Part%20F

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Part F

• Practical Applications

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28. Fan-Cooled Enclosure of a PC System
Physical System
The physical system of interest is a fan-cooled
enclosure containing a Printed Circuit Board
(PCB) array and Power Supply. The system
configuration represents a computer system
package that is typical for personal computers
and workstations. A schematic of the
configuration is shown below
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Network Representation
The network representation of the flow system is
constructed by representing the various paths
that the air follows using the component and
link library provided in MacroFlow.
Flow network representation of the fan cooled
enclosure
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Flow Impedance Characteristics
Flow resistance or impedance characteristics of
various components need to be specified to
complete the network specification. The flow
characteristics of the PCB array and the power
supply are known from empirical measurements and
are expressed in the following form
Analysis has been performed for two cases
corresponding to even and uneven spacing of the
cards in the array. The loss coefficient B for
the Power Supply is constant in both cases and
is equal to 3.5 x 105 Pa/ (m3/s)2. The loss
coefficient for each passage of the PCB array is
listed in Table 28.1.
Each of the cards is assumed to dissipate 50W of
heat while the power supply dissipates 167 W.
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Flow Impedance Characteristics
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Results
Volumetric flow rates for Case I (equally spaced
PCB cards)
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Results
Pressure losses for Case I (equally spaced PCB
cards)
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Results
Bulk temperatures of the air streams exiting the
PCB and the Power Supply for Case I (equally
spaced PCB cards)
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Results
Volumetric flow rates for Case II (unequally
spaced PCB cards)
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Results
Pressure losses for Case II (unequally spaced PCB
cards)
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Results
Temperatures of the air streams exiting the PCB
and the Power Supply for Case II (unequally
spaced PCB cards)
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29. Flow over a Heat Sink
Physical System
Pressure drop and heat transfer characteristics
of heat sinks are determined from wind tunnel
testing as shown in Figure 29.1
The heat sink is situated inside a duct (wind
tunnel). Screens or perforated plates may cover
the inlet and the exit of the duct. The flow
within the duct is driven by a fan situated near
the inlet and its rate is varied by controlling
the opening of the orifice. Further, the duct
size can be varied (by moving the walls or using
different sized ducts) to study the effect of
bypass on the performance of the heat sink.
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Physical System
Figure 29.1.The wind tunnel test cell for
characterizing heat sink performance
Figure 29.2 Cross-sectional view of the heat sink
with bypass
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Physical System
Table 29.1 Geometry of the fin sink manufactured
by Wakefield Engineering
Value (in) Dimension
2.2 Length
4.6 Width
0.75 Fin Height
0.1 Fin Pitch
0.012 Fin Thickness
Experiments have been carried out at Wakefield
Engineering for measuring the pressure drop
through the heat sink over a range of air flow
rates. In the present study, a MacroFlow model
for the wind tunnel test cell has been
constructed for the general case of flow over the
fin sink with the bypass using the methodology
proposed by Butterbaugh and Kang. The results of
the model for the no bypass configuration have
been compared with experimental measurements.
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Network Representation
MacroFlow representation of the test cell used
characterizing the heat sink performance.
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Results
Variation of the pressure drop through the fin
sink with the flow rate with no bypass
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Results
Pressure losses through various parts of the flow
system with bypass
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Results
Variation of the fraction of the flow passing
through the sink as a function of the total flow
rate in the passage in presence of a fixed
bypass ( 0.2 inches on the side and 0.25 inches
above the fin tips )
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Thank You