Tommy Pascal Trimless - Tal.be

with a maximal difference of 4 %. The trend line is in
accordance with Mikheyev’s regression: the maximal
difference between both is 5 %. To verify the metal foam
height’s influence for the 10 PPI metal foam, measurements
have been performed for five different metal foam heights.
The metal foam height’s influence on the heat transfer is
clearly distinguishable. As the metal foam height increases,
the heat transfer improves. The increase is affected by raising
Rayleigh numbers. The increase however diminishes as the
metal foam height increases. Comparing the measurements
for different metal foam heights to the results obtained for a
flat surface, one notices that the latter curve is flatter. Finally
the obtained results are compared to the trend line obtained
by Phanikumar et al. Extrapolating the Nusselt number as a
power function of metal foam height using the test results,
one becomes the values as expected using Phanikumar’s trend
line.
To verify the influence of the metal foam height for the 20 PPI
metal foam, measurements have been performed for three
different heights. The metal foam height’s effect is clearly
visible. The convection coefficient and the corresponding
Nusselt number increase as the metal foam height increases.
The increase is affected by raising Rayleigh numbers.
Figure 1 measurements 20 PPI sample, different foam heights
The conclusion is valid for all trend lines. Moreover Nusselt
number augments as the metal foam height increases.
Finally the results are compared with the Phanikumar’s and
Hetseroni’s trend lines. Surprisingly the heat transfer for
Hetseroni’s 10 mm trend line is higher than the heat transfer
measured at a metal foam height of 12 mm. This can be
explained by the fact that in Hetseroni’s experiments the foam
was not attached to the base plate, it was clamped on both
sides instead . The sample was heated by sending a current
through the metal foam. Therefore the transferred power
was distributed more uniform over the metal foam, moreover
the air could move more freely through the metal foam.
Phanikumar’s trend line corresponds very well to the obtained
test results.
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2012 • 01
Figure 2 measurements brazed test sample vs epoxy glued
The metal foam can be connected to the base plate in
different ways, two of these are investigated in this work: the
first is called ‘brazing the metal foam to the base plate’, while
the second one is called ‘gluing the metal foam to the base
plate using epoxy glue’. The average improvement obtained
by brazing is around 11 % Finally Bhattacharya’s trend line
is compared to the obtained results. The trend line almost
depicts the same increase as does the 12 mm epoxy glued
metal foam trend line.
The influence of the pore density has also been examined. For
each metal foam height the heat transfer is less in the 20 PPI
metal foam. On average the Nusselt number increases with
about 20 % using the 10 PPI metal foam in comparison with
the 20 PPI metal foam.
The two metal foam types have also been compared to a
classical heatsink with a height of 18 mm. One can conclude
that at identical metal foam height 10 PPI metal foam causes
the greatest heat transfer, while the classical heatsink causes
the smallest heat transfer. In the 4,000 – 6,000 interval of
Rayleigh numbers, 10 PPI metal foam gives rise to Nusselt
numbers that are on average 70 % higher than those
corresponding to a classical heatsink. For 20 PPI metal
foam the effect diminishes to about 45 %.
In order to verify the influence of the box’s geometry,
18 experiments have been conducted. To perform these
experiments, metal foam height, box width, box height and
base plate temperature have been varied.
The obtained regression equation was:
© Robin Reynders