**3. Scramjet**

*Hypersonic Vehicles - Past, Present and Future Developments*

tics of the combustor with a cavity.

**2. Waveriders**

The whole process is in the order of zero-dimensional thermodynamic analysis [9], quasi-one-dimensional estimated analysis [10], and three-dimensional computational fluid dynamics analysis [11–13]. The scramjet flowpath is designed with an inward-turning inlet [12], a constant-area circular isolator, a circular combustor with a cavity [11], and a three-dimensional flow-stream traced nozzle [13]. Firstly, geometry parameters and flow conditions of both the inlet and the exit for each subsystem are obtained from the result of the stream function analysis and optimization [14]. Secondly, two design codes are developed, one of which is the quasi-one-dimensional estimation program for the combustor and the other is the aerodynamic force and heat estimation for the whole hypersonic vehicle. Lastly, the CFD method is applied for performance analysis of the jaws inlet, back pressure characteristics of the inlet with a constant-area isolator, and flow field characteris-

In the waverider design, it is the first step to define the generation field and then the streamlines constituting the compression surface of the waverider. In the current study, the design conditions of the vehicle are chosen as follows: height of 25 km and free stream with the inflow Mach number to be 5.0. Thereafter, the shape together with the pressure distribution is determined. Typically, a waverider design process can be divided into: selection and design of the basic flow field in the flow direction, solving of the basic flow field, streamline tracing, and application of the osculating theory in the spanwise direction. After that, points representing streamlines are obtained. Streamlines and compression surface can be generated using CAD tools. For example, in this study, an automatic 3D configuration generation program based on the UG API is developed. Meanwhile, an aerodynamic force estimation program is built. Usually, remodel design of the waverider is needed for a specific purpose. The basic flow field is usually a steady inviscid supersonic flow one, which is the core of the design of a waverider. Basic flow fields used for waverider design can be

*Cone-derived waverider. (upper) Configuration with surface mesh for rapid estimation. (bottom) CFD* 

**32**

**Figure 1.**

*simulation under design conditions.*

Regarding the conceptual design of scramjet, the stream thrust analysis [9] was superior to that of the thermodynamic cycle or first law analyses as it managed to account for several phenomena such as the geometry of the combustor, the velocity, mass of the fuel, and the exhaust outlet pressure not matching the ambient. **Figure 2** briefly introduces the design procedure and method of scramjet.

A scramjet was first designed by using stream thrust analysis, to obtain the overall parameters and flow state parameters at the in-/outlet of each components. When the stream thrust was analyzed, a group of state parameters were determined such as pressure, density, and temperature together with velocity and areas in each component's inlet and outlet. These parameters were delivered to the following two-dimensional components' design of the inlet, isolator, combustor, and nozzle. When the overall dimension of the scramjet internal flow passage was determined, the performance of a scramjet that allowed a supersonic flow to pass through the engine without choking in the inlet throat, combustor, and nozzle were analyzed by the quasi-one-dimensional evaluation program.

Using the above conceptual design method and performance evaluation, the initial design and analysis of a scramjet were performed. The optimization was conducted to generate more practical results based on the specific objectives. The flow chart describing the conceptual design method and optimization process is shown in **Figure 3**. As the design and evaluation of scramjet are highly nonlinear problems,

#### **Figure 2.**

*Flow chart of conceptual scramjet design.*

*Hypersonic Vehicles - Past, Present and Future Developments*

**Figure 3.** *Flow chart of the conceptual design and optimization process.*

**Figure 4.** *Comparison of three optimal vehicle geometry shapes.*

a multi-island genetic algorithm was chosen as the single-objective optimization algorithm, and a nondominated sorting genetic algorithm was selected as the multi-objective optimization algorithm. After the scramjet was designed and evaluated, the optimization was conducted to study how exergy works in the complex integrated system and to find which design variables play relatively important roles in this evaluation system. **Figure 4** shows the vehicle geometry shapes of different optimization objective cases. Three-dimensional design result of the scramjet can be seen in **Figure 5**. The detailed design methods for each part will be introduced in the following.

**35**

**3.1 Inlets**

**Figure 5.**

Various prototypes of hypersonic inlet have been proposed since the 1960s. Kothari introduced the radial deviation parameter and categorized the inlets as inward-turning inlets, outward-turning inlets, and two-dimensional inlets [16]. Unlike the other two groups, inward-turning inlets exhibit accumulation of flows in the central part. The advantages of inward-turning designs, especially those approaching a completely round combustor entrance shape, are several fold. From structural and wetted area perspectives, a more round design provides better performance than a rectangular or two-dimensional configuration. Lower wetted surface area in the combustor for an equivalent level of thrust of course means lower heating loads and lower drag. Moreover, corner flows need be much less of a concern with inward-turning geometries. Low aspect ratios at the isolator, which are characteristic of inward-turning inlets, also result in operational advantages. In **Figure 6**, typical inward-turning inlets such as Busemann, REST, and Jaws

*Conceptual three-dimensional scramjet design: (a) Busemann inlet, circular isolator and combustor, threedimensional asymmetric nozzle. (b) Jaws inlet, circular isolator and combustor, three-dimensional symmetric nozzle. (c) REST inlet, circular isolator and combustor, three-dimensional asymmetric circle-to-rectangle nozzle.*

The performance of designed inlets under on and off design conditions was numerically investigated [12]. **Figure 7** shows the comparison on the performance

inlets were designed and built using CAD tools.

