Modelling of two-phase flow in a steam injector

Full text of publication follows: Two-phase steam and water flow in a steam injector is characterised by a wide spectrum of interface area concentration. The steam injector consists of mixing chamber, nozzle and diffuser. Superheated steam of high velocity and low pressure enters the mixing chamber, where its momentum and energy is transferred into water. Water flows into the mixing chamber in a form of thin annular jet, parallel to the injector walls. Contrary to steam, initial velocity of water is low, approximately few meters per second. In the inlet part of the mixing chamber water flow is annular and boundary of interface is clearly visible. In a some distance from the inlet water film becomes unstable and droplets of water are entrained from the film surface. The flow structure changes into droplet pattern. In the vicinity of the steam nozzle, where shock wave appears, pressure grows rapidly and droplet flow changes into bubble pattern. Inside the shock wave the steam is almost completely condensed. Water pressure downstream of the nozzle is higher than the pressure at the steam injector inlet. Flow in the injector is characterized by high gradients of velocity, pressure, void fraction and very different flow patterns. These changes were observed in the steam injector of 40 cm in length, built in IFFM. The water and steam flow through the steam injector was described by means of one dimensional, two fluid model together with interfacial area transport equation. Source terms in the balance equations of mass, momentum and energy are functions of interfacial area concentration. Source term in interfacial area transport equation is assumed to be in a simple relaxation form. The model is completed by transport equations of mass, momentum and heat fluxes. These equations describe transfer of the fluxes along the axial direction due to gradient of physical variables between phases. Intensity of these fluxes depend strongly on the flow patterns. In the case of annular flow the intensity is negligible, while for the bubble pattern it is significant. The feature of the model includes the existence of real eigenvalues of the equations system, what means the model is of the hyperbolic type. The numerical calculations are based on the experimental data. Part of the boundary conditions come from experiment, the rest is approximated. The solution of the two-phase water and steam flow in the steam injector is performed for the steady-state conditions, though measurements show high fluctuations of physical variables as well as fluctuations of the flow pattern. This effect is especially visible in the jet nozzle. Numerical calculations are performed by means of Runge-Kutta method for the two-phase model described by the system of 10 ordinary differential equations. The results of numerical solution are compared with the experimental data and analysed. (author)