Experimental Investigation On The Stability Characteristics Of Jet Diffusion Flame

Flame stability remains one of the hot issues in combustion research community.Studies on stability limits and stability characteristics of free jet diffusion flame and coflow flame under different conditions have significant scientific and practical values. With the increasing requirement of environmental law, the combustion characteristics of renewable alternative fuel have attracted much attention. The thesis focuses on the stability characteristics of diffusion flame by the following experimental studies.Firstly, the effects of burner tube lip thickness on the flame stability characteristics are experimentally studied. On the combustion experiment rig without coflow air, two kinds of burner tube with inner diameter being 2mm and 3mm, two types of fuels (methane and propane) and two jet conditions (free jet and confined jet)are used to study the effects of the burner lip thickness (ranging from 0.25mm to 16.5mm) on the liftoff and blowout velocities of jet diffusion flame. The results show that the burner lip thickness has a profound influence on flame liftoff velocities,especially the blowout velocities. When the inner diameter is 2mm, the blowout velocities firstly increase as the lip thickness increases from 0.2mm to 2mm, and then decrease when the lip thickness changes from 2mm to 4mm, and lastly remain unchanged when the lip thickness increases from 4mm to 9mm and 16mm. That is to say, the blowout velocity of 2mm inner diameters tubes reaches the maximum when the corresponding burner lip thickness is 2mm. By using the maximum blowout velocity, the normalized blowout velocities of various lip thicknesses of the two fuels have nearly identical variation tendency. When the inner diameter is set to be 3mm,the tendency of the blowout velocities varying with the lip thickness is analogous to that of 2mm inner diameter.Secondly, based on the background of the combusting utilization of low calorific value gas fuel, the liftoff velocities and blowout velocities of jet diffusion flame varying with the diluent concentration (defined as the mole fraction of the diluent gas in the gas mixture of the fuel and diluent gas) are investigated on the experiment rig without coflow air, and various factors, for instance the types of diluent gas (N2 and CO2), types of fuel (methane and propane) and diameter of burner tube (2mm and 3mm) are all taken into consideration. The experiment indicates that there exists a critical dilution concentration and the attached flame cannot turn to stable lifted flame on reaching this dilution concentration. And the value of the critical dilution concentration is influenced by the types of the diluent gas and the fuel and the burner tube diameter. Further, the two critical dilution concentrations of methane and propane are probably controlled by different mechanisms. Besides, based on the critical dilution concentration, the liftoff and blowout velocities of pure fuel, the diffusion flame states of attachment, liftoff and blowout of the diluted fuel could be identified on the two-dimensional graph composed of the jet velocity and the dilution concentration.Thirdly, comparative experimental studies on the free jet flame stability limits and characteristics of a new clean alternative fuel dimethyl ether (DME) and propane,the main component of LPG, are conducted when the diameters of the burner tubes are 1mm, 1.5mm, 2mm, 3mm, respectively. The experiment shows that there ought to exist a critical diameter of the burner tube at the range of 1mm to 1.5mm and the critical diameter can transfer the inexistence of stable DME lifted flame to the existence of stable DME lifted flame. In addition, for the above burner tubes, the flame blowout velocities of DME are about half of that of propane, and the liftoff velocities and reattachment velocities of DME are slightly larger than those of propane. The maximum power of DME lifted flame is approximately one third of that of propane, while the maximum power of DME attached flame are slightly less than that of propane. Next, the flame liftoff and blowout velocities of nitrogen diluted DME or methane varying with the dilution concentration at three kinds of coflow condition, free jet, coflow air velocity equaling to 0.50m/s and 0.88m/s respectively, are experimentally investigated and the diameter of the burner tube is 4mm. And the main results are as follows. The DME and methane free jet diffusion flame manifest as oscillating lifted flame when the dilution concentration is larger,while the oscillating lifted flame is not observed at the presence of coflow air. The states of the above diffusion flames arc all shown on the two-dimensional graph composed of the jet velocity and the dilution concentration. At the three coflow conditions, the jet velocities or the Reynolds numbers corresponding to flame liftoff,blowout and blowoff of the two fuels vary approximate linearly with the dilution concentration. When the coflow air velocity increases from 0.50m/s to 0.88m/s, the flame liftoff and blowout velocities of the two fuels at various dilution concentration all decrease. Moreover, the reduced effect of the coflow air on the flame liftoff and blowout velocities of methane is more evident than that of DME.Lastly, a jet diffusion flame experimental platform with moderate and high temperature coflow air is established. The velocity and temperature of the coflow air can be adjustable and the initial experiment parameters of the coflow air is effectively extended. And the platform can be used for studying the diffusion flame stability limits of gas fuel or pre-vaporized liquid fuel of low-boiling point and the autoignition of jet fuel with the high temperature coflow air. At this platform, the jet diffusion flame stability characteristics of pre-vaporized butanol or ethanol diluted by nitrogen with the coflow air temperature being 452K/520K/570K/640K are studied respectively, and the relationships between the flame liftoff or blowout velocities and the diluent concentration are obtained. The experimental results show that the diffusion flame liftoff and blowout velocities of the pre-vaporized butanol or ethanol diluted by nitrogen increase varying with the coflow air temperature and decrease with the diluent concentration. The flame liftoff and blowout velocities of the pre-vaporized butanol diluted by nitrogen are higher than those of ethanol at the same coflow air temperature and the same diluent concentration. In addition, the standardization flame liftoff velocities (i.e., liftoff velocities divided by the fuel mole fraction in the gas mixture) of the pre-vaporized butanol or ethanol have a uniform linear dependence on the local stoichiometric laminar burning velocity SLst at the four kinds of coflow air temperature. Moreover, the standardization flame blowout velocities of butanol or ethanol respectively have different linear dependence on the local stoichiometric laminar burning velocity at every coflow air temperature.