Experimental investigation on defrosting of a cold flat plate via ultrasonic vibration under natural convection

The present experimental study examines the effect of ultrasonic vibration on the frost formation under a free convective environment. A stainless steel SS ANSI 316 flat surface is tested under 12-24 degrees C dry bulb temperature and 58-84% relative humidity. The experiments are conducted with direct contact and non-contact high-frequency ultrasonic source having continuous or intermittent vibrations. The ultrasonic transducer contains 28 +/- 0.5 kHz resonance frequency. It is found that increasing the relative humidity will increase the frost thickness and influences the frost property. Imposing ultrasonic vibration under high relative humidity and low ambient temperature increases the droplets' circularity with comparatively sparse distribution. Test results indicate that non-contact vibration has no effect on the frost formation. Intensive intermittent contact vibration is the most effective way to suppress frost growth. Compared to continuous contact vibration, either it is applied after each an hour or after every 30 min as an intensive continuous, intensive intermittent contact vibration shows a reduction in frost thickness as much as 24%. In addition, the ultrasonic vibration should be applied after the supersaturation stage since ultrasonic vibration enhances the supercooled process.