New type of dry out crisis in free falling boiling liquid films

Abstract

Here we present measurements of heat-transfer rate and critical heat flux as well as high-speed visualization at intensively evaporating and boiling falling liquid film flow. Research were carried-out at heated surfaces 67 mm wide and 20, 42, 64 mm long along the film flow over the range of inlet Reynolds number from 100 to 2000. Appreciably different crisis phenomena scenarios are observed depending on the heated surface length. Direct experimental measurements and visualization have shown the existence of previously unexplored surface dry out crisis development regime which is characterized by the upstream extrusion of the bubble boiling from the heated surface with the drying front. This type of crisis occurs under the certain range of the operating conditions and heated surface lengths. When the critical heat flux density occurs, large dry spots merge at the lower part of the stream. As a result of the dry spots merging, unstable temperature disturbance is formed. Subsequently it spreads over the whole surface, causing its drying, and critical heat flux is no longer determined by known calculation dependencies and is characterized by significantly smaller values. At such regimes critical heat flux is controlled not by hydrodynamic boiling crisis, but by equilibrium heat flux at which dry spots become unstable. This undesired critical heat flux reduction is potentially avoidable if measures can be taken for artificial liquid redistribution in transversal direction in order to decrease dry spots initial size (thus increase equilibrium heat flux).