Turbulent burning velocities of premixed CH₄/diluent/air flames in intense isotropic turbulence with consideration of radiation losses

This paper presents turbulent burning velocities, ST, of several premixed CH₄/diluent/air flames at the same laminar burning velocity SL=0.1 m/s for two equivalence ratios φ=0.7 and 1.4 near flammability limits with consideration of radiation heat losses from small (N₂ diluted) to large (CO₂ diluted). Experiments are carried out in a cruciform burner, in which the long vertical vessel is used to provide a downward propagating premixed flame and the large horizontal vessel equipped with a pair of counterrotating fans and perforated plates can be used to generate an intense isotropic turbulence in the central region between the two perforated plates. Turbulent flame speeds are measured by four different arrangements of pairs of ion-probe sensors at different positions from the top to the bottom of the central region in the burner. It is found that the effect of gas velocity on ST measured in the central region can be neglected. Simultaneous measurements using the pressure transducer and ion-probe sensors show that the pressure rise due to turbulent burning has little influence on ST. These measurements prove the accuracy of the ST data. At φ=0.7, the percentage of [(ST/SL)CO2-(ST/SL)N2]/ (ST/SL)N2 decreases gradually from -4 to -17% when values of u′/SL increase from 4 to 46, while at φ=1.4 such decrease is much more abrupt from -19 to -53% when values of u′/SL only increase from 4 to 18. The larger the radiation losses, the smaller the values of ST. This decreasing effect is augmented by increasing u′/SL and is particularly pronounced for rich CH₄ flames. When u′/SL=18, lean CO₂ and/or N ₂-diluted CH₄ flames have much higher, 3.6 and/or 1.8 times higher, values of ST/SL than rich CO₂ and/or N ₂-diluted CH₄ flames, respectively. It is found that lean (φ=0.7) CH₄ flames are very difficult to quench globally even when methane mixtures are diluted with 41% CO₂ having large radiation losses. Moreover, measurements of chemiluminescence intensities for these turbulent propagating CH₄/diluent/air flames using the photomultiplier are carried out. The peak light intensities of CH* and C*₂ emitters are found to be qualitatively correlated with these aforementioned ST data of lean and rich turbulent CH₄/diluent/ air propagating flames, respectively. Finally, the scatter plot of ST/SL as a function of u′/SL for the aforementioned N₂- and CO ₂-diluted CH₄ flames can be well compressed and approximated by a general correlation of turbulent burning velocities in a form of (ST-SL)/u′=0.06Da0.58, where Da is the turbulent Damköhler number.