NAMS 2002 Workshop - ICOM 2008

in the vicinity of the rising bubbles were simulated with CFD (Fluent) in
combination with the volume of fluid (VOF) method (constant surface tension,
time step 10 -6 - 10 -4 s). All these numerical simulations were also used to perform
parameter studies by varying geometrical values or operating conditions (e.g.
channel width, bubble diameter) studying their influence on the wall shear
stresses in order to minimise fouling.
Results
For the circulating flow the measured and simulated liquid velocities showed
good agreement. So CFD simulations are an appropriate tool for the optimisation
of module and filtration tank geometry. Furthermore typical problems in the
operation of flat sheet membrane modules became evident such as insufficient
aerated gaps in the outer region of the module. In practice this will lead to an
accelerated fouling in this area and a subsequent permeability reduction of the
total module. By an improved design of the gas sparger a more homogeneous
bubble distribution in the membrane module and an accelerated circulation was
achieved. The rise velocity of bubbles ascending between differently spaced
plates showed that small bubbles move like in an unconfined liquid. Above a
certain diameter, however, which is smaller for narrowly spaced walls, bubbles
briefly slow down as the deceleration effect caused by the walls becomes
dominant. With further increased size, the presence of the walls drastically
changes the bubble shape: they become elongated and flat cap bubbles. Due to
the thus decreased projected area, bubbles with a diameter above 10 mm
overcome the deceleration effect and even achieve higher rise velocities
between plates than in unconfined environments. Although this acceleration is
independent of channel width, the plate distance influences the maximum
possible stable bubble size. Even small bubbles disrupt due to the higher shear
in narrow channels. In order to optimize bubble size and wall distance for fouling
control, the shear rates must be known. From the CFD simulations the maximum
wall shear stresses have been deduced. As expected, highest shear can be
achieved for narrow channels which, however, would become clogged too easily
in sludge systems. For practical applications an optimum bubble size and
membrane gap of both 5 mm is suggested.