Tommy Pascal Trimless - Tal.be

Experimental Performance
study Porifera
Due to its relatively high efficiency, its wide applicability
and especially its long life expectancy, LED lighting has
become one of the most promising technologies
in order to reduce energy consumption. LED can be used in
a wide range of applications. However the efficiency and life
expectancy depend strongly on the LED junction temperature.
The temperature reduction can be achieved in different ways.
The most applied solution is integrating a classical heatsink
with fan. However this solution has several disadvantages. The
solution proposed in this paper is using open cell metal foam.
This is a very light weight
structure, considering it consists for about 95 % out of
air.
APPARATUS AND PROCEDURE
The experimental setup consists of a rectangular heater
element with a length of 254.0 mm (10 inch) and a width of
25.4 mm (1 inch). To make the temperature distribution more
uniform, two copper plates are placed on top of this heater.
Between these two copper plates six thermocouples of type K
are installed. Above the upper copper plate the test sample is
attached.
Under this assembly and to its longest sides, guard heaters
are placed to minimize the heat losses. Three thermocouples
are placed on a copper plate which in turn is placed on the
bottommost guard heaters. On each of the lateral guard
heaters a copper plate with three thermocouples is placed.
Between the heater and the guard heaters and on top of the
lateral guard heaters Microtherm®’s Standard Block insulation
is positioned. Around this entity (except on the top surface)
more insulation is provisioned. Upon this a box was placed
with dimensions of 68.5 cm long, 60.0 cm wide and 45.0
cm high. To improve the conductivity between the different
components, a thermal paste is applied. The temperature
variations across the surfaces do not exceed 0,80 °C for a flat
surface. The main heater’s resistance is about 580 _ ± 15 %.
The main heater’s temperature varies between 55 °C and 95
°C for all measurements performed in this experiment.
The total heat loss is calculated as the sum of the heat loss of
the main heater’s to the guard heaters
(Qloss, 1), the heat loss occurring on the insulation’s top
(Qloss, 2) and the heat losses occurring at the heater’s
ends (Qloss, 3). The first heat loss is estimated conservatively
by calculating the heat loss as if it were a conduction loss
taking into account the temperature difference between the
main heater and the concerned
guard heater, the concerned guard heater’s surface, the
shortest distance between the main heater and the
concerned guard heater and the insulation’s conduction
coefficient (being 0.022 W/mK). This 1D conduction
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model is permitted because the temperature of all
heaters are presumed uniform and the corresponding
heat losses are considered to be small.
The second heat loss is calculated as the surface
integral of the product of the conduction coefficient
and the temperature gradient over the upper 5 mm of
the guard heater’s sides.
The temperature as a function of the area is derived
as the solution of a Robin problem. Solving the problem
shows that the heat loss can be neglected, so further on it is
presumed that Qloss, 2= 0. The third heat loss is calculated in
the same way as the second heat loss. This heat loss can also
be neglected, so further on it is presumed that Qloss,3= 0.
In this paper two types of metallic foam will be examined:
metallic foam of 10 PPI with a porosity of 93.73 % and
metallic foam of 20 PPI with a porosity of 93.31 %. The
maximal heights are respectively 40 mm and 18 mm. To
compare the results to each other as well as to compare them
to the results found in other papers, the Nusselt number will
be computed in function of the Rayleigh number. Therefore
the convection coefficient need be calculated as follows:
To compare the results for a flat surface, the convection
coefficient is calculated in the same manner, with the
exception that radiation heat is subtracted from the heat input
Q. The characteristic length (L*) is determined by following
formula:
RESULTS
To evaluate the test setup, the Nusselt number is calculated
from the measurements for different Rayleigh
numbers for an ordinary flat aluminum plate. In this
figure two trend lines for natural convection adjacent
to a horizontal flat surface are also depicted, namely
Lloyd and Moran’s trend line and Mikheyev’s trend
line. As can be seen, the trend line based on the experimental
data, which has a maximal error of ±3.2 %, is
in good accordance with Lloyd and Moran’s regression.
The trend line is in accordance with this regression