Two-Phase Flow Behavior and Heat Transfer Characteristics in Kettle Reboiler

Effects of the tube array, such as in-line and staggered, on void-fraction distribution and heat transfer coefficient around a tube were experimentally investigated. The test section was vertical duct with inner size of 90 × 90 mm². Diameter of the tubes was 15 mm, and the pitch-to-diameter ratio was 1.5 for both tube bundles. Working fluids were air and water. Experiments were carried out at superficial gas velocity defined at minimum area section, J_g, of 0.10 to 0.89 m/s, superficial liquid velocity, J_l, of 0.1 to 0.3 m/s, under the atmospheric condition. Measurements of void-fraction distribution were carried out using X-ray radiography. In addition, heat transfer coefficient around a tube was measured and the heat transfer coefficients in association with the flow regime and the void-fraction distribution were evaluated. Time-average void-fraction was higher around upstream of a tube than that of downstream at bubbly flow condition for both bundles. Under intermittent flow condition, time-average void fraction at the maximum gap were higher than that around the other points in both tube bundles. For in-line tube bundle, enhancement of the heat transfer clearly appeared between ±90 to 180°. For staggered tube bundle, the heat transfer increased all over the pipe.