Sparse particle methods for turbulent premixed combustion

新颖的燃烧器概念主要是为稀薄预混燃烧而设计的。然而，为了保证点火和操作安全性，需要一些局部燃料分层，从而导致燃料-空气混合物的不均匀性。燃烧室的更高吞吐量和尺寸减小的附加要求可增加湍流水平，使得主要火焰状态从小火焰状态转变为薄火焰状态。目前的燃烧模型通常是基于小火焰假设的，目前没有一个模型可以可靠地预测燃料分层和偏离小火焰region.The拟议的研究将开发一个基于粒子的Monte Carlo方法模拟湍流预混火焰。每个蒙特卡罗（PDF）粒子代表流体元素的瞬时局部解。然后，这些瞬时解允许气相的化学组成的联合概率密度函数（PDF）的近似，从而允许预混火焰的内部结构的近似。速度场的守恒方程在欧拉框架下借助大涡模拟（LES）求解。分子和湍流扩散的建模提出了主要的挑战：将标准混合模型应用于预混燃烧会导致火焰前缘的不受控制和非物理混合，这阻碍了火焰结构甚至燃烧速度的准确预测。所提出的工作的新奇是使用稀疏粒子方法的气相组成PDF的计算。传统的粒子方法每个LES单元需要20到50个粒子，并且它们的数值解可能非常昂贵。当使用稀疏方法时，颗粒数量可以降低到每10个LES单元少于1个颗粒，并且可以显着减少模拟时间。这是通过在参考场上调节混合算子来实现的。正是这个参考场可以用来模拟扩散作为一个多步骤的过程，防止混合的火焰前锋，并确保在预热区的颗粒温度的持续增加，并限制颗粒点火和热释放到reactionzone.First，简单的，统计一维火焰的直接数值模拟（DNS）将有助于新的封闭的发展。然后，DNS配置将扩展到包括燃料分层。在最后一步，新的封闭将适用于真实的实验室火焰，并将与现有的实验数据的帮助下进行验证。

Novel burner concepts are primarily designed for lean premixed combustion. To guarantee ignition and operational safety, some local fuel stratification is needed, however, leading to inhomogeneities of the fuel-air mixture. The additional requirements of higher throughput and a size reduction of the combustion chambers may increase the turbulence levels such that the predominant flame regime shifts from the flamelet to the thin-flame regime. Current combustion models are usually based on the flamelet assumption, and no current model can reliably predict fuel stratification and deviations from the flamelet regime.The proposed study will develop a particle based Monte Carlo method for the modelling of turbulent premixed flames. Every Monte Carlo (PDF) particle represents an instantaneous, local solution of a fluid element. These instantaneous solutions then allow for the approximation of the joint probability density function (PDF) of the chemical composition of the gas phase and thus of the inner structure of the premixed flame. The conservation equations for the velocity field are solved in a Eulerian framework with the aid of large-eddy simulations (LES).The modelling of molecular and turbulent diffusion pose the major challenge: the application of standard mixing models to premixed combustion leads to uncontrolled and unphysical mixing across the flame front that hinder the accurate prediction of the flame structure and even of the burning velocity. The novelty of the proposed work is the use of a sparse particle method for the computation of the gas phase composition PDF. Conventional particle methods require between 20 and 50 particles per LES cell and their numerical solution can be extremely costly. The particle number can be lowered to less than 1 particle per 10 LES cells when using sparse methods, and simulation times can be significantly reduced. This is realized by conditioning of the mixing operator on a reference field. And it is this reference field that can be used to model diffusion as a multistep process that prevents mixing across the flame front and ensures a continuous increase of particle temperature in the preheat zone and limits particle ignition and heat release to the reaction zone.First, direct numerical simulations (DNS) of simple, statistically one-dimensional flames will aid the development of new closures. Then, the DNS configuration will be extended to include fuel stratification. In a final step, the new closures will be applied to real laboratory flames and will be validated with the aid of available experimental data.