Combined experimental and numerical approaches to multi-mode heat transfer between vertical parallel plates

In this paper, the results of an experimental and semi-experimental investigation of steady laminar natural convection and surface radiation between three parallel vertical plates, viz., a central hot plate coated with blackboard paint and two unheated side plates that are polished, symmetrically spaced on each side, with air as the intervening medium, are presented. In the semi-experimental method, the measured wall temperatures are fed as (a) segmental values and (b) average value, into a numerical program for generating the values of convective Nusselt number, and these are compared with the experimental results obtained by calorimetry. The radiative heat transfer rate at the hot surface was computed by the radiosity-irradiation method and this was deducted from the power input to the heater, to obtain experimentally the convective heat transfer rates and hence the convective Nusselt numbers. The experiments were done for six plate spacings ranging from 12.66 to 52.2 mm and for an order of magnitude range of wall-to-ambient temperature difference. The analysis brings out the significance of radiation heat transfer rate even at low temperatures of 310 K. The concordance between the average convective Nusselt numbers obtained from the experimental, and semi-experimental investigation taking into account the variation of temperature along the plate and under the isothermal assumptions, shows that the isothermal approximation itself is adequate. A correlation for the average convective Nusselt number in terms of Grashof number and the aspect ratio is developed based on 133 data for a Grashof number range 2370

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