit time marching. Preliminary results indicate a marked decrease in the amount of coolant available to most of the interior tubes. 12 refs.

Pepper, D.W.; Dyne, B.R.; Brueckner, F.P. (Nevada Univ., Las Vegas (United States) Advanced Projects Research, Inc., Moorpark, CA (United States))

1993-01-01

45

Improved modelling of turbulent forced convective heat transfer in straight ducts  

International Nuclear Information System (INIS)

[en] Ducts with non-circular cross sections are frequently encountered in industrial heat transfer equipment, e.g., compact heat exchangers, cooling channels in gas turbine blades, nuclear reactors, etc. This investigation concerns numerical calculation of turbulent forced convective heat transfer and fluid flow in their fully developed state at low Reynolds number. The authors have developed a low Reynolds number version of the non-linear ?-? model combined with the heat flux models of SED, low Reynolds number version of GGDH and WET in general three dimensional geometries. The wall function approach is abandoned and the friction factor is predicted in agreement with experiments. The Nusselt numbers are also predicted very well. Fully developed condition is achieved by imposing cyclic boundary conditions in the main flow direction. The numerical approach is based on the finite volume technique with a non-staggered grid arrangement and the SIMPLEC algorithm. Results have been obtained with the linear and the nonlinear ?-? model, combined with the Lam-Bremhorst damping functions for low Reynolds numbers. The secondary flow patterns are also of major concern

1997-01-01

46

Effects of buoyancy on the critical heat flux in forced convection  

Energy Technology Data Exchange (ETDEWEB)

The critical heat flux (CHF) for R-113 was measured in forced convection over a flat surface at various orientations for relatively low flow velocities corresponding to Reynolds numbers ranging between 3000-6500 in the test section. As expected, the CHF was found to depend upon the orientation of the buoyancy. Although the buoyancy force acting on the vapor generally dominates over the flow inertia in this flow range, the inertia would continue to be substantial if the gravity was to be reduced significantly. In the experiments of this study, the CHF was determined for heating surface orientations ranging from 0 to 360 deg, for flow velocities between 4 - 35 cm/s, and for subcoolings between 2.8 - 22.2 C. The results presented here demonstrate the strong influence of buoyancy at low flow velocities, which diminishes as the flow velocity and subcooling are increased. In addition, a simple model in which the effects of varying the buoyancy orientation in pool boiling is incorporated is proposed to correlate the CHF at low flow velocities, which finally leads to an analogy between the CHF under adverse gravity and that under microgravity conditions. 18 refs.

Brusstar, M.J.; Merte, H. Jr. (Univ. of Michigan, Ann Arbor, MI (United States))

1994-04-01

47

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

48

A NUMERCIAL COMPARISON OF SINGLE-PHASE FORCED CONVECTIVE HEAT TRANSFER BETWEEN ROUND TUBE AND STRAIGHT MICROCHANNEL HEAT EXCHANGERS  

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,; I. MIRZAIE,; N. POURMAHMOUD; M.RAHIMI EOSBOEE; S. MAJIDYFAR; M. MANSOOR

2010-01-01

49

Adaptive neuro-fuzzy inference system (ANFIS) to predict the forced convection heat transfer from a v-shaped plate  

Science.gov (United States)

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

50

A numerical study of single-phase forced convective heat transfer with flow friction in microchannels  

International Nuclear Information System (INIS)

[en] Three-dimensional simulations of the single-phase laminar flow and forced convective heat transfer of water in microchannels with small rectangular sections having specific hydraulic diameters and distinct geometric configurations were investigated numerically. The numerical results indicated that the laminar heat transfer was to be dependent upon the aspect ratio and the ratio of the hydraulic diameter to the center to center distance of the microchannels. The geometries and operating conditions of that indicated microchannel were created using a finite volume-based computational fluid dynamics technique. The aim of this paper is to obtain computational Nusselt number in laminar flow using Fluent CFD Solver and to validate it with available experimental studies. Acquired numerical results have an admissible compatibility with available experimental studies. In addition, conceivable temperature profiles and pressure drops have been obtained at each Z-location in this model. Then, pressure drop values in the present model were explored for each Reynolds number. Finally, the effects of geometric parameters on the average Nusselt number in the laminar flow were illustrated numerically.

2012-01-01

51

Transient forced convection heat transfer from a circular cylinder embedded in a porous medium  

Energy Technology Data Exchange (ETDEWEB)

Studies of the transient heat transfer past a circular cylinder in a steady-state viscous flow are presented for some fluid saturated fibrous porous media. Numerical results have been obtained according to the Darcy-Brinkman model by means of the finite element method. Analysis of the influence of the Darcy and Peclet numbers on the mean Nusselt number exhibits the successive conduction, transition and convection regimes. The duration necessary to reach the steady-state convection heat transfer appears as a function of the Peclet and Darcy numbers.

Thevenin, J. [Universite Pierre et Marie Curie, Paris (France). Modelisation des Transferts Thermiques

1995-07-01

52

Laminar forced convective/conductive heat transfer by finite element method  

International Nuclear Information System (INIS)

The present study is directed at developing a finite element computer program for solution of decoupled convective/conductive heat transfer problems. Penalty function formulation has been used to solve momentum equations and subsequently transient energy equation is solved using modified Crank-Nicolson method. The optimal upwinding scheme has been employed in energy equation to remove oscillations at high Peclet number. (author)

1982-01-01

53

Burn-out phenomena of a liquid nitrogen forced convective flow in a tube heated in cosine-distribution  

Energy Technology Data Exchange (ETDEWEB)

Forced convective critical heat flux was experimentally studied. Liquid nitrogen was used for the working fluid in a vertical tube the heat flux of which had a cosine distribution. It was examined the rise of the tube wall temperature, the fluctuation of the pressure and the flow pattern for the critical heat flux based on the averaged critical heat flux of the tube. Two phenomena on the critical heat flux were found. One was a burn-out phenomenon occurred when flow pattern changed from slug or churn flow to annular flow at higher heat flux. The other was a DNB phenomenon just after the boiling at lower heat flux. The phenomena on the critical heat flux were experimentally studied and discussed. (author)

Takenaka, Nobuyuki; Fujii, Terushige; Umeki, Isamu [Kobe Univ., Department Mechanical of Engineering, Kobe, Hyogo (Japan)

2000-07-01

54

Burn-out phenomena of a liquid nitrogen forced convective flow in a tube heated in cosine-distribution  

International Nuclear Information System (INIS)

Forced convective critical heat flux was experimentally studied. Liquid nitrogen was used for the working fluid in a vertical tube the heat flux of which had a cosine distribution. It was examined the rise of the tube wall temperature, the fluctuation of the pressure and the flow pattern for the critical heat flux based on the averaged critical heat flux of the tube. Two phenomena on the critical heat flux were found. One was a burn-out phenomenon occurred when flow pattern changed from slug or churn flow to annular flow at higher heat flux. The other was a DNB phenomenon just after the boiling at lower heat flux. The phenomena on the critical heat flux were experimentally studied and discussed. (author)

2000-01-01

55

PERFORMANCE OF A FORCED CONVECTION SOLAR DRIER INTEGRATED WITH GRAVEL AS HEAT STORAGE MATERIAL FOR CHILI DRYING  

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; P. CHANDRASEKAR

2009-01-01

56

Burnout in boiling heat transfer. Part II: subcooled and low quality forced-convection systems  

International Nuclear Information System (INIS)

[en] Recent experimental and analytical developments regrading burnout in subcooled and low quality forced-convection systems are reviewed. Much data have been accumulated which clarify the parametric trends and lead to new design correlations for water and a variety of other coolants in both simple and complex geometries. A number of critical experiments and models have been developed to attempt to clarify the burnout mechanism(s) in simpler geometries and power transients

1977-01-01

57

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

International Nuclear Information System (INIS)

[en] 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

1980-01-01

58

Transient convective heat transfer  

Scientific Electronic Library Online (English)

Full Text Available 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 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 condit (more) ions) 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.

Padet, J.

2005-03-01

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

2005-01-01

60

Constructal design of forced convection cooled microchannel heat sinks and heat exchangers  

Energy Technology Data Exchange (ETDEWEB)

Heat transfer from arrays of circular and non-circular ducts subject to finite volume and constant pressure drop constraints is examined. It is shown that the optimal duct dimension is independent of the array structure and hence represents an optimal construction element. Solutions are presented for the optimal duct dimensions and maximum heat transfer per unit volume for the parallel plate channel, rectangular channel, elliptic duct, circular duct, polygonal ducts, and triangular ducts. Approximate analytical results show that the optimal shape is the isosceles right triangle and square duct due to their ability to provide the most efficient packing in a fixed volume. Whereas a more exact analysis reveals that the parallel plate channel array is in fact the superior system. An approximate relationship is developed which is very nearly a universal solution for any duct shape in terms of the Bejan number and duct aspect ratio. Finally, validation of the relationships is provided using exact results from the open literature. (author)

Muzychka, Y.S. [Memorial University of Newfoundland, St. John' s (Canada). Faculty of Engineering and Applied Science

2005-07-01

 
 
 
 
61

Burnout in boiling heat transfer. II. Subcooled and low-quality forced-convection systems  

International Nuclear Information System (INIS)

[en] Recent experimental and analytical developments regarding burnout in subcooled and low-quality forced-convection systems are reviewed. Many data have been accumulated which clarify the parametric trends and lead to new design correlations for water and a variety of other coolants in both simple and complex geometries. A number of critical experiments and models have been developed to attempt to clarify the burnout mechanism(s) in simpler geometries. Other topics discussed include burnout with power transients and techniques to augment burnout. 86 references

1977-01-01

62

A Dry-Spot Model of Critical Heat Flux Applicable to Both Pool Boiling and Subcooled Forced Convection Boiling  

Energy Technology Data Exchange (ETDEWEB)

A phenomenological model of critical heat flux (CHF) applicable to both pool boiling and subcooled forced convection boiling is developed using the dry-spot model proposed recently and existing correlations for active site density, bubble departure diameter and heat transfer coefficient in nucleate boiling. For the active nucleation site density Kocamustafaogullari and Ishii's model is used including the concept of a suppression factor. The Chen correlation is used for the estimation of total heat flux in nucleate boiling. Comparisons of the model predictions with experimental data for pool boiling of water and subcooled upward flow boiling of water in vertical, uniformly-heated round tubes yield an averaged CHF ratio of 0.93 and a root-mean-square error of 41.3%. The data set compared for CHF in subcooled forced convection boiling covers wide ranges of operating conditions (0.1

Ha, S.J. [Korea Electric Power Research Institute, Taejon (Korea)

1999-07-01

63

Thermal resistance of steam generator tube deposits under single-phase forced-convection and flow-boiling heat transfer  

International Nuclear Information System (INIS)

The heat transfer resistance of a porous deposit can be expressed as the sum of two components: one associated with conduction through the fluid-filled deposit and a second associated with surface roughness. This simple relationship appears to hold under both single-phase forced-convection and boiling heat-transfer conditions. The conductive component of the total deposit resistance is always positive, whereas the roughness component is negative. Values for ? and Rroughness measured in this investigation are as follows. Under single-phase forced-convection heat-transfer conditions, ? = 1.3 ± 0.2 W/mK and Rroughness = -4 x 10-6 m2K/W for magnetite deposits. Under flow-boiling heat-transfer conditions, ? = 0.2 to 0.9 W/mK and Rroughness = -36 x 10-6 m2K/W for magnetite deposits, whereas ? = 2.0 W/mK and Rroughness = -43 x 10-6 m2K/W for deposits composed of approximately equal proportions of copper and magnetite. (author)

1997-01-01

64

Laminar forced convection heat transfer in the floating zone process: Some high Reynolds number results  

International Nuclear Information System (INIS)

[en] Laminar flow and heat transfer in a cylindrical floating zone are computed for iso- and counter rotation of the feed rod and crystal in the absence of natural and Marangoni convection. For differential iso-rotation at high rotational Reynolds numbers, the velocity field exhibits a core region rotating with the average angular velocity of the rotating ends. For equal counter-rotation, the solution loses symmetry about the midplane at a critical Reynolds number; beyond this, multiple asymmetric solutions are to be found. Local and overall Nusselt numbers are computed for a range of Prandtl and Biot numbers. The computed isotherm patterns agree qualitatively with experiment

1987-01-01

65

Experimental determination of heat transfer critical conditions in water forced convection at low pressure in a circular channel  

International Nuclear Information System (INIS)

[en] 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

1973-01-01

66

Determination of heat transfer in pool boiling and forced convective boiling. Laemmoensiirto allas- ja virtauskiehunnassa  

Energy Technology Data Exchange (ETDEWEB)

The scope of this work was present different pool boiling and flow boiling correlations and compare these by calculation. Calculations were done by water and refrigerant R12. Short notes has done concerning different theoretical boiling models. Some basic definations and terminology of two phase boiling phenomen has shown and 6 pool boiling correlation and 20 flow boiling correlation are presented. In the results of calculations in high pressures flow boiling correlations are compared with measurement results collected from litterature. In pool boiling calculation results of three latest correlations, Stephan-Abdelsalam (1978), VDI (1984) and Cooper (1984), mean absolute deviation was within limits 30%-40%. In flow boiling calculation results of four general correlation, Chen (1966), Shah (1976), VDI (1984) and Gungor (1986), mean absolute deviation was within limits 20%-30%. Correlations predicting only concective flow boiling gave 1-1.5 times higher boiling heat transfer coefficients than general correlations in convective boiling. Calculations were done by water and refrigerant R12 by pressure p=0.135. In similar case, convetive flow boiling correlations, which were modified to tackle both nucleate boiling and convection boiling sections in flow boiling, gave 1-2 times higher boiling heat transfer coefficient compared with values predicted by general correlations. 43 figs.

Jaervinen, E.; Fagerholm, N.-E.

1986-01-01

67

Convection heat transfer  

Energy Technology Data Exchange (ETDEWEB)

Emphasizing the integration of mathematical expressions with clear physical associations, this textbook on convective heat and mass transfer reviews the laws of thermodynamics and fluid motions, behavior of laminar and turbulent flows in a variety of conditions, natural free convection in space, and flows through porous media.

Bejan, A.

1984-01-01

68

An analytical study of momentum and heat transfer on turbulent forced convection in rectangular channels with allowance for secondary effects  

International Nuclear Information System (INIS)

A theoretical study was performed to investigate turbulent forced-convective momentum and heat transport in a rectangular channel under fully developed flow and heat transfer conditions. Main emphasis has been devoted to analyse the effects of the anisotropic turbulent transport properties and the turbulence-induced secondary flow on the main flow field and the temperature distribution. The effects of peripheral wall conduction as well as radiation within the channel are included in the analysis. The analysis reveals that only a single secondary current occurs in the trapezoidal symmetry element of the rectangular duct. Furthermore, when the heating extends over one or two oppositely located sides only, the location of the maximum wall temperature is shifted from the corner to the center of the wall. (orig.)

1979-01-01

69

An Experimental Investigation into the Deployment of 3D, Finned Wing and Shape Memory Alloy Vortex Generators in a Forced Air Convection Heat Pipe Fin Stack  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Abstract 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...

70

A numerical study of forced convective heat transfer from a cylinder in a porous medium near an impermeable surface  

Energy Technology Data Exchange (ETDEWEB)

Two-dimensional forced convective flow over a circular cylinder embedded in a saturated porous medium has been numerically studied. The axis of a cylinder is arranged at right angles to the oncoming forced flow. The cylinder is heated to a uniform surface temperature which is different from than that of the fluid in the forced flow ahead of the cylinder. There is an impermeable plane surface above the cylinder, the surface being parallel to the forced flow. This surface is either adiabatic or at a uniform specified temperature that is between the temperature of the liquid flowing over the cylinder and the temperature of the cylinder surface. The study is based on the Darcy type assumptions. The governing equations, written in dimensionless form, have been solved using the finite element method. The solution has as parameters the dimensionless depth of the cylinder from the impermeable surface, the Peclet number and the dimensionless thermal boundary condition at the impermeable surface. Solutions have been obtained for a range of values of all these parameters. The results indicate that the dimensionless impermeable surface temperature has a strong influence of the heat transfer rate from the cylinder at smaller values of the dimensionless cylinder-to-surface distance.

Oosthuizen, P.H.

1999-07-01

71

Convection heat transfer  

CERN Multimedia

Written by an internationally recognized authority on heat transfer and thermodynamics, this second edition of Convection Heat Transfer contains new and updated problems and examples reflecting real-world research and applications, including heat exchanger design. Teaching not only structure but also technique, the book begins with the simplest problem solving method (scale analysis), and moves on to progressively more advanced and exact methods (integral method, self similarity, asymptotic behavior). A solutions manual is available for all problems and exercises.

Bejan, Adrian

2013-01-01

72

Convective Heat Transfer  

CERN Multimedia

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

Forced convective heat transfer with freezing from tandem cylinders in a porous medium  

International Nuclear Information System (INIS)

[en] This paper reports on heat transfer from two circular cylinders of equal diameter buried in a porous medium with a forced flow over them. The cylinders are assumed to be cooled to the same uniform temperature that is, in general, lower than the freezing temperature of the liquid flowing through the porous medium, while the temperature of this liquid in the forced flow ahead of the cylinders is, of course, above this freezing temperature. The cylinders are arranged in tandem such that the approaching forced flow is at right angles to the cylinder axes. The flow has been assumed to be steady and two-dimensional and the usual Darcy type assumptions have been adopted. The fluid properties have been assumed to be constant except that the conductivity of the frozen material is, in general, different from that of the unfrozen material. The fluid velocity and temperature well upstream of the cylinders are assumed to be uniform. The governing equations written in dimensionless form have been solved, subject to the boundary conditions using the finite element method

1990-01-01

74

Combined laminar forced convection and nongray-radiation heat transfer to carbon dioxide flowing in a nonblack, plane-parallel duct  

Energy Technology Data Exchange (ETDEWEB)

Simultaneous forced convection and nongray-radiation heat transfer to flowing carbon dioxide in a nonblack duct is analyzed theoretically based on the two-parameter wideband spectral model for the absorption coefficients of infrared gases. The validity of the P{sub 1} approximation is also addressed. It is found that, for a nonblack duct, the convective Nusselt number is much larger than for pure convection and that the P{sub 1} approximation somewhat overpredicts the total Nusselt number due to the overestimation of radiation effects, irrespective of the surface emissivity of a duct, but its accuracy is fully acceptable.

Kamiuto, K. [Oita Univ. (Japan)

1995-11-01

75

Combined convective heat transfer from short cylinders  

International Nuclear Information System (INIS)

[en] Considerable experimental evidence has been produced recently showing that the free convective heat transfer rate from horizontal circular cylinders becomes influenced by the length to diameter ratio L/D. The major aim of the present study was to determine the influence of the L/D ratio on the conditions under which buoyancy forces cause the heat transfer rate to start to deviate significantly from that existing in purely forced convection

1985-01-01

76

Combined convective heat transfer from short cylinders  

Energy Technology Data Exchange (ETDEWEB)

Considerable experimental evidence has been produced recently showing that the free convective heat transfer rate from horizontal circular cylinders becomes influenced by the length to diameter ratio L/D. The major aim of the present study was to determine the influence of the L/D ratio on the conditions under which buoyancy forces cause the heat transfer rate to start to deviate significantly from that existing in purely forced convection.

Oosthuizen, P.H.; Paul, J.T. [Queen' s Univ., Dept. of Mechanical Engineering, Kingston, Ontario (Canada)

1985-07-01

77

Optimal spacing of parallel boards with discrete heat sources cooled by laminar forced convection  

Energy Technology Data Exchange (ETDEWEB)

This paper shows numerically how to select the optimal spacing between boards mounted in a stack of specified volume, so that the overall thermal conductance between the stack and the forced coolant is maximum. Several configurations are considered: boards with uniform flux, flush-mounted discrete sources, and protruding heat sources. The flow is laminar and the pressure difference across the stack is fixed ([Delta]p). It is shown that for all the board geometries and thermal boundary conditions studied, the optimal board-to-board spacing is correlated by (D[sub opt]/l)[approximately] 2.7 ([Delta]pl[sup 2]/[mu][alpha])[sup [minus]1/4], where l is the effective longitudinal (flow) distance occupied by the discrete sources and the unheated patches contained between them, and [mu] and [alpha] are the viscosity and thermal diffusivity of the fluid. If U[sub [infinity

Morega, A.M.; Bejan, A. (Duke Univ., Durham, NC (United States). Dept. of Mechanical Engineering and Materials Science)

1994-04-01

78

Effects of internal heat generation, thermal radiation and buoyancy force on a boundary layer over a vertical plate with a convective surface boundary condition  

Digital Repository Infrastructure Vision for European Research (DRIVER)

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...

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

79

Mixed Convection Heat Transfer Experiments in Smooth and Rough Vertical Tubes.  

Science.gov (United States)

The mixed convection regime is a transitional heat transfer regime between forced convection and natural convection, where both the forced component of flow, and the buoyancy induced component are important. Aiding flow is when buoyancy forces act in the ...

P. Symolon W. Neuhaus R. Odell

2004-01-01

80

On the generalized correlation equation of two phase heat transfer coefficient in forced-convection boiling  

International Nuclear Information System (INIS)

[en] 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

1992-01-01

 
 
 
 
81

Study of the equivalent diameter concept for heat transfer by forced convection in annular channels  

International Nuclear Information System (INIS)

[en] 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)

1994-01-01

82

Forced convection heat transfer and pressure drop for a horizontal cylinder with vertically attached imperforate and perforated circular fins  

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

83

Forced convection heat transfer and pressure drop for a horizontal cylinder with vertically attached imperforate and perforated circular fins  

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, 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.

2011-01-01

84

Study on forced-convection film-boiling heat transfer (Heat transfer characteristics in high Reynolds number region and critical condition)  

Energy Technology Data Exchange (ETDEWEB)

It has been pointed out that the film-boiling heat transfer is affected by the structures of the vapor flow in vapor film, the liquid-vapor interface and the liquid boundary layer adjacent to the interface. In this study the forced-convection film-boiling heat transfer was investigated experimentally focusing on the heat transfer characteristics in high Reynolds number region and the critical condition on film-boiling heat transfer characteristics. The characteristics of the measured heat-transfer coefficients for the liquid velocity above 1.5 m/s were qualitatively close to the Dittus-Boelter type correlation based upon the turbulent flow. It was found that the critical condition, which was defined as the heat-transfer coefficient increased with the liquid velocity, was about Re{sub l}{sup 0.84}Pr{sub l} = Pe{sub l}* = 6 x 10{sup 4} in the present experimental range. Furthermore, it was noticeable that thickness of vapor film in the film boiling at the critical condition was just as that of the viscous sublayer in turbulent boundary layer of the bulk liquid. (author)

Ohtake, Hiroyasu; Koizumi, Yasuo [Dept. of Mechanical Engineering, Kogakuin Univ., Hachioji, Tokyo (Japan)

1999-07-01

85

Numerical study of natural convection dominated heat transfer in a ventilated cavity: Case of forced flow playing simultaneous assisting and opposing roles  

Energy Technology Data Exchange (ETDEWEB)

Mixed convection heat transfer in a ventilated cavity is numerically studied by solving the mixed convection equations with the Boussinesq approximation. Results are presented in terms of streamlines, isotherms and heat transfer for different combinations of the governing parameters namely, the Reynolds number (10 {<=} Re {<=} 5000), the Rayleigh number (10{sup 4} {<=} Ra {<=} 10{sup 6}) and the relative height of the openings (B = h'/H' = 1/4). The numerical results show the presence of a maximum interaction between the effects of the forced and natural convection and the existence of different flow regimes. The latter are delineated in the Ra-Re plane and the values of Re separating the different regions are determined and correlated versus Ra.

Raji, A. [Faculty of Sciences and Technologies, Department of Physics, University Sultan Moulay Slimane, Team of Flows and Transfers Modeling (EMET), Laboratory of Physics and Mechanics of Materials, B.P. 523, Beni-Mellal (Morocco)], E-mail: abderaji@fstbm.ac.ma; Hasnaoui, M. [Faculty of Sciences Semlalia, Department of Physics, University Cadi Ayyad, UFR TMF, B.P. 2390 Marrakesh (Morocco); Bahlaoui, A. [Faculty of Sciences and Technologies, Department of Physics, University Sultan Moulay Slimane, Team of Flows and Transfers Modeling (EMET), Laboratory of Physics and Mechanics of Materials, B.P. 523, Beni-Mellal (Morocco)

2008-08-15

86

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

2009-01-01

87

Enhancement of forced convection heat transfer in tubes using staged tangential flow injection  

Science.gov (United States)

Experiments using air as the test fluid were carried out to study enhancement in heat transfer in tangential injection induced swirl flow. In the experiments, 25.4 mm (1 in) and 50.8 mm (2 in) diameter tubes were employed. The tubes were heated electrically. The heated length of the tubes varied from 3.05 to 3.66 m. Three series of experiments were conducted. In the 1st series, fluid was tangentially injected in stages through two sets of 5.08 mm (0.2 in) diameter tubular injectors. In the 2nd series, injection was accomplished through a manifold. In the 3rd seried, a 50.8 mm (2 in) diameter tube instead of 25.4 mm (1 in) diameter tube used in series 1 and 2 was used, and fluid injected through a manifold having six 11.1 mm (0.44 in) diameter passages. In all tests, Reynolds numbers varied from 15,000 to 57,000. In comparison to single stage injection, fluid injection at several places along the direction of flow can improve net enhancement in heat transfer. With a manifold type of injection through a chamber, which simulates the application of the method in a multitube heat exchanger, net enhancement was greatly improved over that for tubular injectors.

Dhir, V. K.; Chang, F.; Yu, J.

1990-04-01

88

Forced convection heat transfer of steam in a square ribbed channel  

International Nuclear Information System (INIS)

An experimental study of heat transfer characteristics of steam in a square channel (simulating a gas turbine blade cooling passage) with two opposite surfaces roughened by 60 deg parallel ribs was performed. The ranges of key governing parameters were: Reynolds numbers (Re) based on the channel hydraulic diameter (30000-140000), entry gauge pressure (0.2Mpa-0.5Mpa), heat flux of heat transfer surface area (5kWm-2-20kWm-2), and steam superheat (13 .deg. C-51 .deg. C). The test channel length was 1000mm, while the rib spacing (p/e) was 10, and the ratio of rib height (e) to hydraulic diameter (D) was 0.048. The test channel was heated by passing current through stainless steel walls instrumented with thermocouples. The local heat transfer coefficients on the ribbed wall from the channel entrance to the fully developed regions were measured. The semi-empirical correlation was fitted out by using the average Nusselt numbers in the fully developed region to cover the range of Reynolds number. The correlation can be used in the design of new generation of gas turbine blade cooled by steam

2012-01-01

89

Forced convection heat transfer of steam in a square ribbed channel  

Energy Technology Data Exchange (ETDEWEB)

An experimental study of heat transfer characteristics of steam in a square channel (simulating a gas turbine blade cooling passage) with two opposite surfaces roughened by 60 deg parallel ribs was performed. The ranges of key governing parameters were: Reynolds numbers (Re) based on the channel hydraulic diameter (30000-140000), entry gauge pressure (0.2Mpa-0.5Mpa), heat flux of heat transfer surface area (5kWm{sup -2}-20kWm{sup -2}), and steam superheat (13 .deg. C-51 .deg. C). The test channel length was 1000mm, while the rib spacing (p/e) was 10, and the ratio of rib height (e) to hydraulic diameter (D) was 0.048. The test channel was heated by passing current through stainless steel walls instrumented with thermocouples. The local heat transfer coefficients on the ribbed wall from the channel entrance to the fully developed regions were measured. The semi-empirical correlation was fitted out by using the average Nusselt numbers in the fully developed region to cover the range of Reynolds number. The correlation can be used in the design of new generation of gas turbine blade cooled by steam.

Liu, Jiazeng; Gao, Jianmin; Gao, Tieyu [Xi' an Jiaotong University, Shaanxi (China)

2012-04-15

90

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; R. Rathnasamy

2010-01-01

91

Heat and mass transfer calculations in heavy liquid metal loops under forced convection flow conditions  

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.

2006-01-01

92

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

CERN Multimedia

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

93

Application of Rayleigh-Ritz method to forced convection turbulent heat transfer  

Energy Technology Data Exchange (ETDEWEB)

An analytical model to predict heat transfer rates to an incompressible fluid in turbulent flow, with fully developed velocity profile, between a heated plate and a parallel, insulated plate is developed. The model employs van Driest's mixing length expression near the wall, a constant eddy diffusivity in the core and a constant turbulent Prandtl number. An approximate solution obtained by employing Rayleigh-Ritz method is shown to compare well with the 'exact' solution obtained by numerical integration of the differential equations. The results are compared with the available experimental data and analytical solutions.

Diaz, L.A.; Suryanarayana, N.V.

1981-06-16

94

Experimental Study on Forced Convective Heat Transfer with Low Volume Fraction of CuO/Water Nanofluid  

Directory of Open Access Journals (Sweden)

Full Text Available The present work is an experimental study of steady state convective heat transfer of de-ionized water with a low volume fraction (0.003% by volume) of copper oxide (CuO) nanoparticles dispersed to form a nanofluid that flows through a copper tube. The effect of mass flow rate ranging from (0.0113 kg/s to 0.0139 kg/s) and the effect of inlet temperatures at 100C and 17 0C on the heat transfer coefficient are studied on the entry region under laminar flow condition. The results have shown 8% enhancement of the convective heat transfer coefficient of the nanofluid even with a low volume concentration of CuO nanoparticles. The heat transfer enhancement was increased considerably as the Reynolds number increased. Possible reasons for the enhancement are discussed. Nanofluid thermo-physical properties and chaotic movement of ultrafine particles which accelerate the energy exchange process are proposed to be the main reasons for the observed heat transfer enhancement. A correlation for convective heat transfer coefficient of nanofluids, based on transport property and D/x for 8 mm tube has been evolved. The correlation predicts variation in the local Nusselt number along the flow direction of the nanofluid. A good agreement (±10%) is seen between the experimental and predicted results.

Lazarus Godson Asirvatham; Nandigana Vishal; Senthil Kumar Gangatharan; Dhasan Mohan Lal

2009-01-01

95

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.

1996-01-01

96

Three-dimensional surface of heat transfer for laminar forced and mixed convection from a circular cylinder  

International Nuclear Information System (INIS)

[en] In this paper, results of an experimental investigation of mixed convection in laminar air flow across a tungsten wire are presented. Results are correlated by the 3-D surface equation of heat transfer, and compared with traditional equations for mixed convection which have been suggested by several investigators. Experimental results are given for Reynolds numbers in the range of 0.8 ? Re ? 17.5, Grashof numbers in the range of 1.5 · 10-3 ? Gr ? 25 and buoyancy parameter in the range of 6 · 10-5 ? Gr/Re2 ? 11.5

1992-01-01

97

Experimental study of micro-particle fouling under forced convective heat transfer  

Directory of Open Access Journals (Sweden)

Full Text Available Particulate fouling studies of a hydrocarbon based suspension containing 2 µm alumina particles were performed in an annular heat exchanger having a hydraulic diameter of 14.7 mm. During fouling experiments, the classical asymptotical behavior was observed. It is shown that particle concentration, fluid velocity, and wall temperature have strong influences on the fouling curve and the asymptotic fouling resistance. Furthermore, a mathematical model is developed to formulate the asymptotic fouling resistance in terms of the mass transfer coefficient, thermophoresis velocity, and fluid shear rate. The results demonstrate that the prediction of the new model is in good agreement with the experimental observations.

S. M. Peyghambarzadeh; A. Vatani; M. Jamialahmadi

2012-01-01

98

Experimental study of micro-particle fouling under forced convective heat transfer  

Scientific Electronic Library Online (English)

Full Text Available Abstract in english Particulate fouling studies of a hydrocarbon based suspension containing 2 µm alumina particles were performed in an annular heat exchanger having a hydraulic diameter of 14.7 mm. During fouling experiments, the classical asymptotical behavior was observed. It is shown that particle concentration, fluid velocity, and wall temperature have strong influences on the fouling curve and the asymptotic fouling resistance. Furthermore, a mathematical model is developed to formul (more) ate the asymptotic fouling resistance in terms of the mass transfer coefficient, thermophoresis velocity, and fluid shear rate. The results demonstrate that the prediction of the new model is in good agreement with the experimental observations.

