Heat transfer enhancement by the chimney effect in a vertical isoflux channel

The case of thermal control by means of air natural convection stimulates t he investigation of configurations with the aim at improving the thermal pe rformance. The effect of adding adiabatic extensions downstream of a vertic al isoflux symmetrically heated channel has been experimentally analyzed. O ptimal configurations have been identified through the measured wall temper ature profiles, with reference to the extension and expansion ratios (L/L-h and B/b) of the insulated extensions. Conspicuous maximum wall temperature reductions have been achieved by means of these optimal configurations. In details, percent increases of the heat transfer rate (i.e., average channe l Nusselt number) were of order 10-20% depending on the channel aspect rati o, L-h/b, and imposed wall heat flux. In any case, quite large extensions s hould be added to enhance the heat transfer rate, i.e. about 3.0 times the height of the channel, while the optimal expansion ratio was nearly 2.0 for all the configurations. Composite correlations between the average Nusselt number and the maximum dimensionless wall temperatures and Ra*, the Raylei gh channel number, B/b and L/L-h parameters, in the 1.5 less than or equal to L/L-h less than or equal to 4.0, 1.0 less than or equal to B/b less than or equal to 4.0 and 10(2) less than or equal to Ra*B/b less than or equal to 5.0 x 10(6) ranges, have been evaluated. (C) 2001 Elsevier Science Ltd. All rights reserved.