Modeling of Diesel Fuel Spray Formation and Combustion in OpenFOAM

Licentiatavhandling, 2012

The formation, ignition, and combustion of fuel sprays are highly complex processes and the available models have various shortcomings. The development and application of multidimensional CFD models, that describe the different phenomena have rapidly increased through the use of commercial and public software (e.g. Star-CD, KIVA, FIRE and OpenFOAM). The general approach to spray modeling is given by the Eulerian-Lagrangian method, where the gas phase is modeled as a continuum and the droplets are tracked in a Lagrangian way. The accuracy and robustness of today’s spray models vary substantially and spray penetration simulations and the levels of spray-generated turbulence are dependent on the discretization. The work presented here deals with the prediction of spray formation and combustion with improved models implemented in the free, open source software package OpenFOAM. The VSB2 spray model was implemented and tested under varying ambient conditions. The design criteria of the model were to be unconditionally robust, have a minimal number of tuning parameters, and be implementable in any CFD software package supporting particle tracking. The main difference between the VSB2 spray model and standard spray models is how the interaction between the liquid fuel and hot gas phase is modeled. In the VSB2 spray model, a ’blob’ is defined, containing differently sized droplets; instead of a parcel containing equally sized droplets. Another feature is the definition of a bubble surrounding the blob. The blob just interacts with the gas phase in the bubble instead of with the gas phase in the whole grid cell. The idea is to reduce grid dependency. Furthermore, equilibrium between the blob and the bubble is ensured, which makes the model very robust. Results of spray penetration simulations are compared with data obtained from experiments done at Chalmers University of Technology and with experimental data published by Siebers and Naber from Sandia National Laboratories. The next step was to update the code with a model of the turbulence-chemistry interaction. The volume reactor fraction model (VRFM), a novel partially stirred reactor (PaSR) model, has been implemented in OpenFOAMr. The model defines a reactor fraction based on the mixture fraction, the chemical progress variable, and their variances instead of defining mixing and chemical time scales. The chemistry is described by a reduced n-heptane mechanism with 36 species involved in 81 reactions. The effect of exhaust gas recirculation (EGR) on n-heptane sprays for Diesel enginelike conditions was studied with the VRFM and results obtained from the calculations are compared to experimental data from the Engine Combustion Network.