# Comparison of Dual-Combustion Ramjet and Scramjet Performances Considering Combustion Efficiency
## 3 Results and Discussion
### Combustion Flow Characteristics of the Scramjet  
To study the combustion flow characteristics of the scramjet under Ma 6 and Ma 7 conditions, the commercial software Fluent is used to simulate the scramjet, and the temperature contours, pressure contours, and velocity contours of scramjet 1 and scramjet 2 are shown in Figures 6-8.  
Figure 6: Static temperature contours for scramjet 1 and scramjet 2. 1—Undisturbed zone, 2—Mainstream zone, and 3—Near wall combustion zone.  
The scramjet flow field exhibits the characteristics of a high flow velocity, limited combustion space, and complex shockwave system. When the Mach number increases from 6 to 7, the flow velocity at the combustor inlet increases accordingly, which not only shortens the residence time of the flow in the combustor, but also causes the separation point of the inlet airflow to move downstream, resulting in the first oblique shock wave at the inlet of the combustor moving downstream.  
Based on the temperature contours, the flow field is roughly divided into three zones: an undisturbed zone, a mainstream zone, and a near wall combustion zone. The undisturbed zone is situated before the first oblique shock wave at the combustor entrance. The mainstream zone is situated in the central region after the first oblique shock wave. The near wall combustion zone is the region with high temperature between the main flow zone and the wall surface. The static temperature from high to low is near the wall combustion zone, mainstream zone, and undisturbed zone. Both scramjet 1 and scramjet 2 conditions indicate that the static temperature of the rear cavity is higher than that of the front cavity,  
Figure 8: Velocity magnitude contours for scramjet 1 and scramjet 2.  
Figure 7: Static pressure contours for scramjet 1 and scramjet 2.  
indicating that the region around the rear cavity is more fully burned and more heat is released from this area.  
At the outlet, there is little difference between the pressures of scramjet 1 and scramjet 2. In scramjet 1, because the angle between the shock and flow direction is larger, the compression effect of the flow behind the shock is more obvious, resulting in higher pressure. The high-pressure region under both operating conditions is mainly confined to the middle region of the combustor. The pressure difference between scramjet 1 and scramjet 2 is caused by the different angles of the oblique shock wave. A comparison of the velocity magnitude contours of scramjet 1 and scramjet 2 shows that the two have similar characteristics, but the mainstream of the scramjet 2 has a higher flow velocity.  
Using the A-A section (Figure 2a) as the starting point, the species mass fraction of scramjet 1 and scramjet 2 is obtained, as shown in Figure 9. The mass fractions of each species for scramjet 1 and scramjet 2 are almost identical at the outlet position, signifying that the combustion efficiencies of scramjet 1 and scramjet 2 are basically identical. The CO mass fraction curve first rapidly increases, then slowly increases, and finally stabilizes, whereas the O\({}_{2}\) mass fraction curve first rapidly decreases, then slowly decreases, and finally stabilizes. This shows that, with increasing axial distance, the rate of increase of combustion efficiency decreases.  
In summary, for scramjet, when the Mach number of combustor inlet increases from 2.4 to 2.8, the position and shape of the first oblique shock wave will be significantly changed. That is, the oblique shock moves downstream, and the angle between the oblique shock and the flow velocity becomes smaller. The compression effect of air flow after shock wave is weak and the pressure after shock wave is reduced. However, the mass percentage of species under the two operating conditions is basically the same, indicating that there is little combustion efficiency difference between the two.