Numerical simulation of the fouling process

Abstract

Fouling of heat transfer surfaces causes serious technical and economic problems in industry. The goal of this work is to simulate the aforementioned fouling process using the CFD code FLUENT. The obtained numerical results assist in designing and running heat exchangers. Based on models for the calculation of deposition and removal mass rates [S. Krause, Internat. Chem. Engrg. 33 (1993)], the crystallization fouling of calcium sulfate on flat heat transfer surfaces was simulated. The induction period, which occurs with almost all fouling processes, was therefore not considered. The simulation of real crystal growth requires a continuous variation of the geometric flow model and therefore considerable computational effort. For that reason fictitious crystal growth was simulated instead. This numerical simplification enabled an unsteady simulation to be obtained, of the fouling process and a realistic description of the temporal modification of both the flow and temperature field due to the continuous crystal growth. Based on experimental results of Hirsch [M. Bohnet et al., in: T.R. Bott et al. (Eds.), Understanding Heat Exchanger Fouling and its Mitigation, United Engineering Foundation and Begell House, New York, 1997, pp. 201–208], a model was developed which enables the calculation of the density of the fouling layer not only as a function of the local position within the fouling layer, but also as a function of the time-dependent total thickness of the fouling layer. In addition a model was developed, that enables a realistic distribution of the heat flux along the heat transfer surface during the simulation. Both models provide a more exact description of the complicated fouling process. Results of the numerical simulation are the prediction of the fouling resistance as a function of time and the calculation of the temperature distribution within the fouling layer. In view of the complexity of the fouling process during the incrustation of heat transfer surfaces and the fact that not all influences from the used models could be considered the agreement between calculated and experimentally obtained data is satisfactory.