Leakage And Diffusion Model And Test Research Of High Pressure Underexpanded Hydrogen

Hydrogen is a clean and highly efficient renewable energy,with high conversion efficiency as the main fuel for hydrogen fuel cell vehicles.Hydrogen fuel cell vehicles are superior to traditional fossil fuel vehicles in reducing carbon emissions.However,hydrogen has a wide flammability range(4%-75% vol)and a very low minimum ignition energy(0.017 m J),which poses safety hazards in high-pressure storage and transportation.High-pressure hydrogen leaks from cracks in the high-pressure delivery pipeline,causing combustion or explosion near the leak.The high-pressure hydrogen forms a high-pressure under-expanded jet at the leak.Aiming at the insufficient improvement of the high-pressure under-expanded jet model,this thesis proposes an improved high-pressure under-expanded jet model to study the leakage mechanism and diffusion law of high-pressure hydrogen.The model can predict the flow field characteristics of high-pressure hydrogen leakage and provide a reference for solving hydrogen safety problems.The general virtual nozzle model is suitable for calculating high-pressure hydrogen leakage,but it only assumes that the actual leakage outlet satisfies the mass conservation equation at the virtual leakage outlet,and the friction between the high-pressure hydrogen and the pipeline wall is not considered.Therefore,this paper improves the virtual nozzle model and adds the friction loss term of the airflow wall to the momentum conservation equation.Based on the integral model and the improved virtual nozzle model,a theoretical model of highpressure under-expanded jet is proposed.The high-pressure under-expansion jet model can calculate the hydrogen concentration field and obtain the distribution range of the hydrogen jet.The concentration and velocity of hydrogen along the centerline of the jet conform to the hyperbolic decay law by analyzing the characteristic curve of the high-pressure hydrogen leakage along the centerline of the jet.The radial concentration distribution of the high-pressure under-expanded hydrogen jet on different sections conforms to the Gaussian distribution law.This paper conducts numerical simulation research in three ways,namely,the low-pressure integral leakage model,the high-pressure under-expanded jet model,and the simplified tworegion model based on the basic control equations and numerical models of fluid mechanics.Among them,the inlet conditions simulated by the first two are pure hydrogen flow.The simplified two-region model has two inlets,namely,the core region is a pure hydrogen flow,and the boundary layer region is a flow of hydrogen and air mixed according to the gas flow parameters at the boundary of the Mach plate in the shock wave.By comparing the three simulation methods,the low-pressure integral leakage model can simulate the low-pressure gas jet field.The calculation is simple but the model is difficult to simulate the high-pressure hydrogen jet field.The high-pressure under-expansion jet model can simulate the shock wave structure of the high-pressure hydrogen jet at the nozzle outlet,which has higher requirements on the quality of the grid.The number of grids required for the simplified two-zone model modeling is half of that of the high-pressure leakage simulation,and the calculation is simple.The simplified two-region model can simulate the flow of the core area and the boundary layer in the actual hydrogen jet,which is in line with the actual working conditions of the highpressure hydrogen jet.In order to verify the accuracy of the simplified setting of the equivalent inlet conditions of the two-region model,the gas infrared thermal imaging technology visualization method can be used to study the hydrogen leakage and diffusion laws.The boundary contour formed by the hydrogen jet can be determined by comparing the initial state and the ejected state of the water vapor cloud by the hydrogen jet impacting the high temperature water vapor cloud.Comparing the results of the visual experiment with the shock wave structure of the equivalent entrance of the simplified two-region model,the boundary layers of the two are in good agreement to verify the accuracy of the two-region model.In order to verify the reliability of the high-pressure under-expansion jet model,a highpressure gas leakage test platform was built in this experiment,in which a gas thermal mass flow meter was used to monitor the gas velocity,and the pressure fluctuation was measured by a gas pressure sensor.In this test,helium is used to replace hydrogen in consideration of test safety,and helium concentration sensor is used to measure the gas concentration.The experimental study of the diffusion law of high-pressure under-expansion hydrogen at different pressures,and the curve of the initial diffusion concentration of helium gas with time is obtained at a distance of 10 cm from the nozzle.In the low pressure leakage and simplified two-region model flow field,the hydrogen concentration monitoring point is set at the same position.The comparison results of the two-region model are in good agreement with the test results,while the low-pressure leakage simulation calculation value is low,and the simplified two-region model calculation result is accurate.After the pressure in the leaking tank is stabilized,the test measures the law of high-pressure under-expanded jets at different pressures.The measurement results of the concentration of helium gas show that the attenuation of the jet’s mole fraction along the axial direction conforms to the hyperbolic attenuation law.The high-pressure underexpanded jet model is used to simulate the concentration distribution of hydrogen along the centerline of the jet.Compared with the experimental data.This result verifies the reliability of the high-pressure under-expanded jet model proposed in this paper.It can accurately calculate the gas along the centerline of the horizontal jet concentration.This paper conducts theoretical and experimental research on the leakage and diffusion of high-pressure hydrogen with the support of the major scientific and technological project of Shanxi Province,"Medium-sized hydrogen fuel truck platform complete vehicle flame-proof and explosion-proof safety protection technology"(No.20181102006).The proposed highpressure under-expanded hydrogen jet improved model can calculate the characteristics of the centerline of the jet,and tests have been conducted to verify the reliability of the built model.The research results have reference significance for predicting the flow field characteristics of hydrogen leakage,and lay a theoretical foundation for solving hydrogen safety problems in engineering.The research results have reference significance for predicting the flow field characteristics of hydrogen leakage,and lay a theoretical foundation for solving hydrogen safety problems in engineering.