368 research outputs found

    Annular synthetic jet used for impinging flow mass-transfer

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    An annular synthetic jet was investigated experimentally, both with and without an opposing impingement wall. The experiments involved smoke visualization and mass transfer measurement on the wall by means of naphthalene sublimation technique. Two qualitatively different flow field patterns were identified, depending upon the driving amplitude level. With small amplitudes, vortical puffs maintain their identity for a relatively long time. If the amplitudes are large, breakdown and coalescence of the vortical train is much faster. Also the resultant mass transfer to the impingement wall is then much higher. Furthermore, a fundamental change of the whole flow field was observed at the high end of the investigated frequency range, associated with radical reduction of the size of the recirculation bubble

    Interactive flow behaviour and heat transfer enhancement in a microchannel with cross flow synthetic jet

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    This paper examines the effectiveness in combining a pulsating fluid jet for thermal enhancement in microchannel heat sinks. The proposed arrangement utilises an oscillating diaphragm to induce a high-frequency periodic fluid jet with zero net mass output at the jet orifice hence, termed "synthetic jet". The pulsed jet interacts with the fluid flow through microchannel passages altering their flow characteristics. The present study develops a 2-dimensional finite volume numerical simulation based on unsteady Reynolds-averaged Navier-Stokes equations for examining the microchannel-synthetic jet flow interaction. For a range of parametric conditions, the behaviour of this periodic flow with its special features is identified and the associated convective heat transfer rates are predicted. The results indicate that the pulsating jet leads to outstanding thermal performance in microchannel flow increasing its heat dissipation rate by about 4.3 times compared to a microchannel without jet interaction within the tested parametric range. The degree of thermal enhancement is seen to grow continuously to reach a steady value in the absence of fluid compressibility. The proposed strategy has an intrinsic ability for outstanding thermal characteristics without causing pressure drop increases in microchannel fluid passages, which is identified as a unique feature of the technique.The study also examines and presents the effects of fluid compressibility on the heat removal capacity of this arrangement. The technique is envisaged to have application potential in miniature electronic devices where localised cooling is desired over a base heat dissipation load

    A numerical study of pulse-combustor jet impingement heat transfer

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    A pulsating jet generated by a pulse combustor has been experimentally demonstrated as a technique for impingement heat transfer enhancement relative to a steady jet. The enhancement factor was as high as 2.5. Despite such potential, further studies of this technique have been limited, let alone industrial applications. The ultimate goal of the Pulsed Air Drying project at the Institute of Paper Science and Technology is to develop this technique to commercialization for industrial applications such as paper drying. The main objective of the research in this dissertation is to provide a fundamental basis for the development of the technology. Using CFD simulations, the research studied the characteristics of pulsating single-slot-nozzle jet impingement flows and heat transfer on stationary and moving surfaces. In addition, in order to understand basic flow characteristics of pulse-combustor jets, a simplified model of Helmholtz pulse combustors was developed. The model was used to recommend a strategy to generate a pulsating jet having large amplitude of velocity oscillation. And based on this model, pulsating jets in the simulations were characterized as those at the tailpipe exit of a pulse combustor. The impingement conditions were similar to those in conventional impingement hoods for paper drying. Parameter studies included the effects of jet velocity oscillation amplitude, pulsation frequency, mean jet velocity, tailpipe width, and impingement surface velocity. Simulation results showed that the amplitude of jet velocity oscillation was the most important parameter for heat transfer enhancement, in which two mechanisms were identified: high impinging jet velocity during the positive cycle and strong re-circulating flows in the impingement zone during the negative cycle of jet velocity oscillation. As for the improvement by the pulsating jets relative to steady jets, the maximum heat transfer enhancement and energy saving factors were 1.8 and 3.0, respectively, which were very encouraging for further development of the technology.Ph.D.Committee Co-Chair: Ahrens, Fred; Committee Co-Chair: Patterson, Tim; Committee Member: Aidun, Cyrus; Committee Member: Empie, Jeff; Committee Member: Frederick, Ji

    Cooling Strategies for Heated Cylinders Using Pulsating Airflow with Different Waveforms

