Experimental study on defrosting mechanism of intermittent ultrasonic resonance for a finned-tube evaporator

This paper proposed a new defrosting method based on ultrasonic resonance mechanism, to solve the problem of the unknown mechanism of ultrasonic defrosting for a finned-tube evaporator. Dynamic microscopic process of frost crystals formation and growth under the natural frost condition was first investigated. According to the growth characteristics of the frost crystals, the natural frequencies of frost crystals with different height were calculated in COMSOL software. An ultrasonic transducer of 28 kHz/60 W was adopted as an executor to excite the evaporator, then ultrasonic defrosting experiments and laser vibrometer experiments were carried out under the excitation of the ultrasonic transducer. Finally, experiment of ultrasonic resonance based on intermittent operational was studied to optimize ultrasonic loading method. It was found that the height of the frost crystals were about 0.5 mm after growing for 4 min, the average natural frequency of the frost crystals were about 27.95 kHz, the evaporator and frost crystals on the fin surface were forced vibration at the frequency of 28.2 kHz, which was the actual working frequency of the ultrasonic transducer detected in laser vibrometer experiment, and most frost crystals with certain shape and size were immediately broken up when the ultrasonic vibration applied. The main mechanism of ultrasonic defrosting was the resonance effect of natural frequency of frost crystals and excitation frequency, and the optimal working mode of ultrasonic resonance defrosting was intermittent 4 min, vibration 1 min. The experiment's results also showed that ultrasonic shear stress and acceleration effect of ultrasonic also had defrosting performance, but they were not the main mechanism for ultrasonic defrosting.