Hydrogen blending effects on soot formation in bluff-body stabilized propane swirling flames

The study evaluates three hydrogen blending strategies (Substitution, Addition, and Equal-power) with blending ratios up to 60% in a bluff-body stabilized non-premixed swirling flame and examines their effects on soot emissions. High-speed simultaneous measurements of the flow field, reaction zone, and soot distribution were performed using particle image velocimetry, OH*-chemiluminescence and Rayleigh scattering. The results show that the mean flow field remains largely unchanged for three strategies, whereas the reaction zone shifts downstream by ∼18% as the blending ratio increases to 50%. The soot load monotonically increases with hydrogen addition, whereas it first rises and then declines under the substitution strategy, reaching a maximum at a 40% blending ratio. The probability of soot presence follows a similar trend. Whether fuel power increases or remains constant, hydrogen blending increases soot load, likely due to a local temperature rise. Soot residence time isopleths reveal three distinct transport paths: a loop in the recirculation zone, deposition onto the bluff body, and soot leakage, which mainly occurs near the bluff body during flame liftoff, as captured by high-speed simultaneous measurements. Soot leakage occurs heavily at a 20% hydrogen blending ratio for the substitution strategy, and an increasing trend is observed for the other two strategies, which is governed by the combined effects of soot load and flow-field-controlled leakage probability. We make a unique contribution by proposing the mechanism behind it, which is essential for decarbonizing existing combustors that will continue to rely on hydrocarbon-based fuels for the foreseeable future.