ESEL Paper Review_20130424
By Hong Guo
Mail: hongguo@gist.ac.kr
Phone: (+82) (0)10 82276568
1, Title and Author
Title: Finite element simulation of unsteady flows in secondary settling tanks
Conference: Fifth International conference on CFD in the Process Industries
Authors:
Dorothee KLEINE and B.DAYA REDDY
1 Centre for Research in Computational and Applied Mechanics, University of Cape Town, 7701 Rondebosch. South Africa
2. Summary of Paper
In this paper, the author used a finite element analysis of unsteady flwos in secondary settling tanks. The model takes full account of the distribution and flow feature.
3. Results
? A constant turbulent viscosity was chosen for the Darvill old and new tanks with constant given as Ekama etal
? Boundary condition
? The inlet conditions are idealized, in that all values of the unknowns are assumed to be constant and uniformly distributed across the inflow boundary.
? The gradients or fluxes of velocity and concentration are set equal to zero at the effluent outlet boundary
? The baffles and the effluent weir are treated as a reflecting boundary, so that the normal derivative of the sludge concentration is set equal to zero
? the normal gradients of k and of epsilon are set equal to zero
? The velocity normal to the bottom is set equal to the return activated flow divided by the removal outlet area affected.
? It is assumed that there is negligible change in water surface elevation over the tank, and that the vertical velocity and the horizontal surface traction are et equal to zero at the free surface. The turbulence decreases at the free surface, which leads to a reduced turbulent viscosity.
? In the numerical model this fact is taken into account by allowing the dissipation to vary inversely with depth, as proposed by KAMA
? Wall function are used to mitigate the poor behavior of the k-epsilon model in the near wall region
? Boundary conditions for the concentration are set similarly. The flux at the
bottom is set so as to assure a balance of mass, while walls are treated as
reflecting boundaries
Presentation and discussion of results
? The average element dimensions of approximately 150mm square were
thus sufficient to resolve flow structures which were of the order of 1-2m
? Darvill old tank
? Mass conservation and viscous forces influence the flow pattern in
the settling section, resulting in complete flow circulation with a
strong bottom forward current and a reverse current at the water
surface.
? The steady state case, is reached when the potential and kinetic
energies have reached equilibrium
? The numerical results shows stable behavior, as in the stress test in
the de Hass (1998), the sludge blanket reaches equilibrium and
does not rise to the outlet for the clarified water
? By increasing 13.3% influent flow, the sludge blanket reaches the
effluent outlet and the current coming from the inlet chamber is
raised to the water surface.
? Darvill new tank
? By increasing 13.3% influent flow, the sludge blanket does not
reaches the effluent outlet and the transient phase the sludge
current moves along the bottom to the end of the tank, also causing
complete flow circulation
4. Contribution:
The paper applied the finite element method to complete the numerical analysis
for the secondary sedimentation tank. In addition, the author compares the
experiment data to the numerical result. This paper could be the advanced study ,
which apple the finite element method for sedimentation tank analysis to our
future work.