Investigation On Laminar Burning Characteristics Of Syngas Blended Fuel Premixed Flames

The crude oil resource shortages and environmental crises have prompted studies to find alternative fuels for using in spark-ignition?SI?engines.Natural gas has been widely applied as an alternative fuel for SI engines because it's abundant and convenient to use.However,natural gas has a slower laminar burning velocity,which have negative effects on the improvement of engine thermal efficiency.Hydrogen has been applied as an additive to natural gas because of its high flame propagation speed.However,critics contend that hydrogen storage and transportation are very inconvenient and unsafe for now,and the high cost of hydrogen production also makes it difficult to be accepted as a widely applied alternative fuel for conventional internal combustion?IC?engine powered vehicles.Dissociated methanol,which is one kind of syngas,provides a feasible mean for hydrogen applications on SI engines while utilising the coal in a clean manner for potential vehicle fuels,as well as improving the engine efficiency through recovering the otherwise wasted energy in the exhaust gas.Laminar burning characteristics are critical in determining the flame development process,which will deeply affect the practical combustion phenomena.It is important to understand the laminar burning characteristics of the fuel.The laminar burning characteristics of natural gas-dissociated methanol blended fuel where the dissociated methanol has been considered the representative of syngas was studied under the supports from the national key technology R&D program of China.The constant volume combustion experimental system consists of the combustion chamber,the high-speed schlieren photography system,the intake-exhaust and fuel supply system,the control system,the ignition system,and the combustion pressure data acquisition system.In order to validate the experimental measurements in this study,contrast experiments were conducted and the results were compared to previously published data.The influence of dissociated methanol addition on the laminar combustion characteristics of methane-air mixtures was investigated in a constant volume chamber.The adiabatic flame temperature and thermal diffusivity increased with higher DM fraction for a given equivalence ratio,their combined effects increaselaminar burning velocity of the CH₄-DM-air mixtures.The Markstein length decreased monotonically with an increasing DM fraction,and the reduction significantly increased with the increase of the DM fraction.For lean mixtures with an larger DM fraction,the hydrodynamic instability was mainly dominated by the laminar flame thickness,and the decreasing laminar flame thickness promoted the hydrodynamic instability.The decreasing effective Lewis number promoted the diffusion–thermal instability.Overall,the combination of promoted hydrodynamic instability and diffusion–thermal instability made the flame more unstablewhen DM is added.For stoichiometric and rich mixtures with an increasing DM fraction,the flame instability was dominated by the hydrodynamic instability,and the hydrodynamic instability was mainly dominated by the laminar flame thickness,the decreasing laminar flame thickness promoted the hydrodynamic instability.Overall,the promoted hydrodynamic instability made the flame more unstable with the addition of DM.The influence of initial conditions on the laminar combustion characteristics of methane-dissociated methanol-air mixtures was investigated in a constant volume chamber.The laminar burning velocity increased with an increase in the initial temperature but decreased with an increase of the initial pressure.The initial temperature has little effect on the Markstein length,but the Markstein length decreased significantly with increasing initial pressure.The combination of slightly increasingeffective Lewis number and decreasinglaminar flame thickness and density ratio made the flame instability show no significant change with an increasing initial temperature.The decreasing laminar flame thickness and increasing density ratio promoted the hydrodynamic instability,which made the flame more unstable with an increase in the initial pressure.Based on the experimental results,the formulas to predict laminar burning velocity of CH₄-DM-air mixtures under different conditions have been deduced.The influence of diluents on the laminar combustion characteristics of methane-dissociated methanol-air mixtures was investigated in a constant volume chamber.The adiabatic flame temperature and thermal diffusivity decreased with increasing diluent gas fraction for a given equivalence ratio,their combined effects result in laminar burning velocity decreasing for the CH₄-DM-diluent-air mixtures.The Markstein length of CH₄-DM-diluent-air mixtures slightly decreased with the increase of diluent gas fraction.CO₂ has a more significant impact on the laminar burning velocity and Markstein length decreasing.The flame instability showed no significant change with diluent gas kinds because that CO₂ has more significant impact on promoting the diffusion–thermal instability and suppressing the hydrodynamic instability.The combination of promoted diffusion–thermal instability and suppressed hydrodynamic instability made the flame more unstable with an increasing diluent gas fraction.The elementary reaction sensitivity,reaction rate and radical characteristics was investigated through one-dimensional premixed flame chemical kinetics analysis by using CHEMKIN software and GRI-Mech 3.0 chemical kinetic mechanism,which can be used to analyze the fundamental causes of laminar burning velocity change under different conditions from the macroscopic and microscopic aspect.R38 is the dominant chain branching reaction.R35 and R52 are the dominant chain termination reactions.The maximum growth rate of chain branching reaction R84 was much higher than that of other reactions,which made the laminar burning velocity increased with increasing DM fraction.The concentration of free radicals increased due to the increasing reaction rate of chain branching,which made the laminar burning velocity increased with increasing initial temperature.The maximum growth rate of chain termination reaction with increasing initial pressure is higher than that of chain branching reaction,which made the laminar burning velocity decreased with increasing initial pressure.With the increase of diluent gas fraction,the concentration of free radicals decreased significantly because the reaction rate reduction of chain branching reactions were more significant than that of the chain termination reactions,which will suppress the laminar burning velocity.The reduction rate of the maximum reaction rate of CO₂ dilution was higher than that of N₂ dilution.