AN IMPLICIT-EXPLICIT HYBRID SOLVER FOR A SYSTEM OF STIFF KINETIC-EQUATIONS

A new stiff ordinary differential equation solver has been devised that separates the unknown variables into a fast group and a slow group. The fast variables are solved using the implicit backward-differentiation formulas but with a Jacobian of much smaller dimension than that of the original stiff system. The slow variables are solved using a simple explicit Adams-Bashforth scheme. The method, applied to a stiff atmospheric chemical system, yields an accuracy in the solution comparable to that of the commonly-used LSODE method at a relative tolerance level of 10(-3) and an absolute tolerance level of 10(-7) ppm, with one-third the execution time of LSODE. The method can be further fine-tuned to optimize its accuracy and execution time. As it is, the method should be an excellent candidate for the chemistry solver in air quality, combustion, and reactive flow models. (C) 1994 Academic Press, Inc.