%0 Journal Article
%T Performance Evaluation Criterion at Equal Pumping Power for Enhanced Performance Heat Transfer Surfaces
%A Rajendra Karwa
%A Chandresh Sharma
%A Nitin Karwa
%J Journal of Solar Energy
%D 2013
%I 
%R 10.1155/2013/370823
%X The existing equations for the thermal performance evaluation, at equal pumping power for the artificially roughened and smooth surfaced multitube and rectangular duct heat exchangers, have been critically reviewed because the literature survey indicates that a large number of researchers have not interpreted these equations correctly. Three of the most widely used equations have been restated with clearly defined constraints and conditions for their application. Two new equations have been developed for the design constraints not covered earlier. 1. Introduction Artificial roughness in the form of ribs or baffles has been employed to enhance heat transfer in conventional heat exchangers, gas turbine blade cooling channels, and solar air heaters. However, the enhancement in heat transfer is also associated with significant increase in pressure drop and pumping power. Various thermal performance evaluation criteria, which take account of both heat transfer enhancement and pumping power, have been suggested for these enhanced performance surfaces based on different design constraints and objectives [1¨C6]. However, the criterion based on equal pumping power for the roughened and smooth surfaces suggested by Webb and Eckert [2] has been most widely used [7¨C41]. The performance ratios used by different researchers for the multitube heat exchangers are The Stanton number, St, Nusselt number, Nu, and the friction factor, , without subscript refer to the rough surface and with the subscript, , refer to the smooth surface. The ratio of heat transfer coefficients for the rough and smooth surfaces, , in (2) is replaced by the ratio of the overall heat transfer coefficients, , when heat transfer between two fluids is considered. The heat transfer coefficient or in (2) is calculated at the Reynolds number for the smooth surfaced tubes, which is defined by One of the old references where an equation similar to (1) has been given is that of Walker and Wilkie [1] for flow along the axis of a cluster of heated roughened rods in a smooth channel for the fixed mass flow rate and pumping power as constraints and the flow area as variable. Webb and Eckert [2], while discussing three different design objectives for multitube heat exchangers, proposed (1) for the design objective of enhanced heat transfer rate at equal pumping power for the roughened and smooth tubes. However, they developed this equation for a particular set of constraints and this equation is applicable only when a condition of specified ratio of mass velocities through the smooth and rough tubes is
%U http://www.hindawi.com/journals/jse/2013/370823/