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Abstract
Emissions of NOx and soot from combustion are strongly influenced by the coupled phenomena of radiation heat transfer and turbulence in many practical applications. This thesis presents direct numerical simulation (DNS) studies of these phenomena. The radiative transfer equation (RTE) is solved with the discrete ordinate method (DOM) and the radiation model is coupled with the high-order finite difference DNS code S3D. Three distinct studies are carried out to address different aspects of radiation heat transfer and NOx formation in laminar and turbulent lean premixed flames and the interaction between turbulence and radiation in a series of temporally evolving planar jets. The first study considers the influence of radiation on the laminar flame properties (speed and thickness) in a CO2-diluted oxygen-enriched methane/air planar laminar premixed flame with thermochemical conditions relevant to those of a gas turbine (pressure varied between 1 and 80 atm). The time- and length-scales of radiation are presented and compared with the thermochemical scales found in premixed flames. Radiation is shown to increase the flame speed and reduce the flame thickness, due to the preheating of the reactants through reabsorption of radiation emitted by the products. The influence of pressure on this process is evaluated. In a second study, DNS is utilised to investigate the effect of turbulence intensity on NO formation in two-dimensional (2D) freely propagating CH4/air turbulent premixed flames in decaying isotropic turbulence. Five DNS cases of different turbulent velocities are considered, with a constant turbulence length-scale. NO formation pathways for the low and high turbulence intensity cases are compared. Turbulence is observed to considerably affect the NO formation pathways - especially the NO prompt pathway. The differences between the cases are demonstrated and the main mechanisms are explored. In the third part, a series of a-priori DNS studies of non- and premixed temporally-evolving jets are carried out to address the significance of turbulence-radiation interactions (TRI) in the context of Reynolds-averaged Navier-Stokes (RANS) modelling, especially for highly sooting flames with soot concentrations relevant to those typically encountered in the industry and transport sectors. While emission TRI is extremely important in the highly sooting flames, absorption TRI is also found to play a considerable role and they both need to be taken into account in RANS TRI closures. Overall, the thesis helps to clarify key aspects of the importance of radiation and turbulence in turbulent combustion problems.