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«A Thesis Presented to The Academic Faculty by Shuo Li In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy The G. W. ...»

-- [ Page 1 ] --

A Numerical Study of Micro Synthetic Jet and Its

Applications in Thermal Management

A Thesis

Presented to

The Academic Faculty

by

Shuo Li

In Partial Fulfillment

of the Requirements for the Degree

Doctor of Philosophy

The G. W. Woodruff School of Mechanical Engineering

Georgia Institute of Technology

December 2005

A Numerical Study of Micro Synthetic Jet and Its

Applications in Thermal Management

Approved by:

Professor Marc K. Smith, Advisor Professor Sue Ann Bidstrup Allen Mechanical Engineering Chemical Engineering Professor Fotis Sotiropoulos Professor Ari Glezer Mechanical Engineering Civil Engineering Professor Yogendra Joshi Date Approved: November 2005 Mechanical Engineering To my parents and my grandparents!

iii

ACKNOWLEDGEMENTS

First, I want to thank my advisor, Dr. Marc K. Smith, for without his dedicated instruction, I would not have been able to complete this work. I appreciate his patience to allow me to learn a lot from my own mistakes. I would like to thank Dr. Glezer and his research group for generous help on the experimental data and enlightening discussion. I also want to express my thanks to all my committee members for their understanding and patience.

iv

TABLE OF CONTENTS

DEDICATION...................................... iii ACKNOWLEDGEMENTS.............................. iv LIST OF TABLES................................... viii LIST OF FIGURES

SUMMARY........................................ xvi CHAPTER I INTRODUCTION......................... 1

1.1 Typical thermal management approaches................... 3

1.2 Synthetic jets.................................. 5

1.3 Summary of investigation............................ 7

1.4 Codes and software............................... 9

1.5 Dissertation outline............................... 11 CHAPTER II LITERATURE REVIEW.................... 12

2.1 Numerical study of synthetic jet flow..................... 14

–  –  –

xi

4.17 Synthetic jet model compared with simulation results (f =1000 Hz, dj =1 mm, hc /dc =1/4)................................. 112

4.18 Meshes in example (top: with cavity, bottom: with cavity model)...... 114

4.19 Axial velocity and radial velocity comparison, hc /dc = 1, dj =1 mm, f =250 Hz (Max suction phase)............................. 115

4.20 Axial velocity and radial velocity comparison (Max suction phase) (continued)116

4.21 Axial velocity and radial velocity comparison (Max suction phase) (continued)117

4.22 Axial velocity and radial velocity comparison (Max suction phase) (continued)118

4.23 Axial velocity and radial velocity comparison (Max suction phase) (continued)119

4.24 Axial velocity and radial velocity comparison, hc /dc = 1, dj =1 mm, f =250 Hz (Max blowing stroke)............................. 120

4.25 Axial velocity and radial velocity comparison (Max blowing stroke) (continued)121

4.26 Axial velocity and radial velocity comparison (Max blowing stroke) (continued)122

4.27 Axial velocity and radial velocity comparison (Max blowing stroke) (continued)123

4.28 Axial velocity and radial velocity comparison (Max blowing stroke) (continued)124

4.29 Comparison of vortex evolution in one cycle................. 125

4.30 Comparison of vortex evolution in one cycle (continued)........... 126

5.1 Synthetic-jet impingement heat transfer application............. 130

5.2 H/dj =2 blowing stroke, f =80 Hz, dj =6.35 mm................ 133

5.3 H/dj =2 blowing stroke, f =80 Hz, dj =6.35 mm (continued)......... 134

5.4 H/dj =2 suction phase, f =80 Hz, dj =6.35 mm................ 135

5.5 H/dj =2 suction phase, f =80 Hz, dj =6.35 mm (continued).......... 136

5.6 Velocity vector plots of PIV (right) and CFD simulation (left) H/dj =4, f =80 Hz, dj =6.35 mm................................. 137

5.7 Velocity vector plots of PIV (right) and CFD simulation (left) H/dj =4, f =80 Hz, dj =6.35 mm (continued).......................... 138

5.8 Velocity vector plots of PIV (bottom) and CFD simulation (top) H/dj =4, f =80 Hz, dj =6.35 mm............................. 139

5.9 Vortex trace comparison of PIV and CFD simulation H/dj =4, f =80 Hz, dj =6.35 mm................................... 140

5.10 Geometry of synthetic jet impingement heat transfer validation....... 141

5.11 Surface Heat Transfer Coefficient Comparison................. 142

5.12 Typical meshes in computational domain study, domain 1(top), domain 2(bottom)........................................ 144 <

–  –  –





A numerical study of axisymmetric synthetic jet flow was conducted. The synthetic jet cavity was modeled as a rigid chamber with a piston-like moving diaphragm at its bottom. The Shear-Stress-Transportation (SST) k-ω turbulence model was employed to simulate turbulence. Based on time-mean analysis, three flow regimes were identified for typical synthetic jet flows. Typical vortex dynamics and flow patterns were analyzed.

The effects of changes of working frequency, cavity geometry, and nozzle geometry were investigated. A control-volume model of synthetic jet cavity was proposed based on the numerical study, which consists of two first-order ODEs. With appropriately selected parameters, the model was able to predict the cavity pressure and average velocity through the nozzle within 10% errors compared with full simulations. The cavity model can be used to generate the boundary conditions for synthetic jet simulations and the agreement to the full simulation results was good. The saving of computational cost is significant. It was found that synthetic jet impingement heat transfer outperforms conventional jet impingement heat transfer with equivalent average jet velocity. Normal jet impingement heat transfer using synthetic jet was investigated numerically too. The effects of changes of design and working parameters on local heat transfer on the impingement plate were investigated. Key flow structures and heat transfer characteristics were identified. At last, a parametric study of an active heat sink employing synthetic jet technology was conducted using Large Eddy Simulation (LES). Optimal design parameters were recommended base on the parametric study.

