内设凹槽微通道内的气泡行为与流动沸腾换热特性

Bubble behaviors and flow boiling heat transfer characteristics in microchannel with cavities

运用VOF(volume of fluid)模型与用户自定义函数,数值模拟研究水在加热壁面上V形、梯形、方形和燕尾形凹槽微通道内的流动沸腾,分析凹槽形状对气泡成核、生长、脱离及聚并等行为的影响.研究结果表明:在热流密度为300 kW·m^(-2)时,凹槽均被激活为沸腾核化点,与其他形状凹槽相比,水在燕尾形凹槽微通道内的起始沸腾时间相对较早;在核态沸腾阶段,与梯形凹槽相比,气泡在燕尾形凹槽微通道内的生长和脱离时间分别缩短为7.50和6.70 ms,气泡脱离频率从33.8 s^(-1)提高至66.7 s^(-1),有利于水在微通道内的流动沸腾换热强化;脱离气泡在微通道内的聚并与拉伸行为增大液膜蒸发区域,气泡对液相的扰动增强,但会造成加热壁面上发生局部干涸,降低沸腾换热的稳定性与可靠性.

Volume of fluid(VOF)model and user⁃defined function were applied to simulate the flow boiling of water in microchannel with V⁃shaped,trapezoidal,square and dovetail⁃shaped cavities on the heating wall.The effects of cavity shape on the bubble behaviors of bubble nucleation,growth,detachment and coalescence were analyzed.The results show that when the heat flux on the wall is 300 kW·m^(-2),all cavities are activated as boiling nucleation sites.Compared with the other cavities with V,trapezoidal and square shapes,the onset of boiling time in microchannel with dovetailed cavities is relatively earlier when flow boiling occurs.In the nucleate boiling stage,compared with the trapezoidal cavity,the growth and detachment times of the bubble in the microchannel with dovetailed cavity are decreased by 7.50 ms and 6.70 ms,respectively,while the detachment frequency of the bubble is increased from 33.8 s^(-1)to 66.7 s^(-1),which is helpful to enhance the flow boiling heat transfer of water in microchannel.The coalescence and elongation of the detached bubbles in microchannel can increase the evaporation area of liquid film near the heating wall and improve the disturbance of the liquid phase simultaneously.However,it can cause local drying on the heating wall and reduce the stability and reliability of flow boiling heat transfer in microchannel.