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

使用低温流体的应用通常会遇到障碍或复杂的管道形状，如孔板或会聚-发散喷嘴。为此，对液氦沸腾两相流通过水平管的会聚-发散喷嘴进行了流动可视化研究。管内充满加压液氦，打开安装在管道出口的开关阀，立即发生流动。液氦从储气罐中连续供给，在喷嘴喉部加速流动，压力降低引起相变。通过高速视频图像观察了多泡色散的瞬态演化过程。与液氦和液氮的结果比较表明，在更均匀的气泡混合两相流中，液氦的两相流态比液氮的两相流态形成要快得多。两相流的基本特征是其多尺度结构。因此，在以下尺度上进行了数值分析：用分子动力学方法模拟液氧中的气泡形成。Lennard-Jones势被用作分子间势。为了确定亚稳极限下的数密度，在不同的数密度下进行了数值模拟。结果表明，在双原子分子液体中，气泡的形成密度比单原子分子液体大，这与Lennard-Jones流体安全方程的预测相违背。对不纯液体中球形气泡在不同重力和空隙率下的上升过程进行了直接数值模拟。采用矩形网格系统，用有限差分法求解了Navier-Stokes方程。研究了不同孔隙分数下气泡上升引起的平均速度场。结果表明，随着孔隙率的增加，气泡与气泡之间的相互作用增强，边界厚度变薄。对液氦在管道内充油两相流的二维特性进行了数值研究。建立了基于非定常漂移通量模型的控制方程，并在考虑超流动性影响的情况下，对几种流动特性进行了数值计算。结果表明，由于动量方程中出现的超流动性的影响，压力梯度发生了变化，汽气相在管内迅速扩散。少

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