Peyghambarzadeh, S. M.; Vatani, A.; Jamialahmadi, M.

2012-12-01

99

An experimental comparison between forced convection burn-out in freon 12 flowing vertically upwards through uniformly and non-uniformly heated round tubes  

International Nuclear Information System (INIS)

Some correlations of forced convection burn-out data are based on the approximate linearity of the relationship between burn-out heat flux and the channel-averaged quality at the burn-out point. These correlations perform satisfactorily on data obtained from uniformly heated configurations. Therefore the further inference is sometimes made that the burn-out heat flux is uniquely related to the quality, and that the burn-out in non-uniformly heated configurations can be calculated from measurements made with uniform heating. This report presents burn-out data for Freon 12 flowing vertically upwards through both uniformly and non-uniformly heated round tubes. This data shows that the quality at burn-out does depend on the heat flux profile, and that the inference mentioned above is not justified. (author)

1965-01-01

100

Forced convection heat transfer with slurry of phase change material in circular ducts: A phenomenological approach  

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.

2008-01-01

 
 
 
 
101

Theoretical study to determine the heat transfer by forced convection coefficient in an empirical correlation in single phase, for annular channels  

International Nuclear Information System (INIS)

[en] 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)

1994-01-01

102

Convective heat transfer in buildings: Recent research results  

Science.gov (United States)

Small scale water filled enclosures were used to study convective heat transfer in buildings. The convective processes investigated are: (1) natural convective heat transfer between room surfaces and the adjacent air; (2) natural convective heat transfer between adjacent rooms through a doorway or other openings; and (3) forced convection between the building and its external environment (such as, wind driven ventilation through windows, doors, or other openings). Results for surface convection coefficients are compared with existing ASHRAE coorelations and differences of as much as 20% are observed. Numerical simulations of wind driven natural ventilation exhibit good qualitative agreement with published wind tunnel data.

Bauman, F. S.; Gadgil, A.; Kammerud, R. C.; Altmayer, E.; Nansteel, M.

1982-04-01

103

Heat distribution by natural convection  

Energy Technology Data Exchange (ETDEWEB)

Natural convection can provide adequate heat distribution in many situtations 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 or to reduce the number of heating units required in a building. Natural airflow and heat transport through doorways and other internal building apertures is 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. Natural convective loops that can occur in buildings are described and a few design guidelines are presented.

Balcomb, J.D.

1985-01-01

104

Laminar forced convection wake above a heated plate placed in a channel; Convection laminaire forcee dans le sillage d'une plaque chaude placee dans un canal  

Energy Technology Data Exchange (ETDEWEB)

In this note, a numerical investigation of laminar convection wake above a heated plate placed in a channel is carried out. This configuration is often found for the collection of solar energy through vertical channels. An analytical study based on a 3 region structure is proposed in the immediate neighbourhood of the trailing edge. Velocity and temperature at the centerline channel as well as the pressure gradient are presented in asymptotic expressions. Comparison of these results with numerical solutions enhances the analytical study. (authors)

Doulfoukar, Z. [Faculte des Sciences Ain chock, U.F.R. de Mecanique, Casablanca (Morocco); Achiq, A. [Faculte des Sciences et Techniques, U.F.R. de Mecanique, Mohammadia (Morocco)

2005-06-01

105

Combined free and forced laminar convection in inclined rectangular channels heated from below and cooled from above  

International Nuclear Information System (INIS)

Steady-state numerical results for the solution to the non-linear thermal problem of combined free and forced laminar convection in inclined rectangular channels with constant but unequal surface temperature are presented for an incompressible, viscous fluid whose Prandtl number, Pr = 0.73. Fluid properties are assumed constant, except for density variations with temperature. Maximum values exist for the mean friction factor, Nusselt and Stanton numbers when the inclination to the horizontal lies between 300 and 600 for a given Archimedes number, Ar. Also, for any given inclination a unique solution exists when Ar = 0,50. (Author)

1981-12-18

106

Heat Convection of Compressible Viscous Fluids: I  

Science.gov (United States)

The stationary problem for the heat convection of compressible fluid is considered around the equilibrium solution with the external forces in the horizontal strip domain z 0 < z < z 0 + 1 and it is proved that the solution exists uniformly with respect to z 0 ? Z 0. The limit system as z 0 ? + ? is the Oberbeck-Boussinesq equations.

Nishida, Takaaki; Padula, Mariarosaria; Teramoto, Yoshiaki

2013-09-01

107

Heterogeneous nanofluids: natural convection heat transfer enhancement  

Directory of Open Access Journals (Sweden)

Full Text Available Abstract Convective heat transfer using different nanofluid types is investigated. The domain is differentially heated and nanofluids are treated as heterogeneous mixtures with weak solutal diffusivity and possible Soret separation. Owing to the pronounced Soret effect of these materials in combination with a considerable solutal expansion, the resulting solutal buoyancy forces could be significant and interact with the initial thermal convection. A modified formulation taking into account the thermal conductivity, viscosity versus nanofluids type and concentration and the spatial heterogeneous concentration induced by the Soret effect is presented. The obtained results, by solving numerically the full governing equations, are found to be in good agreement with the developed solution based on the scale analysis approach. The resulting convective flows are found to be dependent on the local particle concentration ? and the corresponding solutal to thermal buoyancy ratio N. The induced nanofluid heterogeneity showed a significant heat transfer modification. The heat transfer in natural convection increases with nanoparticle concentration but remains less than the enhancement previously underlined in forced convection case.

Oueslati Fakhreddine; Bennacer Rachid

2011-01-01

108

[Heat transfer, convection and altitudinal gradient  

UK PubMed Central (United Kingdom)

Measurements of heat loss from fur covered aluminium cylinders were made under barometric pressures ranging from 760 to 368 torr (sea level up to 5.8 km, simulated altitudes). Heat transfer diminished at high altitudes and a relative greater diminution was observed when forced convection was applied. The virtual increase in thermal insulation at high altitudes may be useful to compensate the expected larger difference between body and ambient temperatures.

Bozinovic F; Rosenmann M; Ruiz G

1987-01-01

109

Experimental study of differences in single-phase forced-convection heat transfer characteristics between upflow and downflow for narrow rectangular channel  

International Nuclear Information System (INIS)

[en] The differences in the single-phase forced-convection heat transfer characteristics between upflow and downflow were investigated experimentally with a narrow vertical rectangular channel. The objectives of the experiment were to investigate in both laminar and turbulent flow regions the applicability of existing correlations to and the effects of buoyant force on the heat transfer characteristics in the narrow vertical rectangular channel, which is simulating a subchannel of 2.25mm in gap and 750mm in length in the fuel element of the research reactor, JRR-3 to be upgraded at 20 MWt. As the results, it was revealed that (1) by use of equivalent hydraulic diameter, existing correlations are applicable to a channel as narrow as 2.25mm in gap for turbulent flow though the precision and critical Reynolds number are different among the correlations, and (2) in the laminar flow, the difference in heat transfer characteristics arises between upflow and downflow with Reynolds number less than about 700 and Grashof number larger than about 1,000, giving smaller Nusselt number for downflow than for upflow as the effect of buoyant force. New heat transfer correlations for channel heated from both sides are proposed as lower limits for upflow and downflow, respectively, in the laminar flow. (author)

1985-01-01

110

Measurement and modelling of forced convective heat transfer coefficient and pressure drop of Al2O3- and SiO2-water nanofluids  

Science.gov (United States)

Forced convective heat transfer coefficient and pressure drop of SiO2- and Al2O3-water nanofluids were characterized. The experimental facility was composed of thermal-hydraulic loop with a tank with an immersed heater, a centrifugal pump, a bypass with a globe valve, an electromagnetic flow-meter, a 18 kW in-line pre-heater, a test section with band heaters, a differential pressure transducer and a heat exchanger. The test section consists of a 1000 mm long aluminium pipe with an inner diameter of 31.2 mm. Eighteen band heaters were placed all along the test section in order to provide a uniform heat flux. Heat transfer coefficient was calculated measuring fluid temperature using immersed thermocouples (Pt100) placed at both ends of the test section and surface thermocouples in 10 axial locations along the test section (Pt1000). The measurements have been performed for different nanoparticles (Al2O3 and SiO2 with primary size of 11 nm and 12 nm, respectively), volume concentrations (1% v., 5% v.), and flow rates (3 103Re<105). Maximum heat transfer coefficient enhancement (300%) and pressure drop penalty (1000%) is obtained with 5% v. SiO2 nanofluid. Existing correlations can predict, at least in a first approximation, the heat transfer coefficient and pressure drop of nanofluids if thermal conductivity, viscosity and specific heat were properly modelled.

Julia, J. E.; Hernández, L.; Martínez-Cuenca, R.; Hibiki, T.; Mondragón, R.; Segarra, C.; Jarque, J. C.

2012-11-01

111

Measurement and modelling of forced convective heat transfer coefficient and pressure drop of Al2O3- and SiO2-water nanofluids  

International Nuclear Information System (INIS)

[en] Forced convective heat transfer coefficient and pressure drop of SiO2- and Al2O3-water nanofluids were characterized. The experimental facility was composed of thermal-hydraulic loop with a tank with an immersed heater, a centrifugal pump, a bypass with a globe valve, an electromagnetic flow-meter, a 18 kW in-line pre-heater, a test section with band heaters, a differential pressure transducer and a heat exchanger. The test section consists of a 1000 mm long aluminium pipe with an inner diameter of 31.2 mm. Eighteen band heaters were placed all along the test section in order to provide a uniform heat flux. Heat transfer coefficient was calculated measuring fluid temperature using immersed thermocouples (Pt100) placed at both ends of the test section and surface thermocouples in 10 axial locations along the test section (Pt1000). The measurements have been performed for different nanoparticles (Al2O3 and SiO2 with primary size of 11 nm and 12 nm, respectively), volume concentrations (1% v., 5% v.), and flow rates (3 103Re5). Maximum heat transfer coefficient enhancement (300%) and pressure drop penalty (1000%) is obtained with 5% v. SiO2 nanofluid. Existing correlations can predict, at least in a first approximation, the heat transfer coefficient and pressure drop of nanofluids if thermal conductivity, viscosity and specific heat were properly modelled.

2012-11-26

112

Experimental study of forced convective heat transfer from a vertical tube conveying dilute Ag/DI water nanofluids in a cross flow of air  

Science.gov (United States)

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

113

Terminal project heat convection in thin cylinders  

International Nuclear Information System (INIS)

[en] 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)

1992-01-01

114

Convection driven by internal heating  

CERN Multimedia

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^{-1/5}. 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.

Goluskin, David

2012-01-01

115

Forced and natural convection in aggregate-laden nanofluids  

International Nuclear Information System (INIS)

[en] A number of experimental and theoretical studies of convective heat transfer in nanofluids (liquid suspensions of nanoparticles, typically with features below 100 nm in size) reveal contrasting results; nanoparticles can either enhance or reduce the convective heat transfer coefficient. These disparate conclusions regarding the influence of nanoparticles on convective heat transfer may arise due to the aggregation of nanoparticles, which is often not considered in studies of nanofluids. Here, we examine theoretically forced and natural convective heat transfer of aggregate-laden nanofluids using Monte Carlo-based models to determine how the aggregate morphology influences the convective heat transfer coefficient. Specifically, in this study, it is first shown that standard heat transfer correlations should apply to nanofluids, and the main influence of the nanoparticles is to alter suspension thermal conductivity, dynamic viscosity, density, specific heat, and thermal expansion coefficient. Aggregated particles in suspension are modeled as quasi-fractal aggregates composed of individual primary particles described by the primary particle radius, number of primary particles, fractal (Hausdorff) dimension, pre-exponential factor, and degree of coalescence between primary particles. A sequential algorithm is used to computationally generate aggregates with prescribed morphological descriptors. Four types of aggregates are considered; spanning the range of aggregate morphologies observed in nanofluids. For each morphological type, the influences of aggregates on nanofluid dynamic viscosity and thermal conductivity are determined via first passage-based Brownian dynamics calculations. It is found that depending on both the material properties of the nanoparticles as well as the nanoparticle morphology, the addition of nanoparticles to a suspension can either increase or decrease both the forced and natural convective heat transfer coefficients, with both a 51% increase and a 32% decrease in the heat transfer coefficient achievable at particle volume fractions of 0.05. This study shows clearly that the influence of particle morphology needs to be accounted for in all studies of heat transfer in nanofluids.

2011-01-01

116

Approximate physical burnout model for forced convection of saturated fluid  

Energy Technology Data Exchange (ETDEWEB)

Approximate physical burnout model for forced convection of saturated fluid is considered. Relationships for determining critical heat flux are presented. They correspond satisfactorily with experimental data. Experimental data on burnout in two-phase flow for various fluids including water and helium are presented.

Yagov, V.V.; Puzin, V.A.

1985-03-01

117

Approximate physical burnout model for forced convection of saturated fluid  

International Nuclear Information System (INIS)

[en] Approximate physical burnout model for forced convection of saturated fluid is considered. Relationships for determining critical heat flux are presented. They correspond satisfactorily with experimental data. Experimental data on burnout in two-phase flow for various fluids including water and helium are presented

1985-01-01

118

Numerical investigation of nanofluids forced convection in circular tubes  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Abstract In this paper, developing laminar forced convection flow of a water–Al2O3 nanofluid in a circular tube, submitted to a constant and uniform heat flux at the wall, is numerically investigated. A single and two-phase model (discrete particles model) is employed with either con...

119

Effects of internal heat generation, thermal radiation and buoyancy force on a boundary layer over a vertical plate with a convective surface boundary condition  

Scientific Electronic Library Online (English)

Full Text Available 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 (more) 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.

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

2011-10-01

120

Thermodynamic analysis of optimal curvature ratio for fully developed laminar forced convection in a helical coiled tube with uniform heat flux  

Energy Technology Data Exchange (ETDEWEB)

In the present paper, the optimal curvature ratio for steady, laminar, fully developed forced convection in a helical coiled tube with constant wall heat flux was analyzed by thermodynamic second law based on minimal entropy generation principle. Two working fluids, including air and water, are considered. The entropy generation analysis covers a Reynolds number (Re) range of 100 to 10000, a coil curvature ratio ({delta}) range of 0.01 to 0.3, and two dimensionless duty parameters related with fluid properties, wall heat flux and mass flow rate, {eta}{sub 1} range of 0.1 to 3.0, and {eta}{sub 2}/10{sup 20} range of 0.01 to 1.0. The optimal {delta} for cases with various combinations of the design parameters is given in the present paper. In addition, a correlation equation for the optimal {delta} as a function of Re, {eta}{sub 1} and {eta}{sub 2} is proposed through a least-square-error analysis. For a thermal system composed of helical coiled tubes with fixed Re, wall heat flux and mass flow rate, the optimal {delta} should be selected so that the system could have the best exergy utilization and least irreversibility. (author)

Ko, T.H. [Department of Mechanical Engineering, Lunghwa University of Science and Technology, 300, Wan-Shou Rd. Sec. 1, Kueishan, 33306 Taoyuan, Taiwan (China)

2006-07-15

 
 
 
 
121

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

2012-01-01

122

Burnout conditions in BSR cores under forced convection flow  

International Nuclear Information System (INIS)

[en] The BSR normally operates at a power level of two megawatts with a downward forced convection cooling flow of about 1000 gallons per minute. Because of the relatively low downward coolant velocity, one or two feet per second, the bouyancy produced by the heating of the water as it passes through the core may result in a significant reduction in the critical heat flux as determined by the usual forced convection correlation methods. Previous estimates of the critical heat flux have not taken this bouyancy effect into consideration. It is the purpose of this study to develop a suitable method for estimating the relationship between flow and power level which permits the establishment of limits on operating parameters adequate to prevent burnout and to apply these to the operation of the BSR

1978-01-01

123

The optimal spacing for cylinders in crossflow forced convection  

Energy Technology Data Exchange (ETDEWEB)

In this note I draw attention to a new fundamental aspect of the heat transfer performance of a bundle of parallel cylinders with crossflow forced convection, namely, the maximization of the thermal contact between the bundle and the fluid, when the volume occupied by the bundle is fixed. In the experiments described by Jubran et al. we have seen empirical evidence that the total heat transfer rate is maximum when the cylinder-to-cylinder spacing S has a certain value. This finding is important because it has been overlooked for decades, while forced convection from cylinders in crossflow grew into one of the most researched topics in heat transfer. 8 refs., 3 figs.

Bejan, A. [Duke Univ., Durham, NC (United States)

1995-08-01

124

Mechanistic modeling of CHF in forced-convection subcooled boiling  

Energy Technology Data Exchange (ETDEWEB)

Because of the complexity of phenomena governing boiling heat transfer, the approach to solve practical problems has traditionally been based on experimental correlations rather than mechanistic models. The recent progress in computational fluid dynamics (CFD), combined with improved experimental techniques in two-phase flow and heat transfer, makes the use of rigorous physically-based models a realistic alternative to the current simplistic phenomenological approach. The objective of this paper is to present a new CFD model for critical heat flux (CHF) in low quality (in particular, in subcooled boiling) forced-convection flows in heated channels.

Podowski, M.Z.; Alajbegovic, A.; Kurul, N.; Drew, D.A.; Lahey, R.T. Jr. [Rensselaer Polytechnic Inst., Troy, NY (United States). Center for Multiphase Research

1997-05-01

125

Mechanistic modeling of CHF in forced-convection subcooled boiling  

International Nuclear Information System (INIS)

[en] Because of the complexity of phenomena governing boiling heat transfer, the approach to solve practical problems has traditionally been based on experimental correlations rather than mechanistic models. The recent progress in computational fluid dynamics (CFD), combined with improved experimental techniques in two-phase flow and heat transfer, makes the use of rigorous physically-based models a realistic alternative to the current simplistic phenomenological approach. The objective of this paper is to present a new CFD model for critical heat flux (CHF) in low quality (in particular, in subcooled boiling) forced-convection flows in heated channels

1997-01-01

126

Modelling of natural-convection driven heat exchangers  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Abstract: A lumped model is developed for shell-and-tube heat exchangers driven by natural convection, which is based on a one-dimensional approximation. The heat flux is driven by the logarithmic mean temperature difference. The volumetric air flow rate is driven by the buoyant force. Based on ...

Dirkse, M.H.; Loon, W.K.P., van; Stigter, J.D.; Bot, G.P.A.

127

TWO-DIMENSIONAL FINITE ELEMENT ANALYSIS OF FORCED CONVECTION FLOW AND HEAT TRANSFER IN A LAMINAR CHANNEL FLOW  

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; Pankaj Agarwal

2012-01-01

128

Evaluación del horno de curado de tabaco por convección forzada USCO-MADR/ Evaluating a tobacco-curing oven using a forced-convection heat exchanger USCO - MADR  

Scientific Electronic Library Online (English)

Full Text Available 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 evaluado 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 curad (more) o 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. (more) 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.

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

2010-04-01

129

Forced convective flow and heat transfer of upward cocurrent air-water slug flow in vertical plain and swirl tubes  

Energy Technology Data Exchange (ETDEWEB)

This experimental study comparatively examined the two-phase flow structures, pressured drops and heat transfer performances for the cocurrent air-water slug flows in the vertical tubes with and without the spiky twisted tape insert. The two-phase flow structures in the plain and swirl tubes were imaged using the computerized high frame-rate videography with the Taylor bubble velocity measured. Superficial liquid Reynolds number (Re{sub L}) and air-to-water mass flow ratio (AW), which were respectively in the ranges of 4000-10000 and 0.003-0.02 were selected as the controlling parameters to specify the flow condition and derive the heat transfer correlations. Tube-wise averaged void fraction and Taylor bubble velocity were well correlated by the modified drift flux models for both plain and swirl tubes at the slug flow condition. A set of selected data obtained from the plain and swirl tubes was comparatively examined to highlight the impacts of the spiky twisted tape on the air-water interfacial structure and the pressure drop and heat transfer performances. Empirical heat transfer correlations that permitted the evaluation of individual and interdependent Re{sub L} and AW impacts on heat transfer in the developed flow regions of the plain and swirl tubes at the slug flow condition were derived. (author)

Chang, Shyy Woei [Thermal Fluids Laboratory, National Kaohsiung Marine University, No. 142, Haijhuan Road, Nanzih District, Kaohsiung City 81143 (China); Yang, Tsun Lirng [Department of Marine Engineering, National Kaohsiung Marine University, No. 142, Haijhuan Road, Nanzih District, Kaohsiung City 81143 (China)

2009-10-15

130

FORCED CONVECTION GREENHOUSE PAPAD DRYING: AN EXPERIMENTAL STUDY  

Directory of Open Access Journals (Sweden)

Full Text Available In this research paper, the behavior of heat and mass transfer phenomenon during greenhouse papad drying under forced convection mode has been investigated. Various experiments were performed during the month of April 2010 at Guru Jambheshwar University of Science and Technology Hisar (29o5’5” N 75o45’55” E). Experimental data obtained for forced convection greenhouse drying of papad were used to determine the constants in the Nusselt number expression by using the simple linear regression analysis and, consequently, the values of convective and evaporative heat transfer coefficients were evaluated. The average values of experimental constants C and n were determined as 0.996 and 0.194 respectively. The average values of convective and evaporative heat transfer coefficients were determined as 0.759 W/m2 oC and 23.48 W/m2 oC respectively. The experimental error in terms of percentage uncertainty was also evaluated.

MAHESH KUMAR

2013-01-01

131

Burn-out phenomena of a liquid nitrogen forced convective flow in a tube heated in cosine-distribution  

International Nuclear Information System (INIS)

Liquid nitrogen was used as working fluid in a tube heated in cosine distribution to study burn-out phenomena in the present experiment. Two types of burn-out were observed. One occurred when flow pattern changed from churn flow or slug flow to annular flow. Another one is DNB phenomena when the flow was unstable. (author)

2000-01-01

132

Burn-out phenomena of a liquid nitrogen forced convective flow in a tube heated in cosine-distribution  

Energy Technology Data Exchange (ETDEWEB)

Liquid nitrogen was used as working fluid in a tube heated in cosine distribution to study burn-out phenomena in the present experiment. Two types of burn-out were observed. One occurred when flow pattern changed from churn flow or slug flow to annular flow. Another one is DNB phenomena when the flow was unstable. (author)

Umeki, Isamu; Takenaka, Nobuyuki; Fujii, Terushige [Dept. of Mech. Eng., Kobe Univ., Kobe, Hyogo (Japan)

2000-05-01

133

A 3-D CFD approach to the mechanistic prediction of forced convective critical heat flux at low quality  

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 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)

2005-01-01

134

An experimental investigation of heat transfer in forced convective boiling of R134a, R123 and R134a/R123 in a horizontal tube  

International Nuclear Information System (INIS)

This paper reports an experimental study on flow boiling of pure refrigerants R134a and R123 and their mixtures in a uniformly heated horizontal tube. The flow pattern was observed through tubular sight glasses with an internal diameter of 10 mm located at the inlet and outlet of the test section. Tests were run at a pressure of 0.6MPa in the heat flux ranges of 5-50kW/m2, vapor quality 0-100 percent and mass velocity of 150-600 kg/m2s. Both in the nucleate boiling-dominant region at low quality and in the two-phase convective evaporation region at higher quality where nucleation is supposed to be fully suppressed, the heat transfer coefficient for the mixture was lower than that for an equivalent pure component with the same physical properties as the mixture. The reduction of the heat transfer coefficient in mixture is explained by such mechanisms as mass transfer resistance and non-linear variation in physical properties etc. In this study, the contribution of convective evaporation, which is obtained for pure refrigerants under the suppression of nucleate boiling, is multiplied by the composition factor by Singal et al. (1984). On the basis of Chen's superposition model, a new correlation is presented for heat transfer coefficients of mixture

2004-01-01

135

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)

2004-01-01

136

Mixed Convection Heat Transfer Experiments in Smooth and Rough Verticla Tubes  

Energy Technology Data Exchange (ETDEWEB)

The mixed convection regime is a transitional heat transfer regime between forced convection and natural convection, where both the forced component of flow, and the buoyancy induced component are important. Aiding flow is when buoyancy forces act in the same direction as the forced flow (heated upflow or cooled downflow), while opposing flow is when the buoyancy force is in the opposite direction of the forced flow (cooled upflow or heated downflow). For opposing flow the buoyancy always increases the rate of heat transfer over the forced convection value. For aiding flow, as the heat flux increased, a reduction in heat transfer is encountered until a condition known as laminarization occurs, where the heat transfer is at a minimum value. Further increases in the wall heat flux causes re-transition to turbulence, and increased heat transfer. In this paper, for the first time, experiments were performed to characterize the effect of surface roughness on heat transfer in mixed convection, for the case of aiding flow. A correlation was developed to allow calculation of mixed convection heat transfer coefficients for rough or smooth tubes.

P Symolon; W Neuhaus; R Odell

2004-12-22

137

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; Emmanuel C. Nsofor

2012-01-01

138

Transient conjugated forced convection in turbulent pipe flows  

Energy Technology Data Exchange (ETDEWEB)

This work deals with the problem of transient conjugated forced convection heat transfer in turbulent pipe flows. The external surface of the pipe over a finite heated section is subjected to either uniform heat flux or uniform wall temperature. The governing parameters identified in this work are the Reynolds number Re, the wall-to-fluid conductivity ratio K, the wall-to-fluid diffusivity ratio A, the dimensionless wall thickness {Delta}, and the Prandtl number Pr. A modified low-Re {kappa}-{epsilon} turbulent model is adopted to solve for the fully developed velocity and eddy viscosity distributions. Predicted results show that effects of wall conduction and wall heat capacity have a significant impact on the unsteady heat transfer, especially in the early transient period.

Lee, K.T. [Oriental Inst. of Tech., Taipei (Taiwan, Province of China). Dept. of Mechanical Engineering; Yan, W.M. [Hua Fan Coll. of Humanities and Technology, Taipei (Taiwan, Province of China). Dept. of Mechanical Engineering

1995-02-01

139

Free-jet-induced mixed convection with internal heat sources  

International Nuclear Information System (INIS)

The thermohydraulic conditions in fluids with internal heat sources and forced convection have been experimentally investigated with the aid of holographic interferometry and laser Doppler anemometry. The measurements were carried out in plane fluid layers of segmental cross section onto which a fluid of the same composition flows from above in the symmetry axis in the form of a laminar, plane free jet. The local heat transfer on the curved bottom of the fluid layer, which is cooled on all sides, have been determined from the interference pictures. The convection behaviour of the fluid, including the beam, has been represented by the measured velocity fields and profiles. (orig./GL).

1980-01-01

140

Heat distribution by natural convection  

Energy Technology Data Exchange (ETDEWEB)

Natural convection between spaces in a building can play a major role in energy transfer. Two situations are investigated: convection through a single doorway into a remote room, and a convective loop in a two-story house with a south sunspace where a north stairway serves as the return path. A doorway-sizing equation is given for the single-door case. Detailed data are given from the monitoring of airflow in one two-story house and summary data are given for five others. Observations on the nature of the airflow and design guidelines are presented.

Balcomb, J.D.; Yamaguchi, K.

1983-01-01

 
 
 
 
141

Convective heat transfer in buildings: recent research results  

Energy Technology Data Exchange (ETDEWEB)

Recent experimental and numerical studies of convective heat transfer in buildings are described and important results are presented. The experimental work has been performed on small-scale water filled enclosures: the numerical analysis results have been produced by a computer program based on a finite-difference scheme. The convective processes investigated in this research are (1) natural convective heat transfer between room surfaces and the adjacent air, (2) natural convective heat transfer between adjacent rooms through a doorway or other openings, and (3) forced convection between the building and its external environment (such as, wind-driven ventilation through windows, doors, or other openings). Results obtained at Lawrence Berkeley Laboratory (LBL) for surface convection coefficients are compare with existing ASHRAE correlations and differences of as much as 20% are observed. It is shown that such differences can have a significant impact on the accuracy of building energy analysis computer simulations. Interzone coupling correlations obtained from experimental work reported are in reasonable agreement with recently published experimental results and with earlier published work. Numerical simulations of wind-driven natural ventilation are presented. They exhibit good qualitative agreement with published wind-tunnel data. Finally, future research needs are suggested.

Bauman, F.; Gadgil, A.; Kammerud, R.; Altmayer, E.; Nansteel, M.

1982-04-01

142

Convective heat transfer in buildings: recent research results  

Energy Technology Data Exchange (ETDEWEB)

Recent experimental and numerical studies of convective heat-transfer in buildings are described, and important results are presented. The experimental work has been performed on small-scale water-filled enclosures; the numerical analysis results have been produced by a computer program based on a finite-difference scheme. The convective processes investigated in this research are (1) natural convective heat-transfer between room surfaces and the adjacent air, (2) natural convective heat-transfer between adjacent rooms through a doorway or other openings, and (3) forced convection between the building and its external environment (such as wind-driven ventilation through windows, doors, or other openings). Results obtained at Lawrence Berkeley Laboratory (LBL) for surface convection coefficients are compared with existing ASHRAE correlations, and differences of as much as 50% are observed. It is shown that such differences can have a significant impact on the accuracy of building energy analysis computer simulations. Interzone coupling correlations obtained from experimental work reported in this paper are in reasonable agreement with recently published experimental results and with earlier published work. Numerical simulations of wind-driven natural ventilation are presented. They exhibit good qualitative agreement with published wind-tunnel data. Finally, future research needs are suggested.

Bauman, F.; Altmayer, E.; Gadgil, A.; Kammerud, R.; Nansteel, M.

1983-01-01

143

Convective heat transfer in buildings: recent research results. Rev  

Energy Technology Data Exchange (ETDEWEB)

Recent experimental and numerical studies of convective heat transfer in buildings are described, and important results are presented. The experimental work has been performed on small-scale, water-filled enclosures; the numerical analysis results have been produced by a computer program based on a finite-difference scheme. The convective processes investigated in this research are: (1) natural convective heat transfer between room surfaces and the adjacent air, (2) natural convective heat transfer between adjacent rooms through a doorway or other openings, and (3) forced convection between the building and its external environment (such as wind-driven ventilation through windows, doors, or other openings). Results obtained at Lawrence Berkeley Laboratory (LBL) for surface convection coefficients are compared with existing ASHRAE correlations, and differences can have a significant impact on the accuracy of building energy analysis computer simulations. Interzone coupling correlations obtained from experimental work are in reasonable agreement with recently published experimental results and with earlier published work. Numerical simulations of wind-driven natural ventilation are presented. They exhibit good qualitative agreement with published wind-tunnel data.

Bauman, F.; Gadgil, A.; Kammerud, R.; Altmayer, E.; Nansteel, M.W.