–  –  –

INTRODUCTION

1 Currently, human society relies on electronic systems more than any other time in human history. The importance of the reliability of these electronic systems can never be overestimated. There are a lot of factors that may affect the reliability of electronic systems, such as temperature, humidity, vibration, dust and so on. Thermal overstressing is by far the most common cause of failure in modern electronic systems. The electrical energy consumed by electrical circuits eventually is converted into thermal energy. The accumulation of thermal energy increases the temperature of the electronic component. Because circuits can only operate reliably within a certain temperature range, the heat generated from electronic parts has to be effectively dissipated from the system to maintain a suitable temperature.

In the modern microelectronics industry, there are trends to design and manufacture low power consumption and small-sized systems. Advanced fabrication technologies and the demand of more functionality also push forward system integration to higher levels, like today’s ultra large scale integration (USL). All of these result in higher power densities on microchips. Heat generated from newly developed high-speed, high-power, high-density microchips has reached 100W/cm2. Efficient removal of heat from today’s highly integrated ICs remains a major bottleneck (Tummala et al. [60]). Some new means of heat removal are needed to break this bottleneck.

Another thermal problem in integrated circuits is that the temperature distribution can be quite non-uniform: the temperature at some spots is significantly higher than others, which could damage the electronic component itself or cause the failure of the whole system. This situation is common in high performance microchips, where the base material (silicon) is not good in terms of heat conduction and the packaging structure/technique may not provide a sufficiently low thermal resistance in all directions to maintain a uniform temperature field.

In short, thermal issues are crucial in the design of microelectronic systems. A desired cooling solution for modern electronic systems should be able to efficiently remove large amounts of heat and simultaneously control the local temperature distribution.

–  –  –

Generally, thermal management of an electronic system consists of two levels of mechanisms to reject heat from the integrated circuit (die) to the environment. The first level is local or device-level, in which the heat generated from an integrated circuit is rejected to the packaged surface and substrate through appropriate packaging techniques. In the second level, heat is removed from the packaged IC surface to the ambient.

Numerous transistors, resistors, capacitors and other devices are integrated in a silicon die (chip). In a typical wire-bond package, the silicon die is bonded directly to a substrate material, usually with epoxy. This substrate die assembly is then molded to protect it from environmental hazards and has electrical leads around it to provide electrical connections to the circuit board. Heat generated from the chip is conducted into the substrate, through which heat is removed to the circuit board, and at last to the environment. Once the semiconductor design is finished, it’s infeasible, if not impossible, to do any management to change the device-level thermal characteristics. Therefore the thermal management techniques discussed in this work mainly involve the second level, in which heat is removed from chip surfaces to the ambient.

Although heat conduction is important within microelectronic components, radiation and convection heat transfer are more important mechanism for heat transfer to the ambient. At temperature less than 100 ◦ C, radiation heat transfer is comparable to natural convection heat transfer, but it is much weaker than forced convection heat transfer. Therefore, in second-level thermal management, convection heat transfer is the key mechanism to improve.

Convection heat transfer can be expressed as:

–  –  –

in which, h is the convection heat transfer coefficient, A is the surface area for convection heat transfer, and (Tw − Tf ) is the temperature difference between the heat transfer surface and the fluid. To increase the heat transfer rate Q, we could increase h, A, or (Tw − Tf ).

–  –  –

tions include high speed fan-driven heat sinks, liquid cooling systems, direct immersion cooling, micro refrigeration systems, heat pipes, thermoelectric coolers, etc. These different solutions gain better heat transfer performance by improving the key parameters of convection heat transfer in equation 1.1.

For high-speed, fan-driven heat sinks, the high-speed air flow increases the convection heat transfer coefficient. Using materials with good thermal conductive properties increases the temperature difference. And, if well designed, the heat transfer surface area of the heat sink can also be increased. However, the fan’s relatively high power consumption and noise are two obvious disadvantages.

The heat transfer coefficient for liquid convection heat transfer is much higher than that for air convection heat transfer. A liquid can also provide a larger heat removal capability.

In addition, the temperature increase when fluid flows over a heat transfer surface is smaller because of the larger specific heat Cp of the liquid. These, in turn, increase the temperature difference for convection. Difficulties related to liquid cooling techniques are the complex liquid supply system (reservoir, pipes/hoses) and possible liquid leakage.

Direct immersion cooling utilizes two-phase heat transfer. The circuit board is placed in a fluid with a low boiling point (e.g. Fluorinert FC-72) to take the advantage of very high heat transfer coefficient of boiling heat transfer. The latent heat of phase change also helps to enhance the heat transfer. However one characteristic of boiling heat transfer creates a problem: the high superheat before boiling region may result in thermal overshoot: the temperature of the fluid reaches a much higher point than expected and could destroy the electronic system.

A micro-refrigeration system is able to maintain lower temperature than ambient, but it is difficult to fit the whole system into a small space. Coolant leakage is another potential problem.

A heat pipe is an effective technique to provide high heat-flux heat transfer between two surfaces. However, other techniques are still needed to reject the heat to the ambient. Heat pipes are very useful in situations where conventional heat removal devices are hard to fit

–  –  –

heat sink from directly being mounted on the microprocessor, like in a desktop computer.

The heat pipe is used to transfer the heat from the microprocessor to a heat sink some distance away.

A thermoelectric cooler uses the Peltier effect of some semiconductor materials to create a high temperature pole and a low temperature pole, which can be used to cool the target electronic component. It is compact, with no moving component, and therefore, no noise.



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