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Show COMPUTATION OF ADIABATIC FLAME TEMPERATURES AND OTHER THERMODYNAMIC QUANTITIES Charles K. Westbrook Lawrence Livermore National Laboratory Livermore, California, USA Abst ract The equilibrium final temperature and composition of a given fuel-oxidizer mixture have both theoretical and practical importance for helping to understand the behavior of combustion systems. Both are needed in order to estimate levels of chemical pollutants such as oxides of nitrogen (NO x), and the final temperature is necessary to predict the expected thermal efficiency and heating value of a giv~n combu~tor. Knowledge of the ways that varlOUS radlcal and stable intermediate chemical species vary as operating conditions are changed can also suggest system modifications to improve overall performance. In this paper, the methods available for computing chemical equilibrium conditions such as adiabatic flame temperatures will be reviewed. The dependence of computed values on available thermochemical data bases will be demonstrated. Examples of equilibrium flame temperatures and compositions for mixtures of interest and importance in natural gas combustion will be presented. COMPUTATIONS OF CHEMICAL EQUILIBRIUM have always been important parts of many combustion and chemical engineering studies. In recent years, there have been a number of different developments which have significantly improved the accuracy, reliability, speed, and cost of these computations. In particular, the thermodynamic data bases which are used have been extended and made more accurate, and new computational techniques have been derived which make such calculations much easier and require less time. Some of the most powerful computer programs for computing equilibrium conditions are even available in versions which can execute on now-familiar microcomputers and personal computers. 143 The subject of chemical equilibrium is a va~t and complex scientific discipline, one WhlCh has been examined in a multitude of books and research papers. This brief paper cannot attempt to present any survey of this entire field. However, there are several narrower issues which occur frequently in practical analyses of chemical equilibrium of combustion systems. A situation which often arises is that of needing to predict the final product temperature and chemical composition which should be expected from the oxidation of a given mixture of fuel and oxidizer. In the discussions to follow, the techniques and theory will be illustrated using either methane (CH4) or natural gas as the fuel and molecular oxygen as the oxidizer, but the meth~ds are very general and can directly be applled to other fuels, oxidizers and to multiphase systems. ' In many practical systems, combustion of f~els such as natural gas takes place under' elther constant pressure or effectively constant volume conditions. An example of constant pressure combustion is a common home furnace or gas stove, both of which are open to the atmosphere. In contrast, although the volume of the combustion chamber in an internal combustion engine is not constant throughout the entire engine cycle, the combustion of nearly all of the fuel-air mixture takes place over a ~ery few degrees of crankangle motion, so the lnternal combustion engine can often be treated as a constant volume system. For hydrocarbon fuels, constant volume conditions provide higher product temperatures than constant pressure combustion. Differences in equilibrium temperature and pressure can result in differences in equilibrium species compositions as well. Therefore, analysis of these two conditions, constant pressure and constant volume, can provide information which can be applied to the great majority of combustion environments of practical concern. |