Topology-Dependent Heat Release in Hydrogen/Ammonia Stratified Low-Pressure Direct Injection Flames

Abstract Early heat release in hydrogen/ammonia combustion under low-pressure direct injection is controlled by the coupling between local flow topology and finite-rate chemistry. In this work, post-ignition flame development in a constant-volume bomb was investigated using three-dimensional large eddy simulation (LES) with detailed chemistry and validation based on Schlieren imaging. The first 0–3 ms after ignition was analyzed using a temperature-defined main reaction zone (MRZ, 800–1800 K) and a normalized Q* criterion that separates the MRZ into regions dominated by strain, weak rotation, and strong rotation. The strain-dominated region is the principal carrier of early heat release, accounting for 58–88% of the MRZ volume and 75–94% of the total heat release. Its relative heat release intensity remains at or above unity, ranging from 1.0 to 1.4, and its regional heat release rate reaches approximately 6.0 × 104 W near 1.0 ms. Damköhler numbers remain below unity in all topology classes, whereas Karlovitz numbers increase during flame development, especially in rotation-dominated regions. These results show that the strongest small-scale disturbance is not the primary heat release carrier. Instead, early exothermicity is preferentially sustained in the strain-dominated shear mixing layer. The findings provide a topology-resolved basis for understanding ignition and early heat release in hydrogen/ammonia stratified low-pressure direct injection flames.