A correlation for heat transfer coefficient during stratified steam condensation in large flattened tubes with variable inclination and wall temperature

摘要

This paper experimentally investigates heat transfer coefficient during stratified-flow condensation in a large flattened-tube steam condenser with non-uniform heat flux and wall temperature, and varying inclination angle. The heat transfer test facility is designed and built to provide local measurements of wall temperature; combined with a CFD model, it also provides local heat flux and heat transfer coefficient.The condenser test tube is that commonly used in air-cooled condensers for power plants. The steel tube has inner dimensions of 216 mm height x 16 mm width. In addition, tube inclination is varied from 0 degrees to 38 degrees downwards. The test facility is designed to match the conditions of an operating condenser, with low steam mass flux, realistic development of flow regimes and a large temperature glide on the cooling side.The results show that heat transfer coefficient along the tube wall follows the Nusselt condensation model, while heat transfer through the stratified liquid layer at the tube bottom is predominantly driven by laminar forced convection. Commonly-used correlations for heat transfer coefficient are unable to accurately predict the experimental results, so a new correlation is proposed. The new correlation takes into account heat transfer through the stratified liquid layer at the tube bottom as well as through the condensing film along the tube wall. Recommendations are made for situations where: (1) the wall temperature is known and (2) the wall temperature is unknown. Finally, a recommendation is made for combining the correlation in the stratified condensate layer with a local model for determining condenser capacity. (C) 2019 Elsevier Ltd. All rights reserved.