LDA measurement in air-water downward flow

Local characteristics of the liquid phase in air-water downward flow were investigated in a 50.8 turn inner-diameter round pipe. A laser Doppler anemometry (LDA) system was used to measure axial liquid velocity and its fluctuations. To reduce the measurement uncertainty. the experiments were performed in flow conditions with low void fraction. Titanium dioxide particles with a mean diameter of 2 mum were used as seeding particles to enhance the data rate. Benchmark experiment in the single-phase liquid flow was first carried Out to ensure good performance of the LDA system in the current setup. A total of 13 flow conditions were examined in air-water two-phase experiment. By applying a special setup of the LDA system, it was found that no further signal discrimination process was required to obtain the liquid velocity in the present low void fraction conditions. The comparisons between the liquid flow rates measured by the magnetic flow meter and those obtained from the local measurements showed good agreements. with differences less than 6.0%. The measurement results demonstrated that the presence of the bubbles tended to flatten the liquid velocity radial profile, and the maximum liquid velocity might occur off the pipe centerline, in particular at relatively low flow rates. Furthermore. the axial liquid velocity fluctuations were quite uniform in the radial direction. No significant turbulent reduction in the two-phase downward flow was observed in the current experimental now conditions. (C) 2003 Elsevier Inc. All rights reserved.