Heat transfer enhancement and drag by longitudinal vortex generators in channel flow

Abstract

Triangular and rectangular longitudinal vortex generators were formed by punching small pieces out of flat plate fins so that they stuck out of the plates and formed an angle (angle of attack) with the main flow direction. The fins were mounted on top of each other to form channels, each representing an idealization of a gas-side element of a compact heat exchanger. The effects of single-vortex generators on flow structure, flow losses, and heat transfer were investigated. The vortex structure was observed, the drag—a measure of flow losses—was measured by a balance, and the local heat transfer coefficient was obtained from unsteadt liquid crystal thermography. Vortex generator geometry, angle of attack, and Reynolds number were varied. Stable longitudinal vortices were found up to much higher angles of attack than for corresponding wings in a free stream. The drag induced by the vortex generators was found to be proportional to the projected area and independent of the shape and the Reynolds number. Local heat transfer augmentation of several hundred percent and mean heat transfer enhancement of more than 50% over an area more than 50 times the vortex generator area were achieved. The heat transfer enhancement per unit vortex generator area was highest for delta wings followed by delta winglets and rectangular winglets.