*Airframe-Propulsion Integration Design and Optimization*

*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 5.**

*Hypersonic Vehicles - Past, Present and Future Developments*

a multi-island genetic algorithm was chosen as the single-objective optimization algorithm, and a nondominated sorting genetic algorithm was selected as the multi-objective optimization algorithm. After the scramjet was designed and evaluated, the optimization was conducted to study how exergy works in the complex integrated system and to find which design variables play relatively important roles in this evaluation system. **Figure 4** shows the vehicle geometry shapes of different optimization objective cases. Three-dimensional design result of the scramjet can be seen in **Figure 5**. The detailed design methods for each part will be introduced in

**34**

the following.

**Figure 4.**

**Figure 3.**

*Comparison of three optimal vehicle geometry shapes.*

*Flow chart of the conceptual design and optimization process.*

*Conceptual three-dimensional scramjet design: (a) Busemann inlet, circular isolator and combustor, threedimensional asymmetric nozzle. (b) Jaws inlet, circular isolator and combustor, three-dimensional symmetric nozzle. (c) REST inlet, circular isolator and combustor, three-dimensional asymmetric circle-to-rectangle nozzle.*

#### **3.1 Inlets**

Various prototypes of hypersonic inlet have been proposed since the 1960s. Kothari introduced the radial deviation parameter and categorized the inlets as inward-turning inlets, outward-turning inlets, and two-dimensional inlets [16]. Unlike the other two groups, inward-turning inlets exhibit accumulation of flows in the central part. The advantages of inward-turning designs, especially those approaching a completely round combustor entrance shape, are several fold. From structural and wetted area perspectives, a more round design provides better performance than a rectangular or two-dimensional configuration. Lower wetted surface area in the combustor for an equivalent level of thrust of course means lower heating loads and lower drag. Moreover, corner flows need be much less of a concern with inward-turning geometries. Low aspect ratios at the isolator, which are characteristic of inward-turning inlets, also result in operational advantages. In **Figure 6**, typical inward-turning inlets such as Busemann, REST, and Jaws inlets were designed and built using CAD tools.

The performance of designed inlets under on and off design conditions was numerically investigated [12]. **Figure 7** shows the comparison on the performance

**Figure 6.**

*Several inward-turning inlets design. (a) Busemann. (b) REST. (c) Jaw's.*

#### **Figure 7.**

*Performance parameters of Jaws inlet under different inflow Ma: (a) mass capture ratio, (b) total pressure recovery coefficient, and (c) static pressure ratio (d) temperature ratio.*

of one Jaws inlet under different inflow Mach numbers and two different angles of attack. In **Figure 8**, numerical simulations were carried out for Jaws inlet under different back pressures. In addition, performances of different inward-turning inlets were also compared using numerical simulations.

### **3.2 Isolators**

An isolator is necessary in scramjet to prevent inlet to unstart under high back pressure due to heat release in the combustor. The backpressure can cause the isolator flow to fluctuate violently. As the back pressure exceeds the critical value, the inlet can unstart, which causes the flow field to become unstable and oscillate unsteadily, with the drag increasing sharply and causing the engine to lose thrust.

**37**

**3.3 Combustors**

**Figure 8.**

*Airframe-Propulsion Integration Design and Optimization*

In the isolator, due to the close coupling of the boundary layer and the supersonic core flow through shock waves and expansion waves, the flow structure of a shock train is rather complex even at very simple incoming flow conditions and wall conditions. Correspondingly, it is important to understand the mechanism of the pseudo-shock motion in the isolator. **Figures 9** and **10** show the comparison between numerical simulation results and experimental observation to understand the complex pseudoshock train in the circular and rectangular isolators, respectively. The conceptual design of isolator with a given shape of the inlet is conducted by using the empirical length formula [14]. Meanwhile, the isolator is further truncated with the passive wedge flow control. **Figure 11** shows the simulation results of design without and with the wedge flow control,

*Mach number contours of Jaws inlet flow fields under different back pressures.*

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

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

as shown in the left and right of the figure.

the conceptual design result of the combustor.

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

*Hypersonic Vehicles - Past, Present and Future Developments*

*Several inward-turning inlets design. (a) Busemann. (b) REST. (c) Jaw's.*

**36**

**Figure 7.**

**Figure 6.**

**3.2 Isolators**

*Performance parameters of Jaws inlet under different inflow Ma: (a) mass capture ratio, (b) total pressure* 

of one Jaws inlet under different inflow Mach numbers and two different angles of attack. In **Figure 8**, numerical simulations were carried out for Jaws inlet under different back pressures. In addition, performances of different inward-turning inlets

An isolator is necessary in scramjet to prevent inlet to unstart under high back pressure due to heat release in the combustor. The backpressure can cause the isolator flow to fluctuate violently. As the back pressure exceeds the critical value, the inlet can unstart, which causes the flow field to become unstable and oscillate unsteadily, with the drag increasing sharply and causing the engine to lose thrust.

*recovery coefficient, and (c) static pressure ratio (d) temperature ratio.*

were also compared using numerical simulations.

**Figure 8.** *Mach number contours of Jaws inlet flow fields under different back pressures.*

In the isolator, due to the close coupling of the boundary layer and the supersonic core flow through shock waves and expansion waves, the flow structure of a shock train is rather complex even at very simple incoming flow conditions and wall conditions. Correspondingly, it is important to understand the mechanism of the pseudo-shock motion in the isolator. **Figures 9** and **10** show the comparison between numerical simulation results and experimental observation to understand the complex pseudoshock train in the circular and rectangular isolators, respectively. The conceptual design of isolator with a given shape of the inlet is conducted by using the empirical length formula [14]. Meanwhile, the isolator is further truncated with the passive wedge flow control. **Figure 11** shows the simulation results of design without and with the wedge flow control, as shown in the left and right of the figure.
