ABSTRACT

The effects of droplet diameter, overall (i.e. liquid and gaseous phases) equivalence ratio, and turbulence intensity on the variation of the Minimum Ignition Energy (MIE) for localized forced ignition of uniformly dispersed mono-sized n-heptane droplet-laden mixtures under homogeneous isotropic decaying turbulence have been analyzed based on Direct Numerical Simulation (DNS) data. The MIE was evaluated just for (i) obtaining thermal runway irrespective of the fate of the resulting flame kernel, and (ii) also for a successful self-sustained flame propagation without the assistance of an external energy source following the energy deposition by the ignitor. It has been found that the MIE requirement increases with increasing turbulence intensity and this trend for the MIE increase is especially significant for large values of turbulence intensity. The MIE requirement increases with increasing initial droplet diameter and with decreasing overall equivalence ratio. The MIE requirements for droplet-laden mixtures have been found to be greater than the corresponding value for homogeneous mixture with the same nominal values of initial turbulence intensity and equivalence ratio for the parameter range considered here. This behavior arises due to the deposited energy being partially utilized to supply the latent heat of evaporation and also due to the predominantly fuel-lean composition of the gaseous flammable mixture. This tendency of obtaining fuel-lean mixture strengthens with increasing (decreasing) initial droplet diameter (overall equivalence ratio). It has also been demonstrated that combustion takes place predominantly in the fuel-lean premixed mode although there is a finite probability of having non-premixed combustion in all cases. Moreover, there is a small probability of fuel-rich combustion occurring for small droplets, especially under fuel-rich overall equivalence ratios. The stochastic nature of the ignition event has been demonstrated by considering different realizations of statistically similar turbulent flow fields. The conditions giving rise to a successful thermal runaway/self-sustained flame propagation have been identified by a detailed analysis of the energy budget.

摘要

液滴直径，整体（即液相和气相）当量比和湍流强度对均相下均匀分散的单一尺寸正庚烷液滴混合混合物的局部强制点火最小点火能量（MIE）变化的影响根据直接数值模拟（DNS）数据分析了各向同性衰减湍流。对MIE的评估仅用于（i）获得最终的火焰核的命运的热跑道，以及（ii）在点火器进行能量沉积后，无需外部能源的帮助下，成功的自持火焰传播成功。已经发现，MIE需求随湍流强度的增加而增加，并且对于大的湍流强度值，MIE增加的趋势尤其显着。随着初始液滴直径的增加和总当量比的降低，MIE要求也随之增加。已经发现，对于载有液滴的混合物，其MIE要求要大于均质混合物的相应值，对于此处考虑的参数范围，其初始湍流强度和当量比的标称值应相同。由于沉积的能量被部分地用于提供蒸发潜热，并且还由于气态可燃混合物的主要贫燃料组成而出现这种现象。随着初始液滴直径（总当量比）的增加（减小），获得贫燃料混合物的趋势会增强。还已经证明，燃烧主要以贫燃料的预混合模式进行，尽管在所有情况下都有非预混合燃烧的可能性有限。此外，对于小液滴，发生富燃料燃烧的可能性很小，尤其是在富燃料的总当量比下。通过考虑统计上相似的湍流场的不同实现，已经证明了点火事件的随机性。通过对能量预算的详细分析，已经确定了导致成功的热失控/自持火焰传播的条件。通过考虑统计上相似的湍流场的不同实现，已经证明了点火事件的随机性。通过对能量预算的详细分析，已经确定了导致成功的热失控/自持火焰传播的条件。通过考虑统计上相似的湍流场的不同实现，已经证明了点火事件的随机性。通过对能量预算的详细分析，已经确定了导致成功的热失控/自持火焰传播的条件。