Combustion in meso-scale vortex combustors: Experimental characterization

The combustion characteristics of a series of mesoscale vortex combustors were experimentally studied. The combustors had combustion chamber volumes ranging from 10 to 108 mm³ and were fueled with hydrogen, methane, and liquid nitromethane. Both air and oxygen-enriched air were used as oxidizers. Analysis of the flame structure was obtained from photographic imaging. Stability limits of the combustor were investigated with temperature and OH* chemiluminescence measurements and/or from visual observation of the flame luminosity. Fourier transform infrared (FTIR) spectroscopy of the exhaust gases was performed for quantitative analysis of CO₂, CO, and CH₄ from which chemical efficiencies of the combustion process were obtained. For all combustion chamber sizes, hydrogen-air combustion was stabilized over a wide range of equivalence ratios and flow rates. Stable methane-air combustion was achieved at atmospheric pressure with only the largest combustion chamber; however, when the air was enriched with oxygen, combustion was stabilized in even the smallest combustor chamber. Depending upon the oxygen enrichment, equivalence ratio, and momentum of the inlet reactant jets, the flame structure was observed to change from a micro diffusion flame stabilized at the fuel jet exit to a spinning well mixed flame that engulfed the entire combustor volume. Under conditions where stable operation was achieved, chemical efficiencies ranged from approximately 85-97%. Direct wall injection of liquid nitromethane produced stable combustion at one atmosphere when a small amount of oxygen (6% by volume) was simultaneously injected. Increased combustion chamber pressures are predicted necessary to extend the operational limits and enable combustion of pure liquid nitromethane.