Studies of mechanism of noisy film boiling in He II and its control

本研究的目的是研究氦气在有噪声的膜沸腾过程中产生可听噪声的机理及其控制方法。为此，对沸腾产生的热流动场进行了可视化，并用超导温度传感器测量了温度，用压力传感器测量了压力。研究结果表明：(1)氦气中存在三种不同的膜沸腾模式，即静水压级、无声膜沸腾、噪声膜沸腾和过冷膜沸腾。明确了噪声膜沸腾的起始条件与温度、静压、加热器尺寸等参数的依赖关系。产生巨大可听噪声的原因是过热的加热器上的He II在加热器表面被一个大的蒸汽泡粉碎后立即爆炸蒸发。(2)是…此外，正弦振荡的热扰动叠加在恒定热流力的作用下，导致膜沸腾从无声到有噪声的转变。这表明向噪声沸腾的转变是一种噪声诱发的不稳定性。(3)考察了压力振荡测量数据的FFT分析结果，考虑了三种膜沸腾模式下压力振荡的细节。发现过冷沸腾压力振荡的高频分量(KHz量级)是由低温恒温器中He II的液柱振荡引起的。(4)明确了弱过冷He II从有噪声膜沸腾向无声膜沸腾转变的一般特征。在非常接近波长温度的温度下，随着静压的降低，过冷沸腾直接转变为无声沸腾，没有经过有噪沸腾模式。(5)描述He II有噪沸腾机理的物理数学模型的构建尚未完成，但已经积累了足够的数据。从目前的实验数据来看，噪声沸腾的控制是很有必要的。较少

The objectives of this research are the investigations of the mechanism of loud audible noise generation during noisy film boiling in He II and of the control method of it. Visualization of thermo-fluid dynamic field resulted from boiling and the measurements of the temperature by a superconductive temperature sensor and of the pressure by a pressure sensor are conducted comprehensively for the purpose. The following conclusions are drawn from the present study.(1) There are three modes of film boiling in He II appearing in the order of the hydrostatic pressure level, silent film boiling, noisy film boiling and subcool film boiling. The dependence of the onset condition of noisy film boiling on such parameters as the temperature, the static pressure and the heater size are made clear. The cause of the loud audible noise generation is found to be the explosive evaporation of He II on overheated heater immediately after the crush of a large vapor bubble on a heater surface.(2) It is foun … More d that sinusoidally oscillating thermal disturbance superposed on steady constant heat flux forces to induce the transition from silent to noisy film boiling. This indicates that the transition to noisy boiling is a kind of noise induced instability.(3) The FFT analysis result of the pressure oscillation measurement data is examined to consider the detail of the pressure oscillation caused in three film boiling modes. The high frequency (the order of kHz) components of pressure oscillation in subcool boiling are found to be resulted from the liquid column oscillation of He II in the cryostat.(4) The general feature of the transition from noisy to silent film boillings in weakly subcooled He II is made clear. At the temperatures very close to the lambda-temperature subcool boiling directly turns to silent boiling without passing through noisy boiling mode as the static pressure decreases.(5) The construction of the physico-mathematical model to describe the mechanism of noisy boiling in He II has not been completed, but sufficient amount data are accumulated for this. The control of noisy boiling is reedy to be investigated from the present experimental data. Less