1. Development of Synthetic Air Jet Technology for Applications in Electronics Cooling Dr. Tadhg S. O’Donovan School of Engineering and Physical Sciences Heriot-Watt University 27 th HEXAG Meeting, Heriot-Watt University, Edinburgh

2. Synthetic Air Jet Electronics Cooling 27 th HEXAG Meeting, Heriot-Watt University, Edinburgh 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 What is a Synthetic Air Jet?

3. 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 (O’Donovan 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 Synthetic Air Jet Electronics Cooling 27 th HEXAG Meeting, Heriot-Watt University, Edinburgh Characteristics of a Jet Impingement Cooling

4. Cooling Device Cooling Efficiency [W/m 2 K] 2.4 mm Synthetic Jet96.90 NMB CPU Fan61.52 Shicoh CPU Fan44.30 Synthetic Air Jet Electronics Cooling 27 th HEXAG Meeting, Heriot-Watt University, Edinburgh 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/cm 2. It is anticipated that in the next five to ten years this requirement will increase up to 10 to 40 W/cm 2 In a cooling performance benchmark test by Kercher et al. [3], it has been shown that synthetic microjets outperform conventional CPU fan coolers

5. Stroke Length Reynolds Number Strouhal Number Synthetic Air Jet Electronics Cooling 27 th HEXAG Meeting, Heriot-Watt University, Edinburgh Characterisation of a Synthetic Air Jet

6. Nusselt Number Particle Image Velocimetry (PIV) Synthetic Air Jet Electronics Cooling 27 th HEXAG Meeting, Heriot-Watt University, Edinburgh Experimental Set-up No. 1

7. Phase Locked Particle Image Velocimetry Re = 2670 L 0 = 15 d Re = 2670 L 0 = 7.7 d Synthetic Air Jet Electronics Cooling 27 th HEXAG Meeting, Heriot-Watt University, Edinburgh

8. Synthetic Air Jet Electronics Cooling 27 th HEXAG Meeting, Heriot-Watt University, Edinburgh Experimental Set-up No. 2 Flush Mounted Heat Flux Sensors on a UWT Impingement Surface RdF MicroFoil Heat Flux Sensor Senflex Hot Film Sensor

9. Synthetic Air Jet Electronics Cooling 27 th HEXAG Meeting, Heriot-Watt University, Edinburgh 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.

10. Synthetic Air Jet Electronics Cooling 27 th HEXAG Meeting, Heriot-Watt University, Edinburgh 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

11. Synthetic Air Jet Electronics Cooling 27 th HEXAG Meeting, Heriot-Watt University, Edinburgh Phase Locked Vorticity Plot, H/D = 1, Re = 3700, L 0 /D = 17 Φ = 120° Φ = 180° Φ = 240° Φ = 300° Φ = 0° Φ = 60°

12. Synthetic Air Jet Electronics Cooling 27 th HEXAG Meeting, Heriot-Watt University, Edinburgh Phase Locked Vorticity Plot, H/D = 2, Re = 3700, L 0 /D = 17 Φ = 120° Φ = 180° Φ = 240° Φ = 300° Φ = 0° Φ = 60°

13. Development of an SAJ Electronics Cooler Synthetic Air Jet Electronics Cooling 27 th HEXAG Meeting, Heriot-Watt University, Edinburgh 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

14. Synthetic Air Jet Electronics Cooling 27 th HEXAG Meeting, Heriot-Watt University, Edinburgh

15. 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. Synthetic Air Jet Electronics Cooling 27 th HEXAG Meeting, Heriot-Watt University, Edinburgh Conclusions

16. 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.Jet impingement heat transfer - Part I: Mean and root-mean-square heat transfer and velocity distributions 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.Jet impingement heat transfer - Part II: A temporal investigation of heat transfer and local fluid velocities 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.A pressure-based estimate of synthetic jet velocity Synthetic Air Jet Electronics Cooling 27 th HEXAG Meeting, Heriot-Watt University, Edinburgh References