Numerical solution of steam flow in a nozzle using different non-equilibrium condensation models

摘要

The paper presents three Eulerian models for the two-phase flow of a steam with a non-equilibrium condensation due to the rapid expansion. All models are based on the transport equations for the mass, momentum and total energy of the mixture. The models differ in the number of additional transport equations for the parameters of liquid phase. The models with two and four additional transport equations take into account homogeneous nucleation and growth of existing droplets. The last model with no additional transport equation is based on a “switch” from metastable to equilibrium state, i.e. a “switch” from zero to equilibrium wetness. Although this last model omits the droplet size, it can be particularly interesting for the simplified flow simulations in the first steps of steam turbine design (e.g. simulations of the circumferentially averaged flow in a meridional plane of several turbine stages). Presented numerical results of one- and two-dimensional flows in a convergent–divergent nozzle have been obtained using in-house codes based on the symmetrical operator splitting with a finite volume method used for the convection and a Runge–Kutta method used for time integration of source terms. The result discussion covers the comparison of three presented models in terms of Mach number, pressure and wetness prediction. It further covers the influence of grid density on the prediction of nucleation zone as well as a new thermodynamic closure alternative to the IAPWS-95 formulation.