Interaction Of Surface Radiation Research Articles

Investigations on the interaction of surface radiation and turbulent natural convection in enclosures with multiple positions of source and sink

Investigations on the interaction of surface radiation and turbulent natural convection in enclosures with multiple positions of source and sink

Coupled natural convection and radiation heat transfer of hybrid solar energy conversion system

A hybrid solar energy conversion system described in this study is designed to exceed the efficiency of each type of technology alone by combining advantages of concentrated solar power (CSP) technology and high performance concentrated photovoltaic (CPV) cells. This hybrid system can be tilted at different inclination angles to track the solar power at different latitudes and different time zones. Thermal management is important to CPV cells, as 1000× concentrated solar radiation onto CPV not only leads to higher performance but also generates more waste heat flux. In this work, the effects of natural convection with and without the interaction of surface radiation in the system consisting of PV cells, optical funnels and a heatsink were numerically investigated with an experimentally validated model. The influence of inclination on the overall flow structure and heat transfer was examined. Under the effect of these funnels, the heat transfer was enhanced when the heat was transferred through PV cells to funnels. At the same time, the separation flow between the spaces of funnels would take the heat to the ambient. The minimum average temperature of surrogate PV cells occurred at the inclination angle of 60°. When the inclination angle varied from 0° to 90°, the stagnation point moved from the downward surface of the heat sink towards the edge of heat sink. From 0° to 60°, the stagnation point was at the downward surface of heat sink and there was a dramatic decrease in the average temperature of the surrogate PV cells. When inclination angles were less than 60°, except that there was a separation region above substrate, the side flow could flow in and out freely between adjacent funnels and then formed a pair of separation bubble, or spiral node, on top of the funnels. At these inclination angles, flow velocity was found to be the dominant factor influencing heat transfer because the flow pattern (separation bubble) was the same on top of the funnels. However, at inclination angles of 60° to 90° the stagnation point occurred at the edge of the heat sink, and the surrogate PV temperature increased. When the inclination angle reached 90°, the flow was trapped and recirculated among funnels, and the vortical wake on top of the funnel disappeared. It can be concluded that from 60° to 90°, the dominant factor influencing heat transfer is funnel effect because the flow velocity stayed almost constant.

Interaction of surface radiation with natural convection in tall vertical cavities heated by a linear heat flux

Purpose – A detailed numerical study is conducted on the effect of surface radiation on laminar natural convection in a tall vertical cavity filled with air. The cavity is heated and cooled, through its two vertical walls, by a linear or uniform heat flux q(y) and by a constant cold temperature, respectively. The horizontal walls are considered adiabatic. The paper aims to discuss these issues. Design/methodology/approach – The radiosity method is employed to calculate the net radiative heat exchanges between elementary surfaces, while the finite volume method is implemented to resolve the governing equations of the fluid flow. Findings – For each heat flux q(y) (ascending, descending or uniform), the effect of the emissivity ε (0ε1) on the local, average and maximum temperatures of the heated wall is determined as a function of the average Rayleigh number Ram (103Ram 6×104) and the cavity aspect ratio A (10A80). The effect of the coupling on the flow structures, convective and radiative heat transfers is also presented and analyzed. Overall, it is shown that surface radiation significantly reduces the local and average temperatures of the heated wall and therefore reduces the convective heat transfer between the active walls. Practical implications – The studied configuration is of practical interest in several areas where overheating must be avoided. For this purpose, a simple design tool is developed to estimate the mean and the maximum temperatures of the hot wall in different operating conditions (Ram, A et ε). Originality/value – The originality lies in the study of the interaction between surface radiation and natural convection in tall cavities submitted to a non-uniform heat flux and a constant cold temperature on the active walls. Also, the development of an original simplified calculation procedure for the hot wall temperatures.

