Heat transfer augmentation of a channel flow by active agitation and surface mounted cylindrical pin fins

Abstract

With the intent of enhancing heat transfer, agitation of air flow is produced inside a high-aspect-ratio rectangular channel by means of a translationally oscillating plate. So that the test simulates channel flow between fins of a heat sink for cooling of electronics, the channel is open at one end, allowing oscillatory inflow and outflow at that end. At the other end where the roots of the fins that constitute the heat sink channel walls reside, there is a gap between the moving agitator plate and the channel base. Local heat transfer rates and velocity measurements are made within different regions of the channel. Heat transfer measurements are made on a channel wall augmented with cylindrical pin fins. They compared with equivalent measurement results taken on a smooth wall channel. An increase of 4–7%, based on total wetted area is found in the heat transfer coefficient, generally, when the pin fins are introduced. The change varies according to location within the channel, however. For instance, the heat transfer coefficient in the region near the base of the fins actually decreases 4–5% (again, based on total wetted area) when the pin fins are added. In this region, the flow passing through a tip gap between the agitator tip and the channel base wall, creates strong vorticity and high near-wall shear. These features of the flow are partially blocked by the pin fins, decreasing the effectiveness of the vorticity. Important to note in these comparisons, however, is that the pin fins offer an additional 30% heat transfer area. To help explain the observed trends, velocity measurements were taken with a single-component laser Doppler anemometer in the presence of pin fins as well as in the absence of pin fins. Pin fins lead to a loss of near-wall momentum but create greater mixing and three dimensionality of the near-surface flow.