Near-Field vs Far-Field Stratification Dynamics in Multi-Jet Hydrogen Combustors

Aviation emissions and the hydrogen combustion challenge

Aviation accounts for a growing share of anthropogenic greenhouse gas emissions, and the sector faces mounting pressure to decarbonise without sacrificing the energy density that makes long-haul flight viable. Hydrogen has emerged as a leading candidate fuel: it burns without CO₂, offers a high gravimetric heating value, and can be produced from renewable electricity. Yet the transition from kerosene to hydrogen is not a simple drop-in replacement. Hydrogen's wide flammability limits and high diffusivity produce fundamentally different combustion physics — most critically, a tendency toward mixing-limited rather than kinetically limited combustion in practical combustor geometries.

In a conventional kerosene annular combustor, fuel is injected as a liquid spray that vaporises and mixes with air over a characteristic length scale. Hydrogen, by contrast, enters as a gas with diffusivity roughly six times that of methane. In multi-jet configurations — where discrete fuel injectors are distributed around an annular flame tube — this high diffusivity can produce steep spatial gradients in equivalence ratio before the flame front stabilises. The result is stratification: spatially non-uniform combustion intensity that varies with injector spacing, momentum flux ratio, and downstream distance. For aviation applications, stratification matters because it governs NOₓ formation pathways, combustor stability margins, and the spatial distribution of hot spots that drive thermal loading on turbine components.

Stratification quantification methodology

Quantifying stratification requires a field variable that tracks the spatial non-uniformity of the combustion process without conflating it with purely thermal effects. We introduced H₂O mass fraction as a stratification proxy in Ansys Fluent simulations of an annular multi-jet hydrogen combustor. Water is a direct product of hydrogen oxidation (2H₂ + O₂ → 2H₂O), and its mass fraction field reflects the cumulative history of mixing and reaction along streamlines. Regions of locally elevated H₂O indicate zones where reactants have had sufficient residence time and mixing quality to proceed to near-complete conversion; depressed H₂O zones signal incomplete mixing or locally lean/rich pockets.

The combustor geometry was modelled as an annular flame tube with circumferentially distributed fuel jets and a central pilot zone. Boundary conditions were set to represent cruise-relevant pressure and inlet temperature, with hydrogen injected at multiple azimuthal positions to capture jet–jet interaction. The Reynolds-Averaged Navier–Stokes (RANS) framework with a finite-rate/eddy-dissipation combustion model was used, with mesh refinement concentrated at jet shear layers and the recirculation zone aft of the dome swirler. Steady-state solutions were obtained for multiple injector configurations, and field data were exported for automated post-processing using a Python pipeline that computed spatial statistics of the H₂O distribution.

Two spatial domains were defined to separate near-field and far-field behaviour. The near-field domain extends from the injector plane to approximately one combustor diameter downstream, capturing jet penetration, initial entrainment, and the formation of recirculation zones. The far-field domain spans from one diameter to the combustor exit, where bulk mixing homogenises the flow and the H₂O field approaches a more uniform distribution. Stratification was quantified in each domain using the coefficient of variation (standard deviation divided by mean) of H₂O mass fraction on azimuthally distributed sampling planes.

Near-field vs far-field dynamics

Near-field simulations reveal the strongest stratification signatures. Individual hydrogen jets maintain coherent structure for several jet diameters downstream, producing azimuthal variation in H₂O mass fraction that correlates directly with injector spacing. Jets with higher momentum penetrate deeper before dispersing, creating elongated high-H₂O streaks along their axes, while lower-momentum jets produce broader but shallower reaction zones. Jet–jet interaction in the annular arrangement generates saddle points in the H₂O field where recirculating flow from adjacent jets mixes product gases before fresh reactants arrive — these interaction zones are the primary locus of near-field non-uniformity.

Transition to the far field is marked by a rapid decay in the coefficient of variation. Turbulent diffusion and the annular recirculation pattern redistribute H₂O across azimuthal positions, and by the combustor exit plane the field is substantially more homogeneous. However, the far-field distribution is not uniform: residual azimuthal modes persist at reduced amplitude, indicating that near-field injector layout imprints a fingerprint on exit-plane composition that cannot be fully erased by downstream mixing alone. This has direct implications for turbine inlet temperature profiles and for emissions indexing, where exit-plane uniformity is often assumed in reduced-order models.

Key findings

Three principal findings emerge from this study. First, H₂O mass fraction is an effective and physically grounded proxy for combustion stratification in hydrogen systems — it captures both the spatial extent of reaction and the degree of local mixing completion without requiring explicit flame-front tracking. Second, near-field stratification is dominated by injector momentum and spacing, with the highest non-uniformity occurring within one combustor diameter of the fuel injection plane; far-field decay follows a consistent scaling with turbulent Peclet number across the configurations tested. Third, even at the combustor exit, residual azimuthal non-uniformity of 8–12% (coefficient of variation) persists in the most aggressively stratified configurations, suggesting that combustor design criteria developed for kerosene — which typically target exit-plane uniformity below 5% — may need revision for hydrogen multi-jet architectures.

These results inform ongoing work on hydrogen aviation combustor design at IIT Roorkee and provide a quantitative framework for comparing injector configurations before committing to expensive experimental campaigns. Future work will extend the analysis to transient ignition sequences and couple the combustor exit profiles to downstream turbine and exhaust simulations.