A combined numerical and experimental study of the 3D tumble structure and piston boundary layer development during the intake stroke of a gasoline engine

Due to its positive effect on flame propagation in the case of a well-defined breakdown, the formation of a large-scale tumble motion is an important goal in engine development. Cycle-to-cycle variations (CCV) in the tumble position and strength however lead to a fluctuating tumble breakdown in space and time and therefore to combustion variations, indicated by CCV of the peak pressure. This work aims at a detailed investigation of the large-scale tumble motion and its interaction with the piston boundary layer during the intake stroke in a state-of-the-art gasoline engine. To allow the validation of the flow near the piston surface obtained by simulation, a new measurement technique called “Flying PIV” is applied. A detailed comparison between experimental and simulation results is carried out as well as an analysis of the obtained flow field. The large-scale tumble motion is investigated based on numerical data of multiple highly resolved intake strokes obtained using scale-resolving simulations. A method to detect the tumble center position within a 3D flow field, as an extension of previously developed 2D and 3D algorithms, is presented and applied. It is then used to investigate the phase-averaged tumble structure, its characteristics in terms of angular velocity and the CCV between the individual intake strokes. Finally, an analysis is presented of the piston boundary layer and how it is influenced by the tumble motion during the final phase of the intake stroke

Flame structure analysis and flamelet progress variable modelling of strained coal flames

Strained two-phase pulverised coal flames in a counterflow configuration are investigated numerically. Three operating conditions with different coal-to-primary-air ratios and inlet velocities were evaluated in order to establish different flame regimes. At first, the two-phase flow of the fully resolved reference cases is calculated solving the transport equation for the species and directly evaluating the reaction rates. Different flame structures are identified using the heat release rate and the chemical explosive mode as markers, showing that complex structures with a combination of lean premixed and non-premixed flames can be observed in strained counterflow coal flames. In addition to the fully resolved simulation, the suitability of the Flamelet-Progress Variable (FPV) model is investigated. Both premixed and non-premixed tables are employed. At first, the suitability of the look-up tables is evaluated by means of an a priori analysis, using the fully resolved simulations as reference solutions, showing that the non-premixed flamelet table correctly predicts the structure of the strained coal flames, while the premixed table shows sensible deviations in terms of temperature and species, especially at rich conditions. Finally, the a posteriori analysis shows that the fully coupled FPV model with a non-premixed flamelet look-up table can accurately predict strained coal flames

An abstraction layer for efficient memory management of tabulated chemistry and flamelet solutions

A large number of methods for simulating reactive flows exist, some of them, for example, directly use detailed chemical kinetics or use precomputed and tabulated flame solutions. Both approaches couple the research fields computational fluid dynamics and chemistry tightly together using either an online or offline approach to solve the chemistry domain. The offline approach usually involves a method of generating databases or so-called Lookup-Tables (LUTs). As these LUTs are extended to not only contain material properties but interactions between chemistry and turbulent flow, the number of parameters and thus dimensions increases. Given a reasonable discretisation, file sizes can increase drastically. The main goal of this work is to provide methods that handle large database files efficiently. A Memory Abstraction Layer (MAL) has been developed that handles requested LUT entries efficiently by splitting the database file into several smaller blocks. It keeps the total memory usage at a minimum using thin allocation methods and compression to minimise filesystem operations. The MAL has been evaluated using three different test cases. The first rather generic one is a sequential reading operation on an LUT to evaluate the runtime behaviour as well as the memory consumption of the MAL. The second test case is a simulation of a non-premixed turbulent flame, the so-called HM1 flame, which is a well-known test case in the turbulent combustion community. The third test case is a simulation of a non-premixed laminar flame as described by McEnally in 1996 and Bennett in 2000. Using the previously developed solver âflameletFoamâ in conjunction with the MAL, memory consumption and the performance penalty introduced were studied. The total memory used while running a parallel simulation was reduced significantly while the CPU time overhead associated with the MAL remained low

A Computationally Efficient Implementation of Tabulated Combustion Chemistry based on Polynomials and Automatic Source Code Generation

The simulation of turbulent combustion is a multiphysics and multiscale problem, in which two different domains - fluid mechanics and chemistry - have to be coupled. One solution is a CFD-based simulation framework that performs lookups on tabulated chemistry using flamelets. The tables can become very large when the resolution is increased and modelling parameters and solution values are added. This makes dynamic memory management and its runtime requirements a crucial issue in these simulations. A novel approach for the efficient memory management of tabulated chemistry at reduced computational cost is developed in this study. The original interpolation-focused database is converted into a polynomial description, which is stored in a shared library as a set of functions. This step enables automatic compiler optimization techniques to achieve minimal data movement and the best use of modern computer architecture. The performance and properties of the method are evaluated in a generic test case and a fully coupled flame simulation

Evaluation of radiation modeling approaches for non-premixed flamelets considering a laminar methane air flame

The scope of this investigation is to evaluate different radiation modeling approaches for both Lagrangian and Eulerian flamelet models by comparing them to fully resolved simulation data of a non-premixed flame. The numerical investigations are performed for a well established laminar methane diffusion flame 1. The available experimental data is used to validate the CFD results, which clearly show that radiation must be considered in this flame to accurately describe the flame structure. Based on the validated CFD results the main focus is to analyze the applicability of radiation modeling approaches within the flamelet framework for unity Lewis number and differential diffusion. An unsteady Lagrangian flamelet model with direct integration of the radiation source term as well as an enthalpy defect formulation for steady and unsteady flamelet calculations are considered. Several model variants are introduced and discussed and the corresponding time scales for mixing, radiation and chemistry are analyzed. Based on the Lagrangian flamelet time and the enthalpy defect, both postprocessed from the CFD solution, flamelet calculations are carried out and detailed comparisons to the CFD simulation results are performed for the temperature and several species along the axis and in several radial slices. The results are finally used to evaluate the different approaches concerning their applicability and accuracy for use in coupled CFD-flamelet simulations