Measurements And Modeling Of SupersonicHydrogen Jets Through Rectangular Nozzles

Hydrogen energy is an important energy form with the increasing need to reduce fossil fuel usage and dependence on oil imports,as well as to control pollutant emissions.The commercialization of hydrogen energy has accelerated over the past few years.Safety problems have to be thoroughly studied to ensure that hydrogen energy is as safe as the traditional energy sources.As a basic direction of hydrogen safety research,hydrogen leaks and diffusions are of great significance to the improvement of hydrogen safety standards.Hydrogen is usually stored at very high pressures.Hydrogen leak may occur on pipes,joints or tanks,and through cracks with various shapes.For filling the gap of non-circular nozzle jets,this paper simplifies the actual leak to rectangle nozzles with different aspect ratios(AR),and conducts a series of experimental,simulation,and analytical studies.First,a high-pressure gas release device was built,and a series of changeable nozzles were designed.The equivalent diameters of these nozzles ranged from 0.55 to 1.5 mm,and the aspect ratios ranged from 1 to 12.6.A high-speed camera was used to the nozzle exit sizes.Helium was used as a simulant of hydrogen for safety reasons,since helium has similar physical properties as hydrogen.A feedback pressure system was used to adjust the release pressure,which can maintain the pressure at the target pressure for a period of time to measure the concentration data along the jet centerlines.The maximum test pressure was 3.7 MPa.The results show that the rectangular nozzle jets also have self-similarity as circular nozzle jets.The jet parameters can be normalized by equivalent diameters and release pressures.The diffusion rate of AR1 nozzle was much smaller than those for other nozzles,and the jet diffusion rate through nozzles with aspect ratios other than 1 is not affected by pressure in the pressure range of the experiment.A numerical simulation study has conducted on the hydrogen jet through the rectangular nozzles to investigate the effects of higher pressures and more aspect ratios.The simulation study was based on the ANSYS Fluent,using the RSM turbulence model.and the two-step calculation method to improve the efficiency of the simulation.The numerical model was validated by the experimental results.Then,a series of simulations with different nozzle aspect ratios and test pressures were performed.The simulations well predicted the axis switching phenomenon of the rectangular nozzle jet,and this phenomenon became more obvious with the increasing release pressure.The maximum pressure was 20 MPa in the simulation,which was much higher than those in the experiment.At this pressure,the nozzle aspect ratio will significantly affect the diffusion of the hydrogen jet.A higher nozzle aspect ratio resulted in a greater the diffusion rate.Therefore,a correction factor f(AR,P)was used to normalize the jet centerline distances to consider the effects of the aspect ratio and pressureFor quick prediction of the flow diffusion of the rectangular nozzle jets,a two-dimensional plane integral model was developed to calculate the flow on the major and minor axis planes.Assuming that the non-axisymmetric flow field formed by the rectangular nozzle was composed of numerous small "notional jets",and the flow fields of the "notional jets" were calculated and superimposed to form the integral velocity and concentration fields.To simplify the calculation,only the velocity and concentration fields on the major and minor axes planes were calculated.The velocity field and concentration field calculated by numerical simulation were used as the inlet conditions,and the inlet was discretized into a large number of small "notional jet"inlets.By comparing with the simulation results,the two-dimensional plane integral model accurately and robustly predicted the velocity and concentration distributions of the minor axes plane.This method provides an efficient method for leak diffusion predictions of rectangular nozzle jets,and supplements the physical model for risk assessment which is beneficial to the application of hydrogen safety engineering.