**3.3 Combustors**

In the combustor, injection fuels mix with incoming air and burn to release large amounts of energy. In the conceptual design, the combustor length includes the ignition length and the combustion length. The ignition length can be obtained by multiplying the ignition delay time which referred to Balakrishnan and Williams [17] by the relative velocity between air and fuel. The combustion length can be modeled based on the study of Hasselbrink [18] and Smith [19]. **Figure 12** shows the conceptual design result of the combustor.

**Figure 9.** *Pressure distribution along wall using different turbulent models.*

### **Figure 10.**

*Contour of Mach number in the central symmetry plane.*

#### **Figure 11.**

*Simulation result of circular isolator. (a) Pressure contour of baseline design. (b) Pressure contour of baseline design with wedge control. (c) Velocity distribution of baseline design at x = 300 mm. (d) Velocity distribution of baseline design with wedge control at x = 300 mm.*

According to the combustion in scramjet engines, the time available for fuel injection, mixing, and combustion is very short. It is important to study the flame holding mechanisms. The presence of normal fuel injector inside the combustor generates a detached normal shock toward the upstream direction of the injector. As a result, there is a formation of separation region which may influence the efficiency of the combustor. As shown in **Figure 13**, numerical simulations were performed to understand the related flow structures. Another alternative method for better-mixing phenomena in scramjet combustor is to use cavity flame holders. Numerical studies on the cavity in the combustor were carried out, as can be seen in **Figure 14**. The fuel injection position was numerically investigated to find out an appropriate value, as shown in **Figure 15**.

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**Figure 14.**

**Figure 12.**

**Figure 13.**

*distribution along wall.*

*Airframe-Propulsion Integration Design and Optimization*

*DOI: http://dx.doi.org/10.5772/intechopen.85187*

*Geometry of combustor with cavity in 1/8 size.*

*Simulation of normal inlet under supersonic inflow: (a) Mach contour, (b) streamline, and (c) pressure* 

*Numerical simulation results of cavity under supersonic inflow. (a) Ma contour and streamline. (b) Pressure distribution along the cavity wall. (c) Velocity distribution across the shear layer at x = 25.4 and 38.1 mm.* 

*(d) Velocity distribution across the shear layer at x = 63.5 and 88.9 mm.*

#### **Figure 12.**

*Hypersonic Vehicles - Past, Present and Future Developments*

*Contour of Mach number in the central symmetry plane.*

*Pressure distribution along wall using different turbulent models.*

According to the combustion in scramjet engines, the time available for fuel injection, mixing, and combustion is very short. It is important to study the flame holding mechanisms. The presence of normal fuel injector inside the combustor generates a detached normal shock toward the upstream direction of the injector. As a result, there is a formation of separation region which may influence the efficiency of the combustor. As shown in **Figure 13**, numerical simulations were performed to understand the related flow structures. Another alternative method for better-mixing phenomena in scramjet combustor is to use cavity flame holders. Numerical studies on the cavity in the combustor were carried out, as can be seen in **Figure 14**. The fuel injection position was numerically investigated to find out an

*Simulation result of circular isolator. (a) Pressure contour of baseline design. (b) Pressure contour of baseline design with wedge control. (c) Velocity distribution of baseline design at x = 300 mm. (d) Velocity distribution* 

**38**

**Figure 10.**

**Figure 9.**

**Figure 11.**

appropriate value, as shown in **Figure 15**.

*of baseline design with wedge control at x = 300 mm.*

*Geometry of combustor with cavity in 1/8 size.*

#### **Figure 13.**

*Simulation of normal inlet under supersonic inflow: (a) Mach contour, (b) streamline, and (c) pressure distribution along wall.*

#### **Figure 14.**

*Numerical simulation results of cavity under supersonic inflow. (a) Ma contour and streamline. (b) Pressure distribution along the cavity wall. (c) Velocity distribution across the shear layer at x = 25.4 and 38.1 mm. (d) Velocity distribution across the shear layer at x = 63.5 and 88.9 mm.*

#### **Figure 15.**

*Pressure contour (unit of Pa) and combustion efficiency for four fuel injection positions. (a) Distance between injection nozzle and cavity leading edge of 25 mm. (b) Distance between injection nozzle and cavity leading edge of 30 mm. (c) Distance between injection nozzle and cavity leading edge of 35 mm. (d) Distance between injection nozzle and cavity leading edge of 40 mm. (e) Comparison on the combustion efficiency.*

The cavity has a great influence on the performance of supersonic combustors, such as combustion efficiency, drag characteristics, and flame stability. The impact of the cavity parameter variation on the performance of combustors is complex coupled. A surrogate model-based optimization and parameter analysis of the cavities in three-dimensional supersonic combustors with transverse fuel injection upstream were performed. The length, depth, and sweepback angle of cavities were first designed by orthogonal experiment. Numerical simulations were applied to analyze the performance and flow fields of the test cases. Surrogate models of the combustion efficiency and total pressure recovery coefficient with the design variables were constructed.

