Turbulence Impinging on a Premixed Flame
Although the interaction of an isolated vortex pair with a premixed flame provides important data on the response of the flame to time-dependent stretch and strain, it does not exhibit the full complexity of turbulent flame interaction. The underlying flow field for turbulent flame interactions is considerably more complex than the idealized setting of isolated vortices. AMR methods are a useful tool to accomodate the resolution requirements of a thin flame interacting with turbulent flow. The examples described below illustrate the application of AMR to turbulent premixed hydrogen combustion.
In the first example, we generated a two-dimensional region of periodic isotropic fluid turbulence, and fed the field into the inlet stream of an initially flat horizontally periodic two-dimensional premixed hydrogen flame. This case, while not representing a real laboratory flame, was useful for validating our discretization algorithms, and for assessing a potential chemistry acceleration technique, called PRISM (PRISM, its application within our model, and some of its scaling properties are discussed in a paper published in the Combustion Symposium. Turbulence in the reactant stream induces a dramatic perturbation to the flat flame and modulates the chemistry within the flame. The study concerned the dimensionality of the composition space accessed numerically during the simulation, and how it was affected by the turbulence strength. The two images below, taken at a late time during the interaction, indicate qualitatively the influence that the inlet field has on the flame.
A far more realistic model of flame/turbulence interaction is possible with three-dimensional simulations. The initial objective of our three-dimensional study was to determine the effect of the turbulence on the average fuel consumption rate. Here, we precompute a field of isotropic decaying turbulence in the reactant stream. This field is then allowed to convect into an initially steady premixed hydrogen flame. The complete hydrogen mechanism is simulated (9 species, 27 reactions). The adaptive algorithm is setup to track the flame front and regions of strong vorticity, locally refining the base 32x32x64 grid by up to a factor of four . The two figures below indicate development of the perturbed flame surface, as the turbulence flowing in from the lower boundary begins to impinge on the initially flat hydrogen flame. The work is described in more detail in an article presented at the first MIT conference on Computational Fluid and Solid Mechanics.