The effect of ultrasound irradiation on the convective heat transfer rate during immersion cooling of a stationary sphere

► Ultrasound (US) irradiation increased the cooling rate of a copper sphere notably. ► Higher cooling rates were achieved by increasing the US intensity. ► Acoustic streaming and cavitation were the main mechanisms of increased cooling rate. ► The position of the sphere affected the heat transfer phenomenon significantly. ► Different positions exhibited different cavitation bubble population. It has been proven that ultrasound irradiation can enhance the rate of heat transfer processes. The objective of this work was to study the heat transfer phenomenon, mainly the heat exchange at the surface, as affected by ultrasound irradiation around a stationary copper sphere (k=386Wm−1K−1, Cp=384Jkg−1K−1, ρ=8660kgm−3) during cooling. The sphere (0.01m in diameter) was immersed in an ethylene glycol–water mixture (−10°C) in an ultrasonic cooling system that included a refrigerated circulator, a flow meter, an ultrasound generator and an ultrasonic bath. The temperature of the sphere was recorded using a data logger equipped with a T-type thermocouple in the center of the sphere. The temperature of the cooling medium was also monitored by four thermocouples situated at different places in the bath. The sphere was located at different positions (0.02, 0.04 and 0.06m) above the transducer surface of the bath calculated considering the center of the sphere as the center of the reference system and was exposed to different intensities of ultrasound (0, 120, 190, 450, 890, 1800, 2800, 3400 and 4100Wm−2) during cooling. The frequency of the ultrasound was 25kHz. It was demonstrated that ultrasound irradiation can increase the rate of heat transfer significantly, resulting in considerably shorter cooling times. Higher intensities caused higher cooling rates, and Nu values were increased from about 23–27 to 25–108 depending on the intensity of ultrasound and the position of the sphere. However, high intensities of ultrasound led to the generation of heat at the surface of the sphere, thus limiting the lowest final temperature achieved. An analytical solution was developed considering the heat generation and was fitted to the experimental data with R2 values in the range of 0.910–0.998. Visual observations revealed that both cavitation and acoustic streaming were important for heat transfer phenomenon. Cavitation clouds at the surface of the sphere were the main cause of heating effect. The results showed that closer distances to the transducer surface showed higher cooling r