Evaluation of two-phase flow models for flashing flow in nozzles

A variety of methods and models have been proposed in the literature for sizing relief valves for two-phase flow applications. A typical application involves liquids which flash during discharge as the pressure drops. The predictions from these various methods can vary greatly under such circumstances, and can also deviate as much as 300% from measured values. This paper compares predictions from several versions of the homogeneous equilibrium and non-equilibrium models with data from the literature for flashing of initially subcooled and low quality steam-water mixtures through nozzles of various lengths. The results indicate that the homogeneous equilibrium model gives reasonable results for nozzle lengths greater than about six inches, but non-equilibrium effects must be taken into account for shorter nozzles. Both the Fauske model with no adjustable parameters and the TPHEM model with appropriate values of the non-equilibrium parameter (kne) are shown to give good results for short nozzles and subcooled or low quality inlet conditions.