## 3 Results and Discussion
### Effect of the Shock Wave on the Critical Back Pressure  
When the secondary fluid pressure and the back pressure are 1710 Pa and 3000 Pa, respectively, the variation curve of the entrainment ratio and the back pressure under different primary fluid pressure is shown in Figure 9. According to the previous analysis, the increasing of the primary fluid pressure will cause the normal shock wave moves downstream close to the outlet of the ejector. Meanwhile, the Mach number in the upstream region of the shock wave is also raised (as shown in Figure 7). The structure of the shock wave develops from the inviscid normal shock with lower intensity to the bifurcated oblique shock with higher intensity (as shown in Figure 6). The critical back pressure enlarges with the increasing of the primary fluid pressure, while the entrainment ratio declines. When the primary fluid pressure is 0.34 MPa, the critical back pressure is 3200 Pa; the critical back pressure is 6000 Pa as the primary fluid pressure is 0.46 MPa, increasing by nearly 47%. To ensure the stable operation of the experimental system, a higher primary fluid pressure should be selected as far as possible within the allowable range of the experimental equipment. When the primary fluid pressure is 0.34 MPa, the entrainment ratio is 0.84; the entrainment ratio is 0.44 Pa, as the primary fluid pressure is 0.46 MPa, which decreases by nearly 48%. The critical back pressure is one of the main parameters to determine the working range of the ejector while the entrainment ratio is the parameter to determine the pumping efficiency of the ejector. In the case of ensuring the high pumping efficiency of the ejector, the primary fluid pressure can be appropriately increased to expand the operable range of the ejector. As shown in Figure 10, when the primary fluid pressure is less than 0.40 MPa, the growth rate of critical back pressure is lower than the value when the primary fluid pressure is more than 0.40 MPa. The reason may be that the higher the primary fluid pressure is, the smaller the loss of momentum and kinetic energy produced by the mixed fluid. With the increase of the velocity and the intensity of the shock waves, the mixed fluid resisting the back pressure disturbance is improved. The analysis combined with Figures 6 and 9; the growth rate of the critical back pressure is related to the length of the pseudo-shock wave region. Moreover, the longer the pseudo-shock wave length results in the greater value of the critical back pressure. It indicates that the pseudo-shock wave region has a very significant influence on the critical back pressure. The growth rate of critical back pressure also is related to the primary fluid pressure. When the primary fluid pressure is kept in 0.32 MPa-0.40 MPa, the critical back pressure cure shows a slope of M1 = 0.0012. The slope of the critical back pressure increases to M2 = 0.0031 when the critical back pressure varies between 0.40 MPa-0.46 MPa. The growth rate of the  
Figure 8: The variation of entrainment ratio with the primary fluid pressure. critical back pressure at 0.40 MPa-0.46 MPa accelerates 61% comparing with the 0.32 MPa-0.40 MPa. Consequently, though the critical back pressure is not constantly increased with the primary fluid pressure at the same growth rate. The disturbance caused by the back pressure propagates the inverse pressure gradient from the downstream to the upstream. When the reverse pressure value is lower than the critical back pressure, the shock wave will offset the disturbance caused by the reverse pressure value and decrease the energy loss. Simultaneously, with the increasing of the back pressure, the energy and intensity of the normal shock wave will decrease, the shock wave structure will be changed. Once the adverse pressure gradient value is higher than the critical back pressure value, the adverse pressure gradient will continue to move upstream and cause no normal shock wave to be generated. The inverse pressure  
Figure 10: The influence of the primary fluid pressure on critical back pressure.