Effect of buoyancy force on turbulent modes of complex heat transfer in an air-filled square cavity

Turbulent natural convection with the interaction of surface radiation in square enclosure has been numerically studied. The governing equations are solved contemporaneously by finite difference method to obtain the velocity, temperature and heat flux distributions. Turbulence has been modeled using the standard k-ε model. The change of convective and radiative Nusselt numbers with respect to time has been described. The analysis is carried out over a wide range of Rayleigh number from 108 to 1010. The effect of this key parameter on temperature and velocity distributions, convective and radiative Nusselt numbers has been investigated. The results clearly demonstrate a significant effect of buoyancy ratio on unsteady turbulent heat transfer.

Interaction of Surface Radiation With Laminar and Turbulent Natural Convection in Tall Vertical Cavities: Analysis and Heat Transfer Correlations

The interaction of surface radiation with laminar and turbulent natural convection in differentially heated vertical cavities, filled with air and of large aspect ratio (greater than 10), is analyzed in this study. The k − ωSST turbulence model is used for the formulation of the convection fluid flow and heat transfer, while the governing equations are discretized by the finite-volume method. As an extension of the scarce previous studies, more realistic conditions with a wide range of parameters are considered in the performed simulations. The presented results show the effect of surface radiation on streamlines, isotherms, turbulent kinetic energy, and temperature and vertical velocity profiles, as well as on local and on average convective and radiative heat transfer. Globally, it is found that surface radiation has a weak effect on the dynamic and thermal fields in the major part of the cavity; however, some influence in the upper and lower zones of the cavity is observed. For design purposes, accurate correlations are developed for average convective and radiative Nusselt numbers that cover emissivity of surfaces between 0 and 1, cold wall temperature ranging from 263 K to 303 K, temperature difference between vertical walls ranging from 5 K to 40 K, width of the cavity between 2.5 cm and 7.5 cm, and height of the cavity between 0.25 m and 6 m (this leads to a Rayleigh number ranging from 103 to 2 × 106 and an aspect ratio between 10 and 80).

Interaction of surface radiation with conjugate mixed convection from a vertical channel with multiple discrete heat sources

Important results of a numerical study performed on combined conduction–mixed convection–surface radiation from a vertical channel equipped with three identical flush-mounted discrete heat sources in its left wall are provided here. The channel has walls of identical height with the spacing varied by varying its aspect ratio (AR). The cooling medium is air that is considered to be radiatively transparent. The heat generated in the channel gets conducted along its walls before getting dissipated by mixed convection and radiation. The governing equations for fluid flow and heat transfer are considered without boundary layer approximations and are transformed into vorticity–stream function form and are later normalized. The resulting equations are solved, along with relevant boundary conditions, making use of the finite volume method. The computer code written for the purpose is validated both for fluid flow and heat transfer results with those available in the literature. Detailed parametric studies have been performed and the effects of modified Richardson number, surface emissivity, thermal conductivity and AR on various pertinent results have been looked into. The significance of radiation in various regimes of mixed convection has been elucidated. The relative contributions of mixed convection and radiation in carrying the mandated cooling load have been thoroughly explored.

Alleviating operating temperature of concentration solar cell by air active cooling and surface radiation

In the present paper, a heat transfer model for a multi-junction concentrating solar cell system has been developed. The model presented in this work includes the GaInP/GaAs/Ge triple-junction solar cell with a ventilation system in which air is forced to flow within a duct behind the solar cell assembly and its holders and accessories (anti-reflective glass cover, adhesive material, and aluminum back plate). A mathematical model for the entire system is presented and the finite difference technique has been used to solve the governing equations. Results showed that the interaction of surface radiation and air convection could adequately cool the solar cell at medium concentration ratios. For high concentration ratios, the channel width would need to be narrowed to micro-meter values to maintain the required efficiency of cooling. The conjugation effect has been shown to be significant and has a noticeable effect on the maximum solar cell temperature. Furthermore, the air inlet velocity and channel width were also found to have major effects on the cell temperature.

