Spray Drying Technology.pdf - National University of Singapore

structures can then be visualized using smoke. There can be many options to
introduce smoke into the chamber. When analyzing the atomization region,
Langrish et al. [42] introduced smoke from the air inlet pipe to flow across their
rotary atomizer. Smoke can also be introduced into the chamber via tubes or
‘tube racks’ to be positioned at different regions of interest within the chamber
[16] . Using smoke visualization, they managed to observe small eddy
formations in the recirculation region when swirl was imparted to the flow by
atomizer disk rotation. A key issue in using such visualization method
particularly smoke visualization; is the illumination required to make the
smoke contrastingly visible for analysis. The concentration of smoke in the
region of interest is also another factor which makes this method useful.
3.2 Humidity and temperature measurements
A huge difficulty in measuring the humidity and temperature field is
because of the presence of droplets and particles. A seminal advancement in
this area was contributed by Kieviet et al. [47] in the development of a microseparator.
This device allows particles or droplets in the air to be separated
before reaching humidity and temperature probes which can be inserted into
different regions in the chamber. Separation is achieved by enforcing a tight
flow curvature of the particle-laden air, causing the particles to traverse the air
flow and thus prevents any particle in reaching the measurement sensors.
Incidentally, the development of the micro-separator by Kieviet et al. [47] also
involved use of the CFD technique.
3.3 Final product and deposition flux comparison
For full CFD simulations incorporating particles injections, it is typical to
make comparison of the predicted particle moisture with that of experiments.
So far, there is limited comparison reported in the literature. Quantitative
deposition flux data, hitherto, can be found in numerous reports [59][69][70].
Deposition fluxes were mainly determined by inserting plates into the chamber
and allowing the particles to deposit over a relatively short period of operation
time. Based on the pilot scale experimental deposition fluxes, Kota and
Langrish [28] noted that their simulations produced realistic deposition trends.
However, the CFD prediction only produce reasonable quantitative match to
the experimental data. Qualitative data particularly on regions of high
deposition in industrial scale dryers was also reported by a few workers [71][72] .
Such observations can certainly complement CFD simulations although only
limited comparisons can be made due to the confidentiality of such
commercial operations.
Woo, Huang, Mujumdar, Daud – CFD modeling of spray dryers 21