Experimental Study On The Effect Of Water Mist On The Under-expanded Hydrogen Jet Flames

With the rapid development of economy and growth of population,the continuous increase of energy consumption in the world which leads to two major problems of energy shortage and environmental pollution.As an energy with diverse sources,efficient utilization,clean and environmental protection,hydrogen energy is regarded as the most promising clean energy in the 21st century.However,high-pressure hydrogen is prone to leakage during storage and transportation.If it encounters an open fire or is self-ignited due to shock wave,it is easy to form high-pressure under-expanded hydrogen jet flames,which could cause serious fire accidents.Therefore,it is a major research topic in the field of hydrogen safety and sustainable utilization to effectively reduce the harm caused by high-pressure under-expanded hydrogen jet flames.Water mist,as the most common fire-fighting method in fire accidents,has been applied as early as the 1940s.The application scope of water mist is constantly expanding due to the potential advantage of environmental pollution-free,and it also has important functions such as smoke removal,dust removal,cooling and radiation isolation,which can effectively protect the safety of personnel and equipment.Therefore,in this study,water mist technology was taken as an important means to suppress the harm of hydrogen jet flames,and a series of experiments on the interaction between under-expanded hydrogen jet flames and water mist were carried out to further explore the rationality and effectiveness of water mist in suppressing the harm of hydrogen jet flames.In this study,an experimental platform for the interaction between high-pressure under-expanding hydrogen jet flames and water mist was designed and established,including hydrogen jet flames system,water mist system,image acquisition system,pressure acquisition system,temperature and radiation acquisition system.By changing the type of water mist,horizontal distance,vertical height,pressure and adding K₂CO₃ additive,the influence of water mist on the characteristics of under-expanded hydrogen jet flames was investigated.The research revealed the effects of the fan-shaped water mist on the characteristics of under-expanded hydrogen jet flames.The variations of flame shape,flame horizontal length,plume temperature and radiation of hydrogen jet flames were investigated by changing the expansion angle,horizontal application position and applied pressure of the fan-shaped water mist.The experimental results showed that the hydrogen jet flames had deflection phenomenon when water mist was applied,and the flame tilt angle was defined.With the decrease of the expansion angle of the water mist,the increase of the applied pressure,and the horizontal application position gradually moving away from the hydrogen leakage nozzle,the flame bending angle increased continuously.The horizontal length of hydrogen jet flames,the temperature of plume area and the thermal radiation intensity all decreased significantly.The water mist obviously mitigated the thermal hazard of under-expanded hydrogen jet flames.The experiments showed that the suppression mechanism of fan-shaped water mist on under-expanded hydrogen jet flames was mainly due to the kinetic energy impact of water mist,which reduced the horizontal flame length and then reduces the harm range.The research revealed the effects of conical water mist on the characteristics of under-expanded hydrogen jet flames.The effects of conical water mist on the flame shape,horizontal flame length,plume temperature and radiation of under-expanded hydrogen jet flames under different vertical application heights,horizontal application positions and applied pressures were experimentally studied.It was found that hydrogen jet flames still had deflection phenomenon under the action of conical water mist.When the pressure of water mist was small,the flame expansion phenomenon of the hydrogen jet flames occurred,which caused that the horizontal length of the hydrogen jet flames increased obviously.With the increase of water mist pressure,this phenomenon began to disappear,and the horizontal length of flame gradually shortened.By comparing and analyzing the effects of fan-shaped and solid conical water mist on the characteristics of under expanded hydrogen jet flames,it is found that the suppression effect of solid conical water mist on hydrogen jet flames is more prominent,because on the one hand,solid conical water mist can reduce the harm range of jet flames,on the other hand,water mist also plays an important role in cooling the jet flame and blocking and reducing radiation.The research revealed the effects of conical water mist containing K₂CO₃ additive on the characteristics of under-expanded hydrogen jet flames.A solid conical water mist nozzle was used to investigate the effects of water mist on the under-expanded hydrogen jet flames under different mass fraction of K₂CO₃ additive,horizontal application position and pressure.It is found that the flame color was very bright under the action of water mist containing K₂CO₃ additive.When the water mist pressure was small（0.5 MPa）and the application position was at the flame lift-off position,the hydrogen jet flames still expanded and the flame length increased.Through the comparative analysis with pure water mist,it was found that the water mist containing K₂CO₃ additive had the most prominent inhibitory effect on the temperature and thermal radiation in the flame plume area of hydrogen injection,and the inhibitory effect increased with the mass fraction of K₂CO₃ additive.The action mechanism of water mist containing K₂CO₃ additive on hydrogen jet flames includes not only physical effects,such as kinetic energy impact,cooling and reducing thermal radiation,but also chemical inhibition mechanism,that is,the flame extinguishing active substances produced by the activation and decomposition of K₂CO₃ additive and the small particles released after heating and evaporation will block the chain reaction of hydrogen combustion,so as to achieve the purpose of inhibiting combustion.