## 3 Numerical Analysis of Flow Behaviour inside the Steam Ejector
### Analysis of the Flow Structure inside the Steam Ejector
#### 3.3.2 The Shock-Mixing Layer  
With the aim of deeply understanding the structure and characteristics of the fluid flow inside the ejector, it is necessary to analyse and discuss the shock-mixing layer of which is the key factor affecting the mixing progress of the two streams. The position and the shape of the shock-mixing layer can be reflected through the powerful visualisation function of the simulation method. The secondary fluid did not mix with the primary fluid immediately after entering the mixing chamber as shown in Figure 7a. The shock-mixing layer with a large velocity gradient between the primary fluid and the secondary fluid is captured. It was the boundary layer of the momentum exchange and mixing between the two streams. It was mainly that the high-speed primary fluid transferred its kinetic and  
Figure 5: Fluid flow structure inside the ejector.  
Figure 6: Flow zones distribution in the steam ejector.  
internal energy to the secondary fluid through the viscous transport capacity of the shock-mixing layer. Also, the velocity of the primary fluid decreases gradually through the restraint function of the shock-mixing layer. The borderline of the shock-mixing layer was displayed in Figure 7b. The length range of the shock-mixing layer is from the position of the nozzle outlet to the front of the normal shock wave where the two streams were reaching the co-velocity.  
As indicated in Figure 8, the flow process in the ejector can be divided into the energy exchanging process and the mixing process. In Figure 8, the energy and momentum transmissions of the two streams in front of the normal shock wave were conducted in the exchanging progress and the shock-mixing layer was broken, and the two streams began to mix after the normal shock wave was generated in the mixing progress. However, before the beginning of the mixing process of the primary fluid and the secondary fluid, the shock-mixing layer with a certain thickness can be regarded as a wall surface separating the two fluids. The flow duct of the secondary fluid was a converging-diverging structure like that of the primary nozzle. It was consisting of mixing channel, throat and diffuser as displayed in Figure 7a. The only difference between the actual and the imaginary converging-diverging structure was that the inner wall was a moving wall to provide the maximum energy to accelerate the secondary fluid. The secondary fluid was accelerated from stagnation state to supersonic state relying on continuously absorbing the kinetic energy from the moving wall in the energy exchange process. The position where the secondary fluid reaches the sonic velocity is the choking position and mostly is located at the minimum cross-section of the mixing chamber. In the end, the mixed fluid flow completely mixed after flowing through a series of oblique shock waves and the mixed fluid overcame the back pressure to discharge from the ejector. In summary, the whole flow progress is divided by the position of the normal shock wave, the area before the normal shock wave is the energy exchange process region while the area behind the normal shock wave is the mixing process region.  
Figure 7: The location and range of the shock-mixing layer (a) the contour of Mach number (b) the borderline.