1982-10-01

144

Forced convective drying of willow chips  

Energy Technology Data Exchange (ETDEWEB)

The forced convective drying process of willow chips was described with a deep bed drying model. The model was validated experimentally for bed moisture content, air temperature and relative humidity, and adequately described ambient air drying of a 1 m deep willow chip bed. At the top layers of the chip bed, the model overestimated the drying rate due to vapour condensation which was not incorporated into the model. However, the drying model was an appropriate tool to gain insight into the forced convective drying process of willow chips. The drying costs of willow chips using farm facilities for storage and drying of potatoes were assessed, based on average monthly weather data. March to September was the most suitable period for drying due to favourable ambient weather conditions. In this period, energy costs for drying from a moisture content of 1-0.18kg (water) kg{sup -1} (DM), which corresponds to 50% (wet base) to 15%, ranged from 12 to 25 EURO t{sup -1} (DM), or from 28 to 59 EURO t{sup -1} (DM) when investment costs were partly accommodated. (author)

Gigler, J.K. [Institute of Agricultural and Environmental Engineering, Wageningen (Netherlands); Loon, W.K.P. van; Vissers, M.M. [Wageningen University (Netherlands). Dept. of Agricultural, Environmental and Systems Technology; Bot, G.P.A. [Institute of Agricultural and Environmental Engineering, Wageningen (Netherlands); Wageningen University (Netherlands). Dept. of Agricultural, Environmental and Systems Technology

2000-07-01

145

Unsteady radiative-convective heat transfer on a radiating surface  

International Nuclear Information System (INIS)

[en] Research of radiation-convective heat exchange on radiating surfaces at natural and forced convection is complex mathematical task and here we obtain approximate analytical formulations for this process. We consider two dimensional unsteady heat transfer between solid surface and fluid under the natural laminar convection within optically transparent grey media. Also we assume constant thermo-physical properties except density which is decreasing linearly with temperature. Complex radiative-convective unsteady heat transfer approximately can be considered as a multi-stage process. At the beginning heat transfer coefficient is time dependent but almost independent on longitudinal coordinate. Afterwards heat transfer coefficient becomes dependent on longitudinal coordinate but does not change over time. Analytic formulations obtained for those two stages could be merged along the 'time-space' characteristic basing on the equality of heat flows and temperatures there. Solutions are constructed using asymptotic expansions. Theoretical analysis of the solutions revealed the following: effect of radiation leads to a change in the heat transfer coefficient from the values that are characteristic to the second order boundary conditions to the values that are characteristic for the first order boundary conditions. The rate of this transition depends on ß radiation coefficient. Experimental research confirmed correctness of the simplifications introduced.

2012-11-26

146

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

147

Heat transfers in porous media. Conduction, convection, radiant transfer  

International Nuclear Information System (INIS)

Multiple physico-chemical and transport phenomena take place in porous media. The study of these phenomena requires the knowledge of fluid storage, transfer and mechanical properties of these media. Like all polyphasic heterogenous systems, these properties depend on the morphology of the matrix and of the phenomena interacting in the different phases. This makes the heat transfers in porous media a particularly huge field of researches. This article makes a synthesis of these researches. Content: 1 - classification and characterization of porous media; 2 - modeling of transfer phenomena; 3 - heat transfer by conduction: concept of equivalent thermal conductivity (ETC), modeling of conduction heat transfer, ETC determination; 4 - heat transfer by convection: modeling of convection heat transfer, natural convection (in confined media, along surfaces or impermeable bodies immersed in a saturated porous medium), forced and mixed convection; 5 - radiant heat transfer: energy status equation, approximate solutions of the radiant transfer equation, use of the approximate solutions: case of fibrous insulating materials; 6 - conclusion. (J.S.)

2008-01-01

148

Experimentelle Untersuchung turbulenter Mischkonvektion im Nachlauf einer beheizten Kugel. (Experimental investigation of turbulent mixed convection in the wake of a heated sphere).  

Science.gov (United States)

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 ...

D. Suckow

1993-01-01

149

Forced convection in horizontal porous channels with hydrodynamic anisotropy  

Energy Technology Data Exchange (ETDEWEB)

This paper presents an exact solution for fully developing forced convective flow in parallel-plate horizontal porous channels with an anisotropic permeability whose principal axes are oriented in a direction that is oblique to the gravity vector. A constant heat flux is applied on the channel side walls. Basing this analysis on the generalized Brinkman-extended Darcy model which allows the satisfaction of the no-slip boundary condition on solid wall, it is found that anisotropic parameters K* and {psi} have a strong influence on the flow fields and heat transfer rate, in the limiting case of low porosity media (Da{yields}0). The results indicate that a maximum (minimum) heat transfer rate is reached when the orientation of the principal axis with higher permeability of the anisotropic porous matrix is parallel (perpendicular) to the vertical direction. (Author)

Degan, G.; Zohoun, S. [Universite Nationale du Benin, Cotonou (Benin). LERTI-CPU; Vasseur, P. [University of Montreal (Canada). Ecole Polytechnique

2002-07-01

150

Convective Instability in a Fluid Mixture Heated from Above  

Energy Technology Data Exchange (ETDEWEB)

Convection patterns in ethanol-water mixtures with negative {psi} are studied when the fluid is heated from above. Although the linear analysis predicts that the instability occurs at zero wave number, a large wave number pattern is observed. The onset is supercritical with a threshold that is experimentally indistinguishable from zero. The convection amplitude exhibits damped oscillations for sudden change in the forcing parameter. At the constant Rayleigh number the patterns first coarsen, then exhibit growth of narrow plumes. The instability appears to be related to salt fingering. {copyright} {ital 1998} {ital The American Physical Society}

La Porta, A.; Surko, C.M. [Department of Physics, University of California, San Diego, La Jolla, California 92093 (United States)

1998-04-01

151

Forced and free convection turbulent boundary layers in gas lasers  

International Nuclear Information System (INIS)

[en] Approximate expressions for the effect on optical path length through a turbulent vertical boundary layer caused by the combined presence of forced and free convection were obtained to first order in the asymptotic cases of dominant forced convection and dominant free convection. The effect in both cases is a reduction of the boundary-layer thickness. Characteristic scaling lengths are presented which aid in the optical analysis of the flowfield

1975-01-01

152

Liquid Argon Maximm Convective Heat Flux vs. Liquid Depth  

Energy Technology Data Exchange (ETDEWEB)

In order to help answer questions about the magnitude of heat flux to the liquid argon in a liquid argon calorimeter which could cause boiling (bubbles), calculations estimating the heat flux which can be removed by free convection were made in February, 1988. These calculations are intended to be an estimate of the heat flux above which boiling would occur. No formal writeup was made of these calculations, although the graph dated 3 Feb 88 and revised (adding low-velocity forced convection lines) 19 Feb 88 was presented in several meetings and widely distributed. With this description of the calculations, copies of the original graph and calculations are being added to the D0 Engineering Note files. The liquid argon surface is in equilibrium with argon vapor at a pressure of 1.3 bar, so the surface is at 89.70 K. The liquid is entirely at this surface temperature throughout the bulk of the volume, except locally where it is warmed by a solid surface at a higher temperature than the bulk liquid. This surface temperature is taken to be the boiling temperature of argon at the pressure corresponding to 1.3 bar plus the liquid head; hence it is a function of depth below the surface. The free and forced convection correlations used are 'from Kreith, 'Heat Transfer', for heated flat plates in a large (i.e., no other objects nearby enough to disturb the flow) uniform volume of fluid. Heat flux is a function of plate size, really length along the flow path (since a boundary layer increases in thickness starting from the leading edge of the plate), and orientation (i.e., vertical or horizontal). The maximum heat flux which can be carried away by free convection (i.e., the heat flux above which boiling occurs) is .001 W/sq.cm. at 4 inches below the surface and 0.1 to 0.2 W/sq.cm. 15 feet below the surface. Forced convection over a 1 cm plate with a fluid velocity of 1 cm/sec, or a 10 cm plate at 10 cm/sec, is about like free convection. The line for much higher heat flux is 10 cm/sec flow over a 1 cm plate.

Peterson, T.; /Fermilab

1990-01-12

153

Prandtl Number Dependent Natural Convection with Internal Heat Sources  

Energy Technology Data Exchange (ETDEWEB)

Natural convection plays an important role in determining the thermal load from debris accumulated in the reactor vessel lower head during a severe accident. Recently, attention is being paid to the feasibility of external vessel flooding as a severe accident management strategy and to the phenomena affecting the success path for retaining the molten core material inside the vessel. The heat transfer inside the molten core material can be characterized by the strong buoyancy-induced flows resulting from internal heating due to decay of fission products. The thermo-fluid dynamic characteristics of such flow depend strongly on the thermal boundary conditions. The spatial and temporal variation of heat flux on the pool wall boundaries and the pool superheat are mainly characterized by the natural convection flow inside the molten pool. In general, the natural convection heat transfer phenomena involving the internal heat generation are represented by the modified Rayleigh number (Ra’), which quantifies the internal heat source and hence the strength of the buoyancy force. In this study, tests were conducted in a rectangular section 250 mm high, 500 mm long and 160 mm wide. Twenty-four T-type thermocouples were installed in the test section to measure temperatures. Four T-type thermocouples were used to measure the boundary temperatures. The thermocouples were placed in designated locations after calibration. A direct heating method was adopted in this test to simulate the uniform heat generation. The experiments covered a range of Ra' between 1.5x106 and 7.42x1015 and the Prandtl number (Pr) between 0.7 and 6.5. Tests were conducted with water and air as simulant. The upper and lower boundary conditions were maintained uniform. The results demonstrated feasibility of the direct heating method to simulate uniform volumetric heat generation. Particular attentions were paid to the effect of Pr on natural convection heat transfer within the rectangular pool.

Kang Hee Lee; Seung Dong Lee; Kune Y. Suh; Joy L. Rempe; Fan-Bill Cheung; Sang B. Kim

2004-06-01

154

Prandtl Number Dependent Natural Convection with Internal Heat Sources  

International Nuclear Information System (INIS)

[en] Natural convection plays an important role in determining the thermal load from debris accumulated in the reactor vessel lower head during a severe accident. Recently, attention is being paid to the feasibility of external vessel flooding as a severe accident management strategy and to the phenomena affecting the success path for retaining the molten core material inside the vessel. The heat transfer inside the molten core material can be characterized by the strong buoyancy-induced flows resulting from internal heating due to decay of fission products. The thermo-fluid dynamic characteristics of such flow depend strongly on the thermal boundary conditions. The spatial and temporal variation of heat flux on the pool wall boundaries and the pool superheat are mainly characterized by the natural convection flow inside the molten pool. In general, the natural convection heat transfer phenomena involving the internal heat generation are represented by the modified Rayleigh number (Ra'), which quantifies the internal heat source and hence the strength of the buoyancy force. In this study, tests were conducted in a rectangular section 250 mm high, 500 mm long and 160 mm wide. Twenty-four T-type thermocouples were installed in the test section to measure temperatures. Four T-type thermocouples were used to measure the boundary temperatures. The thermocouples were placed in designated locations after calibration. A direct heating method was adopted in this test to simulate the uniform heat generation. The experiments covered a range of Ra' between 1.5x106 and 7.42x1015 and the Prandtl number (Pr) between 0.7 and 6.5. Tests were conducted with water and air as simulant. The upper and lower boundary conditions were maintained uniform. The results demonstrated feasibility of the direct heating method to simulate uniform volumetric heat generation. Particular attentions were paid to the effect of Pr on natural convection heat transfer within the rectangular pool

2004-01-01

155

A heat exchanger between forced flow helium gas at 14 to 18 K and liquid hydrogen at 20 K circulated by natural convection  

International Nuclear Information System (INIS)

[en] 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

2003-01-01

156

A heat exchanger between forced flow helium gas at 14 to 18 K andliquid hydrogen at 20 K circulated by natural convection  

Energy Technology Data Exchange (ETDEWEB)

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.

Green, M.A.; Ishimoto, S.; Lau, W.; Yang, S.

2003-09-15

157

Experimental study on convective heat transfer of TiO2 nanofluids  

Science.gov (United States)

In this study, nanofluids with different TiO2 nanoparticle concentrations were synthesized and measured in different constant heat fluxes for their heat transfer behavior upon flowing through a vertical pipe. Addition of nanoparticles into the base fluid enhances the forced convective heat transfer coefficient. The results show that the enhancement of the convective heat transfer coefficient in the mixture consisting of ethylene glycol and distilled water is more than distilled water as a base fluid.

Vakili, M.; Mohebbi, A.; Hashemipour, H.

2013-08-01

158

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

International Nuclear Information System (INIS)

[en] 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

1986-01-01

159

Evaluation of heat transfer coefficient in assisting mixed convection on a vertical element with uniform heat flux  

International Nuclear Information System (INIS)

[en] The loss of cooling accident (LOCA) in a nuclear reactor is a case where the heat transfer by forced convection is of the same order of magnitude as the heat transfer by natural convection. Two simple theoretical solutions for evaluating the heat transfer coefficient are presented, assuming a combined velocity and temperature profile of natural and forced convection, within the boundary layer. The following mathematical form is proposed: Y= [1+Xsup(n)]sup(1/n) which is a general expression for a combined effect of wo limiting solutions. (author)

1985-01-01

160

Numerical prediction of convection heat transfer under arbitrary thermal boundary conditions  

Energy Technology Data Exchange (ETDEWEB)

A numerical approach based on adjoint formulations of convection heat transfer is proposed to predict the heat transfer characteristics under arbitrary thermal boundary conditions. The main features of the present method can be summarized as follows: (1) By numerically solving the adjoint problem of forced convection heat transfer, the total heat transfer rate or the temperature at a specific location can be obtained under arbitrary thermal boundary conditions. The computation time for the adjoint problem is equal to that required in a numerical simulation of forced convection heat transfer under a specific thermal boundary condition. (2) For natural or mixed convection heat transfer problem, by introducing the perturbations from the base thermal boundary conditions, the adjoint system can be derived for the perturbations. Thus, by numerically solving the base and the adjoint systems, the change of total heat transfer rate or the change of the temperature at a specific location can be predicted for arbitrary thermal boundary perturbations.

Momose, Kazunari; Kimoto, Hideo

1999-07-01

 
 
 
 
161

Decay heat removal and natural convection in fast breeder reactors  

Energy Technology Data Exchange (ETDEWEB)

This book contains the full texts of the papers presented at a specialists meeting on fast reactor fission product decay heat removal by heat transport systems operated solely by the forces of natural convection. The meeting was sponsored by the American Nuclear Society and held at Brookhaven National Laboratory, February 28 and 29, 1980. The meeting was just 1 month before the first anniversary of the TMI-2 accident (which in a fundamental sense was caused by a temporary failure to remove decay heat), so the contributors felt that their topics were particularly pertinent, even though they were concerned with decay heat removal from another type of reactor. The 27 papers are predominantly about natural convection in liquid metal heat transport systems (since all operating fast power reactors are cooled by liquid sodium). The only exception is a set of three papers about gas cooled fast reactors. In all of the papers, the atmosphere furnishes the ultimate heat sink, so one finally needs to be concerned about natural convection in air. However, this is a relatively well-understood process and nearly all of the attention of the contributors is devoted to the heat transfer processes in the around the reactor core.

Aqrawal, A.K.; Guppy, J.G. (eds.)

1981-01-01

162

Electrohydrodynamic convective heat transfer in a square duct.  

UK PubMed Central (United Kingdom)

Laminar to weakly turbulent forced convection in a square duct heated from the bottom is strengthened by ion injection from an array of high-voltage points opposite the heated strip. Both positive and negative ion injection are activated within the working liquid HFE-7100 (C(4)F(9)OCH(3)), with transiting electrical currents on the order of 0.1 mA. Local temperatures on the heated wall are measured by liquid crystal thermography. The tests are conducted in a Reynolds number range from 510 to 12,100. In any case, heat transfer is dramatically augmented, almost independently from the flow rate. The pressure drop increase caused by the electrohydrodynamically induced flow is also measured. A profitable implementation of the technique in the design of heat sinks and heat exchangers is foreseen; possible benefits are pumping power reduction, size reduction, and heat exchange capability augmentation.

Grassi W; Testi D

2009-04-01

163

Convection heat transfer coefficients at convective drying of porous materials  

Energy Technology Data Exchange (ETDEWEB)

Measurements proved that the convective heat transfer coefficient (h) has a larger value h{sub wet} at the constant drying rate period and after that it falls down to a minimum one: h{sub dry} in the equilibrium dried state. Measurements showed also that the heat of vaporization in the last phase of the falling drying rate period is far greater than it was in the constant drying rate period. The first measurements were made on a gypsum plate. Afterwards the authors carried out measurement research with fine glass powder and cement-perlite plate and determined h{sub wet} and h{sub dry} heat transfer coefficients as a function of Reynolds number. All of these measurements confirmed the conclusion that h{sub wet} is far greater than h{sub dry}.

Szentgyoergyi, S.; Toemoesy, L.; Molnar, O.

2000-07-01

164

Free convection film flows and heat transfer  

CERN Document Server

Presents development of systematic studies for hydrodynamics and heat and mass transfer in laminar free convection, accelerating film boiling and condensation of Newtonian fluids, and accelerating film flow of non-Newtonian power-law fluids. This book provides a system of analysis models with a developed velocity component method.

Shang, Deyi

2010-01-01

165

Turbulent generation and mechanism analysis of forced-convective heat transfer enhancement by applying electric fields in the restricted region near the wall  

Energy Technology Data Exchange (ETDEWEB)

The heat transfer enhancement method of applying electric fields only near a heat transfer wall was numerically investigated. Generation of additional turbulence in the near-wall region occurs by the interaction between migrating electric charges and the turbulent flow of weakly electrically conductive fluids such as refrigerants, oils, and chlorofluorocarbon (CFC) alternatives. Based on electrostatic probe experiments, the authors assumed that the current was mainly transferred by the negative charges. They solved the Navier-Stokes equation with a Coulomb force term, the conservation equation of electric current, the Poisson equation of electric potential, and the energy equation. They used the Large Eddy Simulation (LES) method to represent the turbulence. The numerical analysis showed a heat transfer enhancement of 2.8 times for turbulent flow (Re = 1.8 x 10{sup 4}) when applying 5 kV to the near-wall region, 5 mm from the wall. The simulations for different distances between the coupled electrodes showed that an optimum location of the electrodes exists for achieving the lowest electric power input for a given electric field strength. They also evaluated the heat efficiency in a simple heat exchanger system using this heat transfer enhancement method. For the 5 kV/5 mm condition, where 19% of the total input power was consumed by the electric field, they achieved a heat transfer enhancement of 27 times compared to the case when an equivalent, additional amount of input power would be consumed by the pump to increase the flow rate of the heat-transfer fluid.

Hasegawa, Masato; Yabe, Akira; Nariai, Hideki

1999-07-01

166

Effect of dissolved noncondensables on liquid forced convection in microchannels  

Energy Technology Data Exchange (ETDEWEB)

A method of quantifying the effect of noncondensable desorption on the forced flow of liquids in microchannels subject to a uniform heat flux has been developed. The model is based on the solution of the differential forms of the mass, momentum, energy and noncondensable species conservation equations assuming that the liquid is fully saturated with the noncondensable at the channel inlet. Parametric calculations for conditions encountered in typical microchannel experiments were performed and the results presented. The resulting calculations show that significant noncondensable desorption can take place in microchannel flow resulting in increased liquid velocities and enhanced heat transfer. Experiments were also performed with a 0.76 mm diameter microchannel using both fully degassed water and water saturated with air at the channel inlet. The measured heat transfer coefficients for the air-saturated data were significantly higher than for the fully degassed data in regions where the model predicts significant noncondensable desorption. The forced turbulent convective flow of water in microchannels offers a wide variety of applications including micro-electronic cooling, miniature refrigeration, micro heat exchanger systems and the cooling of fission reactor cores.

Adams, T.M.; Ghiaasiaan, S.M.; Abdel-Khalik, S.I.

1999-07-01

167

Forced-convection boiling tests performed in parallel simulated LMR fuel assemblies  

International Nuclear Information System (INIS)

Forced-convection tests have been carried out at the Oak Ridge National Laboratory using parallel simulated Liquid Metal Reactor fuel assemblies in an engineering-scale sodium loop, the Thermal-Hydraulic Out-of-Reactor Safety facility. The tests, performed under single- and two-phase conditions, have shown that for low forced-convection flow there is significant flow augmentation by thermal convection, an important phenomenon under degraded shutdown heat removal conditions in an LMR. The power and flows required for boiling and dryout to occur are much higher than decay heat levels. The experimental evidence supports analytical results that heat removal from an LMR is possible with a degraded shutdown heat removal system.

1985-01-01

168

A heat transfer correlation for turbulent mixed convection in a vertical tube with flow development effects  

Energy Technology Data Exchange (ETDEWEB)

Numerous experimental and analytical investigations have been undertaken on the subject of heat transfer in the combined free and forced, or mixed convection regime. This paper focuses on the topic heat transfer for aiding turbulent flow. Aiding flow is when buoyancy forces act in the same direction as the forced flow (heated upflow or cooled downflow), while opposing flow is when the buoyancy force is in the opposite direction of the forced flow (cooled upflow or heated downflow). For opposing flow the buoyancy always increases the rate of heat transfer over the forced convection value, while for aiding flow, first a reduction in heat transfer is encountered, then an enhancement. Correlations for turbulent mixed convection in a vertical pipe, and criteria for the onset of mixed convection effects are reviewed. Numerous correlations were found which predict specific data sets, but a correlation which accounts for flow development effects in the regime of degraded turbulent heat transfer is lacking. A new correlation for turbulent mixed convection for aiding flow in a vertical pipe is proposed.

Symolon, P.D.

1997-07-01

169

Natural convection heat transfer in a uniformly heated horizontal pipe  

Science.gov (United States)

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

2013-09-01

170

General analysis of steady laminar mixed convection heat transfer on vertical slender cylinders  

Energy Technology Data Exchange (ETDEWEB)

A general analysis has been developed to study fluid flow and heat transfer characteristics for steady laminar mixed convection on vertical slender cylinders covering the entire range from pure forced to pure natural convection. Two uniquely transformed sets of axisymmetric boundary-layer equations for the constant wall heat flux case and the isothermal surface case are solved using a two-point finite difference method with Newton linearization. Of interest are the effects of the new mixed convection parameter, the cylinder heating/cooling mode, the transverse curvature parameter, and the Prandtl number on the velocity/temeprature profiles and on the local skin friction parameter and the heat transfer parameter. The results of the validated computer simulation model are as follows. Depending upon the magnitude and direction of the buoyancy force, i.e., the value of the mixed convection parameter and the heating or cooling mode applied, natural convection can have a significant effect on the thermal flow field around vertical cylinders. Specifically, strong variations of the local skin friction parameter and reversing trends in the heat transfer parameter are produced as the buoyancy force becomes stronger in aiding flow. The skin friction parameter increases with higher curvature parameters and Prandtl numbers. Similarly, the modified Nusselt number is larger for higher transverse curvature parameters; however, this parameter may reverse the impact of the Prandtl number on the Nusselt number for predominantly forced convection.

Wang, T.Y.; Kleinstreuer, C. (North Carolina State Univ., Raleigh (United States))

1989-05-01

171

Convective cooling of three discrete heat sources in channel flow  

Scientific Electronic Library Online (English)

Full Text Available 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 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 (more) 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.

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

2008-09-01

172

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; Carlos A. C. Altemani

2008-01-01

173

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

International Nuclear Information System (INIS)

[en] 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

1977-01-01

174

Heat transport in bubbling turbulent convection.  

UK PubMed Central (United Kingdom)

Boiling is an extremely effective way to promote heat transfer from a hot surface to a liquid due to numerous mechanisms, many of which are not understood in quantitative detail. An important component of the overall process is that the buoyancy of the bubble compounds with that of the liquid to give rise to a much-enhanced natural convection. In this article, we focus specifically on this enhancement and present a numerical study of the resulting two-phase Rayleigh-Bénard convection process in a cylindrical cell with a diameter equal to its height. We make no attempt to model other aspects of the boiling process such as bubble nucleation and detachment. The cell base and top are held at temperatures above and below the boiling point of the liquid, respectively. By keeping this difference constant, we study the effect of the liquid superheat in a Rayleigh number range that, in the absence of boiling, would be between 2 × 10(6) and 5 × 10(9). We find a considerable enhancement of the heat transfer and study its dependence on the number of bubbles, the degree of superheat of the hot cell bottom, and the Rayleigh number. The increased buoyancy provided by the bubbles leads to more energetic hot plumes detaching from the cell bottom, and the strength of the circulation in the cell is significantly increased. Our results are in general agreement with recent experiments on boiling Rayleigh-Bénard convection.

Lakkaraju R; Stevens RJ; Oresta P; Verzicco R; Lohse D; Prosperetti A

2013-06-01

175

Laminar boundary layers in convective heat transport  

CERN Multimedia

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

176

Two-phase forced-convective fouling under steam generator operating conditions  

International Nuclear Information System (INIS)

Two-phase forced-convective fouling can occur in adiabatic two-phase flow and in diabatic two-phase flow, where it can be a significant contributor to fouling under flow-boiling conditions. For recirculating steam generators (SGs), it is, therefore, of significance to steam separators, tube support plates, tubesheet and the tube bundle. Loop test data are presented on forced-convective fouling rate of iron corrosion products under a range of conditions relevant to the secondary-side of recirculating SGs. The measurements were performed using a number of corrosion products (magnetite, hematite and lepidocrocite) under a range of water chemistry conditions, with several different amines. The measurements were limited to the straight-tube geometry. Comparable fouling data are given for flow-boiling conditions. A SG artefact was examined to corroborate the loop data. The rate constants for the forced-convective fouling measurements are compared with those for flow-boiling fouling. Their relative magnitudes can vary greatly, depending on the chemistry and thermohydraulic conditions. Boiling fouling dominated over forced-convection fouling for hematite and lepidocrocite particles, likely because of particle-bubble interactions. Forced-convective fouling rate was only slightly lower than boiling fouling for magnetite. For the region of cross-flow (upper tube bundle), deposits show significant thickness variation. Four or five deposit thickness peaks are noted, approximately equally spaced circumferentially. It is hypothesized that the fouling pattern is developed due to the cross-flow pattern present in the tube bundle. The possible interactions between the force-convective and nucleate-boiling fouling streams are briefly discussed. A method is presented for the superposition of the forced-convective and nucleate boiling fouling components. This method is based on the Chen heat transfer correlation. (author)

2002-01-01

177

Two-phase forced-convective fouling under steam generator operating conditions  

Energy Technology Data Exchange (ETDEWEB)

Two-phase forced-convective fouling can occur in adiabatic two-phase flow and in diabatic two-phase flow, where it can be a significant contributor to fouling under flow-boiling conditions. For recirculating steam generators (SGs), it is, therefore, of significance to steam separators, tube support plates, tubesheet and the tube bundle. Loop test data are presented on forced-convective fouling rate of iron corrosion products under a range of conditions relevant to the secondary-side of recirculating SGs. The measurements were performed using a number of corrosion products (magnetite, hematite and lepidocrocite) under a range of water chemistry conditions, with several different amines. The measurements were limited to the straight-tube geometry. Comparable fouling data are given for flow-boiling conditions. A SG artefact was examined to corroborate the loop data. The rate constants for the forced-convective fouling measurements are compared with those for flow-boiling fouling. Their relative magnitudes can vary greatly, depending on the chemistry and thermohydraulic conditions. Boiling fouling dominated over forced-convection fouling for hematite and lepidocrocite particles, likely because of particle-bubble interactions. Forced-convective fouling rate was only slightly lower than boiling fouling for magnetite. For the region of cross-flow (upper tube bundle), deposits show significant thickness variation. Four or five deposit thickness peaks are noted, approximately equally spaced circumferentially. It is hypothesized that the fouling pattern is developed due to the cross-flow pattern present in the tube bundle. The possible interactions between the force-convective and nucleate-boiling fouling streams are briefly discussed. A method is presented for the superposition of the forced-convective and nucleate boiling fouling components. This method is based on the Chen heat transfer correlation. (author)

Klimas, S.J.; Pietralik, J.M. [Atomic Energy of Canada Limited, Chalk River, Ontario (Canada)

2002-07-01

178

Mixed convection in a horizontal porous duct with a sudden expansion and local heating from below  

International Nuclear Information System (INIS)

Results are reported for an experimental and numerical study of forced and mixed convective heat transfer in a liquid-saturated, horizontal porous duct. The cross section of the duct has a sudden expansion with a heated region on the lower surface downstream and adjacent to the expansion. Calculated and measured Nusselt numbers for 0.1 1.5 and Ra/Pe1.5. Calculated Nusselt numbers are very close to those for the bottom-heated flat duct, and this result has several important implications for convective heat and mass transfer in geophysical systems and porous matrix heat exchangers.

1996-01-01

179

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)

2008-01-01

180

Heat transfer for forced convection condensation of R123, R134a and their mixtures flowing between two horizontal parallel plates. Numerical study; Transfert thermique pour la condensation du R123, du R134a et de leurs melanges, en ecoulement force entre deux plaques planes horizontales. Etude numerique  

Energy Technology Data Exchange (ETDEWEB)

Film condensation of pure and binary mixtures flowing between parallel plates is treated numerically. The coupled equations of mass, momentum, species and energy conservations of the two phases are solved with an implicit scheme. In this study, we retained the pressure forces, the liquid and vapor interfacial shear stress, the Dufour effect, the inertia and enthalpy convention terms, the turbulence in the two phases and the variation of the physical properties with the temperature and concentration. The results obtained for the condensation of refrigerants R123 and R134a, show strong influence of the composition of mixture on the mean heat transfer coefficient and the total pressure loss. The calculated mean Nusselt number is in good agreement with the experimental correlations of Mochizuki and Inoue and Akers and Rosson. A new correlation for the mean heat transfer for forced convection condensation of pure refrigerants R123 nd R134a and their mixtures between horizontal flat plates is proposed. (authors) 24 refs.

Louahlia, H.; Panday, P.K. [Institut de genie energenique, 90 - Belfort (France)

1996-12-31

 
 
 
 
181

Heat transport by turbulent rotating convection and magnetoconvection in liquid gallium  

Science.gov (United States)

The fluid dynamics of convectively driven planetary dynamos (such as the geodynamo) are strongly influenced by both Coriolis forces due to the planets’ rotation, as well as Lorentz forces caused by the interaction between fluid flow and magnetic fields. We investigate regimes of convective behavior in turbulent rotating Rayleigh-Benard convection in liquid gallium, with and without externally imposed magnetic fields. In particular, we measure heat transfer behavior as characterized by the Nusselt number, Nu, over a broad range of parameters: Rayleigh numbers up to Ra < 108; Ekman numbers as low as 10-6 < E; and Elsasser numbers up to ? < 10.

King, E. M.; Aurnou, J. M.

2010-12-01

182

Regime classification and planform scaling for internally heated mantle convection  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Abstract Highlights ? Internally heated 3-D mantle convection models in a spherical shell with temperature and pressure dependent viscosity have been performed to provide new insights into the various convection regimes. ? We were able to predict the pattern of convection (dominant d...

183

Digital holographic interferometry of convective heat transport  

Energy Technology Data Exchange (ETDEWEB)

Real-time holographic interferometry has been used to study the convective heat transport in a flat plate solar collector. The analysis and interpretation of the interferograms using digital techniques are discussed. This study is to develop an automated digital process for interference pattern analysis, and thus special attention is given to those optical and physical aspects that are relevant to this goal. This includes the structural systematics of the fringes, noise due to spurious optical effects, and the accuracy of digitized optical signals. Some experimental results on automated analysis of interferograms will be discussed.

Choudry, A.

1981-04-01

184

Digital holographic interferometry of convective heat transport.  

UK PubMed Central (United Kingdom)

Real-time holographic interferometry has been used to study the convective heat transport in a flat plate solar collector. The analysis and interpretation of the interferograms using digital techniques are discussed. This study aims to develop an automated digital process for interference pattern analysis, and thus special attention is given to those optical and physical aspects that are relevant to this goal. This includes the structural systematics of the fringes, noise due to spurious optical effects, and the accuracy of digitized optical signals. Some experimental results on automated analysis of interferograms will be discussed.

Choudry A

1981-04-01

185

Simulation of convectively forced gravity waves in comparison with SABER satellite measurements.  

Science.gov (United States)

Gravity waves (GW) are a known coupling mechanism between lower, middle, and upper atmosphere. They are responsible for driving large scale circulations like Brewer-Dobson circulation and contribute almost 60% to the QBO of the inner tropics. Convection is the dominant source for tropical GWs, but deep convection is also one of the most difficult to understand sources of GWs. Especially, the development of atmospheric general circulation models (AGCM) suffers from improvements in the parameterization of convectively forced GWs (cGWs). In this study we present the results of GW ray-tracing calculations of cGWs. For this, we used the Gravity Wave Regional Or Global RAy-tracer (GROGRAT) and the convective source scheme from Yonsei University (South Korea). Furthermore, we used MERRA heating rates, cloud data, and background data for both the calculation of the convective forcing by deep convection and for the atmospheric background of the ray-tracing calculations afterwards. Also, we compare our results with satellite measurements of squared temperature amplitudes as well as momentum flux by the SABER instrument in order to validate our findings over a 10 years period. For the comparison the observational filter of the instrument is taken into account, the influence 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.

