Ultrasounds Used as Promoters of Heat-Transfer Enhancement of Natural Convection in Dielectric Fluids for the Thermal Control of Electronic Equipment
Abstract
:1. Introduction
2. The Use of Ultrasound for Heat Transfer Enhancement: Fundamental Physical Elements
- Acoustic streaming: consists in the alteration of the trajectories of the fluid flow, with its consequent mixing which alters heat transfer;
- Acoustic alteration of the boundary layer: the oscillation of the velocity component perpendicular to the portion of the wall affected by the exchange determines alternating compression and expansion phases of the thermal boundary layer which determines an overall increase of the heat transfer;
- Acoustic cavitation: the phenomenon causes temperature and pressure oscillations due to the propagation of the acoustic waves, which can be quite large, the fluid locally reaches, during the rarefaction phase, the steam condition. Steam bubbles then form: after a certain period of time, they implode violently, causing shock waves and jets of fluid. If the collapse of the bubbles occurs near the thermal boundary layer, it produces strong internal mixing, with consequent variation of the local heat-transfer coefficient.
2.1. Sound and Acoustic Cavitation
2.2. The Mechanisms of Action of Ultrasonic Waves for Heat-Transfer Enhancement
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- the fluid and its phase (gas, liquid or two-phase);
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- the operating conditions of the fluid (such as temperature and pressure);
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- the ultrasonic frequency of the generator, f;
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- the power of the ultrasound generator, Pgen;
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- the geometry of the system;
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- the characteristics of the surfaces;
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- the material of the surface and the possible formation of chemical substances.
3. Experimental Studies on Heat-Transfer Enhancement: General Elements
- Difficulty in modifying the acoustic parameters: the frequency is almost always fixed and often the power cannot be varied; geometry and fluid dynamics influence the actual power released by the generator. It would be useful to modify the generators in such a way that they always supply the set power;
- With the exception of some particular cases, the acoustic generation leads to high PUS/∆Pth ratios, where PUS is the total power given with the ultrasounds and ΔPth represents the heat-transfer enhancement. This occurs because a large part of the acoustic energy is dissipated in the fluid before reaching the exchange surface and before altering it. The possibility could be assessed case by case to switch to mechanical generation or to bring the exchange surface as close as possible to the generation area.
4. Definition of the Experimental Setup and Measurement System
4.1. Experimental Setup
4.2. Measurement Strategy and Control System
5. Measurements for the Electronic Equipment Validation and Heat-Transfer Analysis
5.1. Preliminary Operation
5.2. Thermal Measurements and Data Analysis
6. Conclusions
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
References
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ΔTsub [K] | Pt [W] | EF | Δh/h |
---|---|---|---|
12 | 1.89 | 3.476 | 2.476 |
16 | 2.43 | 2.223 | 1.123 |
20 | 2.52 | 2.260 | 1.160 |
24 | 3.00 | 1.907 | 0.907 |
28 | 3.97 | 1.848 | 0.848 |
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Bartoli, C.; Franco, A.; Macucci, M. Ultrasounds Used as Promoters of Heat-Transfer Enhancement of Natural Convection in Dielectric Fluids for the Thermal Control of Electronic Equipment. Acoustics 2020, 2, 279-292. https://doi.org/10.3390/acoustics2020017
Bartoli C, Franco A, Macucci M. Ultrasounds Used as Promoters of Heat-Transfer Enhancement of Natural Convection in Dielectric Fluids for the Thermal Control of Electronic Equipment. Acoustics. 2020; 2(2):279-292. https://doi.org/10.3390/acoustics2020017
Chicago/Turabian StyleBartoli, Carlo, Alessandro Franco, and Massimo Macucci. 2020. "Ultrasounds Used as Promoters of Heat-Transfer Enhancement of Natural Convection in Dielectric Fluids for the Thermal Control of Electronic Equipment" Acoustics 2, no. 2: 279-292. https://doi.org/10.3390/acoustics2020017