ISS: An Experimental Study of Slip Ratio in Microgravity Two-Phase Flows

Flows involving mixtures of gas and liquid are common in technological applications involving boiling and condensation. For example, refrigerant in air conditioners continually boils and recondenses. Modern computer chips may also use flows of gas and liquid mixtures for efficient cooling. In two-phase flows, the gas phase and the liquid phase move through flow channels at different velocities. Measurements of gas and liquid velocities under boiling conditions have been conducted on Earth, but velocities were inferred from the amounts of gas and liquid moving through the channel. This project will use video tracking of microbubbles onboard the International Space Station (ISS) to measure velocities directly. In microgravity conditions on the ISS, microbubble motion will not be influenced by buoyancy effects, and the microbubbles will trace the liquid flow accurately. New equipment will be designed to directly image the gas and liquid using high-speed video. The velocities of the gas and liquid will then be measured from video analysis. The equipment will first be validated on Earth and then used on the ISS. This project will advance understanding of flow boiling and improve accuracy in designing two-phase systems. The project will also provide research training for students outreach activities for pre-college students. In two phase flow, the ratio of the velocity of gas to that of the liquid is called the Slip Ratio (SR). Existing SR correlations are derived from void fractions. This project will design and conduct experiments to independently measure local vapor and liquid velocities in microgravity on the International Space Station. Measurements will use high-speed visualization and advanced computer-vison analysis. This project will quantify the effects of mass flux, inlet subcooling, and applied heat flux on SR during flow boiling in a transparent quartz channel. It will result in universal SR correlations applicable across different gravity environments and operating conditions. To achieve these research objectives, novel techniques will be developed to facilitate measurements of SR in flow boiling. These include (a) using microbubbles as tracer particles to accurately measure liquid velocities during flow boiling, (b) new laser-based manufacturing methods to create the microslots necessary to generate microbubbles, and (c) computer-vison techniques that use machine learning to track bubble edges to estimate phase velocities accurately. The hardware developed in this project will provide a valuable platform for future studies of flow boiling phenomena in microgravity on the ISS. Better SR correlations will lead to more accurate modeling of two-phase transport in a wide range of terrestrial applications. The project will promote research at the interface between thermal science, space technologies, computer vision, and machine learning. The research results will be accessible to K12 students, faculty, stakeholders, and public and hence will increase the public understanding and appreciation of these technological areas. This project will join the Carolina Master Scholars Adventure Series, which is a pre-university summer program for academically talented rising 6th through 12th graders. Participation in this program will inspire more young Americans to pursue careers in STEM fields. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.