Mechanism and behavior of nucleate boiling heat transfer to the alkalai liquid metals

摘要

A model of boiling heat transfer to the alkali liquid metals is postulated from an examination of the events and phases of the nucleate boiling cycle. The model includes the important effect of microlayer evaporation which causes a wave of temperature depression to penetrate into the heating solids; calculated results predict the periodic boiling behavior in the heating solid as a function of the heat flux, the system pressure, the cavity size, and the thermophysical properties of the liquid and the solid. An experimental program was designed to examine the microscale boiling behavior of sodium and to verify the calculated predictions of the boiling model. Artificial cylindrical cavities are used in most of the test sections; a thermocouple is placed close to the boiling surface and adjacent to the cavity wall, and microscale temperature measurements were obtained for stable boiling of sodium from artificial cavities and also from a natural cavity. Horizontal heating surfaces were made from nickel "A", stainless steel 316, and molybdenum-1/2%-.titanium; the range of saturation pressure is from 20 to 780 mm Hg. Favorable comparisons with the predictions of the boiling model are obtained with data for the bubble period and for the amplitude of temperature oscillation at a thermocouple close to the boiling surface; these results indicate the importance of the microlayer in a model for boiling sodium.