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    Pulsate flow is an effective technique applied for cooling several engineering systems depending on their pulsate frequency. One very sound external flow pulsation application is heat transfer over heated bodies. In present work, an experimental design and numerical model of controlled pulsating flow according to generated pulsating frequency and wave shape around a heated cylinder were performed. The effects of pulsating frequency, amplitude, and mean velocity on the fluid flow and heat transfer characteristics over a heated cylinder were studied. The wave frequency varied from 2 to 12 Hz, and the amplitude varied from 0.2 to 0.8 m/s. Moreover, different waveforms were investigated to determine their effect on wall cooling. For constant wave frequency and amplitude, the most efficient wave in cooling was the sawtooth wave, with the average wall temperature after 30 s was 1.6 °C cooler than that of the forced convection case, followed by the triangular wave at 1.2 °C less. The heat transfer rate and the flow field were drastically influenced by the variations of these parameters. Optimization was conducted for each wave type to find the optimum wave frequency and amplitude. The optimizing showed that, the most efficient wave was the sawtooth with 12°C temperature reduction compared with that of the forced convection case, followed by the triangular. Furthermore, regression analysis was conducted to estimate the relationships between these variables and surface temperature. It was found that the wave amplitude had a greater role in cooling than that of the frequency

    Annular impinging jet with recirculation zone expanded by acoustic excitation

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    Flow visualization and mass transfer (naphthalene sublimation) experiments were performed on acoustically excited annular air jet with diameter ratio Di/Do=0.95. Two different regimes of the time-mean flow field were found, differing in the size of the central recirculation zone, with either the single stagnation point or the stagnation circle. The switching between the two regimes is accomplished by acoustic excitation, under identical geometry conditions. An effective stabilization of the large recirculation zone, as well as remarkable augmentation of average heat/mass transfer by 23%, have been achieved at the excitation Strouhal number Sh=0.94

    No-moving-part hybrid-synthetic jet actuator

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    In contrast to usual synthetic jets, the “hybrid-synthetic jets” of non-zero timemean nozzle mass flow rate are increasingly often considered for control of flow separation and/or transition to turbulence as well as heat and mass transfer. The paper describes tests of a scaled-up laboratory model of a new actuator version, generating the hybrid-synthetic jets without any moving components. Self-excited flow oscillation is produced by aerodynamic instability in fixed-wall cavities. The return flow in the exit nozzles is generated by jet-pumping effect. Elimination of the delicate and easily damaged moving parts in the actuator simplifies its manufacture and assembly. Operating frequency is adjusted by the length of feedback loop path. Laboratory investigations concentrated on the propagation processes taking place in the loop

    Etude et développement de micro-oscillateurs fluidiques pour le refroidissement de systèmes électroniques embarqués

