Fuqiang Liu

This paper proposes an improved spark ignition simulation method to assess the high-altitude relight performance of a multi-swirl staged combustor. A validated RANS-DPM framework provides the cold flow and spray fields. The ignition model incorporates a flame particle survival mechanism, improved droplet evaporation updating, and optimised ignition criteria. A novel combustor dome structure with a pressure atomiser (45°-S-4.0) is proposed. Results show that the optimal configuration improves ignition performance by 20–67% under 3–1% pressure drops at 8 km altitude and extends the successful ignition ceiling from 8 to 10 km. The proposed method effectively captures key mechanisms of high-altitude spray flame ignition, providing a practical design tool for aero-engine combustors.

Inlet distortion significantly influences the overall performance of afterburners. This study numerically investigates the effects of non-uniform inlet velocity and temperature profiles on the flow characteristics, ignition limits, and flame propagation behavior in a strut/cavity-based combustor under sub-atmospheric pressure conditions, with an inlet velocity of 109.9 m/s and an inlet temperature of 1043 K. The results reveal that, compared to a uniform velocity inflow, increasing the velocity non-uniformity raises the ignition fuel-air ratio. However, when temperature distortion is coupled with velocity distortion, the ignition boundary fuel-air ratio decreases. The location of the velocity distortion peak markedly affects the recirculation vortex structure. The ignition equivalence ratio reaches its minimum when the distortion peak is positioned at the upper region, followed by the lower peak, while the middle peak yields the highest value. Despite varying distortion conditions, the flame propagation paths remain qualitatively similar: the flame within the cavity is entrained toward the strut trailing edge by the recirculation zone and subsequently propagates downstream. Among the cases examined, flame propagation is fastest when the distortion peak is located at the middle position, and the addition of temperature distortion further accelerates flame propagation. Nevertheless, an excessively high temperature distortion index may introduce the risk of auto-ignition. Overall, this investigation into flow, mixing, and combustion under non-uniform inlet conditions provides valuable insights for the design and application of strut/cavity-based combustors operating under complex inflow conditions.

This study experimentally investigates combustion instability in a centrally-staged swirl flame under varying stratified ratios (SR). Here, SR is defined as the ratio of the pilot-stage equivalence ratio to the main-stage equivalence ratio. Experiments were performed at a global equivalence ratio of 0.6, with SR values set to 1.9, 2.2 and 2.5. Thermoacoustic oscillations were observed at SR = 1.9 and 2.2, with dominant frequencies of 180 Hz and 182 Hz, respectively, whereas no self-excited oscillation occurred at SR = 2.5. At SR = 2.5, the dominant pressure frequency (184 Hz) and heat-release fluctuation frequency (42 Hz) yielded a ratio of 4.4, indicating clear decoupling between acoustic and combustion responses. With increasing SR, the flame structure changed from a lifted mushroom-shaped flame to a stable V-shaped flame anchored near the swirler exit. Under unstable conditions, pressure oscillations propagated mainly in the longitudinal acoustic mode. POD analysis showed that flame motion at SR = 1.9 was dominated by Mode 1. In contrast, at SR = 2.2, the energy contribution of Mode 1 decreased from 13.8% at SR = 1.9 to 7.3% at SR = 2.2, indicating a transition from single-mode dominance to multi-mode interaction. Increasing pilot-stage enrichment enhances root flame stabilization, suppresses periodic flame oscillation, and promotes the transition from thermoacoustically unstable to stable combustion.

Driven by the demand for wide-speed-range air-breathing propulsion, this study numerically investigates the effect of blockage ratio on the non-reacting flow field, fuel-air mixing, and combustion mode transition in an integrated cavity-strut dual-mode ramjet (DMR) combustor under inflow conditions corresponding to a flight Mach number ( Ma ) of 4.0. Three-dimensional Reynolds-averaged Navier-Stokes (RANS) equations coupled with the Eddy Dissipation Model (EDM) and a single-step global kerosene-air reaction mechanism are employed, and the numerical approach is validated against published experimental data. The results demonstrate that the integrated cavity-strut configuration significantly enhances cavity recirculation and fuel-air mixing compared with the cavity-only configuration, thereby improving flame stabilization and combustion efficiency. The blockage ratio is identified as a key parameter governing the interaction between the cavity recirculation zone and the strut-induced flow structures, thereby affecting mass transport, fuel-air mixing characteristics, and combustion mode distribution. Moderate blockage ratios of σ = 0.08 and 0.10 generate relatively large recirculation zones and more uniform kerosene distribution inside the cavity, resulting in transition-mode combustion and the highest combustion efficiency of approximately 70%. In contrast, σ = 0.06 leads to insufficient heat release and scramjet mode owing to the weak cavity-strut interaction and limited fuel residence time. Although σ = 0.12 achieves a high mixing efficiency of 87%, the excessive blockage strengthens the mainstream flow and deteriorates fuel penetration, causing local fuel-rich accumulation and reduced combustion efficiency. Therefore, an appropriate blockage ratio enhances fuel-air mixing and combustion efficiency, providing critical design guidance for the integrated cavity-strut DMR combustor development.

