Toward accommodating realistic fuel chemistry in large-scale computations

Publisher Summary Chemical mechanisms have been employed in hydrocarbon combustion as a means of understanding the underlying phenomenology of the combustion process in terms of the elementary reactions of individual species. This chapter provides an introduction to most of the mathematical methods that have been used for the construction, investigation, and reduction of combustion mechanisms. The use of algebraic manipulation in techniques, such as the quasi-steady-state approximation (QSSA) and lumping, make the production of a reduced mechanism essential and make subsequent calculations as simple as possible. Computational singular perturbation (CSP) is an alternative to the rate-of-production and sensitivity methods for mechanism reduction and provides an automatic selection of the important reactions as well as time-scale analysis. The simplest and most widely used technique involving the separation of time scales is the QSSA; however, a possible limitation is that it may not provide the minimum low-order system. Chemical lumping can prove very useful in areas, such as the combustion of hydrocarbon mixtures or soot formation. Several programs are available for the investigation and reduction of combustion mechanisms, including MECHMOD, a code for the automatic modification of CHEMKIN format combustion mechanisms, and KINALC, which is an almost automatic program for the investigation and reduction of gas-phase reaction mechanisms. KINALC is a postprocessor to CHEMKIN-based simulation packages SENKIN, PREMIX, OPPDIF, RUN1DL, PSR, SHOCK and EQLIB. Because models in combustion are expected to cover a wide range of conditions, it is natural to expect that a different approach might be used for different cases.

Chapter 4 Mathematical tools for the construction, investigation and reduction of combustion mechanisms

An algorithm for the construction of global reduced mechanisms with CSP data

In this paper, the state of combustion is evaluated and its prospect as a vibrant branch of science is assessed. It is first demonstrated that much progress has been made on the unifying concepts in the theoretical foundation of some elemental combustion phenomena, primarily those involving laminar flames, and that investigations are being conducted through the strong interplay between theory, experiment, and computation, and between physical and chemical interpretations. It is then noted that, being aided by the rapid advance in computational capability, combustion has entered the era of quantitative predictability, especially in the description of chemical kinetics. The needs for comprehensiveness of detailed reaction mechanisms and methodologies for facilitated computation are emphasized, and several facilitating strategies are presented. The essential role of combustion as a major technology driver and in emerging scientific disciplines is then discussed, and the importance of improving the efficiency of the research enterprise and strengthening the education program is emphasized. It is concluded that combustion has much to offer in terms of technological relevance and intellectual challenge, and as such will remain as a robust scientific discipline for years to come.

Combustion at a crossroads: Status and prospects

A piloted turbulent natural-gas diffusion flame is investigated numerically using a 2D elliptic Monte Carlo algorithm to solve for the joint probability density function (PDF) of velocity and composition. Results from simulations are compared to detailed experimental data: measurements of temperature statistics, data on mean velocity and turbulence characteristics and data on OH. Conserved-scalar/constrained-equilibrium chemistry calculations were performed using three different models for scalar micro-mixing: the interaction by exchange with the mean (IEM) model, a coalescence/dispersion (C/D) model and a mapping closure model. All three models yield good agreement with the experimental data for the mean temperature. Temperature standard deviation and PDF shapes are generally predicted well by the C/D and mapping closure models, whereas the IEM model gives qualitatively incorrect results in parts of the domain. It is concluded that the choice of micro-mixing model can have a strong influence on the quali...

Monte Carlo PDF modelling of a turbulent natural-gas diffusion flame

Simplified chemical kinetic schemes are a crucial prerequisite for the simulation of complex three-dimensional turbulent flows, and various methods for the generation of reduced mechanisms have been developed in the past. The method of intrinsic low-dimensional manifolds (ILDM), e.g., provides a mathematical tool for the automatic simplification of chemical kinetics, but one problem of this method is the fact that the information which comes out of the mechanism reduction procedure has to be stored for subsequent use in reacting-flow calculations. In most cases tabulation procedures are used which store the relevant data (such as reduced reaction rates) in terms of the reaction progress variables, followed by table look-up during the reacting-flow calculations. This can result in huge amounts of storage needed for the multi-dimensional tabulation. In order to overcome this problem a storage scheme is presented which is based on orthogonal polynomials. Instead of the use of small tabulation cells and local mesh refinement, the thermochemical state space is divided into a small number of coarse cells. Within these coarse cells polynomial approximations are used instead of frequently used multi-linear interpolation. This leads to a considerable decrease of needed storage. The hydrogen-oxygen system is considered as an example. Even for this small chemical system, a decrease of the needed storage requirement by a factor of 100 is obtained.

/pdf/an-efficient-storage-scheme-for-reduced-chemical-kinetics-2x6fauw00h.pdf

An Efficient Storage Scheme for Reduced Chemical Kinetics Based on Orthogonal Polynomials

The method of intrinsic low-dimensional manifolds to simplify chemical kinetics is applied to laminar methane-air flames. The procedure is based on a mathematical analysis of the reaction system. Neither steady state assumptions for some species, nor partial equilibrium for reactions have to be specified explicitly. The only requirements to the scheme are a detailed reaction mechanism and the number of degrees of freedom desired for the simplified scheme. All necessary information on the ther-mechanical state (species concentrations, temperature, density, etc.) is then given as function of a small number of reaction progress variables, associated with the degrees of freedom. Therefore, less equations (for two or three reaction progress variables instead of for 34 species in case of CH4) have to be solved, thus drastically reducing the computational effort compared to calculations using detailed chemistry. Subsequent use of a tabulation procedure, where all information is stored, guarantees an eff...

Simulation of Laminar Methane-Air Flames using Automatically Simplified Chemical Kinetics

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An Efficient Storage Scheme for Reduced Chemical Kinetics Based on Orthogonal Polynomials

Simulation of Laminar Methane-Air Flames using Automatically Simplified Chemical Kinetics