Heat Sink and Housing for Avalanche Photodiode Cooling

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• Heat Sink and Housing for Avalanche Photodiode
  Cooling

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• Background Information on NOvA project

• The NOvA Experiment requires a high intensity
  neutrino beam
• Uses a 15 kiloton liquid scintillator detector to
  detect emitted neutrinos
• Primary goal of the experiment is to search for
  evidence of muon to electron neutrino
  oscillations

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• Background Information

• Design a heat sink for the Avalanche Photodiode
• Needs to be held at -15C to reduce noise
• APD device detects neutrinos passing through
  detector
• Current system is chilled water cooling which
  needs high maintenance
• Goal is to design a passively cooled solution

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• Mission Statement

• The objective is to deliver a superior cooling
  design for the Avalanche Photodiode system.
• Alternatives being considered use prohibitively
  massive finned heat sinks.
• The ideal design would use a passive cooling
  method
• The project will evaluate current designs,
  formulate a new design and develop a testing
  methodology to prove the merit of the new design.

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• Customer Identification

• The customers for this project are the
  researchers and scientists at FermiLab.
• They will ultimately be the ones deciding on
  using this design for the neutrino detector.

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• Key Customer Requirements

• Maintaining an APD temperature of -15 degrees
  Celsius
• Low maintenance and high reliability
• Lightweight
• Light-tight and air-tight seal
• Solution costs less than current design
• Easy to service and access fixture
• The fixture cannot interfere with adjacent units

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Product Design Specifications
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PDS Highlights
Critical Points

• Heat Dissipation Will it keep the APD at
  -15degrees C?
• Weight Limited to 2.5lb
• Space Constraints Will it interfere with
  adjacent units?
• Air/Water/Light sealing Will the APD be free
  from contamination?
• Reliability Will the new system be better than
  the existing design?

Less Critical Points

• Cost 100 Per unit
• Power Usage
• Maintenance Service Time Is the unit easy to
  fix or replace?

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Concept Generation
Spin offs of the existing system

• Circulate chilled water
• At -15C

• Blow compressed cool air over a heat sink
  attached to the APD

• Circulate a coolant that would quickly evaporate
  if it leaks

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Concept Generation
Other ideas

• Different heat transfer methods

• Use of heat pipes for better heat spreading

• Combine separate units

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Concept Generation
Concepts that show promise

• Circulate chilled water
• At -15C

• Blow compressed cool air over a heat sink
  attached to the APD

• Circulate a coolant that would quickly evaporate
  if it leaks

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Selecting Concepts
Four separate designs considered

• Passive Heat Sink, Dual-TEC
• Redundant Fan system, Dual-TEC
• Passive Heat Sink, Triple-TEC
• Remote Mounted Heat Sink, Dual-TEC

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Modeling a Heat Sink

• 3 Ways to model
• Theory-based analysis
• Physical prototype

• Computer model

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Theory-Based Analysis

• Pros
• Time-tested approach
• A wealth of literature
• Easy to document
• Cons
• Time consuming
• Geometries are often too complex
• Theory often does not correspond
• well with reality

Conclusion Use to predict ballpark values to
verify other analyses
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Computer Model

• Pros
• Allows creation of any
• number of geometries
• Powerful thermal analysis capabilities
• Much cheaper than building actual
• physical model
• Cons
• Programs arent easy to use
• Results are not necessarily correct
• Enables ignorance of actual physical
• principles being used

Conclusion Useful for the bulk of our
analysis, interpret results with caution
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Physical Prototype

• Pros
• Test an actual model in the
• real world
• Most accurate results
• Observe design flaws not previously
• anticipated
• Cons
• Can be expensive
• Advanced manufacturing facilities
• may be required
• May not reflect actual performance
• of final product in use

Conclusion
Too difficult/expensive to build for our purposes
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Plan for future
Creation Process

• Create a sketch of a possible geometry
• Perform basic analysis using methods and
  equations from literature
• Create computer model in program such as Pro/E
• Analyze model using CFD software such as Ansys
  CFX
• Compare results to analysis from 2
• If results are in general agreement and satisfy
  specifications, attempt to
• refine model for further improvement if possible
• Ensure that model conforms to non-thermal related
  specifications
• 8. Finally, develop plan for testing and
  validating real-world model

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Analysis Methods
Analytical Sample
Determine the effective natural convection
heat-transfer coefficient. Find the total
heat-transfer rate of a heat sink design. Source
of Reference Kraus, Allan and Bar-Cohen, Avram,
Design and Analysis of Heat Sinks,
Wiley. Assumptions 1. The heat sink is
vertically oriented, made of sand-cast aluminum
on a 5mm thick, 150mm long, and 41mm wide
base. 2. 4 fins each of 10mm high and 5mm thick
are used. They are spaced 7mm apart. 3. The heat
sink can be assumed isothermal at a temperature
of 330K, and the ambient air is at 300K.
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Analytical Model
Calculation High Lights With the geometry of heat
sink and temperature difference of 30K in the
foregoing, the Rayleigh number RavA is calculated
as RavA 4.64x106 The Nusselt number can be
found as Nu 3.369 0.6 Ra1/4
31.2 The natural convection heat-transfer
coefficient h can be found from Nu as h Nu k/
vA 6.52 W/m2.K The heat dissipation of the heat
sink can thus be found Q hA ?T 6.52 x 0.018
x 30 3.5W.