Numerical investigation of the nozzle expansion state and its effect on the performance of the steam ejector based on ideal gas model

Many research works have generally divided the back pressure range of steam ejectors into critical, subcritical and malfunction modes based on the change of the entrainment ratio. In this study, we innovatively define and compare the abnormal and normal modes of the over-expanded, fully expanded and under-expanded states for a steam ejector using the CFD method and find that the steam ejector can maintain almost the same the secondary fluid mass flow rate under the normal mode of different expanded states. A comparison of different nozzle expansion states is also carried out. The results demonstrate that huge flow separation zones occur in the mixing chamber and diffuser in the under-expanded state, causing a drop in the secondary fluid mass flow rate compared with other expanded states. In addition, the irreversibility analysis illustrates that the steam ejector could minimize entropy production under the over-expansion state to obtain the optimal entrainment rate. More importantly, this paper provides a theoretical reference for maximizing the entrainment ratio or the secondary fluid mass flow rate from the perspective of the nozzle expansion state.