THIN LIQUID FILM DYNAMICS AND BUBBLE BEHAVIOR IN FLOW BOILING

A profound knowledge of the dynamics of a thin liquid film and bubble behavior in flow boiling is crucial in a thorough study of the physical mechanisms of boiling crisis, which is important to the performance and safety of light water reactors. To address this, in the present study a series of tests were carried out at various water and air flow rates under atmospheric pressure and different heat fluxes in a horizontal copper channel. We employed a developed confocal optical sensor system to detect the variations and integrity of the thin liquid film. Additionally, we used a high-speed camera to capture the bubble behavior in detail and analyzed the effect of the film thickness on bubble behavior. According to our observations, the thinning process of the liquid film is governed by the entrainment under adiabatic or lower heat flux conditions, whereas evaporation becomes more pronounced in a higher heat flux system. The critical film thickness of an integral film is found to be related to the condition of the heating surface and the heat flux. Remarkably, the coalescence of bubbles seldom occurs in flow boiling with the thinner liquid film. The bubble departure diameter increases with an increase in the heat flux and liquid mass flow rate, but decreases with an increase in the gas flow rate. The minimum bubble departure diameter observed using high-speed photography was about 90 mu m.