Experimental investigation of mixed convection heat transfer from longitudinal fins in a horizontal rectangular channel: In natural convection dominated flow regimes M. Dogan a , M. Sivrioglu b, * a Department of Mechanical Engineering, Bozok University, 66200 Kampus, Yozgat, Turkey b Department of Mechanical Engineering, Gazi University, 06570 Maltepe, Ankara, Turkey article info Article history: Received 24 October 2008 Accepted 30 May 2009 Available online 10 July 2009 Keywords: Natural convection Fins Fin spacing Fin height Channel Heat transfer abstract Mixed convection heat transfer from longitudinal fins inside a horizontal channel has been investigated in the natural convection dominated region for a wide range of Rayleigh numbers and different fin heights and spacings. An experimental parametric study was made to investigate effects of fin spacing, fin height and magnitude of heat flux on mixed convection heat transfer from rectangular fin arrays heated from below in a horizontal channel. The optimum fin spacing to obtain maximum heat transfer has also been investigated. During the experiments constant heat flux boundary condition was realized and air was used as the working fluid. The velocity of fluid entering channel was kept nearly constant (0.02 6 w in 6 0.025 m/s) using a flow rate control valve so that Reynolds number was always about Re = 250. Experiments were conducted for modified Rayleigh numbers 3  10 7 < Ra * <6  10 8 and Richardson number 600 < Ri < 15,000. Dimensionless fin spacing was varied from S/H = 0.04 to S/H = 0.018 and fin height was varied from H f /H = 0.25 to H f /H = 0.80. The results obtained from experi- mental study show that the dimensionless optimum fin spacing which yields the maximum heat transfer is between S/H = 0.08 and S/H = 0.12. Results also show that optimum fin spacing depends on modified Rayleigh number and fin height. For high modified Rayleigh numbers (Ra * P 5  10 8 ) and low values of fin height (such as H f /H = 0.25) the optimum fin spacing has taken greater values in comparison to the cases of smaller modified Rayleigh numbers. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Fins are often used to enhance the rate of heat transfer dissipa- tion from heated surfaces to air. They can be placed on plane sur- faces, tubes, or other geometries. These surfaces have been used to augment heat transfer by adding additional surface area and encouraging mixing. When an array of fins is used to enhance heat transfer under natural convection conditions, the optimum geom- etry of fins (corresponding to a maximum rate of heat transfer) should be used, provided this is compatible with available space and financial limitations. Advances in printed circuit boards have yielded increasing power dissipation from surfaces in a channel. Rectangular fins are used extensively to increase the rates of natu- ral convection heat transfer from systems, because such fins are simple and cheap, to manufacture. Providing adequate cooling of printed circuits boards has recently motivated experiments on the use of longitudinal fins to enhance heat transfer in rectangular channels. The heat transfer to the fluid flowing through a channel by the heat dissipating surfaces can be obtained mainly by using the mechanisms of heat transfer by forced convection, natural con- vection and by radiative heat transfer. This paper deals with those issues related to the heat transfer obtained mainly by natural convection. An extensive review and discussion of work done on the con- vective heat transfer in electronic cooling were presented by Incropera [1], summarizing various convection cooling options. A great number of experimental and analytical work has been carried out on this problem since Elenbaas [2], first introduced the problem of natural convection between parallel plates. Star- ner and McManus [3], who measured the average heat transfer coefficient not only in horizontal but also in 45° and vertical base positions, have performed the first work on horizontal rect- angular fin arrays. They showed that incorrect application of fins to a surface actually might reduce the total heat transfer to a va- lue below that of the base alone. Welling and Wooldridge [4] conducted a similar experimental study on rectangular vertical fins to determine maximum heat transfer and tried to find opti- mum values for the ratio of fin height to fin spacing. Harahap and McManus [5] observed the flow patterns in two series of 0196-8904/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2009.05.027 * Corresponding author. Tel.: +90 312 2317400x2448; fax: +90 312 2308434. E-mail address: mecits@gazi.edu.tr (M. Sivrioglu). Energy Conversion and Management 50 (2009) 2513–2521 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman