Part F
Practical Applications
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
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
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.
Flow Impedance Characteristics
Results
Volumetric flow rates for Case I (equally spaced PCB cards)
Results
Pressure losses for Case I (equally spaced PCB cards)
Results
Bulk temperatures of the air streams exiting the PCB and the Power Supply
for Case I (equally spaced PCB cards)
Results
Volumetric flow rates for Case II (unequally spaced PCB cards)
Results
Pressure losses for Case II (unequally spaced PCB cards)
Results
Temperatures of the air streams exiting the PCB and the Power Supply
for Case II (unequally spaced PCB cards)
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.
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
Physical System
Table 29.1 Geometry of the fin sink manufactured by Wakefield Engineering
Dimension
Value (in)
Length
2.2
Width
4.6
Fin Height
0.75
Fin Pitch
0.1
Fin Thickness
0.012
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.
Network Representation
MacroFlow representation of the test cell used characterizing the
heat sink performance.
Results
Variation of the pressure drop through the fin sink with the flow rate with no bypass
Results
Pressure losses through various parts of the flow system with bypass
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 (
Thank You