Three-Dimensional Simulation of Diesel Spray Ignition and Flame Lift-Off Using OpenFOAM and KIVA-3V CFD Codes
Journal article, 2008
Three-dimensional simulations of ignition and combustion of a diesel
spray were conducted. The primary goal of the work was to compare two
different CFD codes: OpenFOAM, an object-oriented C++ based code, and
KIVA-3V. The spray is modelled by the Eulerian-Lagrangian approach in
both codes, with several common submodels. Some important sub-models
implemented include \emph{inter alia} a
Kelvin-Helmholtz/Rayleigh-Taylor (KH/RT) model for spray break-up, an
improved spray collision model, and a Partially Stirred Reactor (PaSR)
model for turbulence-chemistry interaction. Both CFD codes solve the
chemical reaction equations in a fully coupled manner. A cubic-shaped Cartesian
mesh was used in the KIVA-3V simulations, while a polyhedral mesh
including a combination of hexagonal and prism-shaped cells was
constructed for the OpenFOAM computations.
The effects of high EGR and ambient temperature on the ignition and flame
lift-off processes of a diesel spray were investigated. Sandia experiments
conducted in a high-pressure and high-temperature constant-volume vessel were
chosen for the simulations and validations. A single spray was injected into
the vessel, and EGR was mimicked by reducing the oxygen concentration. The
diesel reference fuel (n-heptane) was considered. For the study, a medium-size
mechanism involving 83 species and 338 reactions was employed. The mechanism
was validated using the CHEMKIN II package and the reaction rate constants
were adjusted on the basis of measurements of auto-ignition delays of
n-heptane/air mixtures in shock-tube experiments (with equivalence ratios
ranging from 0.2 to 0.4 at 50 bar, and from 0.5 to 2.0 at 13.5 bar and 41.0
bar), laminar flame speeds (1 atm and 3 atm), and flame structure in
burner-stabilized premixed flames (1 atm).
The simulations demonstrate that both CFD codes are capable of spray ignition
and combustion studies, though both show strong
grid-dependence. The numerical results show that the ignition delay,
flame lift-off and combustion temperature of the spray are strongly
influenced by EGR and ambient gas temperature. These predictions are
in agreement with measurements. Nevertheless, differences are observed
between the results predicted by OpenFOAM and those from KIVA-3V, for
example, the flame predicted by the former is thinner and longer than that by the
latter, which requires further investigation.