THE STRUCTURE OF TURBULENT NONPREMIXED FLAMES OF METHANOL OVER A RANGE OF MIXING RATES

The blowoff and electrical connectedness characteristics of pilot-stabilized turbulent nonpremixed flames of methanol fuel are presented. Using the joint Raman-Rayleigh-LIF technique, simultaneous space- and time-resolved measurements of temperature and the mass fractions of CH3OH, CO, CO2, H2, H2O, O2, and N2 are also shown for a number of measurement locations. Flames with slow mixing rates as well as ones close to blowoff are investigated. The results are presented in the form of scatter plots as well as probability density functions conditioned with respect to mixture fraction for a number of mixture fraction ranges selected within the reactive zone for methanol. The local extinction characteristics in the flames are also studied by adopting a temperature threshold below which fluid samples are assumed to be extinguished. It is found that the flame becomes electrically disconnected completely while it remains visibly and aurally stable for a range of velocities just below blowoff. Measurements in regions of the flames where the mixing rates are relatively low show little difference between the turbulent flame data and the laminar flame compositions only on the lean side of stoichiometric. For rich mixtures, however, the peak-measured mass fractions of CO and H2 are about 70% higher than those for laminar flamelets while the mass fractions of CO2 and H2O are lower than those of highly stretched laminar flamelets. Localized extinction remains uniformly low until the flame jet velocity is about 80% of the blowoff velocity. With higher flame jet velocities, localized extinction increases sharply until global blowoff is reached. The flame's approach to global blowoff is mainly bimodal for lean and stoichiometric mixtures but less bimodal for rich ones. This bimodality, however, is less distinct than in methane flames.