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Numerical modelling of multiphase flow in combustion of liquid fuel

The focus of this work is on an integrated simulation approach, which can be applied in numerical simulations of turbulent multiphase droplet flow in practical combustion systems, adopting methods for simulation of dense and dispersed spray in conjunction with combustion and nitrogen emission formation. The first objective was to establish the validated Eulerian multiphase spray modelling approach, which can be applied with confidence in high pressure diesel spray simulations, particularly in dense spray regions. The Eulerian multiphase spray modelling concept, using an approach with fixed droplet size classes, was applied in liquid spray simulations. Several simulations of high pressure diesel injections, combined with different chamber pressures, were carried out and compared with the experimental data. The suitable validated Eulerian multiphase spray method was then coupled with the classic the Lagrangian Discrete Droplet Model (DDM), in conjunction with the classic combustion model and the validated nitrogen oxides (NOx) chemical reaction mechanisms. The NOx reaction mechanisms were investigated and implemented into the CFD code FIRE. The nitrogen scheme was limited to sufficiently few homogeneous reactions to allow effective coupling with the turbulent mixing process. The effects of the turbulent fluctuations on the reaction rates when predicting NOx concentrations were modelled by using the presumed Probability Density Function (PDF) approach. The model was applied to the turbulent non-premixed jet diffusion flame (Sandia flame D) and the nitrogen predictions were compared to the results obtained by the Steady Laminar Flamelet Model (SLFM) and to the experimental data. Furthermore, to demonstrate the capability of the coupling concept, an integrated simulation approach was applied for calculation of the real internal combustion engine, which is particularly challenging for such modelling. Two different CFD simulations, the Eulerian multiphase spray simulation and the single-phase engine simulation (DDM spray in conjunction with the combustion and nitrogen emission models), were coupled and performed simultaneously to take advantage of the capabilities inherent in both simulations.

Computational fluid dynamics; multiphase flow; nitrogen oxides; Eulerian multiphase model; discrete droplet model; diesel spray

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