Kalisch, Silvio; Trinh, Thai; Chun, Hye-Yeong; Ern, Manfred; Preusse, Peter; Kim, Young-Ha; Eckermann, Steven; Riese, Martin

2013-04-01

186

Heat transfer measurements in a forced convection loop with two molten-fluoride salts: LiF--BeF2--ThF2--UF4 and eutectic NaBF4--NaF  

International Nuclear Information System (INIS)

[en] 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

1976-01-01

187

Convective heat transfer and infrared thermography.  

UK PubMed Central (United Kingdom)

Infrared (IR) thermography, because of its two-dimensional and non-intrusive nature, can be exploited in industrial applications as well as in research. This paper deals with measurement of convective heat transfer coefficients (h) in three complex fluid flow configurations that concern the main aspects of both internal and external cooling of turbine engine components: (1) flow in ribbed, or smooth, channels connected by a 180 degrees sharp turn, (2) a jet in cross-flow, and (3) a jet impinging on a wall. The aim of this study was to acquire detailed measurements of h distribution in complex flow configurations related to both internal and external cooling of turbine components. The heated thin foil technique, which involves the detection of surface temperature by means of an IR scanning radiometer, was exploited to measure h. Particle image velocimetry was also used in one of the configurations to precisely determine the velocity field.

Carlomagno GM; Astarita T; Cardone G

2002-10-01

188

Convective heat transfer and infrared thermography.  

Science.gov (United States)

Infrared (IR) thermography, because of its two-dimensional and non-intrusive nature, can be exploited in industrial applications as well as in research. This paper deals with measurement of convective heat transfer coefficients (h) in three complex fluid flow configurations that concern the main aspects of both internal and external cooling of turbine engine components: (1) flow in ribbed, or smooth, channels connected by a 180 degrees sharp turn, (2) a jet in cross-flow, and (3) a jet impinging on a wall. The aim of this study was to acquire detailed measurements of h distribution in complex flow configurations related to both internal and external cooling of turbine components. The heated thin foil technique, which involves the detection of surface temperature by means of an IR scanning radiometer, was exploited to measure h. Particle image velocimetry was also used in one of the configurations to precisely determine the velocity field. PMID:12496015

Carlomagno, Giovanni M; Astarita, Tommaso; Cardone, Gennaro

2002-10-01

189

Endwall convective heat transfer for bluff bodies  

Digital Repository Infrastructure Vision for European Research (DRIVER)

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, ...

Wang, Lei; Salewski, Mirko; Sundén, Bengt; Borg, Andreas; Abrahamsson, Hans

190

Heat transfer by a mixture of a liquid and a gas in forced turbulent convection with slight vaporisation of the liquid phase (1961)  

International Nuclear Information System (INIS)

The vaporisation of an appreciable quantity of a liquid in a turbulent gas stream explains the increase in the heat capacity of the fluid and the improvement in the heat-transfer coefficient. The present study makes it clear that even with a very slight vaporisation, the transfer coefficient can be much increased, the pressure drop remaining nearly constant. (authors)

1961-01-01

191

Forced-to-natural convection transition tests in parallel simulated liquid metal reactor fuel assemblies  

International Nuclear Information System (INIS)

The Thermal-Hydraulic Out of Reactor Safety (THORS) Program at Oak Ridge National Laboratory (ORNL) had as its objective the testing of simulated, electrically heated liquid metal reactor (LMR) fuel assemblies in an engineering-scale, sodium loop. Between 1971 and 1985, the THORS Program operated 11 simulated fuel bundles in conditions covering a wide range of normal and off-normal conditions. The last test series in the Program, THORS-SHRS Assembly 1, employed two parallel, 19-pin, full-length, simulated fuel assemblies of a design consistent with the large LMR (Large Scale Prototype Breeder -- LSPB) under development at that time. These bundles were installed in the THORS Facility, allowing single- and parallel-bundle testing in thermal-hydraulic conditions up to and including sodium boiling and dryout. As the name SHRS (Shutdown Heat Removal System) implies, a major objective of the program was testing under conditions expected during low-power reactor operation, including low-flow forced convection, natural convection, and forced-to-natural convection transition at various powers. The THORS-SHRS Assembly 1 experimental program was divided up into four phases. Phase 1 included preliminary and shakedown tests, including the collection of baseline steady-state thermal-hydraulic data. Phase 2 comprised natural convection testing. Forced convection testing was conducted in Phase 3. The final phase of testing included forced-to-natural convection transition tests. Phases 1, 2, and 3 have been discussed in previous papers. The fourth phase is described in this paper. 3 refs., 2 figs

1990-01-01

192

Forced-to-natural convection transition tests in parallel simulated liquid metal reactor fuel assemblies  

Energy Technology Data Exchange (ETDEWEB)

The Thermal-Hydraulic Out of Reactor Safety (THORS) Program at Oak Ridge National Laboratory (ORNL) had as its objective the testing of simulated, electrically heated liquid metal reactor (LMR) fuel assemblies in an engineering-scale, sodium loop. Between 1971 and 1985, the THORS Program operated 11 simulated fuel bundles in conditions covering a wide range of normal and off-normal conditions. The last test series in the Program, THORS-SHRS Assembly 1, employed two parallel, 19-pin, full-length, simulated fuel assemblies of a design consistent with the large LMR (Large Scale Prototype Breeder -- LSPB) under development at that time. These bundles were installed in the THORS Facility, allowing single- and parallel-bundle testing in thermal-hydraulic conditions up to and including sodium boiling and dryout. As the name SHRS (Shutdown Heat Removal System) implies, a major objective of the program was testing under conditions expected during low-power reactor operation, including low-flow forced convection, natural convection, and forced-to-natural convection transition at various powers. The THORS-SHRS Assembly 1 experimental program was divided up into four phases. Phase 1 included preliminary and shakedown tests, including the collection of baseline steady-state thermal-hydraulic data. Phase 2 comprised natural convection testing. Forced convection testing was conducted in Phase 3. The final phase of testing included forced-to-natural convection transition tests. Phases 1, 2, and 3 have been discussed in previous papers. The fourth phase is described in this paper. 3 refs., 2 figs.

Levin, A.E. (Georgia Inst. of Tech., Atlanta, GA (USA)); Montgomery, B.H. (Oak Ridge National Lab., TN (USA))

1990-01-01

193

Mixed convection transport from an isolated heat source module on a horizontal plate  

Energy Technology Data Exchange (ETDEWEB)

An experimental study of the mixed convective heat transfer from an isolated source of finite thickness, located on a horizontal surface in an externally induced forced flow, has been carried out. This problem is of particular interest in the cooling of electronic components and also in the thermal transport associated with various manufacturing systems, such as ovens and furnaces. The temperature distribution in the flow as well as the surface temperature variation are studied in detail. The dependence of the heat transfer rate on the mixed convection parameter and on the thickness of the heated element or source, particularly in the vicinity of the source, is investigated. The results obtained indicate that the heat transfer rate and fluid flow characteristics vary strongly with the mixed convection variables. The transition from a natural convection dominated flow to a forced convection dominated flow is studied experimentally and the basic characteristics of the two regimes determined. This transition has a strong influence on the temperature of the surface and on the heat transfer rate. As expected, the forced convection dominated flow is seen to be significantly more effective in the cooling of a heat dissipating component than a natural convection dominated flow. The location of the maximum temperature on the module surface, which corresponds to the minimum local heat transfer coefficient, is determined and discussed in terms of the underlying physical mechanisms. The results obtained are also compared with these for an element of negligible thickness and the effect of a significant module thickness on the transport is determined. Several other important aspects of fundamental and applied interest are studied in this investigation.

Kang, B.H.; Jaluria, Y.; Tewari, S.S. (State Univ. of New Jersey, New Brunswick (USA))

1990-08-01

194

Experimental investigation on heat transfer of forced convection condensation of ethanol-water vapor mixtures on a vertical mini-tube  

Science.gov (United States)

In this paper, condensation heat transfer characteristics of ethanol-water vapor mixtures on a vertical mini-vertical tube with 1.221 mm outside diameter were investigated experimentally. The experiments were performed at different velocities and pressures over a wide range of ethanol mass fractions in vapor. The test results indicated that, with respect to the change of the vapor-to-surface temperature difference, the condensation curves of the heat transfer coefficients revealed nonlinear characteristics, and had peak values. At 2 % ethanol mass fraction in vapor, the condensation heat transfer coefficient value of the ethanol-water vapor mixture was found to have a maximum heat transfer coefficient of 50 kW m-2 K-1, which was 3-4 times than that of pure steam. The condensation heat transfer coefficients decreased with increased ethanol mass fraction in vapor. The vapor pressure and vapor velocity had a positive effect on the condensation heat transfer coefficients of ethanol-water vapor mixtures.

Chen, Xiping; Chong, Daotong; Wang, Jinshi; Huang, Ronghai; Yan, Junjie

2013-09-01

195

Boiling of subcooled water in forced convection  

International Nuclear Information System (INIS)

As a part of a research about water cooled high magnetic field coils, an experimental study of heat transfer and pressure drop is made with the following conditions: local boiling in tubes of small diameters (2 and 4 mm), high heat fluxes (about 1000 W/cm2), high coolant velocities (up to 25 meters/s), low outlet absolute pressures (below a few atmospheres). Wall temperatures are determined with a good accuracy, because very thin tubes are used and heat losses are prevented. Two regimes of boiling are observed: the establishment regime and the established boiling regime and the inception of each regime is correlated. Important delays on boiling inception are also observed. The pressure drop is measured; provided the axial temperature distribution of the fluid and the axial distributions of the wall temperatures, in other words the axial distribution of the heat transfer coefficients under boiling and non boiling conditions, at the same heat flux or the same wall temperatures, are taken in account, then total pressure drop can be correlated, but probably under certain limits of void fraction only. Using the same parameters, it seems possible to correlate the experimental values on critical heat flux obtained previously, which show very important effect of length and hydraulic diameter of the test sections. (authors)

1970-01-01

196

Evaluación del horno de curado de tabaco por convección forzada USCO-MADR1 Evaluating a tobacco-curing oven using a forced-convection heat exchanger USCO — MADR  

Directory of Open Access Journals (Sweden)

Full Text Available Teniendo como base la infraestructura existente de un horno tradicional de curado de tabaco, se rediseño e implementó en él un sistema de intercambio de calor por convección forzada que funciona con cisco de café como combustible. Este horno de curado de tabaco por convección forzada USCO-MADR fue evaluado 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.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.

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

2010-01-01

197

Transition from turbulent natural to turbulent forced convection adjacent to an isothermal vertical plate  

Energy Technology Data Exchange (ETDEWEB)

Turbulent mixed convection adjacent to an isothermal vertical flat plate was numerically examined by exploring the effect of small velocity on turbulent natural convection flow. A low Reynolds number {kappa}-{epsilon} turbulence model, equivalent to the one used by Jones and Launder (1973), was employed in the simulation. The effect of increasing the free stream velocity on the velocity and temperature distributions, wall heat transfer, turbulent kinetic energy and its dissipation rate were examined. Results are presented for fluids with Prandtl number Pr = 0.7 at free steam velocities of u{sub {infinity}} = 0, 0.05, 0.25, 0.75 and 2.00 m/s for a Grashof number Gr{sub x} = 1.0 {times} 10{sup 11}. The introduction of small free stream velocity on turbulent natural convection flow suppresses the turbulence and decreases the heat transfer from the wall. As the free stream velocity continues to increase, the flow characteristics change to turbulent forced convection flow. Correlations to predict the heat transfer for the different convective flow regimes are presented.

Patel, K.; Armaly, B.F.; Chen, T.S. [Univ. of Missouri, Rolla, MO (United States)

1996-12-31

198

Pattern formation without heating in an evaporative convection experiment  

CERN Multimedia

We present an evaporation experiment in a single fluid layer. When latent heat associated to the evaporation is large enough, the heat flow through the free surface of the layer generates temperature gradients that can destabilize the conductive motionless state giving rise to convective cellular structures without any external heating. The sequence of convective patterns obtained here without heating, is similar to that obtained in B\\'enard-Marangoni convection. This work present the sequence of spatial bifurcations as a function of the layer depth. The transition between square to hexagonal pattern, known from non-evaporative experiments, is obtained here with a similar change in wavelength.

Mancini, H; Mancini, Hector; Maza, Diego

2003-01-01

199

Mechanical design and fabrication of a heat exchanger. Report of the design and construction of a heat exchanger which will be used in 'Experimental analysis of heat transfer in the boiling in forced convection  

International Nuclear Information System (INIS)

[en] 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)

1991-01-01

200

A study of selected forced-convection supercritical-water heat-transfer correlations for vertical bare tubes based on a wide-range dataset  

Energy Technology Data Exchange (ETDEWEB)

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 - 350{sup o}C, mass fluxes 250 - 3400 kg/m{sup 2}s, heat fluxes 75 - 5,400 kW/m{sup 2}, 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 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)

Farah, A.; King, K.; Gupta, S.; Mokry, S.; Peiman, W.; Pioro, I., E-mail: Amjad.Farah@yahoo.com, E-mail: Krysten.King@gmail.com, E-mail: Sahil.uoit@gmail.com, E-mail: Sarah_Mokry@hotmail.com, E-mail: Wargha.Peiman@gmail.com, E-mail: Igor.Pioro@uoit.ca [Univ. of Ontario Inst. of Tech., Faculty of Energy Systems and Nuclear Science, Oshawa, Ontario (Canada)

2010-07-01

 
 
 
 
201

Convective Heat Transfer in Acoustic Streaming Flows  

Science.gov (United States)

Convective heat transfer due to acoustic streaming has been studied in the absence of an imposed mean flow. The work is motivated by the need to design and control the thermal features of a suitable experimental rig for the containerless processing of materials by heat treatment of acoustically levitated alloy samples at near zero-gravity. First the problem of heat transfer from an isolated sphere (in a standing sound field) is explored in detail. The streaming Reynolds number, Rs, which characterizes the resulting steady flows, is determined from the acoustic signal. A scale analysis is used to ascertain the importance of buoyancy and viscous dissipation. The steady velocity and temperature fields are determined using asymptotic techniques and numerical methods for the limiting cases of RsPHOENICS is adopted for the solution of the complete elliptic form of the governing equations. A study of the effects of a range of acoustic and geometric parameters on the flow and heat transfer is performed and Nusselt number correlations are obtained for air. PHOENICS is also used to study the effects of variable fluid properties and axial side-wall conduction (coupled with radiation). The role of normal/reduced gravity is assessed and suggestions made for terrestrial testing of the levitation apparatus. Finally, with the sample located at a node in the levitation chamber, the effect of the interaction of the streaming flows (on the sphere and the tube walls) is estimated. Representative calculations for the sample heating/cooling rates are presented and compared with existing data in the literature.

Gopinath, Ashok

1992-01-01

202

Effect of free stream concentration on heat and binary mass transfer with thermodynamic coupling in forced convection on a flat plate  

Energy Technology Data Exchange (ETDEWEB)

Simultaneous heat and mass transfer, which arises from injecting a gas into a flowing external stream, has been studied analytically for a flat plate geometry. The studies are centered around the effect of free stream concentration of the injected gas on the thermodynamic coupling, over a wide range of T/sub w//T/sub e/. Exact and linearized approximate solutions were obtained numerically. It was found that the thermal diffusion effect on mass transfer becomes increasingly important as the free stream concentration increases and T/sub w//T/sub e/ departs from unity. On the other hand the diffusion thermo effect on heat transfer was found to be the most important when the free stream concentration is zero and as T/sub w//T/sub e/ approaches unity.

Atimtay, A. (Clarkson College of Technology, Postdam, NY); Gill, W.N.

1981-01-01

203

Analysis of natural convection in volumetrically-heated melt pools  

International Nuclear Information System (INIS)

Results of series of studies on natural convection heat transfer in decay-heated core melt pools which form in a reactor lower plenum during the progression of a core meltdown accident are described. The emphasis is on modelling and prediction of turbulent heat transfer characteristics of natural convection in a liquid pool with an internal energy source. Methods of computational fluid dynamics, including direct numerical simulation, were applied for investigation

1996-01-01

204

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

205

SCALE ANALYSIS OF CONVECTIVE MELTING WITH INTERNAL HEAT GENERATION  

Energy Technology Data Exchange (ETDEWEB)

Using a scale analysis approach, we model phase change (melting) for pure materials which generate internal heat for small Stefan numbers (approximately one). The analysis considers conduction in the solid phase and natural convection, driven by internal heat generation, in the liquid regime. The model is applied for a constant surface temperature boundary condition where the melting temperature is greater than the surface temperature in a cylindrical geometry. We show the time scales in which conduction and convection heat transfer dominate.

John Crepeau

2011-03-01

206

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)

1988-01-01

207

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

Scientific Electronic Library Online (English)

Full Text Available 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 distance 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 existenc (more) e of an optimal geometrical point for the operation of staggered circular tube heat exchangers, as claimed in a numerical study published in the literature.

Marchi, Carlos Henrique; Hobmeir, Maykel Alexandre

2007-03-01

208

Numerical calculation of the drag force induced by natural convection of spheres at low Grashof numbers  

Energy Technology Data Exchange (ETDEWEB)

When a heated solid sphere is introduced into an ambient fluid, a natural convective flow occurs which results in a drag force on the sphere. This study involves the numerical calculation of both the steady-state and the transient natural convective drag force around spheres at low Grashof numbers. Numerical techniques are taken from Geoola and Cornish. An empirical expression is suggested for the total drag coefficient for Grashof numbers ranging from 4 x 10/sup -4/ to 0.5 and Prandtl number = 0.72: log C/sub DT/ = 1.25 + 0.31 log Gr - 0.097(log Gr)/sup 2/. The dimensionless time required to reach 90% of the steady-state drag force can be approximated by the second-order polynomial: log t/sub 90%/ = 1.32 - log Gr - 0.11(Gr)/sup 2/.

Dudek, D.; Fletcher, T.H.

1987-02-01

209

Effect of radiation on forced convection flow of a micropolar fluid over a horizontal plate  

International Nuclear Information System (INIS)

We determine the effect of radiation on forced convection flow of a micropolar fluid over a horizontal plate. The governing nonsimilar boundary-layer equations are solved using the cubic spline collocation method. The solutions are expressed in terms of the local shear stress and local rate of heat transfer. The effect of varying the Prandtl number, the radiation parameter, and the micropolar material parameter are determined. (author)

2000-01-01

210

Effect of radiation on forced convection flow of a micropolar fluid over a horizontal plate  

Energy Technology Data Exchange (ETDEWEB)

We determine the effect of radiation on forced convection flow of a micropolar fluid over a horizontal plate. The governing nonsimilar boundary-layer equations are solved using the cubic spline collocation method. The solutions are expressed in terms of the local shear stress and local rate of heat transfer. The effect of varying the Prandtl number, the radiation parameter, and the micropolar material parameter are determined. (author)

Elbashbeshy, E.M.A. [Ain Shams Univ., Dept. of Mathematics, Faculty of Science, Abbassia, Cairo (Egypt); Bazid, M.A.A. [Suez Canal Univ., Dept. of Mathematics, Faculty of Education (Egypt)

2000-10-01

211

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

212

Prediction of incipient boiling of low pressure water in a horizontal forced convection channel  

Energy Technology Data Exchange (ETDEWEB)

This paper presents both analytical and experimental studies on the phenomenon of incipient boiling in a uniformly heated forced convection channel. Measurements were made using water as the test medium. Based on existing theories, an improved model was formulated for the correlation of the incipient boiling data. The new formulations and the associated analytical procedure tested out in this study have been demonstrated to be potentially versatile and useful for correlation of incipient boiling data from various fluids and systems.

Yin, S.T.; Luh, T.C.

1983-04-01

213

Burn-out analysis. Part I: A burn-out correlation for water in round and rectangular channels uniformly heated at various pressures with forced convection  

International Nuclear Information System (INIS)

The practical objective of research on 'burn-out' is a reliable method giving the maximum safe rating for any water cooled reactor. Experimental work, which began at numerous centres about 10 years ago, has been concerned principally with endeavouring to understand the phenomenon as it applies to simple geometries such as round and rectangular channels. Many millions of pounds have been spent on this work and several thousand separate experimental results obtained. This considerable effort has achieved little real success in providing an explanation of 'burn-out' however. Many conflicting views have arisen and correlations so far developed have been shown to give calculated 'burn-out' heat fluxes varying by a factor of the order of 5> when applied to a typical reactor situation. While some uncertainty may be due to experimental variations, inadequate analytical effort is considered to be the primary cause of the present confused situation. To overcome this various analytical studies are being initiated by the Reactor Development Division at Winfrith and a detailed plan is being evolved for bringing effort to bear on certain fundamental aspects of boiling which have been neglected and which in some oases will require the development of special experimental techniques. This report describes the result of some work already carried out. It concerns an initial examination made on a large group of 'burn-out' data and describes the development of a correlation which predicts 'burn-out' heat fluxes to within an R.M.S. error of less than 10% over a very wide range of operating conditions including pressure. (author)

1961-01-01

214

Fireplace forced air heating apparatus  

Energy Technology Data Exchange (ETDEWEB)

Fireplace forced air heating apparatus comprises a heat exchanger having a pair of tubes, which may serve as andirons, communicating with a plenum within the heat exchanger and extending outwardly to the front of the fireplace. The tubes connect with duct work including a reversible fan, a hot air register, and a baffle, all designed for interchangeable connection to the tubes as required to adapt the installation to various fireplace settings. The tubes and duct work are telescoping to accommodate fireplaces of varying dimension.

Malafouris, D.O.

1980-10-21

215

Mixed convection cooling of a heated, continuously stretching surface  

Energy Technology Data Exchange (ETDEWEB)

An numerical study of the flow and heat transfer characteristics associated with a heated, continuously stretching surface being cooled by a mixed convection flow has been carried out. The relevant heat transfer mechanisms are of interest in a wide variety of practical applications, such as hot rolling, continuous casting, extrusion, and drawing. The surface velocity of the continuously stretching sheet was assumed to vary according to a power-law form, that is, u{sub w}(x)=Cx{sup p}. Two conditions of surface heating were considered, a variable wall temperature (VWT) in the form T{sub w}(x)-T{sub {infinity}}=Ax{sup n} and a variable surface heat flux (VHF) in the form q{sub w}(x)=Bx{sup m}. The governing differential equations are transformed by introducing proper nonsimilarity variables and solved numerically using a procedure based on finite difference approximations. Results for the local Nusselt number and the local friction coefficient are obtained for a wide range of governing parameters, such as the surface velocity parameter p, the wall temperature exponent n, the surface heat flux exponent m, the buoyancy force parameters ({xi} for the VWT case and {chi} for the VHF case), and Prandtl number of the fluid. It is found that the local Nusselt number is increased with increasing the velocity exponent parameter p for the VWT case, while the opposite trend is observed for the VHF case. The local friction coefficient is increased for a decelerated stretching surface, while it is decreased for an accelerated stretching surface. Also, appreciable effects of the buoyancy force on the local Nusselt number and the local friction coefficient are observed for both VWT and VHF cases, as expected. (orig.)

Chen, C.H. [Nat. Huwei Inst. of Technol. Huwei, Yunlin (Taiwan). Dept. of Mech. Eng.

2000-03-01

216

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).

1987-01-01

217

Forced air heating fireplace unit  

Energy Technology Data Exchange (ETDEWEB)

A forced air heating fireplace unit is described. The unit is installed in a framed in opening provided for a fireplace during the erection of a building and including a generally rectangular metal firebox spacedly encased within a metal housing connected at its upper end to a chimney flue. A vertically disposed air inlet pipe enters the flue above the unit and extends downwardly of the rear thereof and introduces air into an air distribution chamber at the lower end thereof from which air is circulated forwardly and upwardly through a series of spaced pipes into the bottom of the firebox below a grate therein so that air within the firebox is heated from the grate and exits upwardly through the main heat duct and chimney flue. A series of heating cores embodying closely spaced parallel aluminum plates are attached exteriorly to the side, rear and top walls of the firebox and extend at right angles thereto in the space between the firebox and housing so that a circulating blower associated with the unit introduces atmospheric air into and upwardly over the heating cores for heating through the fireplace walls. Spaced parallel air transfer pipes extend angularly upwardly and forwardly across the upper end of the firebox and through the rear and front walls thereof so that air flowing upwardly over the unit walls is caused to flow through and is heated in the transfer pipes and exits through the front of the firebox and the fireplace wall into the room and into the main heat duct for transfer to and the heating of other areas. As the transfer pipes are heated from within the firebox and the air flowing therethrough is also heated during its passage over the firebox walls and through the heating cores, maximum heat is extracted from the firebox and is introduced into the areas being heated.

Ebbers, A.C.

1980-09-23

218

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

219

Heat convection on cylinder at high Prandtl numbers  

International Nuclear Information System (INIS)

[en] 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

2003-01-01

220

Vertical natural convection heat transfer data for an enclosed fluid  

Energy Technology Data Exchange (ETDEWEB)

Natural convection flow of enclosed fluids with high temperature gradients can result in extremely high heat transfer rates. This phenomenon must be accurately modeled in order to predict the correct temperature distribution of structures in contact with the convecting flow. Ignoring the heat transfer by natural convection and assuming only the normal molecular heat conduction of stagnant water can result in an underestimate of the heat transfer by several orders of magnitude. Natural convection of enclosed fluids is different than free convection to a non-enclosed (i.e., open) ambient atmosphere since the recirculating fluid flow pattern can have a significant influence on the resultant heat transfer. Rayleigh numbers extending the entire range from conditions where free convection is a second order effect (e.g., Ra < 3000) to those where turbulent free convection dominates (e.g., Ra > 10{sup 5}) were tested and the results described. Inconsistencies between earlier investigations reported in the literature are resolved because of the wide test range capability. Correlations of the data in the form of Nusselt number as a function of Rayleigh number are provided for 0 < Ra < 10{sup l2}.

Coffield, R.D.; Harry, J.A.

1995-12-31

 
 
 
 
221

Method and apparatus for improvements in convective heating  

Energy Technology Data Exchange (ETDEWEB)

The efficiency of a convective heating system employing an elongate firebox is dramatically increased by, inter alia, increasing combustion zone volume to approach that of the firebox and increasing residence time of rising combustibles at ignition temperatures.

Henriques, J.

1980-10-28

222

Mixed convective heat transfer for fluid flowing through annular porous medium  

Energy Technology Data Exchange (ETDEWEB)

The study of the transport phenomena in porous media provides the fundamentals for various branches of science and engineering. The transport phenomena in porous media has critical significance in a variety of practical applications such as agriculture, chemical, petroleum and power engineering, and environmental and material science. The authors have reported currently on the research on the fluid flow and convective heat transfer in porous media. The entrance and non-Darcy`s effects on the forced and mixed convective heat transfer were studied experimentally for air, water and transformer-oil flowing through a vertical annulus packed with porous media beads of different sizes. The results are reported and discussed here briefly.

Wang, B.X.; Du, J.H.; Peng, X.F. [Tsinghua Univ., Beijing (China). Thermal Engineering Dept.

1996-12-31

223

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)

1982-01-01

224

Slow forced and free convection in inclined channels  

International Nuclear Information System (INIS)

Mixed convection at low Reynolds numbers in tilted rectangular channels of intermediate aspect ratios is dealt with. A laminar flow enters into an inclined enclosure whose upper plate is adiabatic whereas a constant heat flux is supplied to the bottom plate. The analytic solution of the linearised problem is first obtained. Subsequently the numerical solution of the pertinent equations is carried on by a computer's time saving iterative procedure. (Author)

1981-12-18

225

Natural Convection Heat Transfer in a Horizontal Layer Heated from Below and Cooled from Side and Above  

International Nuclear Information System (INIS)

[en] The thermo fluid dynamic characteristics of natural convection flow depend strongly on thermal boundary condition such as the spatial and temporal variation of heat flux on the pool wall boundaries. In general the natural convection heat transfer phenomena involving the bottom heat generation are represented by the Rayleigh number, Ra, which quantifies the bottom heat source and hence the strength of the buoyancy force. This work focuses on natural convection in which the density gradient is due to a temperature gradient and the body force is gravitational. The presence of a fluid density gradient in a gravitational field does not ensure the existence of natural convection currents, however, in an apparatus enclosed by two horizontal plates of different temperature. The temperature of the lower plate exceeds that of the upper plate, and the density decreases in the direction of the gravitational force. The LIDO (Liquid Internal Dynamics Operation) tests are conducted in a horizontal circular layer 500 mm in diameter and 220 mm in height using fluid, whose thermophysical properties are typified by the Prandtl number, Pr. The tests cover the range of 3x105 10 and 0.02

2008-01-01

226

Validation of PARET for the modeling of heat transfer under natural convection core cooling  

International Nuclear Information System (INIS)

[en] The PARET code is a one-dimensional, coupled thermal-hydraulic and point-kinetics code, which was originally developed for the analysis of SPERT-I transients and later adapted for the analysis of transient behavior in research reactors. Due to its ease of transportability and relative simplicity of input preparation, it is widely used internationally and is particularly attractive for research reactors with limited computational facilities. The thermal-hydraulic modeling of the current version of PARET accounts for buoyancy forces in the core and external pressure gradients that may arise from density differences between the core inlet and outlet. This feature of PARET makes it a useful tool for the analysis of research reactors cooled by natural convection as well as those cooled by forced convection. Since PARET has been applied to the analysis of the International Atomic Energy Agency 10-MW benchmark cores for protected and unprotected transients and also for the analysis of SPERT-I transients, its forced convection heat-removal model is reliable. However, there has been little experience with the capability of PARET to model heat removal in cores cooled by natural convection. This paper reports the results of some experiments performed at the Malaysian PUSPATI reactor to compare PARET predictions for power increases under natural convection core cooling to measured data

1995-01-01

227

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; S. M. Zuhairi; C. S. N. Azwadi

2010-01-01

228

Natural turbulent convection in a partitioned cavity heated from below  

Energy Technology Data Exchange (ETDEWEB)

This paper concerns the natural turbulent convective flow inside a cavity which is divided into two compartments separated by a horizontal partition with an opening. Vertical walls are adiabatic; horizontal walls are kept at constant and uniform temperature; the lower wall is warmer than the upper wall. The resulting flow is related to Rayleigh-Benard flows; it is controlled by two main parameters, the Rayleigh number (Ra) and the aperture area to wall area ratio (s/S). In the present work, this flow was investigated both experimentally and numerically. Experiments were conducted in a 3D cavity (1.2 x 10{sup 9} {lt} Ra {lt} 3.7 x 10{sup 9}; s/S = 0.032) while 2D direct numerical simulations were performed (10{sup 8} {lt} Ra {lt} 2 x 10{sup 9}; s/S = 0.15). Results lead to the following conclusions: for large values of s/S, the heat transfer is controlled by the boundary layer and Nu is proportional to Ra{sup 1/3}; for small values of s/S, the heat transfer is controlled by buoyancy forces through the aperture and Nu is proportional to Ra{sup 1/2}. One application of such a flow may concern transport of heat or pollutant inside buildings.

Blay, D.; Le Quere, P.

