Critical Review of the Film Flow Model by Okawa et al.: Analysis and Improvement Potentials

Abstract

The film flow model by Okawa et al. is critically investigated. The basis of the study is a database of dryout experiments from the literature, which contains 4460 data points from 21 different sources, covering all influence parameters over a wide parameter range and comprising water as well as non-water fluids. Even though the model is well established and works generally well, three assumptions may be improved. Firstly, the film velocity should be considered in the interfacial shear force for the derivation of the film thickness. Secondly, a linear dependence between the shear entrainment rate and the film thickness is assumed, which cannot be supported by the physically-based Kelvin-Helmholtz approach. And thirdly, an equilibrium between the shear entrainment rate and the deposition rate is assumed for the calculation of the initial entrained fraction at the onset of annular flow. Using an alternative approach for the calculation of the initial entrained fraction, it can be shown that this assumption is not justified. Employing the proposed approaches leads to a more precise description of the conditions at the onset of annular flow, which is the starting point of the integration of the film mass balance in the film flow model, and therefore offers the potential for more accurate predictions of dryout.