Positioning of Bubbles in Microgravity by the Kelvin Force

Abstract

Experiments were performed in July 2003 in the NASA “Weightless Wonde r” KC -135 aircraft through the Reduced Gravity Student Flight Opportunities Program (RGSFOP), to study the influence of the magnetic Kelvin force on the behavior of air bubbles in a paramagnetic liquid in microgravity ( ∝-g). Results indicate that air bubbles within the magnet tank were successfully repelled from the magnet surface in ∝-g because the Kelvin force attracted the liquid to the magnet surface. For one parabola for which the video tape of the experimental re sponse has been analyzed, air bubbles in the control (no -magnet) tank were much larger in ∝-g ( �1 cm diameter, versus 2 to 4 mm) and generally never detached from the bottom of the tank. Average bubble velocity for the magnet tank was 43 mm/s, although the re was considerable scatter in these results. The Kelvin force on a bubble adjacent to the magnet surface for this parabola was computed to be approximately 20 µN, which was judged to be in reasonably good agreement with the computed drag force of 30 µN. Bubbles in the magnet tank also were smaller in diameter for a second parabola that was analyzed (3 to 6 mm diameter, versus �1 cm for the control, no -magnet case). Here, the bubble velocities ranged from about 100 mm/s to nearly zero for the bubbles tha t were repelled from the magnet; this velocity range is similar to that observed for the earlier parabola. I. INTRODUCTION A series of flight experiments was conducted aboard the NASA KC -135 reduced -gravity aircraft to study the influence of the magneti c Kelvin force on the behavior of air bubbles in a paramagnetic liquid in microgravity ( ∝-g). This aircraft flies through a series of parabolic arcs providing about 24 seconds of ∝-g per parabola, thereby allowing a unique environment in which to study the resultant phenomena. The experiment was conceived, designed, constructed, and perform ed by the WVU student Microgravity Research Team (MRT) and their advisors. When there is no dominant body force such as gravity to produce a buoyancy force, vapor bubbles that form during boiling may remain adjacent to heat rejection surfaces, rather than be driven away due to buoyancy. This can reduce the critical heat flux and can cause burnout, possibly damaging the surface that needs to be cooled. The present experiment modeled one possible method for repelling the vapor bubbles from a heat rejection surface in ∝-g, by injecting air bubbles to simulate the vapor bubbles formed due to boiling at a single nucleation site. In the presence of a non -uniform magnetic field, the Kelvin body force is exerted on all magnetically permeable materials. Diamagnetic and paramagnetic fluids are repelled from and attracted to magnetic fields, respectively. The Kelvin force is approximately three orders of magnitude larger for liquids than for gases, leading to a magnetic buoyancy force on bubbles in a paramagnetic liqu id. The present experiment was conceived to determine whether or not the magnetic Kelvin force could be utilized in ∝-g to remove small air bubbles from the bottom of a tank filled with a paramagnetic liquid. This experiment is a first step in determining the effectiveness of the Kelvin force in increasing pool boiling critical heat flux under ∝-g conditions.