CFD Modeling of an Alcohol-Diesel Direct-Injection Dual-Liquid-Fuel Engine using OpenFOAM

Doktorsavhandling, 2021

Legislation for heavy duty combustion engines are becoming more stringent. To keep up with the legislation, new engine technology is required. Furthermore, renewable fuels can also be used together with new engine technology to further reduce emissions.

Computational techniques such as Computational Fluid Dynamics (CFD) provides a powerful and cost effective alternative/complement to traditional engine tests when developing new engine technology. To effectively use CFD for engine development, accurate models for spray formation and chemistry are required. In this work a new direct-injection dual-liquid-fuel engine concept using methanol and diesel was investigated using CFD. The first part of this thesis involved validation and development of our in-house spray model, VSB2. The VSB2 is a Eulerian-Lagrangian model with a minimal amount of tuning parameter. It models the impact of secondary breakup by including a distribution of droplet sizes inside each computational blob. The model was first validated for use with alcohol fuels, where the turbulence parameters were also tuned together with experimental data for later use in engine
simulations. It was shown that the VSB2 spray model could accurately predict the spray formation of an alcohol spray.

Furthermore, the spray model was developed further by implementing a new break up treatment that addressed some conceptual flaws in how the model handles the momentum of a computational blob with different droplet sizes. Previously, each computational blob contained the momentum of all droplet sizes, which had the consequence that smaller droplets would have the same momentum as larger droplets. This was addressed by creating child blobs from the stable droplet sizes. It was shown that this had the effect of enhancing the evaporation and dispersion at lower ambient gas temperatures.

The last part of this thesis was to create a CFD model in OpenFOAM that could model a direct-injection dual-liquid-fuel engine. A tabulated chemistry solver based on the well stirred reactor approach called LOGE-CPV was used to model the chemistry. It was shown that the model could accurately predict global parameters such as in cylinder pressure and rate of heat release of the system, but pollutant formation was predicted poorly when compared to experiments. It was concluded that a turbulent combustion model would be needed to accurately predict pollutant formation. The ignition process was also studied, showing that the pilot diesel could easily ignite the methanol as long as the pilot flame would reach the center of the combustion chamber. It was also shown that further offsetting the pilot injector caused the combustion to become unstable as it was difficult to ignite all
the sprays from the main injector.