Based on the complex system optimization strategy, optimization of the cavity parameters was carried out twice to provide the Pareto front by the non-dominated

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**3.4 Nozzles**

**Figure 16.**

**Figure 17.**

*Pareto front of the cavity optimization.*

baseline design, as shown in **Figure 17**.

*Contour of the mass fraction of H2O at different cross sections.*

*Airframe-Propulsion Integration Design and Optimization*

sorting genetic algorithm (NSGA-II). The results show that the optimal cavity configurations can be divided into narrow deep type, as can be seen in **Figure 16**, shallow long type, and medium deep and long type, which correspond to rapid change section, gentle change section, and extraordinary change section in the Pareto front. The combustion efficiency has a negative correlation with the length of cavities and a positive correlation with the depth of cavities, whereas the total pressure recovery coefficient has the opposite situations. Both combustion efficiency and total pressure recovery coefficient have few positive correlations with the sweepback angle. The combustors in the gentle change section have more uniform pressure distribution and higher total pressure recovery coefficient, which should be preferred when there is no need of high combustion efficiency. Optimized combustor configurations were simulated and verified compared to the

A supersonic nozzle design is a significant work for hypersonic vehicles, which

devotes to produce most of the thrust force and helps to improve the vehicle's internal/external integral level. Two-dimensional (2D) and axisymmetric minimum length nozzles (MLNs) with constant and variable specific heat were designed using the method of characteristics (MOCs) [20, 21], as can be seen in

*DOI: http://dx.doi.org/10.5772/intechopen.85187*

*Airframe-Propulsion Integration Design and Optimization DOI: http://dx.doi.org/10.5772/intechopen.85187*

**Figure 16.**

*Hypersonic Vehicles - Past, Present and Future Developments*

The cavity has a great influence on the performance of supersonic combustors, such as combustion efficiency, drag characteristics, and flame stability. The impact of the cavity parameter variation on the performance of combustors is complex coupled. A surrogate model-based optimization and parameter analysis of the cavities in three-dimensional supersonic combustors with transverse fuel injection upstream were performed. The length, depth, and sweepback angle of cavities were first designed by orthogonal experiment. Numerical simulations were applied to analyze the performance and flow fields of the test cases. Surrogate models of the combustion efficiency and total pressure recovery coefficient with the design

*Pressure contour (unit of Pa) and combustion efficiency for four fuel injection positions. (a) Distance between injection nozzle and cavity leading edge of 25 mm. (b) Distance between injection nozzle and cavity leading edge of 30 mm. (c) Distance between injection nozzle and cavity leading edge of 35 mm. (d) Distance between* 

*injection nozzle and cavity leading edge of 40 mm. (e) Comparison on the combustion efficiency.*

Based on the complex system optimization strategy, optimization of the cavity parameters was carried out twice to provide the Pareto front by the non-dominated

**40**

**Figure 15.**

variables were constructed.

*Pareto front of the cavity optimization.*

**Figure 17.** *Contour of the mass fraction of H2O at different cross sections.*

sorting genetic algorithm (NSGA-II). The results show that the optimal cavity configurations can be divided into narrow deep type, as can be seen in **Figure 16**, shallow long type, and medium deep and long type, which correspond to rapid change section, gentle change section, and extraordinary change section in the Pareto front. The combustion efficiency has a negative correlation with the length of cavities and a positive correlation with the depth of cavities, whereas the total pressure recovery coefficient has the opposite situations. Both combustion efficiency and total pressure recovery coefficient have few positive correlations with the sweepback angle. The combustors in the gentle change section have more uniform pressure distribution and higher total pressure recovery coefficient, which should be preferred when there is no need of high combustion efficiency. Optimized combustor configurations were simulated and verified compared to the baseline design, as shown in **Figure 17**.
