# Enhancing Ignition Reliability with Tail-Groove Strut Designs in Cavity-Based Combustors
## 1 Introduction  
The turbine-based combined cycle (TBCC) engine, which exhibits great potential as a propulsion system for next-generation hypersonic flight [1, 2], has been extensively studied for its applications in space transportation, development, and military operations [3]. TBCC engines, alongside Rocket-Based Combined Cycle (RBCC) engines, represent two of the most promising combined cycle propulsion technologies for modern space exploration [4]. The RBCC engine combines rocket and ramjet engines, enabling the engine to switch between different propulsion modes to achieve maximum performance across a wide range of speeds and altitudes [5], while the TBCC engine consists of turbine and scramjet engines, facilitating efficient operation from takeoff through hypersonic flight. As space technology evolves, these engines are increasingly vital for applications ranging from earth-to-orbit transportation to deep space exploration. Expanding the engine's flight envelope, the most critical challenge--especially for TBCC engines--is to achieve reliable ignition and flame stability within a wide operating range [6]. The turbulent flame velocity of 10 m/s is obviously unable to meet the requirements of self-sustained combustion for the afterburner or ramjet combustor, especially when considering a flow velocity of 100 m/s. Consequently, traditional combustors commonly employ a flame holder, such as cavity [7, 8], to establish a stable pilot flame, igniting the fuel-air mixture for reliable combustion. The usage of a cavity flame holder at subsonic speeds was first proposed by the Air Force Research Laboratory as early as the 1990s [9]. With superior flame stabilization and minimal resistance loss [10, 11], cavity flame holders have been increasingly employed in afterburners and ramjet combustors, addressing challenges encountered at higher subsonic speeds and obtaining a greater performance [12]. Subsequently, a large number ofstudies on cavities have been conducted, such as on cavity configuration [13; 14; 15], injection strategies [16], and flow character [17; 18]. Despite exhibiting satisfactory ignition performance and flame stability, the isolated pilot zone faces challenges in effectively propagating the flame to the core flow [11]. To enhance the expansion of the pilot flame, struts are commonly combined with the pilot flameholder, accelerating the radial and circumferential propagation of the flame across the entire cross-section of the combustion chamber [17]. [PERSON] [19] observed enhanced flame stabilization with cavity-strut flame holders, as demonstrated by OH-planar laser-induced fluorescence and shadowgraphy techniques. [PERSON] [20] showed that cavity-strut flame holders enhance fuel and air mixing, leading to a 21.4% increase in mixing efficiency and a 20.5% improvement in combustion efficiency at the combustor exit. [PERSON] explored the lean ignition [21] and flow [22] characteristics of various cavity-strut flame holders, revealing that the proper cavity-strut combined structure widens the lean ignition limit at high subsonic speeds by simultaneously increasing the mass exchange rate and cavity residence time. Thus, the cavity-strut flame holder stands out as a promising choice for TBCC combustors to broaden the flight envelopes, and numerous related studies have been undertaken [23]. Conversely, the combination of a strut and cavity weakens the ignition and flame-holding performance, which is attributed to the alteration in flow mode and a decrease in residence time [22]. Hence, additional measures are implemented to enhance ignition and flame stability. [PERSON] [24; 25] proposed a novel air-assistant design for the cavity-strut combustor and found that air-entraining can compensate for the flow pattern destruction caused by struts, widen ignitable space, and improve combustion characteristics. [PERSON]'s [26] research delves into the innovative integration of ssel and triangle-lugs (STLs) within struts, showing a significant enhancement in the uniformity and elevation of outlet temperatures.