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A Combined Experimental and Computational Study of Buoyant Jet Diffusion Flames

Li, Jizhao

[Thesis]. Manchester, UK: The University of Manchester; 2010.

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Abstract

In this work, both experimental and computational studies have been performed to investigate the flame dynamics and combustion instability of a laboratory buoyant jet diffusion flame from different prospects. The motivation behind this study was to obtain a better understanding of the dynamics of jet diffusion flames, as part of a long-term effort in achieving flexible fuel utilisation such as interchangeable fuels and achieving more effective combustion control such as better combustion efficiency.In the experimental study of jet diffusion flames, the influences of parameters such as nozzle exit diameter, fuel flow rate, fuel types and burner geometries have been investigated, where the focus was on the effects of fuel mixture on the flame dynamics. The frequency spectra, flame vortex development and flickering frequencies were measured using flow visualisation techniques and data acquisition systems. It was observed that the fuel jet velocity and the type of burner had a weak influence on the pulsation frequency for all the tested diameters. In contrast it has been found that both the ambient condition and fuel variability do have significant effects on the flame flickering frequency. Flame structure and dynamics are very different for the methane, propane and mixed fuel jet flames. Since the measurements of variables such as entrainment properties are difficult to obtain under experimental conditions, it is more effective to deal with such problems numerically. In the second part of this study, the dynamics of the buoyant jet diffusion flame has been investigated by idealised axisymmetric direct numerical simulations (DNS). The physical problem is a fuel jet issuing vertically into an oxidant ambient. Taking the advantages of idealised computational conditions, the effects of nozzle velocity profile, initial momentum thickness, Froude number, Reynolds number and co-flow on the near-field dynamics of a jet diffusion flame have been investigated. The computational cases have shown the development of different vortical structures, which suggest that vortical structures depend on both buoyancy and jet nozzle velocity profile. The flickering frequency and flickering energy results provide supportive evidence of the above finding. The results of the co-flow case indicate no significant flame-vortex interaction, and the flame oscillation is being suppressed. In general, the study suggested that the velocity shear plays a significant role in the near-field flame dynamics, apart from the buoyancy effects.