Analysis of soot filter regeneration by combined numerical and experimental investigations

在这个项目中，我们的目标是预测由于固体和穿透气相之间的化学反应而导致的多孔固体的形态演变。稀释的氧气和NO2通过多孔的煤烟层氧化煤烟，将作为一个具有实际意义的例子。众所周知，即使在等温条件下，烟尘过滤器再生过程中的总体氧化率也是时间的强烈函数。通过结合实验研究和详细的数值模拟，我们将试图从第一性原理出发，证明总反应速率的变化是否（以及在多大程度上）主要源于煤烟的形态变化。因此，该项目旨在将初级煤烟颗粒的动力学与影响煤烟结构中可达表面积的形态变化分开。在该项目中，将建造一个特殊的反应堆，允许在等温条件下逐步氧化烟灰层。通过对废气的分析，可以监测烟尘转化的整体速率。在离散时间，氧化将被中断，部分氧化的烟灰层的整体BET表面将被测量，而无需拆卸样品。因此，我们可以同时得到有效反应速率和整个表面的变化。最近，通过聚焦离子束扫描电子显微镜（FIB-SEM）图像可以评估多孔煤烟结构的详细信息。这可以在微观尺度上以高分辨率重建多孔介质，并为直接数值模拟提供初始条件。建立详细的三维晶格-玻尔兹曼（LB）模型来模拟烟灰氧化过程中烟灰形态的变化。采用离散形式的玻尔兹曼动力学方程来模拟多组分混合气体在孔隙内的输运，而烟灰氧化过程中非均相的烟灰形态变化将作为新的边界条件来模拟。LB方法可以有效地处理复杂的几何形状，因此非常适合于模拟固气相转变。我们可以计算出有效氧化速率并将其与实验值进行比较，并且可以将部分氧化后烟尘样品的FIB-SEM图像与模拟结果进行比较。一旦有了详细的模型，就可以作为开发扩展的面向应用程序的模型的基础。对于各向异性宏观输运参数的推导，将采用渐近均匀化方法。推导出的宏观模型包括经典的质量和能量平衡以及典型结构特性（如BET表面）的平衡方程。

In this project we aim to predict the morphology evolution of a porous solid due to chemical reactions between a solid and a penetrating gas phase. The oxidation of soot by diluted oxygen and NO2, passing through a porous soot layer, will serve as an example of practical relevance. It is well known that the overall oxidation rate during soot filter regeneration is a strong function of time, even under isothermal conditions. By combining experimental investigations and detailed numerical simulations we will try to show from first principles if (and to what extent) the change in the overall reaction rate primarily originates from morphological changes of the soot. Therefor the project aims to separate kinetics on the primary soot particles from morphological changes which affect the accessible surface area in the soot structure.Within the project a special reactor will be constructed that allows to oxidize a soot layer stepwise under isothermal conditions. From analysis of exhaust gas, the integral rate of soot conversion can be monitored. At discrete times the oxidation will be interrupted, and the overall BET surface of the partially oxidized soot layer will be measured without dismounting the sample. Thus, we can simultaneously obtain the effective reaction rates and changes of the overall surface.Detailed information of the porous soot structure has recently become assessable by Focused Ion Beam - Scanning Electron Microscope (FIB-SEM) images. This allows to reconstruct the porous medium in microscopic scale with high resolution and provides initial conditions for direct numerical simulations.A detailed 3D Lattice-Boltzmann (LB) model will be developed to simulate the soot morphology change during soot oxidation. While the transport of a multicomponent gas mixture inside pores will be simulated using the discrete form of the Boltzmann kinetic equation, the heterogeneous soot morphology change during soot oxidation will be modeled as new boundary condition. LB method can handle complex geometries efficiently, thus it is well suited to model solid-gas phase transition.We can calculate the effective oxidation rate and compare it to experimental values, and furthermore, FIB-SEM images obtained from soot samples after partial oxidation can be compared to simulation results.Once a detailed model is available, this will serve as a basis for the development of an extended application-oriented model. For deriving anisotropic macroscopic transport parameters asymptotic homogenization will be used. The derived macroscopic model comprises the classical mass and energy balances as well as balance equations for characteristic structural properties such as BET surface.