  The impact of chemical reactions on the robustness and accuracy of
projection-based Reduced-Order Models (ROMs) of fluid flows is investigated.
Both Galerkin and Least-Squares Petrov Galerkin ROMs are shown to be less
robust in reacting flows as compared to non-reacting flows. In particular,
reacting flow ROMs show a strong sensitivity to \st{the} resolution and are
often unstable. To identify the main underlying causes, a representative
problem that contains the essential physics encountered in typical combustion
dynamics problems is chosen. Comparisons with non-reacting solutions are used
to assess the impact of reactions. Investigations are focused on three
potential areas of significance: 1) preservation of conservation laws; 2) loss
of dissipation; and 3) existence of unphysical local phenomena. Results
indicate that conservation is relatively well-controlled and the global
dissipation in the ROMs is actually larger than that in the underlying CFD
solutions. Spurious local phenomena are, however, highly deleterious.
Specifically, the steep temperature gradients that characterize combustion can
lead to oscillations in local temperatures even in the absence of reactions.
Representative calculations with physics-based temperature constraints verify
that eliminating such excursions results in considerable improvement in both
stability and future-state prediction capability.
