On the development and application of a droplet flamelet-generated manifold for use in two-phase turbulent combustion simulations

A droplet flamelet-generated manifold (DFGM) is developed to account for the effects of finite-rate chemistry of individual droplet combustion during simulations of a turbulent combustion environment. A spherically symmetric droplet model is developed for methanol using the hydrocarbon and nitrogen kinetic mechanisms developed at UC San Diego to account for chemical reaction rates. The inclusion of finite-rate chemistry allows for the capturing of the transition from diffusion to kinetically controlled combustion as the droplet diameter decreases. The droplet model is used to create a DFGM by successively solving the 1D flame equations at varying drop sizes, where the source terms for energy, mixture fraction (Z), and progress variable are cataloged as a function of Z and droplet diameter. A unique coupling of the spherical and planar FGMs is developed and is used to account for individual and group combustion processes simultaneously. Three combustion models are considered when modeling a methanol spray flame; (i) an evaporative model coupled with an unsteady planar flamelet-generated manifold (UFGM), (ii) using only the DFGM method, and (iii) the DFGM coupled with the planar UFGM. The models are compared against one another as well as experimental data for a methanol spray flame with an annular air jet. The DFGM model is shown to agree well with burn rates and normalized flame radii from individual droplet burning experiments. Good overall agreement is observed between the experimental data and the DFGM and coupled UFGM models for temperature, particle size distribution, and OH concentration.