正庚烷-甲烷二元燃料着火特性的模拟研究

Simulation Investigation on the Ignition Characteristics of n-Heptane-Methane Fuel Blends

针对试验中发现的柴油引燃天然气会推迟着火的现象,采用零维模型定量分析了由于加入甲烷所引起的混合气氧分压、定容热容以及化学动力学反应的变化对正庚烷着火的推迟作用,表明了动力学反应的变化对着火推迟有着重要的影响.其次,通过对二元燃料详细机理的耦合分析,对二元燃料动力学反应在着火推迟中的作用机理做出了解释.发现在低温氧化过程中,由于甲烷将活跃的OH转化为稳定的H2O2,同时甲烷不具备与正庚烷类似的低温链分支反应,故降低了系统的反应活性.随着初始温度增加至超过1,000,K,H2O2开始迅速分解,控制着火的重要反应也随之由正庚烷二次加氧后的链分支反应,逐渐向着反应H2O2=OH+OH、CH3+HO2=CH3O+OH、H+O2=OH+O转变,动力学反应对正庚烷着火的影响也由抑制转变为促进.最后,创建了一个包含53步反应、36种组分的简化机理.计算结果表明,该机理在不同初始温度、初始压力、当量比的情况下,可以对二元燃料着火时刻有较好的预测.

It has been found in previous experimental studies that the ignition of diesel is delayed by methane addi-tion. Firstly,a zero-dimensional model was used to analyze the impact of oxygen partial pressure,specific heat and chemical kinetic reactions caused by methane addition on the ignition delay of n-heptane. Results show that the change of kinetic reactions plays an important role in the ignition delay of n-heptane. Secondly,based on the coupling analy-sis of the mechanism of fuel blends,the infulence of kinetic reactions on the ignition delay of n-heptane-methane fuel blends was explained. Results show that OH is converted to H2O2 which is stable due to the existence of methane in low temperature oxidation,and the activity of the whole reaction system deceased since,unlike n-heptane,methane does not have low temperature chain-branching reactions. However,with the increase of initial temperature to higher than 1 000 K,H2O2 becomes unstable and begins to decompose rapidly,the key reactions on the ignition of n-heptane-methane fuel blends change from chain-branching reactions after second oxygen addition to n-heptane to the reactions of H2O2=OH＋OH,CH3＋HO2=CH3O＋OH and H＋O2=OH＋O,and the change of kinetic reactions promotes the ignition of n-heptane instead of suppressing it. Lastly,a reduced mechanism including only 53 reactions related to 36 kinds of species was created to predict ignition delay of n-heptane-methane fuel blends as well as that of each pure fuel. The reduced mechanism test result shows that the ignition time of the n-heptane,methane and the fuel blends is well predicted under various equivalence ratios,initial temperatures and pressures.