1999-07-01

229

An experimental apparatus measuring convective heat transfer coefficient from a heated fine wire traversing in nanofluids  

International Nuclear Information System (INIS)

[en] Most of the previous convection experiments for nanofluids have been performed for internal tube flow with constant heat flux boundary condition. In contrast, a simple experimental apparatus measuring convective heat transfer coefficient from a heated wire to external nanofluids is proposed and its working principles are explained in detail. The convective heat transfer coefficient provided by the present system might be used as a useful indication justifying the adoption of prepared nanofluids as new efficient heat transfer fluids. Validation experiments by comparing convective heat transfer coefficients between the conventional correlation and measured values are carried out for base fluids. Also the effect of increased thermal conductivity of nano lubrication oil on the enhancement of convective heat transfer coefficient is investigated

2011-01-01

230

An experimental apparatus measuring convective heat transfer coefficient from a heated fine wire traversing in nanofluids  

Energy Technology Data Exchange (ETDEWEB)

Most of the previous convection experiments for nanofluids have been performed for internal tube flow with constant heat flux boundary condition. In contrast, a simple experimental apparatus measuring convective heat transfer coefficient from a heated wire to external nanofluids is proposed and its working principles are explained in detail. The convective heat transfer coefficient provided by the present system might be used as a useful indication justifying the adoption of prepared nanofluids as new efficient heat transfer fluids. Validation experiments by comparing convective heat transfer coefficients between the conventional correlation and measured values are carried out for base fluids. Also the effect of increased thermal conductivity of nano lubrication oil on the enhancement of convective heat transfer coefficient is investigated.

Lee, Shin Pyo [Kyonggi University, Suwon (Korea, Republic of)

2011-01-15

231

Forced and free convection flow with viscous dissipation effects: The method of parametric differentiation  

International Nuclear Information System (INIS)

Effect of buoyancy force in a laminar uniform forced convection flow past a semi-infinite vertical plate has been analyzed near the leading edge, taking into account the viscous dissipation. The coupled non-linear locally similar equations, which govern the flow, are solved by the method of parametric differentiation. Effects of the buoyancy force and the heat due to viscous dissipation on the flow and the temperature fields as well as on the wall shear-stress and the heat transfer at the surface of the plate are shown graphically for the values of the Prandtl number ? ranging from 10-1 to 1.0. (author). 20 refs, 3 figs, 2 tabs

1988-01-01

232

Electrically heated convection furnaces for thermal treatment of metals; Elektrisch beheizte Umluftoefen fuer die Waermebehandlung von Metallen  

Energy Technology Data Exchange (ETDEWEB)

As the demand for heavy-duty components and high-tech products made of light metal alloys or nonferrous metals is increasing, the industry needs more convection furnaces, which ensure annealing with excellent temperature distribution inside the furnace and fast heat transfer. The author describes the design of convection furnaces, i.e. the furnace and annealing chamber, the blower, heating system, thermal insulation, control, and also the charge itself. The result of the optimisations were verified by measurements in a specially constructed convection furnace of Messrs. Linn High Therm GmbH. Simulations showed how the process would take place without convection. It is clearly shown that forced convection will lead to shorter heat-up times and a more even temperature distribution inside the furnace. (orig.)

Drott, E. [Linn High Therm GmbH, Werk 2, Bad Frankenhausen (Germany)

2005-07-01

233

Analysis of convective instability and heat transfer characteristics of nanofluids  

Science.gov (United States)

The convective instability driven by buoyancy and heat transfer characteristics of nanofluids are investigated analytically. This paper proposes a factor which describes the effect of nanoparticle addition on the convective instability and heat transfer characteristics of a base fluid. The Bruggeman model based on the mean field approach for expressing the thermal conductivity enhancement is chosen as a lower bound of the thermal conductivity relationship. The results show that as the density and heat capacity of nanoparticles increase and the thermal conductivity and the shape factor of nanoparticles decrease, the convective motion in a nanofluid sets in easily. The heat transfer coefficient of a nanofluid is enhanced by all parameters with respect to the volume fraction of nanoparticles.

Kim, Jake; Kang, Yong Tae; Choi, Chang Kyun

2004-07-01

234

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.

2005-01-01

235

Formulation of nano fluids for natural convective heat transfer applications  

Energy Technology Data Exchange (ETDEWEB)

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.

Wen Dongsheng [Institute of Particle Science and Engineering, University of Leeds, Leeds LS2 9JT (United Kingdom)]. E-mail: d.wen@leeds.ac.uk; Ding Yulong [Institute of Particle Science and Engineering, University of Leeds, Leeds LS2 9JT (United Kingdom)

2005-12-15

236

Determinação do coeficiente convectivo de transferência de calor de figos submetidos ao resfriamento rápido/ Determination of the convective heat transfer coefficient of fig fruits submitted to forced air precooling  

Scientific Electronic Library Online (English)

Full Text Available Abstract in portuguese Este trabalho busca, como função principal, determinar o valor do coeficiente convectivo de transferência de calor através da metodologia de modelagem matemática e simulação numérica associada ao processo experimental previamente realizado, de resfriamento rápido com ar forçado de figos "Roxo de Valinhos". O objetivo foi comparar o resultado com o definido pelo ajuste entre os dados experimentais e o uso de equação empírica da literatura, já realizado. Para (more) a predição da temperatura próximo ao centro das frutas, considerou-se o modelo da lei de Fourier em coordenadas esféricas e se implementaram algoritmos segundo os métodos de diferenças finitas e dos elementos finitos para os modelos unidimensional e tridimensional, respectivamente. O estabelecimento do referido valor se dá a partir da comparação desses dados simulados com os dados experimentais. Os valores resultantes dos dois modelos foram coincidentes e resultaram menores valores residuais que o obtido pela equação empírica; além disso, a representatividade da curva simulada mostra que a atual metodologia é mais precisa que a anteriormente realizada e, portanto, adequada para este e trabalhos futuros. Abstract in english The main purpose of this study is to determine the convective heat transfer coefficient value by using the methodology of mathematical modeling and numerical simulation associated with the experimental process previously performed, of the forced air precooling of the figs type 'Roxo de Valinhos'. The objective was to compare the result with the one determined by fitting an empirical equation from the literature to the experimental data. For the prediction of the temperatu (more) re, near to the center of fruits, the Fourier law model was considered. Algorithms employing the finite differences and finite elements methods were implemented for the one-dimensional and three-dimensional models. The establishment of the referred value was performed through the comparison of these simulated data with the experimental data. The resulting values of the two models were coincident and generated a smaller residual value than the one obtained by the empirical equation. Furthermore, the representation of the simulated curve shows that the methodology of the mathematical modeling and numerical simulation is more accurate than the previous one and suitable for this and future work.

Amendola, Mariangela; Dussán-Sarria, Saul; Rabello, Anderson A.

2009-04-01

237

Computational fluid dynamics analysis of combined conductive and convective heat transfer in model eggs.  

UK PubMed Central (United Kingdom)

Transient temperature and velocity profiles during thermal processes of egg shell bodies filled with a viscous liquid have been studied. Sodium carboxy-methyl cellulose suspension was used as a model system, having well known properties similar to eggwhite. The partial differential equations governing natural convection motion of fluid in a three-dimensional geometry were solved with the commercial computational fluid dynamics package Fluent. A Boussinesq approximation was used to account for the buoyancy force driving the convective motion of the fluid. Simulated temperature profiles were compared with experimentally observed data and were found to be in good agreement. It was observed that natural convection tends to force the slowest heating zone towards the bottom of the egg. Considering the flow properties of the major egg components (albumen and yolk), a similar phenomenon is expected during pasteurization of intact eggs. The effect of the presence of sphere-shaped yolk was also investigated.

Denys S; Pieters JG; Dewettinck K

2004-08-01

238

Coupled heat and mass transfer by mixed convection from a horizontal surface in saturated porous medium  

Energy Technology Data Exchange (ETDEWEB)

Coupled heat and mass transfer in porous media has many important applications in engineering. These include the migration of moisture in fibrous insulation, the spreading of chemical pollutants in saturated soil, underground disposal of nuclear wastes and the extraction of geothermal energy. Here, similarity solutions are reported for coupled heat and mass transfer by free, forced, and mixed convection from a horizontal surface in a saturated porous medium. The present analysis shows that similarity solutions are possible for a surface maintained at a constant heat flux and mass flux condition. The governing parameters for the problem under consideration are the Lewis number Le, the buoyancy ratio N and mixed convection parameter Ra/Pe{sup 3/2}. Depending on the values of N and Le, the heat and mass transfer results may range from the asymptotic free convection limit to that of the forced convection limit. The results are presented in terms of the relation between the transfer coefficients and the governing parameters.

Li, C.T.; Lai, F.C.

1997-07-01

239

Novel insight and numerical analysis of convective heat transfer enhancement with microencapsulated phase change material slurries: laminar flow in a circular tube with constant heat flux  

Energy Technology Data Exchange (ETDEWEB)

This paper presents a novel insight for the forced convective heat transfer enhancement of microencapsulated phase change material slurries flowing through a circular tube with constant heat flux. The influence of various factors is analyzed in detail by using an effective specific heat capacity model, validated with the results available in the literature. It is found that the conventional Nusselt number correlations for internal flow of single phase fluids are not suitable for accurately describing the heat transfer enhancement with microencapsulated phase change material suspensions, and a modification is introduced that enables evaluation for the convective heat transfer of internal flows. (Author)

Hu, X.; Zhang, Y. [Tsinghua Univ., Beijing (China). Dept. of Building Science

2002-07-01

240

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)

1986-01-01

 
 
 
 
241

Numerical simulation of mixed convection in a rectangular enclosure with different numbers and arrangements of discrete heat sources  

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)

2008-01-01

242

Numerical investigations of opposing mixed convection heat transfer in vertical flat channel 1. Laminar mixed convection and transition to vortex flow in case of symmetrical heating  

International Nuclear Information System (INIS)

[en] 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)

243

Heat transfer by thermosolutal natural convection in a vertical composite fluid-porous cavity  

Energy Technology Data Exchange (ETDEWEB)

Many studies have been recently dedicated to double diffusive natural convection in binary mixtures confined in fluid or porous enclosures. The present analysis deals with the case of a partially porous enclosure. In spite of the many potential physical or engineering applications, this configuration has received little attention in the literature. Double diffusive natural convection of a binary fluid is studied in a rectangular enclosure partially filled with a porous matrix. The study is focused on the influence of the porous layer permeability on the overall heat transfer. The numerical results show that the convective flow structure, and consequently the heat transfer, results from a complex interaction between the viscous drag in the porous layer and the driving force enhancement due to the flow penetration.

Goyeau, B.; Gobin, D.

1999-11-01

244

Natural convective heat transfer in an open cavity with a discrete internal heat source  

Energy Technology Data Exchange (ETDEWEB)

An experimental and numerical analysis is performed for natural convection heat transfer from an internal heat source to water in a partially-open cavity. Cavity walls are assumed adiabatic and natural convection caused by a square heat source determines the flow and heat transfer phenomena in the cavity. Experimental and numerical results, thus obtained in terms of Nu-Ra relations, show good agreement. (authors)

Yalcin Uralcan, I. [Istanbul Technical Univ., Faculty of Mechanical Engineering (Turkey)

2000-07-01

245

Solar distillation in forced convection. Simulation and experience  

Energy Technology Data Exchange (ETDEWEB)

Scarcity of water and fossil energies in the Canary Islands leads to an interest in a study on solar assisted desalination. A forced convection humidification-dehumidification process is analysed, where air is forced into an evaporation channel while saline water flows along the bottom, so that a fraction will evaporate to be absorbed by air. Two simulation models are developed for the proposed system. Their output in terms of temperature and humidity are discussed. Evaluation of different alternatives of operation is carried out in terms of exergy efficiency as the basic criterion. Simulation results are also compared with data obtained at a pilot plant. Data measured as well as the efficiencies are similar to those obtained for flat solar collectors. (Author)

Veza, J.M. (Univ. de Las Palmas de Gran Canaria, Las Palmas (Spain)); Ruiz, V. (Univ. de Sevilla (Spain). E.T.S. Ingenieros Industriales)

1993-01-01

246

Effects of Prandtl number on free convection heat transfer from a vertical plate to a non-Newtonian fluid  

Energy Technology Data Exchange (ETDEWEB)

A study has been conducted to analyze the heat transfer characteristics of non-Newtonian power law fluids on the free convective flow over a vertical flat plate. The analysis includes the inertia force in the momentum equation with a finite Prandtl number. In general, it has been found that the average heat transfer increases with increasing Prandtl number.

Huang, Mingjer; Huang, Jhyshean; Chou, Youli; Chen, Cha' okuang (National Cheng Kung Univ., Tainan (Taiwan))

1989-02-01

247

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

Energy Technology Data Exchange (ETDEWEB)

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.

Chung, K.S.; Thompson, D.H.

1980-01-01

248

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).

1981-01-01

249

Combined convective heat transfer in an inclined cavity with a heated rectangular element on the wall  

International Nuclear Information System (INIS)

[en] Mixed forced and free convective heat transfer through a cavity with a rectangular element on one wall heated to a uniform temperature, and the opposite wall cooled to a uniform lower temperature, and with the remaining wall portions adiabatic, has been investigated. The forced flow, which is at the same temperature as the cold wall, enters through the cold vertical wall and also leaves through this wall. The study is based on the use of the two-dimensional Navier-Stokes, equations, the flow assumed steady and laminar. It is assumed that fluid properties are constant except for the density change with temperature which gives rise to the buoyancy force, this being treated using the Boussinesq approximation. These equations have been solved using the finite element method. The solution has, as parameters, the Reynolds number, the Grashof number, the fluid Prandtl number, the aspect ratio of the cavity, the angle of inclination of the cavity, the inlet and outlet duct size, and the aspect ratio of the heated element. Results have only been obtained for a Prandtl number of 0.7. 7 references

1988-01-01

250

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

UK PubMed Central (United Kingdom)

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 B; Zárate L; Gómez-Mares M; Casal J

2011-12-01

251

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

Science.gov (United States)

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

2011-09-21

252

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

1988-01-01

253

Hall effects on combined free and forced convective hydromagnetic flow through porous media  

International Nuclear Information System (INIS)

The effects of Hall current and permeability of the porous medium on combined free and forced convective hydromagnetic flow in parallel plate channel have been studied, when there is a uniform axial temperature variation along the channel walls. The induced magnetic field and heat transfer characteristics in the flow are determined. Expressions for the shearing stress components have also been sought. The effects of porous medium and Hall parameter on the velocity, the induced magnetic field and shearing stress is interpreted with the aid of graphs and a table. (author).

1982-01-01

254

Radiative effects on forced convection flows in micropolar fluids with variable viscosity  

International Nuclear Information System (INIS)

The interaction of forced convection and thermal radiation during the flow of a surface moving continuously in a flowing stream of micropolar fluid with variable viscosity is studied. Two cases are considered: one corresponding to a plane surface moving in parallel with the free stream, the other to a surface moving in the opposite direction to the free stream. The Rosseland approximation is used to describe the radiative heat flux in the energy equation. The viscosity of the fluid is taken as a function of temperature. (author)

2004-01-01

255

Radiative effects on forced convection flows in micropolar fluids with variable viscosity  

Energy Technology Data Exchange (ETDEWEB)

The interaction of forced convection and thermal radiation during the flow of a surface moving continuously in a flowing stream of micropolar fluid with variable viscosity is studied. Two cases are considered: one corresponding to a plane surface moving in parallel with the free stream, the other to a surface moving in the opposite direction to the free stream. The Rosseland approximation is used to describe the radiative heat flux in the energy equation. The viscosity of the fluid is taken as a function of temperature. (author)

El-Kabeir, S.M.M. [South Valley Univ., Dept. of Mathematics, Faculty of Science Aswan, Aswan (Egypt)]. E-mail: elkabeir@yahoo.com

2004-02-01

256

Forced convective transition boiling: review of literature and comparison of prediction methods  

International Nuclear Information System (INIS)

This report reviews the published information on transition boiling heat transfer under forced convective conditions. It was found that transition boiling data have been obtained only within a limited range of conditions and many data are considered unreliable. The data do not permit the derivation of a correlation; however the parametric trends can be isolated from the data. Several authors have proposed correlations valid in the transition boiling region. Most of the correlations are valid only within a narrow range of conditions. A comparison with the data shows that in general agreement is poor. Hsu's correlation is tentatively recommended for low flows and pressures. (author)

1976-01-01

257

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

258

Forced air fireplace heating system  

Energy Technology Data Exchange (ETDEWEB)

A forced air fireplace heating system for a space to be heated circulates air from the space through a lower horizontal plenum which either surrounds or is placed beneath the fireplace grill, and a series of adjustable tubes extend upwardly and forwardly from the rear of the plenum over the fireplace grill and through a front support into the space. A blower may be connected to either side of the plenum to draw air from the space for the purpose of heating and recirculating air back into the space. The system is self-contained and self-supporting within the fireplace requiring no modification or positive attachment to the fireplace or firebox itself; and accordingly it is specifically adaptable for use in combination with a front closure for example of the type having glass doors across the front of the fireplace wherein the plenum extends through a panel section beneath the glass doors and the circulating tubes exit through a panel section above the doors.

Lehrer, J.E.

1981-11-03

259

Toward evaluation of heat fluxes in the convective boundary layer  

Energy Technology Data Exchange (ETDEWEB)

This article demonstrates that vertical profiles of the heat flux in the convective boundary layer can be diagnosed through an integration over height of the time change rates of observed potential temperature profiles. Moreover, the basic characteristics of the convective boundary layer, such as the mixed-layer height z{sub t}, the depth of the interfacial (entrainment) layer, and the heat flux zero-crossing height h{sub 0} can be uniquely evaluated based on a time evolution of potential temperature profiles in the lower atmosphere. 12 refs., 12 figs., 1 tab.

Sorbjan, Z. [Univ. of Oklahoma, Norman, OK (United States)

1995-05-01

260

Measurements of laminar mixed convection flow adjacent to an inclined surface with uniform wall heat flux  

Energy Technology Data Exchange (ETDEWEB)

Measurements of laminar mixed convection flow adjacent to an inclined heated flat plate with uniform wall heat flux are reported. Laser-doppler velocimeter and cold wire anemometer were used to measure simultaneously the velocity and temperature distributions, respectively. Measurements of the air velocity and temperature distributions are presented for a range of buoyancy parameters 0{<=}{xi}{<=}2.91. It was found that both the mixed convection local Nusselt number and local friction coefficient increase as the buoyancy force increases (under the buoyancy assisting condition). The velocity field was found to be more sensitive to the buoyancy force than the thermal field. Predictions from both local similarity and local non-similarity models agree well with the experimental results for the thermal field, but only the predictions from the local non-similarity model agree favorably with the measured values for the flow field. (authors)

Abu-Mulaweh, H.I. [Mechanical Engineering Department, Purdue University at Fort Wayne, 2101 E. Coliseum Blvd., 46805, Fort Wayne, IN (United States)

2003-01-01

 
 
 
 
261

Free convective heat transfer near the critical state  

International Nuclear Information System (INIS)

Experiments were performed on free convective heat transfer from a horizontal, electrically heated tube (d = 8 mm) to the halocarbon refrigerants R12 (CF2Cl2), R115 (C2F5Cl) and RC318 (C4F8) near the critical state. Density varied from approx. half to twice the critical and one supercritical isotherm. As is partially known, at different bulk fluid states either peaks are found in the heat transfer coefficients as a function of heat flux or a behaviour similar to film boiling or to free convection at far subcritical states is observed. The different results can be explained by analyzing the density - and enthalpy-variation within the fluid near the heated wall. (orig.)

1976-01-01

262

Method of operating a convective membrane surface of boiler heating  

Energy Technology Data Exchange (ETDEWEB)

According to the proposed method, the heat carrier with temperature below the sulfuric acid dew point does not enter all the pipes of the convective membrane surface of heating, but through one. These pipes in the process of operation are exposed to sulfuric acid corrosion. The heat carrier does not enter the adjoining pipes. Their temperature remains higher than the sulfuric acid dew point and they do not corrode. After the depletion of the service life of the cooled pipes, supply of heat carrier to them is stopped and it is supplied to the adjoining pipes. This method of operation makes it possible to significantly prolong the service life of the convective membrane surface of heating the hot water unit before its replacement.

Filimonov, A.I.; Gavrilov, A.F.

1980-02-02

263

Environmental Forcing of Supertyphoon Paka's (1997) Latent Heat Structure.  

Science.gov (United States)

The distribution and intensity of total (i.e., combined stratified and convective processes) rain rate/latent heat release (LHR) were derived for Tropical Cyclone Paka during the period 9-21 December 1997 from the F-10, F-11, F-13, and F-14 Defense Meteorological Satellite Special Sensor Microwave Imager and the Tropical Rainfall Measuring Mission Microwave Imager observations. These observations were frequent enough to capture three episodes of inner-core convective bursts and a convective rainband cycle that preceded periods of rapid intensification. During these periods of convective bursts, satellite sensors revealed that the rain rates/LHR 1) increased within the inner-core region, 2) were mainly convectively generated (nearly a 65% contribution), 3) propagated inward, 4) extended upward within the mid- and upper troposphere, and 5) became electrically charged. These factors may have increased the areal mean ascending motion in the mid- and upper-troposphere eyewall region, creating greater cyclonic angular momentum, and, thereby, warming the center and intensifying the system.Radiosonde measurements from Kwajalein Atoll and Guam, sea surface temperature observations, and the European Centre for Medium-Range Forecasts analyses were used to examine the necessary and sufficient conditions for initiating and maintaining these inner-core convective bursts. For example, the necessary conditions such as the atmospheric thermodynamics [i.e., cold tropopause temperatures, moist troposphere, and warm SSTs (>26°C)] fulfill the necessary conditions and suggested that the atmosphere was ideally suited for Paka's maximum potential intensity to approach supertyphoon strength. Further, Paka encountered moderate vertical wind shear (<15 m s1) before interacting with the westerlies on 21 December. The sufficient conditions that include horizontal moisture and the upper-tropospheric eddy relative angular momentum fluxes, on the other hand, appeared to have some influence on Paka's convective burst. However, the horizontal moisture flux convergence values in the outer core were weaker than some of the previously examined tropical cyclones. Also, the upper-tropospheric outflow generation of eddy relative angular momentum flux convergence was much less than that found during moderate tropical cyclone-trough interaction. These results indicated how important the external necessary condition and the internal forcing (i.e., convective rainband cycle) were in generating Paka's convective bursts as compared with the external sufficient forcing mechanisms found in higher-latitude tropical cyclones. Later, as Paka began to interact with the westerlies, both the necessary (i.e., strong vertical wind shear and colder SSTs) and sufficient (i.e., dry air intrusion) external forcing mechanisms helped to decrease Paka's rain rate.

Rodgers, Edward; Olson, William; Halverson, Jeff; Simpson, Joanne; Pierce, Harold

2000-12-01

264

Investigating Convective Heat Transfer with an Iron and a Hairdryer  

Science.gov (United States)

A simple experimental set-up to study free and forced convection in undergraduate physics laboratories is presented. The flat plate of a domestic iron has been chosen as the hot surface, and a hairdryer is used to generate an air stream around the plate. Several experiments are proposed and typical numerical results are reported. An analysis and…

Gonzalez, Manuel I.; Lucio, Jesus H.

2008-01-01

265

Heat transfer by natural convection into an horizontal cavity  

International Nuclear Information System (INIS)

[en] 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)

1998-01-01

266

The effects of magnetic field on forced and free convection flow  

CERN Multimedia

The effects of magnetic field and permeability of the porous medium on unsteady forced and free convection flow past an infinite vertical porous plate have been studied when the temperature of the plate is oscillating with time about a constant nonzero mean value and in the presence of temperature dependent heat source. The governing equations for the hydromagnetic fluid flow and the heat transfer are solved subject to the relevant boundary conditions. The perturbation technique is used to obtain expressions for velocity field, skin friction and Nusselt number. Also, the effects of magnetic parameter, heat source parameter, suction parameter and permeability of porous medium on velocity field, skin friction and Nusselt number are discussed.

Abdel-Khalek, M M

2003-01-01

267

A mechanistic model of the ebullition cycle in forced convection subcooled boiling  

Energy Technology Data Exchange (ETDEWEB)

A mechanistic model is proposed for determining the bubble ebullition process in forced convection subcooled boiling. Using a rigorous analytical approach, the method combines transient heat transfer solutions for the heated wall and for the liquid filling the space vacated by departing bubbles. Accounting for the coupling between these phenomena and for the periodic nature of the process a closed-form solution has been obtained for the parameters characterizing the timing of bubble ebullition. Furthermore, for a give critical diameter of the bubbles, the model calculated the bubble departure frequency. The major parameters taken into account in the model are the: transient heat flux, subcooling, surface characteristics (material and cavity size), pressure, and mass flux. The new model has been validated and parametrically tested for the effects of various parameters on the bubble growth time and the frequency of nucleation. (author)

Podowski, R.M.; Drew, D.A.; Lahey, R.T. Jr.; Podowski, M.Z. [Center for Multiphase Research, Rensselaer Polytechnic Inst., Troy, NY (United States)

1997-12-31

268

A mechanistic model of the ebullition cycle in forced convection subcooled boiling  

International Nuclear Information System (INIS)

A mechanistic model is proposed for determining the bubble ebullition process in forced convection subcooled boiling. Using a rigorous analytical approach, the method combines transient heat transfer solutions for the heated wall and for the liquid filling the space vacated by departing bubbles. Accounting for the coupling between these phenomena and for the periodic nature of the process a closed-form solution has been obtained for the parameters characterizing the timing of bubble ebullition. Furthermore, for a give critical diameter of the bubbles, the model calculated the bubble departure frequency. The major parameters taken into account in the model are the: transient heat flux, subcooling, surface characteristics (material and cavity size), pressure, and mass flux. The new model has been validated and parametrically tested for the effects of various parameters on the bubble growth time and the frequency of nucleation. (author)

1997-10-04

269

Free and forced convective-diffusion solutions by finite element methods  

International Nuclear Information System (INIS)

[en] Several free and forced convective-diffusion examples are solved and compared to either laboratory experiment or closed-form analysis. The problems solved illustrate the application of finite element methods to both strongly-coupled and weakly-coupled velocity and temperature fields governed by the steady-state momentum and energy equations. Special attention is given to internal forced convection with temperature-dependent viscosity and free convection within an enclosure

1976-06-14

270

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; Debbissi Chokri; Nasrallah Ben Sassi

2011-01-01

271

Experimental investigation of forced convection and desiccant integrated solar dryer  

Energy Technology Data Exchange (ETDEWEB)

An indirect forced convection and desiccant integrated solar dryer is designed and fabricated to investigate its performance under the hot and humid climatic conditions of Chennai, India. The system consists of a flat plate solar air collector, drying chamber and a desiccant unit. The desiccant unit is designed to hold 75kg of CaCl{sub 2}-based solid desiccant consisting of 60% bentonite, 10% calcium chloride, 20% vermiculite and 10% cement. Drying experiments have been performed for green peas at different air flow rate. The equilibrium moisture content M{sub e} is reached in 14h at an air flow rate of 0.03kg/m{sup 2}s. The system pickup efficiency, specific moisture extraction rate, dimensionless mass loss, mass shrinkage ratio and drying rate are discussed in this paper. (author)

Shanmugam, V. [Department of Mechanical Engineering, Sathyabama Deemed University, Chennai 600 119 (India); Natarajan, E. [Institute for Energy Studies, College of Engineering, Anna University, Chennai 600 025 (India)

2006-07-15

272

Ohmic potential drop during alkaline water electrolysis at forced convection  

Energy Technology Data Exchange (ETDEWEB)

A survey is given of the results of a comprehensive research program on the alkaline water electrolysis, which has been carried out by the Working Group ''Gas evolution on electrodes'' during the last decade. The research includes both fundamental and practically applicable studies, and is characterized by the combination of experiment and the development of theoretical models. A part of the results, mainly on gas bubble behavior at electrodes, has been presented by Van Stralen and Sillen at the Second, Third and Fourth ''Miami international conference on alternative energy sources''. The present contribution deals with the energy losses during electrolysis at forced convection, which are due to the evolution of gas bubbles. The effect of the gas evolution at the electrode on the reduced resistance, r/r/SUB p/, of the two-phase electrolyte flowing between a vertical working electrode and the diaphragm, has been studied fundamentally by distinguishing two adjacent sublayers: the first one contains the adhering, and the second one the detached bubbles; in addition, one has a remaining layer of pure electrolyte in one-phase flow. A theoretical model for r/r/SUB p/, which applies to two-phase forced convective flow, has been developed. The effect of the bubbles is incorporated by increasing the local thickness of the corresponding one-phase flow velocity boundary layer with a factor, which accounts for the local gas volume fraction. Photographs show, however, that the two-phase boundary layer, which is actually formed along the surface of the electrode, may deviate substantially from calculations.

van Stralen, S.J.D.; Barendrecht, E.; Bongenaar-Schlenter, B.E.; Janssen, L.J.J.; Sillen, C.W.M.P.

1983-12-01

273

Combined free and forced convection inside a two-dimensional multiple ventilated rectangular enclosure  

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; Goutam Saha; Mohammad Ali; Md. Quamrul Islam

2006-01-01

274

Experimental study of the local heat transfer by forced convection in presence of recirculating flows; Contribution a l'etude experimentale du transfert de chaleur local par convection forcee dans le cas d'ecoulements presentant une zone de recirculation  

Energy Technology Data Exchange (ETDEWEB)

The literature review principally deals with the well-documented case of the flow downstream a backward-facing step. Then, the experimental method of determination of the local convective heat transfer coefficient in the case of thermally thick wall is presented. Experimental temperature conditions are applied as boundary conditions of the numerical model of the wall. The heat flux distribution is computed by solving the stead state heat equation. In practice, the wall surface temperature is determined by infrared thermography and an image substraction technique is used to eliminate effects of surroundings. This method is validated in the case of the flow downstream a backward-facing step. Complementary results allows to have a better comprehension of the influence of the upstream flow characteristics on heat transfer in the reattachment region. Finally, this method is applied in the case of a separation of flows at a T junction. The comparison between convective heat transfer and average velocity flow fields obtained by P.I.V. allows to associate the different heat transfer regions to the the flow pattern. The influence of the flow rate in each channel and the influence of the velocity are detailed. (author)

Boizumault, F.

1998-05-01

275

Modeling and analysis of low heat flux natural convection sodium boiling in LMFBRs  

Energy Technology Data Exchange (ETDEWEB)

Flow excursion induced dryout at low heat flux natural convection boiling, typical of liquid metal fast breeder reactor, is addressed. Steady state calculations indicate that low quality boiling is possible up to the point of Ledinegg instability leading to flow excursion and subsequent dryout in agreement with experimental data. A flow regime-dependent dryout heat flux relationship based upon saturated boiling criterion is also presented. Transient analysis indicates that premature flow excursion can not be ruled out and sodium boiling is highly transient dependent. Analysis of a high heat flux forced convection, loss-of-flow transient shows a significantly faster flow excursion leading to dryout in excellent agreement with parallel calculations using the two-dimensional THORAX code. 31 refs., 25 figs., 6 tabs.

Khatib-Rahbar, M.; Cazzoli, E.G.

1982-09-01

276

Heat convection in a set of three vertical cylinders  

International Nuclear Information System (INIS)

[en] 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)

1993-01-01

277

Natural convection burnout heat flux limit for control rods  

Energy Technology Data Exchange (ETDEWEB)

Technical Standard 105-3.05, Safety Circuits, does not require the Septifoil Supply Header Pressure Very Low safety circuit for current charges. This document develops a new requirement for this circuit based on the burnout heat flux of a control rod under natural convective cooling. Specifically, the Septifoil Supply Header Pressure Very Low safety circuit will be required whenever the calculated control rod operating heat flux exceeds 155,000 pcu/ft{sup 2}-hr.

Britt, T.E.

1986-04-14

278

Fluctuations of the heat-flux in turbulent convection  

Science.gov (United States)

We report measurements of the temporal fluctuations of the heat-flux in Rayleigh-Bénard turbulent convection. Although being a global quantity, i.e. spatially averaged on the flow field, we show that its probability density function (PDF) is non-gaussian and asymmetric. The ratio of the rms fluctuations of the heat flux to their mean display a power lax, Ra-? with ??0.25, on more than two decades in Rayleigh number (2 107gaussien des fluctuations.

