A stochastic approach towards large eddy simulation of methanol/air spray flames

The present work attempts to conduct numerical simulations of turbulent two-phase flows in two academic burners with methanol/air spray flames. It is challenging to simulate turbulent spray flames as many complex phenomena such as turbulence-chemistry interaction, spray dispersion and evaporation, and atomisation processes of liquid fuel occur simultaneously. The transported sub-grid scale (sgs) probability density function (pdf ) equation in conjunction with the Eulerian stochastic field method formulated in the context of Large Eddy Simulation (LES) is applied to the experimental configurations operated with different fuel delivery systems, namely a commercial ultrasonic atomiser and a Research Simplex Atomiser (RSA). The performance of the LES methodology as an effective modelling tool for two-phase combusting flows is assessed by simulating a methanol/air turbulent nonpremixed spray flame with a fine droplet distribution achieved by the ultrasonic atomiser. Comparisons of the computed gas phase and droplet statistics with measurements show a good agreement, suggesting that the stochastic dispersion and evaporation models employed in this work are appropriate. The fundamental aspects of this turbulent spray flame such as the occurrence of external group combustion and its development into separate combusting islands are well reproduced. For this particular configuration, the significance of the sgs chemistry model in capturing the development of the flame is highlighted. A systematic, stochastic approach towards modelling a polydisperse droplet distribution associated with the simplex atomiser is developed in this study. The predictive capabilities of the stochastic breakup model are validated by detailed comparisons between the simulated results and the experimental data in terms of droplet and gas phase statistics. The general features of this spray flame including the lift-off height and the formation of a double reaction zone are reasonably well captured, confirming the validity of the LES-pdf formulation when applied to turbulent nonpremixed spray flames. Declaration of Originality I, Dongwon Noh, hereby declare that this Ph.D. thesis entitled “A stochastic approach towards Large Eddy Simulation of methanol/air spray flames” is my own work for the degree of Doctor of Philosophy in English under the guidance and supervision of Dr. A. J. Marquis and Prof. W. P. Jones, Department of Mechanical Engineering, Imperial College London, UK. The present thesis, which I am submitting to Imperial College London, has not been submitted in any form for another degree or diploma or distinction either at this or at any other University. The published and unpublished work of others, of which I have made use, has been acknowledged at the respective place in the text and a list of references used in this thesis can be found in the bibliography. Copyright Declaration The copyright of this thesis rests with the author and is made available under a Creative Commons Attribution Non-Commercial No Derivatives licence. Researchers are free to copy, distribute or transmit the thesis on the condition that they attribute it, that they do not use it for commercial purposes and that they do not alter, transform or build upon it. For any reuse or redistribution, researchers must make clear to others the licence terms of this work.