| For the past few years of the21st Century, it has been a newly hot spot in fire science research field that study on the fire in sub-atmospheric environment. High altitude areas and cargo rooms in planes are the common environment where the pressure is lower than normal atmospheric pressure. Due to the difficulty of putting out fires and organizing rescue after the fire happens, it is crucial to put much more attention on the fire safety problem in sub-atmospheric environment. Hence, it is necessary to focus on the fire behavior under low air pressures to reveal the fire developing pattern to protect people and properties from the damages result from fire in sub-atmospheric environment.As for one of the most important and effective methods of fire prevention and controlling, fire detection technology also needs to be improved by finding out how the detection parameters such as images, sound, light, temperature of combustion change under sub-atmospheric pressure. Because of significant impact of sub-atmospheric pressure on burning development based on earlier research, traditional detection methods are no longer suitable for early sub-atmospheric environment fire detection.To analyze the fire performance under sub-atmospheric pressure, it is necessary to separate each stage of the fire development-pyrolyzation, volatilization and gas combustion, and discuss the impact of air pressure on them individually to set up the relations between pressure and combustion characteristics. Therefore, gas combustion is ideal subject for detail research on low pressure combustion. In addition, as the combined low air pressure cabin could be easier to achieve a serial of different atmospheric pressure environment without high cost and long preparation time, it is wiser to choose the cabin for experimental study on gas combustion in sub-atmospheric environment.Based on the previous work on liquid and solid fires under low air pressure, it is set to study the buoyancy-dominated and momentum-dominated gas jet diffusion flames’combustion characteristics and image parameters under45～100kPa. By theoretical and experimental analyzing the combustion parameters including flame temperature, heat radiation flux, flame height, soot free region’s height ratio, lift-off height, oscillation frequencies and amplitude of various mass flow rate under different low air pressures. Theoretical explanations would be given to describe the influence of sub-atmospheric pressure on jet diffusion flame to provide theoretical instruction and technology advices for fire detection and prevention under sub-atmospheric pressure. The major work has been done in this paper includes theoretical analysis and experimental study.For experimental study, a self designed low air pressure jet flame combustion experiment system was introduced and applied in a low air pressure cabin. The experimental pressure ranges from45to100kPa. To study both buoyancy-dominated and momentum-dominated gas jet diffusion flames, two burners of different nozzle diameters were used. Measurements included thermal couple temperature measurement, high speed schlieren image capture, CCD flame image capture, heat radiation flux recording and mass flow rate controlling. Based on experiment results, a comparison conclusion between buoyancy-dominated and momentum-dominated gas jet diffusion flame under different air pressure was made; in addition, the influence of gas flow rate and co-flow condition was also discussed.It was found that the influence of co-flow condition on combustion was weak, except that with increasing co-flow velocity, the flame height shortened. Both buoyancy and momentum-dominated flames’heat radiation flux increased with Froude number (Fr), and there was a relation between Strouhal number and Fr: St∝Fr-0.48.The difference between momentum-dominated flame and buoyancy-dominated flame was that when atmospheric pressure was lower than70kPa, momentum-dominated flame began to lift off from the burner. Under sub-atmospheric pressures, the oxygen mass density decreased while Fr increased, which resulted in changes of air-fuel ratio and combustion characteristics of the flame. The results indicated70kPa was a turning point for flame temperature, flame height, lift-off phenomena in momentum-dominated flames, which was assumed that under70kPa, the air-fuel ratio changed a lot to make incomplete combustion. Under other circumstance, the flame temperature reduced with growing pressure, while radiation flux increased. For flame height, it was proved that momentum-dominated flame’s height was independent on air pressure(LF∝P∞0), while buoyancy-dominated flame’s height and pressure had a negative exponential relation:LF～P∞α,-0.22<α<-0.18. The fitted relation between frequency and atmospheric pressure was:f∝P∞α (for buoyancy-dominated flames,0.03<a<0.22; for momentum-dominated flames,0.11<α<0.14). Finally, theoretical analysis was improved based on the experimental results. The impact of Fr, co-flow condition, air pressure on jet diffusion combustion was discussed respectively. The difference of buoyancy and momentum-dominated combustion mechanism dominated flames under sub-atmospheric pressure was explained, and a relation chain was set up to illustrate the influence of combustion characteristics on each other under sub-atmospheric pressure. |
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