Incorporating unsteady flow-effects beyond the extinction limit in flamelet-generated manifolds

To study whether the effect of local flow fluctuations can be captured by the flamelet-generated manifold (FGM) approach, especially near the extinction limit, both steady and unsteady non-premixed flamelet simulations with a detailed chemistry model are studied. Two different FGM are constructed in the vicinity and beyond the steady extinction limit. The first manifold is a 2D FGM that is primarily based on steady flamelet simulations, which means it is bounded by the steady extinction limit. Therefore an unsteady extinguishing flamelet simulation is used as a natural continuation of the 2D FGM. The second manifold is derived from a whole series of unsteady flamelets that are subjected to sinusoidally varying strain-rates. It is shown that these unsteady flamelet simulations form a 3D manifold in composition space, which is parameterized with c1, c2 and c3. Furthermore, this 3D FGM accurately predicts a broad range of the strain-rate parameters. Both manifolds are applied to simulate flames that are subjected to sinusoidally varying strain-rate. The results are compared to an unsteady simulation with a detailed chemistry model. Both local observables as well as flame-surface area properties are represented well with both manifolds. However, for species that are related to the slowest time-scales (e.g. NOx) it is shown that a 3D manifold may result in less accurate predictions, and more controlling variables may be needed. It is shown that generating a 4D manifold is relatively easy and that the application of such a 4D manifold may lead to a significant improvement in the predictions of species related to the slowest time-scales. © 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.