Investigation On The Dispersion Behavior Of Hydrogen Vapor Cloud Formed By Large-Scale Liquid Hydrogen Spill

Liquid hydrogen has been widely used as a new generation of aerospace propellant,and shows good application prospects in fuel cells,hydrogen vehicle,etc.However,the flammable and explosive clouds formed when liquid hydrogen spills into surrounding environment seriously threat the safety of personnel and equipment.In the past decades,researchers have made some progress in the experimental and numerical studies of atmospheric dispersion of spilled liquid hydrogen.Objects of this thesis are to further improve the prediction accuracy of the numerical model,to comprehend the dispersion mechanism of vapor cloud,and to establish a prediction method for the safe distance which is suitable for engineering application.The following researches were carried out:1)A numerical model for predicting the dispersion behavior of spilled liquid hydrogen was established.The model solves the continuity equation,momentum equation and energy equation for the mixture,and predicts the mass transfer rate between liquid/gaseous phases using the modified Lee model.The numerical model considers the slip velocity between phases,the distribution of wind speed and turbulence in the atmospheric boundary layer,and the phase change of water in the atmosphere.With the consideration of the phase change of water,the model accuracy is improved in predicting the hydrogen concentration distribution,the time variation of hydrogen concentration and the location of visible cloud.2)The spread mechanism of hydrogen vapor cloud was studied,including the spread and dilution of hydrogen vapor cloud under the interrupted spill and the continuous spill,the effect of turbulent disturbance in the dispersion process,and the temperature-concentration relation of hydrogen.For the interrupted spill,the dispersion of flammable vapor cloud can be divided into the initial spill stage,the main dispersion stage and the disappearance stage.For continuous liquid hydrogen spills,the spread and concentration distribution of flammable vapor cloud will reach a stable state.Source termination has a hysteresis effect on the spread of hydrogen cloud,while the cloud volume deviates from the original trajectory immediately.The spill source vicinity,the top of the cloud,and the cloud leeward side in the low height domain will be significantly affected by turbulent disturbances during cloud dispersion.In the near-ground and near-source domain,the concentration-temperature relation of hydrogen is influenced by the heat conduction from ground,or the heat absorbed by evaporated liquid hydrogen.3)The atmospheric dispersion of spilled liquid hydrogen under different conditions was analyzed,including the wind field,the heat transfer from the ground,the spill source,and the atmospheric humidity.The dilution time of hydrogen cloud increases first and then decreases with the increase of the wind speed.The upwind area of the hydrogen cloud also affects the dilution process,and the cloud dilution rate is determined by the atmospheric turbulence,the turbulent disturbance caused by the rise of the vapor cloud,the upwind area of hydrogen cloud,and the strong turbulent disturbance near the spill source.The variation of ground temperature under natural environment has little influence on the dispersion of hydrogen cloud,due to the large temperature difference between the ground and the liquid pool,and the compensation of the liquid pool radius.The dilution of hydrogen vapor cloud is most sensitive to the liquid mass fraction at the source,followed by the spill rate and the spill amount.The streamwise distance of flammable cloud varies little with relative humidity for large-scale liquid hydrogen spill.The hydrogen concentration on the leeward side is higher than that on the windward side under the same temperature,indicating that the leeward side is more affected by the phase change of water.4)The prediction method for the safe distance was established.The influences of dike and its size on the wind flow field and the cloud dispersion process were studied.Moreover,the protective measures when liquid hydrogen spill occurred were given.Based on the heat balance analysis and the mass conservation law,the theoretical relation for predicting the pool radius is established.The spread extension of the flammable cloud in streamwise direction is fitted as the function of the reference wind speed and the spill rate,with the maximum relative error of 11.76%.The dike enhances the mixing of the hydrogen cloud with the surrounding air inside the dike,and significantly reduces the detachment distance.The detachment distance of the vapor cloud decreases with the increment in dike height,while increasing the height to length ratio of the dike worsens the situation near the source.When liquid hydrogen spill occurs,the dispersion of hydrogen vapor cloud can be promoted by forced ventilation,laying graveled or uneven ground,constructing dike,etc,while the underground heating device has little effect.