Regimes in turbulent non-premixed combustion

在许多非预混燃烧的实际应用中，使用了小火焰模型。然而，仍然存在一个问题，即该模型的基本假设在多大程度上对密集和小尺度湍流有效。在本提案中，应根据直接数值模拟（DNS）对该问题进行评估。为此，将使用暂时演化的平面射流火焰的数值设置。射流由甲烷-氮混合物组成，它与周围的氧化剂流具有相同的氮质量分数。将使用空气或用氮气稀释的空气作为氧化剂。由于大量稀释和由此产生的相对较低的温度，在这些情况下，小火焰假设可能特别关键。将计算两种具有可变路易斯数的情况和两种具有统一路易斯数的情况。除了解出化学反应物质的温度和质量分数的方程外，还解出了混合物分数的方程。混合分数场将被空间填充分解为所谓的耗散单元（DE），其中包含混合分数的局部最小值和局部最大值之间的光滑区域。除了在两个极值点处的混合分数δ Z的标量差外，还将计算这些点之间的线性距离和所得到的元素之间的平均混合分数梯度，这样元素就可以用两个参数δ Z和g来表征。在DNS结果的基础上，可以详细分析单个元素内部存在的反应结构。在由两个参数Z和g组成的状态图中，可以识别出燃烧小火焰、边缘火焰、细尺度混合区和破碎反应区等反应结构。通过对从DNS中得到的这两个参数的联合概率密度函数进行积分，可以计算出在射流火焰的不同区域，这四种不同反应结构发生的概率。这种分析将推广到具有不同湍流特性和不同稀释度的实际火焰。

In many practical applications of non-premixed combustion the flamelet model is used. There remains, however, the question, to which extent the underlying assumptions of this model are valid for intensive and small scale turbulence. In this proposal this question shall be assessed on the basis of Direct Numerical Simulations (DNS). For this the numerical set-up of a temporarily evolving planar jet flame will be used. The jet consists of a methane-nitrogen mixture which has the same nitrogen mass fraction as the surrounding oxidizer stream. As oxidizer air or air diluted with nitrogen will be used. Because of the large dilution and the resulting relatively low temperatures these are situations where the flamelet assumptions could be particularly critical. Two cases with variable Lewis numbers and two cases with unity Lewis numbers will be calculated. In addition to the equations for the temperature and the mass fractions of the chemically reacting species an equation for the mixture fraction will be solved. The mixture fraction field will be decomposed space fillingly into so-called dissipation elements (DE) which contain a smooth region between a local minimum and a local maximum of the mixture fraction. In addition to the scalar difference of the mixture fraction delta Z at the two extremal points, the linear distance between the points and the resulting mean mixture fraction gradient across the elements will be calculated, such that the elements are characterized by the two parameters delta Z and g. The reactive structures which exist within the individual elements can be analyzed in detail on the basis of the DNS results. The reactive structures: burning flamelets, edge flames, fine scale mixing zones and broken reaction zones shall be identified in a regime diagram which is formed by the two parameters delta Z and g. By integrating over the joint probability density function of these two parameters, which is obtained from the DNS, the probability of occurrence of the four different reactive structures shall be calculated for difference regions in the jet flame. This analysis will be extended to flames of practical interest which have different turbulence properties and different dilutions.