Fundamental Study on Micro-Actuator Utilizing Rapid Boiling

本研究的目的是揭示微小薄膜加热器或细丝浸泡在纯液体中通过快速加热产生的沸腾气泡行为，因为这些行为对于理解和设计利用加热器上快速沸腾引起的液气相变化的微致动器是必要的。采用直径为0.1mm、长度为0.25mm的薄膜加热器，用直流电脉冲加热，加热速率最高可达10^9K/s左右，测量和分析了沸腾开始时的温度、有核气泡数的变化、微小气泡的聚集过程、气泡的生长过程和破裂过程以及传热行为。对自发成核后的产物也进行了研究。从而全面揭示了极小型薄膜加热器快速加热时的气泡动力学和传热特性。在此基础上，建立了过热液层蒸发模型，从理论上解释了加热过程中气泡快速成核前的传热和气泡行为。理论和实验结果都表明，最大气泡体积随加热速率的增加而减小。用直径为0.05mm的细丝在很宽的升温速率范围内进行了一系列观察实验，结果表明，气泡的产生和破裂行为根据升温速率大致分为3种类型。讨论了每种类型的微致动器的适用性。由于实际微致动器采用液体混合物代替纯液体，对二元液体混合物中的气泡成核进行了实验研究。

The objective of this study is to reveal the boiling bubble behavior generated on a very small film heater or a fine wire immersed in pure liquids by heating them rapidly, since the behaviors are necessary for understanding and designing the micro-actuator which utilizes the liquid-gas phase change resulted from rapid boiling on the heater. A film heater whose dimensions were 0.1mm in width and 0.25mm in length was heated pulsewise by a direct current with a heating rate up to about 10^9K/s at the maximum and the temperature at the inception of boiling, the variation of nucleated bubble numbers, the aggragation process of tiny bubbles, the growth process and the collapse process of a bubble were measured and analyzed as well as the heat transfer behavior. Those succeeding the spontaneous nucleation were also examined. Thus, the bubble dynamics and the heat transfer characteristics when a very small film heater was heated rapidly were wholy revealed.Further, with the aid of the above experimental results, the superheated liquid layr evaporation model was presented to explain theoretically the heat transfer and bubble behavior preceded by the rapid bubble nucleation on a heater. Experimental results as well as theoreticals showed that the maximum bubble volume decrased with the increase of the heating rate.A series of observation experiments were carried out with a fine wire of 0.05mm in diameter for a very wide range of heating rate and it was shown that the bubble generation and collapse behaviors were divided roughly into 3 types in accordance with heating rate. Applicability of each type for a micro-actuator was discussed. Bubble nucleation in binary liquid mixtures was studied experimentally, since liquid mixtures was used instead of pure liquid in real micro-actuator.