A direct comparison of pair-exchange and IEM models in premixed combustion

Abstract

Two pair-exchange mixing models, viz., the original Curl model and a modification thereof, are compared with the “Interaction-by-Exchange-with-the-Mean” (IEM) model, in the context of homogeneous combustion. The IEM model is attractive because it permits highly “parallelizable” computation, but the consequences of certain peculiarities—such as determinism and the shape-preserving relaxation of the initial pdf of a conserved scalar—need to be examined in the context of combustion. A numerical simulation of a partially stirred reactor (PaSR) is used to directly compare the three models, without the additional errors that contaminate comparisons made in simulations of flowfields. The fuel is 50%CO/50%H 2 (by vol.). The kinetic scheme consists of 11 species and 23 reactions. The stoichiometry of the premixed inflow leads to a PSR temperature of 1740 K, but to blowout in a PFR. The PaSR mixing frequency was varied in the range 10 Hz to 10 4 Hz, by factors of √10. The pair-exchange models predict blowout earlier than does the IEM model. Means computed from the IEM model are less noisy, a consequence of the determinism inherent in the IEM model. Despite these differences, the pdfs and scatterplots of temperature, CO, OH, and O—selected because of their intrinsic importance as well as their influence on NO x emissions—are very similar between the three models. The similarity increases with the mixing frequency, which is significant given that practical (e.g., gas-turbine) combustors operate at high mass-loadings and therefore necessarily at high mixing rates.