An experimental study on frosting and hybrid defrosting of a cold flat plate under natural convection

The purpose of this study is to investigate the frosting characteristics and defrosting performance via a hybrid defrosting technique under free convection. This technique includes both surface treatment and ultrasonic vibration methods. The testing results for slippery surfaces are compared with those of plain stainless-steel SS ANSI 316 surfaces. These surfaces are tested under similar testing conditions with 24 degrees C dry bulb temperature and around 60% relative humidity. The surface is treated by implementing a superhydrophobic coating material. The ultrasonic vibrational source has a high resonance frequency of 28 +/- 0.5 kHz. It is used in direct contact with the cold surface for continuous, intermittent, and mixed vibrations. In the present experiment, the ultrasonic defrosting has been studied extensively using horizontal and vertical plates. The results revealed the following: (I) The nucleation process has delayed around 8 minutes on the superhydrophobic horizontal coated surface and about 10 minutes on the superhydrophobic vertical coated surface due to the reduction in the adhesive surface strength; (II) For plain horizontal surfaces, intensive intermittent vibration is more favorable than continuous vibration when it is applied as a 1-minute intermittent contact vibration followed by 10 minutes cooling; (III) For coated horizontal surfaces, continuous vibration is the most effective way to suppress frost layers; (IV) For vertical surfaces, coating is effective enough for defrosting and there is no need for additional vibration; (V) Using two ultrasonic transducers covers more areas for defrosting, but it increases the circularity of the droplets with comparatively sparse formation for coated surfaces; (VI) Coated superhydrophobic surfaces should be adopted rather the plain surface since it has a proper self-drainage capability which improves defrosting efficiency. (C) 2020 Elsevier Ltd. All rights reserved.