Heat transfer and flow regimes in large flattened-tube steam condensers

An experimental study of steam condensation in a power-plant air-cooled condenser is presented. This is the third of a four-part group of papers. The first two parts (Kang et al., 2017; Davies et al., 2017), published in the same journal present the facility, pressure drop, void fraction, and flow regime results, while this study presents heat transfer results and analysis. A follow-up paper will investigate the effect of varying inclination angle. The condenser test section is half of a flattened steel tube with brazed aluminum fins. The full size of a condenser tube is 10.72 m × 214 mm × 18 mm. The condenser tube is cut in half lengthwise and covered with a polycarbonate window to perform visualization simultaneously with the heat transfer measurements. All tests are performed with condensing pressure slightly above atmospheric. Stratified flow is found for all test conditions and all locations along the condenser, with both filmwise and dropwise condensation along the condenser wall. Steam-side heat transfer coefficient is found to depend on wall-steam temperature difference, and not quality or Reynolds number for vapor. As a result, steam-side heat transfer coefficient does not decrease along the condenser length, as is common for smaller condenser tubes with higher mass flux. This phenomenon disagrees with the predictions of many of the published correlations. Overall condenser heat transfer coefficient is found to decrease along the condenser length, due to an increase in the thickness of the stratified condensate layer.