Flow Boiling Heat Transfer Characteristics Of Mini-Channel Heat Exchanger Under Ultrasound Field

Micro/mini-channel heat exchanger is a new and efficient heat exchanger,which has broad prospects in industrial applications.The ultrasound heat transfer enhancement technology was introduced into the mini-channel heat exchanger,and the flow boiling heat transfer characteristics of pure refrigerant and nano-refrigerant under the action of ultrasound were studied,and the mini-channel heat transfer equipment with better performance was obtained.The research contents mainly include the following points:?1?Flow boiling of pure refrigerant in a mini-channel under the action of ultrasound.A mini-channel experimental section with an ultrasonic transducer delivers stable ultrasonic field to the whole flow region.The flow boiling characteristics of R141b in a mini-channel with or without ultrasound were studied experimentally.The effects of ultrasound on bubble motion were analyzed by visualization with a high-speed camera.The results show that ultrasound can significantly enhance heat transfer,and the enhancement ratio is between 1.02 and 1.88.The heat flux has an important influence on the enhancement of saturated boiling heat transfer by ultrasound,and the enhancement effect decreases with the increase of heat flux.The number of bubbles increases obviously after the action of ultrasound,and the velocity of bubbles in the mini-channel accelerates remarkably.?2?Numerical simulation of ultrasonic sound field and analysis of heat transfer enhancement mechanism.Comsol simulation software was used to model the ultrasonic device and the fluid field,and the sound pressure distribution of different frequencies and different powers in the narrow channel was analyzed.The effect of ultrasound on the initial nucleation and vapor-liquid interface of bubbles was analyzed by the change of sound pressure.The mechanism of heat transfer enhancement by ultrasound was analyzed from the point of view of bubble dynamics.?3?Flow boiling of nanofluids in a mini-channel under the action of ultrasound.TiO₂-R141b nanofluids were prepared by‘two-step method'and Span80 was added to increase the stability of nanofluids.The concentration of nanofluids is 0.1%,0.2%and 0.3%respectively.The flow boiling heat transfer characteristics of nanofluids in a mini-channel with or without ultrasound were studied experimentally.The results show that the compound enhanced ratio of ultrasonic nanofluids ranges from 1.22 to 1.88.Heat flux density has an important influence on the heat transfer enhancement of ultrasonic nanofluids.At low heat flux density,the effect of compound enhancement is close to that of ultrasound only;at medium heat flux,ultrasound and nanofluids cooperate to enhance heat transfer;and at high heat flux,nanofluids play a leading role in heat transfer enhancement.