Properties of inverse nonpremixed pure O2/CH4 coflow flames in a model combustor

In order to evaluate the potential of oxy-combustion of inverse nonpremixed coflow flames using a clean fuel to improve the combustion and emission characteristics compared with the normal coflow flames using air, the fundamental properties of inverse nonpremixed pure oxygen (O-2)/methane (CH4) coflow flames in a model combustor are experimentally investigated. The combustion stability (extinction) limits, structure and nitrogen oxide (NOx) emissions of the inverse coflow flames in the quartz-windowed combustion chamber with a single shear coaxial injector and a fully opened exhaust nozzle are measured using OH* chemiluminescence, a schlieren imaging system and a gas analyzer. Results show four distinct stability regimes and limits: the attached flame regime, the near-blowout flame regime, blowout limits and blowoff limits. The direct flame and OH* chemiluminescence images confirm the cooling effects of chamber walls that cause no steady liftoff flame and only the near-blowout flame. The extinction limits of the inverse CH4/O-2 coflow flames are extended compared with the normal CH4/air coflow flames and reasonable levels of NOx emissions are observed, which supports that oxycombustion of the inverse nonpremixed coflow flames using the clean fuel is acceptable for practical application, providing a useful database for modeling the flames in the confined chamber. (C) 2015 Elsevier Ltd. All rights reserved.