EXPERIMENTAL INVESTIGATIONS OF HEAT-TRANSFER ENHANCEMENT AND FLOW LOSSES IN A CHANNEL WITH DOUBLE ROWS OF LONGITUDINAL VORTEX GENERATORS

Flow structure, heat transfer, and drag by longitudinal vortices gener ated by double rows of delta winglets in transition channel flow are i nvestigated for the reduction of the gas side heat transfer resistance of compact heat exchangers. The experiments consist of flow visualiza tion by laser light sheets, liquid crystal thermography for local heat transfer and balance measurements for drag. Angle of attack and chann el Reynolds number have been varied. Aligned delta winglet double rows show higher heat transfer enhancement than staggered. The critical an gle of attack for the formation of longitudinal vortices is smaller be hind the second row than behind the first. Heat transfer enhancements of 80% and drag increases of 160% have been found on wall areas 40 tim es the winglet area. The ratio of heat transfer enhancement and drag i ncrease is larger for higher Reynolds numbers.