## 5 Conclusions  
The use of steam ejector refrigeration technology to recover waste heat from engine combustion and convert it into useful energy is an effective energy-saving and environmental protection measure. A steam ejector refrigeration system driven by the engine combustion waste heat in the minibus with the cooling capacity of 12.5 kW was proposed. An experimental setup of the steam ejector refrigeration system was established. The computational fluid dynamics (CFD) approach was used to analyse the internal flow characteristic inside the steam ejector which cannot be fully accessed and studied through the experiment research. The influence of fluid flow characteristics on the pumping performance of the steam ejector was comprehensively studied. The reasons for the variation of the entrainment ratio under the operating parameters (i.e., the primary fluid temperature, the secondary fluid temperature and the back pressure) were also discussed. In summary, the following are the key findings from this study: (i) the entire flow area inside the steam ejector was divided into three regions: the inducing zone, the chocking zone and the discharging zone. Also, the energy exchange progress and the mixing progress are determined; (ii) the position and the shape of the shock-mixing layer which the boundary layer separating the primary fluid from the secondary fluid was defined, (iii) in the range of the primary fluid temperature from 134 \({}^{\circ}\)C to 138 \({}^{\circ}\)C, the entrainment ratio was positively correlated with the length of the choking zone. However, though the length of the choking zone increases, the pumping performance of the steam ejector would be reduced when the temperature of the primary fluid exceeds 138 \({}^{\circ}\)C; (iv) The shock-mixing layer only increased with the temperature until 15 \({}^{\circ}\)C, and then a significant drop happened at 20 \({}^{\circ}\)C. It indicates that the entrainment ratio increased from 5 \({}^{\circ}\)C to 20 \({}^{\circ}\)C, in which the improvement from 15 \({}^{\circ}\)C to 20 \({}^{\circ}\)C was marginal; and (v) with the increase of the back pressure, the normal shock moved from the diffuser section to the throat section. When the back pressure is less than the critical value (4600 Pa), the length of the shock-mixing layer is not significantly affected. As the back pressure exceeds the critical pressure, due to the disappearance of the normal shock wave, the choking condition cannot be maintained causing the pumping efficiency drops rapidly until the steam ejector lost its pumping capacity. Generally, this in-depth numerical and experimental study provided major insight structural design optimizations of steam ejectors.  
Conceptualization, Y.H. and L.G.; algorithm, Y.H. and X.W.; software, Y.H., R.C., T.B.Y.C. and H.L.; data curation, Y.H., C.L. and H.L.; formal analysis, Y.H. and A.C.Y.Y.; numerical simulations, Y.H., and R.C.; validation, A.C.Y.Y. and C.L.; writing--original draft preparation, Y.H. and X.W.; writing--review and editing, L.G. and A.C.Y.Y.; visualization, C.L. and A.C.Y.Y.; 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 Wale. All financial and technical support are deeply appreciated by the authors.  
The authors declare no conflict of interest.