Browsing by Author "Parsa, Shadi Habibi"

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Flow and heat transfer study of an impinging piezoelectric fan over a vertical surface
(ASME, 2016) Parsa, Shadi Habibi; Ghaffari, Omidreza; Solovitz, S.; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Parsa, Shadi Habibi; Ghaffari, Omidreza
Piezoelectric fans are low-form-factor cooling devices, which have gained recent attention for electronics cooling. These devices feature a vibrating blade, which sheds vortices from its tip during its motion. The performance of a piezoelectric fan is based on its location, orientation, and operating condition. Thus, we investigated the heat transfer and flow field of an impinging flow produced by a piezoelectric fan. The heat transfer tests are conducted using a vertical, 2.54 cm × 2.54 cm copper heater, which is configured with the piezoelectric fan positioned along its centerline. The fan is operated at its fundamental frequency of 60 Hz, where it achieves maximum heat transfer and fan deflection. There is significant heat transfer degradation with increasing heater-to-fan spacing and off-resonance operating conditions. To better understand this thermal performance, we require information about the flow field produced by this pulsating flow. Hence, we performed particle image velocimetry (PIV) measurements of the flow field for free and impinging cases with different heater-to-fan spacing. We used instantaneous and time-averaged PIV to depict the response in a region within approximately two times the fan oscillation amplitude. In this region, there was a stagnation flow close to the heater, which would result in significant heat transfer. However, this flow also featured high-magnitude velocity vectors towards the sides of the heater rather than towards its center, which would likely result in non-uniform heat transfer.
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Meso-scale cooling devices experimental and numerical heat transfer analysis
(2017-01) Parsa, Shadi Habibi; Arık, Mehmet; Arık, Mehmet; Arslan, M. A.; Ertunç, Özgür; Department of Mechanical Engineering; Parsa, Shadi Habibi
Various thermal management strategies and related technologies are recently developed based on the specific needs, which they are designed to fulfill. Some of these technologies are muffin fans, synthetic jets, and piezoelectric fans. These three technologies are chosen based on the need to compare their core system of heat removing. The muffin fans are the small scale of the normal rotary fans which exploit a number of blades to produce a flow, which performs the cooling. Synthetic jets and the piezoelectric fans both use the oscillation created by a piezoelectric plate to establish a flow. The presence of an oscillating diaphragm in the synthetic jets simulates the breathing process which is done in the lungs of a human being. In the case of piezoelectric fans, the blade is being replaced with an oscillating cantilever beam which periodic movement generates a flow that eventually removes the heat. This study consists of various parts; in the first part, previously mentioned technologies are experimentally compared, in terms of their heat transfer characteristics. In order to test the technologies, an experimental system was designed and manufactured. The test system consists of two main compartments. The heat source and the cooling device; cooling devices are secured on a plexi fixture. This stand is adjustable so that the cooling device can operate in various distances. Fixtures are designed and manufactured to securely hold the devices in position, in order to avoid any extra oscillation during operation. The heat source of the test set up is a square copper heater, placed in front of the cooling technology, which is under the tests. Then, the effect of distance on the heat removal of these cooling devices are examined and reported by putting them at various distances. For the case of the muffin fan, it was observed that the heat transfer coefficient showed inverse relation with distance; higher distance exhibited low heat transfer and thus the fan was only applicable at close distances to the heater. For the synthetic jet, an optimum distance existed after which the heat transfer was not efficient. In the case of the piezoelectric fan, not only increase in the distance resulted on the low heat transfer coefficient but also the heat transfer coefficient depended on the deflection of the piezoelectric fan slab. The effect of slab deflection is studied by the use of two different materials, mylar and metallic substrate fans. The highest Nusselt number for the synthetic jet and the piezoelectric fan is around 60 while for the muffin fan this number is less than 20. The deflection of the piezoelectric slab is so crucial that in the case of the mylar slab the highest achieved heat transfer coefficient is 20 W/m2-K, while for the metallic fan it is approximately three times higher than mylar and it is about 58 W/m2-K. Beside the extensive experimental investigations, the piezoelectric fan is numerically modeled via FLUENT-ANSYS software in order to find the inaccessible results during experiments and better understanding of phenomena. The numerical results are in accordance with the experimental outcomes with a deviation of about 20 percent. The maximum velocity is found to be approximately 1.8 m/s. The flow is also being visualized by the use of the PIV imaging during the heat transfer characteristics examination of the piezoelectric fan. PIV imaging results shed light on side cooling that partially happens at the close distances. The phenomenon of the side cooling is captured after the movement analysis of more than 5000 frames of the operating maylar piezoelectric fan. It is seen that in the distance less than 5 mm there is no time for the formation of the vortex and thus the induced flow is deprived towards the sides. This means that while the heat source is coordinated in the centerline of the fan; the induced flow is cooling the heat source sides and not the heater itself. This is not desirable since the cooling efficiency is lowered at this state. Finally, a high-speed camera has also been used to capture the on spot behavior of the piezoelectric fan over the flow. With the frequency of 3000 frames per second of camera, the flow is captured by the use of a smoke pen. The vortex formation is seen within the various frequencies of the piezoelectric fan. The effect of deflection value is also captured by the sequences of the camera. The formed vortex radius is different in the case of different deflection values.