Research progress on the intensification of heat transfer by ultrasound

Microscale electronic devices offer promising application capabilities in various fields, such as information, aeronautics and astronautics, energy, and chemical engineering. Specifically, the exceptional performance of high-integration and high-frequency devices leads to a significant heat flux enhancement. Conventional air and liquid cooling techniques struggle to meet the efficient heat dissipation requirement, affecting the reliability and safety of microscale electronic devices significantly. Many types of passive heat transfer process intensification strategies have been proposed recently, such as those based on adjusting element structure, surface roughness, surface hydrophobicity, and channel dimension. However, these passive strategies increase flow resistance to some extent, limiting their applicability. Ultrasound has several unique characteristics, including low cost, simple operation, flexible control, strong penetrability, and good biocompatibility. These characteristics make ultrasound a promising candidate for use in national defense, biomedical theranostics, agriculture, food, the environment, and materials. Researchers have paid considerable attention to the integration of ultrasound with heat transfer techniques, which has gradually become one of the key research directions for heat transfer enhancement. This paper aims to provide a comprehensive overview of the research progress on the intensification of the ultrasound-excited heat transfer process. First, the principles of ultrasound-excited heat transfer enhancement are introduced, and two major acoustic phenomena, acoustic cavitation and acoustic streaming, are highlighted. Theoretical and experimental studies on ultrasound-excited single-phase gas convection, single-phase liquid convection, pool boiling, and flow boiling heat transfer process intensification are then summarized, and typical studies in these fields are categorized and discussed in depth. Finally, current challenges and future directions are presented, such as simple numerical simulation models that should consider multiphysics and multidomain constraints for accurately representing the practical heat transfer process, lack of sufficient characterization methods that should develop new and integrated visualization techniques for precisely monitoring heat transfer performance, limited focus on other acoustic phenomena other than acoustic streaming and acoustic cavitation that should provide a comprehensive analysis for revealing the in-depth heat transfer mechanisms, and few attempts and pathways to industrialization that should demand researchers from different disciplines to work together and collaboratively. It is hoped that this review article will not only reveal the unprecedented functionality of ultrasound for heat transfer enhancement but will also provide critical guidelines for the rational and practical design of robust ultrasound heat transfer platforms.