Characteristics of confined bubble behavior and enhanced flow boiling heat transfer in vertical minichannels under ultrasonic field by using bubble edge detection method

Abstract

Confined bubble dynamics plays a pivotal role in flow boiling within minichannels; however, the mechanisms underlying ultrasonic field-induced heat transfer enhancement remain inadequately understood. This study experimentally investigates the effects of ultrasonic fields (frequencies: 23, 28, 32, and 40 kHz) on the growth dynamics of confined bubbles in flow boiling. High-speed visualization and edge detection techniques are employed to qualitatively examine bubble growth, confinement, and elongation. The results reveal that ultrasound modifies bubble growth through: (1) reducing initial bubble size while accelerating late-stage growth; (2) inducing early displacement, delaying thin-film boiling, and extending the nucleation-to-confinement transition from 78 ms to 85 ms at 218.04 kg/(m²·s); (3) suppressing premature elongation and decreasing the diameter-to-length ratio, thereby enlarging the effective evaporation region; and (4) enhancing gasification at the liquid–vapor interface, increasing vapor generation and latent heat removal. A comprehensive analysis of bubble dynamics under varying ultrasonic parameters and confinement effects provides mechanistic insight into ultrasound-enhanced flow boiling. These findings offer guidance for optimizing thermal management in high-power microelectronic cooling systems.