Experimental and Numerical Investigation of NH3/CH4 Mixture Combustion Properties under Elevated Initial Pressure and Temperature

Toward a carbon-free economy, ammonia is proposed as an alternative with high capabilities to replace hydrocarbon fuels. In the present study, the fundamental combustion properties of the ammonia/methane mixture were investigated experimentally and numerically. The experimental procedure was conducted via a constant volume chamber and Schlieren optical method, and CHEMKIN Pro software was employed for the numerical part. An extensive number of study conditions were considered: equivalence ratios: 0.7 to 1.6, ammonia mole fraction: 0.0 to 1.0, initial pressure: 1.0 to 5.0 atm, and initial temperature: 298 to 473 K. Results show that laminar burning velocity decreases non-linearly with ammonia addition. Interestingly, it was found that the decreasing (increasing) effect of pressure (temperature) is diminished (augmented) by adding ammonia. Ammonia’s laminar burning velocity is about 80% lower than methane’s; nonetheless, with moderate ammonia content, e.g., xNH3 = 0.4, and preheating conditions, e.g., Ti = 373–473 K, an ammonia/methane mixture can have the same laminar burning velocity as the standard industrial hydrocarbon fuels. In terms of flame chemistry, the interaction between ammonia and methane predominantly occurs through their competition for O/H radicals, with no significant direct interaction. NO emission has a rising-falling trend with ammonia concentration, reaching the maximum at xNH3 = 0.4. Additionally, the Markstein length strongly correlated with flame thickness and is increased with the ammonia addition. The comprehensive data set of measured laminar burning velocities and Markstein lengths obtained from this study serves as a valuable resource for mechanism validation and enhances our understanding of NH3/CH4 mixture combustion.面向无碳经济发展需求，氨被视作极具潜力的烃类燃料（hydrocarbon fuels）替代方案。本研究通过实验与数值模拟相结合的手段，对氨/甲烷混合燃料的基础燃烧特性开展了系统性研究。实验部分采用定容燃烧弹（constant volume chamber）与纹影光学法（Schlieren optical method）完成，数值模拟部分则依托CHEMKIN Pro软件进行。本研究设置了多组覆盖范围广泛的实验工况：当量比（equivalence ratio）区间为0.7至1.6，氨摩尔分数（mole fraction）区间为0.0至1.0，初始压力区间为1.0至5.0标准大气压，初始温度区间为298至473开尔文。研究结果表明，随着氨掺混比例提升，混合燃料的层流燃烧速度（laminar burning velocity）呈非线性下降趋势。值得注意的是，研究发现氨的掺混会削弱压力对燃烧速度的抑制作用，同时增强温度对燃烧速度的促进效果。纯氨的层流燃烧速度较纯甲烷低约80%；但当氨掺混比例适中（如xNH3=0.4）且燃料处于预热工况（如初始温度Ti=373~473开尔文）时，氨/甲烷混合燃料的层流燃烧速度可达到工业标准烃类燃料的水平。从火焰化学机理（flame chemistry）角度来看，氨与甲烷之间的相互作用主要源于二者对O/H自由基（O/H radicals）的竞争消耗，并未存在显著的直接相互作用。氮氧化物（NO）排放随氨掺混浓度呈先升后降的趋势，并在xNH3=0.4时达到峰值。此外，马克斯坦长度（Markstein length）与火焰厚度（flame thickness）存在显著相关性，且其数值随氨掺混比例的提升而增大。本研究测得的层流燃烧速度与马克斯坦长度的完整数据集，可为燃烧机理验证提供宝贵的参考依据，同时也有助于深化对氨/甲烷混合燃料燃烧过程的认知。