Free convection from a constant heat flux elliptic tube Amr O. Elsayed * , Emad Z. Ibrahim, Sayed A. Elsayed Department of Mechanical Power Engineering, Faculty of Engineering, Zagazig University, Zagazig, Egypt Received 7 August 2002; accepted 11 December 2002 Abstract An experimental investigation is presented on free convection of air around the outer surface of a constant heat flux elliptic tube. The local and average Nusselt number distribution is reported for different valuesofRayleighnumberanddifferenttubeinclinationangles.ThetestRayleighnumber,basedoninput heat flux, ranges from 1:1  10 7 to8  10 7 . Average Nusselt numbers are evaluated and correlated with Rayleigh number for the elliptic tube with vertical major axis. Comparison between the convection char- acteristics of isothermal and constant heat flux elliptic tubes has been presented. Also, the effect of elliptic tubeorientationontheaverageNusseltnumberisshown.ItisfoundthatahighervalueofaverageNusselt number is achieved when the major axis of the tube is vertical. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Free convection; Constant heat flux elliptic tube; Convection of air around tube; Effect of elliptic tube orientation 1. Introduction Free convection is an important phenomenon in many fields of engineering applications, such as solar collectors, thermal energy storage, waste heat utilization, cooling of electronic compo- nents and many others. The study of free convection around heated cylinders is essential for a better thermal design of such heat exchangers, since tubes with circular cross section are most commonly used in engineering work. Most previous studies have been concerned with natural convection around vertical and horizontal circular tubes, e.g. Refs. [1–4]. Recently,tubesofellipticshapehavereceivedmoreattentioninfreeconvectionresearchfields. Fieg and Roetzel [5] showed that the elliptical deformation increases the heat transfer coefficient Energy Conversion and Management 44 (2003) 2445–2453 www.elsevier.com/locate/enconman * Corresponding author. E-mail address: amro9992000@yahoo.com (A.O. Elsayed). 0196-8904/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0196-8904(03)00002-5