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SESES Tutorial September 2012
Figure 2.149: View of the heat exchanger. Visible are two crossed channels which carry a hot
and a cold air flow (arrows).
2.34 Heat Exchange between Air Flows
Air temperature and humidity are key parameters for comfort as well as for many
fabrication processes. The heat exchange between cold and hot air flow plays an important role in temperature control. Fig. 2.149 displays a drawing of a heat exchanger,
which is commercially available. These heat exchangers consist of hundreds of crossed
channels for the hot and cold air flows, separated by thin metal sheets. The channels lay horizontally and have heights between 3 and 15 mm. The efficiency of the
exchanger is expressed with the averaged ratio of hot and cold flows at the in- and
outlet
Tcold,out − Tcold,in
,
φ=
Thot,in − Tcold,in
where values of φ = 0.5 are considered as good. For ISO certification, the efficiency of
heat exchangers needs to be measured. Efficiency predictions by model calculations
are difficult due to the complex and irregular channel shapes and turbulent air flows.
Air conditioning engineers, however, suggest, that the presence of turbulence plays a
minor role for the value of φ. It seem to be the channel structure and dimensions, that
govern the φ value. When modeling just two channels, it is possible to calculate φ as a
function of channel thickness and compare the values to measurements.
Implementation of the Model
In order to simply the matter, we have modeled two air channels only with a given
rectangular flow profile. Implementing a more realistic parabolic flow profile involves either coupled velocity calculations of many air slabs or a complicated parameterization of the flow profile. The numerical example can be found at example/
HeatExchanger.s3d, it defines a cold channel in the center and half a channel of
hot air above and below the cold channel, see the left of Fig. 2.150. In the Geometry
definition section, an initial rectangular geometry is transformed to a rotated rhombus with the following statements
Include "Homotopic.sfc"