Extension of the delayed equilibrium model to flashing flows of organic fluids in converging-diverging nozzles

Flashing flows of organic fluids find application in various energy systems, but the available modelling approaches rely on semi-empirical correlations calibrated for other fluids and operating conditions. This paper extends the state-of-the-art one-dimensional models developed for water to flashing of organic fluids, using R134a as a reference. First, the delayed equilibrium model combined with Grönnerud's friction correlation is identified as the most appropriate approach, reducing the error on mass flow rate with respect to experimental results from 48.3 %, obtained with the homogeneous equilibrium model, to 10.3 %. Then, the traditionally adopted incompressible flow assumption for the metastable liquid phase is replaced by a dedicated thermodynamic model, preventing the appearance of unphysical metastable phase temperatures. Moreover, the delayed equilibrium model is tailored to R134a by tuning the semi-empirical coefficients of the constitutive law. The proposed modifications to the state-of-the-art delayed equilibrium model result in an improvement in the prediction of the pressure profiles and the mass flow rate for flashing flows of R134a, with a decrease in the error on mass flow rate from 10.3 % to 6.1 % with respect to the original formulation.