Development%20of%20Synthetic%20Air%20Jet%20Technology%20for%20Applications%20in%20Electronics%20Cooling

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Development of Synthetic Air Jet Technology for
Applications in Electronics Cooling

• Dr. Tadhg S. ODonovan
• School of Engineering and Physical Sciences
• Heriot-Watt University

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What is a Synthetic Air Jet?

• A flexible membrane or diaphragm forms one end of
  a partially enclosed chamber
• Opposite to the membrane is an opening, such as a
  jet nozzle or orifice plate
• A mechanical actuator or a piezoelectric
  diaphragm causes the membrane to oscillate and
  periodically forces air into and out of the
  opening
• Thus creating a pulsating jet that can be
  directed at a heated surface, such as an
  electronic device

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Characteristics of a Jet Impingement Cooling

• Instabilities in the flow at a jet nozzle develop
  into vortices that impinge on the heated surface
• The breakdown of vortices along the impingement
  surface increases velocity fluctuations normal to
  the impingement surface (ODonovan and Murray
  1, 2)
• These fluctuations result in enhanced heat
  transfer or secondary peaks in the heat transfer
  distribution
• Synthetic air jets are comprised entirely of
  successive vortex rings
• Introduce a stronger entrainment of surrounding
  air than conventional, steady jets
• These factors combine to give superior heat
  transfer characteristics

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• Current technologies to cool state of the art
  circuit chips and multi-chip modules (MCMs) rely
  on global forced air cooling which can dissipate
  0.5 to 1 W/cm2.
• It is anticipated that in the next five to ten
  years this requirement will increase up to 10 to
  40 W/cm2
• In a cooling performance benchmark test by
  Kercher et al. 3, it has been shown that
  synthetic microjets outperform conventional CPU
  fan coolers

Cooling Device Cooling Efficiency W/m2K
2.4 mm Synthetic Jet 96.90
NMB CPU Fan 61.52
Shicoh CPU Fan 44.30
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Characterisation of a Synthetic Air Jet

• Stroke Length
• Reynolds Number
• Strouhal Number

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Experimental Set-up No. 1
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Phase Locked Particle Image Velocimetry
Re 2670 L0 15 d
Re 2670 L0 7.7 d
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Experimental Set-up No. 2

• Flush Mounted Heat Flux Sensors on a UWT
  Impingement Surface
• RdF MicroFoil Heat Flux Sensor
• Senflex Hot Film Sensor

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Heat Transfer Distributions, H/D 2
At this height above the surface the plate lies
in the vortex formation region this results in a
high velocity flow occurring between the vortex
and the plate at a radial distance of r/D
0.7. It can be seen that the mean heat transfer
distribution has a local minimum at the
stagnation point for Reynolds numbers of 2300 and
above.
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Heat Transfer Distributions, H/D 4
At this height above the surface the plate lies
in the vortex are fully formed before
impingement Resulting in a high velocity
fluctuations overall and a peak at the geometric
centre. Some increase in surface heat transfer
fluctuations can be seen in the wall jet flow
region
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Phase Locked Vorticity Plot, H/D 1, Re 3700,
L0/D 17
F 120
F 180
F 240
F 300
F 0
F 60
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Phase Locked Vorticity Plot, H/D 2, Re 3700,
L0/D 17
F 120
F 180
F 240
F 300
F 0
F 60
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Development of an SAJ Electronics Cooler

• Design a synthetic jet array where jets interact
  constructively
• jet diameters, array geometries, frequency of
  oscillation, amplitude etc.
• Encourage the introduction of fresh cold air into
  the confined region by control of the pulsation
  characteristics of the individual jets aligned in
  a channel
• phase, frequency, and amplitude

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Conclusions

• Synthetic Air Jet Cooling can outperform standard
  fan-fin CPU coolers and are more effective than
  similar steady impinging air jets
• The current research addresses the limitations of
  conventional synthetic jet impingement cooling
  systems.
• Recycling of the air in a synthetic jet array
  causes its temperature to continually increase
  which adversely affects the heat removal capacity
  of the jets.
• To ensure that the air being forced over the
  heated surface is sufficiently cool, fresh
  ambient air must be brought in. This is typically
  achieved by introducing a secondary cross-flow of
  air over the heated device via a fan.
• Preliminary results show that synthetic jets can
  operate in clusters or arrays to achieve enhanced
  cooling of surfaces such as electronic devices.

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References

1. T. S. O'Donovan and D. B. Murray, "Jet
   impingement heat transfer - Part I Mean and
   root-mean-square heat transfer and velocity
   distributions," International Journal of Heat and
   Mass Transfer, vol. 50, pp. 3291-3301, 2007.
2. T. S. O'Donovan and D. B. Murray, "Jet
   impingement heat transfer - Part II A temporal
   investigation of heat transfer and local fluid
   velocities," International Journal of Heat and
   Mass Transfer, vol. 50, pp. 3302-3314, 2007.
3. D. S. Kercher, J. B. Lee, O. Brand, M. G. Allen,
   and A. Glezer, "Microjet cooling devices for
   thermal management of electronics," IEEE
   Transactions on Components and Packaging
   Technologies, vol. 26, pp. 359 - 366, 2003.
4. T. Persoons and T. S. O'Donovan, "A
   pressure-based estimate of synthetic jet
   velocity," Physics of Fluids, vol. 19, pp.
   128104-4, 2007.