Study On Premixed Laminar Combustion Characteristics Of Ammonia Under High Pressure

In the context of"dual carbon",ammonia fuel has received extensive attention in the field of internal combustion engines due to its high hydrogen content,good anti-detonating quality,easy liquefaction,easy storage,and the absence of carbon dioxide in its combustion products.The research on the premixed laminar combustion characteristics of ammonia fuel can provide certain theoretical support for the combustion application of ammonia fuel;secondly,optimizing the chemical kinetics mechanism of ammonia fuel can serve to improve the numerical simulation of ammonia fuel engine combustion process and the prediction accuracy of pollutant emissions.However,the existing experimental studies on the laminar combustion characteristics of ammonia fuel premixed mostly focus on low pressure conditions,and the experimental data under high pressure is relatively lacking;Secondly,the prediction accuracy of the laminar combustion characteristics of ammonia fuel by the existing mechanism needs to be further improved,and the understanding of the NO_x generation mechanism is still unclear.Therefore,a high-pressure constant-volume combustion vessel with the pressure release tank was first developed and validated,a high pressure spherical propagating flame test method was given to reduce the influence of flame cracks and other factors.The laminar burning velocity of NH₃/O₂/Ar/He mixed fuel was measured under different initial conditions;Then,based on data from experiments and literature,a new chemical kinetic mechanism was optimized and validated,named the WUT＿NH₃mechanism;Finally,combined with the numerical simulation of a one-dimensional plane flame,the corresponding chemical kinetics analysis was carried out,and the effect of the initial conditions on the NO_x generation was emphatically analyzed.The main research conclusions are as follows:（1）Considering the influence of high pressure on the laminar combustion velocity and flame instability of ammonia flame,the variation law of Lewis number with diluent（He/N₂,He/Ar）ratio and the relationship between Lewis number and adiabatic flame temperature with initial pressure were studied:the results show that,under the conditions of constant equivalence ratio and adiabatic flame temperature,the Lewis number increases gradually with the increase of diluent（He/N₂,He/Ar）ratios;For mixtures with defined composition and content,the initial pressure has little effect on its Lewis number and adiabatic flame temperature.（2）Based on the sensitivity analysis method and basic test data,the most important elementary reactions affecting the laminar combustion velocity of the mixed fuel were explored.The research shows that under lean combustion conditions,the reactions NH₂+NO<=>N₂+H₂O and HNO+OH<=>NO+H₂O have a significant effect on the mechanism prediction results,the reaction NH₂+H+M<=>NH₃+M is a crucial reaction that affects the prediction results of the mechanism under rich combustion conditions.The verification results of the WUT＿NH₃ mechanism showed that it could reasonably predict the laminar combustion velocity of ammonia fuel under lean,rich,high pressure and low pressure conditions.（3）The laminar combustion velocity of NH₃/O₂/Ar/He mixture decreases nonlinearly with the increase of initial pressure,and increases nonlinearly with the increase of adiabatic flame temperature.According to the relationship between the laminar combustion velocity and the initial pressure,the total reaction order of the NH₃/O₂/Ar/He mixture under different equivalence ratios is about 1.5;With the increase of nitrogen dilution rate,the laminar combustion velocity of NH₃/O₂ mixture decreases linearly whenφ=0.7,and the decreasing trend is weakly nonlinear whenφ=1.3.（4）The effect of initial conditions on the chemical kinetics of ammonia premixed laminar combustion was further analyzed.The research shows that with the increase of initial pressure and adiabatic flame temperature,the reaction area of the flame front of the NH₃/O₂/Ar/He mixture becomes narrower,and the reaction rate of each elementary reaction increases.NO and N₂O accounted for a relatively high proportion in the emission,and their contents decreased with the increase of the initial pressure and increased with the increase of the adiabatic flame temperature;The proportion of NO₂in emissions is relatively low,but under high temperature and high pressure conditions,the NO₂ content increases significantly.Besides,a virtual inert substance（written as FN₂）was constructed to separate the physical and chemical effects of nitrogen addition on ammonia combustion.Kinetic analysis showed that both the physical and chemical effects of nitrogen addition would inhibit the formation of nitrogen in combustion products.When the nitrogen dilution rate is less than 0.2,the chemical effect dominates.Further research found that the addition of nitrogen mainly inhibited the occurrence of the reaction NO+N<=>O+N₂,and promoted the reaction of NO and NH_i（i=1,2）,therefore affecting the generation of NO.The research on the laminar combustion characteristics and chemical kinetic mechanism of high-pressure premixed ammonia fuel will help to deepen the understanding of the fuel combustion and pollutant generation process in the engine cylinder,and provide effective theoretical support for the design and optimization of ammonia fuel engines.