Localized Combustion Phenomena of Inverse Nonpremixed Pure O2/CH4 Coaxial Jet Flames at Near-Limit Conditions

Abstract

ABSTRACT The stability of inverse nonpremixed oxygen (O2)/methane (CH4) coaxial jet flames near blowout limits in a model combustor is studied by analyzing signals for their local combustion phenomena using high-speed imaging and dynamic pressure. The power spectral analysis and the statistical analysis are conducted to delineate the stability of the inverse nonpremixed flames. Depending on the O2 to CH4 momentum flux ratio ((O/F)mom), two distinguished combustion phenomena which lead to different liftoff mechanisms are observed in the same burner configuration: the partial liftoff related to the aerodynamic mechanism and the local extinction related to the flame–turbulence interaction. The results of the power spectral density (PSD) show that on approaching the near-blowout limits, the local extinction or the partial liftoff occurs not in a specific frequency, but in a time-varying random distribution. The normalized root mean square (NRMS) based on the frame-integrated visible light signal (S VL) is found to be effective for quantifying the local extinction and the partial liftoff and particularly delineating the stability of the present inverse nonpremixed flames with the partial liftoff.