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.

The separation of multiphase flow constituents in a microgravity environment is of considerable interest as the functionality of many spacecraft systems is dependent on the proper sequestration of interpenetrating gas and liquid phases. Cyclonic separators provide the desired gas-liquid separatory action by swirling the multiphase flow {8211} causing the gas to accumulate along the axis of the vortex as the denser liquid is forced to the walls {8211} thereby allowing segregated extraction of the respective phases. Passive cyclonic separators utilize only the inertia of the incoming flow to accomplish this task. Knowledge gaps regarding their performance, however, preclude their use in operational systems. In the current work, combined experimental, numerical, and phenomenological modeling analyses have been performed to quantitatively describe the performance of these separators. First, a multifluid-VOF CFD technique was developed that could model the unique flow features in microgravity cyclonic separators. This multiscale approach could handle the disperse phase while also capturing the relevant capillarity features at gas core. The CFD techniques were then used to simulate the separator performance under the conditions of steady and unsteady injection of both single-phase and multiphase flow. The results were found to compare reasonably well with experiment. Next, a phenomenological control volume model was developed to capture the relevant physics of the flow in the separator. The approach was found to be a quick way to produce approximate results. Doing so helped to elucidate the fluid physics and allowed for the creation of operational maps that can be used as an aid for future design development. These techniques contributed to new design approaches to improve separator performance. Lastly, a reduced-gravity flight experiment was conducted confirming the performance of the separator design at flow rates impermissible in terrestrial conditions. The control volume model that was developed was compared to the results and good agreement was found. It is hoped that the resulting insight into phase separation, distribution, and control offered by this work will help to afford future designers the latitude to take greater advantage of the benefits offered by the use of multiphase systems in spacecraft applications.