Large-eddy simulation of hydrogen ignition and NOx formation in diesel-pilot dual-fuel combustion

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This study examines the dynamics of diesel/hydrogen dual-fuel combustion in a constant-volume vessel under compression ignition engine conditions, where a small pilot n-heptane (a surrogate of diesel) injection precedes a long-duration hydrogen main injection containing 93.6% of the energy share. We investigate how pilot n-heptane flame and hydrogen injection affect emissions and combustion by systematically varying the dwell time ( Δ t between the end of n-heptane injection and the start of hydrogen injection) while keeping the injection rate profile fixed. Large eddy simulations with detailed chemistry are performed at vessel conditions of 890 K, 52 bar, and 21 vol.% O 2 , and compared against Schlieren and apparent heat-release rate derived from measurements. The simulations show a good agreement with experimental results regarding ignition onset and flame propagation. The pilot n-heptane ignites rapidly at the spray tip, ∼ 0.5 ms after injection; thereafter, the flame propagates upstream toward the injector nozzle. The hydrogen jet ignites nearly immediately upon its intersection with the n-heptane flame. Varying the dwell time between the end of n-heptane injection and the start of hydrogen injection significantly affects hydrogen ignition timing, peak heat release rates, and NO x emissions. Notably, while longer dwell times lead to delayed hydrogen ignition and lower NO x formation, medium dwell times result in higher NO x emissions, showing a complex non-linear relationship between hydrogen-diesel combustion and NO x emissions.

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