Investigation on the heat transfer performance of microchannel with combined ultrasonic and passive structure

•The effect of ultrasonic on the flow and heat transfer process is analyzed.•The synergy of ultrasonic on flow and temperature fields is demonstrated.•Energy-saving properties of ultrasonic enhanced heat transfer is explored.•Coupling the acoustic and flow fields of the microchannel through weak format. With efficient heat dissipation capacity, the microchannel heat sink (MCHS) can be exploited and applied in the development of new energy technologies. With 2.8 MHz high-frequency ultrasonic, the flow and heat transfer performance in different kinds of microchannel was studied. These microchannels included rectangular straight microchannel, 90° fan-shaped and triangular combined cavity microchannel and 90° fan-shaped and triangular combined cavity circular fin microchannel. Furthermore, the comprehensive performance of the microchannel was analyzed and evaluated in detail from different aspects such as flow characteristics, heat transfer characteristics, field synergy and efficiency analysis. At the low Reynolds number, the acoustic streaming effect induced by ultrasonic could destroy the wall boundary layer and improve the heat transfer between the fluid and the wall. However, with the increase of Reynolds number, the flow velocity gradually dominated the heat transfer process. Meanwhile, the ultrasonic mainly acted on improving the synergy between the flow and temperature fields. Moreover, the combination of cavity structure and ultrasonic was conducive to increasing the action depth of acoustic wave in the fluid, which was much easier to induce the acoustic streaming effect. Thus, the effect of ultrasonic enhancement could be greatly strengthened. More importantly, the efficiency of using ultrasonic to enhance heat transfer was higher than that of pump power. This work can contribute to the mechanism and improve the efficiency of active and passive enhancement of microchannel heat transfer by using high-frequency ultrasonic.