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

Abstract

Large scale experiments were performed with the support of CERN to simulate and understand the LHC HeII cooling circuit. 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 and 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 this case, liquid droplets entrainment occurs. Droplets falling on the tube wall augment the wetted surface. So we infer that for higher vapour velocities, the stratified two-phase flow model cannot 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 loop was built and experiments were performed. First results are presented here, including the observation of the stratified-annular flow transition.