A study of the mechanism of flashing flow by experiment and theoretical analysis

Abstract

A new theory of flashing flow is described which assumes that all bubble nuclei are present when the pressure is reduced. Steam formation occurs by conduction of heat to the surfaces of these nuclei, followed by evaporation at the surface. As the bubbles grow, the two phase mixture is accelerated in the duct, producing a pressure drop and a further increase in the superheat. An experimental study suggests that when the superheat is small, nucleation of bubbles does not occur within the bulk of the fluid and that steam formation only occurs at existing interfaces. In a desalination plant, growth of bubbles swept from one stage to the next or flashing at the free surface is likely to be most important. Measurements of the growth of steam bubbles in superheated sea water show good agreement with the theoretical predictions during the initial stages but the theory underestimates the growth rate further away from the change in pressure.