PDF simulations of turbulent combustion incorporating detailed chemistry (original) (raw)
Related papers
Computationally efficient implementation of combustion chemistry usingin situadaptive tabulation
Combustion Theory and Modelling, 1997
A computational technique is described and demonstrated that can decrease by three orders of magnitude the computer time required to treat detailed chemistry in reactive flow calculations. The method is based on the in situ adaptive tabulation (ISAT) of the accessed region of the composition space-the adaptation being to control the tabulation errors. Test calculations are performed for non-premixed methane-air combustion in a statisticallyhomogeneous turbulent reactor, using a kinetic mechanism with 16 species and 41 reactions. The results show excellent control of the tabulation errors with respect to a specified error tolerance; and a speed-up factor of about 1000 is obtained compared to the direct approach of numerically integrating the reaction equations. In the context of PDF methods, the ISAT technique makes feasible the use of detailed kinetic mechanisms in calculations of turbulent combustion. The technique can also be used with reduced mechanisms, and in other approaches for calculating reactive flows (e.g. finite difference methods).
Combustion Theory and Modelling, 2007
A computational fluid dynamics (CFD) tool for performing turbulent combustion simulations that require finite-rate chemistry is developed and tested by modelling a series of bluff-body stabilized flames that exhibit different levels of finite-rate chemistry effects ranging from near equilibrium to near global extinction. The new modelling tool is based on the multi-environment probability density function (MEPDF) methodology and combines the following: the direct quadrature method of moments (DQMOM); the interaction-by-exchange-with-the ...
Proceedings of the Combustion Institute, 2007
A hybrid large-eddy simulation/filtered-density function (LES-FDF) methodology is formulated for simulating variable density turbulent reactive flows. An indirect feedback mechanism coupled with a consistency measure based on redundant density fields contained in the different solvers is used to construct a robust algorithm. Using this novel scheme, a partially premixed methane/air flame is simulated. To describe transport in composition space, a 16-species reduced chemistry mechanism is used along with the interaction-by-exchange with the mean (IEM) model. For the micro-mixing model, typically a constant ratio of scalar to mechanical timescale is assumed. This parameter can have substantial variations and can strongly influence the combustion process. Here, a dynamic timescale model is used to prescribe the mixing timescale , which eliminates the timescale ratio as a model constant. Two different flame configurations, namely, Sandia flames D and E are studied. Comparison of simulated radial profiles with experimental data show good agreement for both flames. The LES-FDF simulations accurately predict the increased extinction near the inlet and re-ignition further downstream. The conditional mean profiles show good agreement with experimental data for both flames.
Computational Frameworks for Advanced Combustion Simulations
Fluid Mechanics and Its Applications, 2011
Computational frameworks can significantly assist in the construction, extension and maintenance of simulation codes. As the nature of problems addressed by computational means has grown in complexity, such frameworks have evolved to incorporate a commensurate degree of sophistication, both in terms of the numerical algorithms that they accommodate as well as the software architectural discipline they impose on their users. In this chapter, we discuss a component framework, the Common Component Architecture (CCA), for developing scientific software, and describe how it has been used to develop a toolkit for simulating reacting flows. In particular, we will discuss why a component architecture was chosen and the philosophy behind the particular software design. Using statistics drawn from the toolkit, we will analyze the code structure and investigate to what degree the aims of the software design were actually realized. We will explore how CCA was employed to design a high-order simulation code on block-structured adaptive meshes, as well as a simulation capacity for adaptive stiffness reduction in detailed chemical models. We conclude the chapter with two reacting flow studies performed using the above-mentioned computational capabilities.
