| Syngas is a kind of clean and renewable fuel.Because of its wide range of sources,high combustion efficiency and low pollution emissions,it is considered to be a potential alternative fuel which attracts the attention of a large number of scholars.The main combustible components of syngas are hydrogen and carbon monoxide,but the composition and content of syngas vary greatly because of the difference between gasification process and gasification raw materials,so the combustion and explosion characteristics of syngas are complex and dangerous.In order to realize the safe utilization of syngas,there is an urgent need to carry out related research work on the combustion and explosion characteristics of syngas.Based on the small-scale experimental platform,the propagation dynamic characteristics of premixedsyngas-air explosion flame in the ductline are studied in details in this paper.(1)The equivalent ratio and the volume fraction of hydrogen significantly affect the flame propagation time,the maximum flame propagation velocity and the maximum explosion overpressure of premixed syngas-air explosion.The propagation time of premixed syngas-air explosion flame in the duct decreases with the increase of hydrogen volume fraction.The hydrogen volume fraction has a strong effect on the flame propagation time when the hydrogen volume fraction is small(<0.5).The effect of the hydrogen volume fraction on flame propagation time is relatively mild when the volume fraction of hydrogen is larger(?0.5).The maximum explosion overpressure increases linearly with the increase of hydrogen volume fraction.For the effect of equivalence ratio,when the volume fraction of hydrogen is small(<0.5),the minimum flame propagation time,the maximum flame propagation velocity and the maximum explosion overpressure are obtained when the equivalence ratio is 2.0.When the volume fraction of hydrogen is larger(?0.5),the minimum flame propagation time,the maximum flame propagation velocity and the maximum explosion pressure are obtained when the equivalence ratio is 1.5.(2)The dynamic characteristics of flame propagation in premixed syngas-air explosion such as duct's boundary conditions,obstacles and ignition position have significant influence.The downstream opening of the duct alleviates the flame oscillation,shortens the flame propagation time,and reduces the maximum explosion overpressure,but increases the flame propagation speed.The opening at both ends of the duct can restrain the acceleration and oscillation of the flame,and the inhibitory effect is more obvious when the ignition position is close to the opening at one end.Obstacles change the flame structure of premixed syngas-air explosion,shorten the flame propagation time,increase the flame propagation speed and increase the maximum explosion overpressure.(3)The tulip flame in the obstacle duct and the open duct at both ends is a typical hydraulic dynamic phenomenon caused by the competition between fluids.After the flame passes through the obstacle,the small flame unit drawn near the obstacle competes with the small flame unit propagating to the unburned area,which leads to the reversal of the obstacle flame front.Obstacles promote the formation of tulip flames.In the open duct at both ends,the overpressure curve P1 in the negative pressure oscillation confirms that there is significant fluid competition in the process of flame propagation on the left and right sides.In addition,the remarkable instability and tensile phenomenon of the flame propagating to the side wall of the semi-open duct confirm that the pressure wave can not act directly on the flame surface,forcing the flame front to reverse and deform.Supporting tulip flame is a typical hydrodynamic phenomenon.(4)The phenomenon of distorted tulip flame is found in the semi-open duct for the first time,indicating that the distorted tulip flame can be formed in both the closed duct and the semi-open duct,but they have different formation and development mechanisms.In the closed duct,the distorted tulip flame is only formed when the maximum flame velocity is more than 31.27 m·s-1.The interval between the flat flame formation time and the distorted tulip flame initiation time is a constant,which is 4.03 ms.Flame tip velocity coincides with the overpressure curve.These characteristics indicate that the distorted tulip flame in the closed duct is a typical pressure wave correlation phenomenon,and the pressure wave is the direct cause of the distorted tulip flame repetition.There is only one distorting process of the tulip flame in the semi-open duct.The distorted tulip flame in the semi-open duct is formed in the rising stage of overpressure without significant oscillation characteristics.The Kelvin-Helmholtz instability caused by the impact of the air flow from the outside of the duct to the flame in the tube is the main reason for the distortion of the tulip flame in the semi-open duct.(5)Pressure wave plays a key role in the acceleration and oscillation of premixed syngas-air explosion flame.When the flame propagates from the open end to another opening,the premixed syngas-air explosion flame will develop at a uniform speed without pressure wave disturbance.At this time,Darrieus-Landau instability and buoyancy are the main factors affecting the propagation of premixed syngas-air explosion flame.When the flame propagates from the open end to the closed end,the coupling between the pressure wave and the flame surface leads to the strong acceleration and severe oscillation of premixedsyngas-air explosion flame.(6)The existing flame propagation model is used to predict the flame propagation dynamic characteristics of premixedsyngas-air explosion.Flame instability and flame-duct wall coupling have great influence on the flame propagation model of premixedsyngas-air explosion.Under the lean burn condition,the thermal-diffusion instability significantly affects the dynamic characteristics of premixedsyngas-air explosion flame tip and the flame propagation time to the side wall of the duct so that the model results are larger than the experimental results.When lean burn or large equivalent ratio(?=0.6,?=2.5 and?=3.0),the heat loss caused by the coupling between flame and duct wall significantly affects the reversal of flame front. |
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