Elektroniklerin soğutulmasında yüksek frekanslı jetlerin sıvı akışı ve ısı transferi araştırması

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.