Analytic solutions for gas-phase chemical mechanism compression

A mathematical technique for combining unoxygenated hydrocarbons in gas-phase chemical reaction mechanisms is presented. Unlike earlier methods of combining hydrocarbons, the new method directly alters the differential equations for the species of the chemical mechanism. The new system of differential equations is mathematically equivalent to the original system, but contains fewer variables, representing a considerable decrease in computer memory. Processing time requirements were greatly decreased for scalar compiler tests, and relatively unchanged for vector processor tests. Numerical integrations were performed using an uncompressed reaction set, a reaction set condensed using the new method, and a reaction set parameterized using an older method (the “integrated reactivity weighting” of the RADM mechanism). The new method produces results nearly identical to the original system of equations, while integrated reactivity weighting can make large errors. These include large underpredictions of ozone concentrations in low NOx environments. Two examples of the new method in which eight hydrocarbons are replaced by four and then by a single condensed variable are given. The maximum error between the uncompressed and compressed systems over a 2 d simulation (for species not approaching the low concentration limit) was 1.8 × 10−3%. The tests imply that current methods of lumping hydrocarbons may underestimate ozone production by as much as a factor of two, depending on the initial conditions. In contrast, the new method allows large numbers of hydrocarbons to be combined into a single variable, with negligible losses in accuracy.