Thermal Acoustic Oscillations (TAO) are a common event in cryogenic systems that can have significant adverse effects on the performance of the system. TAOs are sustained pressure oscillations that can occur in tubes containing gas, closed at one end (the warm end) that have a very large temperature gradient along the length of the tube. This geometry is found in many places in cryogenic systems, including in pressure taps, instrumentation feedthroughs, fill lines and relief valve lines.
TAOs begin when the temperature gradient causes cold gas in the tube to warm and expand, thus increasing in pressure. This increased pressure then pushes the gas into the colder end of the tube, causing the pressure in the warmer end to fall. The gas then moves back to the warmer end to occupy this now lower pressure space. Under the proper conditions of tube size and temperature gradient, sustained pressure oscillations can be set up. These pressure oscillations can result in very high pressures that can damage equipment. Thermal acoustic oscillations are also very efficient in transferring heat from the warm tube wall to the cryogenic system, resulting in higher-than-expected heat leaks and higher-than-expected boiloff rates of liquid cryogens.
These oscillations do not occur under all conditions, and significant work has been done that allows the prediction of tube geometries and temperature gradients that will result in thermal acoustic oscillations. Conceptually, there are two regions of stability in which sustained thermal acoustic oscillations won’t develop. If the tube is small enough, frictional forces on the gas moving in the tube damp out the oscillations, preventing them from being established. If the tube is large enough, the heat transfer from the wall into the larger volume of gas isn’t enough to set up the oscillations. TAOs develop between these two regions. Awareness of TAOs and the proper design of cryogenic systems to avoid them can eliminate TAOs as a significant problem in cryogenics.
A very good description of TAOs, including a methodology for predicting them, can be found in Thermal Acoustic Oscillations in Cryogenic Systems, Y. Gu, PhD Thesis, University of Colorado (1993) and “Experimental Verification of Stability Characteristics for Thermal Acoustic Oscillations in a Liquid Helium System,” Y. Gu & K.D. Timmerhaus, Adv. Cryo. Engr., Vol. 39 (1994). Other studies of this phenomenon may be found in the following publications: “Stability Limit for Thermally Driven Acoustic Oscillation,” T. Yazaki, A. Tominaga, Y Narahara, Cryogenics, July, 1979; “Thermal Effects due to Oscillations of Gas Columns,” Y. Narahara, A. Tominaga, F. Mizutani, Y. Yazaki, Cryogenics, Vol. 28 (1988) and “Experiments on Thermally Driven Gas Oscillations,” T. Von Hoffman, U. Lienert, H. Quack, Cryogenics, August, 1973. A description of the elimination of TAOs in cryogenic equipment is given in “Elimination of Thermal Acoustic Oscillations in Cryogenic Pumps,” T.J. Miller and Y. Gu, Adv. Cryo. Engr., Vol. 51 (2006).
