氨气近极限燃烧特性研究不仅是氨燃料高效利用的基础，还可为构建和优化详细的化学反应动力学模型提供指导。该文基于定容燃烧弹实验平台开展了氨气近极限燃烧特性实验，获取了当量比为0.8~1.2时的NH3和空气的预混气(NH3/空气)及其在2种稀释气(体积分数5%的N2或Ar)条件下的球形火焰图像及层流燃烧速度。利用CHEMKIN软件包开展数值模拟，选用多种机理，计算了层流燃烧速度值并开展相应的化学反应动力学分析。结果表明，N2对火焰传播的抑制作用强于Ar，最小层流燃烧速度约为2.9 cm/s; 实验值与模拟值整体符合度较高，基于Mei 2021机理对不同稀释剂条件下燃烧速度进行的预测效果良好; N2稀释和Ar稀释的反应路径与敏感性反应系数略有差距，反应活性自由基O、OH、H浓度对层流燃烧速度具有重要影响。
Objective: Ammonia fuel is a promising alternative fuel. Research on the near-limit combustion characteristics of ammonia is the basis for the efficient use of ammonia fuel and provides guidance for the construction and optimization of the detailed and comprehensive chemical reaction kinetic models of ammonia. Herein, the near-limit combustion characteristics of ammonia gas were investigated experimentally and computationally. Methods: The constant volume combustion bomb method was used to experimentally measure the laminar burning velocity (SL) of the near-limit premixed ammonia/air/dilution mixture. A high-speed camera with a frame rate of 10 000 fps was employed to record images of the outwardly propagating spherical flame. MATLAB code was used to process the images and extract the flame radius. In addition, a corresponding computational study, including the prediction of the SL of the near-limit ammonia/air/dilution premix and analysis of chemical reaction kinetics, was conducted with the CHEMKIN code package. During the simulation, CURV and GRAD values were set to 0.02 and at least 500 grid points were used. Results: The SL values and flame images of NH3/air, 95% (NH3/air)/5% N2, and 95% (NH3/air)/5% Ar with an equivalence ratio (ϕ) range of 0.8—1.2 were obtained. A buoyancy effect on the flame of NH3/air plus dilution gas (N2/Ar) was observed, and N2 suppressed flame propagation more than Ar. As ϕ increased, SL values first increased and then decreased, and the SL peak value (maximum) was at ϕ=1.1. For the NH3/air premixed mixture, the experimental values were in overall good agreement with nine chemical reaction kinetic mechanisms of NH3, and the predicted values of the Han 2020 mechanism were in the best agreement with the experimental values. For NH3/air/dilution(N2/Ar), the predicted values of the Mei 2021 mechanism were in the best agreement. Sensitivity and reaction pathway and flux analyses were performed under different dilution conditions by using the Mei 2021 mechanism. The reaction H+O2$\rightleftharpoons$O+OH had a strong promoting effect on SL, whereas the reaction NH2+NO$\rightleftharpoons$N2+H2O had a strong inhibiting effect. The sensitivity coefficients, as well as the fluxes of each reaction branch, were slightly different under the two dilution conditions. By comparing adiabatic reaction temperatures, the increase in SL for Ar dilution compared with that for N2 dilution was found to be mainly due to the difference in thermophysical properties between Ar and N2. Conclusions: N2 has a stronger inhibitory effect on flame propagation than Ar, a buoyancy effect on the flame of the NH3/air/dilution (N2/Ar) premix exists, and a minimum SL of approximately 2.9 cm/s is obtained. Numerical simulation results indicate that the experimental and simulated values are generally in good agreement. The Mei 2021 mechanism predicts the SL of NH3/air/dilution(N2/Ar) well. A slight difference exists between the reaction pathways and sensitivity reaction coefficients for N2 and Ar dilution, and the concentrations of the active free radicals O, OH, and H significantly affect laminar burning velocities.
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