Investigation Of The Premixed Laminar Flame Characteristics And Chemical Kinetics Of Oxygenated Fuel Blends

Fuel combustion will be the main form of energy utilization for a long time in the future.The development of advanced power plant is very important for energy efficient utilization and environmentally sustainable development.In the research of engine fuel design,oxygenated fuel is considered as an ideal alternative fuel.In practical application,oxygenated fuel is often mixed with gasoline and diesel fuel in different ways.Systematic study of fuel combustion characteristics and chemical reaction kinetics is a necessary prerequisite for the development of combustion technology.Most of the previous studies focused on the single fuel,while the research on the mixed fuel was very limited.The research on the coupling effect and competition/promotion mechanism of the components in the combustion reaction of the mixed fuel was still inconclusive.Laminar combustion is an important content in the study of combustion mechanism and an important means to develop and verify the kinetic mechanism of chemical reaction of fuel combustion.Supported by the National Natural Science Foundation of China(NSFC),this research is aimed at the basic problems in the application of oxyfuel combustion for the needs of internal combustion engine fuel design and high efficiency clean combustion technology development.In this study,three kinds of oxygenated fuels which have great application prospects:polyoxydimethyl ether(PODE),dimethyl carbonate(DMC),n-butanol(NB)and commercial 92#gasoline were investigated.The premixed laminar flame characteristics and combustion chemical reaction kinetics of oxygenated fuels with different functional groups,hydrocarbon fuels and their mixtures were systematically studied by means of experiment and simulation.The effects of molecular structure of oxygenated fuel on laminar combustion characteristics(flame propagation,laminar flame velocity,Markstein length,flame instability,flame chemical structure,etc.)were investigated;the formulation of surrogate model fuel was proposed to characterize the laminar combustion characteristics of practical fuel;the influence of oxygated fuel mixing on laminar combustion velocity and flame instability of hydrocarbon fuel was determined;the internal thermodynamic and kinetic mechanisms of the influence of fuel molecular structure on laminar combustion characteristics were revealed.Based on the spherical expansion flame and high-speed schlieren test method,the experimental platform of fuel premixed laminar combustion was designed and built,and the flame image processing algorithm was developed to realize the experimental measurement of fuel/air mixture Premixed Laminar Combustion Characteristics under a wide range of initial conditions.Based on chemical dynamics software CHEMKIN-PRO,a platform for flame chemical analysis of combustible mixture was built,which realized the construction and reduction of chemical reaction mechanism of oxygenated fuel blends and the simulation analysis of laminar flame chemical structure.The results show that:1.The molecular structure of oxygenated fuel has a significant effect on the laminar combustion characteristics.Under the same conditions,the order of laminar combustion velocity of three kinds of oxygenated fuel is PODE₃>NB>DMC,and the kinetic mechanism of this result comes from the fuel specific initial pyrolysis reaction path and the resulting intermediate products with different reactivity.The laminar flame instability of the three oxygenated fuels is also different.The flame stability of PODE₃ is better than that of DMC and NB because of its faster flame speed and stronger flame stretching.2.The three-component-model fuel(TRF)with n-heptane,isooctane and toluene as representative components can accurately characterize the laminar combustion characteristics of the practical fuel in the whole equivalence ratio range,and its performance is better than that of the two-component-model fuel(PRF)with n-heptane and isooctane.The low heat value,H/C ratio and molecular weight of the practical fuel should be taken as the target parameters,and the proportion of each family component in the practical fuel should be taken into account.3.The influence of mixing different oxygenated fuels on the laminar combustion characteristics of hydrocarbon fuels is different.Mixing PODE₃ and NB can improve the laminar combustion velocity of hydrocarbon fuel,while mixing DMC can reduce the laminar combustion velocity of hydrocarbon fuel.The laminar combustion velocity of the blended fuels changes linearly with the volume mixing ratio of the oxygenated fuel.The laminar flame stability of the three oxygenated fuels involved in this study is better than that of hydrocarbon fuels(TRF).When the mixing ratio is small,the oxygenated fuel has little effect on the flame stability of hydrocarbon fuels.When the mixing ratio reaches 50%,the oxygenated fuel helps to improve the laminar flame stability of hydrocarbon fuels.4.The cross-reaction between the individual component of blended fuels is generated by the interaction between the combustion species pools of various fuel components.The special combustion reaction path of oxygenated fuel can increase the concentration of active species,especially HO₂,HCO and CH₂O,in laminar flame.These species can affect the consumption reaction path of hydrocarbon fuel,and then affect the laminar combustion characteristics and combustion products.5.Based on the method of combination of the transferable core combustion mechanism and the fuel specific combustion reaction mechanism can quickly construct the combustion reaction mechanism of blended fuels.The combined mechanism reduction method used in the study can accurately extract the main characteristics of fuel combustion reaction,and the developed mixed fuel reduced mechanism can accurately simulate the laminar combustion characteristics,spontaneous ignition characteristics and pollutant generation characteristics of PODE₃/DMC/NB/TRF mixed fuel.