C. Balaji

Interaction of surface radiation with free convection in a square cavity

Abstract A numerical investigation of free convection in a rectangular enclosure has been carried out based on Gosmans 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 Hottels 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.

Correlations for free convection and surface radiation in a square cavity

In the present study the results of a numerical investigation of combined surface radiation and free convection in a square cavity with air as the intervening medium are reported. The computations have been performed for 103 ≤ Gr ≤ 106, with the emissivities of all the walls varying between 0 and 1. Surface radiation reduces the convective heat transfer across the cavity and at the same time contributes to the overall heat transfer by direct radiant heat transfer across the cavity. This “ual” nature of radiation has been qualitatively highlighted in an earlier investigation by the authors. The objective of the present study however, is to give comprehensive correlations for convection and radiation based on the numerical calculations of the coupled problem. The basis for the choice of the form of the various terms in the correlations has also been brought out.

Interaction of radiation with free convection in an open cavity

Abstract This paper reports the numerical results of the fundamental problem of interaction of surface radiation with free convection in an open cavity with air as the intervening medium. Rayleigh numbers (based on height) in the laminar range 104–108 have been considered in the present study. Surface radiation was found to alter the basic flow pattern as well as the overall thermal performance substantially. A physical insight into the effect of radiation has been provided and correlations have been developed for convective as well as radiative transfer based on the numerical results.

EFFECT OF SURFACE RADIATION ON CONJUGATE MIXED CONVECTION IN A VERTICAL CHANNEL WITH A DISCRETE HEAT SOURCE IN EACH WALL

Abstract The results of a numerical analysis of the problem of two-dimensional, steady, incompressible, conjugate, laminar, mixed convection with surface radiation in a vertical parallel-plate channel, provided with a flush-mounted, heat generating, discrete heat source in each wall, are presented here. Air, a radiatively non-participating medium, is used as the cooling agent. A computer code based on the finite volume method is written exclusively for solving the above problem. The effect of surface emissivity, aspect ratio, discrete heat source position and modified Richardson number on the fluid flow and heat transfer characteristics is explored. Useful correlations are evolved for the maximum temperature of the left and the right channel walls, the mean friction coefficient and the forced convection component of the mean friction coefficient.

Conjugate Mixed Convection With Surface Radiation From a Vertical Plate With a Discrete Heat Source

The results of a numerical study of the problem of two-dimensional, steady, incompressible, conjugate, laminar, mixed convection with surface radiation from a vertical plate with a flush-mounted discrete heat source are reported. The governing equations, written in vorticity-stream function form, are solved using a finite-volume based finite difference method. A hybrid grid system has been employed for discretization of the computational domain. The effects of (i) the magnitude and location of the heat source, (ii) the material and surface properties of the plate, and (iii) the free-stream velocity on both heat transfer and fluid flow have been studied. Based on a large set of (more than 550) numerical data, correlations have been developed for maximum and average non-dimensional plate temperatures and mean friction coefficient. A method for evaluating the forced convection mean friction coefficient component, which may be used in estimating the power input required for maintaining the flow, has been proposed

Combined conduction, convection and radiation in a slot

This paper reports the results of a numerical study of the fundamental problem of combined mode heat transfer in a vertical slot. The paper is a sequel to the work of the authors on problems of interaction of natural convection with radiation and conduction in closed and open cavities. The present paper considers the intractions between all three modes of heat transfer and, hence, represents the most general case of interaction. The slot simulates a typical intermediate fin in a fin array or a typical section in an extruded heat sink commonly employed in practice. A detailed parametric study has led to useful correlations. The nature and extent of the coupling between the various modes of heat transfer are also elucidated.

