Numerical And Visual Experimental Investigation On The Fluid Flow And Shock Wave Characteristics Inside The Ejector

With the rapid development of shipping industry,new requirements for energy conservation and emission reduction of ships were proposed by the International Maritime Organization.Ocean-going ships consume a lot of energy.How to improve the ship energy efficiency and reduce the emission of harmful substances is a hot topic.The ejector is an effective energy saving device,which is widely used for many applications,especially in the ship waste heat refrigeration and seawater desalination.However,the fluid flow and mixing process inside the ejector is very complex.The mechanism of the special phenomenon inside the ejector is not clear.Therefore,the characteristics of the fluid flow and the shock wave inside the ejector were investigated by the numerical simulation and visualization experiments.The main research contents are as follows.Firstly,the numerical model of the ejector was established.The effects of working condition on the flow field characteristics and the working performance of the ejector were investigated.On this basis,the influences of the mixing chamber length on the flow field characteristics and the working performance of the ejector under different Mach number at nozzle exit,constant-area section length and diffuser length were studied.The results show that there is an optimal value for the mixing chamber length of the ejector.And the optimal value for the mixing chamber length is related to the Mach number at nozzle exit.The higher the Mach number at nozzle exit is,the smaller the optimal value for the mixing chamber length is.Also,the performance of the ejector decreases with the increase of the Mach number at nozzle exit.The critical back pressure is more sensitive to both the length of the constant area mixing section and the length of the diffuser than the entrainment ratio.For a given length of mixing chamber,the critical back pressure will increase with increasing the length of constant area mixing section and the length of diffuser.However,the increment of critical back pressure will be smaller and smaller when the diffuser length exceeds a certain value,such as 200 mm.Secondly,a visualized experiment system of the steam ejector was built.The back-flow phenomena inside the steam ejector was investigated by using the high-definition camera and infrared thermal imager.The experimental results show that the performance of the ejector decreases rapidly with the increase of the back pressure at the subcritical condition.At the same time,a large number of vortexes are formed at the inlet of the mixing chamber,which lead to the liquid drops on the wall of the mixing chamber flow back.The liquid drops on the wall of the mixing chamber will become smaller and smaller until they disappeared.There is a significant temperature difference between the wall of the mixing chamber and the diffuser.The temperature on the wall of the mixing chamber inlet is much lower than that on the wall of the diffuser.The temperature on the wall of the mixing chamber will increase as increasing the back pressure.So,the liquid drops on the wall will be smaller and smaller.Finally,a visualized experiment system of air ejector was built based on a schlieren method.The effects of working conditions,area and shape of the nozzle exit on the structure of the shock wave and the working performance of the air ejector were investigated.The experimental results show that continuous expansion wave and compression wave are formed at the nozzle exit,which repeatedly superimposed to form a series of strong shock chains.The structure of the shock chain is related to the working conditions of the ejector and the shape of the nozzle outlet.As the primary pressure increases,the number of shock waves increases and the shock chain becomes longer.As the secondary pressure increases,the shock chain becomes shorter but the expansion angle becomes larger.When the nozzle in an under?expanded state,the entrainment ratio increases but the vacuum capacity decreases.When the nozzle in an over-expanded state,the shock intensity becomes the weakest and the ejector performance is also the worst.The lobe nozzle can effectively improve the turbulence ability of the primary flow at nozzle outlet and greatly improve the performance of the ejector.In this thesis,the cokParative studies show that the four-lobe nozzle has the highest turbulence degree at the nozzle exit,which has the greatest ikPact on the performance improvement of the ejector.CokPared with the ejector with Laval nozzle,the mass flow rate of the secondary fluid is increased by 101%,the entrainment ratio is increased by 117%,and the lowest entrainment pressure is reduced by 5.03%when the primary fluid pressure is 550 kPa and the secondary fluid pressure is 70 kPa.