Starting transient simulation of a vacuum ejector-diffuser system under Chevron effects

The vacuum ejector-diffuser system has been widely used in many applications such as refrigeration systems, high altitude test facilities and fluid transportation devices. In the present study, the starting transient flows of supersonic vacuum ejectordiffuser system, and its performance characteristics were simulated and analyzed by numerical methods. Newly designed chevron lobes were installed at the inlet of the primary stream of the vacuum ejector-diffuser system for the purpose of the performance improvement. A CFD method based on transient scheme has been applied to simulate the equilibrium flows and flow dynamics behavior of the secondary chamber. Primary numerical analysis results show that the chevrons get a positive effect on the vacuum ejector performance: less starting time and secondary chamber equilibrium pressure are found in chevron transient flow, compared with the convergent nozzle. The flow characteristics inside the ejector system are discussed using pressure history, vortices behavior, transient flow pattern and Reynolds stress distributions. INTRODUCTION As one of the important devices applied in the refrigeration systems [1,2], high altitude test facilities and fluid transportation industries [3], the vacuum ejector-diffuser system has been widely used due to its many incomparable advantages: such as no moving parts and even increasing pressure without mechanical energy [4-5]. The vacuum ejector-diffuser system is used to propel one low pressure secondary stream using high pressure primary stream through pure shear action. The mixing stream will flow out the ejector exit with a higher pressure than the secondary stream itself [6-7]. While in the vacuum ejector, the entrainment ratio between two streams is not so high like that of the normal ejector-diffuser system, and the secondary flow inlet of the vacuum ones is not supplied infinitely. This is due to its objective that not only to propel more secondary stream but also pull down the secondary chamber pressure as much as possible. In this case, the design of a vacuum ejector can not only base on the highest entrainment ratio (Rm), but also the volume of secondary chamber and pressure history during the operating process. In most of the earlier works [8-9], the secondary chamber was assumed to be of infinite size that can supply mass continuously. However in the most practical applications, the secondary chamber of the vacuum ejector has a finite volume. In this practical scenario, the starting transient flows of the vacuum ejector seems more meaningful than the final steady state itself [9-11]. In the present study, the steady cases with an infinite secondary stream was simulated to obtain the stable flow field. Then the secondary flow inlet was closed using wall boundary condition so that the left secondary mass can be pumped out due to the strong shearing action created by primary flow. Then the pressure inside the secondary chamber will be reduced. Present study is to simulate and analyse the starting transient flows of supersonic vacuum ejector-diffuser system, and its performance characteristics by numerical methods using a commercial software Fluent. Newly designed nozzle with several chevron lobes were installed at the inlet of the primary stream of the vacuum ejector-diffuser system for the purpose of its performance improvement. A CFD method based on transient scheme has been applied to simulate the equilibrium flows and flow dynamics behaviour of the secondary chamber. The flow characteristics inside the ejector system are discussed using pressure history, vortices behaviours, transient flow pattern and Reynolds stress distributions. RSM turbulent model was applied to simulate shear stress actions and vortices generation process, respectively, to demonstrate those effects on the starting transient process.