Environmental Engineering Reference
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Figure 2.13 Surface and air temperatures and flow velocities in a double fa¸ade
In the external gap with a 0.175m distance between the outer glass pane and the
blind, the flow velocities varied between 0.05 and 0.6m s 1 ; in the inner, wider gap
of 0.325m distance the flow velocities were between 0.1 and 0.3m s 1 . The maxi-
mum temperature increase of the air gap was 8 K. Due to the high temperatures in
the double fa¸ade, the inner surface of the heat-protecting glazing increases from
night temperatures of 22 Cto28 C during the day, causing additional cooling loads
(see Figure 2.13). The boundary conditions correspond to the first 2 days shown in
Figure 2.12.
Further measurements on the south-eastern fa¸ade gave similar results. The mean
temperature increase in the air gap was 6.5 K at an average irradiance of 560Wm 2 ,
while the gap entry temperature was 2.8 K higher than ambient air. The mean flow
velocity in the narrow gap was 0.41m s 1 and 0.14m s 1 in the wider gap. The blind
temperature reached 45 Cmaximum. In the laboratory with air inlet through the com-
plete fa¸ade gap (44% cross-section), the blind temperature was only 5-6 K above the
inlet air temperature. A reduction of the inlet air free cross-section down to 14%, which
is close to the 10% opening in the Zeppelin Carre building, reduced flow velocities and
increased temperatures. Typically, blind temperatures were then about 8 K above inlet
air temperatures andmaximum temperature increases of the fa¸ade air were 5 K. These
slightly lower temperature levels indicate that flow conditions on site in real buildings
are more turbulent than laboratory measurements with nearly undisturbed air flow
entry into the fa¸ade. Finally, measurements were taken to determine the g -value of
the building fa¸ade. The short-wave irradiance transmission was measured using two
pyranometers inside and outside of the fa¸ade. The secondary heat flux can only be
directly measured if no irradiance strikes the sensor; that is, for the completely closed
blinds. From those heat flux measurements the internal heat transfer coefficient was
determined at 10Wm 2 K 1 and used to calculate the other secondary heat fluxes.
The averaged results are listed in Table 2.4.
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