Abstract
Local heat transfer around a tube in in-line and staggered tube bundles were investigated in two-phase flows under adiabatic and atmospheric pressure conditions. The working fluids were air and water. The test section was vertical duct with inner dimensions of 90 × 90 mm2. In-line and staggered tube bundles, each containing five columns and eight rows, were employed for the measurements as the test sections. The tube diameter of each was 18 mm, and the pitch-to-diameter ratio was 1.25 for both bundles. The local heat-transfer coefficients were measured using a platinum wire electrode placed on a tube that could be rotated. Improvement of the heat transfer caused by liquid agitation in bubbly flow was remarkable for the in-line tube bundle, but not for the staggered tube bundle. In intermittent flow, the heat transfer was improved significantly for the both bundles. This is because the liquid phase is agitated by the reverse flow generated by the passage of large bubbles. However, this effect became smaller as the gas phase flow rate increased, especially for the staggered arrangement. It is considered that the lower the void fraction is, the more remarkable the improvement of the heat transfer coefficient is due to the liquid agitation by bubbles motion.
