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

Abstract

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 emissions. This work investigates the effects of hydrogen (H2) addition on soot formation in n-heptane in low strain rate counterflow diffusion flames with different oxygen mole fractions (XO2=0.3 ∼ 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. Measurements of n-heptane flames with addition of helium (He), as an inert gas with a specific heat capacity similar to that of H2, 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 H2 or helium (He) addition. The reduction of PAHs and soot is larger in n-heptane/H2 flames due to H2 chemical effects, absent in the n-heptane/He flames, hampering the formation of peri‑condensed structures. The effect of O2 concentration 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 helium addition since the H2 soot inhibiting chemical effect prevails over the H2 temperature effect which instead promotes soot formation.