Latest Developments on He II Co-Current Two-Phase Flow Studies

Abstract

Large scale experiments were performed at CEA Grenoble with the support of CERN to simulate and understand the HeII cooling circuit of the LHC. This paper describes the latest results obtained in HeII co-current two-phase flow configuration. First we summarize thermal and hydraulic behaviour of flows obtained in a 40 mm I.D., 86 m long tube inclined at 1.4% which resembles closely the LHC heat exchanger tube. For low vapour velocities, the flow pattern is found to be stratified. A model based on this observation has been developed which fits very well the measured pressure losses. However the wetted surface predicted by the model underestimates the measured one, notably for high vapour velocities. In that case, liquid droplets entrainment takes place. Droplets landing on the tube wall increase the wetted surface. Thus we infer that for higher vapour velocities, the stratified two-phase flow model should not be applied anymore. In order to validate the range of availability of the model, and begin to draw a flow pattern map, a 20 mm I.D. horizontal test sector was built and experiments were performed. First results are presented here, including the observation of the stratified-annular flow transition. ABSTRACTLarge scale experiments were performed at CEA Grenoble with the support of CERN to simulate and understand the HeII cooling circuit of the LHC. This paper describes the latest results obtained in HeII co-current two-phase flow configuration. First we summarize thermal and hydraulic behaviour of flows obtained in a 40 mm I.D., 86 m long tube inclined at 1.4% which resembles closely the LHC heat exchanger tube. For low vapour velocities, the flow pattern is found to be stratified. A model based on this observation has been developed which fits very well the measured pressure losses. However the wetted surface predicted by the model underestimates the measured one, notably for high vapour velocities. In that case, liquid droplets entrainment takes place. Droplets landing on the tube wall increase the wetted surface. Thus we infer that for higher vapour velocities, the stratified two-phase flow model should not be applied anymore. In order to validate the range of availability of the model, and begin to draw a flow pattern map, a 20 mm I.D. horizontal test sector was built and experiments were performed. First results are presented here, including the observation of the stratified-annular flow transition.

He II Two Phase Flow in an Inclinable 22 m Long Line

“…2), which was adopted for wall friction factor in our two-phase flow model. The interfacial friction factor introduced in the model follows the Hanratty correlation as previously reported 3,4 . In situ calibration of the thermometers located along the two-phase flow enabled temperature measurements with 10 -3 K accuracy.…”

“…In the framework of LHC cooling scheme studies, previous experiments were performed 2,3,4 using a 40 mm I.D., 1.4 % slope helical pipe and a He II two-phase flow model was constructed. Obtained results gave us confidence in this co-current two-phase flow model and justified further experiments to fully validate the LHC cooling scheme.…”

TESLA & ILC Cryomodules

“…Evaporation from the surface of the saturated helium liquid results in vapor flow within the 2-phase pipe over the liquid surface to an exit port connecting to the HGRP. Studies done at CEA Grenoble for CERN's LHC magnet cooling [15] provided the result that a 5 meters/sec vapor "speed limit" over liquid is a conservative "rule of thumb" not to entrain liquid droplets into the vapor. For LCLS-II, the closure of each 2-phase pipe in each cryomodule, limiting the 2-phase pipe vapor flow to that generated by one cryomodule, keeps the vapor flow rate quite low, less than 2 meters/sec.…”

Cooling Strings of Superconducting Devices Below 2 K: The Helium II Bayonet Heat Exchanger

“…In complement of this application-oriented tests, more thorough investigations of twophase flow of saturated helium II were conducted at CEA Grenoble, France, on a test loop featuring 40-mm and 20-mm diameter heated cylindrical pipes, comprehensively equipped with diagnostics equipment 11,12 . The diagnostics include wetted-perimeter indicators, as well as visual observation sections featuring CCD video cameras, which enabled to check the flow patterns inferred from hydraulic and thermal measurements.…”