Non-equilibrium Phenomena in Two-Phase Flashing Flows of Organic Fluids

Abstract

Flashing flows can be found in several energy systems, but accurate and comprehensive models for the phenomenon do not exist yet. A few models have been developed for water and CO2 flashing flows, but they are not exhaustive nor directly applicable to flashing flows of organic fluids due to the substantial differences in typical operating conditions and in thermophysical and thermodynamic properties of the fluids. These properties influence the behaviour of the two-phase flow during the flashing expansion and govern the interphase transfer phenomena. In this paper we present a comparison between results from a purposely developed 1D homogeneous equilibrium model and recent experimental data on flashing flows of R134a through a converging-diverging nozzle. The comparison between the computed mass flow rates and pressure distributions with the experimental data shows that the homogeneous equilibrium model is not able to accurately reproduce flashing flows of refrigerants and indicates a significant deviation of the system from equilibrium conditions, with underestimations of the mass flow rate up to 50% with respect to the experimental values. The difference between experimental and calculated mass flow rates is used to quantify the degree of non-equilibrium of the system and to identify the sources of non-equilibrium. The results suggest that the finite vaporization rates and the subsequent evolution of the liquid phase through metastable conditions are the primary sources of non-equilibrium in flashing flows of refrigerants and therefore, models that can relax the equilibrium assumption are needed to accurately predict the behaviour of these flows.