Transported probability density function (TPDF) simulation with sensitivity analysis has been conducted for turbulent non-premixed CH₄/H₂ flames of the jet-into-hot-coflow (JHC) burner, which is a typical model to emulate moderate or intense low oxygen dilution combustion (MILD). Specifically, two cases with different levels of oxygen in the coflow stream, namely HM1 and HM3, are simulated to reveal the differences between MILD and hot-temperature combustion. The TPDF simulation well predicts the temperature and species distributions including those of OH, CO and NO for both cases with a 25-species mechanism. The reduced reaction activity in HM1 as reflected in the peak OH concentration is well correlated to the reduced oxygen in the coflow stream. The particle-level local sensitivities with respect to mixing and chemical reaction further show dramatic differences in the flame characteristics. HM1 is less sensitive to mixing and reaction parameters than HM3 due to the suppressed combustion process. Specifically, for HM1 the sensitivities to mixing and chemical reactions have comparable magnitude, indicating that the combustion progress is controlled by both mixing and reaction in MILD combustion. For HM3, there is however a change in the combustion mode: during the flame initialization, the combustion progress is more sensitive to chemical reactions, indicating that finite-rate chemistry is the controlling process during the autoignition process for flame stabilization; at further downstream where the flame has established, the combustion progress is controlled by mixing, which is characteristic of nonpremixed flames. An examination of the particles with the largest sensitivities reveals the difference in the controlling mixtures for flame stabilization, namely, the stoichiometric mixtures are important for HM1, whereas, fuel-lean mixtures are controlling for HM3. The study demonstrates the potential of TPDF simulations with sensitivity analysis to investigate the effects of finite-rate chemistry on the flame characteristics and emissions, and reveal the controlling physio-chemical processes in MILD combustion.

已对湍流非预混合CH ₄ / H ₂进行了带有灵敏度分析的运输概率密度函数（TPDF）模拟喷射热气流（JHC）燃烧器的火焰，它是模拟中等或强烈的低氧稀释燃烧（MILD）的典型模型。具体来说，模拟了两种气流中氧气含量不同的情况，即HM1和HM3，以揭示MILD和高温燃烧之间的差异。通过TPDF模拟，可以很好地预测两种情况下25种物种的温度和物质分布，包括OH，CO和NO的温度和物质分布。从峰值OH浓度反映出的HM1中反应活性的降低与气流中氧气的减少密切相关。关于混合和化学反应的颗粒水平局部敏感性进一步显示出火焰特性的显着差异。由于抑制了燃烧过程，HM1对混合和反应参数的敏感性不如HM3。具体而言，对于HM1，对混合和化学反应的敏感性具有可比较的大小，这表明在MILD燃烧中，燃烧过程既受混合又受反应控制。但是对于HM3，燃烧模式发生了变化：在火焰初始化期间，燃烧进程对化学反应更加敏感，这表明有限速率化学反应是火焰稳定的自动点火过程中的控制过程。在形成火焰的更下游，燃烧过程是通过混合来控制的，这是非预混合火焰的特征。对具有最高灵敏度的颗粒进行检查后发现，稳定火焰的控制混合物存在差异，即化学计量混合物对于HM1非常重要，而贫燃料混合物则对HM3具有控制作用。这项研究证明了通过敏感性分析进行TPDF仿真的潜力，以研究有限速率化学对火焰特征和排放的影响，并揭示在MILD燃烧中控制物理化学过程的过程。