| Because of accident or man-made causes, combustions or explosions of combustible hydrocarbon gas occur from time to time in the production, transportation and storage processes. These always lead to huge losses and damage. Ducts in the industrial duct networks usually consist of non-straight sections, such as T junctions,90°bend or crosses. The understanding of the influence of these configurations on flame propagation characteristics is important for industrial, operational and personnel safety. Also flame propagation in the duct is always affected by the flow. The interaction between flame and flow usually involves flame acceleration, flame structure variation and flame instability. Accordingly, premixed propane-air flame propagation in a combustion duct with a 90°bend has been investigated both numerically and experimentally.Firstly, the basic mechanisms of flame propagation in duct were analyzed theoretically.Secondly, a series of experiments of propane-air premixed flame in closed and semi-closed ducts were performed.(1) Study of the influences of initial conditions on the propagation behaviorThe experiments were carried out with premixed propane and air mixture at atmospheric conditions. High speed camera and Schlieren image technology were used to record the dynamic process of flame propagation flame structure variation. Fine thermocouples, ion probes and PCB pressure transducers were distributed along the duct to measure the experimental temperature, ion current and overpressure. The experimental work has provided a unique data set which includes combustion regimes ranging from laminar flame to turbulent flame. The effects of equivalence ratio, pressure relief vent and ignition position which affect the flame propagation and transition have been found.(2) Study of the influences of different opening areas on the propagation behaviorDifferent opening areas of pressure relief vent were considered when flame propagated in semi-closed duct. The opening areas were from closed to totally open with an incremental interval of 10% opening area. The flame propagation velocity would be influenced by comprehensive effects of the opening area and system pressure. Two maximum values of flame propagation velocity both increased with the increase of the opening area. In the process, the flame structure variation was mainly induced by the interaction between the flame and vortex, the vortex's movement and development, Rayleigh-Taylor instability, Kelvin-Helmholtz instability and thermo-diffusion instability.Thirdly, numerical studies were performed using the computational fluid dynamics code-Fluent with Realizable k-εmodel. The computational results agreed very well qualitatively with the experimental results. The essential characteristics observed in experiments were well captured in simulation. The primary focus of these simulations was the interactions among the vortices, reverse flow and flame-induced flow which directly caused the transformation of the flame structure. The computed results indicated that for ignition in horizontal section, two developing vortices formed in this part resulted in formation of tulip flame, and a big recirculation region existed in the bend induced the flame tip to move towards the upper side. For ignition in the vertical section, the flow velocity downstream of the bend remained the same. Also, a single upward moving vortex caused the inversion of the flame front. After it met the upper side wall, it decayed and vanished. Fluent can therefore be used to enhance the understanding of premixed gas flame propagation in ducts.
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