Aumaître, S.; Fauve, S.

1999-06-01

279

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

280

Effects of vortex destabilization on convective heat transfer  

Energy Technology Data Exchange (ETDEWEB)

Local convective heat transfer coefficients were evaluated for flow geometries resulting in self-sustained Tollmien Schlichting waves. Navier-Stokes, continuity and energy equations were used to obtain the fluid temperature field. The objective of this work was to determine how vortex destabilization modified local heat transfer. Increases in heat transfer were very sensitive to geometrical parameters including symmetry, period, amplitude of sections and the presence of Tollmien Schlichting waves. It was found that heat transfer increased locally in symmetric channels at the detachment point and slightly upstream of the reattachment point. 2 figs., 3 refs.

Creff, R.; Blancher, S. [Laboratoire de Transferts Thermiques (France)

1995-12-31

 
 
 
 
281

Unsteady natural convection in a partially heated rectangular cavity  

Energy Technology Data Exchange (ETDEWEB)

The present numerical study is concerned with two-dimensional unsteady natural convective flow in a rectangular enclosure with a partially heated vertical wall. It has been assumed that the fluid is initially at a uniform temperature and motionless. Then, at time zero, the heated element is heated to a higher uniform temperature, while the opposite wall is held at the initial fluid temperature with the remaining wall surfaces being assumed adiabatic. The development of the flow following this temperature change has been studied numerically. Results have been obtained for a number of heat surface element sizes and locations, cavity aspect ratios, and Rayleigh numbers.

Kuhn, D.; Oosthuizen, P.H. (Queen' s Univ., Kingston, Ontario (Canada))

1987-08-01

282

Convective heat transfer near the entrance region of a tube for water near the critical point  

Energy Technology Data Exchange (ETDEWEB)

Numerical modeling is carried out to investigate the characteristics of convective heat transfer for the turbulent developing flow of water near the critical point in a tube with or without buoyancy force for constant wall temperature. The modeling includes the effects of the thermodynamic and transport property variations. The effects of proximity to the critical point and wall temperature variation on the buoyancy force are considered as well as varying properties near the pseudocritical temperature. For turbulent diffusivity, a modified mixing length model including density fluctuation is used. Emphasis is on effects very close to the inlet (z/D < 10). The results show heat transfer and fluid flow characteristics which include velocity profiles, heat transfer coefficient and friction factor distribution along the tube. Effects of gravity force on turbulent eddy viscosity distribution inside the tube are also studied. Results using several models for turbulent Prandtl number are compared.

Lee, S.H.; Howell, J.R.

1997-07-01

283

Analysis of cross-flow mixed convection with applications to building heat transfer  

Energy Technology Data Exchange (ETDEWEB)

A numerical simulation model has been developed for partial enclosure with restricted inlet and outlet simulating the building fluid flow and heat transfer scenario. Computed results are presented for a number of geometric configurations over a wide range of Reynolds and Rayleigh numbers and validated with available experimental data. The physical processes were modeled by solving equations for the conservation of mass, momentum, and energy with appropriate boundary conditions. The properties of the fluid were assumed to remain approximately constant over the range of operation and the buoyancy was incorporated using the Boussinesq approximation. The k-{var_epsilon} model was used for the simulation of turbulence. The computed results included the local velocity and temperature and the variation of local heat transfer coefficient along the heated side wall. Computed results showed excellent agreement with experimental data. The flow pattern within the enclosure was found to be quite complex in nature and consisted of a core flow due to forced convection near the central region of the enclosure and strong buoyancy induced flow near the heated side walls. It was found that as the flow rate through the enclosure increased, the enhancement of heat transfer above that for natural convection alone, also increased. The variation of the local heat transfer coefficient over the heated surface was found to be strongly affected by the recirculation of portions of the forced flow within the enclosure as well as the impingement to or separation of flow from the side walls in some regions.

Gao, S.; Rahman, M.M.

1999-07-01

284

Investigation of combined free and forced convection in a 2 x 6 rod bundle during controlled flow transients  

Energy Technology Data Exchange (ETDEWEB)

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 (a modified version of the COBRA-IV code).

Bates, J.M.; Khan, E.U.

1980-10-01

285

Convective heat transfer with buoyancy effects from thermal sources on a flat plate  

Energy Technology Data Exchange (ETDEWEB)

An experimental study is carried out on the thermal interaction between two finite-size heat sources, located on a flat plate that is well insulated on the back. Both the horizontal and the vertical orientations of the surface are studied by measuring the flow velocities, the temperature field, and the local heat flux. The investigation is directed at the pure natural convection circumstance (no forced flow velocity) and the buoyancy-dominated mixed-convection circumstance (presence of a relatively small forced flow velocity). Large temperature gradients occur in the vicinity of the heat sources, resulting in a substantial diffusion of heat along the plate length. However, the effect of conduction is found to be highly localized. The orientation of the surface has a very strong effect on the interaction of the wakes from the heat sources for the circumstances considered. An upstream source is found to have a very strong influence on the temperature of a downstream source in the vertical surface orientation but has a much weaker influence in the horizontal orientation. In the latter circumstance the presence of a small forced flow velocity may actually increase the temperature of a downstream source by tilting the wake from the upstream source toward the downstream source. 25 refs.

Tewari, S.S.; Jaluria, Y. (Rutgers University, New Brunswick, NJ (USA))

1991-06-01

286

Simulation of heat transfer and convection during sapphire crystal growth in a modified heat exchanger method  

Science.gov (United States)

Quasi-steady-state (QSS) and transient models, developed using the CrysMAS code, are employed to study the effects of transport mechanisms and cold finger design on the temperature distribution, melt flow field, and melt-crystal interface shape during the crystal growth of sapphire by a small-scale, modified heat exchanger method (HEM). QSS computations show the importance and effects of various heat transfer mechanisms in the crystal and melt, including conduction, internal radiation, and melt convection driven by buoyant and Marangoni forces. The design of the cold finger is demonstrated to have significant effects on growth states. Notably, transient computations on an idealized heat transfer model, supplemented with QSS calculations of a model with rigorous heat transfer representation, show that non-uniform growth conditions arise under uniform cooling of the system via a linear decrease in furnace set points. We suggest that more uniform HEM growth conditions may be achieved by using non-linear cool-down strategies.

Zhang, Nan; Park, Hyun Gyoon; Derby, Jeffrey J.

2013-03-01

287

Mixed convection heat transfer in concave and convex channels  

Energy Technology Data Exchange (ETDEWEB)

Mixed convection heat transfer studies in the literature have been primarily confined to pipe and rectangular channel geometry's. In some applications, however, heat transfer in curved channels may be of interest (e.g., nozzle and diffuser shaped passages in HVAC systems, fume hoods, chimneys, bell-shaped or dome-shaped chemical reactors, etc.). A numerical investigation of laminar mixed convection heat transfer of air in concave and convex channels is presented. Six different channel aspects ratios (R/L = 1.04, 1.25, 2.5, 5, 10, and {infinity}) and five different values of Gr/Re{sup 2} (Gr/Re{sup 2} = 0, 0.1, 1, 3, 5) are considered. Results are displayed in terms of streamline and isotherm plots, velocity and temperature profiles, and local and average Nusselt number estimates. Numerical predictions reveal that compared to straight channels of equal height, concave channels of low aspect ratio have lower heat transfer at relatively low values of Gr/Re{sup 2} and higher heat transfer at high values of Gr/Re{sup 2}. When compared to straight channels of equal heated length, concave channels are always found to have lower heat transfer and for all values of Gr/Re{sup 2}. On the other hand, predictions for convex channels revealed enhancement in heat transfer compared to straight channels of equal height and/or equal heated length for all values of Gr/Re{sup 2}.

Moukalled, F.; Doughan, A.; Acharya, S.

1997-07-01

288

Mixed convection film condensation from downward flowing vapors onto a sphere with uniform wall heat flux  

Science.gov (United States)

A model is developed for the study of mixed convection film condensation from downward flowing vapors onto a sphere with uniform wall heat flux. The model combined natural convection dominated and forced convection dominated film condensation, including effects of pressure gradient and interfacial vapor shear drag has been investigated and solved numerically. The separation angle of the condensate film layer, ?s is also obtained for various pressure gradient parameters, P* and their corresponding dimensionless Grashof 's parameters, Gr*. Besides, the effect of P* on the dimensionless mean heat transfer, will remain almost uniform with increasing P* until for various corresponding available values of Gr*. Meanwhile, the dimensionless mean heat transfer, is increasing significantly with Gr* for its corresponding available values of P*. For pure natural-convection film condensation, is obtained. Zusammenfassung Es wird ein Modell zur Untersuchung der Mischkonvektion bei Filmkondensation von Dämpfen an einer Kugel entwickelt, die unter gleichförmigen Wärmefluß daran abwärts strömen. Das Modell verbindet die durch natürliche und durch erzwungene Konvektion bewirkte Filmkondensation unter Einschluß von aus einem Druckgradienten resultierenden Effekten sowie von Dampfschubspannungen an der Phasengrenze. Die numerische Lösung liefert den Separationswinkel des Kondensatfilms ?s für verschiedene Druckgradienten-Parameter P* und zugehörige Grashof-Parameter Gr*. Der Einfluß von P* auf den mittleren Wärmeübergangsparameter bleibt bis ziemlich gering, auch wenn Gr* zwischen 0.01 und 100 variiert. Für reine natürliche Filmkondensation erhält man:

Hsu, C. H.; Yang, S. A.

289

ASME proceedings of the 32nd national heat transfer conference (HTD-Vol. 346). Volume 8: Fundamentals of convection; Turbulent heat transfer; Mixed convection heat transfer  

Energy Technology Data Exchange (ETDEWEB)

This volume contains a portion of the over 240 ASME papers which were presented at the conference. For over 40 years, the National Heat Transfer Conference has been the premiere forum for the presentation and dissemination of the latest advances in heat transfer. The work contained in these volumes range from studies of fundamental phenomena to applications in the latest heat transfer equipment. The following topics are covered in this volume: fundamentals of convection; turbulent heat transfer; and mixed convection heat transfer. Separate abstracts were prepared for most papers in this volume.

Oosthuizen, P.H.; Chen, T.S.; Acharya, S.; Armaly, B.F.; Pepper, D.W. [eds.

1997-07-01

290

Anomalous heat transport and condensation in convection of cryogenic helium.  

UK PubMed Central (United Kingdom)

When a hot body A is thermally connected to a cold body B, the textbook knowledge is that heat flows from A to B. Here, we describe the opposite case in which heat flows from a colder but constantly heated body B to a hotter but constantly cooled body A through a two-phase liquid-vapor system. Specifically, we provide experimental evidence that heat flows through liquid and vapor phases of cryogenic helium from the constantly heated, but cooler, bottom plate of a Rayleigh-Bénard convection cell to its hotter, but constantly cooled, top plate. The bottom plate is heated uniformly, and the top plate is cooled by heat exchange with liquid helium maintained at 4.2 K. Additionally, for certain experimental conditions, a rain of helium droplets is detected by small sensors placed in the cell at about one-half of its height.

Urban P; Schmoranzer D; Hanzelka P; Sreenivasan KR; Skrbek L

2013-05-01

291

Thermal convection experiments with internal heating  

International Nuclear Information System (INIS)

[en] Experiments have been conducted to measure the effect of an imposed stabilizing or destabilizing temperature difference upon the heat transfer and temperature distribution in an internally heated layer. The experiments thus far conducted have shown that large increases in downward heat transfer can occur with an imposed stabilizing temperature difference. In PAHR applications consideration should be given to the possibility of having upper surfaces consisting of materials other than solidified fuel since large increases in downward heat transfer can occur. Since relatively good agreement has been found between the experimental measurements and the power integral predictions the latter could serve as a method for extrapolating and interpolating the data for PAHR situations

1975-11-13

292

Free convection from a constant heat flux elliptic tube  

Energy Technology Data Exchange (ETDEWEB)

An experimental investigation is presented on free convection of air around the outer surface of a constant heat flux elliptic tube. The local and average Nusselt number distribution is reported for different values of Rayleigh number and different tube inclination angles. The test Rayleigh number, based on input heat flux, ranges from 1.1x10{sup 7} to 8x10{sup 7}. Average Nusselt numbers are evaluated and correlated with Rayleigh number for the elliptic tube with vertical major axis. Comparison between the convection characteristics of isothermal and constant heat flux elliptic tubes has been presented. Also, the effect of elliptic tube orientation on the average Nusselt number is shown. It is found that a higher value of average Nusselt number is achieved when the major axis of the tube is vertical.

Elsayed, Amr O. E-mail: amro9992000@yahoo.com; Ibrahim, Emad Z.; Elsayed, Sayed A

2003-09-01

293

Free convection from a constant heat flux elliptic tube  

International Nuclear Information System (INIS)

An experimental investigation is presented on free convection of air around the outer surface of a constant heat flux elliptic tube. The local and average Nusselt number distribution is reported for different values of Rayleigh number and different tube inclination angles. The test Rayleigh number, based on input heat flux, ranges from 1.1x107 to 8x107. Average Nusselt numbers are evaluated and correlated with Rayleigh number for the elliptic tube with vertical major axis. Comparison between the convection characteristics of isothermal and constant heat flux elliptic tubes has been presented. Also, the effect of elliptic tube orientation on the average Nusselt number is shown. It is found that a higher value of average Nusselt number is achieved when the major axis of the tube is vertical.

2003-01-01

294

Enhancement of convective heat transfer by using microencapsulated PCM slurry  

Energy Technology Data Exchange (ETDEWEB)

As a method to make use of a high latent heat of a liquid-solid phase change material in a confined convective heat transfer, a phase change material, lauric acid, was encapsulated in a tiny hollow sphere, and mixed with water, forming a slurry of microencapsulated phase-change material (MCPCM slurry). Four different sizes (200, 100, 50, and 25 {mu}m ID) of MCPCMs were tested in this study. Thermal characteristics of the pure lauric acid and lauric acid MCPCMs were tested by a differential scanning calorimeter. Viscosity of the slurries of water and the capsules was measured by a rotating viscometer. The slurries were also tested in a convective-heat-transfer-test apparatus. (author). 11 refs., 5 figs., 1 tab.

Choi, E.S. [Myongji University, Yongin (Korea); Yoo, J.H. [Graduate School, Myong Ji University, Yongin (Korea)

1998-11-01

295

Blow-up criterion for two-dimensional heat convection equations with zero heat conductivity  

Directory of Open Access Journals (Sweden)

Full Text Available In this article we obtain a blow-up criterion of smooth solutions to Cauchy problem for the incompressible heat convection equations with zero heat conductivity in $mathbb{R}^2$. Our proof is based on careful H"oder estimates of heat and transport equations and the standard Littlewood-Paley theory.

Yu-Zhu Wang; Zhiqiang Wei

2012-01-01

296

Coupled natural convection and atmospheric wind forced advection in above ground reacting heaps  

Energy Technology Data Exchange (ETDEWEB)

A CFD code CFX-4.1 has been employed to simulate a flow and pressure field around an above ground heap. Due to porosity of the heap material a wind driven forced gas advection field within the heap will result. If the heap material contains oxidizable material as in mine wastes and coal stockpiles, then natural-convection induced by heat of oxidation will also occur. The CFD wind flow model is coupled with a model which simulates the heat and mass transfer within heaps containing oxidizable material. This study shows that even under relatively moderate wind flows the traditional assumption that the heaps are immersed in a static atmosphere becomes questionable. 4 refs., 5 figs.

Anne, R.D.; Pantelis, G. [Australian Nuclear Science and Technology Organization, Menai, NSW (Australia)

1997-07-01

297

Combined free and forced convection flow in a rotating channel with arbitrary conducting walls  

Directory of Open Access Journals (Sweden)

Full Text Available Combined free and forced convection flow of a viscous incompressible electrically conducting fluid in a rotating channel is studied. Analytical solution for the velocity and induced magnetic field is obtained in closed form. Asymptotic behavior of the solution for the velocity and induced magnetic field is analyzed for large values of magnetic parameter M2 and rotation parameter K2 to gain some physical insight into the flow pattern. The expressions for the shear stress and critical Grashof number at both the plates and mass flow rates are also derived. Numerical values of rate of heat transfer at both the plates are obtained with the help of MATLAB software. The numerical values of velocity and induced magnetic field are displayed graphically whereas that of shear stress at the upper plate, mass flow rate, critical Grashof number and rate of heat transfer at both the plates are presented in tabular form for various values of flow parameters.

G.S. Seth; N. Mahto; Md. S. Ansari; R. Nandkeolyar

2010-01-01

298

Convection of a binary fluid saturating a shallow porous cavity subjected to cross heat fluxes  

Science.gov (United States)

In this work, natural convection in a differentially heated binary mixture is studied analytically and numerically. The fluid is subjected to the Soret effect and is contained in a shallow rectangular porous cavity. All four faces are exposed to uniform heat fluxes, opposite faces being heated and cooled, respectively. Analytical solutions for the stream function, temperature and concentration fields are obtained using a parallel flow assumption in the core region of the cavity and an integral form of the energy and constituent equations. Numerical confirmation of the analytical predictions is also obtained. Results are presented first in the presence of a vertical temperature gradient (a {=} 0) for which the solution takes the form of a standard Bénard bifurcation. For this situation, steady bifurcations are either pitchfork or subcritical, depending on the separation parameter varphi and Lewis number Le. The imperfection brought by a horizontal temperature gradient (a {?} 0) to the bifurcation is then investigated. Both the nonlinear analytical model and the numerical solution indicate that, depending on a, varphi and Le, the onset of motion occurs through subcritical bifurcations. The existence of transcritical bifurcations is also demonstrated. The special case where the buoyancy forces induced by the thermal and solutal forces are opposing and of equal intensity (varphi {=} {-}1) is also discussed. For this particular situation, the supercritical Rayleigh number for the onset of convection is predicted on the basis of a linear stability analysis. Multiple steady states near the threshold of convection are found.

Bahloul, A.; Vasseur, P.; Robillard, L.

299

A boundary layer solution of a forced convection problem in a composite channel partly filled with a porous medium  

Energy Technology Data Exchange (ETDEWEB)

Forced convection in a composite region part of which is occupied by a clear fluid and part by a fluid saturated porous medium has recently attracted considerable attention and became a subject for numerous investigations. This interest is due to many important thermal engineering applications related to this problem. Solid matrix heat exchangers, the use of porous materials for heat transfer enhancement, fault zones in geothermal systems and solidification of binary alloys are few to mention in this respect. In this paper a problem of fully developed forced convection in a parallel-plate channel partly filled with a homogeneous porous material is considered. The flow in the porous material is described by nonlinear Brinkman-Forchheimer-extended Darcy equation. Utilizing the boundary layer approach, analytical solutions for the flow velocity, the temperature distribution, as well as for the Nusselt number are obtained.

Kuznetsov, A.V.

1999-07-01

300

Second Law Analysis in Convective Heat and Mass Transfer  

Directory of Open Access Journals (Sweden)

Full Text Available This paper reports the numerical determination of the entropy generation due to heat transfer, mass transfer and fluid friction in steady state for laminar double diffusive convection, in an inclined enclosure with heat and mass diffusive walls, by solving numerically the mass, momentum, species conservation and energy balance equations, using a Control Volume Finite-Element Method. The influences of the inclination angle, the thermal Grashof number and the buoyancy ratio on total entropy generation were investigated. The irreversibilities localization due to heat transfer, mass transfer and fluid friction is discussed for three inclination angles at a fixed thermal Grashof number.

M. Magherbi; H. Abbassi; N. Hidouri; A. Ben Brahim

2006-01-01

 
 
 
 
301

Combined heat and mass transfer by natural convection in a saturated thermally stratified porous medium  

Energy Technology Data Exchange (ETDEWEB)

A numerical study is presented for the combined heat and mass transfer by natural convection adjacent to a vertical surface embedded in a stably thermally stratified, fluid-saturated, low-porosity medium. A wide range of thermal stratification levels is considered in flows with both aiding and opposing buoyant forces. The thermal stratification was shown to have a profound influence on the heat and mass transfer rates and on the direction of flow and transport. The underlying physical phenomena are explained while providing the Nusselt and Sherwood number data.

Angirasa, D.; Peterson, G.P. [Texas A and M Univ., College Station, TX (United States). Dept. of Mechanical Engineering; Pop, I. [Univ. of Cluj (Romania). Faculty of Mathematics

1997-02-21

302

Combined heat and mass transfer by natural convection in a saturated thermally stratified porous medium  

Energy Technology Data Exchange (ETDEWEB)

A numerical study is presented for the combined heat and mass transfer by natural convection adjacent to a vertical surface embedded in a stably thermally stratified, fluid-saturated, low porosity medium. A wide range of thermal stratification levels is considered in flows with both aiding and opposing buoyant forces. The thermal stratification was shown to have a profound influence on the heat and mass transfer rates, and the direction of flow and transport. The underlying physical phenomena are explained while providing the Nusselt and Sherwood number data.

Angirasa, D.; Peterson, G.P. [Texas A and M Univ., College Station, TX (United States). Dept. of Mechanical Engineering; Pop, I. [Univ. of Cluj (Romania). Faculty of Mathematics

1996-12-31

303

Convective flat-plate solar heat collector for cauliflower drying  

UK PubMed Central (United Kingdom)

A forced convective, flat-plate, solar heat collector has been developed and tested for drying cauliflower. Its main components were galvanised iron sheet with black paint, transparent glass over it and a closed duct. Four panels were arranged in parallel to each other and the hot air was transferred to drying chamber by means of forced air blower. Solar time (14:363 h), standard time (+5:30 h), solar declination angle (-7.15 degrees), total sunshine duration (11:48 h) and solar heat collector thermal efficiency were calculated using solar collector theory. Ambient and collector outlet air temperature as well as relative humidity (RH) were measured at an interval of 1 h for 28-day experimental period. The temperature rise of the drying air ranged from a minimum of 1.7 degrees C to a maximum of 13.5 degrees C and the RH reduction was from a minimum of 1.59% to a maximum of 10.5% by passing through the solar collector at a mass flow rate of 4.81 m3 s(-1). The average mid-day thermal efficiency was around 16.5% and the peaks out of this average are due to the inertia effect that keeps the collector temperature almost constant for some minutes when the radiation falls. A linear correlation between the temperature difference from ambient (T-T(a)) and the radiation I, as well as inlet and outlet air temperature and RH were obtained. Cauliflower was blanched for 3 min in boiling water and dipped in sodium chloride, potassium metabisulphite and sodium benzoate for 15 min in 1.0% preservative concentration level before drying cauliflower in solar dryer. The treatments were found to be significantly different (probability P<0.0001) for all preservatives. It was clear that potassium metabisulphite was the best among sodium chloride and sodium benzoate. The relation between moisture ratio and drying time was found and drying constant K was calculated.

Kadam DM; Samuel DVK

2006-02-01

304

Numerical analysis of laminar forced convection in a spherical annulus  

International Nuclear Information System (INIS)

[en] 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

1980-10-24

305

The effect of heat generation in inclined slats on the natural convective heat transfer from an isothermal heated vertical plate  

International Nuclear Information System (INIS)

[en] Natural convective heat transfer from a wide heated vertical isothermal plate with adiabatic surfaces above and below the heated surface has been considered. There are a series of equally spaced vertical thin, flat surfaces (termed 'slats') near the heated surface, these surfaces being, in general, inclined to the heated surface. There is, in general, a uniform heat generation in the slats. The slats are pivoted about their centre-point and thus as their angle is changed, the distance of the tip of the slat from the plate changes. The situation considered is an approximate model of a window with a vertical blind, the particular case where the window is hotter than the room air being considered. The heat generation in the slats in this situation is the result of solar radiation passing through the window and falling on and being absorbed by the slats of the blind. The flow has been assumed to be laminar and steady. Fluid properties have been assumed constant except for the density change with temperature that gives rise to the buoyancy forces. The governing equations have been written in dimensionless form and the resulting dimensionless equations have been solved using a commercial finite-element package. The solution has the following parameters: (1) the Rayleigh number (2) the Prandtl number (3) the dimensionless heat generation rate in the slats per unit frontal area (4) the dimensionless distance of the slat center point (the pivot point) from the surface (5) the dimensionless slat size (6) the dimensionless slat spacing (7) the angle of inclination of the slats. Because of the application that motivated the study, results have only been obtained for a Prandtl number of 0.7. The effect of the other dimensionless variables on the mean dimensionless heat transfer rate from the heated vertical surface has been examined. (author)

2003-01-01

306

Natural convection heat exchangers for solar water heating systems. Technical progress report, February 1, 1996--March 31, 1996  

Energy Technology Data Exchange (ETDEWEB)

This progress report describes the thermodynamic testing and modeling of a thermosyphon heat exchanger used in solar water heating systems. Testing of a four tube-in-shell thermosyphon heat exchanger was performed in two parts. The first portion of the test increased the collector fluid while the storage tank remained isothermal. After the collector fluid temperature was raised to 95 C, the second part of the test allowed the storage tank to gain heat. The test was performed for two collector flow rates. Measured values included collector side forced flow rate, temperature differences across the heat exchanger, vertical temperature distribution in the storage tank, vertical water temperature profile in the heat exchanger, and pressure drop on the thermosyphon side of the heat exchanger. The overall heat transfer coefficient-area product (UA) values obtained confirmed that models which assume UA depends solely on thermosyphon flow rate do not adequately characterize thermosyphon heat exchangers. This is because heat transfer in thermosyphon exchangers occurs in the mixed convection, rather than forced flow, regime. A linear regression equation was developed to better predict UA using the Prandtl, Reynolds, and Grashof numbers and dimensionless parameters based on fluid properties calculated for the average hot and cold leg temperatures. 9 figs.

Davidson, J.H.

1998-06-01

307

Experimental investigation of forced-convection in a finned rhombic tube of the flat-plate solar collectors  

DEFF Research Database (Denmark)

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. The temperature distribution along the finned tube for the fluid and the wall were also illustrated.

Taherian, Hessam; Yazdanshenas, Eshagh

2006-01-01

308

Mixed convection heat transfer processes in ventilated construction elements; Zur Waermeuebertragung bei Mischkonvektion in luftdurchstroemten Bauteilen  

Energy Technology Data Exchange (ETDEWEB)

When condensation effects with initially inexplicable causes occurred during experimental studies on ventilated facades, the team around Prof. Edgar R. F. Winter and the development engineers from the company Gartner, Gundelfingen, decided to investigate this phenomenon. As part of a research project lasting several years, studies on ventilated vertical gaps with rectangular section were therefore carried out with the aim of deriving fundamental statements about the heat transport due to convection in ventilated claddings and windows, both for winter and summer boundary conditions. The results not only contribute to the energetic evaluation of double-skin windows and facades. They are also applicable to the heat transport processes in other technical areas where free, forced or mixed convection occurs. (orig.)

Schwab, A.

2002-12-01

309

Fully developed combined heat and mass transfer natural convection between parallel plates with asymmetric boundary conditions  

Energy Technology Data Exchange (ETDEWEB)

An analysis of fully developed flow between parallel plates is presented for combined heat and mass transfer natural convection with asymmetric boundary conditions. Closed form solutions are obtained for uniform wall temperature and concentraction (UWT/C) and uniform heat and mass fluxes (UH/MF). For UWT/C, the solution is independent of Prandtl, Schmidt and Grashof numbers. For UH/MF, an analytical solution can be obtained only for equal Prandtl and Schmidt numbers. By assuming a purely parabolic velocity profile, an approximate solution for general Prandtl and Schmidt number is obtained that reduces to the analytical solution as a special case. The channel flow rate decreases in proportion to the square root of the Grashof number and depends on the Prandtl and Schmidt numbers as well as the boundary conditions. The highest Nusselt and Sherwood numbers occur for asymmetric boundary conditions, opposite of the results for forced convection duct flows.

Nelson, D.J.; Wood, B.D.

1985-01-01

310

Conjugate problems in convective heat transfer  

CERN Multimedia

The conjugate heat transfer (CHT) problem takes into account the thermal interaction between a body and fluid flowing over or through it, a key consideration in both mechanical and aerospace engineering. Presenting more than 100 solutions of non-isothermal and CHT problems, this title considers the approximate solutions of CHT problems.

Dorfman, Abram S

2009-01-01

311

INTEGRAL TRANSFORM SOLUTION FOR THE FORCED CONVECTION OF HERSCHEL-BULKLEY FLUIDS IN CIRCULAR TUBES AND PARALLEL-PLATES DUCTS  

Directory of Open Access Journals (Sweden)

Full Text Available The thermal entry region in laminar forced convection of Herschel-Bulkley fluids is solved analytically through the integral transform technique, for both circular and parallel-plates ducts, which are maintained at a prescribed wall temperature or at a prescribed wall heat flux. The local Nusselt numbers are obtained with high accuracy in both developing and fully-developed thermal regions, and critical comparisons with previously reported numerical results are performed.

QUARESMA J. N. N.; MACÊDO E. N.

1998-01-01

312

Liquid oil painting: Free and forced convection in an enclosure with mechanical and thermal forcing  

CERN Document Server

A fluid dynamics video is linked to this article, which have been submitted to the Gallery of Fluid Motion as part of the 65th American Physical Society meeting of the Division of Fluid Dynamics, held in San Diego, California, USA, over 17-20 November 2012. The video serves to visualize flows generated in a rectangular enclosure that are subjected to both mechanical and thermal forcing through a common horizontal boundary. This system exhibits features consistent with either horizontal convection or lid-driven cavity flows depending on the ratio between thermal and mechanical stirring, and three different cases are visualized in the linked videos.

Sheard, Gregory J; King, Martin P

2012-01-01

313

Forced and combined convection of water in a vertical seven-rod bundle with P/D = 1. 38  

Energy Technology Data Exchange (ETDEWEB)

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 (Gr{sub q}) from 1.3 x 10{sup 5} to 3 x 10{sup 6}, 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 {plus minus}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: Nu{sub C,L} =(Nu{sub F,L}{sup 3} + Nu{sub N,L}{sup 3}){sup 1/3}, for upflow and Nu{sub C,L}=(Nu{sub F,L}{sup 2} -Nu{sub N,L}{sup 2}){sup 1/2}, for downflow, where Nu{sub C,L}, Nu{sub F,L} and Nu{sub N,L} are the Nusselt number for combined laminar flow, forced laminar flow and natural laminar flow respectively. These correlations are within {plus minus}11 and {plus minus}15% of the upflow and downflow data, respectively. (author).

El-Genk, M.S.; Bedrose, S.D.; Rao, D.V. (New Mexico Univ., Albuquerque, NM (USA). Dept. of Chemical and Nuclear Engineering)

1990-06-01

314

A study on calibration of heat flux sensor by using convective heat transfer  

International Nuclear Information System (INIS)

The objective of this work is to propose calibration facility in which a thin film type heat flux sensor can be calibrated under convective flow condition by using a small wind tunnel with the constant temperature plate condition. A small wind tunnel has been built to produce a boundary layer shear flow above a constant temperature copper plate. 12-independent copper blocks, thin film heaters, insulators and temperature controllers were used to keep the temperature of flat plate constant at a specified temperature. Three commercial thin film-type heat flux sensors were tested. Convective calibrations of these gages were performed over the available heat flux range of 1.4?2.5 kW/m2. The uncertainty in the heat flux measurements in the convective-type heat flux calibration facility was ±2.07%. Non-dimensional sensitivity is proposed to compare the sensitivity calibrated by manufacturer and that of experiment conducted in this study.

2004-01-01

315

Analysis of natural convection heat transfer and flows in internally heated stratified liquid pools  

Energy Technology Data Exchange (ETDEWEB)

In this paper, natural convection flows and heat transfer in a liquid pool, with two superposed immiscible fluid layers, are analyzed. The objective of the study is to examine the effect of interfacial hydrodynamics and to develop a method which enables energy splitting to be evaluated in a stratified liquid pool. The thermal convection, with and without an internal heat source, in a rectangular cavity with different pairs of fluids was numerically simulated by a CFD code FLOW-3D. It was found that the code performs very well for prediction of heat transfer coefficients for different conditions. The hydrodynamic coupling between immiscible layers was found to have minor, if any, impact on the natural convection heat transfer for the conditions examined. Calculated results were used to develop, and validate, a new correlation for energy splitting and for heat transfer in stratified liquid pools.

