Free convective heat transfer from a constant heat flux vertical circular tube with different entrance restrictions length Hussein A. Mohammed a, * , Yasin K. Salman b a University Tenaga Nasional, College of Engineering, Mechanical Engineering Department, Km 7, Jalan Kajang, Puchong, 43009 Kajang, Malaysia b Baghdad University, College of Engineering, Nuclear Engineering Department, Baghdad-Al-Jaderyia, Iraq Received 29 May 2006; accepted 10 December 2006 Available online 18 April 2007 Abstract Free convection from the inside surface of vertical circular tube with a uniformly heated surface under constant wall heat flux was investigated experimentally for laminar air flow in the ranges of Ra L from 6.9 · 10 8 to 5 · 10 9 . The effect of restrictions length placed at upper side of the heated tube on the surface temperature distribution, the local and average heat transfer coefficients were studied. The experimental apparatus consists of an aluminum tube with 900 mm length and 30 mm inside diameter. The entry restrictions included a circular tube of the same diameter as the heated tube but with different lengths of 60 cm, 90 cm, 120 cm, 150 cm and 180 cm. It was found that the surface temperature along the tube surface had higher values for the restriction with a length of 180 cm and smaller values for the restriction with a length of 120 cm. The results show that the local Nu x and average Nusselt number Nu were higher for the restriction with a length of 120 cm and lower for the restriction with a length of 180 cm. The results obtained were correlated by dimensionless groups as Log Nu L against Log Ra L for each case, and a general correlation for all cases was proposed. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Free convection; Constant heat flux; Vertical circular tube; Upper entrance restriction; Different entrance section lengths 1. Introduction Free convection heat transfer has always been of partic- ular interest among heat transfer problems. In free convec- tion processes, fluid motion is caused by density variations resulting from the temperature distribution between the fluid and the contacting surface. Natural convection from cylinders or tubes of circular shapes have been receiving growing interest in the last few decades because of its employment in many practical fields in the area of energy conservation, design of solar collectors, heat exchangers, nuclear engineering, cooling of electrical and electronic equipments and many others [1]. Heat transfer studies of natural convection from circular tubes are necessary for better thermal design of industrial applications. Experi- ments using different fluids and different values of length to diameter ratio in both the isothermal surface and con- stant wall heat flux conditions are still needed to provide a complete investigation of the problem. This research was conducted in an attempt to fill a part of the existing gap and provides experimental data by experimentally investigating free convection heat transfer from the inside surface of uniformly heated constant wall heat flux in a ver- tical tube to air. The available work on free convection from the inside surfaces of vertical tubes open at both ends with a restriction at the entry is limited. Extensive analyti- cal and experimental work has been conducted with fluids in a tube subject to different boundary conditions, but the specific problem that will be considered here is that of con- stant wall heat flux. Dyer [2] presented a theoretical and experimental study of laminar air flow free convection in heated open ended vertical ducts. The temperature and velocity fields and the relationship between the Nu and Ra numbers were obtained by solving the governing equations by a step by step numerical technique. The 0196-8904/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2006.12.024 * Corresponding author. Tel.: +6 03 89216271; fax: +6 03 89212116. E-mail address: hussein@uniten.edu.my (H.A. Mohammed). www.elsevier.com/locate/enconman Energy Conversion and Management 48 (2007) 2233–2243