HEAT TRANSFER ENHANCEMENT BY MULTILOBE VORTEX GENERATORS: EFFECTS OF LOBE PARAMETERS

Abstract

A three-dimensional computational method is developed to study the flow and heat transfer in a circular tube with a multilobe vortex generator inserted. Governing equations are discretized by the finite volume technique. The irregular lobe geometry is treated using curvilinear nonstaggered grids. Examination of the flow field reveals that the radial and counterradial flows induced inside and outside the lobe are forced to form axial vortices downstream of the vortex generator. Because of the transport of the high-speed, low-temperature core flow toward the tube wall by the vortex, heat transfer is enhanced. Also because of the vortex flow, the high-temperature wall flow is carried away from the wall to mix with the low-temperature core flow. Consequently, very effective augmentation in the heat transfer and mixing of flow temperature results. It is shown also that since the slope of the lobe is increased by enlarging the lobe penetration, reducing the lobe length, and making a concave contour geometry, the circulation, representing the strength of the axial vortex induced by the lobe, is promoted, leading to higher heat transfer and flow mixing. By increasing the lobe number both the circulation and Nusselt number (Nu) decrease because of the narrowed lobe passage.