Numerical Simulation Of Heat Exchanger Fouling

Fouling of heat exchangers is still an ongoing problem that causes tremendous energy loss and substantial costs in a wide variety of industries, although continuous progresses have been made to understand fouling mechanisms and pursue fouling mitigation approaches. In the past decades, numerous studies have been carried out to explore how surface characteristics, solution chemistry, and operating conditions can be manipulated to inhibit fouling layer growth, which usually require laborious and lengthy experimental schemes. Those efforts may be greatly facilitated if a predictive model is available. The comprehensive 3D dynamic data offered by the model, most of which are not easily attainable using current experimental techniques, could improve our fundamental understanding of fouling mechanisms. Moreover, thorough and systematic in silico experiments can be designed to identify the most effective fouling inhibition pathways. In this thesis, we introduce two major modeling efforts in this area.1. A new computational fluid dynamics(CFD) model is developed to characterize a crystallization fouling process mathematically. The introduced method incorporates a pseudo-dynamic scheme where the dynamic fouling process is approximated as a set of sequential steady-state processes taken place in a continuously varying geometric domain. This unique approach allows the characterization of mass, momentum and heat conservations of a continuous flow of liquid over a growing fouling layer. Dynamic evolution of the fouling layer surface(even with a complex shape) and its intricate interactions with hydrodynamics and fouling kinetics can then be rigorously taken into account. The introduced model was validated using the experimental data for a calcium sulphate fouling system. Furthermore, the effects of the solution chemistry and operating conditions on fouling resistance evolution were quantified through a comprehensive parametric study. As a predictive tool, this model could be especially useful for the identification of effective fouling mitigation or even elimination strategies.2. The fouling layers on heat exchanger surfaces exhibit complicated structures, which essentially determine flow hydrodynamics, fouling kinetics and hence the heat transfer performance. Numerical models developed so far for the fouling process, however, are based exclusively on the assumption of an impermeable fouling layer with a uniform porous structure. In order to quantitatively evaluate the effect of fouling layer structure on fouling dynamics, this work systematically investigated four representative schemes for fouling layer characterization: i.e., a homogeneous porous medium that is impermeable to water(Ho Im), a heterogeneous porous medium that is impermeable to water(He Im), a homogeneous porous medium that is permeable to water(Ho Pe), and a heterogeneous porous medium that is permeable to water(He Pe). Under the same operational conditions, four models offer significantly different prediction results on the fluid velocity, temperature distribution and fouling resistance. It is concluded that numerical model development should take the fouling layer structure into account, and the scheme of He Pe that best resembles a real fouling layer structure should be a promising option.