Study of Special Features of Development and Collapse of Vapor Film on Hemispherical Surfaces

Local processes, taking place during a change of the boiling regime on a hot hemispherical surface, have been studied. The experiments have been performed at atmospheric pressure and the initial temperatures of the heated surface and the cooling water equal to 600−1000 K and 288−368 K, respectively. Hemispheres made of stainless steel and copper were used in the experiments. It has been established that the process of film collapse is extremely diversified and greatly depends on the state and thermophysical properties of the surface. It includes both a smooth change of film boiling and an explosive collapse of the vapor film with discharge of jets. The latter regime was observed only on oxide-coated surfaces. Oxidic film thicknesses have been experimentally evaluated. It has been demonstrated that, at an explosive collapse of the vapor film, the rate of cooling of the hemisphere amounted to ∼300 deg/sec; at that, the heat flux density exceeded 10 W/m2 . The fluctuation behavior of the volume of hemispherical vapor cavity has been found. The boundaries of its existence and variation ranges of characteristic parameters have been determined.