Steady and transient flamelet modelling of turbulent non-premixed combustion

Abstract

Laminar flamelet models have shown to be useful for the prediction of detailed chemical processes in turbulent combustion. Recently the existence of laminar flamelets in turbulent flow has been questioned. This is because there are several different ways to implement the flamelet concept and the implication of some modelling assumptions are not yet fully understood. Also, most existing flamelet models are based on a 1D and quasi-steady description of the local flame structure. These steady flamelet models are limited in predicting transient or unsteady effects as for example local extinction due to high strain rate with subsequent re-ignition of partially premixed fluid elements. Here the flamelet concept is discussed in view of these questions. It is shown that unsteady effects can be included by solving the transient Peters equations with an initial guess that represents a partially premixed state. Partially premixing is considered by a simple model that is based on a single reaction progress variable and that is similar to the IEM model. The use of the Peters equations is of advantage in comparison to the commonly used counterflow diffusion flame formulation. The results of the steady flamelet model applied to the ETH-Sandia hydrogen jet flame demonstrate that the scalar dissipation rate is superior to the strain rate. The steady and transient flamelet models are applied to the piloted Masri-Bilger Methane-air jet flame. The results show that the behaviour of this flame is only captured by the transient flamelet model and that, therefore, the flamelet concept can be extended if transient effects are considered.