Experimental Investigations on Sooty Flames at Elevated Pressures

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Abstract

This study addresses the influence of elevated pressures, fuel type, fuel flow rate and co-flow air on the flame structure and flickering behaviour of laminar oscillating diffusion flames. Photomultipliers, high speed photography and schlieren, accompanied with digital image processing techniques have been used to study the flame dynamics. Furthermore, the effects of pressure on the flame geometry and two-dimensional soot temperature distribution in a laminar stable diffusion flame have been investigated, utilising narrow band photography and two-colour pyrometry technique in the near infra-red region. This study provides a broad dataset on the diffusion (sooty) flame properties under pressures from atmospheric to 16 bar for three gaseous hydrocarbon fuels (methane, ethylene and propane) in a co-flow burner facility.It has been observed that the flame properties are very sensitive to the fuel type and flow rate at elevated pressures. The cross-sectional area of the stable flame shows an average inverse dependence on pressure to the power of n, where n was found to be 0.8±0.2 for ethylene flame, 0.5±0.1 for methane flame and 0.6±0.1 for propane flame. The height of a flame increases firstly with pressure and then decreases with further increase of pressure. It is observed that the region of stable combustion was markedly reduced as pressure was increased. An ethylene flame flickers with at least three dominant modes, each with corresponding harmonics at elevated pressures. In contrast, methane flames flicker with one dominant frequency and as many as six harmonic modes at elevated pressures. The increase in fuel flow rate was observed to increase the magnitude of oscillation. The flickering frequency, however, remains almost constant at each pressure. The dominant flickering frequency of a methane diffusion flame shows a power-law dependence on chamber pressure.It has been observed that the flame dynamics and stability are also strongly affected by the co-flow air velocity. When the co-flow velocity reached a certain value, the buoyancy driven flame oscillation was completely suppressed. The schlieren imaging has revealed that the co-flow of air is able to push the initiation point of outer toroidal vortices beyond the visible flame to create a very stable flame. The oscillation frequency was observed to increase linearly with the air co-flow rate.The soot temperature results obtained by applying the two-colour method in the near infra-red region shows that in diffusion flames the overall temperatures decrease with increasing pressure. It is shown that the rate of temperature drop is greater for a pressure increase at lower pressures in comparison with higher pressures.

Layman's Abstract

Today, combustion still provides most of the energy consumed in the world. Many systems, such as diesel engines and gas turbine combustors, operate at high pressures to increase their efficiency and decrease their physical size. However, current understanding of the influence of pressure on flames characteristics is still limited. This study provides a broad dataset on the diffusion (sooty) flame properties under pressures from atmospheric to 16 bar for three gaseous hydrocarbon fuels (methane, ethylene and propane) in a co-flow burner facility. The present PhD research is directly related to problems currently faced by the energy industry and in combustion systems; where elevated pressures and variable fuel and air flow rates lead to combustion instabilities and enhancement in soot formation, which are yet to be fully understood.

Keyword(s)

Co-flow air; Combustion instabilities; Diffusion flames; Flame dynamics; Flickering frequency; Flickering suppression; Fuel effects; High pressure; Near infra-red spectrum; Soot concentration; Soot temperature; Sooty flames; Two-colour method

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