2008

Simplified Modeling of Oxidation of Hydrocarbons

Only a few dozen molecular and radical species are included in calculations.

A method of simplified computational modeling of oxidation of hydrocarbons is undergoing development. This is one of several developments needed to enable accurate computational simulation of turbulent, chemically reacting flows. At present, accurate computational simulation of such flows is difficult or impossible in most cases because (1) the numbers of grid points needed for adequate spatial resolution of turbulent flows in realistically complex geometries are beyond the capabilities of typical supercomputers now in use and (2) the combustion of typical hydrocarbons proceeds through decomposition into hundreds of molecular species interacting through thousands of reactions. Hence, the combination of detailed reaction-rate models with the fundamental flow equations yields flow models that are computationally prohibitive. Hence, further, a reduction of at least an order of magnitude in the dimension of reaction kinetics is one of the prerequisites for feasibility of computational simulation of turbulent, chemically reacting flows.

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Some of the Constituent Radicals included in the simplified model are listed with some of their pertinent thermodynamic properties. The listed properties, all in units of kilojoules per mole, are the heat of formation (δh0), the heat of combustion (δhc), and the heat of component-to-free transition (δhf).Other pertinent properties, omitted for the sake of simplicity, are coefficients in an equation for partial molar heat capacity as a function of temperature.
In the present method of simplified modeling, all molecular species involved in the oxidation of hydrocarbons are classified as either light or heavy; heavy molecules are those having 3 or more carbon atoms. The light molecules are not subject to meaningful decomposition, and the heavy molecules are considered to decompose into only 13 specified constituent radicals, a few of which are listed in the table. One constructs a reduced-order model, suitable for use in estimating the release of heat and the evolution of temperature in combustion, from a base comprising the 13 constituent radicals plus a total of 26 other species that include the light molecules and related light free radicals. Then rather than following all possible species through their reaction coordinates, one follows only the reduced set of reaction coordinates of the base.

The behavior of the base was examined in test computational simulations of the combustion of heptane in a stirred reactor at various initial pressures ranging from 0.1 to 6 MPa. Most of the simulations were performed for stoichiometric mixtures; some were performed for fuel/oxygen mole ratios of 1/2 and 2. The following are among the conclusions drawn from the results of these simulations:

  • The release of heat in combustion of heptane is modeled adequately.
  • A simplified, low-dimensional chemistry model depends on an adequate representation of a reduced rate set. The net constituent rate is nearly quasi-steady and can be split into an incubation zone of modest temperature rise followed by a fast-reaction zone of high temperature.
  • The incubation region is characterized by reaction times of the order of milliseconds — similar to diffusion time scales. Therefore, chemistry is expected to be significantly coupled with flow processes during incubation. This coupling gives rise to several issues that must be resolved in further development of a simplified model.
  • In the fast-reaction zone, the coupling between chemistry and flow processes is weak, and combustion is determined primarily by the mixing rate. The temperature profiles in the fast-reaction zone tend to be independent of the details of behavior during incubation.

The development of the model is not yet complete. To close the model system of equations, it will be necessary to determine effective mean source strengths for light molecules and light radicals resulting from decomposition of heavy molecules. The final model will thus focus on reactions of the light species; the necessary rates are expected to be well determined insofar as kinetic interactions among light species prevail.

This work was done by Josette Bellan and Kenneth Harstad of Caltech for NASA’s Jet Propulsion Laboratory.
NPO-44750

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