A Computational Study of the Heat Transfer Characteristics of Offset-Strip Fin Cores

摘要

Enhanced extended surfaces such as offset-strip fins (OSF) are effective in increasing the area density as well as altering the convective flow behavior to provide higher heat transfer coefficient in compact heat exchangers. This is achieved by periodic disruption and reattachment of the new thin boundary layer on the fin plate of each offset-strip fin module. The heat transfer characteristics and the flow physics inside the OSF cores is revisited in this computational analysis for laminar air flows (Pr = 0.7) and their performance is compared with plain fins. A simplified model of thin fins is used to study the effect of fin geometric parameters, viz., offset-fin length l, fin separation s and fin thickness t. The parametric variation is restricted to the practical range of fin density (8 fpi to 22.6 fpi) and low blockage ratio to the flow (<20%) while systematically increasing the offset-fin length (1 < l/s < 35). The results show that the short offset-fin length provides higher heat transfer enhancement compared to the plain rectangular fins, while the fin thickness and fin separation show negligible improvement in heat transfer rate for a constant offset-fin length ratio (l/s). The offset-fin effect diminishes as Reynolds number decreases or the offset-fin length becomes very large, as the OSF performance asymptotically approaches towards that of a plain rectangular fin. The OSF cores are shown to reduce the heat transfer surface area by 30% - 50% while keeping a constant pressure drop as that for a plain rectangular fin. A practical case with short offset-fin length (∼ 3mm) having a squarer cross-section fins (s/h >0.5) with intermediate to low fin density (8 fpi to 12 fpi) provides smaller pressure drop gradients as well as larger heat transfer enhancement capacity for a constant heat duty application.