Research On The Leakage And Dispersion Characteristics Of Hydrogen And The Spatiotemporal Distribution Of Hydrogen Concentration In Confined Space

As a kind of secondary energy source,clean,efficient and sustainable hydrogen energy has received increasing attention under the background of the energy crisis and environmental issues.However,due to its wide combustible range,low minimum ignition energy and fast burning velocity,hydrogen can easily cause fire and explosion accidents once it leaks and accumulates in a confined space.This thesis focuses on the accident scene of hydrogen leakage and dispersion in underground parking garages,and the simulation experiment system of hydrogen leakage and dispersion in confined space is built,and the corresponding numerical simulation models are established.By means of experimental and numerical methods,the hydrogen leakage and dispersion behavior in confined space and its influencing factors,as well as the spatiotemporal distribution of hydrogen concentration,are systematacially studied.In this thesis,the dispersion process of hydrogen(replaced by helium)in confined space and the dominant mechanism are studied first.Then,the influence mechanism of various conditions on the variety of helium concentrations on the ceiling is revealed.Finally,the characteristics of the spatiotemporal distribution of the hydrogen concentration under different conditions are analysed,and the evolution law of the critical hydrogen concentration is obtained,which provides a scientific basis for the prevention and control of hydrogen leakage and dispersion in confined spaces.The main contents and conclusions are as follows:First,based on the simulation experiment system of hydrogen leakage and dispersion in a confined space established in this thesis,helium release experiments at different initial leakage rates and leakage directions are conducted to obtain the variety of helium concentrations at different locations during and after the leakage.If the leakage lasts for a long time,the helium concentration will go through three stages,namely,rapid growth,slow growth and a relatively stable state.Due to the obstruction of the beam on the ceiling,the helium-air mixture generates a "filling front" and moves down,and then this filling process repeats in adjacent areas.In the case of vertical downwards leakage,the helium jet moves in a radial direction after hitting the ground and eventually spreads upwards due to buoyancy.Different initial leakage rates lead to different helium concentration distributions.With the increase of the initial leakage rate,the momentum-dominated length of the helium jet increases gradually,and the helium concentration near the ceiling decreases gradually.After the leakage is stopped,the helium concentration shows an exponential decay trend,and the helium concentration attenuation curves at different positions gradually coincide after a certain time.Furthermore,helium release experiments under different leakage conditions are conducted,and the influence of the leakage flow rates,leakage hole diameters,leakage directions and leakage locations on the helium concentration near the ceiling of the underground garage is analysed.The results show that the degree of influence of the leakage flow rates and leakage hole diameters on the helium concentration near the ceiling is related to the leakage direction.When the leakage direction is upwards vertically,the larger the leakage hole diameter,the higher the helium concentration.When the leakage direction is downward vertically,the larger the leakage flow rate,the higher the helium concentration.Relatively speaking,among the three leakage directions,namely,upwards vertically,horizontal and downwards vertically,horizontal leakage causes the lowest helium concentration near the ceiling.When the leakage direction is upwards vertically,the helium concentration at different measuring points has an exponential relationship with the dispersion path length from the point to the center of the ceiling above the leakage source.Finally,the characteristics of the spatiotemporal distribution of hydrogen concentration and the variety of the critical concentration of hydrogen under different leakage directions and leakage locations are obtained through the numerical simulation of hydrogen leakage and dispersion behavior in underground parking garages.The hydrogen dispersion process follows the cyclical pattern of accumulation-dispersion-accumulation and the hydrogen only spreads to the adjacent area of the ceiling.The presence of the beam reduces the dispersion range of hydrogen,but prolongs the accumulation time of hydrogen near the ceiling.The volume whose hydrogen concentration is above the alarm concentration of the hydrogen detector(1%)increases rapidly in the early stage of leakage,but its growth rate slows down and even levels off in the later stage.The volume whose hydrogen concentration is above the lower flammable limit of hydrogen(4%)is related to the leakage direction.The volume whose hydrogen concentration is above the lower explosive limit of hydrogen(18.3%)increases sharply within a very short time after leakage,and then increases slowly continually.After the leakage is stopped,the volume whose hydrogen concentration exceeds the above three limits corresponding to horizontal leakage is the smallest among the three leakage directions,and the volume whose hydrogen concentration exceeds that corresponding to leaking in the corner of the garage is the smallest among the three leakage locations.