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

A new stiff ordinary differential equation solver has been devised tha t separates the unknown variables into a fast group and a slow group. The fast variables are solved using the implicit backward-differentiat ion formulas but with a Jacobian of much smaller dimension than that o f the original stiff system. The slow variables are solved using a sim ple explicit Adams-Bashforth scheme. The method, applied to a stiff at mospheric chemical system, yields an accuracy in the solution comparab le to that of the commonly-used LSODE method at a relative tolerance l evel 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-tun ed to optimize its accuracy and execution time. As it is, the method s hould be an excellent candidate for the chemistry solver in air qualit y, combustion, and reactive flow models. (C) 1994 Academic Press, Inc.