Turbulent burning velocities of premixed CH_4/diluent/air flames in intense isotropic turbulence with consideration of radiation losses

摘要

This paper presents turbulent burning velocities, S_T, of several premixed CH_4/diluent/air flames at the same laminar burning velocity S_L = 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 (N_2 diluted) to large (CO_2 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 S_T 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 S_T. These measurements prove the accuracy of the S_T data. At φ = 0.7, the percentage of [(S_T/S_L)_(CO_2) - (S_T/S_L)_(N_2)]/(S_T/S_L)_(N_2) decreases gradually from —4 to —17% when values of u'/S_L increase from 4 to 46, while at φ = 1.4 such decrease is much more abrupt from —19 to —53% when values of u'/S_L only increase from 4 to 18. The larger the radiation losses, the smaller the values of S_T. This decreasing effect is augmented by increasing u'/S_L and is particularly pronounced for rich CH_4 flames. When u'/S_L = 18, lean CO_2 and/or N_2-diluted CH_4 flames have much higher, 3.6 and/or 1.8 times higher, values of S_T/S_L than rich CO_2 and/or N_2-diluted CH_4 flames, respectively. It is found that lean (φ = 0.7) CH_4 flames are very difficult to quench globally even when methane mixtures are diluted with 41% CO_2 having large radiation losses. Moreover, measurements of chemiluminescence intensities for these turbulent propagating CH_4/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 S_T data of lean and rich turbulent CH_4/diluent/air propagating flames, respectively. Finally, the scatter plot of S_T/S_L as a function of u'/S_L for tne aforementioned N_2- and CO_2-diluted CH_4 flames can be well compressed and approximated by a general correlation of turbulent burning velocities in a form of (S_T — S_L)/u' = 0.06 Da~(0.58), where Da is the turbulent Damkoehler number.