CTRC Breakthroughs

Two-Phase Heat Transfer from Arrays of Impinging Jets

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High heat flux dissipation rates at low pumping power have been previously demonstrated with two-phase jet impingement cooling. However, design of thermal management systems based on this technology requires a model that accounts for boiling heat transfer within arrays of jets. This project developed and experimentally validated a semi-empirical model for two-phase heat transfer from arrays of impinging jets. The model predicts experimental data to within 5% and has been packaged as a software tool for use by CTRC members that allows variation of the working fluid, operational parameters, and impinging array geometry.

Low Noise Amplifiers

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A novel linear air flow amplifier device has been developed and has been optimized for application in data center server rack cooling to replace high speed axial fans. A modular experimental test facility has been developed for objectively comparing the pressure/flow (PQ) performance, flow velocity and turbulence field, acoustic noise emission and heat transfer effectiveness of axial fans, commercial annular air flow amplifiers and the new linear air flow amplifier developed here. In parallel to the experimental measurements, computational fluid dynamics (CFD) simulations have been conducted with hybrid RANS/LES and aeroacoustic simulation methods. The numerical approach has been validated by means of experimental flow velocity measurements using particle image velocimetry (PIV). Based on these verified analytical models and correlations, an Excel-based prediction tool was developed for use with both axial fans and air amplifiers, enabling the user to vary dimensions and control variables (e.g., fan speed, inlet pressures), and verify the performance in terms of PQ curves, acoustic noise emission level and sound quality in real time.

Air and Impingement Cooling Approaches Coordinated Miniature Piezofan

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Fluidic coupling between neighboring piezofans (see figure) and its impact on the heat transfer performance is being explored in this project. This coupling phenomenon causes an increase in vibration amplitude up to 40 percent compared to an isolated single fan, which causes a further increase in heat transfer performance. There appears to be an optimum condition for separation distance between these two fans (pitch) where the performance is greatest. Fan coupling can indeed be exploited to improve the thermal performance obtained when multiple fans operate in close proximity.

Anti-Noise Synthetic Jet Enhanced Heat Sink

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Synthetic jets use an oscillating diaphragm to pull and push air out of a cavity with a small orifice opening; an outward air jet is formed at this orifice location, and can be used to cool surfaces. By placing a pair of orifices next to each other, the pressure waves that generate acoustic noise can be strategically attenuated. Our study shows the potential for joint optimization of heat transfer performance and noise emission reduction using adjacent synthetic jets. The noise attenuation outcomes of this research will help overcome the noise issue in applying synthetic jets in heat sinks. The attached image shows the cooling profile provided to a surface for a dual synthetic jet that impinges air onto a hot surface. At high air velocities the cooling becomes more intense. By testing at a wide range of operating parameters to predict performance and behavior, the cooling efficiency can be optimized.

Control of Cooling Fan Noise by Radiation Efficiency Control

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The noise level caused by axial cooling fans can be controlled by careful consideration of installation conditions as well as improving the aerodynamic design of fans. Such an installation effect is due to the dipole-like acoustic features of axial fans and this was experimentally proven by confirming the out-of-phase sound wave propagation between the front and back fields of an axial fan. Utilizing this result, an axial fan was able to be numerically modeled as a simple dipole source so that the installation effect of an axial fan could be simulated with very low computing cost. The red points in the figure indicate measurement points for directivity pattern of an axial fan mounted on ISO 10302 plenum.