MULTICOMPONENT SPRAY-TURBULENCE INTERACTION

Licentiate thesis, 2018

Blended fuels are gaining importance in automotive industry owing to stringent legislations for emissions. It is important to understand the fuel spray formed as a consequence of injecting blended fuels into the engine (for direct injection engines) and also the influence of fuel spray on combustion properties. Fuel sprays are sought to be understood by formulating and modelling the physical processes involved in its formation and testing the predictions obtained from models using computer simulations. Complementing this procedure, experiments are performed under predefined boundary conditions either in single or multi cylinder engines or in constant volume spray chambers when deeper insight into sprays is required. The experiments are used to validate the models and also report any newly observed physical phenomenon which can be then investigated using the models.

This work presents the computaional and modelling efforts for multicomponent fuel sprays whose behavior is studied in constant volume combustion vessel. Lagrangian-Eulerian framework is followed where the liquid fuel is modelled using Lagrangian approach and the gas phase is modelled using Eulerian approach. The focus of this work is on Lagrangian liquid phase modelling and it's interactions with the gas phase. The spray modelling is done using VSB2 stochastic blob and bubble (VSB2) model which is developed with the aim of minimising tuning parameters by treating spray and it's submodels as one entity. The VSB2 model also removes overshoot or undershoot in predicted quantities by using relaxation equations based on thermodynamic equilibrium. The methods for modelling secondary breakup, evaporation and momentum transfer of liquid droplets are outlined in this work. Specifically computational method for differential evaporation in multicomponent fuel sprays is discussed.

The VSB2 model is validated against experiments performed in constant volume combustion vessels for multicomponent fuel sprays. Differential evaporation was predicted corrrectly by the model within acceptable limits when compared to experiments on component gasoline-diesel fuel blend. Effects of non-ideal vapor liquid equilibrium on multicomponent fuel evaporation of ethanol and iso-octane blend was also studied, and the predictions showed reasonable agreement with experiment. Ethanol was observed to have a strong influence on iso-octane and deviation from ideal behavior was strong for higher ethanol percentage and in these cases ideal vapor liquid equilibrium was seen to predict incorrect results.