Numerical simulation on the direct contact condensation in a steam-water two-phase ejector with non-condensable gas

Abstract

The steam-water condensing ejector is widely used in many industrial fields and production processes. In the practical operation of ejectors in many systems such as nuclear reactors and underwater gas-ejection devices, the steam pressure is much higher than those in common direct heating systems or refrigeration systems. In addition, some non-condensable gas is inevitably mixed into steam jet. With the presence of non-condensable gas, the flow field and performance of ejector are significantly influenced by the changes in heat transfer characteristics. This paper presents a three-dimensional numerical simulation of the high-pressure steam-water condensing ejector with non-condensable gas based on inhomogeneous multiphase model. The steam inlet pressure, mass flow rate and non-condensable gas fraction are in the range of 0.70–2.40 MPa, 1019–3310 kg/(m2s) MPa and 0–0.05, respectively. The changes in steam plume shapes, distributions of thermal hydraulic parameters and pressure ratio are investigated under different boundary conditions. Numerical results show that the steam plume inside ejector transforms from ellipsoidal shape to divergent shape with the increase of steam inlet mass flow rate and the decrease of back pressure. The choking phenomenon in throat is found at divergent steam plume and the changes of flow field in diffuser do not spread upstream to the mixing chamber, so the thermal hydraulic parameters in mixing chamber and pressure ratio of ejector keep unchanged. In addition, the pressure ratio first increases from 1.08 to 2.34, then decreases to 1.99 with the increase of steam mass flux. As back pressure rises, the pressure ratio drops from 2.00 to 1.30 at low steam inlet mass flux; while first increasing linearly from 1.98 to 2.41 and then decreasing to 1.78 at high steam inlet mass flux. With the increase of non-condensable gas mass fraction, the pressure ratio slowly decreases from 1.48 to 1.39 at low steam mass flux while rapidly decreasing from 4.00 to 1.65 at high steam mass flux.