Coupling tabulated chemistry with Large Eddy Simulation of turbulent reactive flows
Comptes Rendus Mécanique, 2009
A new modeling strategy is developed to introduce tabulated chemistry methods in LES of turbulent premixed combustion. The objective is to recover the correct laminar flame propagation speed of the filtered flame front when subgrid scale turbulence vanishes. The filtered flame structure is mapped by 1-D filtered laminar premixed flames. Closure of the filtered progress variable and the energy balance equations are carefully addressed. The methodology is applied to 1-D and 2-D filtered laminar flames. These computations show the capability of the model to recover the laminar flame speed and the correct chemical structure when the flame wrinkling is completely resolved. The model is then extended to turbulent combustion regimes by introducing subgrid scale wrinkling effects on the flame front propagation. Finally, LES of a 3-D turbulent premixed flame is performed.
Proceedings of the Combustion Institute, 2020
The use of the Eulerian Stochastic Fields (ESF) method to model the sub-grid turbulence-chemistry interaction (TCI) in the LES context can be computationally expensive if detailed chemistry mechanisms are involved. This work aims to assess whether it is possible to neglect the modelling of the TCI on sufficiently refined meshes while using finite rate chemistry, provided that at least 80 % of the turbulent kinetic energy scales are resolved. Turbulent non-premixed methane-air flames showing a moderate degree of local extinction are selected as benchmark. Results obtained for the Sandia flame E with and without transporting the ESF on three different meshes are discussed. Sensible deviations are visible on the fuel-rich side from section x/D = 30, by reducing the grid refinement. The influence of three finite rate chemistry solvers is further investigated on flame D, without the sub-grid scale chemistry model. All simulations are in good agreement with the experimental data and show a weak dependence on the chemistry involved. A trade-off assessment between computational time and accuracy is provided, in order to extend the validation to a more severe extinction regime.
Using detailed mechanisms to include chemical kinetics in computational fluid dynamics simulations is required for many combustion applications, yet the resulting computational cost is often extremely prohib- itive. In order to reduce the resources dedicated to this stage, we investigated the coupling of the dynamic adaptive chemistry (DAC) reduction scheme with the in situ adaptive tabulation (ISAT) algorithm. This paper describes the tabulation of dynamic adaptive chemistry (TDAC) method which takes advantage of both ISAT and DAC to reduce the impact of the mesh and the oxidation mechanism on the computa- tional cost, particularly for unsteady applications like internal combustion engines. In the context of homo- geneous charge compression ignition (HCCI), we performed simulations on simplified 2D cases using various n-heptane mechanisms and on a real case mesh using a detailed 857-species iso-octane mechanism. Compared to the direct integration of the combustion reactions, results are in very good agreements and a speed-up factor above 300 is obtained. This is significantly better than what was reported for ISAT and DAC which illustrates the synergy of the two methods. In addition, an experimental validation has also been performed with low load HCCI data. Accordingly, the TDAC method is a significant improvement for the computation of the combustion chemistry in engine simulations and allows the use of detailed mechanisms with practical case meshes in simulations that are inconceivable using direct integration.
Combustion and Flame, 2005
A consistent hybrid large-eddy simulation/filtered-density-function approach (LES-FDF) is formulated for variable-density low-Mach-number flows. The LES-FDF approach has been proposed as a suitable method for finite-rate-chemistry-based predictive modeling of turbulent reactive flows. Due to the large computational grid associated with LES, use of Lagrangian schemes is numerically expensive. In this work, a highly efficient parallel Lagrangian implementation is used for the simulation of a nonpremixed flame. This bluff-body-stabilized flame is characterized by complex flow fields that interact strongly with the combustion mechanism. A LES grid size of 1 million computational cells and roughly 15 million notional particles is used to simulate a time-accurate variable-density flow. The hybrid approach predicts the time-averaged velocity and root mean square (RMS) velocity components quite accurately. Species profiles including hydroxyl radical compare well with experimental data. Consistency and accuracy are established by comparing particle and Eulerian density, mixture fraction, and RMS mixture fraction fields. Scalar FDFs at select locations are shown to be well approximated by the presumed beta function used in typical combustion LES.