Background Two phase flow, flow of both a liquid and gas state, is crucial in countless space applications including power cycles, the storage and transfer of liquid fuels, cryogens, propellants, life support systems, and thermal control systems. However, truly steady and fully developed flow has not yet been achieved in low-g experiments due to limitations in hardware, size, cost, and time. Presently, necessary applications have been either over-designed or designed to completely avoid two- phase flow conditions.

Research Previous experimentation has been overwhelmingly dominated by pressure driven flow. Large complex systems have been used to force a liquid slug or a gas bubble through a long straight tube. Bubble Tube To Syringe Pump To Syringe AB Valve A Valve B These systems can be large, difficult to control, expensive, complex, and unable to achieve steady-state conditions. Jing-Den Chen, 1986

Fluid Behavior As a liquid slug passes through a tube, a thin film is deposited along the inside walls. This film thickness fluctuates depending upon the gravitational conditions. If correctly predicted and modeled, system parameters such as pressure drop and heat transfer rate can be calculated simplifying designs by reducing unnecessary size, weight, and cost. In reduced gravity, typically neglected forces become more dominant and may even control the flow regime. In the case of two-phase slug flow, surface tension becomes a prevailing force and must be included in all analysis. θ V L R rR r R aR a h hh h h

More Fluid Behavior In a reduced gravity environment, it is difficult to assure of the location of liquid and gas phases in a two-phase system. Similar to a 1-g environment, an applied external force can be exploited to collect and position the liquid into a single continuous slug. An unusual idea of applying a simple centrifugal force on a closed circular loop became the new model for this experiment. This unique solution eliminates the control concerns of pressure driven flow, reduces the size, weight and complexity of the experiment, and will ultimately allow for a steady-state flow field.

1g Design The simplicity of the reduced gravity design allows for testing and modeling in a 1g environment. Steady flow is easily achieved by mounting a tube of fluid on a circular disk. To further predict fluid behavior in reduced gravity, the disk is tilted at various angles, therefore lessening the effect of gravity in the direction of flow. In addition, various tube sizes, fluid viscosities, and rotational speeds are examined to determine the most appropriate combination for the one chance reduced gravity experiment.

1g Results For verification purposes, the 1g data was compared with several published predictions. The closed circuit loop demonstrated that steady-state slug flow is easily achieved and well-predicted by established analyses.

A New Solution A small disk holding several sealed tubes of a liquid and gas mixture is mounted above a larger plate. By rotating the large plate, a centrifugal force is applied to the small disk positioning one continuous slug to the outside rim. The second smaller disk can then be rotated resulting in two-phase slug flow. This simple design allows for fine control of both rotational speeds while quickly providing steady-state, fully-developed slug flow.

Reduced Gravity Design The experiment is enclosed in an aluminum frame and undergoes a rigorous safety evaluation to NASA specifications. Due to the unusual environment aboard the reduced gravity aircraft, individual components must be designed to withstand severe loadings, as high as nine times gravity in certain directions.

Analysis A remote camera above the small disk records and transmits data to an onboard video display. While the flow is monitored during the flight, the images are analyzed at a later date. The video is downloaded and the flow properties scrutinized using image analysis software. From these images, film thicknesses, dynamic contact angles, deposition rates and transient and steady state flow regimes are examined.

References Balakotaiah, Vemuri and Larry Witte. “Studies on Two-Phase Flows at Normal and Microgravity Conditions.” 1998/1999. Institute for Space Systems and Operations. 29 June Chen, Jing-Den. “Measuring the Film Thickness Surrounding a Bubble Inside a Capillary.” Journal of Colloid and Interface Science. 109 (1986): Colin, C., J.Fabre, and A.E. Dukler. “Gas-Liquid Flow at Microgravity Conditions.” Int. J. Multiphase Flow. 17 (1991): Hallinan, Dr. Kevin, and Jeffery S. Allen and Jack Lekan. “Capillary-Driven Heat Transfer (CHT) Investigation, MSL-1, STS-83.” NASA. August, Incropera, Frank P., and David P. Dewitt. Fundamentals of Heat and Mass Transfer. New York: John Wiley & Sons, 2001: 357. Taylor, G.I. “Deposition of a viscous fluid on the wall of a tube.” Journal of Fluid Mechanics. 10 (1961): Young, Donald F., and Bruce R. Munson and Theodore H. Okiishi. A Brief Introduction to Fluid Mechanics. New York: John Wiley & Sons, 2001: Special Thanks : The NASA Reduced Gravity Student Opportunity Program, the Oregon Space Grant Program, Mark Weislogel, Portland State University’s College of Engineering and Computer Science, Infinity Images, FMC Allen Machinery, and Chehalem Machine Works