【作者】 林曦鹏；

【导师】 柯道友；

【作者基本信息】 清华大学 ， 动力工程及工程热物理， 2013， 博士

【摘要】 微机电技术特别是MEMS加工技术的发展为微尺度相变换热的研究提供了很大的方便。微通道或受限空间内可控微气泡的快速核化、生长和塌陷产生的驱动力已经被应用于热喷墨打印、气泡微泵、微阀以及微混合器等器件的设计中，对建造生物微分析系统（μ-TAS）和芯片实验室（Lab on a chip）起到非常重要的作用，具有广阔的应用前景。本文实验和理论研究了在恒定加热和脉冲加热两种条件下PDMS玻璃微槽内FC-72液体的流动沸腾特征以及光滑Pt微加热片上的气泡核化生长行为。研究发现无论是在恒定加热或者较快的脉冲加热下，光滑Pt微加热片上非常湿润液体FC-72的气泡核化都需要很高的过热度，核化在加热片中心最高温度位置处开始并且该温度最大值可以通过经典动力学成核理论进行预测。恒定加热条时铂微加热片上的沸腾曲线只存在膜态沸腾和部分过渡沸腾区域，在初始气泡形成以后沸腾可以在低于起始核化温度下继续进行。流动沸腾中微加热片上生长的蒸气泡会周期性脱落，在微通道内形成泡状流，弹状流和环状流等流型，在500μm宽度通道内出现了300μm宽度通道内没有出现的双气柱气泡脱落现象。气泡的脱落位置以及脱落频率随加热量增大而增大，在高过冷度下脱落频率增快，脱落位置更加靠近加热片。核化在脉冲加热开始后一段时间开始，在脉冲加热结束后仍然能够持续直到表面过热度低于约50oC。脉冲加热下微通道内气泡的生长特征主要可以分为：典型单气泡生长，薄蒸气膜连接气泡，极小气泡喷射以及长气泡的两倍周期生长运动等特征。在单气泡初始核化生成时候能产生一个很强的冲击波，可以应用于微制动器的设计中。气泡在核化形成初期的2ms内是惯性控制生长，其生长速率有个阻尼振荡的过程，而在2ms以后开始逐渐转入传热控制阶段，随着加热热流密度增加气泡生长速度变大，而过冷度增加以及流量的增大则对气泡生长有限制作用，流量减小能将惯性控制阶段缩短。结合Fluent软件和VOF两相模型，建立了一个数值模型对微通道内微加热片上的脉冲加热气泡动力学进行模拟。计算结果和实验结果比较符合，但是不能很好的描述一些特殊现象，需要发展一个更精细的模型来进行模拟。更多还原

【Abstract】 The development of MEMS technologies has greatly facilitated the study ofmicroscale phase change and heat transfer mechanisms. The rapid controlledbubble nucleation, growth and collapse in microchannels and confined spacescan generate powerful driving forces. These forces have been used to designthermal ink-jet printers, micro-bubble pumps, micro-valves and micro-mixerdevices. These mechanisms also play very important roles in biologicalmicro-analysis systems (μ-TAS) and lab-on-a-chip devices with many possibleapplications.This project experimentally and theoretically studied the flow boilingcharacteristics of FC-72in PDMS-glass microchannels with constant or pulsedheating. The bubble behavior on a smooth platinum microheater was alsostudied. The study showed that high wall superheats were required fornucleation of FC-72bubbles on the smooth Pt microheater for both constantheating and rapid pulsed heating. A bubble first nucleated at the center of theheater at the maximum temperature location with the nucleation temperaturewell predicted by the classical kinetics of nucleation theory. The boiling curveon the Pt microheater for constant heating had only a film boiling region andpart of the transition boiling region. Boiling could be maintained at walltemperatures less than the nucleation temperature after an initial bubble formedon the heater.The growing vapor bubble on the microheater periodically departed andflowed down in the microchannel to generate bubbly flow, slug flow and annularflow in the microchannel. A special bubble departure mode with two vaporcolumns was observed in the500μm wide microchannel. The departure positionand frequency increased as the heating increased, with higher subcoolingscausing a higher departure frequencies and departures closer to the heater.Boiling started a short time after the heating pulse started. The boiling thencontinued after the end of the heating pulse until the surface superheat was lessthan about50.0oC. The bubble growth characteristics with pulsed heating in the microchannel can be divided into typical single bubble growth, a thin vapor filmconnected to the bubble, a very small bubble jet and an elongated bubble at theflow rates when the bubble was shed at half the frequency of the heating pulse.The rapid initial bubble growth at nucleation produced a strong shock wavewhich could be useful for the design of micro-actuators. The initial bubblegrowth was inertia control growth for about2.0ms with the growth rate thenfollowing a damped oscillation process. The bubble growth mode thentransitioned to heat transfer controlled growth.A3-D numerical model was developed in Fluent using the VOF two-phasemodel to simulate the bubble dynamics in the microchannel for pulsed heating.The numerical results agree reasonably well with the observations, but do notaccurately describe some of the complex special phenomena; thus, moresophisticated models are needed to accurately simulate the bubble dynamics.更多还原