Chemical Kinetics in the N,atidnal Combustion Code

The National Combustion Code (NCC) is intended as a tool to enable the modeling of a wide variety of reacting flow problems. To this end, a variety of different models for the chemical reaction and turbulence/chemistry interaction have been included in the code. seven variables: Pressure (P), Velocities (U, V and W), Turbulence quantities (K and E) and Enthalpy (H). To function as a combustion code, several other calculations must be performed. In this paper, the different NCC models are described, along with their strengths and weaknesses. Firstly, the transport equations for the species must be calculated. The number of these equations can range from one upward, though currently NCC sets an upper limit of 12 species. Simply in terms of transport equations, it can be seen that for 12 species, the code now has 19 equations to solve, compared to 7 for just flow. Introduction Two of the important components of a combustion code are how the chemistry is modeled and how the turbulence-chemistry interaction is modeled. The degree of complexity of these models are balanced against the amount of computational resources available to the user. In the National Combustion Code (NCC) the flow solver calculates the transport equations for The second set of calculations to be performed are the species source terms, or rate equation evaluations. These equations are generally a set of very stiff, highly non-linear expressions involving exponents, powers and log evaluations. Because of this, rate evaluations are an espensive part of combustion calculations and much time and effort is spent in simplifying and optimizing these expressions. * Senior Research Associate, Member AIAA t Senior Researcher, Member AIAA $ Engineering Specialist, Associate AIAA Copyright 01999 by the American Institute of Aeronautics and Astronautics, Inc. No copyright is asserted in the United States under Title 17, U.S. Code. The U.S. Government has a royalty-free licence to exercise all rights under the copyright claimed herein for Governmental Purposes. All other rights are reserved by the copyright owner. Lastly, because of the exothermic nature of the combustion process, there is a significant variation of the fluid thermodynamic and transport properties which must be accounted for. The properties of individual species are most commonly stored as polynomial functions of temperature i, but models for the properties of mixtures must be included for any case with more than one species present. These models also involve exponential and power evaluations, and so consume significant CPU resources. American Institute of Aeronautics and Astronautics (c)2000 American Institute of Aeronautics & Astronautics or published with permission of author(s) and/or author(s)’ sponsoring.organization. In addition to the combustion, additional models also add to the computational load, including the modeling of the interaction of turbulence and chemistry. Two of these models involve the use of probability density functions (PDFs) which result in the need to evaluate the reaction rates many additional times in the code execution. In this paper, the models for the chemistry source terms and the models used for the turbulencechemistry interaction are described. An overview of each model ,is given, as well as the advantages and limitations of the model, combined with the potential applicability to various combustion prob: lems. Combustion Chemistry