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
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.