The separation of two-phase flow is essential for many fluid systems in the microgravity environment. Unlike the situation on earth, gas and liquid cannot separate spontaneously due to the absence of buoyancy in space. Therefore, phase separators must be designed to complete this task. The passive cyclonic separator is a prominent technology for gas-liquid separation in microgravity for the reason that it is free of maintenance and power consumption. However, the complex flow physics within the separator presents significant challenges in designing the device and characterizing its performance. In the present study, numerical simulations and analytical modeling are combined to construct a system of control volume equations that can quantitatively describe the key features of the separator, as well as defining the parameters for design purposes. The numerical simulations are conducted utilizing an open-source computational fluid dynamics (CFD) software OpenFOAM. Both two dimensional (2D) axisymmetric modeling and three-dimensional (3D) modeling are performed. The CFD work in the current study focuses on pure-liquid injection cases in which gas cores would still form without small gas bubbles. Therefore, the volume of fluid (VOF) approach is used to capture the large scale gas-liquid interface. Since the swirling flow inside the separator is mostly turbulent, a Reynolds stress transport model (RSTM) is adopted for the simulations. The CFD technique provides enough amount of data to study the important parameters of the separator, which facilitate and complete the analytical modeling. The analytical modeling is developed based on control-volume approximations. This approach helps clarify the fluid physics by transforming the complicated physical phenomena into simple control-volume equations representing the mass and angular momentum conservations as well as pressure balance at the interface. With the aid of CFD results, the control-volume equations are closed. The predictions produced by solving the equations are reasonably accurate compared to the experimental results. Then, the equations are further improved to include gas-liquid two-phase injection situations. In addition, the surface tension effect is also investigated and included in the analysis.

This Volume 5 of the successful book package "Multiphase Flow Dynamics" is devoted to nuclear thermal hydraulics which is a substantial part of nuclear reactor safety. It provides knowledge and mathematical tools for adequate description of the process of transferring the fission heat released in materials due to nuclear reactions into its environment. It step by step introduces into the heat release inside the fuel, temperature fields in the fuels, the "simple" boiling flow in a pipe described using ideas of different complexity like equilibrium, non equilibrium, homogeneity, non homogeneity. Then the "simple" three-fluid boiling flow in a pipe is described by gradually involving the mechanisms like entrainment and deposition, dynamic fragmentation, collisions, coalescence, turbulence. All heat transfer mechanisms are introduced gradually discussing their uncertainty. Different techniques are introduced like boundary layer treatments or integral methods. Comparisons with experimental data at each step demonstrate the success of the different ideas and models. After an introduction of the design of the reactor pressure vessels for pressurized and boiling water reactors the accuracy of the modern methods is demonstrated using large number of experimental data sets for steady and transient flows in heated bundles. Starting with single pipe boiling going through boiling in the rod bundles the analysis of complete vessel including the reactor is finally demonstrated. Then a powerful method for nonlinear stability analysis of flow boiling and condensation is introduced. Models are presented and their accuracies are investigated for describing critical multiphase flow at different level of complexity. Therefore the book presents a complete coverage of the modern Nuclear Thermal Hydrodynamics. This present third edition includes various updates, extensions, improvements and corrections.