Study On Sedimentation Form And Erosion Law Of Solid Air In Liquid Hydrogen

Air inevitably penentrates into liquid hydrogen system in the process of liquid hydrogen charging and gas hydrogen displacement.Due to low temperature of liquid hydrogen,air will exist in solid state.If the proportion of oxygen in the solid air is greater than that in the air,liquid hydrogen system will have the risk of explosion.And oxygen-riched solid air will further increase the risk of system.Therefore,simulating the morphgology of solid air and the distribution of oxygen have guiding significance for determining oxygen safety threshod and reducing security risk in liquid hydrogen system.On the other hand,solid air will cause erosion and wear on variable cross-sections such as elbows,when flowing with liquid hydrogen.So the analysis of erosion law is conducive to optimizing pipeline design and further enhancing system safety.Numerical simulation is mainly adapt in this research.The method combining cellular automann and lattice Boltzmann method is used in crystal growth of solid air.Firstly,the original way of solute distribution is modified according to the gradient of oxygen concentration.The solidification model is established by referring to nitrogen-oxygen phase diagram.Taking flow field into consideration,the distribution of oxygen is studied at a certain cooling rate during the growth of solid-air crystal.Also the effect of different cooling rate,initial oxygen concerntration,intensity of flow and muti-grain growth are discussed.Results show that growth rate and peripheral oxygen concentration increase with cooling rate.The increase of initial oxygen concentration results in smaller concentration cooling and slower growth rate.Both peak oxygen concentration and oxygen concentration in front of interface keep near-linearly increasing with initial oxygen concentration.The interaction between crystals exits during the period of muti-grain growth.It shows that crystals inside are surrounded by high concentration diffusion layer of crystals outside,so their growth is inhibited compared with crystals outside.Peak oxygen concentration also increases near linearly with initial oxygen concentration.when intial oxygen concentration is21%,peak oxygen concentration reachs 26.9% after 0.54 s.Considering the existence of natural convection and forced convection in the solidification field,crystals of solid air exhibit asymmetric growth.When forced convection occurs,upstream side grows rapidly,while the growth downstream side is inhibited.For the distribution of oxygen and nitrogen,oxygen released at the upstream side is translated to the downstream side under the influence of flow field,so the gradient of oxygen concentration at the upstream side is much larger than that at the downstream side.Increasing inlet velocity can enhance the asymmetry of solid-air crystal and increase overall solidification rate.Initial oxygen concentration has little effect on crystal asymmetry.When natural convection exists,solid air grows slowly on the upper side,but fast on the underside.In this case,the asymmetry of solid-air crystal is enhanced by intial oxygen concentration.The peak concentration of solidification field changes little with time under different initial oxygen concentration,because natural convection promotes the diffusion of oxygen in the solidification field.Both peak oxygen concentration and solidification rate increase with the increase of intensity of flow.With the solidification time increasing,the difference of peak concentration corresponding to different intensity of flow tends to increase further.In the case of multi-grain growth,the average flow velocity and peak oxygen concentration under natural convection are smaller than that under forced convection,but the solidification rate is slightly higher than that under forced convection.When the solidification time is from 0.1s to1.19 s,the inner crystal equivalent diameter ranges from 2.73μm to 17.74μm.Finally,Selection of solid-air particle size is based on the equivalent diameter range of grain growth,and the effect of solid air on elbow erosion is studied by discrete phase model(DPM)in Fluent.The results show that the wear mainly occurs in the range of 60 to 90 degree on the outer wall of the elbow,and the wear rate is positively correlated with the mass flow rate of particles and a power function relationship with liquid hydrogen flow rate.The fitting curve can be used to predict the erosion wear of solid air on the elbow.The maximum wear rate of elbow decreases with the increase of curvature radius,while the average wear rate increases first and then decreases.The reduction of elbow wear can be achieved by Inhibition of solid air generation,reduction of liquid hydrogen flow rate,refinement of solid air particles and increase of curvature radius.