advanced building skins 14 | 15 June 2012 - lamp.tugraz.at - Graz ...

3.2 Development of the Numerical Model
Advanced Building Skins
The most important task while developing the numerical model is to ensure its correct functioning.
Some numerical details, for example the heat transfer from the pipe to the foam or the heat transfer
from the foam to the surface, is numerical non-critical and thus in the scope of testing the single
components not further investigated. The most challenging part of the numerical simulation is the heat
transfer from the fluid to the inner wall of the pipe. This transfer is dependent on the properties of the
fluid, the velocity and the geometry. Based on a circular pipe, there are two possible ways for a
numerical simulation. The more accurate way is to simulate the fluid with all its changing properties
depending on its position in the pipe. Such a simulation leads to more realistic total values, but the
simulation itself becomes very complex. Thus the second way, the numerical simulation with the
abstraction, that fluid is assumed to be uniform over the width of the pipe, is chosen. The results of the
numerical simulation, using the simplified heat transfer from the fluid to the pipe, are proofed by using
calculation methods given by ‘[2]’. After combining the models of all components to create a
sandwich panel with an integrated solar system, the accuracy of the whole model is verified by the use
of a constructed prototype.
The prototype is a 0.5 mm thick steel sheet with a pipe system underneath. The pipe is made of copper
and has an inner diameter of 6 mm and an outer diameter of 8 mm. It is glued directly to the steel sheet.
The total length of the pipe is 10.61 m. The total size of the prototypes surface is 1 m x 1 m.
To avoid changing influences of the sun and wind, the prototype is operated inside the building. In fact,
the prototype is not used as a collector but as a heater instead. Constantly heated water circles with a
defined velocity. The temperatures are measured at the inlet, the outlet and the air. The schematic setup
of the prototype is shown in figure 2.
Figure 2: Schematic set-up of the prototype
For the numerical simulation of the prototype the surrounding air is assumed to be stationary. Also
heat losses from radiation are neglected. Because of the fact, that it is not possible to glue the pipe
always directly to the steel sheet with direct contact between both metals, a constant, filled with glue,
gap of 0.1 mm is assumed. Table 1 shows the results of the comparison between the measured and
calculated outlet temperatures. In addition to the measured outlet temperatures, a thermal image of the
prototype during test number 4 and the associated solution of the numerical calculation are shown in
figure 3. The comparison of the results of the numerical simulation with the measurements of the
prototype confirms the correct functioning of the developed model.
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