Two-Dimensional Effects and Design Criteria for Convective Extended Surfaces

Abstract An analysis of the thermal performance of convective extended surfaces is presented that takes into account the primary function of the fins, which is to augment the heat transfer. The measure of the heat transfer augmentation is expressed by the removal number Nr, which we proposed should be O(10). It is shown that in order to satisfy this criterion the transverse Biot number hw/k must be of the order of 0.01. Further, it is argued that the fin is fully utilized if the aspect number is O(1). These criteria, Bi; = O(0.01) and u = 0(1), offer guidelines for designing fins and are applicable to longitudinal, annular, and pin fins, of uniform thickness or tapered profile. They must substitute the criterion that is frequently used, Bi < I, which was derived from the analysis of longitudinal fins. A very important consequence of this investigation is that fins designed to meet the above criteria can be analyzed using the classical one-dimensional method without introducing any appreciable error.