Propagation of nitrogen gas in a liquid helium cooled vacuum tube following sudden vacuum loss – Part I: Experimental investigations and analytical modeling

• Published in: International journal of heat and mass transfer 2016-05, Vol.96 (C), p.573-581
• Main Authors: Dhuley, R.C., Van Sciver, S.W.
• Format: Article
• Language: English
• Subjects: Accidental vacuum loss; Atmospherics; Beam-line; Conservation; Liquid helium; Mass transfer; Mathematical analysis; Particle accelerator; Propagation speed; Reservoirs; Superconducting; Tubes; Vacuum tubes
• ISSN: 0017-9310; 1879-2189
• DOI: 10.1016/j.ijheatmasstransfer.2016.01.077

•N2 gas propagation after loss of vacuum is studied in tubes at 295K, 77K, and 4.2K.•Propagation speed of N2 gas is orders of magnitude slower in 4.2K tube.•Speed in 4.2K tube decreases exponentially while it is constant at 295K and 77K.•The front deceleration is due to gas condensation inside the vacuum tube. This paper describes propagation of near atmospheric nitrogen gas that rushes into a liquid helium (LHe) cooled vacuum tube after the tube suddenly loses vacuum. The loss-of-vacuum scenario resembles accidental venting of atmospheric air to the beam-line of a superconducting particle accelerator and is investigated to understand how the in-flowing air will propagate in such geometry. In controlled experiments, we simulated loss of vacuum by rapidly venting a large reservoir of nitrogen gas (a substitute for air) to a vacuum tube immersed in a LHe bath at 4.2K. The resulting rise in the tube pressure and temperature were measured by pressure probes and thermometers arranged along the tube length. The data show that the propagation of nitrogen gas in the LHe cooled vacuum tube is orders of magnitude slower than in the same tube at room temperature. Interestingly, the gas front speed in the LHe cooled tube also decreases along the tube. A gas propagation model developed by employing conservation of mass identifies mass transfer (gas condensation in the tube) as the principal cause of the slow propagation as well as of the front deceleration. Some limitations of this analytical model are discussed in the context of quantifying the propagation speed. The speed obtained from direct measurements is seen to decrease exponentially along the tube. This exponential decay form of the propagation speed well represents the data from experiments that have different mass flow rates of nitrogen gas flowing into the vacuum tube after venting.