Experimental investigation of equivalence ratio and reynolds number dependence on oscillatory behaviors in mesoscale circular tubes

An experimental investigation into the dependence of oscillatory modes on equivalence ratio and flow speed for fuel-rich mesoscale flames was conducted. The study was performed in the context of micro-combustion for autonomous power sources that will have the size of commercially available batteries, but significantly higher power density, because they will employ the high power density of hydrocarbon combustion. 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 (steady or oscillating mode) 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 boundary equivalence ratio values decreasing with increasing Reynolds number. The effect of tube length on oscillatory behavior and stability was also examined by repeating all experiments in 35mm, 70mm, and 210mm long tubes. Increasing the tube length decreased the stability of oscillations for both fuels, although the effect was less for propane.