Experimental Study on Heat Transfer Performance of Solidification Heat Exchanger Enhanced by Ultrasonic

Solidification heat exchangers offer an effective solution to the operational limitations of surface water-source heat pumps (SWSHPs) in cold northern regions. However, under near-freezing conditions, their engineering application is hindered by heat transfer deterioration and flow channel blockage caused by external tube icing. Thus, investigating the potential of ultrasonic technology for anti-icing, de-icing, and heat transfer enhancement is critical. In this study, an experimental system including a coil-type solidification heat exchanger and 28 kHz low-frequency ultrasound was established. The effects of ultrasonic power (0–300 W) and secondary refrigerant (ethylene glycol) inlet conditions—specifically temperature (-8 to -5 °C) and velocity (0–0.5 m/s)—on de-icing, anti-icing, and heat transfer were tested and analyzed. The results indicate that: (1) Ultrasonic power exhibits a significant capability to regulate ice layer thickness. Under the tested de-icing conditions, 300 W ultrasonic power achieved a de-icing rate of 97.27% within 6 minutes. Under anti-icing conditions, ultrasonic power in the range of 200–300 W reduced the maximum ice layer thickness by 65.8%–85.8%. (2) As ultrasonic power increased from 0 to 200 W, the overall heat transfer coefficient of the heat exchanger rose rapidly, and a maximum enhancement ratio of 1.75 was obtained. However, that the enhancement effect diminished beyond 200 W demonstrates a distinct power threshold characteristic. (3) The flow velocity and inlet temperature of the secondary refrigerant exhibit a coupling effect on heat transfer performance and ultrasonic enhancement. When the inlet temperature exceeded -6 °C, the overall heat transfer coefficient increased monotonically with velocity. Conversely, in the range of -7 to -6 °C, the coefficient peaked at a velocity of approximately 0.4 m/s, reflects the competition effects between internal convective heat transfer and the thermal resistance of external icing. This study elucidates the synergistic mechanism by which low-frequency ultrasound enhances heat transfer and regulates ice formation through cavitation micro-jets and acoustic streaming. These findings provide a valuable reference for the application of SWSHPs in cold regions.