How to Use Synthetic Jets for Local Thermal Management

As todays electronics generate more heat inside ever smaller spaces, engineers are challenged to find new ways to effectively cool the components [1]. There have been continued advances in high heat flux technologies [2], but commercial, consumer-oriented systems continue to focus on air cooling for its reliability, acoustics, cost and portability [3]. To support the increasing power dissipation of components and systems, designers must evaluate different cooling solutions within the dimensional constraints of the system. Available cooling solutions have their own advantages and disadvantages, such as reliability and noise. In this article, we discuss synthetic jets, an air cooling technology that can provide a high local heat transfer coefficient at a low flow rate with low acoustics and high reliability [1].

Synthetic Jets

Synthetic jets are formed by the periodic suction and ejection of fluid out of an orifice bounding a cavity by the periodic motion of a diaphragm built into one of the walls of the cavity [1], as shown in Figure 1. The operation of a synthetic jet can be divided into two phases. First is the ejection phase, which is shown in Figure 1 (a) to (c). Second is the suction phase, which is shown in Figure 1 (d) and (e). A coherent vortex is created and convected downstream from the jet exit in the ejection phases. The vortex is created by the movement of the diaphragm, which forces air to exit through an orifice. The suction phase occurs once the vortex flow has propagated well downstream. The diaphragm moves back, thereby entraining the ambient fluid from the vicinity of the orifice into the diaphragm volume.

When the bulk of the high speed air has moved away from the orifice, avoiding re-entrainment, ambient air from around the orifice is drawn into the orifice [1]. Thus, a synthetic jet is a zero-mass-flux jet comprised entirely of the ambient fluid. It can be conveniently integrated with the surfaces that require cooling without the need for complex plumbing. The periodic motion of the synthetic jet diaphragm can be achieved using several techniques, including piezoelectric, electromagnetic, electrostatic and combustion driven pistons [1]. The most commonly used actuators are 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 [1].

particle image velocimetry data formation of a synthetic jet

Figure 1. Particle Image Velocimetry Data Formation of a Synthetic Jet Showing the Ejection Phase (a) to (c); and Entrainment (d) to (e) [1].

The principle of jet ejectors or jet pumps [4] has been known for several decades. A jet ejector consists of a primary high momentum jet that drives a secondary airflow through a channel as shown in Figure 2 (a). 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 Figure 2 (b) 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 [1].

 

Principle of Operation of a Jet Ejector

Figure 2. Principle of Operation of a Jet Ejector (a) and Calculation of Ratio of Induced Secondary Channel Flow to Jet Flow in a Jet Ejector (b) [1].

In conventional jet ejectors, the primary jet is created using a pressure source ducted into the entry of a channel [1]. 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 or pressure supplies. During the ejection phase 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 ambient [1]. During the suction phase, the low pressure in the jet cavity results in additional flow entrainment, which is forced out during the subsequent blowing stroke.

Application to Electronics Thermal Management

Synthetic jets have two main areas of application in the thermal management of electronics. The first is when a heat sink is designed with integrated synthetic jets. The second is when synthetic jet is used in conjunction with a fan.

When a heat sink is integrated with synthetic jets, it offers a number of advantages over heat sinks designed for use either with fans or without fans. Natural convection sinks are normally much larger than synthetic jet heat sinks that provide the same thermal performance [5]. Table 1 shows published data  comparing LED cooling solutions for natural convection and for synthetic jet heat sinks. Along with providing a smaller size solution, the synthetic jet is an active solution which gives more design freedom to the final product. A natural convection heat sinks thermal performance is directly influenced by the heat sink shape and orientation. As an active solution, the synthetic jet enables an orientation independent design [5]. Some synthetic jet integrated solutions are shown in Figure 3.

Comparison of a Synthetic Jet Cooled Heat Sink and Natural Convection Heat Sinks

Table 1. Comparison of a Synthetic Jet Cooled Heat Sink and Natural Convection Heat Sinks. Data Adapted from [5].

Note that Cheung, et al. used a straight fin passive heat sink. To reduce the thermal resistance, they further increased the height of an already tall heat sink. This is not a very effective method because the air is already warm at the original heat sink height. Through proper design and material property choices, natural convection heat sinks can be excellent alternatives to synthetic jet heat sinks, although they will be limited in form, orientation and size compared to active heat sinks.

Heat Sink Designs with Integrated Synthetic Jets

Figure 3. Heat Sink Designs with Integrated Synthetic Jets. Shown are the SynJet® MR16 (a) [6] and the SynJet® Low Profile Cooler with Heat Sink (b) [7].

