LES-FGM modelling of non-premixed auto-igniting turbulent hydrogen flames including preferential diffusion

Abstract

Tabulated chemistry methods are a well-known strategy to efficiently store the flow thermochemical properties. In particular, the Flamelet-Generated Manifold (FGM) is a widely used technique that generates the database with a small number of control variables. In order to build such a manifold, these coordinates must be monotonic in space and time. However, the high diffusivity of hydrogen can prevent such requisite. There have been many studies that successfully included non-unity Lewis effects in FGM, but mostly in the context of premixed flames. The problem of accounting for differential diffusion in purely non-premixed auto-igniting hydrogen flames still has to be investigated thoroughly. To avoid the non-monotonicity of control variables (the progress variable, in particular), one practical workaround is to perform the tabulation on zero-dimensional (0D) reactors rather than on one-dimensional (1D) flamelets. Various works already implemented and tested such 0D-based manifold, but mainly in the context of spray engines, where most of the composition is lean and information past the flammability limit is not relevant. The present work aims at investigating, for the first time, the applicability of a tabulation based on homogeneous reactors to study auto-igniting turbulent hydrogen jets.

Three different techniques to extrapolate the data beyond the flammability limit are evaluated in 1D simulations and assessed against detailed chemistry results. It is shown that a combined use of homogeneous reactors at the lean side and an extrapolation with 1D flamelets on the richer side is required to capture both chemistry and diffusive effects accurately in pure hydrogen flames. Then, this manifold is coupled to Large-Eddy Simulation (LES) of three-dimensional turbulent temporal evolving planar jets and evaluated against direct numerical simulation with detailed chemistry. Good agreement is found, in terms of both ignition delay and the following steady-state burning process. Further analyses are carried out on statistics and modelling. In particular, the sensitivity of the LES solution to filter width, turbulence-chemistry interaction and multidimensional flame effects is investigated to provide new relevant insights on modelling non-premixed auto-igniting turbulent hydrogen flames.

Novelty and Significance Statement

The novelty of this research is represented by a detailed and systematic numerical study on turbulent non-premixed auto-igniting hydrogen flames by means of tabulated chemistry, including preferential diffusion. There have been works that successfully accounted for non-unity Lewis effects in tabulated chemistry, but mainly in the context of premixed flames. As regards non-premixed flames, few works included preferential diffusion, but did not model any igniting phenomena at all. In order to conduct such work, we rely on a manifold based on homogeneous reactors (HR-FGM). This is the first time that such method is used to study hydrogen combustion. Therefore, the insights on statistics and modelling of turbulent auto-igniting hydrogen jets by means of HR-FGM coupled to LES, and including preferential diffusion, make this work of high relevance.