COOLING USING SYNTHETIC JETS
Introduction
As increasing functionality is packaged into ever-shrinking electronics, cooling requirements rise exponentially. While there have been continued advances in high heat flux technologies, commercial, consumer-oriented systems continue to focus on air cooling for reasons of reliability, acoustics, cost and portability.
In order to support the increasing power dissipation levels, designers use high-speed fans with noise, reliability and weight penalties. It has become clear that heat exchange to the ambient has to be more efficient, i.e., better air-side heat transfer with lower air flow rates. A novel air-cooling technology, called a synthetic jet, which extends the envelope of air cooling by providing high heat transfer at low flow rates with low acoustics and high reliability.
Synthetic Jets
Synthetic jets are formed by periodic suction and ejection of fluid out of an orifice bounding a cavity by the time periodic motion of a diaphragm that is built into one of the walls of the cavity. During the ejection phase (the first three frames from left to right), a coherent vortex, accompanied by a jet, is created and convected downstream from the jet exit. Once the vortex flow has propagated well downstream, ambient fluid from the vicinity of the orifice is entrained (the last two frames). The bulk of the high-speed air has moved away from the orifice, avoiding re-entrainment, while quiescent air from around the orifice is sucked into the orifice. Thus, a synthetic jet is a “zero-mass-flux” jet comprised entirely of the ambient fluid and can be conveniently integrated with the surfaces that require cooling without the need for complex plumbing.
The far field characteristics are similar to conventional turbulent jets. The time periodic motion of the aforementioned diaphragm can be achieved using several techniques, including piezoelectric, electromagnetic, electrostatic and combustion driven pistons. The most commonly used actuator is piezoelectric and electromagnetic. For a given form factor, piezoelectric diaphragms hold an advantage in weight and power consumption, while electromagnetic actuators have better noise and reliability performance.
Synthetic Jet Ejectors
The principle of jet ejectors or jet pumps has been known for several decades now. A jet ejector consists of a primary high momentum jet driving a secondary airflow through a channel as shown in the 1st figure. The low pressure created by a primary jet discharging into the channel results in entrainment of quiescent ambient flow, thus creating an increase in overall flow rate at the channel exit. This is also shown in the 2nd figure where the computed induced flow is plotted as a function of channel width in a channel flow driven by a high momentum jet. The overall flow rate can be an order of magnitude higher than the jet flow itself, depending on the operating conditions.
In conventional jet ejectors, the primary jet is created using a pressure source ducted into the entry of a channel. The use of synthetic jets as the primary jet is an attractive option since the only input to the primary jet is electrical, requiring no plumbing and pressure supplies. During the blowing stroke of the synthetic jet, the jet ejector phenomenon is similar to steady jet ejectors, wherein a primary high momentum jet creates a low pressure in a channel resulting in the entrainment of fluid from the quiescent medium.
During the suction stroke, the low pressure in the jet cavity results in additional flow entrainment, which is forced out during the
subsequent blowing stroke.
Synthetic Jet Thermal Performance Data
Research performed over the last several years has shown significant improvements in air-side heat transfer compared to steady flows or fan-type flows. In a channel cooling experiment, Mahalingam et al. showed that synthetic jet ejectors have much higher heat transfer coefficients than steady flows with Reynolds numbers of the same magnitude based on the mean channel flow. The graph shows that synthetic jet driven channel flows exhibit higher Nusselt numbers than predicted for steady turbulent channel flow by the Gnielinski correlation.
Heat Sink Integrated with Synthetic Jets
A synthetic jet-based PCI-E half-height graphics card cooler was built and tested against a fan-sink solution of the same form factor (90 x 50 x 12 mm). The figure shows the thermal and acoustic performance of the synthetic jet solution when compared with the fan solution. The thermal resistance was based on the temperature measured by a thermocouple embedded in the base of the heat sink and inlet ambient to the cooling module. The acoustic data were measured in a hemi-anechoic chamber at 0.5 m using a binaural head outside the computer chassis. The A-weighted sound pressure level (SPL) is significantly lower for the synthetic jet. For a given SPL-A of 40 dBA, the synthetic jet exhibits 12% better thermal performance than the fan.
The synthetic jets also perform better when comparing Sones for a given performance level, which is a measure of how loud a sound actually feels to the observer. In a separate test performed at the 2°C/W performance level, the power consumption for the synthetic jet solution was 640 mW, while the power consumption for the fan solution was 672 mW.
Spot Cooling
A 2 x 2 array of hot spots was cooled with a remote synthetic jet module measuring 50 x 50 x 25 mm, placed 300 mm away from the array. The flexible tubing forming the synthetic jets was 6 mm in diameter. The schematic of the cooling setup as well as thermal performance is shown below. During the initial time period the synthetic jet is off while the heaters are energized to 2.5 W each.
The synthetic jets are then turned on and the temperature of the heaters, measured using thermocouples, rapidly drops to a steady-state value 2.5 times lower than natural convection. The power consumption for cooling the entire array was 308 mW.
Fan Augmentation
Synthetic jets can be used to improve the thermal performance as well as noise and reliability of the fan-cooled systems by reducing flow bypass as well as increasing the local heat transfer from the heat sinks. A practical application is in the case of servers where the main fans drive airflow over heat sinks that cool the main processors as well as other components in the chassis.
To test the efficacy of synthetic jet augmentation, a synthetic jet assembly was retrofitted into a commercially available Newisys 4300 quad-socket, 3U, AMD Opteron rack-mounted model server. Due to the space constraints and PCB layout within the server, two of the processors have reduced-height ducted heat sinks, which conform to a 1U form factor, and two of the processors have full height 3U heat sinks. The synthetic jet was located directly in front of these shorter heat sinks without modifying the existing server chassis architecture.
The figure below shows the case-to-ambient thermal resistance of the CPUs with and without jet augmentation at different fan speeds. The inlet velocities vary approximately linearly in the range of 2.8 to 3.8 m/s over the range of fan speeds. At an idling speed of 5500 RPM the thermal resistance drops from about 0.43°C/W to about 0.35°C/W, enabling an increase in the processor power from 70 W to 85 W for a constant case temperature of 65°C. At the full speed of 9000 RPM, the performance goes from 0.33 to 0.30°C/W, enabling an increase in processor power from 92 W to 100 W at a temperature of 65°C.
Augmenting the performance with synthetic jets can also enable a significant performance improvement in terms of power drawn for cooling. The power consumed by the synthetic jets for this application was approximately 14W. Using the synthetic jets to reduce the speed of the system fans from 9000 to 5500 RPM resulted in a reduction in power consumption of the system fans from 108 to 48 W, resulting in a net decrease of 42% in the cooling power requirement including the synthetic jets. It would be significant to note here that the actuators used for these tests were not designed for use as synthetic jet actuators, and hence consumed 2–3 times more power than custom actuators that are currently under development.
Conclusion
This method of producing an unsteady, turbulent air jet enables compact and unique form factors for cooling electronics. Also, since synthetic jets are inherently pulsating and turbulent, they produce higher heat transfer coefficients than typical fan flows and can remove heat from a surface with lesser airflow.
I would like to thank my group members Adnan Imtiaz, Ajitesh Jachak and Vedant Aher for their immense contribution in successfully detailing this blog.
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