Aspects of Thermal Design and

1
Aspects of Thermal Design and Analysis of
AMS-02 Crew Operations Post(III)
A.- S. Wu and M.- C. Yu
November 4, 2004
2
Outline

• Content
  
  Page
• 1. Introduction
  3
  
• 2. System Description
  4
• 3. Thermal Control Concept and Thermal Design
  Description 6
• 4. Thermal Requirements
  8
• 5. Boundary conditions
  9
• 6. Thermal Loads
  10
• 7. Model Description and thermal Analysis
  12
• 8. Analysis Results
  16
• 9. Conclusions
  
  24
• Reference
  
  27

3
1. Introduction

• Since the existed acoustic noise of the ISS cabin
  is beyond the specifications, and fortunately
  ducted cooling air is provided by the Express
  Rack locker to cool the installed ACOP, the
  cooling system for ACOP will discard the cooling
  fans to meet the requirements of no noise.
• The sketch of ACOP is shown as in Figure 1.

Figure 1 The current design of cooling system for
ACOP .
4
2. SYSTEM DESCRIPTION

• The size of two square ports of the inlet and the
  outlet for the ducted cooling air is 110 x 110 mm
  at the back side of ACOP.
• The ports are fitted with screens with an open
  area ratio of 60.02.
• The friction coefficient of pressure loss for the
  screens
• is around 1.0.
• A typical flow rate, 15 cfm, of the cooling air
  with a conserved pressure of 10.2 psia is
  compressed into the inlet of ACOP.
• Two conduits are designed to connect the ports of
  the inlet and the outlet with the fin channels of
  heat sink in order to reduce the pressure loss.

5
2. SYSTEM DESCRIPTION

• At both sides of ACOP chassis extrude 56 fins
  respectively to be the heat sinks in order to
  increase both of the heat transfer area and the
  heat transfer coefficient.
• The thickness of the aluminum alloy fins is 1.5mm
  with height and length of 60mm by 162mm.
• The gap between two neighboring fins is 2.5mm.
• The cooling air comes into the inlet, by the
  conduit, through the fins to take away the power
  dissipation, generated on the boards and in the
  hard disk drivers and conducted to the chassis
  and the extruded fins, and comes out to the front
  chamber to cool the LCD panel, and then goes
  through the fins of the opposite side, by the
  opposite conduit and finally goes out to the Rack
  locker via the outlet port.

6
3. THERMAL CONTROL CONCEPT AND THERMAL DESIGN
DESCRIPTION

• In ACOP, there are three main subsystems to
  consume power rate.
• Four hard disk drivers, installed in the upper
  chassis of ACOP, are used to record the data
  collected by AMS-02.
• One single board computer, three compact-PCI 6U
  PMC carrier boards, and one power distribution
  board consisted of the controlling module of
  ACOP, are arranged in the bottom of ACOP.
• One LCD monitor is fixed on the front panel of
  ACOP to show the required information.

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3. THERMAL CONTROL CONCEPT AND THERMAL DESIGN
DESCRIPTION

• The power dissipation of parts on the boards is
  nearly conducted through the copper layers of
  power planes and of ground planes to the board
  edge via the spacer ,fixed to the chassis by a
  card-locker, to the chassis and spreads to the
  fins, and finally transfers to the cooling air.
• The commercial hard disk driver has a control
  board and a driver. Both of them will consume
  power. The board power dissipation also conduct
  to the edge of the HDD edge through the board.
  The driver uses spreader to conduct the heat to
  the HDD edge.

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4. THERMAL REQUIREMENTS

• According to the thermal management design of
  J-crate of AMS-02, the worst condition for the
  crate wall temperature can reach 50. Under this
  condition, the thermal modelling results and the
  TVT measurements show that the part temperature
  is under the specification of the data sheet.
• Currently the electronics designers cannot
  provide the temperature requirements without
  further information of the board and of the
  parts. However, the crate wall temperature of
  AMS-02 can be the temporary temperature
  requirement for the chassis of ACOP.

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5. Boundary conditions

• The cooling air temperature supplied by the AAA
  will be in the range of 18.3 to 29.4 C, cited in
  5.3.1.3.2 of reference 1.
• Here the cooling air temperature is assumed to be
  30C for a conserved calculation of the model.
• The back-plate, top-plate, two side-plate, and
  bottom-plate of ACOP are assumed to insult from
  the cabin air.
• The tip plate to enclose the fin channels is also
  considered to insult from the surrounding air.
• Only the front-plate, seeing the open space of
  the cabin, has a natural convection with the
  cabin by 0.965 W/m2 ? , referred to 5.3.1.1.3 of
  reference1.

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6. THERMAL LOADS

• Table 1 shows the power dissipation of boards
  and HDDs of ACOP.
• The total typical power dissipation of ACOP is
  estimated to be around 59.86W, removing the
  consumption of fan boards.
• For the boards, the power dissipation is divided
  into two equal parts and uniformly distributes on
  the slots to be the thermal load.
• For the HDDs, the power dissipation is also
  divided equally to load on the mounting slots of
  the chassis uniformly as shown in Figure.
• The power consumption of the LCD monitor is
  uniformly put on the surface of the square panel.

Figure 2 Thermal load of the model.
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7. MODEL DESCRIPTION AND THERMAL ANALYSIS

• I-DEASTMGESC code is applied to solve the
  computation task.
• The analyzed domain is meshed into around 224
  thousand elements by mapping method.
• For the solid and the fluid, 67,045 hexagonal
  elements and 157,671 trihedral elements are
  meshed respectively.
• Based on the electrical analogy, thermal model of
  the solid domain is established to construct a
  resistance-capacitance thermal network.
• A hybrid approach is developed in the code by
  utilizing the element based finite difference
  method to simulate conduction, and surface
  convection.
• The thermal code is coupled with the element
  based finite volume method flow solver, which
  models air flow, turbulence, fluid conduction,
  and advection.

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7. MODEL DESCRIPTION AND THERMAL ANALYSIS

• Although the Reynolds number of the fin channel
  flow is calculated to be around 200 to show most
  regions of the channels belong to a laminar flow,
  the inlet and the outlet of the channels give the
  characteristic of turbulence flow due to the
  entry effect.
• The turbulence model of the fixed turbulent
  viscosity model is adopted in the code, while in
  the laminar flow region, the turbulence effect
  will be much smaller the laminar viscosity
  effect.
• Where denotes the dynamic viscosity,
  denotes the density, Vm is a mean flow velocity
  scale, and Lt is a turbulent eddy length
  scale.

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7.MODEL DESCRIPTION AND THERMAL ANALYSIS

• Both the thermal and momentum wall functions
  utilized to calculate the heat transfer
  coefficient for the solid surface and the cooling
  air, described by B.A. Kader in 19812.
• The front plate is also treated as the aluminum
  alloy to conduct the power dissipation of the LCD
  monitor effectively.
• The thermal conductivity of aluminum alloy 6061T6
  is given by a constant value of 159 W/m ? ,
  neglecting the insignificant variation with
  temperature in the domain.
• The thermal conductivity of air is calculated by
  the internally established data-base in ESC code.
  

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7. MODEL DESCRIPTION AND THERMAL ANALYSIS

• Three cases of the cooling air with 12, 15, and
  18cfm respectively are solved to see the working
  temperature of ACOP under the limited conditions
  and the nominal condition.
• The inlet cooling air temperature is fixed to be
  30 ? with a conserved consideration for these
  three cases.

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8. Analysis Results

• The cooling air velocity field is shown in Figure
  3. The predicted results show that the maximum
  velocity is around 0.9 m/sec in the fin channel.
  The air flows very smoothly in the inlet conduit
  and in the inlet fin channel. However, at the
  outlet of the left fin channels produces
  significantly turbulent eddies due to an abrupt
  expansion.
• Theoretically Both the flow velocity and the
  Reynolds number are low enough to produce a
  laminar flow in the fluid domain.
• However, at the channel outlet the eddies will
  yield a turbulence effect to enhance the heat
  transfer rate.

Figure 3 The predicted velocity field of the
cooling air
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8. Analysis Results

• The calculated heat transfer coefficient h is
  around 28 W/m2 ? at the central part of the fins
  due to a thin gap of 2.5mm between the
  neighboring fins.
• According to the semi-empirical equation for the
  Nusselt number(Nu) of the fully developed flow in
  channels, the calculated heat transfer
  coefficient by hands is around 32 W/m2 ? with a
  conserved constant value of Nu5.6 for a channel
  laminar flow with an aspect ratio of 8.

Figure 4 Heat transfer coefficient on the
ACOP chassis and fins.
Figure 5 Heat transfer coefficient on the
ACOP front plate.
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8. Analysis Results

• Thus, the calculated h value by ESC code is less
  than that of the semi-empirical correlation.
• The computation results of this report are more
  conserved than the real solutions.