Bypass combustion implementation in a variable cycle engine offers a viable solution for operational mode transitions. Compared to the main combustor, the relative pressure and inlet temperature of bypass combustor proposed a challenge to its flame anchoring and high efficiency. Through integrated numerical and experimental approaches employing high-speed imaging techniques, proper orthogonal decomposition (POD), and dynamic mode decomposition (DMD) methods, this investigation demonstrates that the integrated strut–cavity configuration exhibits stable and efficient ignition performance across broad operating conditions, and the stabilization mechanism of cavity-anchored flame was systematically investigated. The results show that: (1) the integrated strut–cavity configuration demonstrates superior ignition performance across broad inflow conditions, with inlet temperature emerging as the primary governing factor for the fuel–air ratio of lean ignition boundary. (2) The cavity ignition sequence progresses through three distinct phases: initial radial downward migration of flame kernels, subsequent upward envelopment and propagation of flame, and final flame oscillation, with ignition delay times remaining consistent (about 6.5–7 ms) across most operating conditions. (3) Differential flame propagation mechanisms manifest during the second phase (flame envelopment and propagation) under varying relative pressure drops. Lower pressure-drop-conditions exhibit bilateral flame roll-up patterns, whereas elevated relative pressure drop scenarios demonstrate strut-proximal unilateral flame propagation, a phenomenon explained by numerical simulations of the flow field and fuel distribution. (4) POD and DMD analyses identify persistent dominant modal structures in stabilized cavity flames. Under elevated conditions, the flame demonstrates a pronounced tendency to anchor proximal to the strut rear wall, and turbulence intensification amplifies high-frequency oscillation components while maintaining the low-frequency dominant modes characteristic of cavity combustion.

Currently, steady-state analysis predominates in combustion chamber design, while dynamic combustion characteristics remain underexplored, and there is a lack of a comprehensive index system to assess dynamic combustion behavior. This study conducts a numerical simulation of the dynamic characteristics of the combustion chamber, employing a method combining large eddy simulation (LES) and Flamelet Generated Manifold (FGM). The inlet air temperature, air flow rate, and fuel flow rate were varied by 1%, 2%, and 3%, respectively, with a pulsation period of 0.008 s. The effects of nine different inlet parameter pulsations on both time-averaged and instantaneous combustion performance were analyzed and compared to benchmark conditions. The results indicate that small pulsations in the inlet parameters have minimal impact on the steady-state time-averaged performance. In the region near the cyclone outlet, which corresponds to the flame root area, pronounced unsteady flame characteristics were observed. Fluctuations in inlet parameters led to an increase in temperature fluctuations near the flame root. Analysis of the outlet temperature results for each operating condition reveals that inlet parameter fluctuations can mitigate the inherent combustion instability of the combustion chamber and reduce temperature fluctuations at the outlet hot spot.

High-altitude relight is a critical challenge for aero-engines, directly impacting the safety and emergency response capabilities of aircraft. This paper systematically reviews the physical mechanisms, key factors, and relevant prediction models of high-altitude relight, highlighting the detrimental effects of extreme conditions such as low pressure and temperature on fuel evaporation rates, flame propagation speeds, and turbulent combustion processes. A comprehensive overview of the current state of high-altitude relight research is presented, alongside recommendations for enhancing the ignition performance of aero-engines under extreme conditions. This paper focuses on the development of ignition prediction models, including early empirical and semi-empirical models, as well as physics-based models for turbulent flame propagation and flame kernel tracking, assessing their applicability in high-altitude relight scenarios. Although flame kernel tracking has shown satisfactory performance in predicting ignition probability, it still overly relies on manually set parameters and lacks precise descriptions of the physical processes of flame kernel generation. Future studies on some topics, including refining flame kernel modeling, strengthening the integration of experimental data and numerical simulations, and exploring the incorporation of new ignition technologies, are needed, to further improve model reliability and predictive capability.