Thermo-acoustic oscillations (Taconis oscillations or TAO) are an undesired phenomenon characteristic for cryogenic systems operated under high temperature gradients, where one end of the pipe is in continuous contact with the extremely cold process fluid and the second end of the pipe is directly exposed to the ambient temperature. As a consequence of TAO, an extraordinary heat exchange occurs between cold mass and surrounding atmospheric air outside vacuum box. One of the components especially prone to this undesired situation are cryogenic valves.
There exist different measures against TAO that can be implemented for the valves used under deep cryogenic conditions. A fully effective yet very conservative solution is the assembly of several convection brakes along the valve stem. Preventing the free mixing of gas layers on the different temperature levels eliminates the risk of TAO completely. However, further theoretical research on the character of phenomena had shown that there are possibilities to simplify the existing solution whilst maintaining its full functionality.
Therefore, in cooperation with SHINE project team in China, WEKA AG investigated TAO in a cryogenic valves assembly in which oscillatory behaviour had been experimentally confirmed. In the first step, the general nature of oscillations had been thoroughly examined based on existing publications and previous experiments. The possibilities of the optimization had been assessed using numerical simulation techniques. Afterwards, various solutions were then developed and subsequently manufactured.
At SHINE, WEKA tested the new solutions against TAO under real cryogenic conditions of approximately 4K at 2bar. A series of measurements and design modifications were performed to test different approaches under real conditions and to determine if thermo-acoustic oscillations within the system can be prevented or mitigated. The measurement results demonstrated that the temperatures at the warm end of the valve remained stable, effectively preventing ice formation and indicating the stable operation of the system. This publication illustrates the actions performed by WEKA to evaluate the effectiveness of optimized solutions.

Keywords
Thermo-acoustic oscillations (TAO)|High temperature gradients|Cryogenic valves|Undesired oscillatory behavior|Gas layer separation