# Comparison of Dual-Combustion Ramjet and Scramjet Performances Considering Combustion Efficiency
## 3 Results and Discussion
### Combustion Flow Characteristics of DCR  
Figures 10-12 display the temperature contours, pressure contours, and velocity contours for DCR 1 and DCR 2. The largest difference between the contours of the scramjets and DCRs is in the combustion mode. That is, the scramjet is in near wall combustion mode, while the DCR is in central combustion mode. Moreover, the temperature gradient, pressure gradient, and velocity gradient of DCRs are smaller than those of the scramjets. This shows that, in the combustion mode of DCR, the heat release is relatively uniform.  
Figure 9: Species mass fraction for (**a**) scramjet 1 and (**b**) scramjet 2.  
the pressure is lower compared to DCR 1. The higher temperature stems from the higher total temperature of the free flow, and the lower pressure stems from the excessive total pressure loss when the free flow reaches the subsonic velocity. In the supersonic combustor, the temperature contours, pressure contours, and velocity contours of DCR 1 and DCR 2 exhibit similar characteristics. As the flow moves to the outlet, the temperature difference between DCR 1 and DCR 2 gradually decreases. This is because when the temperature increases, the chemical reaction equilibrium shifts toward that of an endothermic reaction, resulting in an increase in the degree of dissociation of CO\({}_{2}\).  
For DCR 1, the static pressures at the inlet and throat of the subsonic combustor are 400 and 330 kPa, respectively; for DCR 2, they are 370 and 270 kPa, respectively. In the supersonic combustor, the static pressure difference between DCR 1 and DCR 2 gradually decreases as the flow gradually flows to the outlet. In the supersonic combustor, the velocity in the central zone of the DCRs is higher than that in the near wall zone. The velocity distribution of DCR is more uniform than that of the scramjet, and its velocity is higher.  
The species mass fractions for DCR 1 and DCR 2 are shown in Figure 13a,b, respectively. When the Mach number increased, the mass fraction of CO\({}_{2}\) and H\({}_{2}\)O decreased, while the mass fraction of CO and O\({}_{2}\) increased. Both the reduction and increase in DCR are greater than that of scramjet, indicating that the combustion efficiency of DCR decreases more after the Mach number increases. Notably, when the DCR is at position x = 0.5-0.6 m, under both sets of operating conditions, the mass fractions of O\({}_{2}\) and CO significantly increase and decrease, respectively. This is because the inlet of the DCR supersonic chamber is located at x = 0.59 m. When the gas enters the supersonic chamber, the mass fraction of O\({}_{2}\) increases sharply after being supplemented. On the one hand, the concentration of CO was diluted; on the other hand, the rate of CO reaction to produce CO\({}_{2}\) was accelerated, and the combined effect of the two made the mass percentage of CO decrease sharply.  
In summary, compared to the scramjet, the DCR has higher combustion efficiency and more uniform heat release. As a result, the DCR has a smaller gradient of temperature, pressure, and velocity than the scramjet. When the Mach number increases, the degree of CO\({}_{2}\) dissociation of DCR increases due to increased static temperature, resulting in a more obvious decrease in combustion efficiency.