Experimental study on flame propagations and cellular instabilities in syngas-air premixed flames

Abstract

Experiments were conducted in a closed combustion chamber to determine the unstretched laminar burning velocities and the Markstein lengths of various syngas-air mixtures; to investigate the effects of hydrogen enrichment, the hydrocarbon such as methane-propane-butane additions, the carbon dioxide-nitrogen-helium dilutions on cellular instabilities of the syngas-air premixed flames at room temperature and elevated pressures; to define some important parameters such as critical radius, critical Peclet number, critical initial pressure to elucidate the cellularity of these mixtures; to examine some explosion characteristics such as the explosion pressure, the combustion duration, the maximum rate of pressure rise, the deflagration index of various syngas-air mixtures, hydrocarbon-added and diluted syngas-air flames. In this study, the propagating spherical flame was imaged using schlieren photography and recorded using a high-speed digital camera. Experimental results indicate that the 50H2:50CO syngas-air premixed flame has the maximum unstretched laminar burning velocity of 184.4 cm/s at equivalence ratio of 2.0. Increasing contents of H2 and CO concentrations will respectively increase and decrease the laminar burning velocities of the flames. With adding hydrocarbons and diluents, the laminar burning velocities of the syngas-air premixed flames decrease and the reductions are different with various hydrocarbons and diluents. The experimental results were compared with several available chemical kinetic mechanisms, then a priority of the existing reaction mechanisms were addressed based on the experimental data. In addition, the present experimental data show that the Markstein lengths of the 50H2:50CO syngas-air premixed flames vary from negative to positive values with increasing of equivalence ratios, and the transition from unstable to stable flame occurs approximately at equivalence ratio of 0.8. The Markstein lengths significantly increase with the increase of hydrocarbon addition. Furthermore, the Markstein length of He-diluted syngas-air flame almost keeps the same value with adding He concentration. Whereas, the additions of CO2 and N2 diluents cause the Markstein lengths to decrease hence make the flames more sensitive to flame stretch effects. In the present investigation, the cellular instabilities for various hydrocarbon-added and diluted syngas-air flames were experimentally determined and evaluated in the viewpoint of the hydrodynamic and diffusional-thermal instabilities. Also, experimentally measured Peclet numbers were compared with the predicted results for fuel-lean flames. It is clear that the flame front destabilizations are promoted when enriching hydrogen in the hydrocarbon-syngas-air mixtures. The onsets of cellular instabilities are delayed for hydrocarbon additions to syngas-air flames, especially for heavy hydrocarbon such as propane and butane. The promotion of flame front stability for C3H8- and C4H10-added syngas-air flames is mainly caused by the modulation of diffusional-thermal instability due to the significant increase of the effective Lewis number. In addition, the experimental results exhibit that flame front instabilities of the syngas-air flames are delayed when diluted with helium, while the flames with CO2 and N2 dilutions give similar behavior to the syngas-air flames. The history of pressure with time in a combustion process were recorded and utilized to determine some important parameters of the combustion characteristics in this study. Experimental results show that the peak pressure and explosion time respectively increase and decrease with the increase content of hydrogen component in the syngas fuel blends. With the increase of initial pressure, the maximum explosion pressures, the maximum rate of pressure rises and the deflagration indexes of all flames increase, and shorter combustion durations are obtained. Hydrocarbon additions and dilutions to syngas-air premixed mixtures make the flames reduce the deflagration index, thus they can be recognized as potentials to reduce the explosion hazards of syngas-air flames.