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By Olivier Pelletier, Manager of Heat Transfer Research, and Fredrik
Strömer, CFD Engineer, SWEP International AB, Sweden
Compact brazed heat exchangers
The Compact Brazed Heat Exchanger (CBE) is one of the most efficient
ways to transfer heat from one medium to another. CBEs can be used in
many different applications for both heating and cooling. They are routinely
used in supermarket refrigeration units, air-conditioners, and domestic
tap water heaters. A CBE consists of many corrugated plates combined to
create complex channels through which a hot fluid and a cold one can be
distributed. Without ever mixing, the fluids come into close proximity
inside the CBE, and heat is transferred from one to the other as they
flow side-by-side.
CFD is a powerful tool for improving the heat transfer efficiency of
a CBE because it allows the flow in the CBE channels to be simulated in
three dimensions. The design of the CBE must offer both mechanical strength
and high thermal performance, so it is important to be able to simulate
new patterns and geometries quickly and efficiently. CFD readily allows
this to be done. Predictions can be validated experimentally, giving engineers
a high level of confidence in them.
CBE channel with tetrahedral cells
Pathlines in the CBE channel colored by velocity
SWEP is the world’s leading manufacturer
of CBEs, and has been
using CFD since 1997. FLUENT was chosen
because of its ability to import
geometries from 3D CAD packages.
The automatic tetrahedral meshing
feature was also important. At that time,
only a small region of the heat
exchanger could be simulated - the
region between four braze points, corresponding
to an area of about
50mm2 – and periodic boundary
conditions were frequently used. With
improvements in the software and speed
increases in affordable computer systems,
it became possible to simulate
entire channels. During the past five
years, the size and complexity of our
models have increased, yet the time
spent on meshing and solving them
has been drastically reduced.
Simulations using FLUENT have been
very important in the development of
new, improved CBEs that offer better
heat transfer and shorter time-to-market.
The ongoing effort with CFD has
also had an organizational impact. A
closer cooperation has evolved between
the Design and the Heat Transfer
Research departments, who share the
common goal of making better use
of the simulation possibilities. The design
department uses Mechanical Desktop
5.0, which generates ACIS-based
geometries that are easily meshed by
GAMBIT. The heat transfer research
department uses FLUENT 6.0 to generate
a solution that gives information
about the flow, which is then used to
improve the design. The design engineers
make modifications and the
process repeats itself. There are still limitations
in the development process,
however. For example, it is still not practical
to simulate the entire CBE,
which consists of many parallel channels,
ports and connections. Yet with
the speed of computers continuing to
rise, even this prospect will soon become
possible.
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