July 29, 2019
Over five decades ago, the first humans were launched into space and successfully survived, due primarily to the environmental control and life support systems (ECLS) on board the spacecrafts. All human-in-the-loop systems such as EVA suits and space habitats require fluids, both one-phase and two-phase, to maintain the crew’s health and well-being. One and two-phase fluid flows are also germane to systems such as cryogenic fuel systems and in-situ resource utilization (ISRU). As NASA and international collaborators plan to return crews to the Lunar surface and eventually the Martian surface, ECLS and other fluid-based systems will need to operate in microgravity as well as in the partial gravity levels of the Moon and Mars (16 g and 38 g respectively).
When studying two-phase fluid dynamics in convection or buoyancy-driven flows, the gravitational acceleration levels of interest can be split into four regimes: terrestrial gravity (defined as 1g), microgravity (10-6 g), partial gravity (between 0 g and 1 g), and hypergravity (above 1g). This research is focused on 1 g, microgravity and variable partial gravity. Bubble formation and rise velocity are fundamental mechanisms of heat and mass transfer in two-phase fluids in 1 g and can be used to create models of these flows in partial gravity environments. In microgravity models, surface tension forces and effects dominate the bubble formation and movement, while in terrestrial models, the buoyant force dominates the bubble formation and movement. The parametric relationships between fluid flows in 1 g and microgravity are not well understood. The authors’ experimental objectives include creating experimentally-verified computational models of gas behavior in a liquid under varying gravitational environments. This poster will present the results from Phase I of an experimental protocol in steady-state 1 g, discuss Phase II, a proposed experiment on board the International Space Station, and summarize Phase III, a fluids experiment on the surface of the Moon.
The experimental protocol being studied by the authors is the formation of nitrogen bubbles at an orifice and their movement in water at various gravitational acceleration levels. High-speed video data of bubble formation at an orifice and its rise in water after separation have been successfully captured. From the high-speed video data and assuming an axisymmetric bubble, important parameters have been extracted such as bubble position, bubble volume, bubble shape, bubble velocity, contact angle, volumetric flow rate of nitrogen into the bubble, and bubble surface area. Initial Computational Fluid Dynamic (CFD) models have also been created to model the physical experiment. The CFD work utilizes OpenFOAM’s InterFoam solver. The InterFoam solver is a two-phase, incompressible, isothermal, immiscible solver which uses the volume of fluid (VOF) method.
Future Phase II research will compare the bubble models created by the 1 g Phase I data with data obtained in an artificial partial gravity analog. Even though drop towers, and parabolic flights can create transient, partial gravity environments, long-term steady-state partial gravity conditions cannot be simulated on the surface of the Earth. A Low Earth Orbit (LEO) centrifuge has long been considered the best analog for partial gravity, short of actually being on the surface of either the Moon or Mars. LEO centrifuges have been used for biological research on board the Space Shuttle/SpaceLab for decades, as well as on the International Space Station (ISS). This research project focuses on designing a fundamental fluids experiment to be implemented in an artificial gravity centrifuge on board the ISS. While centrifugal acceleration can approximate partial gravity, fluid flow modeling, however, must take into account the Coriolis effect. Still, centrifugation will create the best approximation to date. Creating long-term partial gravity conditions using a centrifuge will enable the capture of bubble movement after detachment and bubble-to-bubble interactions once multiple bubbles detach from the orifice.
The authors are planning to utilize Techshot’s Multi-use Variable-gravity Platform (MVP) system which is currently performing artificial partial gravity experiments on board the ISS in an EXPRESS rack. The authors’ proposed experiment will be housed in an MVP experimental module and would operate for 21 days at various gravity levels maintained by the MVP system, with the data collected by a flow visualization system. MVP itself is an innovative platform being flown on board the ISS. The proposed research would be the first fluids research project to be flown on board MVP. Based on discussions with Techshot’s engineering and integration teams, the proposed experiment is expected to fly on board the MVP system within 24 months.
Phase III of this research plan includes housing a fluids experiment in an “AggieSat”, built at the Texas A&M University, which would be transported to the surface of the Moon and used to measure the effects of steady-state 16 g on heat and mass transfer in liquids.
The data and models which will be produced as a result of this proposed research could be used to inform both in-space and terrestrial applications. Bubble models in varying acceleration environments which account for buoyant forces, surface tension, and centrifugal effects will aid in the development of life support systems for Lunar and Martian surface missions, ISRU process development, cryogenic fuel management, and inform the future of ECLS in artificial gravity habitats. Although bubble formation has been studied extensively in Earth’s gravity and to a degree in microgravity, there are no experimentally-tested models in steady state partial gravity levels. This research aims to create the first experimentally-verified models of bubble formation in partial gravity environments.
Burke, Paul. Varnum-Lowry, Daniel. Microgravity and Partial Gravity Fluid Physics: Bubble Formation and Movement in Variable Gravity Environments. Poster presented at: The International Space Station (ISS) Research and Development conference (ISSR&D 2019) held in Atlanta, Georgia.; 2019 July 29 – Aug 1;