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

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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