Numerical Study On The Mixing Characteristics Of The Steam Streams In The Steam Ejector

The marine diesel engine are the main energy consumption equipment in the course of ship operation.With the increasing requirements of the International Maritime Organization on the carbon emission of ships,with the increasing requirements of the International Maritime Organization on the carbon emission of ships,improving the energy efficiency of Marine diesel engines has become the research focus at the present stage.The steam ejector has the advantages of simple structure,remarkable energy-saving effect,and small environmental pollution,so it has a wide range of applications in the utilization of ship waste heat.Therefore,improving the performance of steam ejectors can make better use of ship waste heat and promote energy saving and emission reduction.The performance of a steam injector is closely related to the mixing of the two internal streams.But at present,there is still no concrete description of the complex mixing process in the steam ejector.The effect of the mixing of two streams on its performance is still unclear.In this thesis,the mixing characteristics of steam ejectors are studied by numerical simulation.The main research contents are as follows:Firstly,the numerical model of the steam ejector is established,and the mixing process and mixing mechanism of two streams in the steam ejector are studied.The mixing process in a three-dimensional steam ejector is numerically simulated using the material transport equation by establishing a 3D steam ejector model and a computational model of entropy yield.The simulation results show that the working fluid and the ejecting fluid start to mix at the nozzle exit,forming a mixing layer.Due to the velocity difference,a large number of turbulent vortices are generated at the boundary and inside the mixing layer.Turbulent vortices constantly break and shrink,thus promoting fluid mixing and causing a large amount of energy loss.Outside the mixing layer,there exists a certain unmixing region between the working fluid and the ejecting fluid.This indicates that the working fluid and the ejecting fluid first mix locally in the mixing chamber to produce the mixing layer.Then the working fluid and the ejecting fluid exchange mass and energy with the mixing fluid at the boundary of the mixing layer,and finally reach a uniform state.Secondly,the influence of operation conditions on the mixing characteristics of two steam streams in the steam ejector is studied.The mixing characteristics of steam ejectors at different operation conditions were studied by numerical simulation.The results show that the decrease of primary temperature and the increase of secondary temperature lead to the increase of mass transfer capacity of primary fluid.The maximum mixing efficiency of the steam ejector is 1.03 in the given operating condition range.In the critical state,the mixing region of the primary fluid decreases at the action of back pressure,while the mixing region of the secondary fluid remains unchanged.However,when the back pressure is greater than3454.5 Pa,the mixing of the secondary fluid in the mixing chamber is blocked,the mixing performance of the steam ejector is reduced,and the steam ejector enters the subcritical working state.Finally,the influence of structural parameters on the mixing characteristics of two streams in the steam ejector is studied.The mixing characteristics of steam ejectors at different nozzle exit shapes,mixing chamber angles,throat distance,and constant area section length were studied by numerical simulation.The results show that the mixing zone of two steam streams is directly determined by the structural parameters of the steam ejector.The petal nozzle increases the contact area between the primary fluid and the secondary fluid at the nozzle exit and enhances the mixing rate of the two fluids in the mixing chamber.The square nozzle increases the mixing strength of the two streams at the nozzle exit,but decreases the mixing rate in the mixing chamber.The distance between the throat nozzle directly affects the mixing area of the two steam streams in the mixing chamber.The smaller mixing area makes the mixing of the two steam streams restricted by the equal section.The unmixed length ratio increases by 2.05 times in the given throat distance range.Mixing chamber angle and constant area section length have little effect on mixing in the steam ejector.When the mixing chamber angle is 7,the mixing performance of the steam ejector is the best,and the unmixing length ratio increases only 22.8% in the given mixing chamber angle range.The length of the constant area section cannot affect the mixing area of two steam streams in the mixing chamber.When the length of the constant area section is 160 mm,the mixing layer in the steam ejector is the most fully developed,and the unmixed length ratio increases only 34.9% within the given constant area section length range.