Experimental And Kinetic Studies On Combustion Of Nitromethane

Due to the high energy density,energetic materials are the important power sources for the rocket launch,propulsion,and blasting in the fields of military,national defense,and avi-ation.The development and design of modern weapons are inextricably linked to the research on energetic materials.To endow the modern weapon with higher maneuverability,endurance,and blasting,it is necessary to understand the combustion mechanism of energetic materials in depth.Given the intricate reaction network,there are plenty of obstructions when exploring the combustion mechanism of real energetic materials directly.Consequently,researchers are usually motivated to study the combustion of small molecules as a first step,and construct the combustion mechanism of larger energetic molecules using a hierarchical method.As a liq-uid energetic material,nitromethane(referred as to NM)is the simplest nitro-paraffin with the molecular formula of CH₃NO₂,widely used as fuel,additive,and propellant.Investigation on the combustion properties of nitromethane not only facilitates understanding the role of nitro functional group,but also provides the basis for studying the combustion mechanism of more complicated energetic materials.Furthermore,as a high performance additive,exploring the effect of nitromethane addition on the combustion properties of other fuels,such as hydrocar-bon and alcohol fuels,is also helpful to optimize the engine output with a more suitable fuel formulation.This dissertation mainly investigates the combustion properties of nitromethane under dif-ferent temperatures,pressures,equivalence ratios,and dilution levels with a shock tube heated at 50?.Using the tunable diode laser absorption spectroscopy technique,the concentration of primary N-containing product,nitric oxide(referred as to NO),is monitored during the pyrol-ysis of nitromethane.Based on the measured results,the literature models are evaluated and then improved.In addition,the promotion effect of nitromethane on the ignition of methane,ethane,and ethanol is explored,and the mechanism behind this enhancement is also illumi-nated via sensitivity,reaction pathway,and rate of production analyses.In the dissertation,the main research contents and innovations are as follows:1)The two-stage ignition delay times of nitromethane are measured at a temperature range of 931–1558 K and a pressure range of 4–16 atm,and the effects of equivalence ratio(0.2–2.0)and dilution level(93–97%Ar)on the ignition are also investigated.The recent proposed models are evaluated,and the rate constants of an important elementary reaction(CH₃+NO=H₂CN+OH),which is critical to the nitromethane ignition under fuel-rich condition,are up-dated.The updated model can well reproduce the experimental measurements of nitromethane at the extend working conditions,including ignition delay times,laminar flame burning ve-locities,and species concentration profiles.According to the reaction kinetic analyses,the mechanism of two-stage ignition during the nitromethane combustion is clarified:after the C–N bond fission of nitromethane,the interconversion reactions between NO₂and NO(NO₂+H=NO+OH and NO+HO₂=NO₂+OH)account for the first-stage ignition;the chain branching reaction,H+O₂=O+OH,is responsible for the second-stage ignition,similar to the combustion of hydrocarbon fuels.2)Combined with the heated shock tube,a laser absorption spectroscopy apparatus is assembled.The absorption cross-section of NO is calibrated at a temperature range of 1006–1790 K and a pressure range of 2.5–3.7 atm with a detection wavelength of 1900.08 cm^-1.Based on the calibrated cross-section,the concentration profiles of NO during the pyrolysis of0.60%CH₃NO₂/Ar and 1.05%CH₃NO₂/Ar are measured at different temperatures(1013–1418K),and a detection limit of 100 ppm is achieved.The chemical kinetics of NO formation in the literature models is analyzed and evaluated.The results demonstrate that NO is the major N-containing product during the pyrolysis of nitromethane,and the two reactions,CH₃NO₂=CH₃+NO₂and CH₃+NO₂=CH₃O+NO,are sensitive to the formation of NO.3)The promotion effect of nitromethane on methane ignition is studied,and the effects of temperature(995–1962 K),pressure(3.8–17.6 atm),and equivalence ratio(0.5–2.0)are also explored.The addition of nitromethane can dramatically reduce the ignition delay time and ignition temperature of methane,and the promotion effect increases with increasing blending ratio of nitromethane.The available models from the literature are evaluated,and the rate constants of a key reaction(HNO+NO₂=NO+HONO)are recalculated using conventional and variation transition state theory at the G4 level of theory.The kinetic analyses indicate that CH₃and OH radicals are rapidly produced after the decomposition of nitromethane via C–N bond fission.These radicals enhance the consumption pathway of methane via the hydrogen abstraction reactions,thus leading to a faster ignition delay time.4)The promotion effect of nitromethane on ignition of ethane and ethanol is investigated at the stoichiometric ratio,and the ignition delay times of mixtures with different nitromethane blending ratios are measured.The experimental results indicate that a small amount of ni-tromethane can markedly promote the ignition of ethane and ethanol,but this promotion reaches the maximum at a blending ratio of 0.5.With further increasing blending ratio,the promotion effect of nitromethane decreases.The developed model in this dissertation is evaluated and validated against the measured experimental results.The reaction kinetic analyses demon-strate that the establishment of radical pool is slow during the ignition processes of ethane and ethanol.As an initiator,addition of nitromethane provides abundant radicals,which promote the ignition process.During the ignition process of nitromethane,however,most radicals are consumed after its first-stage ignition,and the new radical pool needs to be re-established.Ethane or ethanol serves as a stable source of free radicals,and the addition of them can accel-erate the re-establishment of radical pool of nitromethane.Therefore,a synergistic promotion on the ignition is observed between the nitromethane and ethane or ethanol.