Heat transfer and critical phenomena during evaporation and boiling in a thin horizontal liquid layer at low pressures

Abstract

The results of an experimental study of heat transfer and high-speed video recording of evaporation and boiling processes in horizontal liquid films of n-dodecane are presented for the wide ranges of layer height and pressure. Evaporation regimes at low reduced pressures were characterized by formation of dry spots and structures with the shape of “funnels” (depressions with a hemispherical bottom on the layer surface) and “craters” in the layers. In contrast to dry spots, the surface of “craters” is covered with a residual layer of liquid. The paper presents regime maps indicating the regions of dry spots, “funnels”, “craters”, and nucleate boiling observed for each layer height depending on the reduced pressure and heat flux density. It is shown that in the region of low reduced pressures, the Kutateladze formula describes change in hydrodynamics in the layers, whose height is equal to the Laplace constant or higher, by the regime where only “craters” remain in the layer. In the region of reduced pressures less than 7.4 · 10−5 (133 Pa) the critical heat fluxes (CHF) decrease with decreasing pressure. In the range of reduced pressures from 7.4 · 10−5 (133 Pa) to 5.5 · 10−3 (104 Pa), the CHF depends weakly on the pressure. With an increase in the layer height, CHFs increase sharply, and when achieving a constant value, are described by the Yagov dependence obtained for pool boiling of liquids. For the reduced pressure above 5.5 · 10−3 (104 Pa), in the layers with a height exceeding the Laplace constant, the CHF is the same as that calculated by the Kutateladze and Yagov formulas for nucleate boiling crisis. The slope of the curve of heat flux dependence on the temperature head depends on the layer height at both evaporation and nucleate boiling.