The effects of different entrance sections lengths and heating on free and forced convective heat transfer inside a horizontal circular tube ☆ Hussein A. Mohammed a, ⁎ , Yasin K. Salman b a Baghdad University, College of Engineering, Mechanical Engineering Department, Baghdad, Al-Jaderyia, Iraq b Baghdad University, College of Engineering, Nuclear Engineering Department, Baghdad-Al-Jaderyia, Iraq Available online 19 April 2007 Abstract An experimental study was done for hydrodynamically fully developed and thermally developing laminar air flows in a horizontal circular tube has a 30 mm inside diameter and 900 mm heated length (L/D = 30) under a constant wall heat flux boundary condition, with different aluminum entrance section pipes (calming sections) having the same inside diameter as test section pipe but with variable lengths of 600 mm (L/D = 20), 1200 mm (L/D = 40), 1800 mm (L/D = 60), and 2400 mm (L/D = 80). The Reynolds number ranged from 400 to 1600 and the heat flux is varied from 60 W m - 2 to 400 W m - 2 . This paper examines the effects of the entrance sections lengths and heating on the free and forced convection heat transfer process. The surface temperature data were measured and heat transfer rates at different heat flux levels as well as different Reynolds numbers were calculated and correlated in the form of relevant parameters. The buoyancy force has a significant effect on the heat transfer and the combined convection factor was approximately varied form 0.13 ≤ Gr/Re 2 ≤ 7.125. It was found that the surface temperature increases as the entrance section length increases. It was inferred that the heat transfer decreases as the entrance section length increases due to the flow resistance and the mass flow rate. The proposed correlation was compared with available literature and with laminar forced convection and showed satisfactory agreement. © 2007 Elsevier Ltd. All rights reserved. Keywords: Free and forced convection; Fully developed flow; Horizontal circular tube; Entrance section length effect 1. Introduction Combined free and forced convection in the tube entrance region has many diverse industrial and engineering applications, such as heat exchangers, electronic equipment and chemical processes [1]. It is long recognized that gravity plays an important role in characterizing combined convection tube flows. If the orientation of the tube is not vertical, the buoyancy force induces the secondary flow, which enhances heat transfer between the wall and the fluid, such effects are most pronounced in horizontal flows. Combined convection in horizontal tubes gives rise to secondary International Communications in Heat and Mass Transfer 34 (2007) 769 – 784 www.elsevier.com/locate/ichmt ☆ Communicated by W.J. Minkowycz. ⁎ Corresponding author. University Tenaga Nasional, College of Engineering, Mechanical Engineering Department, Km7, Jalan Kajang - Puchong, 43009 Kajang, Selangor, Malaysia. E-mail address: hussein@uniten.edu.my (H.A. Mohammed). 0735-1933/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.icheatmasstransfer.2007.03.005