Technical Summary: Numerical Simulation Study on Ignition Process of Hydrogen Micro-Mixed Diffusion Combustion Chamber

Problem Statement

Under the strategic goal of "dual carbon," hydrogen fuel is increasingly viewed as a critical component for transforming aircraft engine energy structures. However, hydrogen's high reactivity, low ignition energy, and wide ignition range present significant safety challenges, particularly within the confined spaces of combustion chambers. Ignition in such environments carries a high risk of overpressure (defined in this study as a local pressure increase exceeding 5% of the initial pressure) and potential explosion. While micro-mixed diffusion combustion technology offers a solution by dividing flames into micro-regions to enhance mixing and reduce emissions, research regarding the specific ignition risks and explosion characteristics of pure hydrogen in these confined spaces remains in a "blank stage." Existing literature largely focuses on hydrogen-doped fuels or unconfined leakage scenarios, leaving a gap in understanding the ignition dynamics of pure hydrogen micro-mixed chambers under varying injection and ignition strategies.

Methodology

The study employs a numerical simulation approach using ANSYS-FLUENT software to investigate the ignition process of a hydrogen micro-mixed diffusion combustion chamber.

• Physical Model: The simulation domain is based on an extended array of the single three-dimensional injector structure from Cranfield University, featuring a 260 mm long combustion chamber, a 70 mm tail convergence section, and six sets of injectors (each with air and hydrogen inlets).
• Numerical Setup:
  • Algorithm: Pressure-based velocity-coupled Coupled algorithm.
  • Turbulence Model: Selected based on prior research by Yilmaz Harun et al., ensuring accuracy for hydrogen combustion.
  • Combustion Model: Eddy Dissipation Concept (EDC) to capture chemical reactions in turbulent flow.
  • Chemistry: The ÓConaire complex hydrogen reaction mechanism (10 components, 21 reaction steps).
• Boundary Conditions: The equivalence ratio is fixed at 0.4. Inlet conditions include a total air temperature of 422 K and pressure of 243.47 kPa. The ignition source is modeled as a high-temperature heat source (2000 K, 3 mm radius).
• Safety Criterion: Referencing GB/T 42368-2023, an explosion is deemed to occur if the instantaneous pressure ratio ($P_r = P_m / P_{t3}$) reaches or exceeds 1.05.
• Verification: Grid independence was verified using structured grids of 0.2 mm, 0.3 mm, and 0.5 mm. The 0.3 mm grid was selected as it provided results consistent with the 0.2 mm grid while meeting accuracy requirements.

Key Contributions and Results

The study systematically analyzes combustion and explosion characteristics under different injection-ignition timings (stable filling, end of filling, half-filling) and different starting loads (full, 1/2, 2/3 working conditions).

1. Ignition Timing and Mixture Homogeneity

• Stable Filling (Time A): Under stable filling, the flame propagates relatively smoothly but experiences backfire in the early stage. Overpressure events occur, with the highest peak pressure ($P_r = 1.185$) appearing at $t = 75.5$ ms near the hydrogen outlets of injectors ④ and ⑤.
• End of Filling (Time B): Ignition at the end of filling results in a highly non-uniform mixture with concentration gradients. This leads to flame separation and significantly higher overpressure risks compared to stable filling. The highest pressure peak reached $P_r = 1.48$ at $t = 194.98$ ms in the injector pipeline of injector ①.
• Half-Filling (Time C): This condition presents the most unstable flow field, with pure air in the second half of the chamber. Ignition fails to achieve flame connection, and the flame tends to blow out. The risk of overpressure is highest here, with a peak pressure of $P_r = 1.54$ at $t = 207.47$ ms. The study concludes that ignition at the half-filling time is unsafe and unreliable.

2. Starting Loads and Injection Strategies

• Full Working Condition: Exhibits moderate overpressure risks with frequent backfire.
• 1/2 Working Condition: Reducing the number of active injectors (lower half only) decreases average temperature and reaction heat but paradoxically increases the number and amplitude of overpressures due to flow field instability.
• 2/3 Working Condition: Using the middle four injectors with a symmetric ignition position (Ignition Position 2) significantly reduces risks. The average pressure pulsation amplitude drops to 0.61 kPa, and only one minor overpressure event ($P_r = 1.072$) occurs, which is attributed to a specific geometric stress concentration near injector ④.

3. Optimal Ignition Scheme

The study identifies a specific sequential ignition strategy as the optimal solution:

1. Open only the two middle hydrogen injectors (③ and ④) to establish a stable, symmetric flame.
2. After 50 ms, open the adjacent injectors (② and ⑤) to expand the combustion.
   Results: This symmetric, staged approach eliminates backfire phenomena entirely during the detection period. No overpressure events ($P_r \geq 1.05$) were observed. The reaction heat and pressure fluctuations remain smooth, and the flame propagates stably without the violent oscillations seen in asymmetric or non-sequential strategies.

Significance and Claims

The paper claims to provide the first theoretical support for the safe and reliable ignition of China's aviation hydrogen fuel micro-mixed combustion chambers, addressing a previously unexplored research gap.

• Mechanism of Overpressure: The study clarifies that overpressure is directly correlated with backfire. It occurs when local high temperatures from backfire encounter edges, corners, injector steps, and walls, causing pressure accumulation.
• Role of Homogeneity: The research demonstrates that mixture inhomogeneity, inappropriate ignition timing, and asymmetric injection conditions are primary drivers for increased overpressure frequency and amplitude.
• Practical Recommendation: The authors conclude that reducing the number of hydrogen injection ports under symmetric conditions is an effective method to mitigate overpressure. Specifically, the "two-middle-first, then expand" ignition scheme is proposed as the optimal strategy to minimize combustion instability and explosion risks.

The study emphasizes that while the 2/3 load condition with symmetric injection reduces risk, the sequential ignition of two middle groups followed by expansion offers the most stable process with the lowest overpressure risk, effectively preventing the conditions that lead to hydrogen explosions in confined micro-mixed chambers.