Browsing by Author "Ghaffari, Omidreza"

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Effect of actuator deflection on heat transfer for low and high frequency synthetic jets
(IEEE, 2014) Ikhlaq, Muhammad; Ghaffari, Omidreza; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Ikhlaq, Muhammad; Ghaffari, Omidreza
Synthetic jets are being investigated over the last four decades. Researchers have been interested in its unique applications for a wide range of flow control to thermal management of electronics applications. Synthetic jets are made up of actuators such as piezoelectric, magnetic, or linear piston technology etc. In this study, we performed an experimental and numerical investigation of a piezoelectric disk deflection over a range of frequencies in order to understand the performance for low and high frequency synthetic jets. First, we performed a numerical analysis of a piezoelectric based synthetic jet and, validated computational result with experimental findings. Numerical models are performed by using commercial finite element software. To understand the size effect on the operating frequency, three jets with different sizes are manufactured and examined. Two different low frequency synthetic jets manufactured in our laboratory and a commercially available high frequency jet are included in the present study. Heat transfer performance is given as an enhancement over natural convection heat transfer. The heat transfer enhancement factor of each of these jets with respect to natural convection is measured over a 25.4×25.4 (mm) vertical heater. Finally, power consumption of the low and high frequency synthetic jets were measured and compared. It is found that disk deflection and operating frequency are directly related to heat transfer enhancement factor, if the Helmholtz frequency of a cavity has no effect on the performance of a jet. The Helmholtz frequency of each jet was calculated to ensure that it has no effect on the synthetic jet, but we found that the commercial synthetic jet took partial advantage of Helmholtz phenomena to enhance the performances at high frequencies.
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An experimental study of impinging synthetic jets for heat transfer augmentation
(World Scientific Publishing Co, 2015-07) Ghaffari, Omidreza; Ikhlaq, Muhammad; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Ghaffari, Omidreza; Ikhlaq, Muhammad
According to recent trends in the field of miniature electronics, the need for compact cooling solutions compatible with very thin profiles and small footprint areas is inevitable. Impinging synthetic jets are recognized as a promising technique for cooling miniature surfaces like laptops, tablets, smart phones and slim TV systems. Effect of jet to cooled surface spacing is crucial in cooling performance as well as predicting Nusselt number for such spacing. An experimental study has been performed to investigate the cooling performance of two different synthetic jets actuated with piezoelectric actuators cooling over a vertical surface. Results showed that a major degradation of heat transfer when jets are close to the surface is occurred. Slot synthetic jets showed a better performance in terms of coefficient of performance (COP) than semi-confined circular jets for small jet to surface spacing. Later, a correlation is proposed for predicting Nu number for a semi-confined circular synthetic jet accounting the effects of Re number (500≤Rej≤1150500≤Rej≤1150), jet-to-surface spacing (H∕D=2H∕D=2 and H∕D=4H∕D=4) and the stroke length (1.75≤L0∕D≤4.751.75≤L0∕D≤4.75 and L0∕H<2.5L0∕H<2.5). It is found that correlation can provide predictions with an R2R2 value of over 98%.
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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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An investigation into flow and heat transfer for a slot impinging synthetic jet
(Elsevier, 2016-09) Ghaffari, Omidreza; Solovitz, S. A.; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Ghaffari, Omidreza
According to the latest trends in miniature consumer and military electronics, there is a need for compact cooling solutions to meet performance requirements at compact volumes. Successful technology must feature a thin profile and a small footprint area, while still removing a significant amount of heat dissipation. Impinging synthetic jets driven by a piezoelectric membrane are a promising method for cooling small-scale electronics. In this paper, we explore the thermal response of a miniature synthetic jet impinging upon a vertical heater. In addition, we study the local flow field using the particle image velocimetry (PIV) technique to couple heat transfer with fluid dynamics. Heat transfer results show that the maximum cooling performance occurs with a jet-to-surface spacing of 5 ⩽ H/Dh ⩽ 10, which is associated with the flow consisting of coherent vortex structures. There is a degradation of heat transfer for closer jet-to-surface spacings, such as H/Dh = 2. This was due to the incomplete growth of the vortices, along with re-entrainment of warm air from the impinging plate back into the jet flow. There was also some warm air sucked back into the jet during the suction phase of the synthetic jet. For a fixed value of Reynolds number, cooling was improved at high Stokes numbers, but with a reduced coefficient of performance.
