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

• Published in: Combustion and flame Vol. 143; no. 1-2; pp. 106 - 118
• Main Authors: SHY, S, YANG, S, LIN, W, SU, R
• Format: Journal Article
• Language: English
• Published: United States 01.10.2005
• Subjects: 03 NATURAL GAS; COMBUSTION; FLAMES; METHANE; TURBULENCE; VELOCITY
• ISSN: 0010-2180; 1556-2921
• DOI: 10.1016/j.combustflame.2005.05.007

This paper presents turbulent burning velocities, S{sub T}, of several premixed CH{sub 4}/diluent/air flames at the same laminar burning velocity S{sub L}=0.1 m/s for two equivalence ratios f=0.7 and 1.4 near flammability limits with consideration of radiation heat losses from small (N{sub 2} diluted) to large (CO{sub 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{sub 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{sub T}. These measurements prove the accuracy of the S{sub T} data. At f=0.7, the percentage of [(S{sub T}/S{sub L}){sub CO{sub 2}}-(S{sub T}/S{sub L}){sub N{sub 2}}]/(S{sub T}/S{sub L}){sub N{sub 2}} decreases gradually from -4 to -17% when values of u{sup '}/S{sub L} increase from 4 to 46, while at f=1.4 such decrease is much more abrupt from -19 to -53% when values of u{sup '}/S{sub L} only increase from 4 to 18. The larger the radiation losses, the smaller the values of S{sub T}. This decreasing effect is augmented by increasing u{sup '}/S{sub L} and is particularly pronounced for rich CH{sub 4} flames. When u{sup '}/S{sub L}=18, lean CO{sub 2} and/or N{sub 2}-diluted CH{sub 4} flames have much higher, 3.6 and/or 1.8 times higher, values of S{sub T}/S{sub L} than rich CO{sub 2} and/or N{sub 2}-diluted CH{sub 4} flames, respectively. It is found that lean (f=0.7) CH{sub 4} flames are very difficult to quench globally even when methane mixtures are diluted with 41% CO{sub 2} having large radiation losses. Moreover, measurements of chemiluminescence intensities for these turbulent propagating CH{sub 4}/diluent/air flames using the photomultiplier are carried out. The peak light intensities of CH{sup *} and C{sup *}{sub 2} emitters are found to be qualitatively correlated with these aforementioned S{sub T} data of lean and rich turbulent CH{sub 4}/diluent/air propagating flames, respectively. Finally, the scatter plot of S{sub T}/S{sub L} as a function of u{sup '}/S{sub L} for the aforementioned N{sub 2}- and CO{sub 2}-diluted CH{sub 4} flames can be well compressed and approximated by a general correlation of turbulent burning velocities in a form of (S{sub T}-S{sub L})/u{sup '}=0.06Da{sup 0.58}, where Da is the turbulent Damkohler number.