Study On Combustion-Induced Rapid Phase Transition By Methane/Hydrogen/Air And Its Unsteady Oscillation Characteristics

In the process of exploitation,transportation and utilization of natural gas,once leaking and encountering fire source,explosion may occur,which will also cause Combustion-induced Rapid Phase Transition under certain conditions.Combustion-induced Rapid Phase Transition of methane under oxygen-enriched conditions is a phenomenon accompanied by a sudden rise of maximum pressure,which can reach 40 MPa,and has great harmfulness.Combustion-induced Rapid Phase Transition occurs not only in oxygen-enriched conditions but also in adding hydrogen conditions.Because hydrogen has smaller ignition energy than methane and its ignition and explosion limits are much larger than methane,adding hydrogen to methane can not only reduce the ignition energy required by methane gas,but also significantly broaden the combustion concentration limit and increase the combustion speed of mixture.The explosion relief device of the equipment will fail,which will lead to more dangerous safety accidents.In view of this,the law and formation mechanism of fast phase transition and overpressure oscillation induced by combustion of methane/hydrogen/air premixed gases are deeply explored by combining theoretical and experimental research methods,which has certain theoretical guiding significance for enriching and perfecting the theory of flammable explosion and preventing and controlling flammable safety accidents.In this thesis,a closed,explosion-releasing and visualization experimental system is constructed firstly.Seven kinds of volume fraction of hydrogen(0%,5%,10%,15%,20%,25% and 30%)and three kinds of fully open,semi-open and fully closed explosion vents with different proportions are selected for experimental analysis under different working conditions.The experimental results show that: Under closed conditions,when the volume fraction of hydrogen is 0% and 5% in the opaque pipeline,the pressure peak is lower than the adiabatic pressure value,and the pressure oscillation period and amplitude are smaller and be the same order.When the volume fraction of hydrogen is 10%,15%,20%,25% and 30%,the pressure peak remains basically the same and far exceeds the theoretical adiabatic pressure.The pressure oscillation period and amplitude are much smaller than those of the previous two conditions,appearing very typical combustion-induced rapid phase transition.The peak pressure remains unchanged with the change of hydrogen content under the condition of half-opening.With the increase of hydrogen content,the peak reaction pressure first decreases gradually,and then increases with the increase of hydrogen content.The arrival time of the pressure peak advances with the increase of the hydrogen volume fraction,and the pressure oscillation amplitude also increases gradually,indicating that adding hydrogen to methane will promote the pressure oscillation and the reaction rate.As the venting ratio increases,the pressure peak gradually increases,and the pressure peak arrival time is postponed.The characteristics of pressure oscillation period and amplitude under different hydrogen volume and detonation discharge conditions are consistent with those of typical combustion-induced rapid phase transition under closed conditions.It is determined that the unsteady pressure oscillation under detonation discharge conditions belongs to combustion-induced rapid phase transition.Under the visual conditions,the pressure peak law is consistent with the opacity experiment under different venting conditions.With the increase of volume fraction of hydrogen,the time for the flame to form each structure is advanced and the time spent in the tube is gradually reduced.Changing the venting area has little effect on the time of each structure before the tulip structure.The turbulent excitation effect of the discharge area on the non-combustible gas from the flame to the outflow pipeline is greater,and the period time decreases with the decrease of the venting ratio.In the initial stage of flame propagation,flame is a shape flame,which is mainly in laminar state.There is no significant difference in flame speed with the change of hydrogen content.Turbulence appears in the later stage of flame propagation,and flame accelerates obviously.With the increase of hydrogen content,flame reaches its maximum speed faster.Flame shape changes to plane flame shape,flame speed decreases,and then the rising rate is higher.The results showed that the reactivity of the reactants increased and the combustion rate of methane increased significantly when hydrogen was added to the reactants.In the process of flame propagating in the tube,the flame front velocity is greater than the flame skirt velocity in the initial continuous stretching process.The positive projection area of the flame front on the pipe wall increases continuously,and then the difference between the flame front velocity and the flame skirt velocity decreases gradually.The positive projection area of the flame front on the pipe wall decreases continuously and forms a planar flame shape.The tense reaches the minimum.At this stage,there is obvious phenomenon of combustion-induced rapid phase transition,and the pressure oscillation reaches the maximum.It may be that the larger the projection area of the flame front on the wall of the pipeline,the larger the heat transfer between the high-temperature flame front and the condensate film of the reaction product water on the wall,resulting in the rapid phase transition of the water film more intense.Therefore,the occurrence of fast phase transition induced by combustion may be closely related to the positive projection area of the flame front on the inner wall of the pipeline.