| Jet diffusion flames can be easily controlled due to nonpremixing of air and fuel. Thus, by controlling the supply of fuel and oxidizer, it can be easy to regulate the reaction temperature and combustion process effectively, thereby enhances the combustion efficiency and reduces the production of combustion pollutants (such as NOx). So diffusion flames are wided employed in the pratical combustion system. Controlled jet diffusion flames can bring great convenience to people’s life and industry, but uncontrolled ones would bring great losses of life and property. With China’s rapid economic development and acceleration of urbanization, the national economy has a growing appetite for LNG, thus a long and complex gas supply system is set up to meet the increasing demand. However, in recent years, fire accidents occur frequently due to gas pipeline burst, giving rise to a great loss of life and property. Investigating the behavior characteristics of jet diffusion flame is of great significance to the design and operation of industrial combustion equipment, as well as related fire safety issues. Moreover, the fire problem in some special circumstances are also worth to concern. For example, in the plateaus, the special low relative oxygen and low pressure environment makes jet diffusion flame behavior quite differ from that in normal environment. In addition, in the course of space exploration, spacecraft fire has always been a great threat to the running safety of equipments and astronauts’ lives. In microgravity, as thermal buoyancy indeced convection disappears, the combustion behavior may be totally different from that in ground. Although, much attentions have been paied on these issues, the instabilities of the flame (including blow-out limit) is not revealed, so it is valuable to carry out investiagions about these.This paper concerns behaviors of jet diffusion flame in different special environments (including reduced pressure in plateaus, cross air flow and microgravity) by experimental researches, combining with theoretical analysis. To conduct these researches, some experimental facilities are designed:(1) Experimental equipments for free jet diffusion flame in Hefei and Lhasa;(2) Wind tunnel for jet flame in cross flow in Lhasa and Hefei;(3) Apparatus in microgravity of jet diffusion flame. Based on observations of characteristic parameters (the flame shape, profile of center line temperature, flame radiation, flame liftoff and blowout), combining with the classical theory of fire and combustion dynamics, the evolution mechanism of jet diffusion flame behavior in different special environments are revealed, and dimensionless model are established to characterize the behaviors. Specific workes include:The behaviors of jet diffusion flame, including flame height, flame temperature and flame liftoff, in reduced and normal pressures are investigated. By studying the differences of flame height in Hefei and Lhasa, it is found that flame height in reduced pressure is significantly higher than that in normal atmospheric pressure. By analyzing basic physical models of fire plume, a method is established to quantify the differences of air entrainment in the two environments. And the deviations in the flame heights are converged with the calibration of entrainment coefficient ratio. The conception of virtual origin is induced. By considering the coupling effect of flame heat release rate, flame size and atmospheric environment, a dimensionless model of virtual origion in both pressure environments is deduced. And it is used to calibrate the McCaffery’s classical model of flame plume temperature. Moreover, it is confirmed that the flame will lift heigher in lower pressure environment. By analyzing the underlying physical mechanism and key factors affected by pressure, the fundamental principle of flame lift-off phenomen are revealed, and Kalghatgi’s classical model is revised.The flame evolution process differences of nonpremixed turbulent jet flames in cross flows are studied by conducting experiments at atmospheric and sub-atmospheric pressures. It’s found that the visual character would significantly change with the increasing of cross wind velocity in both pressure atmosphere. It is also found that, as cross air flow speed increases from zero, the flame trajectory-line length firstly decreases to a minimum level and then transits to invariable (for relative small nozzle,3mm) or to increasing (for relative large nozzle,4mm and5mm in this study) in normal pressure, however almost monotonically decreases until the flame is blown-out in the sub-pressure atmosphere. The flame height in jet direction decreases monotonically with cross air flow speed and then reaches a steady value in both pressures. For the transitional state of flame trajectory-line length with cross air flow, the critical cross air flow speed is found to have a linear relation with the fuel jet velocity, meanwhile independent of nozzle diameter. Correlation models are proposed for the flame height in jet direction and the flame trajectory-line length for both ambient pressures. Moreover, new discovery is observed that the blow-out limit first increased and then decreased as the fuel jet velocity increased in both pressures, however, the blow-out limit of the air speed of the cross flow is much lower under the sub-atmospheric pressure than that under standard atmospheric pressure with the domain of the blow-out limit shrinking as the pressure decreases (or the domain is embodied by the normal pressure). A theoretical model is developed to characterize the blow-out limit of nonpremixed jet flames in a cross flow based on a Damkohler number, defined as the ratio between the mixing time and the characteristic reaction time. A satisfactory correlation is obtained that includes the effects of the air speed of the crossflow, fuel jet velocity, nozzle diameter and pressure.Finally, a drop tower experiments are conducted to investigate flame behaviors of jet diffusion flame in microgravity. It’s confirmed that the flame is higher in height, brighter in luminance and has more soot production in microgravity. Meanwhile, the blowout process induced by coflow in microgravity is also concerned, and findings can be made that the liftoff are harder and the critical flame blow-out coflow air velocity is significantly larger than that in normal gravity. |
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