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A numerical investigation of the effects of imperfect premixing on flame propagation.

机译：不完全预混对火焰传播影响的数值研究。

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Gasoline direct-injection engines and lean-premixed prevaporized combustors have recently emerged as promising technologies for improving combustion efficiency and pollutant emission. In both applications, the time available for the mixing of fuel and oxidizer prior to combustion may be insufficient for the mixture to acquire a homogeneous composition, and this thesis addresses the propagation of premixed flames under such conditions.; Previous numerical studies, owing to severe spatio-temporal limitations, have been unable to provide a clear picture of what to expect from the effects of imperfect premixing on turbulent flames, and have not found agreement with existing experimental data. In this study, direct numerical simulations (DNS) of flame propagation in inhomogeneous mixtures with imposed velocity fluctuations are carried out. To avoid aforementioned limitations and to account for important flame dynamics such as the hydrodynamic instability, a configuration that permits larger domains and longer times than previously accessed by DNS is employed. The simulated flows are unavoidably restricted to two dimensions. One-step chemistry and Fick's diffusion law are considered, along with unity Lewis number assumption.; It is observed that relatively weak fluctuations in composition alone may lead to a large increase in flame wrinkling and propagation speed, consistent with experimental findings. The hydrodynamic instability caused by gas expansion is found to be responsible for such strong flame response. However, the impact of composition fluctuations diminishes as velocity fluctuations present become more intense, and eventually imperfect premixing ceases to affect the flame brush speed. This effect is explained based on the numerical results, and a criterion for the transition from one regime to the other is provided. It is also found that imperfect premixing always affects the structure of the flame brush. A gradual change in the composition of the unburnt mixture across the flame brush is observed and explained, and a model of this effect is developed and found to agree well with the numerical results. Finally, the effect of mixture stratification on flame propagation is also briefly investigated. Mixture stratification is found to impact on propagation speed, even when velocity fluctuations present become more intense.
机译：汽油直喷发动机和稀薄预混的预蒸发燃烧器最近已成为提高燃烧效率和污染物排放的有前途的技术。在这两种应用中，燃烧前燃料和氧化剂混合的可用时间可能不足以使混合物获得均质的成分，因此本文着眼于在这种条件下预混合火焰的传播。由于严重的时空限制，先前的数值研究无法清楚地看到不理想的预混对湍流火焰的影响，并且与现有的实验数据不一致。在这项研究中，进行了火焰在不均匀混合物中传播速度波动的直接数值模拟（DNS）。为了避免上述限制并考虑到重要的火焰动力学（例如流体动力学不稳定性），采用的配置比DNS以前访问的域更大，时间更长。模拟流不可避免地限于二维。考虑了一步化学反应和菲克扩散定律，以及统一的路易斯数假设。观察到单独的相对较弱的波动可能导致火焰起皱和传播速度大大增加，这与实验结果一致。发现由气体膨胀引起的流体动力不稳定性是造成这种强烈火焰响应的原因。但是，随着当前速度波动的加剧，成分波动的影响逐渐减弱，最终不完美的预混合将不再影响火焰刷的速度。根据数值结果说明了这种效果，并提供了从一种状态过渡到另一种状态的准则。还发现不完美的预混合总是会影响火焰刷的结构。观察并解释了火焰刷上未燃烧混合物成分的逐渐变化，并开发了这种效果的模型，发现与数值结果非常吻合。最后，还简要研究了混合物分层对火焰传播的影响。发现混合物分层会影响传播速度，即使存在的速度波动变得更加强烈时也是如此。

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