摘要
为探究有无超声波作用下微细通道内纳米流体流动沸腾传热特性,该研究设计了一种可以放置超声波换能器的微细通道试验段,运用超声波振荡法制备了纳米颗粒质量分数为0.1%、0.2%、0.3%的均匀稳定TiO2/R141b纳米制冷剂。在设计系统压力为152 kPa,有效热流密度的范围为10.8~22.7 kW/m^2,超声功率为50 W,超声频率为23 kHz,质量流率为121.1 kg/(m^2·s),入口温度为35℃的工况下,在截面宽度为2 mm的矩形微细通道内进行流动沸腾试验。研究结果表明:纳米颗粒质量分数为0.2%时的传热系数较高,强化传热效果较好,超声波作用下仍是质量分数为0.2%的纳米流体强化传热效果较好,相对于无超声情况下R141b平均饱和沸腾传热系数最大提高了89.9%。热流密度对超声波强化传热效果有很大影响,不同热流密度下强化效果有明显差距,声场作用下纳米制冷剂的平均饱和沸腾传热系数随有效热流密度的增大呈先增后降的趋势。通过COMSOL软件对通道内汽液界面的声场进行了模拟,模拟结果表明超声波在汽泡中的传播较弱。对于质量分数为0.2%的纳米制冷剂,进出口超声作用下超声强化因子最大为1.46。该研究结果可为通过施加超声波提高微细通道换热性能提供新思路。
An ultrasonic vibration field can be used to enhance the heat transfer efficiency in microchannels.This study aims to investigate the flow-boiling heat transfer characteristics of nanofluids in the microchannels with or without ultrasonic wave.A microchannel test was designed for the section that can be placed in an ultrasonic transducer.An ultrasonic vibration method was selected to prepare the uniform and stable TiO2/R141b nano-refrigerant with the mass fraction of 0.1%,0.2%and 0.3%.The flow-boiling parameters of nanofluid were measured in the microchannels under ultrasonic wave.A flow boiling experiment was performed on a rectangular microchannel with a cross-sectional width of 2 mm,where the design system pressure of 152 kPa,the effective heat flux density ranged from 10.8 to 22.7 kW/m^2,the ultrasonic power of 50 W,ultrasonic frequency of 23 kHz,mass flow rate of 121.1 kg/(m^2·s),and inlet temperature of 35℃.Different enhancement effects of heat transfer can be achieved under the nanofluids with different mass fractions.The reason is that the nanoparticles can be used to enhance heat transfer,while increase the thermal resistance avoided by heat transfer,and thereby the increase of thermal resistance can reduce the heat transfer efficiency.The results show that the heat transfer coefficient reached the highest,when the mass fraction of nanoparticles was 0.2%,where the heat transfer enhancement effect can be the best.The nanofluid with a mass fraction of 0.2%under the action of ultrasound indicated the optimal enhancement effect of heat transfer,compared with the case of no ultrasound,where the average saturation boiling heat transfer coefficient of R141b increased by 89.9%.The heat flux posed a great influence on the enhanced heat transfer effect of ultrasonic.There was a significant difference in the enhancement effect under different heat fluxes.The average saturated boiling heat transfer coefficient of nano-refrigerant under the action of sound field increased first and then decreased,with the increase of effective heat flux density.The sound field of vapor-liquid interface in the microchannel was also simulated by COMSOL software.The simulation results show that the propagation of ultrasonic waves in the bubble was weak.When the effective heat flux density below 15.2 kW/m^2,the ultrasonic wave can enhance the heat transfer via the increase in the breakaway frequency of the bubble,whereas,the average saturated boiling heat transfer coefficient increased,as the effective heat flux density increased.After the effective heat flux density was 15.2 kW/m^2,the strengthening effect of ultrasound began to weaken,due to the increase of bubbles in the microchannel.When the effective heat flow density reached 19.8 kW/m^2,the change of flow pattern can lead to an uniform heat transfer,due mainly to the elongation of bubble flow in the microchannel.In the nano-refrigerant with a mass fraction of 0.2%,the enhanced heat transfer effect increased successively for the imported ultrasonic wave,and the ultrasonic wave of the inlet and outlet.When the ultrasonic wave was applied to the inlet,the average saturated boiling heat transfer coefficient increased by 26%,whereas,increased by 46%under the action of ultrasonic import and export.The findings can provide new ideas to improve the heat transfer performance of microchannels when applying ultrasonic waves.
作者
罗小平
喻葭
王文
Luo Xiaoping;Yu Jia;Wang Wen(School of Mechanical and Automotive Engineering,South China University of Technology,Guangzhou 510640,China)
出处
《农业工程学报》
EI
CAS
CSCD
北大核心
2020年第19期50-57,共8页
Transactions of the Chinese Society of Agricultural Engineering
基金
国家自然科学基金(21776096)
广东省自然科学基金(2019A1515011053)。
关键词
超声波
流动沸腾
微细通道
纳米粒子
传热
ultrasonic
flow boiling
microchannels
nanoparticles
heat transfer
