Local heat transfer coefficient during stratified flow in large, flattened- tube steam condensers with non-uniform heat flux and wall temperature

摘要

Steam condensation heat transfer coefficient (HTC) in a large, flattened tube with non-uniform heat flux and wall temperature, and variable inclination angle is determined experimentally. The condenser tube is that typically used in an air-cooled condenser for power plants. The steel tube has an elongated-slot cross section with inner dimensions of 216 x 16 mm. Water vapor and liquid flow co-currently through a 5.7 m long air-cooled conditioning section, followed by a 0.12 m long water-cooled test section. The long conditioning section creates conditions in the test section that mimic the conditions in an operating condenser - allowing for the realistic development of flow regime and void fraction. HTC is then determined in the water-cooled section. The water-cooled section is designed as a crossflow heat exchanger to match the temperature and heat flux conditions of an air-cooled condenser. Visualization sections at the tube inlet and outlet allow determination of flow regime and void fraction. The flow is found to be stratified for all conditions. Tube inclination angle is varied from 0 to 38 degrees downwards. Inlet quality in the water-cooled section ranges from 0 to 0.74. HTC is found to increase by more than 400% along the condenser height. In addition, inclination angle, wall-steam temperature difference, inlet water-steam temperature difference, water temperature glide and vapor quality are all found to affect the condensation HTC. (C) 2019 Elsevier Ltd. All rights reserved.