Heat transfer enhancement by the chimney eect in a vertical iso¯ux channel A. Auletta a , O. Manca a , B. Morrone a , V. Naso b, * a Dipartimento di Ingegneria Aerospaziale, Seconda Universit a degli studi di Napoli Real Casa dell'Annunziata, Via Roma 29-81031, Aversa CE), Italy b Dipartimento di Energetica, Termo¯uidodinamica applicata e Condizionamenti ambientali Universit a degli studi Federico II, Piazzale Tecchio 80125, Naples, Italy Received 4 February 2000; received in revised form 24 November 2000 Abstract The ease of thermal control by means of air natural convection stimulates the investigation of con®gurations with the aim at improving the thermal performance. The eect of adding adiabatic extensions downstream of a vertical iso¯ux symmetrically heated channel has been experimentally analyzed. Optimal con®gurations have been identi®ed through the measured wall temperature pro®les, 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 con®gurations. In details, percent increases of the heat transfer rate i.e., average channel Nusselt number) were of order 10±20% depending on the channel aspect ratio, L h =b, and imposed wall heat ¯ux. In any case, quite large extensions should 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 con®gurations. Composite correlations between the average Nusselt number and the maximum dimensionless wall temperatures and Ra , the Rayleigh channel number, B=b and L=L h parameters, in the 1:5 6 L=L h 6 4:0; 1:0 6 B=b 6 4:0 and 10 2 6 Ra B=b 6 5:0 10 6 ranges, have been evaluated. Ó 2001 Elsevier Science Ltd. All rights reserved. 1. Introduction Nowadays more recent investigation trends in natu- ral convective heat transfer are oriented towards either the seeking of new con®gurations to enhance the heat transfer parameters or the optimization of standard con®gurations. This is true in particular for thermal design in cooling of electronics as it has been well ex- plained in [1±3] recently. In fact, the continuously growing heat transfer rates to be dissipated force re- searchers to focus their eorts to improve the heat transfer coecients, without renouncing the cheap and reliable natural convection mechanism. A con®guration where natural convection heat transfer has been widely studied is the heated vertical parallel plates, as reported in [4±6] and reviewed more recently in [7]. This review does not need to be repeated here. Several authors tried to optimize the thermo-geo- metrical con®gurations to enhance the heat transfer, both from the experimental side and from the numerical one, as for example in [2,3,8]. One of the techniques employed to improve the heat transfer rate is the placement of adiabatic extensions downstream of the heated channel. This induces an in- crement of the ¯ow rate due to the well-known chimney eect. Haaland and Sparrow [9] were the ®rst research- ers who showed that a higher ¯ow rate of ¯uid through a con®ned open-ended enclosure can be induced by the chimney eect. They accomplished a numerical solution for natural convection ¯ow in a vertical channel with a point heat source channel plume problem) or distrib- uted heat sources situated at the channel inlet chimney problem). Oosthuizen [10] numerically studied the heat transfer enhancement caused by the addition of straight adiabatic extensions at the exit of isothermal International Journal of Heat and Mass Transfer 44 2001) 4345±4357 www.elsevier.com/locate/ijhmt * Corresponding author. Tel.: +39-081-768-2302; fax: +39- 081-239-0364. E-mail address: vinaso@unina.it V. Naso). 0017-9310/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII:S0017-931001)00064-3