Study On Fine Flame Structure Behavior And Flame Accelerating Mechanism Of Premixed Propane-air

With the adjustment of Chinese energy strategy, more and more alternative energy, such as the LPG and LNG would be used instead of coal resource. In the transportation and usage of flammable gas resource, the gas explosion often occurs due to the accidental ignition. The gas explosion is one of the most destructive disasters in our life and industry manufacture, which always lead to big losses and damages. During the gas flame propagation, the flame accelerating increases the pressure quickly and finally leads to explosion disaster. It's difficult for us to control the disaster case when the explosion takes place, therefore it's necessary to study the inner mechanism of flame accelerating and further to prevent the gas explosion accident.During the flame accelerating propagation, the flow influences the heat and mass transfer in combustion directly. The flow-flame interaction involves the flame structure and flame propagation and flame instability, etc. Accordingly, the study in this paper focuses on the inner dynamic mechanism of flame accelerating, such as the flame instability and the flame microstructure characteristics.The experimental system on premixed flame fine structure was built up, which was used to explore the dynamic behavior of premixed flame propagation, the flame front change and the flame instability characteristics.Firstly, the theory analysis and numerical simulation were carried out to describe the basic behavior on flame propagation. In the experimentation, the premixed propane-air flame propagation behavior was studied as the following:(1) Study on pressure wave influence on premixed propane-air flame propagation behavior under different pressure relief caseThe high speed Schlieren photograph method was used to record the dynamic process of flame propagation clearly, including the precise flame front position, flame structure, and so on. The research disclosed that just the co-flow pressure wave and the reflect pressure wave caused the flame speed fluctuations in the propagation process. In addition, the pressure played an important role on the Tulip flame structure.(2) Study on the interaction between rarefaction wave and premixed propane-air flameIn the research, the co-flow and counter-flow rarefaction wave were induced to intervene in the flame by different ignition scheme. The results show that the rarefaction wave influences the premixed flame structure and propagation greatly. The rarefaction wave intervened in flame structure when they encountered, which hastened the laminar-turbulent flame transition.The co-flow rarefaction wave decreased the flame speed on the whole, even led to reverse propagation. When the turbulence became strong enough, the flame accelerated again. On the other hand, the counter-flow rarefaction wave speedup the flame propagation on the whole, accompanied with great fluctuations. In a word, the rarefaction wave hastened the transition from laminar to turbulence greatly, simultaneously led pressure fluctuation in combustion pipe, which further accelerated the laminar-turbulence transition.(3) Study on equivalence ratio influence on premixed propane-air propagation behaviorIn the case of equivalence ratio, the flame propagating forwards with symmetrical flame front structure. While the lean fuel flame front became asymmetrical firstly, after then the Tulip flame structure came into being. The rich fuel flame turned into symmetrical Tulip flame structure firstly, and subsequently became asymmetrical gradually. The results show that the buoyancy plays an important role on flame front symmetry. In the case of equivalence ratio, the buoyancy effect was not big enough relative to the great flame speed, but which influenced the lean fuel and rich fuel flame greatly for their small flame speed, and finally induced the asymmetrical flame structure.In addition, the high speed Schlieren photograph system, the ion current probe technology and super thin thermocouple system were used to explore the fine flame structure characteristics in the propagation process, which helps to disclose the inner mechanism of flame instability behavior.(1) A typical flame instable structure came into being during the flame propagation in combustion pipe. The baroclinicity effect and Mach effect induced by pressure wave produce eddies, which leads to the deformed and instable flame structure.(2) The result shows that, during the course of flame structure change, the laminar-turbulence transition appeared. In early stage of flame propagation, the small scale vortex increased the combustion reaction intensity, but did little influence on the flame front structure. While the large scale vortex at the later stage thickened the flame front, simultaneously accompanied with fluctuations in reaction zone.In conclusion, through the small scale experimental system, the premixed propane-air flame propagation behavior and the fine flame structure were studied in detail, all which help to disclose the inner dynamic mechanism of flame structure characteristic, and further to prevent the gas explosion disasters.