Effects of an acoustic field on vapour bubbles in microgravity
Correu electrònic de l'autorvicencartigasgmail.com
Tutor / director / avaluadorGonzález Cinca, Ricardo
Tipus de documentProjecte Final de Màster Oficial
Condicions d'accésAccés restringit per acord de confidencialitat
One of the most critical points when considering long-term space exploration missions is the use and storage of propellants. Typically, cryogenic propellants (CP) are a suitable option for this kind of mission due to their high efficiency, but the tanks in which they are contained are exposed to very harsh environments (e.g. radiation or extreme temperatures). Multi-layer insulators (MLI), which could be used to counteract these conditions, would not be enough to avoid deterioration of the tank. Thus, CP, which needs to be stored at very low temperatures (e.g. liquid hydrogen is stored at 20K), might suffer localized boiling due to heat leaks in the tank walls. Boiling leads to bubble formation, which under reduced gravity stays attached to the tank. The accumulation of vapour bubbles can present risks for tank chill down, engine restart, propellant loading or space propellant management. One of the possible solutions to avoid these risks is to detach the vapour bubbles and move them to a colder part of the tank where they would change to the liquid state by condensation. The proposal of this Master Thesis is the study of the effect of acoustic wave acting on a bubble in microgravity conditions. This study had been realized in the context of other works carried out in the UPC Microgravity Lab addressed to the development of an acoustic technology to control vapour bubbles. Previous studies in the same field have been given significant results in order of displacement and localization of the bubbles. However, few experiments have been reported to study the interaction of boiling and acoustics under microgravity conditions. Thus, this project aims obtain more information about this phenomenon. In order to be able to observe this effect, vapour bubbles were created using a heater and boiling a refrigerant inside a Test Cell applying different voltages to the heater. The acoustic field is induced using piezoelectric transducer that creates the acoustic wave inside the Test Cell. This acoustic field depends on the vibration frequency applied. The transducer was installed in the orthogonal axis respect the heater surface, with the same direction than buoyancy but in opposite sense. This configuration was adopted to observe how the acoustic force acted against the buoyancy. In microgravity, buoyancy would be low enough that the acoustic field should be the only force acting on the bubble, so the effect of the acoustic field on the bubble would be observable. Different kinds of tests have been performed for the several elements that compose the experimental setup. Heater selection, Test Cell validation and piezoelectric transducer analysis will be the main components studied separately and simultaneously in gravity and microgravity conditions. Microgravity conditions were achieved using a drop tower built in the lab. The expected results of the tests were to observe the bubble staying in the same vertical position or even moving in the opposite buoyancy direction during the microgravity phase due to the acoustic force.