Explosive regime of the development of instability leading to vapor-film collapse on a heated solid hemispherical surface

Abstract

We investigated the local processes proceeding in the case of changing the boiling regime on a heated cylindrical rod with a hemispherical end immersed in water ( T = 285 K) at a depth equal to the radius of the hemisphere. The rod was heated in ambient air up to temperature T ≤ 1000 K. Vapor-film escape was observed visually using a microscope and two video cameras installed below and aside from the hemisphere immersed in the liquid. The pressure arising in the liquid and the vapor-film thickness were measured using fiber-optical sensors [1]. In our experiments, we used metallic rods made of steel, pure copper, and copper covered by PSr62 silver solder. After immersing the hemisphere in a liquid, freeconvection flow formed near the heated surface. Then, wave formations appeared on the vapor‐fluid interface. The amplitude and characteristic length of the surface waves were on the order of magnitude of a vapor-film thickness. The process of vapor-film escape and passing to bubble boiling proceeded following two different scenarios. According to one of them, the wave perturbances having the form of isolated solitons or wave trains were enhanced with time and enveloped the entire hemisphere immersed in the water. Afterwards, a vapor-film explosion occurred and the nucleate-boiling regime was established. In a number of cases, the explosive boiling of the film was accompanied by the formation of a jet flow directed from the lower end of the hemisphere into the fluid. The explosive escape of the vapor film accompanied by the formation of the jet flow can be repeated (up to 30 times) in intervals of 0.3 to 1 s. In the other regime, the wave perturbances, once arisen, attenuated gradually and passed relatively smoothly to the nucleate-boiling regime. Smooth passage to nucleate boiling was observed only at the first immersion in water for a new hemisphere or for that freshly cleaned from oxides. In repeated experiments with surfaces having the oxide film, the vapor escaped explosively. In Figs. 1‐3, we show photographs of a vapor film before explosion and with various types of its escape. The experiments were carried out under normal pressure, and the temperatures of the cooling water and heated surface were 293 and 795 K, respectively. The characteristic values of the vapor‐fluid jet velocity (Fig. 3) attained 0.3 m/s. Vibrations of the vapor-film surface were detected both at the second boiling crisis with quiet vapor-film escape and with vapor explosion. The characteristic longitudinal dimension of the wave structures attained