An empirical solution to turbulent natural convection and radiation heat transfer in square and rectangular enclosures

The effects of natural turbulent convection with the interaction of surface radiation in a rectangular enclosure have previously been numerically and theoretically studied. The analyses were carried out over a wide range of enclosure aspect ratios ranging from 0.0625 to 16, different enclosure sizes, with cold wall temperatures ranging from 283 to 373 K, and temperature ratios ranging from 1.02 to 2.61. The work was carried out using four fluids (Argon, Air, Helium and Hydrogen; whose properties vary with temperature).These can be used to calculate Nusselt number for pure natural convection and also to calculate the ratio between convection to radiation heat transfer for both square and rectangular enclosures.This work extends these results by providing an empirical solution for the case of radiation and natural convection in square and rectangular enclosures and also provides a correlation equation to calculate the total Nusselt number for these cases. This method allows the simple calculation of either the total heat transfer rate, given the fluid, the geometry and the temperatures of the hot and cold walls, or via a straightforward iterative technique, the temperature of one wall given the other wall temperature and the total heat transfer.

INTERACTION OF SURFACE RADIATION WITH CONJUGATE MIXED CONVECTION FROM A VERTICAL PLATE WITH MULTIPLE NONIDENTICAL DISCRETE HEAT SOURCES

Some of the important results of a numerical investigation into interaction of surface radiation with conjugate mixed convection from a discretely heated vertical plate with three nonidentical heat sources are provided here. The heat sources with identical rate of volumetric heat generation are placed flush-mounted along the plate in the descending order of their height from the bottom to the top ends of the plate. The heat sources are positioned at the leading edge, center, and the trailing edge of the plate, while the cooling medium considered is air, which is assumed to be radiatively transparent. The governing equations for fluid flow and heat transfer are initially converted into vorticity-stream function form and are later solved using the finite volume method coupled with Gauss-Seidel iterative solver. A computer code is written for the purpose. The effects of modified Richardson number, surface emissivity, and thermal conductivity on temperature distribution, peak temperature, drag coefficient, and relative contributions of mixed convection and radiation to heat dissipation are studied.

Investigations on Multimode Heat Transfer From a Heated Vertical Plate

The interaction of surface radiation and conduction with natural convection heat transfer from a vertical flat plate assembly, with an embedded heater, has been investigated, both experimentally (using differential interferometer) and numerically (using FLUENT), in the present work. In the absence of radiation, the asymptotic limits that can be attained by the heated plate are isothermal and isoflux conditions. High values of plate thermal conductivity tend to make the surface isothermal, where as, lower values of thermal conductivity tend to make it isoflux. Irrespective of the thermal conductivity of the plate, an increase in the emissivity reduces the average temperature of the plate and brings the plate toward isothermal condition. A new methodology has also been proposed to determine the thermophysical properties, emissivity and thermal conductivity, the consistency of which is tested by carrying out experiments for various heat inputs and comparing the estimated values with those available in literature.

Interaction of surface radiation with combined conduction and convection from a discretely heated L-corner

Prominent results pertaining to the problem of multi-mode heat transfer from an L-corner equipped with three identical flush-mounted discrete heat sources in its left leg are given here. The heat generated in the heat sources is conducted along the two legs of the device before being dissipated by combined convection and radiation into air that is considered to be the cooling agent. The governing equations for temperature distribution along the L-corner are obtained by making appropriate energy balance between the heat generated, conducted, convected and radiated. The non-linear partial differential equations thus obtained are converted into algebraic form using a finite-difference formulation. The resulting equations are solved simultaneously by Gauss–Seidel iterative solver. A computer code is specifically written to solve the problem. The computational domain is discretised using 101 grids along the left leg, with 15 grids taken per heat source, and 21 grids along the bottom leg. The effects of surface emissivity, convection heat transfer coefficient, thermal conductivity and aspect ratio on local temperature distribution, peak device temperature and relative contributions of convection and radiation to heat dissipation from the L-corner are studied in detail. The point that one cannot overlook radiation in problems of this class has been clearly elucidated.

