An experimental study on the effects of agitation on convective heat transfer

In this study, agitation is produced inside a channel by a plate that is periodically oscillating normal to the channel side walls. The test channel simulates a deep-finned rectangular channel open on one end to a plenum and with a gap to allow flow over the tip of the agitator plate. The purpose of agitation is to strongly mix the near-wall flow, to thin the thermal boundary layer and to increase the convective heat transfer coefficient. Heat transfer and velocity measurements are made within different regions of the channel to study the effectiveness of such agitation. The entry region which is closest to the open end (plenum) is characterized by unsteadily driven periodic flow. The base region close to the channel base and agitator tip gap has high vortical activity and turbulent flow. The central region between the two has an unsteadily driven channel flow in one direction of oscillation and is rich in advected turbulence in the other direction. A parametric study is done to identify parameters that are critical to enhancing heat transfer. The amount of agitation produced in the channel directly scales with increasing frequency. Agitation is found to scale almost entirely with agitation velocity, the product of amplitude and frequency, with amplitude being only slightly more important than frequency in a few cases. Though this study finds application in electronics cooling where agitation can be used inside finned, air-cooled heat sinks to enhance heat transfer with walls, the results could be applied to any similar situation with such enhancement of heat or mass transfer with active surfaces. Very few experimental studies can be found in the literature on flow agitation effects on wall transport.