Positivity issues in adaptive solutions of detailed chemical schemes for engine simulations

Reliable and efficient solution of chemical kinetics is one of the main computational kernels in engine simulations. The chemical schemes are characterized by high degrees of stiffness, due to the very different reaction rates, and include intermediate species with low density, whose accurate solution is needed to well understand the combustion process. In this context, the non-negativity property of numerical methods for ordinary differential equations is crucial to avoid spurious numerical instabilities and very high accuracy requests. In this work we discuss results obtained by using a recent version of the VODE package, which allows to impose non-negativity constraints on the solution, in simulations of combustion when a moderate size chemical scheme is used. The impact of positivity requirements on the total mass preservation and the computational cost is analyzed for a classic model problem for which experimental data are available for validation.