On the Flame Stability and Laminar Burning Speeds of Syngas/O2/He Premixed Flame

Askari, O., Wang, Z., Vien, K., Sirio, M., & Metghalchi, H. (2017). On the Flame Stability and Laminar Burning Speeds of Syngas/O2/He Premixed Flame. Journal of Fuel. 190, 90-103.

Fundamental properties such as flame structure and laminar burning speed of syngas/O2/He premixed flames have been investigated. Synthetic gas, also known as syngas, is a mixture of hydrogen and carbon monoxide that has been used in the present study. Three different mole fractions of 5%, 10% and 25% of hydrogen in the syngas were used in this research. Experiments were done in a cylindrical and a spherical chambers. Flame structure studies were made in the cylindrical vessel. The cylindrical vessel was coupled with a Z-shape schlieren system, equipped with a high speed CMOS camera, which has the capability of capturing pictures up to 40,000 frames per second, in order to study flame structure and instability. Hydrodynamic and thermo-diffusive instabilities on the flame front during propagation of flames have been observed and studied. Helium’s smaller molar heat capacity and higher thermal conductivity than nitrogen results in a more stable and smooth flames. The increase range of smooth flames can be used to validate chemical kinetics mechanisms at higher pressures and temperatures. Pressure rise data as a function of time during the combustion process was obtained through a pressure transducer in the spherical vessel was the primary input of the multi-shell model used to calculate the laminar burning speed for the smooth flames. Power law correlations over a wide range of pressures (from sub-atmospheric up to 7.3 atm), temperatures (298 K up to 650 K), and equivalence ratios (0.6–3.0) have been developed for laminar burning speeds of smooth H2/CO/O2/He flames. At equivalence ratios corresponding to maximum laminar burning speeds, the experimental laminar burring speeds show a positive relationship with pressure. Experimental laminar burning speeds of H2/CO/O2/He mixtures have been compared with numerical values calculated by free flat flame simulation using two chemical kinetics mechanisms and some discrepancies discovered for equivalence ratios greater than 2. The effect of helium as a diluent on flame morphology and laminar burning speeds of syngas are compared with two diluents, nitrogen as well as nitrogen that is further diluted with synthetic exhaust gas recirculation (SEGR).