Development of a new empirical correlation for the prediction of the onset of the deterioration of heat transfer to supercritical water in vertical tubes

摘要

Heat transfer to supercritical water is relevant for modern power applications such as supercritical water reactors, supercritical boilers in fossil-fired power plants or future concepts for supercritical solar-thermal power plants. The phenomenon of Deteriorated Heat Transfer (DHT), caused by the strong variation of fluid properties at pressures above the critical pressure is still not thoroughly understood to date. At the same time it is of great importance for plant design and operation. Disagreements exist regarding its definition, its prediction and the mechanisms leading to its occurrence. This study evaluates the performance of empirical correlations for the a priori prediction of the heat flux causing the onset of DHT in heated vertical tubes with an internal upward flow of supercritical water and proposes a new correlation. Also, definitions from literature are discussed, and the most frequently used quantitative definition: α < 0.3. α_(NHT) is selected for correlation analysis and development. It is emphasized that the predictive capability of a correlation for the deterioration of heat transfer is limited when no definition for this phenomenon is provided. For the evaluation of the existing correlations and the development of the new correlation, heat transfer data consisting of 4018 data points from 14 independent references is considered. This data set is reduced to 26 independent experiments reflecting the heat flux causing the onset of DHT. The newly developed correlation has a mean average relative deviation of 16.6% and predicts 76.9% of the DHT data within ±20%. This is a significant improvement compared to the best existing correlation with a mean average relative deviation of 20.4% and which predicts 69.2% of the DHT data within ±20%.