Invention Journal of Research Technology in Engineering & Management (IJRTEM) www.ijrtem.com ǁ Volume 1 ǁ Issue 7 ǁ
ISSN: 2455-3689
Analysis of Natural Convention Heat Transfer Enhancement in Finned Tube Heat Exchangers Mahendra J. Sable Professor, Mechanical Engineering Department, Government College of Engineering, Jalgaon, 425001, India
ABSTRACT: Most of the engineering problems require high performance heat transfer components with progressively less weight, volumes, accommodating shapes and costs. Air cooled heat exchangers are subjected to air on outer side of heat exchanger surface on in heat recovery systems like economizers gases are subjected on one side of tube surface. On air or gas side heat transfer coefficient is less. Extended surface (fins) are one of the next exchanging devices that are employed extensively to increase heat transfer rates from tubular heat exchangers. The rate of heat transfer depends on the surface area of fin available for exchanging the heat transfer rate from the primary surface of cylindrical shape. Present study focuses on enhancement of heat transfer by using both circular and elliptical type of fins. The present paper attempts to examine trend of heat transfer coefficient experimentally and by using CFD software for various types of elliptical fins with i) varying elliptical ratio, ii) changing orientation of mounting of heat exchanger tube with elliptical fins, iii) varying spacing or fin density. KEY WORDS: Natural convection, Heat transfer enhancements, Elliptical fin, Fin orientation, Fin density.
INTRODUCTION The majority of passive techniques employ special surface geometry or fluid additives for enhancement of heat transfer without direct application of external power. Whenever it is difficult to increase the rate of heat transfer either by increasing heat transfer coefficient or by increasing the temperature difference between the surfaces and surrounding fluid, the fins are commonly used. Fin and tube heat exchangers are widely used to transfer heat between liquid and gas. Examples include condensers and evaporators in HVAC & R applications (heating, ventilation, air conditioning and refrigeration, water and oil cooling in vehicles or process plants etc. The dominant heat transfer resistance (up to 85% of the total thermal resistance) in these process is located on the gas side due to the lower thermal conductivity of the gas. In order to improve the heat transfer rate the exterior surface area is increased by adding fins. The disadvantage of adding fins is that the gas side frictional pressure drop increases. N. Nagarani et al.[3] has analyzed the heat transfer rate and efficiency for circular and elliptical annular fins for different environmental conditions.. Elliptical fin efficiency is more than circular fin. If space restriction is there along one particular direction while the perpendicular direction is relatively unrestricted elliptical fins could be a good choice. He stated the advantages of elliptical fin as (i)In cross flow, the aerodynamically shaped tubes will reduce the pressure drop of the external fluid to a great extent in the sub critical range. S. R. MCILWAIN [4] in his study compared heat transfer around a single serrated finned tube and a plain finned tube. He stated that with comparable bundle geometry, serrated fin finned tube heat exchangers transfer more heat than plain finned ones. H. Huisseune and C. T’Joen [5], in there study experimentally determined the heat transfer and friction correlation of a single row heat exchanger with helically finned tubes. The transversal tube pitch was parametrically varied. Gulay Yakar and Rasim Karabacak [6], studied the effects of holes placed on perforated finned heat exchangers at different angles on the Nusselt and Reynolds numbers. A. Nuntaphan , T. Kiatsiriroat , C.C. Wang [7], in their experimental work, studied heat transfer and friction characteristics of crimped spiral finned heat exchangers with dehumidification using forced convection . Their study experimentally examines the air-side performance of a total of 10 cross flow heat exchangers having crimped spiral configurations under the dehumidification. The effect of tube diameter, fin spacing, fin height, transverse tube pitch, and tube arrangements are examined. Rene Hofmann, Friedrich Frasz, Karl Ponweiser [8],in their study compared heat transfer and pressure drop in forced convection of different shaped fins on tube heat exchangers. Three different finned-tube shapes were investigated. The I-shaped helical, U-shaped helical and I-shaped serrated fins. They found that the heat transfer capacity of U-shaped finned-tube is better than that of an I-shaped finned-tube. The comparison of the serrated and solid finned tubes shows, the dimensionless heat transfer coefficient for serrated finned tube rows in staggered arrangement is greater than that for the solid tube bundle. Gregory J. Zdaniuk, Louay M. Chamra, Pedro J. Mago [9],in their experimental work determined the performance of heat exchanger with inside helical fins at different helix angles between 25° and 48° and at different fin height to diameter ratios between 0.0199 and 0.0327. M.J.Sable, S.K.Bhor[10], performed computational analysis for enhancement of natural convection heat transfer on vertical heated plate by multiple V-fin array. They used CFD analysis because it eliminates the cost associated with experimental setups for different geometries of fins and varying variables such as fin height, thickness, fin spacing which saves time and cost of experimentation. Thus in the context of above work, it was decided to determine heat transfer performance of elliptical fins for varying elliptical ratios and for changing orientations and fin spacing’s, both experimentally and using CFD analysis which has not been attempted so far.
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