Study on Instabilities of Cryogenic Two-Phase Flows in a Pipe

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Applications using cryogenic fluid generally encounter obstacles or complex pipe shapes such as an orifice or a converging-diverging nozzle. Therefore, a flow visualization study on boiling two-phase flow of liquid helium passing through the convergent-divergent nozzle of a horizontal pipe was carried out. The pipe is filled with pressurized liquid helium and flow immediately occurs by opening the on-off valve installed in the pipe outlet. Liquid helium is continuously supplied form the storage tank, the flow is accelerated at the nozzle throat and the phase change is induced by a pressure decrease. The transient evolution of the multi-bubble dispersion was observed from a high-speed video image. Comparison with results of liquid helium and liquid nitrogen indicated that the two-phase flow state of liquid helium is much more rapidly formed in a more homogeneous bubbly mixture two-phase flow than that of liquid nitrogen.The fundamental feature of two-phase flows is its multi-scale struc … More ture. Thus, numerical analyses were carried out in the following scales :The bubble formation in liquid oxygen was simulated by the Molecular Dynamics method. Lennard-Jones potential is used as the intermolecular potential. Simulations were carried out at various number densities in order to determine the number density at the limit of metastability. The results showed that in the liquid of diatomic molecules the bubble is formed at large number density than in the liquid of monatomic molecules against the prediction of the equation of sate of Lennard-Jones fluid.The direct numerical simulations of rising spherical bubbles in an impure liquid were conducted for various gravities and void fractions. The Navier-Stokes equations were solved by the finite difference method with the rectangular grid system. The mean velocity field induced by rising bubbles was investigated for various void fractions. The results showed that the boundary thickness becomes thinner in the higher void fraction due to the stronger bubble-bubble interaction.The two-dimensional characteristics of the oiling two-phase flow of liquid helium in a duct were numerically investigated. The governing equations based on the unsteady drift-flux model were conducted and several flow characteristics were numerically calculated, taking into account the effect of superfluidity. The results showed that the vapor gas phase rapidly spreads throughout the inner flow duct because of the change of the pressure gradient due to the effect of superfluidity which appears in momentum equations. Less