## 4 Results and Discussion
### The Influence of the Secondary Fluid Temperature  
The Mach number contour of the flow field is shown in Figure 12, as the primary fluid temperature and back pressures of the steam ejector are 138 \({}^{\circ}\)C and 3500 Pa, respectively. With the increase of secondary fluid pressure, the length of the shock-mixing layer firstly increases and then decreases. The position of the normal shock moves downstream, and the position of the shock train locates in the mixing chamber section while the choke occurs in the throat section. It shows that the secondary fluid pressure has a significant effect on the flow characteristics of the ejector.  
Otherwise, the length of the primary jet core and the shock-mixing layer increases with the increasing of the secondary fluid temperature when it is less than 20 \({}^{\circ}\)C. As in the analysis above, the shock-mixing layer is the main momentum and energy exchange region, w  
Figure 11: The contours of Mach number under various primary fluid temperature.  
Figure 12: The contours of Mach number for various secondary fluid temperature.  
and momentum transfer between the two streams are determined. The primary and secondary fluids will not be mixed until the shock-mixing layer disappears. The longer the length of the shock-mixing layer is, the more degree energy and momentum transmission would be. On the other hand, with the secondary fluid temperature increases, the position of the normal shock wave gradually moves downstream indicating that the ability of ejector to resist back pressure disturbance is enhanced and the ejector will operate more stably. As indicated by the variation of the entrainment ratio with the secondary fluid temperature in Figure 13, the increasing of the length of the shock-mixing layer causes the normal shock wave moving downstream and its intensity then the entrainment ratio increases except the temperature of the secondary fluid is at 20 \({}^{\circ}\)C. The entrainment ratio is not only influenced by the length of the shock-mixing layer but also relates to the area from the end of shock mixing layer to the beginning of the normal shock wave where the energy and momentum exchanging progress finished and the two streams starts to mix. In this area, the ability of the primary fluid pumping the secondary fluid is continuously enhanced and the mass flow rate of the secondary fluid is also increased resulting in the pumping performance of the ejector will be better and better.  
The ratio of the primary fluid pressure to the secondary fluid pressure is generally referred as \"expansion ratio\", which is a dimensionless parameter to analyse the flow behaviour and to reflect the efficiency of a steam ejector [15]. As the value of expansion ratio increases, it means more secondary fluid is entrained given the same amount of primary fluid is being consumed. The pumping performance of the ejector will be greatly improved. Comparing Figure 12 with Figure 13, the steam ejector should be operated at relatively higher secondary fluid pressure to maintain the Mach number in a higher value and to pump more mass flow rate of the secondary fluid. The higher of the secondary fluid pressure is, the mixing progress will be, and the larger secondary fluid mass flow rate will be obtained.