An experimental study on the effects of agitation in generating flow unsteadiness and enhancing convective heat transfer

Agitation is produced inside a channel by a plate that is periodically oscillating normal to the channel side walls. The test channel is a rectangular cavity open on one end to allow inflow and outflow of air, as driven by the plate movement. Heat transfer and velocity measurements are made within different regions of the channel to study the effectiveness of agitation in promoting heat transfer from the channel side wall. The purpose of agitation is to strongly mix the near-wall flow, to thin the thermal boundary layer and increase the convective heat transfer coefficient. Velocity measurements using laser Doppler velocimetry are made to document the fluctuations of velocity within the agitated cavity. Variations of ensemble-averaged velocity throughout the cycle identify the unsteady sloshing of the flow. Cycle-to-cycle variations about the ensemble mean computed as an RMS and resolved in time within the cycle period present the changing turbulence levels throughout the agitation cycle. The ensemble-averaged mean velocity variations show periods of acceleration, deceleration and flow reversal during a cycle as a result of agitator movement. Turbulence is found to increase toward the end of the acceleration phase and persist through the deceleration phase. Intensities of sloshing and turbulence are used to explain the measured convective heat transfer coefficients. ANSYS FLUENT simulations supply velocity contours and flow visualization. This study finds application in electronics cooling where agitation can be used inside air-cooled heat sinks to enhance heat transfer to through-flow driven by a fan.