INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS Int. J. Numer. Meth. Fluids 2011; 65:1145–1159 Published online 10 February 2010 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/fld.2232 Thermally developing flow in finned double-pipe heat exchanger Mazhar Iqbal 1, ∗, † and K. S. Syed 2 1 College of E&ME,(DBS&H), National University of Science and Technology, Islamabad, Pakistan 2 CASPAM, Bahauddin Zakariya University, Multan, Pakistan SUMMARY A numerical solution of the convective heat transfer in the thermal entry region of the finned double-pipe is carried out for the case of hydro-dynamically fully developed flow when subjected to uniform wall temperature boundary condition. Adaptive axial grid size is used in order to cater for the variation of large solution gradients in the axial direction. It has been observed that the thermal entrance region is highly effective and there is a substantial enhancement in the heat transfer coefficient. A maximum of 76.4877% increase has been observed in the thermal entrance region as compared with the fully developed region for 24 fins and H ∗ = 0.6 when ˆ R = 0.25, whereas for ˆ R = 0.5 the maximum increase is 75.0308% for the same number of fins of same height. It has been observed that no geometry consistently perform better throughout the entrance region. However, the geometries that have optimal performance in the fully developed region perform better in the developing region on average terms. Results show that the Nusselt number and the thermal entrance length are dependent upon various geometrical parameters such as ratio of radii of the inner and the outer pipe, fin height and the number of fins. The limiting case results match well with the literature results. This validates our numerical procedure and computer code. Copyright 2010 John Wiley & Sons, Ltd. Received 17 October 2008; Revised 18 September 2009; Accepted 8 November 2009 KEY WORDS: heat transfer; Nusselt number; entrance length; entrance region; fully developed flow; thermally developing flow INTRODUCTION Convective heat transfer occurs in a variety of engineering problems such as space heating, air- conditioning, power production, chemical processing, and many other engineering phenomena, and is of particular importance where viscous fluids are heated or cooled. Since the heat transfer in these types of fluids is generally low, therefore, there is a need for augmentation. One of the most effective methods of enhancing the convective heat transfer is the use of extended surfaces. The longitudinal finned tubes have been a widely used geometry. There is literature evidence indicative of the fact that a significant level of heat transfer enhancement may be achieved through this type of finned tube as compared with the plane tubes. Masliyah and Nandakumar [1] studied the heat transfer characteristics for laminar forced convection of fully developed flow in an internally finned circular tube with axially uniform heat flux and peripherally uniform temperature using finite element method. They have reported that the influence of the fin height is pronounced for its large values. Sparrow and Charmchi [2] studied the heat transfer in the externally finned tube and reported a substantial heat transfer enhancement as compared with smooth tube. Zeitoun and ∗ Correspondence to: Mazhar Iqbal, College of E&ME, (DBS&H), National University of Science and Technology, Islamabad, Pakistan. † E-mail: iqbalmazhar107@yahoo.com Copyright 2010 John Wiley & Sons, Ltd.