Modeling Spray C and Spray D with FGM within the framework of RANS and LES

In this study, two different diesel-like igniting sprays are investigated: Engine
Combustion Network (ECN) Spray C and D. In particular, this study focuses on
the respective performances of the RANS and LES models to predict a turbulent,
igniting spray using the OpenFOAM platform. The breakup model, discretization
schemes, and case setups, including the combustion model, are kept constant
in order to mitigate any potential effect on the simulation apart from intrinsic
differences due to turbulence modeling. A classic κ-ε model is applied for the
RANS approach, while a dynamic structure model is used to solve the
momentum equation in the LES approach. The κ-ε model constants are
tuned to obtain a suitable prediction of inert experiments. Both approaches
exhibit a reasonable agreement with the inert experiments regarding the global
spray characteristics, the liquid length, and the vapor penetration. However, the
transient local properties, including the spatial distribution of mixture fraction
variance and the species distributions, are not identical. For reacting conditions,
the Flamelet Generate Manifold (FGM) model is adopted in both the LES and
RANS simulations, using several enthalpy levels as the fourth dimension in the
tabulation to account for local heat loss. The results show good agreement
between the two turbulence models, in terms of liquid length, vapor
penetration, and lift-off length, while a short ignition delay is registered for
both sprays and turbulence frameworks. Turbulence–chemistry interaction
(TCI) is considered by applying a presumed probability density function (β-
PDF) to the mixture fraction, and is found to play a key role in the reproduction
of species distribution in the domain.