Measurement of laminar flame speeds through digital particle image velocimetry: Mixtures of methane and ethane with hydrogen, oxygen, nitrogen, and helium

A digital particle image velocimetry technique that is appropriate for the experimental derivation of fundamental flame properties was implemented. The technique allows for the determination of the instantaneous flowfield and is essential for fluid mechanics measurements in reduced gravity environments. Measurements of laminar flame speeds were conducted in the stagnation flow configuration just before a flame undergoes a transition from planar to Bunsen flame. Results obtained for lean CH4/air and C2H6/air flames were found to be in close agreement with previous laser Doppler velocimetry data. Subsequently, measurements were conducted for the CH4 and C2H6 flames by independently varying the equivalence ratio and flame temperature to distinguish between temperature and concentration effects. The laminar flame speeds were also calculated using the GRI-Mech 3.0 mechanism. It was convincingly shown that under high-O-2 and low-temperature conditions, the experimental laminar speeds are overpredicted by the simulations especially for C2H6 flames. Additional experiments were conducted by adding H-2 to lean C2H6/air flames and by diluting those mixtures by either He or N-2 to vary the flame temperature. While for the He dilution case, the predictions noticeably overpredict the experiments, for N-2 dilution, closer agreement was observed. Analyses of the flame structures revealed that for those fuel-lean flames, the burning rate largely depends on the competition of the two-body branching and three-body termination reaction between H and O-2. It was not possible to point to possible kinetic deficiencies other than referring to uncertainties associated with the rates and collision efficiencies of three-body reactions. The high-O-2 low-temperature region is of interest not only to lean-premixed combustion, but also to flame ignition, and requires further exploration.