Numerical Simulation of the Fouling on Structured Heat Transfer Surfaces (Fouling)

Abstract

In a preliminary calculation the simple case of parallel flow past flat plates is investigated. Using this simplification numerous fouling experiments under different conditions were performed. The measurements were taken using aqueous solutions of CaSO4 and serve for the verification of the numerical simulation. The objective of this work is to make a contribution to a good and fast prediction of the crystal growth on flat and structured heat transfer surfaces. For the numerical simulation the CFD code Fluent is used. The simulation enables an unsteady calculation of the fouling process and a realistic description of the temporal modification of the flow and temperature fields due to the continuous crystal growth. EXPERIMENTAL SET-UP The numerical simulation of the crystal growth is based on models for the calculation of the deposition (Krause, 1993) and removal (Bohnet, 1990) mass rates. Based on experimental results of Hirsch (Bohnet et. al., 1999), 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. All models are implemented into the simulation with the help of the programming user interface of the CFD code. During the experimental and numerical investigations the operating parameters like flow rate, surface temperature, concentration of the salt solution and geometry of the flow channel are varied. The induction period and the effects of aging which occur with almost all fouling processes are not considered. Result of the numerical simulation is the prediction of the fouling resistance as function of time. 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.