Local void fraction and heat transfer characteristics around tubes in two-phase flows across horizontal in-line and staggered tube bundles

This study investigated the local characteristics of void-fraction distribution and heat transfer around tubes in two-phase flows under adiabatic conditions using vertical duct test sections with inner dimensions of 90 × 90 mm2. Two kinds of test sections, in-line and staggered tube bundles, each containing five columns and eight rows, were employed for the measurements. The tube diameter of each was 15 mm, and the pitch-to-diameter ratio was 1.5 for both bundles. The working fluids were air and water, and the experiments were performed under atmospheric pressure in a temperature range of 20-25 °C. Superficial liquid velocity, JL, and gas velocity, JG, ranged from 0.1 to 0.3 m/s and 0.03 to 1.19 m/s, respectively. Two-dimensional void-fraction distributions were obtained using X-ray radiography and the local heat-transfer coefficients were measured using a platinum wire electrode placed on a tube that could be rotated. In the experiments, the time-averaged void fraction increased at the maximum and vertical minimum gaps for the in-line tube bundle, whereas the void fraction increased upstream of the tubes for the staggered tube bundle. In the bubbly flow condition, enhancement of the heat transfer by bubbles motion clearly occurred between ±90 and 180° for the in-line tube bundle, and increased all over the pipe for the staggered tube bundle. The increase in the local heat transfer coefficients by bubbles motion was more apparent for the in-line tube bundle. The average heat transfer coefficient in the staggered tube bundle was higher than that in the in-line tube bundle in the bubbly flow regime, whereas the results were opposite in the intermittent flow regime.