Investigation On The Typical Structure Of Internal Flow Field And Pumping Performance Of Steam Ejector

Water vapor injection refrigeration system first appeared in the early 20th century,which was a certain application of environmental protection equipment.But because of their own low efficiency,large area and other serious defects,which made the use of water vapor injection refrigeration system was greatly limited.In recent years,with the development of industry,environmental problems and energy problems have become urgent problems,thus energy saving and environmental protection equipment becomes more and more important.At the same time,theoretical and experimental research on steam ejector has been further developed.Consequently great progress has been made in structural improvement and pumping efficiency,which has accelerated the wide application of jet pumps in petrochemical,metallurgy,textile,refrigeration and other fields,simultaneously promotes the research of jet refrigeration system and makes it enter the period of rapid development.In recent years,with the development of computer technology,the numerical simulation of the flow field inside the ejector as well as the pumping performance has been taken into account,and a great deal of research results have been accumulated.This provides an important theoretical basis for the performance improvement and structural optimization of the ejector.In this paper,the main ejector of steam injection refrigeration system was utilized as the calculation model.The influence of different operating parameters and geometric parameters on the ejection coefficient and the critical pressure was analyzed numerically by using the computational fluid dynamics(CFD)method.Meanwhile the typical structure of the field and its influence on the pumping performance of the ejector was also investigated.The main research studies contained in this paper are as follows:(1)In order to verify the accuracy of the simulation,the wall pressure distribution obtained by the jet refrigeration experiment system was compared with the wall pressure distribution simulated by the ejector model.It could be seen that the experimental results and the simulated values were in good agreement with the simulation results,which Indicated that the simulation results were relatively reliable.(2)The influence of the jet core on the internal flow field of ejector under different operating parameters and its effect on the pumping performance were researched.The results showed that when the working steam pressure is changed,there is an optimum working vapor pressure value(0.25MPa)which enables the ejector to obtain the highest entrainment ratio.When the working steam pressure reaches a certain value(0.36MPa),the mass flow rate of the secondary fluid no longer changes,Because when the steam pressure is too large,at the congested position,the jet core will rapidly expand and the effective area would be reduced.The presence of the jet core can suppress the continued increase in the mass flow rate of the exhaust gas,and the mass flow rate maintains the constant value.When the back pressure is lower than 5000Pa,the size of the jet core is constant and the Mach number of the near-wall fluid is constant,so the entrainment ratio does not change.When the back pressure exceeds 5000Pa,the jet core at the choking position disappears and the Mach number decreases,consequently the pumping performance of ejector declines.(3)The influence of boundary layer separation on the internal flow field of ejector under different operating parameters and different geometric parameters and its effect on the pumping performance of the jet pump were studied.While,the nature of the boundary layer detachment phenomenon,the mechanism of occurrence and the degree of separation of the boundary layer were also revealed.The results of the study demonstrated that boundary layer separation can occur when changing the back pressture,throat diameter and nozzle position.The higher wall reverse pressure gradient will induce the separation of the boundary layer,and the region of boundary layer separation partially occupies the overcurrent area of the secondary fluid in the mixing chamber,resulting in the loss of energy caused by the vortex,leading to a decrease in the pumping performance of the ejector.(4)The influence of shock phenomenon on the critical pressure of steam ejector under different working steam pressure was investigated.It was revealed that the position of the shock in the diffuser and the shape of the shock were important for the critical pressure of the ejector.When the shocking position is shifted downstream,and the shock pattern is close to the bifurcated oblique wave,consequently the intensity of the shock wave is high,and the ejector has relatively high critical pressure.In this paper,through the study of the typical structure of the internal flow field(jet core,shock train,boundary layer separation),the influence mechanism of different operating parameters and geometric parameters on the pumping performance(entrainment ratio,critical pressure)of the ejector can be deeply understood,which provides a comprehensive understanding and theoretical basis for the structural improvement and performance optimization of the ejector.