Abstract
This study aims to investigate the bubble-induced acoustic streaming effect on heat transfer enhancement in microchannel flow with cavities and membrane design. Rectangular microchannel devices with single cavity with membrane and multiple staggered cavities with membrane were fabricated with PDMS as the material by soft lithography. The cavities with membrane were positioned at the sidewall of the microchannel, and the bubble-induced acoustic streaming was driven by a piezoelectric powered by a power supply. The design of the membrane at the exit of cavity is to suppress the expansion of the bubble during the exciation by piezoelectric and heating. The flow field and heat transfer analysis of the microchannel were analyzed by using the flow visualization (FV) technique, Micro Particle Image Velocimetry (µ-PIV) and Temperature-Sensitive Paint (TSP) technique. The magnitude and affected area of bubble-induced acoustic streaming velocity flow field under Reynolds number 2,4,6,8 are measured. It is found that the effect of bubble-induced acoustic streaming is suppressed with the increase of Reynolds number from the experimental result of single cavity with membrane. The effect range of bubble-induced acoustic streaming in x direction is about 206 µm and approximately 187.5 µm in y direction at the Reynolds number conditions of 2 and 4. The effect range of bubble-induced acoustic streaming in x direction reduces to 107 µm and about 75 µm in y direction at the Reynolds number condition of 6. The bubble-induced acoustic streaming is not obvious at Reynolds number of 8. The velocity profiles of multiple staggered cavities with membrane have been examined at the center of microchannel(y/W=0) at Reynolds number 2,4,6,8 with bubble-induced acoustic streaming. The bubble-induced acoustic streaming at Reynolds number of 4 can provide most effective disturbance in the flow field, and it can be observed that the effect of bubble-induced acoustic streaming is continued and increased while the flow passing cavities. The enthalpy change of flow through the microchannel with mulitiple staggered cavities with membrane have also been meaured at Reynolds number conditions of 2,4,6,8 with constant heat flux of 0.2 W/mm2 boudnary condition at the bottom of microchannel. The highest enthalpy change between microchannel inlet and exit is at Reynolds number of 4, which can reach about 27.04%. The local Nusselt (Nu) number variation has been obtained by TSP technique and the local Nu number would gradually increase while the flow passing the cavities, the same as obsereved in the velocity field. Heat transfer enhancement would reach to about 84.3% at center of microchannel (y/W=0) after the flow moving to the location of last cavity.