Effect of area ratio of the primary nozzle on steam ejector performance considering nonequilibrium condensations

Abstract

The formation and evaporation of nanodroplets in steam ejectors is neglected in many numerical simulations. We analyse the influence of a primary nozzle on steam ejector performances considering phase change processes. The numerical model is validated in detail against experimental data of supersonic nozzles and steam ejectors available in the literature. The results show that the first nonequilibrium condensation is observed within the primary nozzle, while under-expanded supersonic flow causes a second nucleation-condensation process to achieve a large liquid fraction of 0.26 in the steam ejector. The compression process of the supersonic flow results in a steep decrease of the degree of subcooling leading to droplet evaporations. The condensation and evaporation processes repeat alternatively depending on the flow behaviour in the mixing section. The increasing area ratio leads to the transition of the flow structure from under-expanded flows to over-expanded flows in the mixing section. The droplet diameter is about 7 nm in the constant section and the entrainment ratio can reach approximately 0.75 for an area ratio of 8, which achieves a good performance of the steam ejector.