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物或复杂的管道形状，如孔口或收敛-扩张喷嘴。为此，开展了液氦沸腾两相流经水平管缩放喷嘴的流动可视化研究。管道中充满了加压的液氦，通过打开安装在管道出口的开关阀，立即产生流量。液氦从储罐中源源不断地供应，在喷嘴喉部加速流动，并通过压力下降引起相变。从高速视频图像中观察到了多气泡弥散的瞬时演化。与液氦和液氮的实验结果对比表明，在均匀的气泡状混合两相流中，液氦的两相流态比液氮更快地形成，其基本特征是多尺度结构…更真实的说法。为此，采用分子动力学方法对液氧中气泡的形成进行了数值模拟。分子间势采用Lennard-Jones势。在不同的数密度下进行了模拟，以确定亚稳极限处的数密度。结果表明，与Lennard-Jones流体状态方程的预测相反，双原子分子液体中气泡的形成密度大于单原子分子液体中气泡的形成密度。对不同重量和含气率的不纯液体中上升的球形气泡进行了直接数值模拟。采用矩形网格有限差分法求解Navier-Stokes方程。研究了不同含气率下上升气泡诱导的平均速度场。结果表明，气泡-气泡相互作用越强，空泡分数越高，边界厚度越薄。对管内液氦加油两相流的二维流动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，并考虑了超流的影响，对几种流动特性进行了数值计算。结果表明，由于动量方程中的超流效应引起压力梯度的变化，汽气相在内流管道中迅速扩散。较少

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