International Journal of Thermal Sciences 41 (2002) 1101–1111 www.elsevier.com/locate/ijts Heat and fluid flow resulting from the chimney effect in a symmetrically heated vertical channel with adiabatic extensions Antonio Auletta, Oronzio Manca ∗ Dipartimento di Ingegneria Aerospaziale, Seconda Università degli studi di Napoli, Real casa dell’Annunziata, Via Roma 29, 81031 Aversa (CE), Italy Received 4 May 2001; accepted 7 January 2002 Abstract An experimental study on a channel-chimney system was carried out in order to elucidate the behavior of heat transfer and fluid flow. The results are presented in terms of local air temperature measurements inside the symmetrically heated channel and between the adiabatic extensions. Different fluid motion regions are observed inside the chimney. Inflows of air are detected in the lower extension ratio, particularly for large values of the ratio of the width of chimney to that of the heated channel. Some typical configurations show the presence of a vortex structure for an expansion ratio greater than one close to the corner regions in the chimney. Some monomial correlation equations between the local Nusselt number, the channel Rayleigh number and the geometric parameters are proposed. The dimensionless parameters are in the following ranges: 10 2  Ra ∗ (B/b)  10 6 ; 1.5  L/L h  4.0; 1.0  B/b  4.0, in which L is the total height of the system, L h is the height of the heated channel, B is the width of the chimney and b is the width of the heated channel. A good agreement between the correlation and the experimental data is observed.  2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Natural convection; Vertical channel; Experimental analysis; Enhancement of heat transfer; Uniform heat flux; Temperature measurements 1. Introduction The use of adiabatic extensions downstream of a heat sink or a heated vertical channel enhances natural convec- tive transfer in the heated part of a system [1–6]. Optimal configurations can be obtained for heat sink-chimney sys- tems [1–4] and channel-chimney systems [4,5]. With ref- erence to heated vertical channel-chimney systems, their thermal performance depends on the Elenbaas number (or channel Rayleigh number) as well as two non-dimensional geometric parameters: the extension ratio (total height of the system / height of the channel) and the expansion ratio (chimney spacing / channel spacing). Several studies have been carried out on the chimney effect both numerically and experimentally [1–3,5–12], as recently reviewed in [6]. The experimental studies are briefly reviewed in the following. * Corresponding author. E-mail addresses: antonio.auletta@unina2.it (A. Auletta), manca@unina.it (O. Manca). The heat sink-chimney system was studied experimen- tally in [3,4]. In [3] a vertical parallel-plate finned heat sink with a chimney was investigated. Results obtained using air confirmed to within 11% theoretical predictions of the over- all heat transfer and location of optima given in [1]. Flow- field measurements validated momentum transfer and cold inflow at the chimney exit. The measured average chimney velocity was found to be close to that predicted by the the- ory. Periodic cold inflow at the chimney exit was quantified by its frequency and found to reduce overall heat transfer by approximately 4%. The experiments confirmed that a chim- ney, when combined with free convection heat sinks resulted in enhanced heat transfer, and therefore permitted the use of smaller geometric dimensions. In [4] the performance a of a pin-fin heat sink with a chimney was obtained. Increased chimney height was shown to reduce the heat-sink temper- ature by as much as 30%. The experiments were compared with theoretical predictions and reasonable agreement was observed for overall heat transfer, optimal heat-sink poros- ity and fluid velocity. Finally, the phenomenon of periodic cold inflow at the chimney exit was observed and quantified. Results confirmed the same conclusions obtained in [3]. 1290-0729/02/$ – see front matter  2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII:S1290-0729(02)01396-0