Interaction of Surface Radiation with Conduction and Convection from a Vertical Channel with Multiple Discrete Heat Sources in the Left Wall

This article reports the results of a numerical study of the problem of interaction of surface radiation with conduction and convection from a vertical channel equipped with three flush-mounted discrete heat sources in the left wall. The governing equations for temperature distribution along each of the two channel walls have been deduced based on appropriate energy balance between various energy interactions in which the channel is involved. Radiosity-irradiation formulation is employed to tackle radiation-related calculations, with the view factors involved therein obtained using Hottel's crossed-string method. The governing nonlinear partial differential equations are converted into algebraic form through a finite-volume-based finite-difference method, and are subsequently solved using Gauss-Seidel iterative technique. An optimum grid system, with 26 grids along each of the three discrete heat sources and 11 grids along each of the two non-heat source portions of the board, is used for discretization of the computational domain. The effects of various pertinent parameters, viz., aspect ratio, volumetric heat generation, thermal conductivity, surface emissivity, and convection heat transfer coefficient, on temperature distribution along the left board, maximum temperature in the channel, and relative contributions of convection and surface radiation to heat dissipation from the channel are probed.

Effect of Surface Radiation and Partition Resistance on Natural Convection Heat Transfer in a Partitioned Enclosure: An Experimental Study

The heat transfer across an air-filled partitioned square enclosure is studied experimentally using a differential interferometer. The partition was located centrally inside the enclosure, parallel to the two isothermal differentially heated vertical walls, and extended the full height of the enclosure. The top and bottom horizontal walls of the enclosure were maintained adiabatic. A parametric study has been carried out using different partitions, focusing attention on the effect of partition thermal resistance as well as the interaction of surface radiation and natural convection, and on the total heat transfer between the vertical walls, inside the enclosure. Correlations, valid for the laminar range, are proposed.

Interaction of surface radiation and variable property natural convection in a differentially heated square cavity – a finite element analysis

A finite element solution on the interaction of surface radiation and variable property laminar natural convection is presented. Finite element formulation of the governing equations, associated with variable property natural convection, and incorporation of the radiative boundary conditions has been extensively discussed. The study also aims to highlight the limiting value of the terminal temperature difference (TTD), below which the natural convection heat transfer becomes the sole heat transfer mode, i.e. the effect of surface radiation can be neglected. The effects of variations of emissivity and TTD are also presented.

Interaction of Surface Radiation and Free Convection in an Enclosure With a Vertical Partition

The present two-dimensional study allows for both surface radiation and free convection and attempts to throw more light on the effect of aspect ratio as well as the emissivities of the various surfaces on the interaction of these two modes of heat transfer in partitioned rectangular enclosures

Interaction of surface radiation with free convection in a square cavity

A numerical investigation of free convection in a rectangular enclosure has been carried out based on Gosman's finite volume method with a 21 × 21 nonuniform grid. A radiation model has been included in the analysis to study the effect of surface radiation on the heat transfer characteristics of the enclosure. The model is general since it takes into account different emissivities for the side walls and for the top and bottom walls. The nonuniform grids for convection have been retained for radiation, notwithstanding the fact that the evaluation of view factors becomes highly tedious. The view factors are evaluated by Hottel's crossed-string method. Nonuniform grids for both convection and radiation routines ensure grid compatibility and yield convergent solutions. The model has thrown light onto the importance of surface radiation even at low emissivities and temperature levels and provides an explanation of the discrepancies between the experimental and theoretical correlations. A parametric study of surface radiation effects is also presented.

Interaction of surface radiation with convection in crystal growth by physical vapor transport

Growth of single crystals from vapor in closed ampoules is governed by an intricate interplay between mass, momentum and heat transfer processes. The objective of this study is to examine and isolate the effects of surface radiation heat transfer on the vapor transport process using a mathematical model. The model consists of a set of coupled nonlinear partial differential equations for conservation of mass, momentum, energy and species, and the integrodifferential equations which represent radiative exchange. It depends on five important physical parameters. These are Grashof number, Prandtl number, Schmidt number, aspect ratio and the radiation-conduction number. The effects of these dimensionless groupings are systematically investigated. From the cases examined, it is concluded that surface radiation can change the flow structure appreciably. This is especially true in microgravity environment where radiation competes primarily with conduction in modifying the thermal profiles. The numerical results also show that in the presence of radiation, the top heating configuration (source on top) is no longer stable and that near the growing crystal, radiation-induced vortices can introduce significant nonuniformities in the growth flux.