Nonpremixed Flamelet Statistics at Flame Base of Lifted Turbulent Jet Nonpremixed Flames

摘要

Nonpremixed flamelet statistics at the flame base of lifted turbulent nonpremixed flames are investigated experimentally using a planar temperature Rayleigh scattering method. A methane/hydrogen mixture is supplied from a tube of 3.2mm I.D. into the surrounding air flow so as to form two lifted turbulent nonpremixed flames having exit velocities of 50 m/s (Re - 4 200) and 80 m/s (Re - 6 700), respectively. Temperature data are related to maximum temperature at several flame positions based on the instantaneous flame base tip as containing information on the reaction region of each nonpremixed flamelet at each position. The statistics in terms of maximum temperature, thermal dissipation rate, scalar dissipation rate, and flame brush are discussed with reference to the modeling of the flame base structure. The scalar dissipation rate has log-normal statistics and the quenching scalar dissipation rate is lower than the critical value predicted using the uniformly strained counter nonpremixed flame. These statistics are compared to those obtained using a model proposed by Muller et al., which combines the flamelet model and the scalar field variable to predict lifted turbulent nonpremixed flames. The comparison has shown that the model can qualitatively predict lifted turbulent nonpremixed flames, but modification is required in order to obtain more accurate quantitative prediction taking into account the edge flame structure, the statistics of scalar dissipation rate, and the statistics of flame brush.