Oxygen effects on soot formation in H2/n-heptane counterflow flames

Blending of hydrocarbon fuels with hydrogen is a practically feasible way to rapidly integrate a carbon neutral energy vector in the transport sector and to effectively reduce harmful particulate matter emis-sions. This work investigates the effects of hydrogen (H 2 ) addition on soot formation in n-heptane in low strain rate counterflow diffusion flames with different oxygen mole fractions (XO2= 0.3 similar to 0.45). Flame temperature, soot volume fraction (fv), monocyclic (MAHs), and polycyclic aromatic hydrocarbons (PAHs) are measured by using OH-2C-PLIF/thermocouple, LII/LE, and PAH-LIF methods, respectively. Measure-ments of n-heptane flames with addition of helium (He), as an inert gas with a specific heat capacity similar to that of H 2, are also carried out and compared. Numerical simulations are then performed using a soot discrete sectional model coupled with detailed gas-phase kinetics to interpret the data obtained. Temperature and soot volume fraction profiles are better predicted by 2D simulations, which account for the non-negligible impact of radial diffusion and buoyancy under low strain rate conditions. Then, it is shown that both the fv and PAH yields almost linearly decrease by increasing H 2 or helium (He) addition. The reduction of PAHs and soot is larger in n-heptane/H2 flames due to H 2 chemical effects, absent in the n-heptane/He flames, hampering the formation of peri-condensed structures. The effect of O 2 concentra-tion in the oxidizer stream is also studied. Experimental and simulation results show that PAHs and soot increase with XO2 for all flames. Finally, the effects of H2/He addition varying XO2 on soot formation are investigated. With the increase of XO2, the difference in the peak flame temperatures of n-heptane/H2 and n-heptane/He flames decreases. Consequently, compared to the neat n-heptane flame, the reduction of peak fv becomes more and more pronounced in the case of hydrogen addition rather than the he-lium addition since the H 2 soot inhibiting chemical effect prevails over the H 2 temperature effect which instead promotes soot formation.(c) 2023 The Combustion Institute. Published by Elsevier Inc. All rights reserved.