湍流脉动对化学反应影响的数值研究

Numerical Research for Effects of Turbulent Fluctuations on Chemistry

关键词:  湍流脉动  低温机理  化学反应路径通量分析  点火延迟 

Key words:  Turbulent fluctuation  Low temperature chemistry  Analysis of chemical pathway fluxes  Ignition delay

摘要:

为了研究湍流脉动对化学反应的影响,在对冲火焰中引入了时空发展的扰动,其中H₂O₂扰动模拟组分脉动,拉伸率扰动在燃料和氧化剂边界温度均匀的条件下模拟速度脉动,在非均匀条件下模拟温度脉动。正庚烷自点火的计算结果显示:H₂O₂扰动可以缩短点火延迟时间。通过RO₂同素异形化以及C₂H₃氧化的反应路径通量分析,发现H₂O₂扰动不会改变反应路径间的极限竞争关系。均匀温度边界条件下的拉伸率扰动通过恶化热量和自由基的损失抑制中温机理,延长点火所需时间。非均匀温度边界条件下的拉伸率扰动能产生附加的温度扰动,点火延迟时间随扰动频率的变化关系复杂。通过RO₂同素异形化的反应路径通量分析可知,该扰动能够改变反应路径间的极限竞争关系,也能逆转点火进程。由此可以推断,在工程应用中,若湍流脉动较强,点火可能会从单级转化为多级。 

Abstract:

In order to study the effects of turbulent fluctuations on chemistry, time and space dependent perturbations were introduced to a counterflow flame. The H₂O_2and strain rate perturbations were used to mimic species and velocity fluctuations, respectively. The strain rate perturbation can also mimic the temperature fluctuation if the fuel and oxidizer boundary temperature are nonuniform. N-heptane autoignition is calculated in such a perturbed flame. The results show that the H₂O_{2perturbation} is able to reduce the ignition delay time. The analysis of the chemical pathway fluxes for RO_{2isomerization} and C₂H_3oxidization indicates that the H₂O_{2perturbation} cannot change the extreme branching ratios of chemical pathways. The results of the strain rate perturbation with uniform boundary temperature show that the intermediate temperature chemistry is retarded due to the aggravation of the heat and radical losses. For the case of the strain rate perturbation with nonuniform boundary temperature,a complex relation between the ignition delay time and the frequency due to the additional temperature perturbation is found. According to the analysis of the chemical pathway fluxes for RO_{2isomerization}, the strain rate perturbation in this case is able to change the extreme branching ratio of chemical pathways and moreover to reverse the ignition process. This reversion can lead to the conclusion that a multi-stage ignition is more likely to occur than a single-stage one if the turbulent fluctuation is at a higher level in engineering applications. 