Performance assessment of ultrasonic waves for bubble control in LOX tanks

An efficient long-term storage of cryogenic propellants is a challenge for future space exploration missions. In long duration missions in low Earth orbit, issues associated to long-term storage, such as the cryogenic propellant loss due to boil-off, will require a proper management (Salerno 1999, Motil 2007). The vapour bubbles formed as a result of boiloff can generate foam structures, which could be hazardous in different operations in orbit. Since current heat insulation technologies are not able to provide a sufficient control of boil-off for long times, other techniques are required to minimize the effects of boiling in fuel tanks. An approach recently proposed by the UPC Laboratory of Microgravity consists in the use of acoustic fields for the control and elimination of bubbles. Bubble dynamics can be managed by the application of an acoustic field (Leighton 1974, Crum 1975). In the proposed technique, the force due to the acoustic wave generated by a piezoelectric transducer detaches the bubbles from the tank walls and moves them to the subcooled liquid where they collapse. This technique is currently under study in microgravity conditions at non-cryogenic temperatures. To be applicable in space, the technology has to be validated at cryogenic temperatures. However, numerous attempts to generate a valid acoustic signal at low temperatures have been performed without success. This is due to two facts: on one hand, piezoelectric materials are known to work lousy at the desired cryogenic conditions; on the other hand, the acoustic matching layer material loses its transmission properties at low temperatures, and consequently no acoustic signals can be transmitted into the fuel tank. However, recent studies have shown that epoxy resin-based acoustic matching layers can exhibit an increase in the transmission coefficient at cryogenic conditions, and experimental results show that the amplitude of the transmitted signal at low temperatures can increase by a factor of 1.5 the amplitude obtained at room temperature.