An experimental and numerical study of the effects of heat loss and unsteadiness on laminar strained flames

A combined experimental and detailed numerical study was conducted on the effects of heat loss and unsteadiness on strained laminar flames at normal- and microgravity. Results are of interest to a variety of fundamental combustion phenomena including flammability limits. Furthermore, valuable information is provided in the context of turbulent combustion for conditions under which the flamelet concept is applicable. The majority of previous studies on flamelets have been focused on steady and adiabatic conditions, even though unsteadiness and heat loss are inherently present in any realistic flowfield. The experiments included the use of the opposed-jet and single-jet configurations in which the strain rate is a well-defined and well-controlled parameter. Velocity measurements were conducted through the use of laser Doppler velocimetry at normal-gravity and extinction strain rates. The counterflow technique was also introduced in micro-gravity through an involved experimental apparatus that allowed for the study of extinction of near-limit flames under conditions that could not be assessed in normal-gravity. The C-shape response of the extinction strain rate vs equivalence ratio was quantified for Le < 1 flames by assuring that upstream heat losses were not present. For Le > 1 flames, a monotonic response was found. Experiments were also conducted at normal-gravity on the effect of downstream heat loss on the propagation and extinction of laminar strained premixed flames. The effect of monochromatic velocity unsteadiness was experimentally studied for non-premixed strained flames and theoretically derived scaling arguments were confirmed. Furthermore, the flames were found to resist to extinction at high frequencies, confirming again theoretical predictions. The experiments were modeled by using detailed description of chemical kinetics, molecular transport, and thermal radiation. The effect of various radiation models on the flame response was assessed. Such models included the assumptions of optically thin and optically thick limits, as well as the mean Planck mean absorption coefficient and detailed narrow-band formulations.