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.
Highlights
A supersonic ejector is a static fluid device designed to achieve high energy efficiency in thermal engineering systems
The primary nozzle plays a significant role in a steam ejector, in which the steam expands to the supersonic flow in the divergent portion
The wet steam model shows that the degree of subcooling can reach 50 K to generate a maximum liquid fraction of 0.26 in the steam ejector
Summary
A supersonic ejector is a static fluid device designed to achieve high energy efficiency in thermal engineering systems. Numerically studied the relationship between the mixing process and entrainment characteristics in steam ejectors based on the dry gas model and realizable k-ε turbulence model. Liu et al [15] evaluated the effect of an area ratio on entrainment ratios in a steam ejector using the dry gas model and realizable k-ε turbulence model. Employed realizable k-ε, RNG k-ε and k-u SST turbulence models, respectively, to compare the dry gas and condensing flow models in evaluating ejector performances. Wang et al [19] investigated the effect of the outlet diameter of the nozzle on the condensation parameters in a supersonic nozzle, which can hardly be extended to a steam ejector as the flow behaviour in the mixing section and the entrainment performance are not discussed in their numerical simulations. The transition of the flow structures from underexpanded flows to over-expanded flows is reported due to the increase of the area ratio of the primary nozzle in the steam ejector
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