Investigation of ultrasonic array defrosting method based on synergism of standing wave intermittent phase-stagger and multi-frequency for finned-tube evaporator

Abstract Frost dramatically deteriorates the operating performance of an air source heat pump unit and leads to additional energy consumption. Results of our previous study show that intermittent ultrasonic vibration can greatly improve the defrosting efficiency and operation performance. To meet the ultrasonic defrosting demands of large-scale evaporators and to eliminate the standing wave effect of ultrasonic defrosting, an optimised ultrasonic array method for defrosting is proposed based on synergism of standing wave intermittent phase-stagger and multi-frequency excitation. Firstly, the surface vibration characteristics and defrosting effect difference of copper plate with standing wave intermittent phase-stagger in random mode and regular mode are investigated using a plate model. On this basis, the surface vibration characteristics and defrosting effect of a finned-tube evaporator are analysed in both phase-stagger modes. Finally, to optimise further the defrosting effect of the ultrasonic array, the standing wave intermittent phase-stagger and multi-frequency excitation are integrated to realise chaotic motion of the standing wave on the evaporator surface. Finite element analysis results and experimental results indicate that the standing wave effect of ultrasonic array defrosting can be eliminated using random intermittent phase-stagger, and the defrosting efficiency under random intermittent phase-stagger is superior to the regular method. Moreover, when arrayed ultrasonic transducers are controlled by synergistic random intermittent phase-stagger and multi-frequency excitation, there is no dead defrosting angle on the evaporator surface, the average frost layer thickness is approximately 0.282 mm, and it is 81.1% of mode I (intermittent and multi-frequency excitation).