Experimental Study of the Two-Phase Flow and Induced Vibrations in a Square-Pitch Tube Bundle in Air-Water Cross-Flow

Abstract Experimental studies of two-phase cross-flow induced vibrations in tube bundles are relevant for the design and operation of U-tube steam generators (SGs) of nuclear power plants. This article presents experimental campaigns on a 5x5 square pitch tube-bundle subjected to air-water flows. Keeping a water superficial velocity constant and equal to 0.13m/s, the air superficial velocity was varied from 0.02 to 1.22m/s: this allowed to transition from bubbly to intermittent flows. A first campaign was dedicated to study the two-phase flow features, measured through dual-optical probes and wire mesh sensor. Average and local void fractions, gas velocities and gas-liquid interface sizes were measured. The frequency and rising velocity of the large air structures in intermittent flow were obtained, along the free duct region upstream of the tube bundle and within the tube bundle. A frequency of 3-4 Hz was measured, with rising velocity reaching 2.5 m/s for the largest gas structures. Their values were found to be very similar between the free duct region and the tube bundle region. A second experimental campaign was dedicated to the study of the vibrational response of the tubes. Two different vibrating configurations were studied: either with only one central flexible tube, or with two flexible tubes next to each other. The tubes are rigid cylinders mounted on a flexible blade, allowing deformation in the lift direction. The displacement was measured by accelerometers. The different behavior between the two vibrating configurations was studied. The vibration spectra do not show vibration harmonics corresponding to the large air structures.