





Please Visit Our Sponsor Gift Shop Thermoacoustic (Thermal Acoustic) OscillationThermoacoustic Oscillations (TAO's) occur in cryogenic systems where a long tube open at the cold end is extended to the closed end at the warm boundary. This usually happens in the fill line or the vent line where the tube is capped off (at the hot end), or the flow rate in the tube is small. TAO's can be very damaging, as the oscillations are accompanied by a considerable heat conduction down the tube. This can increase the heat leak of the system by several orders of magnitude, thus decreasing time between servicing or replenishing the cryogenic dewar. Theory The TAO phenomenon was known as early as 1804. Rayleigh provided an explanation for the heatdriven oscillation based on a critical value now known as the Rayleigh's number. Keesom also mentioned the effect of TAO's in 1942. Taconis has offered a qualitative explanation of the oscillations. In 1949, Krammers attempted to use sound theory to provide a quantitative description of the phenomenon. He concluded that no useful results can be derived from the linear stability theory, due to the neglect of nonlinear terms. Twenty years later, Rott showed that it was only for the special combination of the material constants of helium gas that Krammers had failed.Using a second order (in viscous effect) but still linear theory, Rott was able to derive the stability curves for helium gas in a long tube with piecewise step change of temperature. For temperature profiles other than a step function, NIST has developed a computer program based on Rott's analysis. The radius to Stokes boundary layer ratio = r (Cr/l_{c}h_{c})^{1/2}. Where r is the tube radius, C the speed of sound and l_{c} is the length of the cold tube. The density and the viscosity of the cold gas are represented by r and h_{c} respectively The stability curves for helium and nitrogen were formulated by Rott. As shown in the following figures for single phase (Figure 1) and two phase (Figure 2) TAO's (for various hot to cold tube length ratios).
Since the heat capacity ratio of helium and neon are the same g = C_{p}/C_{v} = 1.666 and the temperature dependence of the viscosity of helium and neon are identical h_{c} = aT^{1+}^{b} and the Prandtl number of the two gases are the same Pr = h_{c} C_{p} / k = g / (1.77 g 0.45) the author of this paper found that stability criteria derived for the helium gas can also be applied to neon gas. The Experiment An experiment was set up to verify the analysis. The results are summarized in the following table
References: I.E. Spradley and S.W.K. Yuan, Prediction of Thermal Acoustic Oscillations (TAO's) in the CLAES Solid CO_{2} / Neon System, Adv. In Cryogenic Engineering, vol. 37, Part A, Plenum Press, New York, 1992 pp 257. Abstract Download Do You Know Someone Who Needs A Job in Engineering, Physics or Chemistry? About the author Dr. Sidney Yuan is a consultant in the field of Low Temperature Physics and Cryogenics, and has written a Book on Cryogenics and published extensively in the field. EMail. Bookmark This Page Send This Page To A Friend Place Your Ad Here For As Little As $1 Per Day About Us  Add URL  Advertise with Us  Auction  Awards  Contact Us  Discussion Forum  Links  Search This Site  Send This Page  Shop  Top Ten Sites Copyright 2000 Yutopian, All Rights Reserved 