Mahalingam et al. have published data for comparing a synthetic jet based PCI-E half-height graphics card cooler and a fan-sink equivalent [1]. They found that the A-weighted sound pressure level (SPL) was significantly lower for the synthetic jet, as shown in Figure 4. Also, for an SPL-A of 40 dBA, the synthetic jet solution had a 12% better thermal performance than the fan-sink. In additional tests conducted at a 2 K/W thermal resistance for both solutions, the power consumption for the synthetic jet solution was 640 mW, while the power consumption for the fan solution was 672 mW.

Plot of the Acoustic Performance of a Synthetic Jet

Figure 4. Plot of the Acoustic Performance of a Synthetic Jet and Fan-Sink Plotted Against the Thermal Resistance [1].

When a synthetic jet is used in conjunction with a fan, it can decrease the thermal resistance and noise, and increase the reliability of fan cooled systems [1]. The synthetic jet achieves this by reducing flow bypass of a heat sink and increasing the local heat transfer coefficient of the heat sinks. Mahalingam et al. [1] have published data on where synthetic jets were applied to a server. They found that augmention resulted in a reduction in the thermal resistance as well as a reduction in the power needed to run the fans. Using the synthetic jets, the fan speed was reduced from 9000 to 5500 RPM and resulted in a reduction in power consumption from 108 to 48 W. This further reduced the cooling required.

Advantages and Disadvantages of Synthetic Jets

The cooling solution design for a product is a combination of various factors. These include mass, volume, thermal resistance, ambient temperature, component temperature, cooling method used, reliability, life time, cost, transport cost and performance of electronics. Most of these factors are dependent on each other; however they all influence the final design of the product.

It has been shown that the synthetic jet cooling method can reduce the heat sink size and therefore the heat sink mass without sacrificing thermal performance. Because a synthetic jet is an active cooling solution, the heat sinks thermal performance is better than similarly sized natural convection heat sinks. This means greater design freedom which is critical for LED consumer products. The industrial (visual) design of a lamp is very important in the buying impulse of a consumer.

A factor that has not been discussed is the reliability of a synthetic jet. Jones [8] has published a white paper for the reliability for the Nuventix SynJet, in which it was compared to data published by leading fan manufacturers. It was found that many vendors fail to specify reliability data for fans at high ambient temperature. For small fans, it was found that the L10 reliability at 60ºC is around 50,000 hours, while for the Nuventix SynJet is at least 300,000 hours at 60ºC.

L10 Reliability to Other Leading Air Movers

Figure 5. Comparison of SynJet L10 Reliability to Other Leading Air Movers [8].

For a comparison of natural convection, fan-based and synthetic jet cooling solutions, Table 2 shows the results based on the noise, reliability, thermal resistance, cost, power consumption and size of the aforementioned cooling solutions. The cooling solutions from Table 2 are compared via an X system, where the higher the count, the better.

Comparison of Natural Convection, Fan and Synthetic Jet Cooling Solutions

Table 2. Comparison of Natural Convection, Fan and Synthetic Jet Cooling Solutions.

Summary

This article has discussed the operational principle of a synthetic jet. The use of synthetic jets offers advantages in thermal performance, orientation, form factor of the final product, size and mass. Power consumption and noise are also small for a synthetic jet. However, the advantages of synthetic jets must be weighed against the unit price of a synthetic jet module.

References

1. Mahalingam, R., Heffington. S., Jones, L. and Williams, R, Synthetic Jets for Forced Air Cooling of Electronics, Electronics Cooling, May 2007

 

2. Lasance, C. and Simons, R., Advances in High-Performance Cooling for Electronics, Electronics Cooling, November 2005.

 

3. Bar-Cohen, A., Computer-Related Thermal Packaging at the Millennial Divide, Electronics Cooling, January 2000.

 

4. Gosline, J. and OBrien, M., The Water Jet Pump, Univ. of California Publ. Engrg., 1934.

 

5. Cheung, C., Noska, B. and van der Heide, K., Comparison of Passive and Active Cooling Effectiveness, LED Professional Review Magazine, Sep-Oct 2009.

 

6. Synjet® MR16 LED Cooler with HS, Nuventix Data Sheet, http://www.nuventix.com/files/uploaded_files/pf_SM16S-CM005-xxx%20MR_16%20%20Rev_2.3.pdf, 2009.

 

7. SynJet® Low Profile Cooler with HS, Nuventix Data Sheet, http://www.nuventix.com/files/uploaded_files/pf_SSCCS-CM005-xxx%20Low%20Profile%20cooler%20Rev2%203%20datasheet.pdf, 2009.

 

8. Jones, L., Nuventix SynJet Ultra-High Reliability Cooling White Paper, http://www.nuventix.com/technology/papers/high-reliability-cooling/, 2009.

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