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An investigation into flow and heat transfer of an ultrasonic micro-blower device for electronics cooling applications
(Elsevier, 2016-05-08) Ghaffari, Omidreza; Solovitz, S. A.; Ikhlaq, Muhammad; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Ghaffari, Omidreza; Ikhlaq, Muhammad
As compact electronics increase in functionality, new electronics cooling approaches must be more effective, and they must be lower in form factor. In this paper, we investigated the cooling performance of a miniature ultrasonic micro-blower impinging upon a vertical heater. We studied the temperature response at different operating conditions, determining the optimal thermal conditions. We further examined the local flow field using the particle image velocimetry (PIV) technique at the same operating conditions, providing explanations for the heat transfer response in terms of the fluid dynamics. Heat transfer measurements show that the maximum cooling performance occurs at a jet-to-surface spacing ratio of 15 < H/D < 30, and the performance slowly decays when the jet is located further away. The preferred operating frequency of the piezoelectric cooling device occurs at an ultrasonic frequency of over 20 kHz, meaning that this device can function outside the human hearing range. The PIV results demonstrate that the jet profile in the near field deviates significantly from a traditional turbulent free jet. In the far field, it nearly matches the self-similar, fully-developed jet profile. The jet cooling performance is sensitive to the frequency, with the thermal performance dropping by a factor of six when varying by less than 1 kHz from the peak. At the optimal heat transfer condition, the coefficient of performance is measured at approximately three, which is lower than that of some synthetic jets.
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An investigation into fluid flow and heat transfer of high frequency synthetic jets for electronics cooling
(2016-04) Ghaffari, Omidreza; Arık, Mehmet; Arık, Mehmet; Yaralıoğlu, Göksenin; Başol, Altuğ; Solovitz, S. A.; Koşar, A.; Department of Mechanical Engineering; Ghaffari, Omidreza
Modern electronics have been decreasing in size for decades, so their cooling systems must continually improve in efficiency too. In particular, compactness is vital, which is challenging because typical thermal management uses relatively large fans and heat sinks. For more advanced liquid cooling, additional coolant and structure are required, somewhat counteracting the improvement with liquid heat transfer. Ideally, thermal management should be economical, low volume, and localized on the powered devices. Fortunately, some recent advances in synthetic jet devices may provide a potential solution. Synthetic jets use an oscillating structure near an orifice, which produces a periodic jet outflow and sink inflow. When averaging over time, this leads to an axial jet, which can be directed towards a powered device. Unlike traditional impingement cooling, the jet is supplied by ambient fluid, as opposed to an additional coolant. We developed a series of thermal, structural and flow experimental setups in order to test two distinct in house manufactured synthetic jet devices along with one commercial ultrasonic jet. The in house manufactured slot synthetic jets used in this study have a different topology than the previous slot actuators, sandwiching two circular disks together. While a large number of earlier devices placed their orifices normal to the oscillating piezoelectric disk, this new approach placed the orifice along the circumference of the sandwiched pair. Hence, the jet direction was perpendicular to the piezoelectric disk deflection. We later performed a series of CFD simulations to examine the performance of in-house made novel slot synthetic jet and validated it with experimental data. Along with measurements of deflection and thermal performance, we used time-averaged and phase-locked PIV to study the flow physics and their effects on heat transfer. We focused on synthetic jet behavior at several frequencies, both at and away from resonant conditions, which may help in selecting device conditions with lower acoustic noise. For a slot synthetic jet the degradation of heat transfer for small jet-to-surface