Gubaidullin, A.A. Jr.; Dinh, T.N.; Sehgal, B.R.

1999-07-01

316

Gating heat transport by manipulating convection in a magnetic nanofluid  

Science.gov (United States)

Gating thermal transport is a key requirement in smart heat exchangers used in a variety of applications such as electronics and energy generation. Here, we demonstrate a high on-off ratio thermal valve using magnetic nanofluids actuated by a non-uniform magnetic field. Using nanofluids comprised of magnetic nanoparticles in paraffin oil, we obtain on-off ratios as high as 16, which is more than 5-fold higher than that seen in comparable nanofluids with uniform magnetic fields. Analysis of these results using heat transfer modeling shows that the remarkable enhancement arises from magneto-thermally activated convection due to field gradients. Such convective thermal gating could be promising for applications.

Seshadri, Indira; Gardner, Alex; Mehta, Rutvik J.; Swartwout, Richard; Keblinski, Pawel; Borca-Tasciuc, Theo; Ramanath, Ganpati

2013-05-01

317

The fouling of heat exchanges in natural convection applications  

Energy Technology Data Exchange (ETDEWEB)

The fouling cycle for heat exchangers in natural convection applications using potable water through compact flat-plate heat exchangers was studied. This arrangement is commonly used in modern solar domestic hot water systems. Fouling was defined as the deposition of unwanted materials on heat transfer surfaces which leads to decreased system performance and increased costs. The causes of heat exchanger fouling and methods of reducing its effects were examined. It was shown that temperature and velocity of the water stream were the two most significant factors affecting the fouling of heat exchangers. The investigators concluded that to minimize fouling the water supply should have a low concentration of inverse soluble salts and maintain a fairly neutral pH. 8 refs., 3 tabs., 4 figs.

Purdy, J.M.; Harrison, S.J. [Queen`s Univ., Kingston, ON (Canada). Dept. of Mechanical Engineering

1998-05-01

318

CFD analysis of the convection flow in the pan during induction heating and gas range heating  

UK PubMed Central (United Kingdom)

One difference between induction heating (IH) and gas range heating (GRH) is that, because the heating location of the pan is different, the convection in the pan is different. In particular, in cooking situations in which solids are mixed with liquids, such as the boiling of soup containing solid foods, the natural convection flow in the pan may affect food quality. The convection in the pan during IH and GRH was analyzed by using computational fluid dynamics (CFD), and the temperature distribution and the flow velocity distribution in the pan were obtained. These results were then compared with the measurement results. In GRH, the heat flux at the wall of the pan was obtained by experiment, and it was used for the analysis as a boundary condition. In IH, the heat generation distribution obtained by electromagnetic field analysis was used for the analysis as the boundary condition. Differences in convection flow in the pan during IH and GRH were observed by CFD modeling especially at the initial stage of heating. The simulated convection in the pan showed the same tendency as the captured movement of the tracer particle using flow visualization.

Kawakami H; Llave Y; Fukuoka M; Sakai N

2013-06-01

319

Computer code CONDIF (release 1) for transient convective-conductive heat transfer (finite element modelling). [LMFBR  

Energy Technology Data Exchange (ETDEWEB)

This report describes the finite element computer code CONDIF developed at J.R.C. Ispra to solve time-dependent natural and forced convection problems in an arbitrary plane or axisymmetric domain. The basic objective in developing CONDIF-01 was to create a flexible tool for the analysis of some aspects of the Post Accident Heat Removal (PAHR) problem following a hypothetical meltdown of a fast-reactor core. Of particular importance in this area is the molten pool cooling problem which requires the analysis of transient conductive-convective heat transfer in a fluid and its enclosing structure. The present report contains three sections. Section 1 deals with the thermal energy equation. A weak variational form of this equation is introduced which forms the basis for spatial discretization using finite elements. Specialized equations are then presented for three-node triangles and four-node quadrilaterals. The explicit time integration scheme is discussed and the combined effects of space and time discretizations are analyzed in detail. Section 2 is concerned with the numerical solution of the incompressible Navier-Stokes equations. Section 3 is devoted to a presentation of numerical applications. Standard problems of natural and forced convection are solved and the solutions obtained with CONDIF are compared to solutions derived in the literature by means of other numerical techniques.

Donea, J.; Giuliani, S.

1980-01-01

320

Measurements in the transition vortex flow regime of mixed convection above a horizontal heated plate  

Energy Technology Data Exchange (ETDEWEB)

Experimental results covering the transition vortex flow regime of mixed convection over a heated, horizontal flat plate are presented. A criterion for the onset of vortex instability as a function of critical Reynolds and Grashof numbers was established with the aid of a flow visualization technique. The three-dimensional nature of tis flow regime was documented through both velocity and temperature measurements using laser-Doppler and hot/cold-wire anemometers, respectively. A higher buoyancy force, through a higher plate temperature or a larger downstream distance and/or a lower free-stream velocity, intensifies the strength of the vortices. Velocity and temperature profiles through vortex peaks and valleys are reported to quantify the behavior of these vortices. It has been found from these measurements that the two-dimensional laminar mixed convection flow changes into a transitional three-dimensional vortex flow in a relatively short distance from the leading edge of the plate. The vortex three-dimensional flow continues to intensify as the buoyancy force increases and then develops into a two-dimensional fully turbulent flow at the end of the transition regime. These findings place an upper limit on the applicabilityof the two-dimensional, laminar boundary layer flow analysis for mixed convection a heated horizontal flat plate.

Moharreri, S.S.; Armaly, B.F.; Chen, T.S. (Univ. of Missouri, Rolla (USA))

1988-05-01

 
 
 
 
321

Experimental study of convective heat transfer of compressed air flow in radially rotating ducts  

Energy Technology Data Exchange (ETDEWEB)

The convective heat transfer of pressurized air flow in radially rotating serpentine channel is investigated experimentally in the present study. The main governing parameters are the Prandtl number, the Reynolds number for forced convection, the rotation number for the Coriolis force induced cross stream secondary flow and the Grashof number for natural convection. To simulate the operation conditions of a real gas turbine, the present study kept the parameters in the test rig approximately the same as those in a real engine. The air in the present serpentine channel was pressurized to increase the air density for making up the low rotational speed in the experiment. Before entering the rotating ducts, the air was also cooled to gain a high density ratio of approximately 1/3 in the ducts. This high density ratio will give a similar order of magnitude of Grashof number in a real operation condition. The local heat transfer rate on the four channel walls are present and compared with that in existing literature.

Hwang, G.J,; Tzeng, S.C.; Mao, C.P.

1999-07-01

322

Estimation of the boundary conditions in convectional heat transfer problems  

Energy Technology Data Exchange (ETDEWEB)

This paper describes the method of estimating boundary quantities in convectional heat transfer problems. There are a lot of possible formulations of inverse problems. Two of them are concerned here: evaluating of boundary heat flux from the internal temperature measurements (inverse formulation of energy equation), and estimation of inlet velocity profile from internal velocity measurements (inverse formulation of momentum equation). The complete algorithm of both formulations are presented. Due to the character of problem (preliminary studies) investigations of stability of solution are not complete. Only partial results are shown.

Szczygiel, I.

1997-07-01

323

Convective Heat and Mass Transfer in Rotating Disk Systems  

CERN Document Server

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

324

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

Science.gov (United States)

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

325

Development of convective heat transfer correlations for common designs of solar dryer  

International Nuclear Information System (INIS)

Highlights: ? Separate experimental methods of hcpf evaluation are proposed for different dryers. ? Correlation for hcpf in terms of dimensionless numbers for each dryer is proposed. ? Single correlation for hcpf representing different dryer designs is also developed. ? Levenberg–Marquardt algorithm is used to develop temperature dependent correlation. ? Close agreement of experimental and predicted hcpf validates proposed correlations. - Abstract: The knowledge of convective heat transfer coefficient hcpf (absorber plate to flowing air) is necessary to predict or evaluate thermal performance of any solar dryer. In order to determine hcpf, laboratory models of direct (cabinet), indirect and mixed mode solar dryer are designed and constructed to perform no-load steady state experiments for natural and forced air circulation. The dryers are operated under indoor simulation conditions for absorbed thermal energy and air flow rate for the range of 300–800 W/m2 and 1–3 m/s, respectively. Separate methods depending on mode of heat utilisation are proposed for determination of hcpf for different dryers. Correlations of hcpf in terms of dimensionless numbers are developed for each dryer operating under natural and forced convection. Levenberg–Marquardt algorithm is used to develop temperature dependent correlations. A close agreement between experimental and predicted hcpf values obtained from proposed correlations for natural convection dryers demonstrates their reliability. However, for forced convection dryers, there is a need to use temperature dependent Nu–Re correlation for more accurate results. The low uncertainty ranging from 0.3% to 0.8% in the determination of hcpf confirms the accuracy of experimental data obtained for various dryer designs operated under different conditions.

2012-01-01

326

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

International Nuclear Information System (INIS)

[en] 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

1980-07-30

327

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 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.

2011-01-01

328

Heat transport measurements in turbulent rotating Rayleigh-Benard convection  

Energy Technology Data Exchange (ETDEWEB)

We present experimental heat transport measurements of turbulent Rayleigh-Benard convection with rotation about a vertical axis. The fluid, water with Prandtl number ({sigma}) about 6, was confined in a cell which had a square cross section of 7.3 cm x 7.3 cm and a height of 9.4 cm. Heat transport was measured for Rayleigh numbers 2 x 10{sup 5} < Ra < 5 x 10{sup 8} and Taylor numbers 0 < Ta < 5 x 10{sup 9}. We show the variation of normalized heat transport, the Nusselt number, at fixed dimensional rotation rate {Omega}{sub D}, at fixed Ra varying Ta, at fixed Ta varying Ra, and at fixed Rossby number Ro. The scaling of heat transport in the range 10{sup 7} to about 10{sup 9} is roughly 0.29 with a Ro dependent coefficient or equivalently is also well fit by a combination of power laws of the form a Ra{sup 1/5} + b Ra{sup 1/3} . The range of Ra is not sufficient to differentiate single power law or combined power law scaling. The overall impact of rotation on heat transport in turbulent convection is assessed.

Ecke, Robert E [Los Alamos National Laboratory; Liu, Yuanming [Los Alamos National Laboratory

2008-01-01

329

Analysis of mixed convection flows within a square cavity with uniform and non-uniform heating of bottom wall  

Energy Technology Data Exchange (ETDEWEB)

A penalty finite element analysis with bi-quadratic elements is performed to investigate the influence of uniform and non-uniform heating of bottom wall on mixed convection lid driven flows in a square cavity. In the present investigation, bottom wall is uniformly and non-uniformly heated while the two vertical walls are maintained at constant cold temperature and the top wall is well insulated and moving with uniform velocity. A complete study on the effect of Gr shows that the strength of circulation increases with the increase in the value of Gr irrespective of Re and Pr. As the value of Gr increases, there occurs a transition from conduction to convection dominated flow at Gr=5.10{sup 3} and Re=1 for Pr=0.7. A detailed analysis of flow pattern shows that the natural or forced convection is based on both the parameters Ri (Gr/Re{sup 2}) and Pr. As the value of Re increases from 1 to 10{sup 2}, there occurs a transition from natural convection to forced convection depending on the value of Gr irrespective of Pr. Particularly for higher value of Grashof number (Gr=10{sup 5}), the effect of natural convection is dominant up to Re=10 and thereafter the forced convection is dominant with further increase in Re. As Pr increases from 0.015 to 10 for a fixed Re and Gr (Gr=10{sup 3}), the inertial force gradually becomes stronger and the intensity of secondary circulation gradually weakens. The local Nusselt number (Nub) plot shows that the heat transfer rate is very high at the edges of the bottom wall and then decreases at the center of the bottom wall for the uniform heating and that contrasts lower heat transfer rate at the edges for the non-uniform heating of the bottom wall. It is also observed that Nu{sub l} shows non-monotonic behavior with both uniform and non-uniform heating cases for Re=10 at higher value of Pr. The average Nusselt number plot for the left or right wall shows a kink or inflexion at Gr=10{sup 4} for highest value of Pr. Thus the overall power law correlation for average Nusselt number may not be obtained for mixed convection effects at higher Pr. (authors)

Basak, T. [Indian Institute of Technology Madras, Dept. of Chemical Engineering, Chennai (India); Roy, S.; Kumar Sharma, P. [Indian Institute of Technology Madras, Dept. of Mathematics, Chennai (India); Pop, I. [Cluj, Univ., Faculty of Mathematics (Romania)

2009-05-15

330

Analysis of mixed convection flows within a square cavity with uniform and non-uniform heating of bottom wall  

International Nuclear Information System (INIS)

[en] A penalty finite element analysis with bi-quadratic elements is performed to investigate the influence of uniform and non-uniform heating of bottom wall on mixed convection lid driven flows in a square cavity. In the present investigation, bottom wall is uniformly and non-uniformly heated while the two vertical walls are maintained at constant cold temperature and the top wall is well insulated and moving with uniform velocity. A complete study on the effect of Gr shows that the strength of circulation increases with the increase in the value of Gr irrespective of Re and Pr. As the value of Gr increases, there occurs a transition from conduction to convection dominated flow at Gr=5.103 and Re=1 for Pr=0.7. A detailed analysis of flow pattern shows that the natural or forced convection is based on both the parameters Ri (Gr/Re2) and Pr. As the value of Re increases from 1 to 102, there occurs a transition from natural convection to forced convection depending on the value of Gr irrespective of Pr. Particularly for higher value of Grashof number (Gr=105), the effect of natural convection is dominant up to Re=10 and thereafter the forced convection is dominant with further increase in Re. As Pr increases from 0.015 to 10 for a fixed Re and Gr (Gr=103), the inertial force gradually becomes stronger and the intensity of secondary circulation gradually weakens. The local Nusselt number (Nub) plot shows that the heat transfer rate is very high at the edges of the bottom wall and then decreases at the center of the bottom wall for the uniform heating and that contrasts lower heat transfer rate at the edges for the non-uniform heating of the bottom wall. It is also observed that Nul shows non-monotonic behavior with both uniform and non-uniform heating cases for Re=10 at higher value of Pr. The average Nusselt number plot for the left or right wall shows a kink or inflexion at Gr=104 for highest value of Pr. Thus the overall power law correlation for average Nusselt number may not be obtained for mixed convection effects at higher Pr. (authors)

2009-01-01

331

Forced convection of radiating gas over an inclined backward facing step using the blocked-off method  

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; Gandjalikhan Nassaba Seyyed Abdolreza

2013-01-01

332

Numerical predictions of natural convection in a uniformly heated pool  

International Nuclear Information System (INIS)

In the event of a core meltdown accident, one of the accident progression paths is fuel relocation to the lower reactor plenum. In the heavy water new production reactor (NPR-HWR) design the reactor cavity is flooded with water. In such a design, decay heat removal to the water in the reactor cavity and thence to the containment may be adequate to keep the reactor vessel temperature below failure limits. If this is the case, the accident progression can be arrested by retaining a coolable corium configuration in the lower reactor plenum. The strategy of reactor cavity flooding to prevent reactor vessel failure from molten corium relocation to the reactor vessel lower head has also been considered for commercial pressurized water reactors. Previously, the computer code COMMIX-LAR/P was used to determine if the heat removal rate from the molten cerium in the lower plenum to the water in the cavity was adequate to keep the reactor vessel temperature in the NPR-HWR design below failure limits. It was found that natural convection in the molten pool resulted in heat removal rates that kept the peak reactor vessel temperature about 400 degrees C below the steel melting point. The objective of the work presented in this paper was to determine whether COMMIX adequately predicts natural convection in a pool heated by a uniform heat source. For this purpose, the experiments of free convection in a semicircular cavity of Jahn and Reeneke were analyzed with COMMIX and code predictions were compared with experimental measurements. COMMIX is a general purpose thermalhydraulics code based on finite differencing by the first order upwind scheme.

1993-01-01

333

Natural convection in an enclosure with localized heating from below  

Energy Technology Data Exchange (ETDEWEB)

Natural convection heat transfer in a cylindrical enclosure, heated partially from below by a disk shaped heating surface and cooled from the top and the side, was investigated experimentally and numerically. Heat transfer measurements are presented for Rayleigh numbers ranging from 10{sup 8} to 2 {times} 10{sup 10} with water as the test fluid. The test results were correlated by an equation of the form Nu{sub D} = 0.16 Ra{sub D}{sup 1/3} for a Prandtl number of 2 and aspect ratio of 1.01. The flow field was numerically analyzed using FLUENT code. The {kappa}-{epsilon} model for predicting the turbulent flows with buoyancy was used.

Ulucakli, M.E. [Lafayette Coll., Easton, PA (United States). Dept. of Mechanical Engineering

1996-12-31

334

Mixed convection heat and mass transfer in radially rotating rectangular ducts  

Energy Technology Data Exchange (ETDEWEB)

Heat transfer in rotating ducts is encountered in many engineering applications, such as cooling of turbomachinery, gas turbines, and other rotating systems. The present work investigates mixed convection heat and mass transfer in the entrance region of radially rotating rectangular ducts with water film evaporation along the porous duct walls. Mechanisms of secondary vortex development in the ducts under various conditions are examined by a vorticity-velocity numerical method. Emphasis is placed on the rotation effects, including both Coriolis and centrifugal buoyancy forces, and the mass diffusion effect on the flow structure and heat transfer characteristics. Results are presented in particular for an air-water vapor system under various conditions. Predicted results show that the effects of liquid film evaporation along the porous duct walls on the mixed convection heat transfer are rather substantial. The magnitude of the evaporative latent heat transfer may be 10 times greater than that of sensible heat transfer. The predictions also demonstrate that the distributions of Nu, Sh{sub z}, and fRe are closely related to the emergence, disappearance, growth, and decay of the rotating-induced secondary vortices. Additionally, a higher Nu{sub z} is found for a rectangular duct with a larger aspect ratio ({gamma} = 2) due to the relatively stronger secondary flows.

Lee, K.T. [Oriental Inst. of Tech., Taipei (Taiwan, Province of China). Dept. of Mechanical Engineering; Yan, W.M. [Huafan Univ., Taipei (Taiwan, Province of China). Dept. of Mechanical Engineering

1998-11-27

335

Development and performance evaluation of forced convection potato solar dryer  

International Nuclear Information System (INIS)

This research paper deals with the design development and testing of a forced convection solar dryer, for drying and converting to flour of high moisture content vegetables like potatoes. The angle of solar collector was made adjustable for the absorption of maximum solar radiation by the absorber plate. The air flow rate was controlled by adjustable gate valve to find the optimum flow rate for dehydration of the product. The penetration of solar radiation raised the temperature of the absorber plate of the dryer to 110 deg. C during the operation under stagnation or no load conditions. The maximum air temperature attained in the solar air heater, under this condition was 80 deg. C. The dryer was loaded with 12 Kg of blanched potato chips having an initial moisture content of 89.75%, and the final desired moisture content of 6.95% was achieved within five hours without losing the color of potato chips, while the moisture contents reduction was from 89.75% to 33.75% for five hours in open sun drying under shade. The drying cost for 1 Kg of potatoes was calculated as Rs. 245 and it was Rs. 329 in the case of an electric dryer. The life span of the solar dryer was assumed to be 20 years. The cumulative present worth of annual savings over the life of the solar dryer was calculated for blanched potato chips drying, and it turned out be Rs.163177.67/- which was much higher than the capital cost of the dryer (Rs. 25000). The payback period was calculated as 0.89 years, which was also very small considering the life of the system (20 years). (author)

2011-01-01

336

Thermo capillary and buoyancy convection in a fluid locally heated on its free surface; Convection thermocapillaire et thermogravitaire dans un fluide chauffe localement sur sa surface libre  

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

337

On free convection heat transfer with well defined boundary conditions  

Energy Technology Data Exchange (ETDEWEB)

The scaling of free convection heat transfer is investigated. The non-dimensional groups for Boussinesq and fully compressible variable property free convection, driven by isothermal surfaces, are derived using a previously published novel method of dimensional analysis. Both flows are described by a different set of groups. The applicability of each flow description is experimentally investigated for the case of the isothermal horizontal cylinder in an air-filled isothermal enclosure. The approach taken to the boundary conditions differs from that of previous investigations. Here, it is argued that the best definition of the boundary conditions is achieved for heat exchange between the cylinder and the enclosure rather than the cylinder and an arbitrarily chosen fluid region. The enclosure temperature is shown both analytically and experimentally to affect the Nusselt number. The previously published view that the Boussinesq approximation has only a limited range of application is confirmed, and the groups derived for variable property compressible free convection are demonstrated to be correct experimentally. A new correlation for horizontal cylinder Nusselt number prediction is presented.

Davies, M.R.D.; Newport, D.T.; Dalton, T.M.

1999-07-01

338

Confinement-Induced Heat-Transport Enhancement in Turbulent Thermal Convection  

Science.gov (United States)

We report an experimental and numerical study of the effect of spatial confinement in turbulent thermal convection. It is found that when the width of the convection cell is narrowed, the heat-transfer efficiency increases significantly despite the fact that the overall flow is slowed down by the increased drag force from the sidewalls. Detailed experimental and numerical studies show that this enhancement is brought about by the changes in the dynamics and morphology of the thermal plumes in the boundary layers and in the large-scale flow structures in the bulk. It is found that the confined geometry produces more coherent and energetic hot and cold plume clusters that go up and down in random locations, resulting in more uniform and thinner thermal boundary layers. The study demonstrates how changes in turbulent bulk flow can influence the boundary layer dynamics and shows that the prevalent mode of heat transfer existing in larger aspect ratio convection cells, in which hot and cold thermal plumes are carried by the large-scale circulation along opposite sides of the sidewall, is not the most efficient way for heat transport.

Huang, Shi-Di; Kaczorowski, Matthias; Ni, Rui; Xia, Ke-Qing

2013-09-01

339

Inverse convection problem for determining wall heat flux in annular duct flow  

Energy Technology Data Exchange (ETDEWEB)

An inverse problem for unsteady forced convection in an annular duct is presented. The simulated temperature data taken at the outer wall are used to estimate the time-dependent, axially-varying surface heat flux distribution at the inner wall of the annular passage. No prior information on the functional form of the unknown wall heat flux is needed in the inverse method. The effects of the functional form of the wall heat flux and the measurement errors on the accuracy of the estimation are investigated. It is shown that the reconstruction of the timewise and spatial variations of the wall heat flux is satisfactory even if the estimated function is not smooth.

Li, H.Y.; Yan, W.M.

1999-07-01

340

Local instantaneous convective heat transfer characteristics of radial reattaching nozzles  

Energy Technology Data Exchange (ETDEWEB)

An experimental investigation of pulsating radial reattaching (PRJR) nozzles was made to determine local instantaneous heat flux and surface temperature characteristics for different geometric, flow, and pulsation conditions. Local instantaneous heat flux and surface temperature measurements were made using a heat flux microsensor with a time constant of 6 microseconds. The experimental apparatus consisted of air flowing through an annular feeding tube, which exited via a mechanically pulsed nozzle diverter and impinged upon a heated plate at a specified distance below. cases were run which varied the gap based Reynolds number 1,936 to 3,983, pulsation rate 5 to 21 Hz, Strouhal number 0.02 to 0.09, non dimensional nozzle-to-plate ratio 1.161 to 1.3, gap height variation to nozzle radius ratio of 0.31, and exit angle 0 and 20{degree}. The heated plate onto which the flow impinged was supplied with a constant heat flux of 4.0 kW/m{sup 2}. Local instantaneous convective heat transfer coefficients varied by as much as 40% when compared to local time average coefficients. Fast Fourier transform analysis identified that the dominant frequency observed in power spectrum plots of heat flux corresponded to the pulsation frequency with other subharmonics also present. Also the data showed a phase lag in local heat transfer and surface temperature minima and maxima as a function of radial distance from the nozzle centerline.

Furlow, J.S.; James, D.L.

1999-07-01

 
 
 
 
341

Combined free and forced convective hydromagnetic flow through a porous channel with Hall and wall effects  

International Nuclear Information System (INIS)

An investigation of the hydromagnetic free and forced convection in a parallel plate channel formed by two finitely conducting parallel porous walls taking Hall effects into account, the liquid being permeated by a transverse magnetic field, is made. There is uniform axial temperature variation along the walls. An exact solution of the governing equations is obtained. The flow phenomenon has been characterized by the non-dimensional numbers like M2 (Hartmann number), G(Grashof number), ?sub(o)(suction parameter), thetasub(1), thetasub(2) (conductance parameters) and m (Hall parameter). The induced magnetic field and the heat transfer characteristcs in the flow are also determined. Expressions for the shearing stress components have also been sought. The effect of Hall parameter on the velocity, the induced magnetic field and shearing stress is interpreted with the aid of graphs and a table. (author).

1980-01-01

342

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

Energy Technology Data Exchange (ETDEWEB)

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.

Domanus, H.M.; Sha, W.T.

1981-01-01

343

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

International Nuclear Information System (INIS)

[en] 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

1980-07-30

344

Entropy Generation Due to Laminar Incompressible Forced Convection Flow Through Parallel-Plates Microchannel  

Directory of Open Access Journals (Sweden)

Full Text Available 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 the microchannel is found to decrease as Knudsen number increases, and it is found to increase as Reynolds, Prandtl, Eckert numbers and the nondimensional temperature difference increase. The contribution of the viscous dissipation in the total entropy generation increases as Knudsen number increases over wide ranges of the flow controlling parameters.

Osamah Haddad; Mohammad Abuzaid; Mohammad Al-Nimr

2004-01-01

345

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

International Nuclear Information System (INIS)

[en] 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

1981-01-01

346

A new model for forced-convection condensation on integral-fin tubes  

Energy Technology Data Exchange (ETDEWEB)

Integral-fin tubes are extensively used in shell-and-tube condensers for refrigeration. This work investigates the effects of vapor shear during pure vapor external condensation on horizontal integral-fin tubes. More than 220 experimental data-points in a wide range of operative conditions and enhanced surface geometries are reported together with the visual observation of the condensate flow patterns. The effects of vapor shear are relevant only for vapor Reynolds numbers greater than 70,000--100,000, while heat transfer enhancement is linked to the geometry of the extended surface. A simple semi-empirical equation was developed to account for the shear stress contribution in forced-convection condensation: this equation, applied in conjunction with the model by Briggs and Rose (1994) for stationary vapor condensation, displays a good ability in reproducing all the available data with relevant vapor velocities.

Cavallini, A.; Doretti, L.; Klammsteiner, N.; Longo, G.A.; Rossetto, L. [Univ. di Padova (Italy). Ist. di Fisica Tecnica

1995-12-31

347

Development of a Forced-Convection Liquid-Fluoride-Salt Test Loop  

Energy Technology Data Exchange (ETDEWEB)

A small forced-convection molten-fluoride-salt loop is being constructed at Oak Ridge National Laboratory to examine the heat transfer behavior of FLiNaK salt in a heated pebble bed. Objectives of the experiment include reestablishing infrastructure needed for fluoride-salt loop testing, developing a unique inductive heating technique for performing heat transfer (or other) experiments, measuring heat transfer characteristics in a liquid-fluoride-salt-cooled pebble bed, and demonstrating the use of silicon carbide (SiC) as a structural component for salt systems. The salt loop will consist of an Inconel 600 piping system, a sump-type pump, a SiC test section, and an air-cooled heat exchanger, as well as auxiliary systems needed to pre-heat the loop, transport salt into and out of the loop, and maintain an inert cover gas over the salt. A 30,000 Hz inductive heating system will be used to provide up to 250 kW of power to a 15 cm diameter SiC test section containing a packed bed of 3 cm graphite spheres. A SiC-to-Inconel 600 joint will use a conventional nickel/grafoil spiral wound gasket sandwiched between SiC and Inconel flanges. The loop system can provide up to 4.5 kg/s of salt flow at a head of 0.125 MPa and operate at a pressure just above atmospheric. Pebble Reynolds numbers of up to 2600 are possible with this configuration. A sump system is provided to drain and store the salt when not in use. Instrumentation on the loop will include pressure, temperature, and flow measurements, while the test section will be instrumented to provide pebble and FLiNaK temperatures.

Yoder Jr, Graydon L [ORNL; Wilson, Dane F [ORNL; Peretz, Fred J [ORNL; Wilgen, John B [ORNL; Romanoski, Glenn R [ORNL; Kisner, Roger A [ORNL; Holcomb, David Eugene [ORNL; Heatherly, Dennis Wayne [ORNL; Aaron, Adam M [ORNL

2010-01-01

348

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

Digital Repository Infrastructure Vision for European Research (DRIVER)

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.

D. H. Riahi

349

The Role of the Velocity Gradient in Laminar Convective Heat Transfer through a Tube with a Uniform Wall Heat Flux  

Science.gov (United States)

|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…

Wang, Liang-Bi; Zhang, Qiang; Li, Xiao-Xia

2009-01-01

350

The Role of the Velocity Gradient in Laminar Convective Heat Transfer through a Tube with a Uniform Wall Heat Flux  

Science.gov (United States)

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…

Wang, Liang-Bi; Zhang, Qiang; Li, Xiao-Xia

2009-01-01

351

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

CERN Multimedia

In this paper, a 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 forcing term to the MRT-LB equation of the flow field in the moment space. The proposed MRT-LB model is validated by numerical simulations of several two-dimensional convection problems in porous media. The numerical results predicted by the present MRT-LB model agree well with those reported in the literature.

Liu, Q; Li, Q

2013-01-01

352

Convective heat-transfer enhancement by a corona discharge  

Energy Technology Data Exchange (ETDEWEB)

This paper reports on an experimental study conducted to examine heat-transfer enhancement utilizing a corona discharge in a channel flow. A series of high- potential wire electrodes equally spaced were located along the center line of the channel at angles to the primary-flow direction. A theoretical analysis was performed on the electric flow and temperature fields, taking account of the interfield interactions. First, the combined flow field, where a secondary cellular flow occurred due to the interaction between the primary flow and the ionic wind, was characterized in conjunction with the primary-flow velocity and the applied voltage. Second, the enhancement of convective heat transfer was investigated, especially at low Reynolds numbers, and its mechanism was clarified. On the basis of these results, the heat-transfer performance was presented as a function of the amount of pumping power and the corona discharge power.

Takimoto, A.; Tada, Y.; Hayashi, Y. (Dept. of Mechanical Systems Engineering, Kanazawa Univ. (JP)); Yamada, K. (Sumitomo Electric Industry Co., Ltd. (JP))

1991-01-01

353

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

354

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

355

Free convection film boiling heat transfer from a rotating surface  

Energy Technology Data Exchange (ETDEWEB)

A boundary layer model of laminar, subcooled, free convection film boiling from a rotating sphere has been developed. The conservation equations for the vapor and liquid were simplified, transformed into ordinary differential equations using an integral approach, and solved numerically. The theoretical variation of vapor film thickness with heater temperature and the resulting boiling fluxes were investigated. An experimental facility was built for the purpose of verifying the validity of the theoretical model and good agreement was found between the model and the experimental data at low rpm. The instability of the vapor film near the minimum heat flux for a rotating surface flux was also investigated.

