Two-Phase Flow and Flash Vaporization

Leaking liquids may be subcooled or superheated [7], The temperature of a superheated liquid lies above its vaporization temperature at ambient pressure. For a subcooled liquid the inverse is true. 

With subcooled liquids the jet disintegrates into droplets due to aerodynamic forces after a certain distance of flight. Liquid vaporizes from these droplets. They 

trajectories for the jet droplets result, which may differ. Depending on the elevation of the leak above the ground, the direction of emission, the type of material and the droplet size, the droplets may vaporize before they reach the ground. Droplets which reach the ground form a pool from which further liquid vaporizes in the course of time. Due to these processes droplets are formed which differ in trajectory and extent of vaporization. 

If a superheated liquid is released a certain fraction vaporizes on depressurization (flash) the remainder is cooled down because it has to provide the enthalpy of vaporization. The heat balance based on the assumption of adiabatic depressurization is 

Equation (10.38) provides the vapour quality under equilibrium conditions. In practical emission simations part of the liquid is discharged as spray. The droplets then formed receive their enthalpy of vaporization from the surrounding air. In this way the fraction of vaporized liquid is substantially increased. Often one assumes that the fraction of liquid spray is equal to that vaporized by flash vaporization. If the fraction vaporized by flashing is small, about 5 %, one may assume that the quantity vaporized due to spray formation is two or three times that vaporized by flashing. 

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