On the mass exchange mode controlled by high-speed compressive mainstream/cavity interaction in a laboratory dual-mode scramjet combustor
Introduction
In a dual-mode scramjet, the combustion process must be stable over a wide range of conditions, including subsonic to supersonic transition in a dual-mode combustor [1]. The stable mode transition process is crucial to enhance the stability and reliability of the dual-mode scramjet engine [2]. The cavity-based flameholder in a dual-mode scramjet is a promising technology due to its effectiveness in stabilizing the flame without excessive total pressure loss [3,4]. The mass and heat exchange between the mainstream and the cavity is crucial to flame stabilization in a dual-mode scramjet, which is controlled by the cavity shear layer [3,[5], [6], [7], [8]].
The effects of the compressibility on the turbulent free shear layer have been well established. The convective Mach number was raised to describe the degree of the compressibility by Bogdanoff [9] and Papamoschou and Roshko [10]. In this equation, and are the upper and lower free stream velocity, respectively, and and are the respective speed of sound. Papamoschou and Roshko noted that the growth rate of the shear layer decreases rapidly as the convective Mach number increases, and Clemens and Mungal [11] and Rossmann et al. [12] support this conclusion. Goebel and Dutton [13] adopted the Laser Doppler velocimetry measurements to gain the velocimetry data of the compressible shear layer. They showed that the crosswise turbulence intensity and the turbulent shear layer stress decreased as the convective Mach number increased, but the streamwise turbulence was reduced slightly. In the work of Elliott and Samimy [14], the streamwise turbulence intensity was shown to diminish with increasing Mach number.
However, the cavity shear layer is different from the free shear layer in three aspects [15]. First, the boundary condition of the cavity shear layer is the recirculation region, and that varies spatially and temporally, whereas, in the free shear layer, both the streams are considered uniform and constant [15]. Second, the shear layer impinging on the cavity trailing edge will affect the upstream shear layer [16]. Third, the cavity geometry also plays a vital role in the development of the shear layer, and the geometry effects have not been explored in the free shear layer [[16], [17], [18]].
The development of the cavity shear layer is highly dependent on the free-stream velocity and the geometry of the cavity. Several studies on the mainstream flow over a rectangular cavity have been investigated for various conditions.
In the flow condition at low speed, particle image velocimetry (PIV) was applied to investigate the flow over a rectangular cavity at free-stream speeds from 0.26 to 0.4 m/s, and the vortex structures in the cavity shear layer were captured [19,20]. Bian et al. [21] performed similar work but at a higher speed (7.4 and 11.3 m/s). They observed the time evolution of large-scale vortices forming immediately downstream of the leading edge. Ashcroft and Zhang [22] performed the qualitative and quantitative flow field study for the free-stream velocity at 32, 37, and 42 m/s. They revealed that the growth rate of the cavity shear layer has the same trend in the three inflow conditions. The shear layer development includes three stages: exponential growth immediately away from the leading edge, linear growth in the center of the cavity, and no growth near the trailing edge. Hammad [23] captured the flow structure of subsonic flow (25.7, 51.6, and 66.5 m/s) over a shallow open cavity. The modes of impinging and non-impinging interaction between the vertical structures in the shear layer and the trailing edge were studied. It was observed for the impinging type of interaction that cavity recirculation influenced the flow and propagated the flow disturbance from the leading edge to the trailing edge.
In a supersonic flow condition, Lahr et al. [24] adopted hydroxyl tagging velocimetry (HTV) to measure the velocity field of a Mach 2 airflow over a cavity and observed the growth rate of the supersonic shear layer decreased with increasing Mach number due to compressibility effects. Ye et al. [25] investigated the turbulence flow characteristics in a dual-mode scramjet combustor also using HTV. They pointed that the turbulence fluctuation velocity in the subsonic mode is higher than that of the supersonic mode. Tan et al. [26] revealed that the peak value of streamwise turbulence intensity and Reynolds shear stress decreased sharply with the increase of convective Mach number in the weakly compressible mixing layer. Zhao et al. [27] employed nano-planar laser scattering and PIV techniques to observe the flow field of the scramjet combustor at the inlet velocity of Mach 2.68. They evaluated the mass exchange between the cavity and free stream by the mass flow rate on the lip surface of the cavity. They observed mainstream fluid entrained near the trailing edge and cavity fluid entering the mainstream in the middle of the cavity. This phenomenon of mass exchange between the cavity and mainstream was also observed in Cai et al. [28].
