| Summary: | Experiments were performed to evaluate the effects of swirl on mixing in a Kraft recovery
boiler. A 1/28 geometrical scale physical model with water as the working fluid was studied by
means of laser Doppler velocimetry, L D V , and particle tracking. Flow field and mixing
characteristics were measured in the boiler model for three alternative conditions: i) the
"balanced" case where all the primary and secondary, ports were open, ii) a slight swirl case in
which 2 secondary corner ports, out of 18, were closed, and iii) a "strong swirl" case in which 4
secondary corner ports were closed. Such port closures are possible in an actual boiler through
clogging and clearing of air ports in normal operations.
A comparison of all three cases showed that the flow patterns in the boiler were
significantly different between balanced and swirling flow. The balanced case showed general
upward flow in the boiler model whereas the swirl cases showed a downward central core. In
general, the vertical component of velocity was much more unsteady for the balanced case than
for the other two cases, so that the balanced case showed higher upward R M S and kinetic energy
values than the other two cases. Mixing, as quantified by a parameter involving the flux of a
polystyrene tracer injected through one of the open secondary ports, was high for all cases.
However, less mixing was observed for the balanced case than the swirl cases.
The observations described above lead to the conclusions that: i) mixing is not necessarily
improved by increased unsteadiness or kinetic energy, a relationship that is superficially attractive,
and ii) a moderate degree of swirl, by introducing a more regular flow pattern and greater mixing
into the boiler, can be advantageous for combustion and therefore for recovery boiler operations. Experimental checks on the continuity of fluid flow and particle flow indicated significant
errors in the balanced flow case, probably due to the highly unsteady flow in this case with
resulting measurement difficulties.
The measurements were compared to numerical simulations of the flow field and the
mixing in the boiler. In all cases, trends observed in the experiments were followed in the
computational results. The swirling flow cases exhibited downward flow cores as well as lower
kinetic energy and more complete mixing than the balanced case.
The agreement between computational and experimental results provides confidence in the
trends found and suggests that the effects of mixing due to swirl in a recovery boiler can be
refined using numerical simulations.
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