| Ejector is widely used in many applications,such as air-conditioning systems,aerospace,multi-effect desalination systems and chemical industries for its simple structure,low energy consuming and reliability.Critical back pressure plays a crucial role in the performance ejector systems,but few systematical investigations were conducted to study ejector that operates under variable conditions.In order to promote the development of this issue,the main work of this paper are described as follows:1.The effects of critical back pressure with variations of primary flow pressure on ejector performance were investigated in an ejector refrigeration system.The entrainment ratio first increases and then decreases with the growth of primary flow pressure at specific back pressure.Furthermore,both optimum primary flow pressure model and optimum entrainment ratio model were put forward based on experimental results and validated by seven groups of experimental data reported in literature,which is significant for the prediction and control of ejector operation.2.In order to study the relationship between superheated refrigerant and ejector performance,Computational Fluid Dynamics(CFD)technique was adopted to investigate the correlation using R134a as refrigerant.The results indicated that the increase of entrainment ratio is almost proportional to the rising of superheated level.Therefore,it can be concluded that the superheated level of primary flow is effective for the improvement of ejector performance.3.A wet steam model was utilized to research the relationship between the degree of superheat and homogeneous condensation within ejector primary nozzle.The homogeneous condensation process from nucleation to droplet growth stage was deeply analyzed.The homogeneous condensation process in ejector primary nozzle releases a large amount of latent heat,which enhances static pressure,reduces Mach number and lowers static temperature of the supersonic flow.Take both mass flow rate and dryness fraction into consideration,the selection of proper superheated level is a careful balancing act.The growth of superheated level from 5 K to 30 K causes 40.22%delay in the location and 43.92%reduction in the intensity of the condensation shock,which is good for the flow stability as well as nozzle outlet properties.4.An adaptive-NXP(nozzle exit position)ejector was proposed for the first time to enhance its performance by self-adjusting the position of primary nozzle with the variation of primary flow pressure.Simulation results indicated that there is an optimum range of NXP for ejector to achieve its highest performance.Secondary flow rate first rises and then drops while primary flow rate remains unchanged with the increase of NXP at fixed working conditions.Furthermore,a correlation between optimum NXP and primary flow pressure was developed to provide designing parameters for the ANXP ejector. |
