Experimental study on frosting suppression for a finned-tube evaporator using ultrasonic vibration

Abstract

An experimental study was conducted to investigate the possibility and the effect of frost release from a finned-tube evaporator by using ultrasonic vibrations in natural convection. A compressor was adopted as cold source, and a finned-tube evaporator was placed horizontal on the frames inside a chamber. The aluminum fins surface was covered with hydrophilic coatings whose contact angle of the water drop of the surface is 37.7°, and the fins had wing-cases to enlarge the heat-exchange area. The thickness of a fin was approximately 0.1 mm, and the gap distance between any two fins of the evaporator was about 1.55 mm in average. An ultrasonic transducer with resonance 28.2 kHz and rating power 60 W was adopted as a vibrator to excite the evaporator. A microscopic image system was used to observe and record the whole frosting process with/without ultrasonic vibrations, a method of frost thickness measurement on the double sides of a fin was present and realized through digital image processing technology. From microscopic view, the deposited frost nearly block the fins gaps when the compressor worked for 32 min without ultrasonic vibration, while almost 2/3 of the gaps between two neighbor fins were unblocked when the compressor worked for 92 min with ultrasonic vibration, which shows that the frost deposited on the fins surfaces could be suppressed with high frequency ultrasonic vibrations. From macroscopic view, the frost growing on the fins are being vibrated to fall off and accumulated under the evaporator to form frost piles. Although the fins above the braze tubes were deposited with frost, the fins between any two neighboring tubes were unblocked. Experiments show that the basic ice layer on the fins cannot be removed with ultrasonic vibrations, but frost crystals and frost branches on the ice layer can be fractured and removed effectively, the frost growing on the evaporator can be suppressed by using ultrasonic vibrations. The main mechanism of ultrasonic frost suppression maybe mainly attributed to the high frequency ultrasonic mechanical vibrations that can break up frost crystals and frost layers, then frost will fall off with the gravity, not to ultrasonic cavitation effect or heat effect.