Background
Figure 1: Flight Personnel and Payload of First Day of Testing Payload containing SwRI and Texas A&M University (TAMU) experiments loaded onto the zero-G aircraft before flights begun. Pictured are Dr. Eugene Hoffman (left), Dr. Bonnie Dunbar (center), and Dr. Akbar Whizin (right).
Returning to the Moon or future missions to Mars, with their lower gravitational acceleration, requires expanded knowledge of the effects of reduced gravity on fluid behaviors in these environments, particularly two-phase fluids in contact with solid surfaces. Decades of research in microgravity fluid physics have provided extensive data for model and technology validation for space flight systems, but there are little to no data available across a range of partial gravity levels for lunar or Martian terrestrial systems. For example, bubbles that remain in contact with surfaces can lead to reduced heat and mass transfer in heat exchangers, coalescence of small bubbles into larger ones that can damage equipment, and increased boiloff of cryogenic propellants in storage tanks. Research on partial gravity two-phase fluid physics applies to lunar and Martian surface power systems, life support systems, cryogenic fuel management, and In Situ Resource Utilization (ISRU).
Texas A&M University (TAMU) and SwRI (Divisions 18 and 15) have executed a joint research program to study how bubbles detach from surfaces in reduced gravity. TAMU’s experiment focused on the dynamics of bubble detachment from typical surfaces while SwRI’s experiments sought to demonstrate possible improvements to bubble detachment with engineered surfaces. All experiments were conducted during a two-day test campaign in April 2024 on a reduced-gravity flight.
Approach
SwRI’s experiment was designed to heat stainless steel surfaces and boil FC-72 (an inert heat transfer fluid) aboard an aircraft that simulates reduced gravity by flying a parabolic profile. The surfaces were approximately 0.5-inches square. In addition to bare stainless steel, three surface modifications were explored including two with unique surface morphologies and one with a hydrophobic coating. The experiment was mounted in a payload rack with TAMU’s experiment as shown in Figure 1. During the flight, high-speed video footage of bubble nucleation and detachment volume as well as various temperature and acceleration measurements were captured.
Accomplishments
Figure 2: Associated Bubble Sizes at Corresponding Gravitational Levels A series of photographs revealing the exponential growth in bubble size with decreased gravitational condition.
Overall, an exponential growth in bubble size with reduced levels of gravity was witnessed, as shown in Figure 2. The increase in bubble size was a result of the reduced buoyancy force acting on the bubble, thus requiring a larger volume of gas to be generated before overcoming the surface tension force and detaching from the surface. Furthermore, it was witnessed that engineering surfaces with an appreciable amount of surface undulations decreased the bubble size at detachment, while smoother surfaces had the opposite effect. The surface condition was believed to influence the surface-tension force which, in turn, affected the required buoyancy force to detach from the surface.