Study On Overpressure Characteristics And External Explosion Mechanism Of Vented Lean Hydrogen Explosion In Pipeline

The potential explosion hazard of flammable gas in industrial production is a practical problem that must be considered and resolved in engineering design.If the potential explosion hazard is not properly dealt with,the consequences can be catastrophic in terms of casualties,property,and equipment damage.The vented explosion technology,as one of the most widely used and effective strategies,is an important research topic in industrial explosion protection.Besides,hydrogen,as renewable energy due to its properties of a wide range of sources,clean,low-carbon,flexible,and efficient,is expected to play a key role in leading human beings to transition to a more sustainable low-carbon energy.It is very important for the realization of hydrogen safety and hydrogen economy to conduct further research of mechanism and technology of vented hydrogen explosion,establish fundamentals experiments data,and then developing comprehensive vented hydrogen explosion standards.To investigate the characteristics of vented hydrogen explosions,a series of vented explosion experiments of fuel-lean hydrogen/air mixture was conducted in a self-designed and built experimental platform.The effects of vent coefficient,hydrogen concentration,and obstacles were investigated.In the experiment,the internal and external flow fields of the pipeline were recorded using a high-speed camera and schlieren system,and the pressure inside and outside the pipeline was monitored by a pressure acquisition system.The characteristics of the internal and external flow field and its interaction,the characteristics of internal and external pressure,and the mechanism of pressure peak in the process of vented hydrogen explosions in the pipeline were studied and revealed.The research work in this paper is detailed as follows.Firstly,experiments were carried out on an empty pipeline with different vent coefficients and hydrogen concentrations.It was found that with the vent failure,two types of typical overpressure peaks,P_ex1 and P_ex2,occur outside the vent.The former was caused by the outward propagating pressure wave formed by the hot combustion products inside the vessel pushing the unburned gas when the vent rupture.The latter was caused by the blast wave generated by the internal flame propagating to the vent and igniting the unburned mixture discharged from the pipeline.Vent coefficient and hydrogen concentration significantly affect the internal pressure dynamics.For vent diameter of68.0 mm,three kinds of typical pressure peak values P_V,P_hel,and P3 were observed in the internal pressure histories.Among them,the formation of P_V was associated with the vent rupture;the formation of P_hel was associated with the Helmholtz oscillation occurring inside the vessel;P3 was caused by the increased combustion reaction and local pressure rise due to flame destabilization as the flame propagates to sensor PT2.With the decrease of vent diameter and the increase of hydrogen concentration,the influence of external combustible gas cloud combustion on internal pressure dynamics gradually increased.For instance,in the vented explosion experiment of the vent diameter 34.0mm,only one other type of pressure peak P_ex was observed in addition to the pressure peak P_V associated with the vent failure,the formation of which was associated with the combustion of a gas cloud outside the vent.Compared with the pressure peak under different test conditions without obstacle,it was found that the pressure peak P_V and the maximum pressure P_maxin the container increase with the increase of hydrogen concentration and decrease with the increase of vent area.The pressure peak P_ex2 increases with the increase of hydrogen concentration.At low hydrogen concentration,the effect of vent area on P_ex2 is negligible,and as the hydrogen concentration gradually increases,P_ex2 increases with the decrease of the vent area.Secondly,experiments were performed on fuel-lean hydrogen vented explosion with different vent coefficients and different hydrogen concentrations in the presence of an obstruction.By comparing the experimental phenomena and patterns of hydrogen vented explosion without obstacle,the effect of the obstacle on the hydrogen vented explosion of the pipeline was revealed.It was found that the presence of the obstacle caused a new pressure peak P_Obs to appear inside the vessel.The P_Obs dominated the maximum pressure inside and became stronger as the hydrogen concentration increased.In a 68.0 mm diameter vented explosion experiment,four types of typical pressure peaks,P_V,P_hel,P_Obs,and P_ex,were observed inside the pipeline.As the vent area decreases and the hydrogen concentration increases,the pressure peak P_hel gradually disappeared,while the pressure peaks P_Obs and P_ex gradually coupled into a larger pressure peak P5.For the flow field outside the vent,two types of typical pressure peaks P_ex1 and P_ex2 also occurred.Comparing the magnitude of the peak pressure in different test conditions with an obstacle,it was found that the pressure peak P_V and the maximum pressure P_max inside the pipeline increase with the hydrogen concentration and decrease with the increase of the vent area.Pressure Peak P_ex2 increases with increasing hydrogen concentration.However,the size of the vent has a positive and negative effect on the magnitude of P_ex2:on the one hand,the reduction of the vent diameter increases the discharge gas outflow velocity and the intensity of turbulence outside the vent,which increases P_ex2;on the other hand,the increase of the discharge gas outflow velocity increases the propagation distance of the unburned gas outside the vent,the dilution effect of the external air on the discharged unburned gas increases,the hydrogen concentration of the combustible gas cloud decreases and the P_ex2 decreases.