spacings, like H/Dh = 2, was due to the reduced growth of the vortices. In addition, there was re-entrainment of warm air next to the impinging plate by the vortex back into the jet flow, and some warm fluid was sucked back into the orifice. In a slot jet case the heat transfer is maximum for a jet-to-surface spacing for 5 ≤ H/Dh ≤ 10, which is associated with flow dominated by coherent vortices that grow to full strength before detachment or impact with the wall. At the diaphragm resonant condition and below, the flow structure was similar at all phases, with a single vortex present between the orifice and the wall. This response suggests that there is a critical jet-to-surface spacing for this behavior, Hcrit = Uo/2f, which should relate to the optimal thermal condition. By tuning the actuator frequency to the wall spacing, the vortices can reach the wall in phase at the end of the outstroke. When well-tuned, the thermal response is governed primarily by Reynolds number. There is a superior cooling performance at high Stokes number with the same ReU0 number. The maximum cooling performance of a circular synthetic jet at the close heater to jet distance was not observed at the structural frequency of the jet where the maximum velocity occurs. It occurs at frequencies greater than the structural frequency. Based on efficiency (COP) comparison, a slot synthetic jet has a better cooling performance compared to a circular jet and ultrasonic micro-blower due to the difference in the flow physics of the two jets so the slot synthetic jet is a better candidate for electronics cooling applications. For the micro blower jet the preferred operating frequency of the piezoelectric actuator occurs at an ultrasonic frequency of 25 kHz, meaning that this device can function with low noise. The micro-blower axial velocity profile shows similar behavior to high Reynolds number turbulent free jets in the far field, including a self-similar profile. But in the near field, there has been a significant deviation with turbulent free jets. The average Nusselt number increases sharply up to H/D = 10, and then it shows a gradual increase till H/D=15. There is a fairly flat maximum region for 15 ≤ H/D ≤ 30, followed by a gradual decay for H/D > 30. The heat transfer increases more than three times by moving the jet from H/D = 2 to H/D = 10. This reveals that the jet performance is highly sensitive to the jet-to-surface spacing. The jet cooling performance is sensitive to the frequency, though there is a 1 kHz wide band of similar thermal response about the peak. The coefficient of performance at the best operating heat transfer condition is about 2, which is less than the value of 14 seen for a slot synthetic jet. Thus, while this micro-blower can greatly reduce noise, it has a significant performance penalty.
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An investigation into momentum and temperature fields of a meso-scale slot synthetic jet for a small jet-to-surface spacing
(Begell House Inc., 2014) Ghaffari, Omidreza; Doğruöz, M. B.; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Ghaffari, Omidreza
Impinging synthetic jets have been identified as a promising technology for cooling miniature structures. Recognizing their thermal performance on the target surface requires a fundamental understanding of the momentum field produced by the pulsating coolant flow which is dependent on the distance between the nozzle exit and the wall. It was earlier reported that the cooling performance of a synthetic jet is highly sensitive to this distance, i.e. as the nozzle-to-plate distance is reduced the jet performance degrades, however the fundamental mechanism for this behavior has not been well-understood. Therefore, a computational study is performed to investigate the flow and thermal fields of a meso-scale slot synthetic jet for a small jet-to-surface spacing of H/Dh = 2. Spatial discretization is implemented via a second order upwind scheme and a second-order implicit scheme is used for temporal discretization to ensure stability. The results show that the pulsating flow at the nozzle exit generates vortices and these vortices seem to have effect on the target surface profiles before they get dissipated. Mean surface profiles are also determined and their applicability at various frequencies is discussed.
