Applications of a numerical method in study of combustion instabilities

Abstract

This thesis explores the capabilities of an large Eddy Simulation (LES) method in study of flame dy- namics with three test cases. The method, BOFFIN-LES, comprises a fully compressible formulation to account for acoustic wave propagation. A transported probability density function (pdf )/ Eulerian stochastic fields method is employed for turbulence-chemistry interaction, which has the merit that chemical source terms appear in a closed form so that no additional modelling for the chemical re- action is required. This approach is shown to be independent of flame burning regime and therefore highly applicable in the study of partially premixed flames with multiple regimes. Combustion instabilities remain a central issue in designing and constructing successful lean combus- tion systems, which are driven and controlled by complex physical mechanisms including small-scale stochastic turbulent fluctuations and large-scale coherent structures. In order to address the capability of the employed LES method in study of combustion instabilities from different perspectives, three test cases are investigated: a highly strained turbulent flame with local-extinction and re-ignitions, noise generation in a resonator with a non-isentropic nozzle and finally a series of lab-scale swirling flames undergoing thermo-acoustic and hydrodynamic instabilities. The findings of this work strongly suggest that the employed LES method is an effective and reliable tool to describe combustion dynamics related problems in elementary studies as well as complex flame configurations. The LES study has been shown to be capable of well predicting the unsteady flame local-extinction and re-ignition with a relatively small number of stochastic fields, and the influence of flame stoichiometry were also successfully reproduced. The methodology with proper acoustic boundary treatments predicts the direct and indirect noise generating process to a good level of accuracy in the context of low Mach number flows. In the application to swirling flames, the LES successfully reproduces thermo-acoustic and hydrodynamic instabilities, with the driving mechanism clearly identified. The LES well captures the iso-thermal flow dynamics and the flame topology under various operating conditions, with a good prediction of the thermo-acoustic frequencies in all the cases. The effect of Hydrogen enrichment on modifying the flame topology and changing the thermo-acoustic instability features are well predicted by the simulations. Different mode of precessing vortex cores (PVC) are detected, and their periodic excitement, evolution and effect on the flame stabilisation are discussed with great details. To conclude, the LES study provides many useful insights into the investigated unsteady swirling flames, which involves complex interactions of unsteady combustion, acoustic fluctuations, flow dynamics and solid boundaries. All the test cases are performed with virtually the same set of model parameters, which potentially eliminates the requirement of tuning or adjusting the model parameters according to a certain setup.