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The purpose of this thesis is to provide better insight into isosurface interactions in premixed turbulent flames. These interactions for a suitable isosuface corresponding to the reaction zone can be interpreted as flame-flame interactions. Isosurface interactions can be identified with the help of Morse theory of critical points and the local topology at critical points can be evaluated. In previous studies, the interactions have been categorised into four different groups, namely reactant pocket, tunnel formation, tunnel closure and product pocket. The effect of isosurface interactions on flame propagation is analysed by expanding the equations for Surface Density Function (SDF) and displacement speed in the vicinity of a critical point and Direct Numerical Simulation (DNS) is used to validate the theoretical results. In particular, the formation of isolated pockets of reactants or products is associated with flame pinch--off events which cause rapid changes in the flame surface area. In the past, the analysis of pocket formation in turbulent premixed flames has been carried out in two dimensions. In this thesis, the topological analysis is carried out in three dimensions with emphasis on the formation and subsequent burnout of reactant pockets. The terms of the SDF transport equation show singular behaviour which was also observed in the previous two--dimensional analysis. Singular behaviour is also observed in the terms of the displacement speed equation close to reactant pocket burnout. The theory is compared against DNS data from hydrogen--air flames and good agreement is obtained. Histograms showing the frequency of occurrence of each topological event are presented for different hydrogen and hydrocarbon flame cases. The analysis is first carried out in a twin flame setup with two flames propagating towards each other. It is observed that the relative frequency of occurrence of each type of topology changes with changes in turbulence intensity. With increasing turbulence intensity in hydrocarbon flames, the fraction of product pockets and tunnel formation events increases whereas the fraction of reactant pockets and tunnel closure events decreases. The results for hydrocarbon flames are compared with those for hydrogen flames and the differences are explained both qualitatively and quantitatively. Self-interactions in single hydrocarbon flames are also analysed and it is found that a significant number of self interactions exist within individual flames. The analysis is further extended to systematic variations in integral length scale as well as turbulence intensity. |
