Operational regimes of rich methane and propane/oxygen flames in mesoscale non-adiabatic ducts

An experimental investigation was conducted of the oscillatory flames observed in narrow circular ducts in order to determine the mode of oscillation as a function of equivalence ratio and flow speed for fuel-rich methane-oxygen and propane-oxygen mixtures. In order to study flame stabilization in narrow ducts of diameter on the order of the quenching diameter, premixed fuel and oxygen were burned in an optically accessible straight quartz tube with an inner diameter of a few mm, one end of which was open to the atmosphere. Methane-oxygen and propane-oxygen flames were examined and compared, and various oscillating and steady flame formations were observed. It was observed that for methane the boundaries between behavioral regimes were dependent almost exclusively on equivalence ratio, with the boundary equivalence ratio values remaining nearly constant for different Reynolds numbers; for propane the boundaries were dependent on both equivalence ratio and Reynolds number, with the limit equivalence ratio values decreasing with increasing Reynolds number. The effect of tube length on oscillatory behavior and stability was also examined by experimenting in 35, 70, and 210 mm long tubes. Increasing the tube length decreased the stability of oscillations for both fuels, although the effect was less intense for propane. It was also observed that oscillations were only established in mesoscale ducts made of low conductivity materials, a result that was rationalized in terms of the pertaining Biot and Fourier numbers.