The modeling of ash deposition trends, rates, and shapes has been the target of numerousinvestigations since slagging and fouling constitute a major penalizing factor in boilerperformance. Also, they are the most usual cause of unscheduled downtimes.Unfortunately, the ash deposition is a rather challenging phenomenon to model due to itscomplicated and multidisciplinary nature which may combine complex species chemistry(ash aerosol formation), mineralogy, ash particle drag and impaction mechanics, and fluiddynamics. This motivates the research of ash deposition models.Ash deposition issues can be studied by using computational fluid dynamics (CFD).These tools are particularly attractive due to their versatility, although their current stateof-the-art is still somewhat inaccurate and qualitative, as it has been pointed out by someauthors. It is certainly challenging and ambitious to achieve satisfactory foulingpredictions. This thesis presents a CFD model for ash deposition trends and deposit shapepredictions. The model has been used with the aim to explain relevant fouling phenomenain kraft and coal-fired boilers. Special care has been taken regarding an accurate solvingof the flow patterns around tube arrays and on the necessary grid resolution for properdiscrete particle trajectory tracking. The necessity for unsteady flow simulation isemphasized, remarked and justified due to the swinging flow patterns.The model validity was tested with some experimental work. The challenges which aroseregarding the implementation and determination of the original phenomena are pointedout and detailed, as well as a comparison between the experimental and modeled results.Especially, a proper determination of the thermal conductivity and the solid fraction orporosity of the deposits are essential for reliable model results.Thermophoresis was found to be the main impaction mechanism for submicronparticulate. On the other hand, it becomes negligible for larger particles (from 3 micronsand larger) whose arrival rates are mainly influenced by inertial impaction. In addition,the importance of accurate flow solving was remarked by non-uniform distributions ofparticle sticking efficiency and arrival rates around the tube perimeters. The self-limitingnature of fouling (i.e., fouling rates tend to decrease with the time) is briefly quantifiedand explained. The deposit growth over relatively long fouling times is simulated andstudied with the usage of dynamic mesh routines.
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