Regimes in turbulent non-premixed combustion

在非预混燃烧的许多实际应用中，都采用小火焰模型。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适用于强湍流和小尺度湍流。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，将使用暂时演变的平面射流火焰的数值设置。射流由甲烷-氮气混合物组成，该混合物具有与周围氧化剂流相同的氮气质量分数。将使用空气或用氮气稀释的空气作为氧化剂。由于大的稀释和由此产生的相对较低的温度，这些情况下，小火焰的假设可能是特别关键的。将计算具有可变刘易斯数的两种情况和具有单位刘易斯数的两种情况。除了化学反应物质的温度和质量分数的方程外，还将求解混合物分数的方程。混合分数场将被空间填充地分解成所谓的耗散元素（DE），其包含混合分数的局部最小值和局部最大值之间的平滑区域。除了在两个极值点处的混合物分数Δ Z的标量差之外，还将计算点之间的线性距离和所得到的跨元件的平均混合物分数梯度，使得元件由两个参数Δ Z和g表征。在DNS结果的基础上，可以详细分析单个元件中存在的反应性结构。反应性结构：燃烧火焰、边缘火焰、细尺度混合区和破碎反应区应在由两个参数Δ Z和g形成的状态图中识别。通过对从DNS获得的这两个参数的联合概率密度函数进行积分，应计算射流火焰中不同区域的四种不同反应结构的发生概率。这种分析将扩展到具有不同湍流特性和不同稀释度的实际感兴趣的火焰。

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.