Hydrogen Explosion Characteristics With Concentration Gradient In Cubic Enclosure

With the continuing progress and development of the society,the energy requirement is growing rapidly,and ensuring sufficient energy supply to meet the development needs has become a hot spot in the world today.As one of the new energy sources in recent years,hydrogen energy has great potential and broad prospects,and has received increasing attention and application.In the industrial production and the application of hydrogen,hydrogen is often sealed and stored in an enclosed space,and the device leaks into a closed or semi-closed space to form an explosive mixture due to its own defects or human factors.Once the hydrogen-air mixture reaches the explosion limit,an explosion accident occurs when a sufficient intensity of the ignition source is encountered.Since the density of hydrogen is small,the hydrogen in the mixture floats on the top of the space when the leak occurs,so the mixing is not uniform,or it is very likely that a certain concentration gradient is formed in the space.Once the mixture with the concentration gradient encounters the ignition source,the explosion may bring even more damage.Therefore,it is a great significance to study the explosive combustion characteristics of hydrogen in a closed space with a concentration gradient,which can provide explosion-proof methods for hazardous chemical plants,tunnels and other places.In this paper,a cubic chamber with a side length of 0.5 m was designed.The effects of hydrogen concentration,diffusion time and openings on the explosion characteristics of hydrogen/air premixed gas and mixed gas with concentration gradient are studied experimentally.The flame images were captured by a high speed camera,and the overpressure were detected by the pressure transducers.In the uniform cases,the flame shape propagates outward in a spherical shape.When the flame surface is 5 cm away from the square wall surface,the wall has a significant influence on the flame shape,and the flame shape evolves into a square shape.For the lean hydrogen mixture,the wrinkles appear on the flame front due to the thermal diffusion instability and the hydraulic instability.As the radius of the spherical flame increases,the wrinkles become more and more obvious.With the increase in the hydrogen concentration,the flame propagation speed increases first and then decreases,and moreover the similar trend can be found for the rate of pressure rise.At a concentration of about 30%,the rate of pressure rise reaches a maximum value.It is worth noting that when the hydrogen concentration is very low,significant pressure oscillations occur following the pressure peak,which may be caused by flame instability and unstable heat release rate.In the cases with an opening,the flame initially propagates outward in a spherical shape.When approaching the wall surface,the flame surface near the opening expands outward until the opening is opened,and the leaking hydrogen-air mixture gas is ignited outside the opening which leads to the second explosion.The maximum length of the external flame increases as the concentration increases.The pressure waves generated by the secondary explosion and the backflow of the external air after the membrane rupture induce the formation of two pressure peaks.The pressure peak with the opening is lower than the pressure peak without the opening.As the concentration increases,the effect of the opening on the pressure peak is getting weaker.As to hydrogen combustion explosion with a concentration gradient in the closed chamber,the hydrogen flame morphology for the average concentration of 22.2% is very different from that in homogeneous case.Due to the influence of the concentration gradient,the flame shape is slightly narrow.For the average concentrations of both 22.2 % and 35.7%,the diffusion time has a positive effect on the flame propagation speed.In other words,as the diffusion time increases,the gradient decreases and the flame propagation speed increases.Not all gradients promote flame propagation compared to the uniform case.For the lean hydrogen mixture,the peak explosion pressure in homogeneous case is larger than that in inhomogeneous case.For the rich hydrogen mixture,the peak explosion pressure in homogeneous case is smaller than that in inhomogeneous case.As to a hydrogen explosion with an opening,a mushroom-like flame structure first forms for concentration gradient with the average value of 22.2%,and then the flame surface expands towards the opening,which is significantly different from that in the homogeneous case.The maximum overpressure changes with the diffusion time.The opening has a less effect on the peak pressure in the cases with the opening than in the cases without the opening.Unlike the homogeneous case,with the increase of the hydrogen concentration,the effect of this gradient is not significantly weakened.