Mechanism Investigation And Application On Energy Transfer Process Subject To Fouling

Depositing or contaminant is a very common phenomenon in the nature and also a very important problem in the environment. The existence of fouling can bring heat transfer equipments a series of economic losses, and has become an important aspect of design, operation and maintenance in heat transfer equipments. To investigate the impact of fouling on energy transfer process has become especially important. The existing studies for the fouling neglect the impact of mass transfer on the heat transfer process, especially for the beginning of the forming period of fouling, and only consider the increasing of thermal resistance in heat conduction process subject to fouling. However, mass transfer which can affect the heat transfer process exists at the beginning of forming period of fouling. Moreover, there is a phenomenon that the heat transfer performance at the beginning of the forming period of fouling is better than that of clean heat transfer surface, which has been proved by some experiments. And it is often explained that it is the result of the change of surface roughness due to fouling at the beginning of the forming period of fouling. But the experimental results by Ceylan proved that the duct roughness due to fouling is not effective in the augmentation of the heat transfer. So it needs to find a new approach to explain this phenomenon theoretically. As an extension of the previous works, the objective of this paper is based on the combination of thin film model reflecting heat and mass transfer synchronously with the asymptotic fouling model proposed by Kern and Seaton, to investigate the convective heat transfer characteristics through a duct/on a board at the forming period of fouling. A quantitative expression of the convective heat transfer coefficient through a duct/on a board considering fouling mass transfer process is presented. Meanwhile, the effect factors of convective heat transfer process through a duct/on a board are discussed. And the results are compared with that without fouling, which explains the phenomenon that the heat transfer performance is better than that of clean heat transfer surface at the beginning of the forming period of fouling theoretically. In addition, critical fouling layer thickness is firstly derived in this paper and its equation has been provided. The effect factors of critical fouling layer thickness are analyzed. Also, a new theoretic explanation to the enhancement heat transfer phenomenon is presented in the viewpoint of critical fouling layer thickness.On the other hand, the optimal cleaning cycle of heat exchangers has always been approached from the perspective of the first law at present. However, heat transfer is inevitably accompanied with exergy transfer in a heat exchanger. And determining the optimal cleaning cycle of heat exchangers from the perspective of energy quality has not been reported. This paper presents a comprehensive thermo-economic study of heat exchangers cleaning cycles by introducing the exergy transfer and heat transfer effectiveness. The total cost equation has been derived to determine the optimal cleaning cycle of heat exchangers. In addition, the results are compared with that based on the first law and the effect factors of the optimal cleaning cycle are discussed. The research reported in this paper provides a reference to the cleaning of heat exchangers in engineering.