1. Introduction

As one of the qualified candidates for hypersonic propulsion system, scramjet combustor has attracted a large amount of attention all over the world. As is now well known, realizing high efficiency and steady combustion in a supersonic combustor always remains as a critical issue for scramjet engines. Numerous researches have indicated that the additions of cavity in combustor and the transverse fuel injection upstream of the cavity can promote the stabilization of

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

combustion and flame [1–3]. Whereas, it is difficult yet for the fuel to reach properly mixing within the supersonic flow by the mechanical methods [4, 5]. Moreover, it is a great challenge for matching the transverse fuel jet up with the cavity under off-design condition [4]. Furthermore, certain stagnation pressure loss will appear using the approaches. Taken all account, some new methods are imminently needed for keeping stable and highly efficient combustion in combustor with the least penalties adding to the flowfield.

the discharge path, this type of discharge plasma is called "quasi-DC discharge plasma." However, it is a type of low temperature arc plasma in fact. The main properties of quasi-DC

Plasma-Assisted Combustion

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As depicted in Figure 1, the domain of quasi-DC discharge plasma filament is simplified as a cuboid region presented in red dashed lines for simulation based on its appearance. In Figure 1, the symmetric plotted line indicates that the cathode and anode are symmetrical for

Because the quasi-DC discharge plasma releases a large amount of heat concentrated in its discharge path, the discharge path goes into very hot. This high temperature domain (i.e., the discharge path) obstructs the supersonic inflow due to a thermal blocking occurred. Hereby, the quasi-DC discharge plasma behaves as a virtual blockage in the high speed flow of scramjet combustor, which is called as "the dominant thermal blocking mechanism" [17]. On the foundation of dominant thermal blocking mechanism, the individual plasma filament can

Generally speaking, the plasma input power and the effective heat power that transfers into circumambient gas are quite different. Besides, the percentage of loss commonly varies with the power supply and environment, so it is not a suitable way to use the plasma input power as the heat source value of quasi-DC discharge plasma in simulation. In order to straightforwardly describe the plasma heat strength, the average temperature of the plasma zone is acquired by using the numerical simulation here. This way is feasible when the average temperature of the plasma zone is within a reasonable range [10, 16, 18, 19]. Hence, based on the thermal blocking mechanism, a certain temperature which denotes the actuating strength (i.e., the input power) is specified for the plasma filament domain when the actuator works. The electrodes are flush mounted and do not have influence on the main flow themselves. The length and the section dimensions of individual plasma filament heat source are 20 mm length and 3 3 mm, respectively. The plasma filament locates 40 mm upstream of the fuel orifice center. Besides, it is generated near the wall in the center of the combustor, which means the symmetric plane of plasma filament is within the symmetric xy-plane of combustor. The quasi-DC discharge plasma working under a pulsed mode shows better performance than a continuous mode [19], so a plasma control frequency F<sup>c</sup> = 8 kHz with duty cycle ratio D = 2/5 is

chosen here. And Tpl = 2500 K is specified as an optimal actuating strength.

Figure 1. Computed configuration of quasi-DC discharge plasma in case one.

discharge plasma are described in [10–12, 15].

be simulated as a volumetric heat source [10, 12].

the central line of combustor wall and so does the plasma filament.

It is rather promising to adopt a discharge plasma for supersonic flow and/or combustion control in aerospace field [6–8]. It has been widely considered that plasma-assisted combustion is one of the most promising approaches for enhancing ignition and combustion in the environment of scramjet combustion [4, 6, 9]. As a further promising method, there are three advantages, that is: rapid response, less inertia, and flexibility [10]. Former studies clearly show that the quasi-DC discharge plasma can availably modify supersonic flow in a controllable manner among the discharge plasma mentioned above [8, 11], whereas the DBD plasma is commonly used in low-speed flow environment [12, 13]. If we can combine the quasi-DC discharge plasma with cavity and transverse fuel jet together, some new phenomena will surely appear, which may help in the improvement of combustor performance.

There in three primary mechanisms of the plasma effect on flow and combustion can be summarized: (1) momentum transfer, (2) fast local ohmic heating of the medium, and (3) active particles [6, 7, 14]. Ignoring the magnetic field, the mechanism of quasi-DC discharge plasma affecting a supersonic flow is mainly its fast local heating rather than the electrostatic force (i.e., the momentum transfer mechanism) [10, 15, 16].

This chapter aims at investigating the changes of the fuel jet, cavity, and whole scramjet combustor led by the quasi-DC discharge plasma based on the dominant thermal blocking mechanism. Here, a short cavity downstream of a fuel jet orifice is considered in order to simulate the combustor flowfield more realistic. The plasma is set as a controllable heat source. The k-ω shear stress transport (SST) model together with finite rate chemical reaction model is used simulating the turbulent flow and combustion. The flow structures, equivalent ratio, product distribution, stagnation pressure, and combustion efficiency in the combustor are all obtained and analyzed using the 3D numerical simulation which can acquire some data that are difficult to measure in experiments.
