Study On The Fouling Characteristic Of Heat Transfer Surface

Heat exchanger fouling is a common problem in industry. Fouling deposition is usually very intricate process, as there are lots of factors affecting the fouling accumulation. It affects on heat transfer, and causes the extreme waste of energy and economic losses. So fouling has aroused peoples' attentions in the domain of heat transfer but is still an unsolved problem. This dissertation discussed the fouling process of calcium carbonate on heat transfer surface. A series of theoretical and experimental studies were carried out systematically in this work. The main achievements of the dissertation are as follows:Firstly, an on-line monitoring apparatus of fouling thermal resistance was developed. Tests had been undergone to achieve the heat transfer and friction performances of four types of enhanced tubes (arc line tube, corrugated tube, convergent-divergent tubes I and convergent-divergent tubes II) under clean conditions. Dimensionless correlations for forced convection heat transfer and friction factor inside the above four types enhanced tubes were obtained from the test results. Moreover, the comparative fouling experiments were carried out between the above four types enhanced tubes and their respective corresponding plain tubes. The experimental working fluid was man-made hardness water or MgO particles solution, and the experiments were done under the conditions of the same liquid velocity, temperature at the water tank outside the tubes and inlet temperature of the working fluid inside the tubes. The relations between fouling resistances of all these tubes and time were gained from the results of the comparative fouling experiment. The experimental results showed that four enhanced tubes had lower deposition rate, longer induce period and smaller asymptotic fouling thermal resistances. Moreover, four enhanced tubes significantly enhanced the heat transfer under both clean and fouled conditions.Secondly, the formation of fouling is closely related with the characteristic of heat transfer surface. So the parameters of surface, such as surface roughness, contact angle, surface energy and interfacial energy were measured and calculated. The effects of all these parameters on the formation of fouling were studied by the fouling experiments of several different types coat surfaces. At the same time, the SEM micrographs of surfaces were analyzed on clean and fouled condition. The experimental results showed that there was an important impact of the material's surface energy upon fouling formation, but there was not distinct regularity in other parameters. Overall, low surface energy of coat surfaces had advantage in anti-fouling effect.Finally, the numerical simulation of CaCO₃ fouling was carried out by using the CFD software to plain tube. Base on the change of CaCO₃ concentration, the deposition rate and the removal rate of the CaCO₃ fouling with time were achieved. By ignoring the induction period, the fouling resistance with time was also obtained from the fouling sediment mass. At the same time, in order to confirm the correctness of the computed results, the experimental confirmation was carried out, and predicted values agreed with the computed results.