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An investigation into momentum and temperature fields of a meso-scale synthetic jet
(IEEE, 2014) Ghaffari, Omidreza; Doğruöz, M. B.; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Ghaffari, Omidreza
Thermal management has become a critical part of advanced micro and nano electronics systems due to high heat transfer rates. More constraints such as compactness, small footprint area, lightweight, high reliability, easy-access and low cost are exposed to thermal engineers. Advanced electronic systems such as laptops, tablets, smart phones and slim TV systems carry those challenging thermal needs. For these devices, smaller thermal real estates with higher heat fluxes than ever have created issues that current thermal technologies cannot meet those needs easily. Therefore, innovative cooling techniques are necessary to fulfill these aggressive thermal demands. Synthetic jets have been studied as a promising technology to satisfy the thermal needs of such tight electronics devices. The effect of nozzle-to-surface distance for a synthetic jet on its cooling performance has neither been studied extensively nor been well-understood. In a few available experimental studies, it was reported that synthetic jet performance is very sensitive to this distance and when the jet gets closer to the hot surface its performance degrades. Therefore, a computational study has been performed to understand the flow physics of a small-scale synthetic jet for a jet-to-surface spacing of H/Dh=5. Spatial discretization is implemented via a second order upwind scheme and a second order implicit scheme is used for temporal discretization to ensure stability. It is found that pulsating flow at the nozzle exit generates vortices and these vortices seem to have minimal effect on the target surface profiles. Local surface pressure, velocity, turbulence profiles and heat transfer coefficient distributions are determined, then the effects of jet frequency as well as near-wall vortices are discussed.
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A numerical study of a single unsteady laminar slot jet in a confined structure
(ASME, 2013) Ghaffari, Omidreza; Doğruöz, M. B.; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Ghaffari, Omidreza
With the inherit advantages of air cooling, jet impingement can produce a factor of two or higher heat transfer than conventional fan flow over bodies. Therefore, impinging jets can solve a number of electronics thermal issues. Those jets produce complex flow and thermal structures leading to non-uniform and non-monotonic profiles on target surfaces. A numerical study is performed to investigate the flow and heat transfer characteristics of an unsteady laminar impinging jet emanated from a single high-aspect ratio rectangular (slot) nozzle in a confined arrangement. The spacing between the target plate and the nozzle is such that the jet would still be in its potential core length as it was in a free axial jet. Following the initial transients, flow and heat transfer parameters still vary considerably in time that the instantaneous and time-averaged values of surface profiles are not identical. Instantaneous surface pressure distributions exhibit that the stagnation point translates periodically around the initial jet-symmetry line and the surface profiles demonstrate off-center (non-stagnation point) peaks.
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Predicting heat transfer for low- and high-frequency central-orifice synthetic jets
(IEEE, 2016-04) Ikhlaq, Muhammad; Ghaffari, Omidreza; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet; Ikhlaq, Muhammad; Ghaffari, Omidreza
As electronic devices are becoming more compact each day, the more effective and efficient active cooling technologies are needed. Microfluidic devices, such as synthetic jets, serve as a potential candidate to fulfill the thermal management needs of the next generation electronics. An experimental and computational study has been performed for circular central-orifice synthetic jets. First, a series of experiments was performed to quantify the actuator deflection, air velocity, heat transfer augmentation, and power consumption for central-orifice synthetic jets. Later, a computational study was performed utilizing the same boundary conditions in order to predict the deflection of the diaphragm. The experiments were conducted on three different types of synthetic jets, namely, low-, medium-, and high-frequency synthetic jets. Although a number of correlations were proposed for the prediction of Nu number for slot synthetic jets, no correlation was found to predict the average Nu number for a synthetic jet with a round orifice. Therefore, two correlations were developed, one for low- and medium-frequency synthetic jets and the other for high-frequency synthetic jets to predict the heat transfer coefficient as a function of the geometry, position, and operating condition for impinging flows. The proposed correlations are able to predict the impingement heat transfer of a synthetic jet with an accuracy of ±25% for a wide range of operating conditions and geometrical variables. Normalized frequency had the minimum impact on the average Nu number of a high-frequency synthetic jet compared with dimensionless distance, both have moderate impact on low- and medium-frequency jets.