A study of the effects of diluents on near-limit H₂-air flames in microgravity at normal and reduced pressures

A combination of microgravity experiments and computational simulations were used to study effects of diluents on the near-limit properties of laminar, premixed hydrogen/air flames. The experiments were conducted in a short-drop free-fall laboratory facility that provided at least 450 ms of 10^-2 g conditions. Outwardly propagating spherical flames were used to measure near-limit laminar burning velocities at various fuel-equivalence ratios and pressures with reactants containing varying concentrations of He, Ar, N₂, and CO₂ as fire suppressants. Burning velocities were also computed using the steady, one-dimensional laminar premixed flame code PREMIX with detailed chemical kinetics, transport properties, and radiative heat loss based on an optically thin assumption. Measured and computed results both showed the suppressant effectiveness to increase in the order He, Ar, N₂, and CO₂. This is attributed to both the increasing specific heats and the decreasing transport rates of the gases. The suppressants can also decrease the Markstein number, especially for CO₂, causing the flames to become more susceptible to preferential-diffusion instability. The resulting increase in flame surface wrinkling increases the burning velocity, thus counteracting the desired effect of the flame suppressant. The agreement between measured and computed laminar burning velocities was better than it was near the limit. Sensitivity analyses suggest that inaccuracies in three-body termination rates for H + O₂ + M = HO₂ + M reactions and in mass diffusion coefficients for H₂ diffusion are the most likely explanation for the near-limit differences.