The cavity configuration determines the flow turbulence characteristics, mass exchange properties, mixing efficiency and flame stability. In the work of Beresh et al. [17,18], the width effects on turbulence characteristics were measured. It was concluded that the length-to-width ratio affects the location of the recirculation region and the turbulence characteristics of the cavity shear layer. The exchange of momentum and heat can be improved by increasing the cavity length-to-depth ratio [8,29]. Two different cavity depths are compared to show the effects of cavity depth on the development of the cavity shear layer. They found that the shear layer in the smaller cavity has a larger thickness, and the more significant oscillation in the smaller cavity can promote the mixing efficiency [30]. The residence time of air in the scramjet combustor could be improved by the spherical and step cavity [31]. Wang et al. [32] pointed the retention time of the initial flame kernel is reduced by the combined effects of the cavity recirculation zone and the cavity shear layer and the mainstream in the rear-wall-expansion cavity. Roos et al. [33,34] placed the injector downstream of the cavity and found that this configuration can improve mixing efficiency relative to the flat plate configuration.
Although the flow in the actual dual-mode Scramjet combustor is three-dimensional (3-D) [35], unsteady [36], and reactive [[37], [38], [39]], Ashcroft and Zhang [22] proved that in the cross-center plane of the model, the flow feature could be nominally two-dimensional in the cross-central plane of the model. In Bian, Driscoll et al. [21], the recirculation zone is seen from the time-averaged velocity. The time-averaged shear layer momentum thickness was used to calculate the growth rate of the cavity shear layer, which was consistent with the time-resolved method. The time-averaged velocity was also used to analyze the mass exchange characteristics of the cavity-strut flameholder [27]. Oamjee and Sadanandan [40] revealed that though the non-reactive flow studies cannot be used to determine the combustion efficiency of a system, they can still deliver valuable insight into the mixing efficiency of the system. Most of the interpretations from a non-reactive flow study can be obtained from a reactive flow study. Ye, Shi et al. [25] proved that the turbulence fluctuation velocity in the supersonic mode was the same as in the non-reacting flow. The above explanations can prove that the 3-D effects, unsteady phenomenon, and reactive effects have no apparent impact on studying the recirculation within the cavity and mass transport progress.
The geometry effects and compressibility effects on the performance of dual-mode scramjet combustors have been investigated in recent years, as summarized herein. Unfortunately, investigation of high subsonic inflow conditions is scarce due to the difficulty of velocity measurement and the design of the flow conditions. Understanding the flow structure and properties of mass exchange at high subsonic inflow conditions is significant to broadening the ignition conditions of the dual-mode scramjet in the subsonic mode.
Herein, an experimental investigation was conducted in a direct-connected supersonic research facility. The velocity field of the subsonic flow over a rectangular cavity with the length-to-depth ratio from 0.8 to 1.5 was measured using PIV. The time-averaged velocity field, turbulent characteristics of the cavity shear layer, and progression of mass exchange between the cavity fluid and mainstream were measured. This study documents the flow structure of a subsonic flow over a rectangular cavity at inlet velocities of Ma 0.3 and 0.5, geometry effects and weak compressibility effects on the turbulence characteristics of the cavity shear layer, and the mass exchange mode between the cavity fluid and mainstream.
Section snippets
Direct-connected test experiment and cavity models
The experiments were conducted in a supersonic direct-connected supersonic test facility in the Hypersonic Innovation Technology Laboratory of Shang Hai Jiao Tong University. The schematic of the flow facility is shown in Fig. 1(a). The design Mach numbers are 0.3 and 0.5. The details of the experimental conditions are listed in Table 1. It should be noted that, for the condition of Ma = 0.3 and L/D = 0.8, the case is named Ma0.3 L/D0.8, and the other cases are named in the similar way. The air
Time mean velocity field
Example data are shown in Fig. 3 for the cavity with a 1.2 length-to-depth ratio. In Fig. 3, the mean velocity vectors of the shear layer, the streamlines of the mean velocity field, the mean unit velocity field (normalized with the free stream velocity), and the overall mean velocity vectors are shown. The error in Fig. 3(b) is due to the severe reflection on the trailing edge, which negatively influences subsequent analysis.
The vector field in Fig. 3(a) and (d) captures the shear layer
Conclusions
The PIV technology was adopted to investigate high subsonic inflow conditions, which included the development of the cavity shear layer and the mass exchange process between the cavity fluid and the mainstream for inflow Mach numbers of 0.3 and 0.5. Weak compressibility and geometry effects on the cavity shear-layer growth rate were discussed based on the experimental data. The mass exchange process between the cavity fluid and mainstream was also determined. It was determined that the mass
Funding
This work was supported by the National Natural Science Foundation of China for Turbulent Combustion Major Research Project (Grant No. 91941301) and the National Natural Science Foundation of China (Grant No. 51906146).
Role of the funding source
The funder had no role in experimental design, model establishment, data analysis, manuscript writing, or decisions to submit articles for publication.
Declaration of competing interest
The authors declare that we have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work benefited from fruitful discussions with B. Yu, the authors would like thank the anonymous referees for the valuable comments. Also, we thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript.
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