Computational study of the effect of cavity geometry on the supersonic mixing and combustion of ethylene

Highlights

• Role of cavity configurations on non-reacting and reacting flow characteristics.

• Cavity length and aft wall angle enhances fuel mixing.

• Fuel dispersion and residence time provide insights on enhanced mixing.

• Mixing and kinetics limited regions are identified for the combustor configurations.

• Role of detailed chemistry model over single step chemistry is demonstrated.

Abstract

In this numerical study, the supersonic combustion of ethylene in three model combustor configurations namely, baseline (no cavity), square cavity and inclined cavity are investigated. To this end, 3D, compressible, turbulent, non-reacting (with fuel injection) and reacting flow calculations using single step and 10-step chemical kinetics in conjunction with a one equation turbulence model have been carried out. In the mixing study, predictions of the flow features with fuel injection, fuel trajectories, contours of total pressure loss along the combustor are compared between the combustor models. In the combustion study, contours of heat release are compared across the combustor models. Overall performance metrics such as mixing efficiency, total pressure loss and combustion efficiency are also compared between the combustor models. The comparison of combustor top wall static pressure and exit total pressure predictions with experimental data reported in the literature are also presented and discussed. The results clearly show that the model combustor with a shallow and inclined aft wall cavity has the highest residence time and maximum heat release. In addition, the role of a smaller L/D ratio cavity is shown to be minimal on the predictions of residence time and the heat release.

Introduction

Air-breathing engines are increasingly considered as a potential alternative propulsion device in place of rockets in the lower hypersonic regime (M $<$ 8) for the space transportation vehicles [1], [2], [3]. Supersonic combustion ramjet (scramjet) engine is the most logical choice for the propulsion of such sustained hypersonic flights. The successful development of such hypersonic vehicle depends to a large extent on the development of an efficient propulsion system, which in turn depends on the efficient mixing and combustion of different fuels like hydrogen [4], [5], Kerosene [6], [7], and ethylene [8], [9] injected into a supersonic cross flow in the combustor. In the lower hypersonic flight regime, gaseous hydrocarbon fuels are advantageous due to their ease of handling and high volumetric energy content [2]. Different fuel injection/flame-holding strategies like strut, ramp, cavities, rearward facing steps, have been proposed and extensively studied. Although efficient fuel dispersion (mixing) is the primary purpose of all those strategies, the associated total pressure loss must be within the acceptable limits to implement them in full-scale scramjet engines. Among them, recessed cavities exhibit better mixing characteristics with lower total pressure loss when compared to other strategies [6], [10]. A brief review of the recent literature on the mixing and combustion characteristics of a hydrocarbon fuel such as ethylene in the presence of cavities is presented next.

Cavities in supersonic combustors were reported to enhance the mixing rate [11], [12], ignition [4], heat release [5], [9], [13] and influence the engine starting characteristics [14]. In addition, direct cavity fueling has been shown to have more favorable cavity flame-holding stability characteristics [15], [9]. However, this method of fuel injection can cause additional complexity in terms of cavity entrainment. The role of cavity geometric parameters and injection location on the cavity flow characteristics [10] and mixing and combustion characteristics of ethylene [16], [17] have been reported in the literature. Wang et al. [8] investigated the combustion instabilities inside an ethylene fueled supersonic combustor at Mach 2.1 conditions. Different fuel injection schemes were investigated. They reported that the fuel injection scheme and combustion oscillation mode are directly correlated, and for low stagnation inflow temperatures it not only affects mixing but also the combustion efficiency and stability. Zhao et al. [16] experimentally investigated the ethylene flame flashback phenomena in a cavity based scramjet combustor at Mach 5.5 flight conditions. In their study, the fuel was injected upstream of the cavity, and the flame flash back was reported to be more sensitive to the temperature fluctuation downstream of the cavity due to enhanced combustion activity. Chang et al. [17] experimentally investigated the ethylene flame-holding in a double-ramp supersonic flows in a shock tunnel. The stagnation temperature was varied from 1270 to 1810 K. Their results highlighted that the flame pattern is influenced by the flow stagnation temperature and the location of the fuel injection.

Recently, Liu et al. [18] experimentally investigated the supersonic combustion of ethylene in an axisymmetric combustor with a cavity at Mach 4.5. They reported that the enhanced heat release in the cavity configuration affects only the near wall region, and does not alter the core flow characteristics. The inlet distortion and shock wave interaction effects on the ethylene fuel-air distribution and ignition was experimentally investigated in a cavity based scramjet combustor at Mach 3 flow conditions by McGann et al. [19]. They reported that the inlet distortion affect the cavity shear layer, and in turn, the fuel distribution within the cavity. An et al. [20] experimentally investigated the ignition characteristics of ethylene fuel in a cavity based scramjet combustor at Mach 2.5 flow conditions. They reported that the flame holding characteristics were enhanced by the presence of cavity, however, more centralized fuel distribution will be critical for fuel ignition at supersonic flow conditions. All these studies indicate the increasing interest in cavity based supersonic combustors for hypersonic flights with a focus on the fuel distribution inside the combustor and its effect on the heat release and flame pattern. Huang [21] reviewed the advances in the cavity-based scramjet combustors and emphasized that the more details on the cavity-based fuel mixing and flame stabilization are needed for the design/development of future hypersonic airplanes. Therefore, further studies are warranted to gain additional insights that are difficult, if not impossible, to obtain experimentally, and this serves as the motivation for the present work.

The objective of the present work is to numerically investigate the mixing and combustion characteristics of ethylene fuel injected from a non-circular port in three different model combustor configurations. Two cavity geometric configurations namely square and inclined cavity are investigated and the results are compared with those of the baseline case (i.e., combustor model without cavity) to highlight the role of cavities on the supersonic mixing and combustion of the fuel. Although hybrid approach like large eddy simulation (LES)-Reynolds Averaged Navier Strokes (RANS) is tested for the supersonic mixing and combustion [22], they are computationally expensive for practical applications. Reynolds-averaged Navier–Strokes (RANS) based approach is employed in this study as it is extensively tested and established for aerospace applications [23], [24], and owing to its computationally less expensive nature. Furthermore, an inclined cavity configuration is chosen as it has attracted significant interest for the development of supersonic combustors [11], [12], [9], [13], [18]. In addition, a systematic evaluation of the sensitivity of the RANS predictions to the chemistry model has also been carried out. The results obtained using the multi-step chemistry model are compared with those of the single-step model for different model combustor configurations. The numerical predictions with both the chemistry models are validated with the experimental data reported in the literature.