Orozco, J.; Francisco, H. (Florida International Univ., Miami, FL (United States))

1992-08-01

356

Free and forced convective cooling of pipe-type electric cables. Volume 1: forced cooling of cables. Final report  

Energy Technology Data Exchange (ETDEWEB)

A multi-faceted research program has been performed to investigate in detail several aspects of free and forced convective cooling of underground electric cable systems. There were two main areas of investigation. The first one reported in this volume dealt with the fluid dynamic and thermal aspects of various components of the cable system. In particular, friction factors for laminar flow in the cable pipes with various configurations were determined using a finite element technique; the temperature distributions and heat transfer in splices were examined using a combined analytical numerical technique; the pressure drop and heat transfer characteristics of cable pipes in the transitional and turbulent flow regime were determined experimentally in a model study; and full-scale model experimental work was carried out to determine the fluid dynamic and thermal characteristics of entrance and exit chambers for the cooling oil. The second major area of activity, reported in volume 2, involved a feasibility study of an electrohydrodynamic pump concept utilizing a traveling electric field generated by a pumping cable. Experimental studies in two different configurations as well as theoretical calculations showed that an electrohydrodynamic pump for the moving of dielectric oil in a cable system is feasible.

Chato, J.C.; Crowley, J.M.

1981-05-01

357

Mixed convection film condensation from downward flowing vapors onto a sphere with uniform wall heat flux  

Energy Technology Data Exchange (ETDEWEB)

A model is developed for the study of mixed convection film condensation from downward flowing vapors onto a sphere with uniform wall heat flux. The model combined natural convection dominated and forced convection dominated film condensation, including effects of pressure gradient and interfacial vapor shear drag has been investigated and solved numerically. The separation angle of the condensate film layer, {phi}{sub s} is also obtained for various pressure gradient parameters, P{sup *} and their corresponding dimensionless Grashof `s parameters, Gr{sup *}. Besides, the effect of P{sup *} on the dimensionless mean heat transfer, Nu(Re){sup -1/2} will remain almost uniform with increasing P{sup *} until P{sup *}=2/9Gr{sup *} for various corresponding available values of Gr{sup *}. Meanwhile, the dimensionless mean heat transfer, Nu(Re){sup -1/2} is increasing significantly with Gr{sup *} for its corresponding available values of P{sup *}. For pure natural-convection film condensation, Nu(Re{sub w}/Gr){sup 1/3}=1.706 is obtained. (orig.). With 7 figs. [Deutsch] Es wird ein Modell zur Untersuchung der Mischkonvektion bei Filmkondensation von Daempfen an einer Kugel entwickelt, die unter gleichfoermigen Waermefluss daran abwaerts stroemen. Das Modell verbindet die durch natuerliche und durch erzwungene Konvektion bewirkte Filmkondensation unter Einschluss von aus einem Druckgradienten resultierenden Effekten sowie von Dampfschubspannungen an der Phasengrenze. Die numerische Loesung liefert den Separationswinkel des Kondensatfilms {Phi}{sub s} fuer verschiedene Druckgradienten-Parameter P{sup *} und zugehoerige Grashof-Parameter Gr{sup *}. Der Einfluss von P{sup *} auf den mittleren Waermeuebergangsparameter Nu(Re){sup -1/2} bleibt bis P{sup *}=2/9Gr{sup *} ziemlich gering, auch wenn Gr{sup *} zwischen 0.01 und 100 variiert. Fuer reine natuerliche Filmkondensation erhaelt man: Nu(Re{sub w}/Gr){sup 1/3}=1.706. (orig.)

Hsu, C.H. [Department of Mold and Die Engineering, National Kaohsiung Institute of Technology, Kaohsiung (Taiwan, Province of China); Yang, S.A. [Department of Mold and Die Engineering, National Kaohsiung Institute of Technology, Kaohsiung (Taiwan, Province of China)

1997-06-01

358

Mixed Convection Heat Transfer on the Outside of a Vertical Cylinder  

International Nuclear Information System (INIS)

[en] An experimental study was made of turbulent heat transfer from a vertical cylinder placed in a square channel. The flow medium was water flowing upwards. Basic differential equations governing the mixed flow heat transfer phenomena in a vertical annulus are presented. A dimensional analysis is done to find the dimensionless variables affecting the relative magnitude of the effect of buoyancy on forced convection heat transfer. Dimensionless equations correlating the experimental data ana incorporating a buoyancy parameter of the form Gr/Re2 are presented. Reynolds number range covered is 690 to 129,500 and the Rayleigh num- ber range covered is 109 to 4.2 x 1013 . Effect of different length parameters, like hydraulic diameter and distance of the measuring point from the inlet of the test section, on dimensionless equations are studied

1965-01-01

359

Heat transport measurements in turbulent rotating Rayleigh-Benard convection.  

UK PubMed Central (United Kingdom)

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)heat transport, the Nusselt number, 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.

Liu Y; Ecke RE

2009-09-01

360

Forced and Free Convection Flow with Viscous Dissipation Effects: The Method of Parametric Differentiation.  

Science.gov (United States)

Effect of buoyancy force in a laminar uniform forced convection flow past a semi-infinite vertical plate has been analyzed near the leading edge, taking into account the viscous dissipation. The coupled non-linear locally similar equations, which govern t...

M. A. Hossain O. Arbad

1988-01-01

 
 
 
 
361

The role of the velocity gradient in laminar convective heat transfer through a tube with a uniform wall heat flux  

International Nuclear Information System (INIS)

[en] 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 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

2009-01-01

362

The role of the velocity gradient in laminar convective heat transfer through a tube with a uniform wall heat flux  

Energy Technology Data Exchange (ETDEWEB)

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 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.

Wang Liangbi; Zhang Qiang; Li Xiaoxia [Department of Mechanical Engineering, Lanzhou Jiaotong University, Lanzhou, Gansu 730070 (China)], E-mail: lbwang@mail.lzjtu.cn, E-mail: zhangq@mail.lzjtu.cn, E-mail: lixx@mail.lzjtu.cn

2009-07-15

363

The structure of convection in the spherical mantle with internal heating  

Science.gov (United States)

Thermal convection in the mantle is caused by the heat transported upwards from the core and by the heat produced by the internal radioactive sources. According to the data on the heat transfer by the mantle plumes and geochemical evidence, only 20% of the total heat of the Earth is supplied to the mantle from the core, whereas most of the heat is generated by the internal sources. Along with the models that correctly allow for the internal heat sources, there are also many publications (including monographs) on the models of mantle convection that completely ignore the internal heating or the heat flux from below. In this study, we analyze to what extent these approximations could be correct. The analytical distributions of temperature and heat flux in the case of internal heating without convection and the results of the numerical modeling for convection with different intensity are presented. It is shown that the structure of thermal convection is governed by the distribution of the heat flux in the mantle but not by the heat balance, as it is typically implicitly assumed in most works. Heat production by the internal sources causes the growth of the heat flux as a function of radius. However, in the spherical mantle of the Earth, the heat flux decreases with radius due to the geometry. It turned out that with the parameters of the present Earth, both these effects compensate each other to a considerable extent, and the resulting heat flux turns out to be nearly constant as a function of radius. Since the structure of the convective flows in the mantle is determined by the distributions of heat flux and total heat flux, in the Cartesian models of the mantle convection the effective contribution of internal heating is small, and ignoring the heat flux from the core significantly distorts the structure of the convective currents and temperature distributions in the mantle.

Trubitsyn, V. P.; Evseev, A. N.; Evseev, M. N.; Evseeva, A. V.

2013-09-01

364

Modeling a forced to natural convection boiling test with the program LOOP-W. [LMFBR  

Energy Technology Data Exchange (ETDEWEB)

Extensive testing has been conducted in the Simulant Boiling Flow Visualization (SBFV) loop in which water is boiled in a vertical transparent tube by circulating hot glycerine in an annulus surrounding the tube. Tests ranged from nonboiling forced convection to oscillatory boiling natural convection. The program LOOP-W has been developed to analyze these tests. This program is a multi-leg, one-dimensional, two-phase equilibrium model with slip between the phases. In this study, a specific test, performed at low power where non-boiling forced convection was changed to boiling natural convection and then to non-boiling again, has been modeled with the program LOOP-W.

Carbajo, J.J.

1984-01-01

365

Interaction of mixed convection in two-sided lid driven differentially heated square enclosure with radiation in presence of participating medium  

Energy Technology Data Exchange (ETDEWEB)

The current study addresses the mathematical modeling aspects of transport phenomena in steady, two-dimensional, laminar flow accompanied by heat transfer in a lid-driven differentially heated cavity in presence of radiatively absorbing, emitting and scattering gray medium. The walls of the enclosure are considered to be opaque, diffusive and gray. Mixed convection is the outcome of the interaction of forced convection induced by the moving vertical hot and cold wall with the natural convection induced due to the differentially heated enclosure. Two different orientations of the wall movement have been considered to simulate opposing and aiding mixed convection phenomenon and to study its interaction with radiation. Vorticity-stream function formulation of N-S equation has been employed. The discrete ordinate method has been used in modeling the radiative transport equation followed with finite volume method as discretisation technique. The effect of influencing parameters on fluid flow and heat transfer has been studied. (orig.)

Mahapatra, S.K.; Nanda, P. [University College of Engineering, Mechanical Engineering Department, Burla, Orissa (India); Sarkar, A. [Jadavpur University, Mechanical Engineering Department, Kolkata (India)

2006-06-15

366

Conjugate mixed convection heat and mass transfer in brick drying  

Energy Technology Data Exchange (ETDEWEB)

In this study, a numerical methodology for the solution of conjugate heat and mass transfer problem is presented. Fluid flow, heat and mass transfer over a rectangular brick due to transient laminar mixed convection has been numerically simulated. The coupled non-linear partial differential equations, for both gas phase and solid are solved using finite element procedure. Flow is assumed to be incompressible, two-dimensional, laminar. Analysis has been carried out at a Reynolds number of 200 with Pr = 0.71. The effect of buoyancy on the brick drying has been investigated. Velocity vectors, streamlines in the flow field and temperature and moisture contours and temperature distribution along the solid surface are presented. It is observed that there is considerable effect of buoyancy during drying. The results indicate a non-uniform drying of the brick with the leading edge drying faster than the rest of the brick. (orig.)

Suresh, H.N.; Aswatha Narayana, P.A. [Dept. of Applied Mechanics, I.I.T., Madras (India); Seetharamu, K.N. [School of Mechanical Engineering, Universiti of Sains, Tronoh (Malaysia)

2001-04-01

367

Comparison between one-dimensional uncoupled and convection-conduction conjugated approaches in finned surface heat transfer  

Scientific Electronic Library Online (English)

Full Text Available Abstract in english 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, (more) 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.; Zaparoli, Edson L.

2000-01-01

368

Comparison between one-dimensional uncoupled and convection-conduction conjugated approaches in finned surface heat transfer  

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.

Cláudia R. Andrade; Edson L. Zaparoli

2000-01-01

369

Buoyancy and wall conduction effects on forced convection of micropolar fluid flow along a vertical slender hollow circular cylinder  

Energy Technology Data Exchange (ETDEWEB)

This paper presents a numerical analysis of the flow and heat transfer characteristics of forced convection in a micropolar fluid flowing along a vertical slender hollow circular cylinder with wall conduction and buoyancy effects. The non-linear formulation governing equations and their associated boundary conditions are solved using the cubic spline collocation method and the finite difference scheme with a local non-similar transformation. This study investigates the effects of the conjugate heat transfer parameter, the Richardson number, the micropolar parameter, and the Prandtl number on the flow and the thermal fields. The effect of wall conduction on the thermal and the flow fields are found to be more pronounced in a system with a greater buoyancy effect or Prandtl number but is less sensitive with a greater micropolar material parameter. Compared to the case of pure forced convection, buoyancy effect is found to result in a lower interfacial temperature but higher the local heat transfer rate and the skin friction factor. Finally, compared to Newtonian fluid, an increase in the interfacial temperature, a reduction in the skin friction factor, and a reduction in the local heat transfer rate are identified in the current micropolar fluid case. (author)

Chang, Cheng-Long [Department of Mechanical Engineering, Hsiuping Institute of Technology, Dali, Taichung 41280, Taiwan (China)

2006-12-15

370

Numerical investigation of Al2O3/water nanofluid laminar convective heat transfer through triangular ducts  

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; Noie Seyyed Hossein; Talaii Elham; Sargolzaei Javad

2011-01-01

371

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.

2012-06-06

372

Experimental investigation of turbulent mixed convection in the wake of a heated sphere  

International Nuclear Information System (INIS)

[en] 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.)

1993-01-01

373

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).

1981-01-01

374

Non-Darcy forced convection boundary layer flow over a wedge embedded in a saturated porous medium  

Energy Technology Data Exchange (ETDEWEB)

The present paper deals with non-Darcian, forced convection, boundary layer flow over a wedge with a variable free stream, taking into account the effects of convective inertia, solid boundary, and porous inertia as well as Darcy flow resistance. Three novel methods, namely, the extended series method together with Shanks' transformation, the local nonsimilarity method, and the finite difference approximations combined with the Keller box method, are employed to integrate local nonsimilarity momentum and energy equations governing the flow. The results thus obtained are compared with the local skin friction and the local Nusselt number at the surface. Effects of different physical parameters on the local rate of heat transfer coefficient against the Darcy drag parameter [xi] are examined for fluids of Prandtl number [sigma] ranging from 0.7 to 100.0.

Hossain, M.A.; Banu, N. (Univ. of Dhaka (Bangladesh). Dept. of Mathematics); Nakayama, A. (Shizouka Univ., Hamamatsu (Japan). Dept. of Energy and Mechanical Engineering)

1994-10-01

375

Effects of Bubbles Between Electrodes on Alkaline Water Electrolysis Efficiency Under Forced Convection of Electrolyte  

Energy Technology Data Exchange (ETDEWEB)

The effects of forced convection of electrolyte on efficiency of alkaline water electrolysis were experimentally investigated, and forced convective two-phase flows between electrodes were visualized. Electrode was made of Ni with Pt plating ({approx} 1 {mu}m thickness), with 100 mm height and 30 mm width. KOH solution (8.5 wt%, 20 C) was used for electrolyte. The experimental conditions were as follows; electrodes space {delta}=1-30 mm, current density {phi}=0.1-2.0 A/cm{sup 2}, flow velocity u{sub l}=0.1-2.0 m/s. The obtained experimental results show that flow velocity of forced convection clearly affects the efficiency of water electrolysis, i.e. as flow velocity becomes larger, the efficiency of water electrolysis becomes higher. The optimum condition of water electrolysis in the forced convective system was also found, that was function of current density, electrode space and flow velocity. This tendency was found to be the same qualitatively with two-phase flow modeling for natural convective system. (authors)

Niro Nagai; Masanori Takeuchi [University of Fukui, 3-9-1 Bunkyo, Fukui 910-8507, (Japan); Tetsuya Furuta [Graduate School of University of Fukui, 3-9-1 Bunkyo, Fukui 910-8507, (Japan)

2006-07-01

376

Effects of Bubbles Between Electrodes on Alkaline Water Electrolysis Efficiency Under Forced Convection of Electrolyte  

International Nuclear Information System (INIS)

The effects of forced convection of electrolyte on efficiency of alkaline water electrolysis were experimentally investigated, and forced convective two-phase flows between electrodes were visualized. Electrode was made of Ni with Pt plating (? 1 ?m thickness), with 100 mm height and 30 mm width. KOH solution (8.5 wt%, 20 C) was used for electrolyte. The experimental conditions were as follows; electrodes space ?=1-30 mm, current density ?=0.1-2.0 A/cm2, flow velocity ul=0.1-2.0 m/s. The obtained experimental results show that flow velocity of forced convection clearly affects the efficiency of water electrolysis, i.e. as flow velocity becomes larger, the efficiency of water electrolysis becomes higher. The optimum condition of water electrolysis in the forced convective system was also found, that was function of current density, electrode space and flow velocity. This tendency was found to be the same qualitatively with two-phase flow modeling for natural convective system. (authors)

2006-01-01

377

Development of laminar mixed convection flow in a horizontal rectangular duct with uniform bottom heating  

International Nuclear Information System (INIS)

[en] Using a vectorized finite-difference marching technique, the steady-state continuity, momentum, and energy equations are solved numerically to evaluate the effects of buoyancy-induced secondary flow on forced flow in a horizontal rectangular duct with uniform bottom heating. Combined entry region conditions are considered, and the secondary flow is found to consist of longitudinal plumes and vortices that first develop at the vertical sidewalls and subsequently propagate to interior spanwise positions. Sequential stages of the secondary flow development are computed in detail and used to interpret the nonmonotonic longitudinal distribution of the spanwise average Nusselt number. The distribution is characterized by oscillations that, under certain conditions, are damped and yield a fully developed Nusselt number that substantially exceeds the value for pure forced convection

1987-01-01

378

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

379

Studies on convective heat transfer through helical coils  

Science.gov (United States)

An experimental investigation on steady state convection heat transfer from vertical helical coiled tubes in water was performed for laminar flow regime. Three coils with curvature ratios as 0.0757, 0.064, 0.055 and range of Prandtl number from 3.81 to 4.8, Reynolds number from 3,166 to 9,658 were considered in this work. The heat transfer data were generated from 30 experiments conducted at constant water bath temperature (60 °C) for different cold water flow rates in helical coils. For the first time, an innovative approach of correlating Nusselt number with `M' number is proposed which is not available in the literature and the developed correlations are found to be in good agreement with the work of earlier researchers. Thus, dimensionless number `M' was found to be significant to characterize the hydrodynamics of fluid flow and heat transfer correlations in helical coils. Several other correlations based on experimental data are developed. To cover wide range of industrial applications, suitable generalized correlations based on extended parameters beyond the range of present experimental work are also developed. All these correlations are developed by using least-squares power law fit and multiple-regression analysis of MATLAB software. Correlations so developed were compared with published correlations and were found to be in good agreement. Comparison of heat transfer coefficients, friction factor and Nusselt number for different geometrical conditions is presented in this paper.

Pawar, S. S.; Sunnapwar, Vivek K.

2013-08-01

380

Heat transfer enhancement of turbulent natural convection adjacent to vertical heated plate  

International Nuclear Information System (INIS)

Enhancement of heat transfer was investigated experimentally on natural convection adjacent to a vertical heated plate. In order to promote heat transfer from the heated plate, a V-shaped divertor plate of which the edge faced upstream was attached onto the surface of the vertical plate. The divertor plate redirects high-temperature fluids toward both sides of the plate and introduces low-temperature ambient fluids behind the plate instead. These two mechanisms enhance heat transfer, in particular, in the region behind the divertor plate. Local heat transfer coefficients around the divertor plate were measured using water as a test fluid. The coefficients behind the divertor plate reached 2.5 times of those without the divertor plate. The optimum heights and angles of the divertor plate that make the heat transfer maximum were also studied experimentally. (author)

1996-01-01

 
 
 
 
381

Oscillatory Rayleigh-Marangoni convection in a layer heated from above: numerical simulations with an undeformable free surface.  

UK PubMed Central (United Kingdom)

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.

Boeck T; Jurgk M; Bahr U

2003-02-01

382

Temperature dependent viscosity effects on laminar forced convection in the entrance region of straight ducts  

Energy Technology Data Exchange (ETDEWEB)

A parametric investigation is carried out on the effects of temperature dependent viscosity in simultaneously developing laminar flow of a liquid in straight ducts of arbitrary but constant cross-sections. Viscosity is assumed to vary with temperature according to an exponential relation, while the other fluid properties are held constant. Different cross-sectional geometries are considered, corresponding both to three-dimensional (rectangular, trapezoidal and hexagonal) and to axisymmetric (circular and concentric annular) duct geometries. Uniform wall temperature boundary conditions are imposed on the heated/cooled walls of the ducts. A finite element procedure is employed for the solution of the parabolized momentum and energy equations. Computed axial distributions of the local Nusselt number and of the apparent Fanning friction factor for ducts of the considered cross-sections are presented with reference to both fluid heating and fluid cooling. Numerical results confirm that, in the laminar forced convection in the entrance region of straight ducts, the effects of temperature dependent viscosity cannot be neglected in a wide range of operative conditions. (author)

Nonino, C.; Del Giudice, S.; Savino, S. [Dipartimento di Energetica e Macchine, Universita degli Studi di Udine, Via delle Scienze 208, 33100 Udine (Italy)

2006-11-15

383

An experimental investigation of forced convection flat plate solar air heater with storage material  

Directory of Open Access Journals (Sweden)

Full Text Available Solar air heater (SAH) is a heating device that uses the heated air in the drying of agriculture products and many engineering applications. The purpose of the present work is to study a forced convection flat plate solar air heater with granite stone storage material bed under the climatic conditions of Egypt-Aswan. Experiments are performed at different air mass flow rates ; varying from 0.016 kg/s to 0.08 kg/s, for five hot summer days of July 2008. Hourly values of global solar radiation and some meteorological data (temperature, pressure, relative humidities, etc.) for measuring days are obtained from the Egyptian Meteorological Authority, Aswan station. Inlet and outlet temperatures of air from a SAH have been recorded. In this work, attempt has been made to present the temperature distribution in non dimensional form that makes it useable for any region and not restricted to local conditions. The variation of solar radiation, air heater efficiency, Nusselt number and temperature distribution along the air heater are discussed. Comparisons between the calculated values of outlet air temperatures, average air temperatures and storage material temperatures and the corresponding measured values showed good agreement. Comparison between current work and those in previous investigations showed fair agreement.

Aissa Walid; El-Sallak Mostafa; Elhakem Ahmed

2012-01-01

384

Second law analysis of forced convection in a circular duct for non-Newtonian fluids  

Energy Technology Data Exchange (ETDEWEB)

The second law characteristics of fluid flow and heat transfer inside a circular duct under fully developed forced convection for non-Newtonian fluids are presented. Heat flux is kept constant at the duct wall. Analytical expressions for dimensionless entropy generation number (N{sub S}), irreversibility distribution ratio (F), and Bejan number (Be) are obtained as functions of dimensionless radius (R), Peclet number (Pe), modified Eckert number (Ec), Prandtl number (Pr), dimensionless temperature difference ({omega}), and fluid index (m or n). Spatial distributions of local and average entropy generation number, irreversibility ratio, and Bejan number are presented graphically. For a particular value of fluid index, n=1 (or m=2), the general entropy generation number expression for a non-Newtonian power-law fluid reduces to the expression for Newtonian fluid as expected. Furthermore, entropy generation minimization is applied to calculate an optimum fluid index (n{sub EGM}). A correlation is proposed that calculates n{sub EGM}as a function of group parameter (EcxPr/{omega}) and Peclet number (Pe) within +/-5% accuracy. Finally, for some selected fluid indices, the governing equations are solved numerically in order to obtain Nusselt number. It is observed that the numerically obtained asymptotic Nusselt number shows excellent agreement with the analytically obtained Nusselt number. (author)

Mahmud, Shohel; Fraser, Roydon Andrew [Department of Mechanical Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ont. (Canada)

2006-09-15

385

Bubble departure size in forced convective subcooled boiling flow under static and heaving conditions  

International Nuclear Information System (INIS)

Highlights: ? Experiment was carried out on bubble departure size under heaving motion. ? High-speed camera was mounted on heaving platform to capture bubble departure size. ? Trends of mass flux and inlet subcooling on bubble departure size were studied. ? Flow rate fluctuation and acceleration variation affected bubble departure size. ? A modified model was proposed to predict bubble departure size in heaving conditions. - Abstract: A visual study of bubble departure size in forced convective subcooled boiling flow under static and heaving conditions was presented. High-speed digital images of flow boiling phenomena were obtained, which were used to measure bubble departure diameter. Experiments were conducted at atmosphere pressure in a narrow rectangular channel, with mass flux ranging from 300 to 710 kg/m2 s, heat flux ranging from 65 to 298 kW/m2 and inlet subcooling ranging from 20 to 40 K. The heaving frequency, which is generated by a six degrees-of-freedom platform, ranged from 0.2 to 0.61 Hz. The results indicated that decreasing mass flux and increasing heat flux had a tendency to increase bubble departure diameter under static condition. In heaving motion, bubble departure size was affected by additional heaving acceleration and flow rate fluctuation. A bubble departure model was proposed to predict the bubble departure diameter under static and heaving conditions by considering the additional acceleration and flow rate fluctuation. The proposed model agreed well with the experimental data within the averaged relative deviation of ±17.5%.

2012-01-01

386

Modelling unsteady turbulent forced convection inside tubes using a low-Reynolds-number k ? model  

International Nuclear Information System (INIS)

[en] The study of turbulent forced convection in unsteady flow regimes is of both fundamental interest and considerable practical importance because of the widespread occurrence of such flow regimes in engineering systems. Since the complexity of the equations governing turbulent flow, obviously precludes direct analysis, even for steady flow, investigators must turn to experimentation and modelling in order to study this problem. Whilst a considerable body of work does exist on the hydrodynamics of unsteady turbulent shear flow, very few studies also consider heat transfer and in consequence it is not well understood. The present study was motivated by unsteady heat transfer measurements made in water pipe flow undergoing harmonic oscillation. Although the experiment was complicated by variable viscosity and density effects, the results suggested that measured Nusselt numbers could differ significantly from those obtained using a pseudo-steady correlation (a correlation strictly applicable to steady flow evaluated at the instantaneous conditions of an unsteady flow). This study primarily aims to assess the degree to which the widely used low-Reynolds-number k-? turbulence model reproduces the previous experimental data. (Author)

1992-01-01

387

Natural convection heat transfer for a staggered array of heated, horizontal cylinders within a rectangular enclosure  

International Nuclear Information System (INIS)

[en] 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

1996-01-01

388

Double-diffusive natural convection in a porous enclosure partially heated from below and differentially salted  

International Nuclear Information System (INIS)

This paper reports numerical results of two-dimensional double-diffusive natural convection in a square porous cavity partially heated from below while its upper surface is cooled at a constant temperature. The vertical walls of the porous matrix are subjected to a horizontal concentration gradient. The parameters governing the problem are the thermal Rayleigh number (Ra=100 and 200), the Lewis number (Le=0.1, 1 and 10), the buoyancy ratio (-10?N?10) and the relative position of the heating element with respect to the vertical centerline of the cavity (?=0 and 0.5). The effect of the governing parameters on fluid characteristics is analyzed. The multiplicity of solutions is explored and the existence of asymmetric bicellular flow is proved when the heated element is shifted towards a vertical boundary (?=0.5). The solutal buoyancy forces induced by horizontal concentration gradient lead to the elimination of the multiplicity of solutions obtained in pure thermal convection when N reaches some threshold value which depends on Le and Ra

2004-01-01

389

Experimental investigation of natural convection induced by internal heat generation  

International Nuclear Information System (INIS)

Dilatation of a convection cell with respect to its Rayleigh number, one of the problems in internally heated convection, was quantitatively investigated by analyzing temperature field in a cell. The temperature field visualized by a thermo-chromic liquid crystal (TLC) expresses the cell dilatation. A calibration system was developed to convert the visualized photographs of the temperature field to the temperature field. A calibration curve correlating color information extracted from the photograph and temperature was determined from the approximately linear temperature distribution in the horizontal fluid layer using the hue method. Photos taken at various internal Rayleigh numbers were converted to the temperature field by the obtained curve. Extracting individual cells from a temperature field achieves a quantitative expression of the cell dilatation as the variation of the wavenumber of the cell with Rayleigh number increases. The temperature profile in a cell shows that high temperature areas appear at the apexes of the cell, largely different from the profile obtained by linear theory

2005-01-01

390

Effective thermal conductivity of a heat generating rod bundle dissipating heat by natural convection and radiation  

Energy Technology Data Exchange (ETDEWEB)

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 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.

Senve, Vinay [Department of Mechanical Engineering, Indian Institute of Science, Bangalore 560 012 (India); Narasimham, G.S.V.L., E-mail: mecgsvln@mecheng.iisc.ernet.in [Department of Mechanical Engineering, Indian Institute of Science, Bangalore 560 012 (India)

2011-10-15

391

Effective thermal conductivity of a heat generating rod bundle dissipating heat by natural convection and radiation  

International Nuclear Information System (INIS)

[en] 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 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.

2011-01-01

392

Mixed convection in the thermal entrance region of symmetrically and asymmetrically heated vertical flat duct with upward or downward air-flow  

Energy Technology Data Exchange (ETDEWEB)

A numerical investigation has been conducted on the effect of body force on pure forced convection of the upward or downward air-flow in the thermal entrance region between vertical parallel plates with uniform wall temperature. The governing equations based on the usual Boussinesq approximation are solved for the symmetrically and asymmetrically heated parallel plates. Numerically predicted friction factors C{sub f} and local Nusselt numbers Nu{sub x} are compared with their counterparts, C*{sub f} and Nu*{sub x}, for pure forced convection.

Naito, Etsuro; Nagano, Yasutaka

1999-07-01

393

Influence of the dielectrophoretic force on thermal convection  

Energy Technology Data Exchange (ETDEWEB)

The dielectrophoretic effect can be used to produce a buoyancy force in a fluid, since the dielectric strength of the fluid is a function of temperature. This effect is in many aspects similar to the gravitational force and can be used to investigate thermal flows in complex force fields. The strength of this buoyancy force is experimentally measured. The experimental results show that side effects, like flows induced by charge injection or chain formation of fluid molecules, can be avoided. Theory and experiment are in good agreement. For future applications, this set-up is a reliable test experiment to check the suitability of available fluids. (orig.)

Sitte, B.; Rath, H.J. [Center of Applied Space Technology and Microgravity (ZARM), University of Bremen, Am Fallturm, 28359 Bremen (Germany)

2003-01-01

394

Evaporation and Marangoni driven convection in small heated water droplets.  

UK PubMed Central (United Kingdom)

Evaporation dynamics of small sessile water droplets under microgravity conditions is investigated numerically. The water-air interface is free, and the surrounding air is assumed to be quasisteady. The droplet is described by Navier-Stokes and heat equations and its surrounding water/air gaseous phase with Laplace equation. In the thermodynamic conditions of the simulations presented herein, the evaporative mass flow is nonlinear. It shows a minimum that indicates the existence of qualitative changes in the evaporative regimes although the droplet is sessile. Due to temperature gradients on the free interface, Marangoni motion occurs and generates inside the droplet convection cells that furthermore exhibit small fluctuating motion as evaporation goes on.

Girard F; Antoni M; Faure S; Steinchen A

2006-12-01

395

Laminar free convection from a uniformly heated horizontal cylinder  

International Nuclear Information System (INIS)

[en] The steady laminar free convection flow around a uniformly heated circular cylinder which is placed horizontally in an infinitely extended incompressible fluid otherwise at rest is numerically investigated at high Grashof numbers. The Boussinesq approximation is assumed and the fluid flow with Pr = 0.72 is calculated for Grd = 8 x 105, 8 x 106 and 8 x 107 using Leonard's difference approximation scheme, where Pr is the Prandtl number and Grd the Grashof number based on the diameter of the circular cylinder and the temperature difference from the surrounding fluid. From the calculated result the flow separation bubble present at the upper stagnation point is observed to gradually extend as the Grashof number increases. Corresponding values of the Nusselt number are also evaluated and are found to be consistent with those given by existing empirical formulas. (author)

1987-01-01

396

Convection forced by a descending dry layer and low-level moist convergence  

Science.gov (United States)

A narrow line of convective showers was observed over southern England on 18 July 2005 during the Convective Storm Initiation Project (CSIP). The showers formed behind a cold front (CF), beneath two apparently descending dry layers (i.e. sloping so that they descended relative to the instruments observing them). The lowermost dry layer was associated with a tropopause fold from a depression, which formed 2 d earlier from a breaking Rossby wave, located northwest of the UK. The uppermost dry layer had fragmented from the original streamer due to rotation around the depression (This rotation was also responsible for the observations of apparent descent-ascent would otherwise be seen behind a CF). The lowermost dry layer descended over the UK and overran higher ?w air beneath it, resulting in potential instability. Combined with a surface convergence line (which triggered the convection but had less impact on the convective available potential energy than the potential instability), convection was forced up to 5.5 km where the uppermost dry layer capped it. The period when convection was possible was very short, thus explaining the narrowness of the shower band. Convective Storm Initiation Project observations and model data are presented to illustrate the unique processes in this case.

Russell, Andrew; Vaughan, Geraint; Norton, Emily G.; Ricketts, Hugo M. A.; Morcrette, Cyril J.; Hewison, Tim J.; Browning, Keith. A.; Blyth, Alan M.

2009-03-01

397