## 4 Conclusions  
In this work, the engine combustion waste heat as the heat source is driven by the steam ejector refrigeration system, which can improve the energy utilization and save cost, as is proposed. The effect of the shock wave structure on the pumping performance of the steam ejector is thoroughly studied. The shock train and co-velocity region locating in the mixing chamber and throat sections is defined as the pseudo-shock region. While the other part is the oblique-shock region consisting of a single normal shock wave and a series of the oblique shock waves locating in the subsonic diffuser section. The shock train, the single normal shock wave and the oblique shock wave are the three types of shock waves, respectively. In conclusion, the main findings of this study are as follows: (i) the shock wave structure is thoroughly studied by the experimental-CFD method. An experimental system of the ejector refrigeration was established and the CFD model was verified. Based on the ideal gas model, the k-w SST turbulence model is chosen, as it can predict the complex flow in the ejector; (ii) the structure of the shock wave is divided into two regions: pseudo-shock region and the oblique-shock region. Three types of shock waves are determined as the shock train, the normal shock wave and the oblique shock wave, respectively; (iii) the length of the pseudo-shock region increases with the increasing of the primary fluid pressure when the pressure is less than 0.34 MPa, then maintains constant. Due to a part of the energy of the mixed fluid is consumed, the entrainment ratio and the pumping performance of the steam ejector decreases. The bifurcation shock waves increase, and the energy of the normal shock wave decreases with the increase of the primary fluid pressure when the primary fluid pressure is more than 0.34 MPa; (iv) as the primary fluid pressure increases, the entrainment ratio declines and the critical back pressure and the length of shock waves increasing of the steam ejector increase; (v) the idealized primary fluid pressure is between 0.32 MPa and 0.36 MPa and the idealized Mach number is between 1.6 and 2.1. The growth rate of the critical back pressure with the primary fluid pressure at 0.40 MPa-0.46 MPa accelerates 61% compared with the pressure at 0.32 MPa-0.40 MPa. With the utilization of CFD analysis, the critical operating primary fluid pressure of the steam ejector system was identified. It can enhance the effectiveness of converting combustion waste heat to recyclable and useful energy sources in a steam ejector refrigeration system. Future works can be carried out to further enhance the understanding of the shock structure via experimentation using high-speed photography techniques.  
Conceptualization, Y.H. and X.W.; algorithm, Y.H. and L.G.; software, Y.H., C.W., R.C. and H.L.; data curation, Y.H., A.C.Y.Y. and H.L.; formal analysis, Y.H. and A.C.Y.Y.; numerical simulations, Y.H. and X.W.; validation, A.C.Y.Y., C.L. and; writing--original draft preparation, Y.H. and X.W.; writing--review and editing, A.C.Y.Y., L.G., R.C. and H.L.; visualization, L.G. and R.C.; supervision, J.T. and G.H.Y.; funding acquisition, X.W., J.T. and G.H.Y.  
This research was funded by the National Natural Science Foundation of China (51775098), the Australian Research Council (DP160101953) and the Australian Research Council Industrial Transformation Training Centre (ARC IC170100032) in the University of New South Wales. All financial and technical support are deeply appreciated by the authors.  
The authors declare no conflict of interest.