THE USE OF ACFD APPROACH PROBLEMS INVOLVING SURFACE RADIATION AND FREE CONVECTION

The possibility of combining the method of matched asymptotic expansions with Computational Fluid Dynamics (CFD), a technique called ACFD, has been explored, for a new class of problems that involve interaction of radiation with convection. A specific problem of natural convection with surface radiation from an open cavity has been considered to demonstrate the efficacy of the ACFD approach. The work clearly demonstrates the adequacy of the approach and it appears to be an attractive option in the analysis of such multi-mode problems in heat transfer

Comparison of Various Methods for Simultaneous Retrieval of Surface Emissivities and Gas Properties in Gray Participating Media

An inverse radiation analysis for simultaneous estimation of the radiative properties and the surface emissivities for a participating medium in between infinitely long parallel planes, from the knowledge of the measured temperatures and heat fluxes at the boundaries, is presented. The differential discrete ordinate method is employed to solve the radiative transfer equation. The present analysis considers three types of simple scattering phase functions. The inverse problem is solved through minimization of a performance function, which is expressed by the sum of squares of residuals between calculated and observed temperatures and heat fluxes at the boundaries. To check the performance and accuracy in retrieval, a comparison is presented between four retrieval methods, viz. Levenberg-Marquardt algorithm, genetic algorithm, artificial neural network, and the Bayesian algorithm. The results of the present analyses indicate that good precision in retrieval could be achieved by using only temperatures and heat fluxes at the boundaries. The study shows that the radiative properties of medium and surface emissivities can be retrieved even with noisy data using Bayesian retrieval algorithm and artificial neural network. Also, the results demonstrate that genetic algorithms are not efficient but are quite robust. Additionally, it is observed that an increase in the error in measurements significantly deteriorates the retrieval using the Levenberg-Marquardt algorithm.

Sensitivity of tropical cyclone Jal simulations to physics parameterizations

In this study, the sensitivity of numerical simulations of tropical cyclones to physics parameterizations is carried out with a view to determine the best set of physics options for prediction of cyclones originating in the north Indian Ocean. For this purpose, the tropical cyclone Jal has been simulated by the advanced (or state of science) mesoscale Weather Research and Forecasting (WRF) model on a desktop mini super computer CRAY CX1 with the available physics parameterizations. The model domain consists of one coarse and two nested domains. The resolution of the coarse domain is 90 km while the two nested domains have resolutions of 30 and 10 km, respectively. The results from the inner most domain have been considered for analyzing and comparing the results. Model simulation fields are compared with corresponding analysis or observation data. The track and intensity of simulated cyclone are compared with best track estimates provided by the Joint Typhoon Warning Centre (JTWC) data. Two sets of experiments are conducted to determine the best combination of physics schemes for track and intensity and it is seen that the best set of physics combination for track is not suitable for intensity prediction and the best combination for track prediction overpredicts the intensity of the cyclone. The sensitivity of the results to orography and level of nesting has also been studied. Simulations were also done for the cyclone Aila with (i) best set of physics and (ii) randomly selected physics schemes. The results of the Aila case show that the best set of physics schemes has more prediction skill than the randomly selected schemes in the case of track prediction. The cumulus (CPS), planetary boundary layer (PBL) and microphysics (MP) parameterization schemes have more impact on the track and intensity prediction skill than the other parameterizations employed in the mesoscale model.

A holistic optimization of convecting-radiating fin systems

A convecting-radiating fin array, which stands vertically outside of a horizontal rectangular duct, has been analyzed for various design constraints. Fully developed turbulent flow is considered inside the duct. This study takes into account the variation of fluid temperature along the duct, which has been ignored in most of the earlier studies. The one-dimensional governing equation for temperature distribution along the fin is solved for all the fins of the fin array and the total heat transfer rate per unit system mass. total entropy generation rate and optimum fin height based on maximum heat transfer rate per unit system mass are evaluated from the derived temperature profiles. These quantities are then correlated as functions of geometric and flow parameters for three types of fin profile. Optimum solutions are generated based on (i) maximum heat dissipation rate per unit system mass and (ii) minimum entropy generation rate. A procedure to combine these two optima in order to obtain a holistic optimum is also discussed.