Application of asymptotic model for the prediction of fouling rate of calcium sulfate under subcooled flow boiling

In this work, a large number of experiments are performed to measure the heat transfer reduction and fouling resistance of CaSO4 aqueous solutions in a vertical upward annulus under subcooled flow boiling condition. Experiments are designed so that the effects of different parameters such as solution concentration (1.5 g/l to 2.2 g/l), wall temperatures (102-115 degrees C), heat flux (up to 400 kW/m(2)), and flow velocity (0.5-2 m/s) would be clarified. The experimental results were then used to develop a mathematical model to predict the fouling rate, based on an asymptotic model and Chen model. It was found that the advantage of the asymptotic model in comparison to the Chen model is not only its simplicity and more accurate prediction of flow boiling heat transfer coefficient, but also the physically sounder prediction of nucleate boiling fraction (NBF) which is a crucial concept in the fouling processes under boiling conditions. Furthermore, the frequency factors (k(o)) used in the fouling kinetics proved to be the same in the boiling and non-boiling area of the heat transfer surface. This leads to a physically meaningful and mechanistic model for the prediction of the CaSO4 fouling rate under subcooled flow boiling condition. The experimental data are in good agreement with the model predictions with an absolute average error of about 15%. (C) 2011 Elsevier Ltd. All rights reserved.