Doctoral thesis from the field of thermodynamics

13/05/2015

A computational fluid dynamics (CFD) model for particulate fouling in high solid content heat exchangers has been developed in Ulla Ojaniemi's doctoral thesis. The model is applicable in practical industrial heat exchangers.
 
Particulate fouling is generally considered as a serial process of transport of particles into the vicinity of the wall, adherence on the surface and possible re-entrainment from the surface. The CFD fouling model was first implemented as a detailed two fluid Eulerian model, which included all the relevant near-wall forces affecting on the colloidal particles and requiring an extremely fine mesh near the fouling surface. For modelling particle transport, the generally accepted models were applied. Particle adhesion on the surface was described by a mass transfer coefficient based on the XDLVO theory.
 
Based on the experience gained from the detailed model, a wall function approach was developed for calculating the near-wall particle transport in order to avoid the use of excessively small computational cells. The wall function model was compared to a detailed CFD model and to experimental results from a fouling test apparatus. Deposition of sub-micron calcium carbonate (CaCO3) particles on a heated stainless steel AISI 316L surface in water based suspension was used as a case study. Comparisons were made with several heat fluxes and mass flow rates applying two different high particle concentrations. As a practical case, the wall function approach was applied in the modelling of industrial corrugated heat exchanger equipment with liquid of very high particle content. For including the non-Newtonian viscosity behaviour of the high solid content slurry, the model for viscosity was derived from the experiments.
 
The re-entrainment of the particles from the surface takes place, if the hydrodynamic forces exceed the adhesion forces even temporarily. Shear stress is a dominant force affecting the re-entrainment. For evaluating the shear strass in detail, the test apparatus was modelled using large eddy simulation (LES) method in addition to the standard k-ε turbulence model. The re-entrainment model presented in the literature was applied in order to evaluate the effect of surface roughness and surface energy on the re-entrainment. The model was modified for CFD application, and it was applied in fouling modelling of the practical heat exchanger. The effect of surface roughness on adhesion was studied based on the models presented in literature. In addition, the effect of particle size distribution on the fouling rate was examined.

Public examination on 21 May

The public examination of Ulla Ojaniemi's doctoral thesis Modelling particulate fouling in heat exchanger with high solid content liquid suspension takes place on 21 May at 12 at Aalto University School of Engineering, address: Konetekniikka 1, hall K215, Otakaari 4, 02150 Espoo. Opponent is TkT Jouni Pyykönen.



 

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