Heat transfer mechanism of nucleated bubbles on vertical heating surface induced by 2.4 MHz waves in HFE-7100 liquid

This research investigates the heat transfer mechanisms of nucleated bubbles on a vertical heating surface using 2.4 MHz ultrasonic waves. The experiments, conducted in a closed rectangular chamber filled with HFE-7100, a hydrofluoroether fluid, varied the surface heat flux from 4.91 kW/m² to 12.06 kW/m². The results revealed a notable reduction in temperature on the vertical heating surface due to the influence of 2.4 MHz ultrasound, leading to an average surface temperature decrease of up to 5.3 °C across the entire heat flux range. Analysis of acoustic streaming, flow patterns, and thermally nucleated bubbles using particle image velocimetry (PIV) and a high-speed camera demonstrated the pronounced impact of ultrasonic waves on heat transfer. Our findings highlight the crucial role of 2.4 MHz ultrasonic waves in influencing bubble behavior and enhancing heat transfer on the vertical heating surface. This enhancement is achieved by disturbing the near-wall flow, particularly at lower heat fluxes, with a peak velocity ratio of 5.27 times. This disturbance increases average velocities, indicating potential improvements in heat transfer. Consequently, our study showed a maximum heat transfer enhancement of 83 %. At higher heat fluxes, the interaction with the waves becomes more complex, increasing velocities but limiting streaming coverage in the lower region of the heating surface. The combined effect of 2.4 MHz ultrasound and nucleate boiling convection not only enhances near-wall heat transfer but also amplifies fluid mixing within the chamber. The examination of bubble evolution with and without 2.4 MHz ultrasonic waves underscored the role of acoustic streaming in sweeping bubbles, reducing the size of nucleation sites, and reducing their crowd density, particularly at lower heat fluxes.