VOL. 4, NO. 5, JULY 2009 ISSN 1819-6608 ARPN Journal of Engineering and Applied Sciences © 2006-2009 Asian Research Publishing Network (ARPN). All rights reserved. www.arpnjournals.com EXPERIMENTAL INVESTIGATIONS IN A CIRCULAR TUBE TO ENHANCE TURBULENT HEAT TRANSFER USING MESH INSERTS Naga Sarada S., Kalyani K. Radha and A. V. S. Raju Department of Mechanical Engineering, JNTUH College of Engineering, Hyderabad, Andhra Pradesh, India E-Mail: kalyaniradha@gmail.com ABSTRACT The present work shows the results obtained from experimental investigations of the augmentation of turbulent flow heat transfer in a horizontal tube by means of mesh inserts with air as the working fluid. Sixteen types of mesh inserts with screen diameters of 22mm, 18mm, 14mm and 10mm for varying distance between the screens of 50mm, 100mm, 150mm and 200mm in the porosity range of 99.73 to 99.98 are considered for experimentation. The Reynolds number is varied from 7000 to 14000. Correlations for Nusselt number and friction factor are developed for the mesh inserts from the obtained results. It is observed that the enhancement of heat transfer by using mesh inserts when compared to plain tube at the same mass flow rate is more by a factor of 2 times where as the pressure drop is only about a factor of 1.45 times. Keywords: tube, heat transfer, mesh, turbulent flow, pressure drop, augmentation. INTRODUCTION In the recent years, considerable emphasis has been placed on the development of various augmented heat transfer surfaces and devices. This can be seen from the exponential increase in world technical literature published in heat transfer augmentation devices, growing patents and hundreds of manufacturers offering products ranging from enhanced tubes to entire thermal systems incorporating enhancement technology. Energy and materials saving considerations, space considerations as well as economic incentives have led to the increased efforts aimed at producing more efficient heat exchanger equipment through the augmentation of heat transfer. Among many techniques investigated for augmentation of heat transfer rates inside circular tubes, a wide range of inserts have been utilized, particularly when turbulent flow is considered. The inserts studied included twisted tape inserts, coil wire inserts, brush inserts, mesh inserts, strip inserts etc. The utilization of porous inserts has proved to be very promising in heat transfer augmentation. One of the important porous media characteristics is represented by an extensive contact surface between solid and fluid surfaces. The extensive contact surface enhances the internal heat exchange between the phases and consequently results in an increased thermal diffusivity. Different types of porous materials are extensively studied in forced convection heat transfer due to the wide range of potential engineering applications such as electronic cooling, drying processes, solid matrix heat exchangers, heat pipe, enhanced recovery of petroleum reservoirs etc. however the experimental work carried out in this area is limited. Experimental studies conducted for heat transfer and pressure drop of laminar flow in horizontal tubes with/without longitudinal inserts (Shou-Shing Hsieh et al., 2003). They reported that enhancement of heat transfer as compared to a conventional bare tube at the same Reynolds number to be a factor of 16 at Re <= 4000, while a friction factor rise of only 4.5. Hsieh and Kuo (Shou- Shing Hsieh et al., 2003) conducted experimental investigations for the augmentation of tube side heat transfer in a cross flow heat exchanger for turbulent flow of air by means of strip type inserts. They found that longitudinal strip inserts perform better than crossed strip (CS) and regularly interrupted strip (RIS) inserts for high Reynolds number (Shou-Shing Hsieh et al., 2003). Hsieh and Wu (Shou-Shing Hsieh et al., 2000) conducted experimental studies on heat transfer and flow characteristics for turbulent flow of air in a horizontal circular tube with strip type inserts (longitudinal and Crossed Strip inserts). They reported that friction factor rise due to inclusion of inserts was typically between 1.1 and 1.5 from low Re (=6500) to high Re (=19500) with respect to bare tube. The experimental investigations of Hsieh and Liu (Shou-Shing Hsieh et al., 1996) report that Nusselt numbers were between four and two times the bare values at low Re and high Re respectively. Bogdan I.Pavel (Bogdan I. Pavel et al., 2004) experimentally investigated the effect of metallic porous inserts in a pipe subjected to constant and uniform heat flux at a Reynolds number range of 1000-4500. The maximum increase in the length-averaged Nu number of about 5.2 times in comparison with the clear flow case and a highest pressure drop of 64.8Pa were reported with a porous medium fully filling the pipe. Angirasa (Devarakonda Angirasa, 2001) performed experiments that proved augmentation of heat transfer by using metallic fibrous materials with two different porosities namely 97% and 93%. The experiments were carried out for different Reynolds numbers (17,000-29,000) and power inputs (3.7 and 9.2 W). The improvement in the average Nusselt number was about 3-6 times in comparison with the case when no porous material was used. Fu (Fu.H.L et al., 2001) experimentally demonstrated that a channel filled with high conductivity porous material subjected to oscillating 53