Experimental study of natural convection heat transfer from an enclosed assembly of thin vertical cylinders Muhammad Arshad * , Mansoor Hameed Inayat, Imran Raﬁq Chughtai Department of Chemical and Materials Engineering, Pakistan Institute of Engineering and Applied Sciences, P. O. Nilore, Islamabad 45650, Pakistan article info Article history: Received 29 December 2009 Accepted 26 July 2010 Available online 11 August 2010 Keywords: Heat transfer Natural Convection Cylinder Assembly Turbulent boundary layer abstract Steady state heat transfer by natural convection was investigated experimentally from an enclosed assembly of thin vertical cylinders at high Rayleigh numbers. The published literature lacks any infor- mation regarding such a study in the turbulent boundary layer regime. All the available literature is for the laminar region for an enclosed assembly. The enclosed assembly consisted of a 3  3 array of vertical cylinders immersed in a large volume tank of water. All the cylinders were electrically heated. Uniform heat ﬂuxes at various power levels were applied to each cylinder and the surface temperature at different positions along the cylinders were recorded. The experimental results show that the surface temperature distribution increases axially up to a certain length, then decreases due to mixing which increases the heat transfer. The heat transfer coefﬁcient, Nusselt number and modiﬁed Rayleigh number have also been presented for the experimental data. Empirical correlations between overall Nusselt number and average modiﬁed Rayleigh number have been developed for an assembly of cylinders. This empirical correlation for an assembly is valid for 1.28  10 12  Ra * L  1.18  10 13 . The maximum natural convection at central cylinder and Chimney effects in an assembly are observed during these experiments. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction The natural convection heat transfer occurs due to the ﬂuid motion induced by the density gradient in a heating or cooling process. Natural convection from vertical cylinders has many engineering applications such as heat loss from process piping, HVAC system, cooling of electronic components, steam or electrical heating in process vessels, heat removal from spent nuclear fuel bundles and cooling of nuclear reactor core after loss of coolant accident etc. Natural convection from vertical cylinder and a vertical plate differs signiﬁcantly due to the curvature effects [1]. For short cylinders having small length to diameter (L/d) ratio, the natural convection from a single vertical cylinder is similar to that of a plate. The heat transfer correlations of vertical plate are applicable to short cylinder without any modiﬁcations. However, for the thin (long) cylinders, special treatment is required due to the signiﬁcant curvature effects. Moreover, the heat transfer around a single thin cylinder differs from that of a similar cylinder in an assembly due to affect of heat transfer of other cylinders in assembly. In the present study, the natural convection from thin cylinders in an assembly has been experimentally investigated. The natural convection characteristics of each cylinder prior to its ﬁtting in the assembly were experimentally veriﬁed. This was done by comparing, the Nusselt numbers obtained from experiments, with the previously reported data [2e4]. Natural convection heat transfer from a single vertical cylinder has been investigated extensively both numerically and experi- mentally as reviewed by Popiel [1]. Arshad et al. [5] presented the temperature proﬁles and the heat transfer coefﬁcient for single vertical cylinder immersed in stationary water. The present study for single cylinder is the continuation of the earlier work. The Nusselt numbers for single cylinder at different modiﬁed Rayleigh numbers are in good agreement with the previous experimental results. Hence, these cylinders can be used for further experimental study in an assembly. In the present study, heat transfer by natural convection through assembly of vertical cylinders was experimentally inves- tigated at various values of heat ﬂux. A little literature was found for natural convection heat transfer through assemblies of vertical cylinders. Keyhani et al. [6] experimentally investigated free convection in different conﬁgurations of rod bundles and reported the correlations for Nusselt number, however they studied only the laminar region (Gr < 7  10 9 [1]). Isahai & Hattori [7] numerically studied natural convection with few experiments, using vertical cylinders of different triangular pitch at modiﬁed Grashof number (Gr*) from 10 to 10 8 . They used boundary ﬁtted coordinate system * Corresponding author. Tel.: þ92 333 6154636; fax: þ92 51 2208070. E-mail address: marshad2000@gmail.com (M. Arshad). Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng 1359-4311/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2010.07.031 Applied Thermal Engineering 31 (2011) 20e27