Conjugate turbulent natural convection and surface radiation in rectangular enclosures heated from below and cooled from other walls, typically encountered in Liquid Metal Fast Breeder Reactor (LMFBR) subsystems, have been investigated by a finite volume method for various aspect ratios. The formulation comprises the standard two equation k-ε turbulence model with physical boundary conditions (no wall functions), along with the Boussinesq approximation, for the flow and heat transfer. As far as radiation is concerned, the radiosity - irradiation formulation for a transparent fluid of Prandtl number 0.7 has been employed. The conjugate coupling on the walls has been handled by using a fin type formulation. The Rayleigh number based on the width of the enclosure is varied from 108 to 1012 and the aspect ratio is varied from 0.5 to 2.0. Detailed results including stream lines, temperature profiles, and convective, radiative and overall Nusselt numbers are presented. A correlation for the mean convection Nusselt number in terms of Rayleigh number and aspect ratio is proposed for design purposes. The influence of the wall emissivity and the external heat transfer coefficient on the heat transfer from the enclosure has also been investigated. © 2006.

C. Balaji

Department of Mechanical Engineering

Approximation theory

Boundary conditions

Fins (heat exchange)

Mathematical models

Prandtl number

Turbulent flow

BOTTOM HEATED ENCLOSURE

Surface radiation

TRANSPARENT MEDIA

TURBULENT NATURAL CONVECTION

Natural convection

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

International Journal of Heat and Mass Transfer

Two-dimensional numerical studies of flow and temperature fields for turbulent natural convection and surface radiation in inclined differentially heated enclosures are performed. Investigations are carried out over a wide range of Rayleigh numbers from 108 to 1012, with the angle of inclination varying between 0° and 90°. Turbulence is modeled with a novel variant of the k-ε closure model. The predicted results are validated against experimental and numerical results reported in literature. The effect of the inclination of the enclosure on pure turbulent natural convection and the latter's interaction with surface radiation are brought out. Profiles of turbulent kinetic energy and effective viscosity are studied to observe the net effect on the intensity of turbulence caused by the interaction of natural convection and surface radiation. The variations of local Nusselt number and average Nusselt number are presented for various inclination angles. Marked change in the convective Nusselt number is found with the orientation of enclosure. Also analyzed is the influence of change in emissivity on the flow and heat transfer. A correlation relevant to practical applications in the form of average Nusselt number, as a function of Rayleigh number, Ra, radiation convection parameter, N RC and inclination angle of the enclosure, φ is proposed. © 2007 Springer-Verlag.

Heat and Mass Transfer/Waerme- und Stoffuebertragung

Interaction of turbulent natural convection and surface thermal radiation in inclined square enclosures

In case of heavy water moderated nuclear reactors, the decay heat of a fuel bundle can be transferred to the moderator available outside calandria vessel. Radiative heat transfer accounts for significant amount of total energy exchange from the fuel bundle during LOCA (loss of coolant accident) situation. In order to simulate one channel of an advanced reactor comprising 54 nuclear rods of 3.5 m length, scaling analysis has been used to scale it down to an 18 rod bundle facility of 1 m test length. The experimental analysis, in order to examine the contribution of energy transfer by thermal radiation and natural convection in removing decay heat of fuel/heater rods using moderator as a heat sink, is presented. It focuses mainly on the role of moderator in removal of decay heat, for situation with air alone in the annular gap between pressure tube and calandria tube coupled with flowing and standstill/boiling moderator situations. Four experimental tests have been presented in this paper. It is seen that, 80% of heat energy is transferred from the heater bundle of rods to the enclosing pressure tube by means of radiation mode, out of this 20-45% of energy is carried away by the moderator. It depends mainly on the resistances involved in the heat flow path and the test set up conditions. © 2015 Elsevier Ltd. All rights reserved.

Progress in Nuclear Energy

Evaluation of contribution of thermal radiation in transferring decay heat to the moderator, in case of 18 rod bundle facility, simulating advanced nuclear reactors

During a loss of coolant accident in nuclear reactors, radiation heat transfer accounts for a significant amount of the total heat transfer in the fuel bundle. In case of heavy water moderator nuclear reactors, the decay heat of a fuel bundle enclosed in the pressure tube and outer concentric calandria tube can be transferred to the moderator. Radiation heat transfer plays a significant role in removal of decay heat from the fuel rods to the moderator, which is available outside the calandria tube. A single heater rod test facility is designed and fabricated as a part of preliminary investigations. The objective is to anticipate the capability of moderator to remove decay heat, from the reactor core, generated after shut down. The present paper focuses mainly on the role of moderator in removal of decay heat, for situation with air alone in the annular gap of pressure tube and calandria tube. It is seen that the naturally aspirated air is capable of removing the heat generated in the system compared to the standstill air or stagnant water situations. It is also seen that the flowing moderator is capable of removing a greater fraction of heat generated by the heater rod compared to a stagnant pool of boiling moderator. © 2010 Elsevier Inc.

Experimental Thermal and Fluid Science

Experimental investigations on decay heat removal in advanced nuclear reactors using single heater rod test facility: Air alone in the annular gap

A thermal lattice Boltzmann method based on the BGK model has been used to simulate high Rayleigh number natural convection in a square cavity. The model uses the double populations approach to simulate hydrodynamic and thermal fields. The traditional lattice Boltzmann method on a uniform grid has unreasonably high grid requirements at higher Rayleigh numbers which renders the method impractical. In this work, the interpolation supplemented lattice Boltzmann method has been utilized. This is shown to be effective even at high Rayleigh numbers. Numerical results are presented for natural convection in a square cavity with insulated horizontal walls and isothermal vertical walls maintained at different temperatures. Very fine grids (wall y+ &lt; 0.3) have been used for the higher Rayleigh number simulations. A universal structure is shown to exist in the mean velocity turbulent boundary layer profile for y+ &lt; 10. This agrees extremely well with previously reported experimental data. The numerical results (for Rayleigh numbers up to 1010) are in very good agreement with the benchmark results available in the literature. The highlight of the calculations is that no turbulence model has been employed. © 2005 Elsevier Ltd. All rights reserved.

Simulation of high Rayleigh number natural convection in a square cavity using the lattice Boltzmann method

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. © 1993.

International Journal of Heat and Fluid Flow

Interaction of surface radiation with free convection in a square cavity

Applied Thermal Engineering

Natural convective heat transfer in square enclosures heated from below

Numerical Heat Transfer, Part A: Applications

INTERACTION BETWEEN NATURAL CONVECTION AND RADIATION IN A SQUARE CAVITY HEATED FROM BELOW

Renewable Energy

Natural convection in enclosures with partial partitions

Conjugate turbulent natural convection with surface radiation in air filled rectangular enclosures

Journal	International Journal of Heat and Mass Transfer
ISSN	00179310
Open Access	No