Increases in microprocessor power dissipation coupled with reductions in feature sizes due to manufacturing process improvements have resulted in continuously increasing heat fluxes [1]. The ever increasing heat flux has necessitated the development of new thermal management technologies. Although some of the solutions for high flux cooling problems based on liquid methods such as spray and evaporative cooling; air cooling is still commonly used due to its availability and low costs.
Over the last several years synthetic jets also known as Zero-Net Mass Flux(ZNMF) actuators have been researched as an alternative to fans as air moving devices and have been shown to be highly effective for cooling of electronics in a very small form factor. Synthetic jets, seen in Figure 1, are formed by periodic, suction and ejection one and after of fluid through an orifice bounding a small cavity, by the periodic motion with respect to time of a diaphragm that is built into one of the walls of the cavity. A unique feature of these jets is that they are formed entirely from the working fluid of the flow system in which they are deployed. Unlike conventional jets, synthetic jets are ‘zero-mass-flux’ in nature and produce fluid flow with no mass addition to the system and without the need for complex plumbing [2], and their unique attributes make them attractive fluidic actuators for a broad range of flow control applications.
Figure 1: General flow behaviors in a standard synthetic jet
Over the last few years, the stream wise and span wise evolutions of finite span synthetic (zero net mass flux) jets were investigated experimentally over a broad range of length and time-scales at various Reynolds numbers, stroke lengths, slit widths, and formation frequencies. The velocity and vorticity fields were measured in two planes, across the slit (i.e., along the short axis of the orifice) and along the slit (i.e., along the long axis). The effect of the slit aspect ratio on the development of the synthetic jet, and the spatial evolution of secondary three-dimensional vortical structures in the flow field were investigated.
In this study, a review for the evolution of synthetic jets, its usage of purpose in combination of an air cooling method deployed by different actuator systems and the flow control in electronics where the thermal management applications utilized, will be chronologically presented in series of related investigations.
1.1 Literature Review for Piezoelectric Actuators:
Piezoelectricity is a property of certain classes of crystalline materials such as Rochelle salt and Tourmaline which are natural crystals of Quartz. Piezoelectric materials can also be manufactured into ceramics such as Barium Titanate and Lead Zirconate Titanates (PZT) or plastics such as PolyVinyliDene Fluoride (PVDF). A voltage is produces proportion to the pressure from the crystalline structure of the piezoelectric material when mechanical pressure applied. Conversely, when an electric field is applied, the structure changes shape producing dimensional changes in the material. Piezoelectric materials can thus be used as either an actuator or a sensor. Their applications range from simple buzzers and furnace igniters to cell phones, vibration dampening and medical imaging devices. New designs, materials, and refined fabrication process for manufacturing have improved the performance of these devices.
The constant improvement of piezoelectric technology has been advantageous for the development of synthetic jet actuators [3]'[7]. When compared to other conventional possible control devices such as air pumps and voice coils, the use of piezoelectric devices have the advantages of having faster response, good reliability, low cost, and a reduction in weight and space [3]'[7]. Chen et al. [8], [9] have attained a maximum air jet velocity of approximately 40 m/sec using a 23mm diameter Murata piezoelectric type 7BB-50M-1 bonded to a 50mm diameter brass shim driven with a sine wave at a frequency of 1160 Hz. Their study shows that for limiting cases the jet velocity may be scaled by the peak-to-peak displacement of the actuator.
Studies such as the ones mentioned above and many others that utilize a sinusoidal wave drive input require relatively high frequencies to match the actuators resonance frequency or the cavity’s Helmholtz frequency. Helmholtz frequency is the natural frequency that fluid tends to oscillate into and out of a container dependent on the area opening, cavity volume and length of the opening port. The high frequencies required to form a synthetic jet however consume more power and also physically limit the oscillation amplitude of the diaphragm that in turn limits the amount of air volume displaced. Furthermore the operating frequency is limited to a narrow resonance peak to give enough actuator displacement. The performance of three piezoelectric actuators is currently being explored by Mossi et al [4]'[7] they are the Bimorph, Thunder@ and RFD. These actuators are similar in that they are circular with a diameter of 6.35 cm and use the same active element, Lead Zirconate Titanate (PZT) type 5A. The geometry and overall free displacement characteristics of these piezoelectric actuators make them easy to implement into a relatively simple design. The focus of this study is based on the two actuators, Bimorph and Thunder. The Bimorph model T2 16-A4NO-573X manufactured by Piezo Systems Inc., has the largest capacitance of 130nF and is 4.1 mm thick consisting of two bonded piezoelectric layers with nickel electrodes. Thunder@ is a pre-stressed curved Unimorph composed of three layers that include a 0.254mm thick layer of stainless steel, a 0.254mm thick layer of PZT type 5A and a .0254 layer of perforated copper, laminated with a polyimide adhesive between each layer [21-241. The resulting actuator, Thunder is saddle shaped with a capacitance of 80nF. Many studies are available on synthetic jets, Bimorphs, and Thunder actuators.
...(download the rest of the essay above)