Ultrasonic influence mechanism of a cold surface frosting process and an optimised defrosting technique

摘要

Frost formation on a cold surface is a complicated heat-and mass-transfer process, which is unsteady, nonlinear, and simultaneously accompanied with phase transformation and moving boundaries. A thorough understanding of the influence of ultrasonic vibration on the cold surface frosting process and frosting regularity forms the basis for developing a high-efficiency, energy-saving defrosting strategy. However, traditional research on the cold surface frosting process focuses on analysing the environmental parameters influencing the frost layer thickness and density; the ultrasonic influence at different stages of the process has not been comprehensively and thoroughly studied. Therefore, we begin by investigating the characteristics of the cold surface frosting process at different stages, based on the heterogeneous nucleation theory on a cold surface and thermodynamics theory of frost layer growth. A phase transformation kinetics model of heterogeneous nucleation and a thermodynamic model of frost growth on a cold surface are established under ultrasonic excitation. By combining the theoretical model of frosting and the experimental results of microscopic visualisation, the ultrasonic influence mechanism at different frosting stages is revealed and pertinence ultrasonic defrosting and frost suppression strategies are developed. Finally, the environmental influence on frosting characteristic under ultrasonic excitation is analysed, which lays a theoretical foundation for the subsequent study on the optimisation of the ultrasonic defrosting effect.