| In many compact heat exchangers, interrupted-fin surfaces are used to enhance air-side heat transfer performance. One of the most common interrupted surfaces is the louvered fin. The goal of this work is to develop a better understanding of the flow and its influence on heat transfer and pressure drop behavior for both louvered and convex-louvered fins. Specifically, this research explores the end effects of heat exchanger walls; the effect of vortex shedding on heat transfer and pressure drop; and the effect of using convex louvers rather than flat (offset-strip) fins. Flow visualization is performed using dye in a water tunnel, and pressure drop is measured in a wind tunnel. Heat transfer is inferred from mass transfer data obtained using the naphthalene sublimation technique. Mass transfer data are acquired on a row-by-row basis through the louvered arrays over a Reynolds number range (based on louver pitch) of 75--1400, and local mass transfer data on fins are acquired for the convex-louver geometry over a Reynolds number range (based on hydraulic diameter) of 200 to 5400. Compared to flow far from the walls where spanwise periodic conditions exist, flow near array walls is louver-directed to a lesser degree and is characterized by deviations in flow velocity, large separation and recirculation zones, and an earlier transition to unsteady flow. At low Reynolds numbers, heat transfer is lower for louvers near the walls than for louvers far from them due to the large separation zones. At higher Reynolds numbers unsteady flow causes increases in heat transfer for louvers near the walls. The walls cause a large pressure drop increase at all Reynolds numbers. In the transitional Reynolds number range, transverse vortices shed from fins far from array walls are smaller and result in much less mixing than vortices shed near array walls or in the similar offset-strip geometry. These small-scale vortices have little effect on heat transfer. For the convex-louver geometry, the results clarify the effects of boundary-layer restarting, shear-layer unsteadiness, transverse vortices, and separation, reattachment and recirculation on heat transfer. |
