Characterisation of hydrogen jet flames under different pressures with varying coflow oxygen concentrations

Abstract

The elevated temperature of hydrogen combustion increases the formation of thermal NO×. Moderate or intense low oxygen dilution (MILD) combustion is known to reduce NO× emissions and increase thermal efficiency. Pressure is often also used for increasing thermal efficiency. The impact that pressure has on fluid dynamics and chemical kinetics is especially relevant in MILD combustion conditions. Hydrogen jet flames issuing into a hot and vitiated coflow were imaged using OH∗ chemiluminescence at different pressures (1–7 bar) and oxygen levels (3–9% by vol.). Laminar flame simulations complemented the experiments. The observed mean radial OH∗ width increased with increased pressure, but only at O₂ content less than 9%, suggesting that pressure has greater influence on kinetics when oxygen is reduced. The integrated OH∗ signal strength remained constant at 3% coflow O₂, despite an apparent increase in flame width, suggesting a spatial broadening of the flame with pressure. Numerical results indicate that at 3–6% O₂, conditions for MILD combustion of H₂ are met across a wide range of strains and pressures, supporting the experimental observations for 3% O₂.