Numerical Study On Hydrogen Leakage And Dispersion In Confined Space Of Hydrogen Powered Ship

In recent years,environmental pollution and fossil energy depletion have become increasingly serious worldwide.Shipping is currently an important part of world transportation and cargo trade,but most of the existing ships still use heavy oil or diesel as propulsion fuel.According to statistics,the shipping industry produces a large amount of air pollutants and greenhouse gases in human activities.With the gradual increase in maritime trade transactions,shipping emissions are on the rise in the future.The International Maritime Organization and governments have realized the serious threat posed by ship emissions to the world environment,and have introduced several rules and regulations to limit carbon emissions and pollutant emissions from ships one after another.Therefore,the energy transformation problem of shipping industry is urgent and serious.Hydrogen energy,with its unique advantages of clean and pollution-free,high energy density and high energy conversion efficiency,has gradually come into people’s view and become an important solution of clean alternative energy for ships,and various types of hydrogen-powered ships have come out one after another in recent years.As a major scenario of hydrogen energy application in the future,hydrogen-powered ships have relatively harsh sea conditions and the physical and chemical characteristics of hydrogen gas,such as easy leakage,easy diffusion,easy combustion and easy explosion,all of which pose great safety risks for the use of hydrogen energy in the field of ships.In this thesis,a series of numerical simulations are conducted to investigate the distribution law of hydrogen diffusion and provide a theoretical basis for the safe dispersion of hydrogen gas in the restricted space of ships.It is of great significance to the formulation of hydrogen energy-related codes and standards in the ship industry and the safe application of hydrogen energy in ships.The hydrogen leakage simulation study is carried out in the restricted space of the fuel cell compartment by numerical simulation of a yacht powered by hydrogen fuel cell as the main propulsion.Under the low leakage volume,the buoyancy effect dominates the hydrogen diffusion process,showing the flow state of a dense differential jet,and following the boundary layer principle,the hydrogen concentration stratification phenomenon is presented at the top of the compartment.With the increase of the leakage volume,the hydrogen flowed in a forced plume state dominated by the momentum flux,and completed the uniform distribution throughout the confined space relatively quickly,without forming the concentration stratification phenomenon.Under the condition of low hydrogen leakage volume: changing only the location of the leak hole,the distribution of hydrogen cloud and the location of the leak hole show spatial isotropy,but the volume share of the hazardous area is not affected by the location of the leak hole;changing only the location of the single natural vent,the natural vents at the left and right symmetrical positions have similar effects on hydrogen dispersion,following the symmetry law of physical structure,but the natural vent location has little effect on the volume share of hydrogen in the chamber.In the process of increasing the number of natural vents from one to six,the ventilation efficiency is significantly improved,the time required to reach the steady state is shortened,the concentration stratification phenomenon gradually disappears,and the volume share of clouds in the hazardous area continues to decrease,and the hydrogen dispersion effect is positively correlated with the number of vents.Under the condition of high hydrogen leakage: natural ventilation alone cannot meet the demand of safe hydrogen dispersion in the restricted space of the ship;when single fan forced ventilation is used,gradually increasing the fan air supply can significantly reduce the hydrogen concentration in the restricted space,and the inward air supply condition has better dispersion effect than the outward air supply condition,but neither of the two working modes can achieve safe dispersion when the hydrogen leakage is high;when two fans forced The simulation study found that the working mode of the front fan venting outward and the rear fan venting inward is more conducive to increasing the gas momentum flux in the restricted semi-enclosed chamber and achieving rapid and safe dispersion in the case of accidental hydrogen leakage with high leakage rate;when both fans are placed on one side,the farther the fan is from the leakage hole,the better the dispersion effect;when both fans are placed on the left and right sides of the chamber,the more the dispersion effect is.When the two fans are placed on the left and right sides of the chamber,the fluctuation of hydrogen mole fraction in the chamber is reduced,and the closer the two fans are placed to the leak hole,the better the hydrogen dispersion effect.