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8. Analysis Results

• With a mean value of h around 28 W/m2 ? and a
  large value of total fin area, 2.138 m2, the
  predicted chassis/fin temperature is around 36 ?
  at the inlet region of the left fin channels, and
  is around 40?at the outlet region of the right
  fin channels.
• The maximum temperature occurs at the central
  chassis for mounting the HDDs to be around 41 ?.
• At the central chassis of ACOP top side, the
  cooling is not introduced to cool down the
  working temperature. However, the thermal
  conductivity of chassis is high enough to conduct
  the power dissipation of HDDs effectively to the
  rest regions of the chassis, lead to a
  significant reduction of the working temperature.

Figure 6 Working temperature of
the ACOP chassis and fins.
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8. Analysis Results

• Figure 5 shows the predicted temperature profile
  of the front panel. Although the power
  dissipation of the LCD monitor is provided by
  6.3W with a smaller mean heat transfer
  coefficient of 8 W/m2 ?, than that of the
  channels of 28 W/m2 ?, the heat can conduct to
  the rest regions of the ACOP enclosure
  effectively, resulting into a maximum temperature
  of around 40 ? at the center of the LCD monitor.

Figure 7 The predicted temperature profile
of the ACOP front plate.
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8. Analysis Results

• Figure 6 shows the predicted temperature profile
  of the cooling air. The cooling air is gradually
  heated in ACOP from the inlet to the outlet. At
  the inlet conduit, the heat transfer rate is very
  small. But in the left channels, the air
  temperature is still under the low temperature
  condition. Thus, the heat transfer capability is
  strong in this region. The air temperature
  increases by around 7? to be 37? at the outlet
  of the left channels. In the front chamber, the
  air is heated by around 1?, and in the right
  channels, the air temperature is high as compared
  to the ACOP inlet condition, can take away a
  small portion of the power dissipation, lead to a
  small increase of the air temperature to be
  around 40.2? at the outlet of the right channels.

22
Figure 8 the predicted temperature profile of the
cooling air
23
8. Analysis Results

• The system pressure loss is calculated to be
  9.4Pa due to wall friction, entry transition,
  outlet expansion if the pressure loss due to the
  screens at the ACOP inlet and outlet is
  neglected.
• Under the open area ratio of 0.602 of the
  screens, the friction coefficient is assumed to
  be 1. Thus, Two screens of the inlet and the
  outlet will yield a pressure loss of 0.72 Pa.
• The total system pressure loss is than predicted
  to be 10.12 Pa with a flow rate of 15cfm and a
  low limited pressure of 10.2 psia.

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9. Conclusions

• The introduction of fin channels extruded out
  from ACOP chassis can provide two enhancing
  effects on the heat transfer rate from chassis to
  the cooling air.
• The apparently significant effect is due to a
  large increase of the area value for forced
  convection of the cooling air.
• The other enhancing effect of heat transfer
  results from a reduction of the channel gap,
  leading to an increase of the heat transfer
  coefficient.
• Both effects make the effective thermal
  resistance between the cooling air and ACOP be
  low enough, leading to a maximum increase
  temperature by 11 ? at the chassis center for
  mounting the HDDs.

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9. Conclusions

• Although the actual heat transfer coefficient of
  the cooling air can be larger than the modelling
  results for up to 34, the small temperature
  difference between the wall and the cooling air,
  which ranges from 1 to 3 ?, results into an
  uncertainty of 0.3 to 1 ? at the predictive
  results. The uncertainty of around 1 ? is
  acceptable as an allowable temperature of around
  50 ? on the chassis is specified. 

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9. Conclusions

• From the computation results, under the worst
  case with the minimum air flow rate 12cfm and
  with the maximum supplied air temperature 30?,
  the calculated maximum chassis temperature around
  43.4 ? is still below the crate wall temperature
  of AMS-02 J-crate 50 ?. Thus, the currently
  thermal management design is appropriate for ACOP
  to dissipate the power consumption effectively
  without any installed fans and needs a maximum
  pressure loss around 15 Pa for the maximum flow
  rate 18cfm, which is below the recommended value
  0f 25 Pa

27
Reference

• 1. Expedite the Processing of Experiments to
  Space Station (EXPRESS) Rack Payloads Interface
  Definition Document , SSP 52000-IDD-ERP
• 2. B.A. Kader, Temperature and Concentration
  Profiles in Fully Turbulent Boundary Layers,
  Int. J. Heat Mass Transfer, V.24, No.9,
  PP.15411544,1981