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    Dans le domaine aéronautique, les contraintes sur le refroidissement sont multiples. L'efficacité d'un système de refroidissement ne se résume plus au simple taux de chaleur dissipée, mais englobe d'autres facteurs comme la compacité, le poids, la robustesse, le coût de maintenance ainsi que la durabilité. Une conception du système de refroidissement qui intègre ces aspects pourrait diminuer les coûts de fonctionnement, notamment la consommation de kérosène, et donc réduire l'impact environnemental du vol. La multiplication de systèmes embarqués dans l'aéronautique amène des contraintes supplémentaires pour leur refroidissement. Dans ce contexte, les actionneurs fluidiques présentent un fort potentiel. Ces travaux portent plus précisément, sur l'utilisation de jets pulsés produits par des oscillateurs fluidiques pour refroidir une surface chauffée. Plusieurs travaux sur les jets d'impact ont montré qu'il était possible d'améliorer la dissipation thermique en introduisant des pulsations dans l'écoulement. Il manque cependant un consensus dans la littérature autour de l'ensemble des conditions opératoires propices à l'amélioration des performances. D'où la nécessité de mener une étude sur l'écoulement produit par ces dispositifs fluidiques et le refroidissement qui en résulte. En amont de cela, il est nécessaire de se pencher sur l'effet de certains paramètres liés à la géométrie du l'oscillateur sur son mode de fonctionnement, en commençant par la caractérisation de l'écoulement pulsé produit par l'oscillateur. AK cette fin, un prototype d'oscillateur est réalisé en fabrication additive puis caractérisé via une reconstruction spatiale 2D et 3D du champ de vitesse à l'aide d'un seul fil-chaud et d'une sonde de pression placée au niveau des canaux de retours. Cette méthode de mesure nous permet de mettre en évidence des structures cohérentes et suivre leur évolution. En marge de cette étude, un réseau de neurones artificiels profond, ayant des fonctions d'activations sinusoïdales atypiques, est utilisé pour créer une représentation implicite du champ de vitesse. L'oscillateur ainsi caractérisé a alors été utilisé pour refroidir une plaque en verre chauffé. Des tests sont pratiqués sur des jets stationnaires et des jets pulsés de même débit massique moyen. Une amélioration considérable des performances est observée pour des faibles distances d'impact et des hautes fréquences de pulsation. Des simulations numériques sont ensuite réalisées en utilisant des méthodes statistiques en un point (dites RANS) et des modèles hybrides LES/RANS. En vue de concevoir un système de refroidissement compact et capable de cibler des composants de tailles submillimétriques, des versions micrométriques de ces mêmes oscillateurs ont été conçues et fabriquées ainsi qu'une instrumentation électronique à même de les caractériser. Rares sont les études menées sur les microjets d'impact alors qu'aucune étude n'a pu être recensée à ce jour sur les microjets d'impact pulsés ni sur les micro-oscillateurs fluidiques gazeux. Le défi est donc double : de montrer que les micro-oscillateurs à gaz peuvent fonctionner à cette échelle et de les utiliser pour refroidir des composants dissipateurs de chaleur. À cela vient s'ajouter un problème non moins ambitieux, celui d'instrumenter l'oscillateur ainsi que la surface d'impact chauffée. Étant donné que la fréquence d'oscillation à cette échelle-là se mesure en kilohertz et que les fluctuations de température sont relativement faibles, des capteurs thermiques à base de couches de polysilicium fortement dopé ont donc été produits. Bien que leur haute sensibilité thermique ait été déjà démontrée, il est question ici d'améliorer leur temps de réponse. Pour ce faire, les capteurs ont été partiellement désolidarisés du substrat en silicium. Cette amélioration de la dynamique du capteur a été obtenue au prix d'une structure fragilisée qu'il a fallu prendre en compte dans les étapes technologiques suivantes.Thermal management in the aerospace industry is subject to a number of constraints. The suitability of a cooling system does not only depend on the heat flux that it can evacuate, but also includes such aspects as compactness, weight, sturdiness, cost of maintenance and durability. Taking these factors into consideration contributes to reducing fuel consumption, thus reducing the carbon footprint of the airplane. With this in mind, fluidic actuators were developed for electronics cooling applications on-board airplanes. In other words, the aim is to cool heated surfaces using the periodic unsteady flow produced by no-moving-parts fluidic oscillators. Previous studies had shown the possibility of enhancing jet impingement heat transfer by introducing a periodic perturbation in the flow. Nevertheless, the exact experimental conditions that lead to this improvement remain somewhat inconsistent across different studies. For this reason, this study tackles both the flow features of the pulsed impinging jet as well as their effects on heat transfer. In preparation, the oscillator is characterized by assessing its response to changes in design parameters and experimental conditions. This was followed by a two- and three-dimensional reconstructions of the velocity field outside the device using a hot-wire and a pressure transducer mounted onto one of the feedback loops. Using this technique, it was possible to deduce certain flow characteristics as well as detect and track the evolution of large coherent vortices produced by the pulsed jet. The data from these exhaustive measurements was then used to train a deep neural network that uses sinusoidal activation functions. The result is an implicit representation of the flow that could be useful to designers when the oscillator is only part of a larger system. The oscillators were then used to cool a heated plate whose temperature was measured using an infrared camera. Both steady and pulsed jets were studied for a large range of frequencies, impact distances and flow rates. Remarkable enhancement was observed for small impact distances and high frequencies. Simulations where then performed using both RANS and hybrid LES/RANS approaches. In the second part of this work, a miniaturized version of the oscillator was produced that can efficiently target small electronic components. Impinging microjets have rarely been studied, while little to no works could be found on pulsed microjets of air or no-moving-parts microfluidic oscillators. The goal of the present study is then twofold, to prove that functional microfluidic oscillators with air as working fluid can be produced and that they can efficiently cool a heated surface. From an experimental standpoint, this requires proper instrumentation capable of acquiring measurements at the spatial and temporal scales of the system. For this end, high-sensitivity thermal sensors were implemented inside the microfluidic device as well as on the heated target surface. The current iteration of these sensing elements involves partially suspending them over the substrate on which they were built in order to reduce their thermal inertia. The carefully suspended structures were shown to withstand the subsequent fabrication steps despite undergoing high temperatures and pressures

    이중 오리피스에서 발생하는 두 합성 제트의 유동 상호작용에 관한 실험적 연구

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    학위논문 (석사) -- 서울대학교 대학원 : 공과대학 기계공학부, 2020. 8. 황원태.Multiple orifice synthetic jet devices are becoming widely utilized for active flow control and jet impingement cooling, due to its mixing performance resulting from the vortices and jet interaction. Therefore, understanding the flow interaction between multiple synthetic jets is crucial in maximizing the potential for many industrial applications. In the present study, an experimental investigation on flow interaction between two synthetic jets generated from a dual-orifice device is performed. The influence of the orifice spacing (s/D = 1.2, 2.0, and 3.0) and the dimensionless stroke length (L0/D = 13.7, 19.0, and 28.3) is analyzed at a fixed Reynolds number of Re0 = 3700. Phase-locked particle image velocimetry (PIV) is used to obtain time- and phase-averaged flow fields. The jet interaction is enhanced as the orifice spacing and the dimensionless stroke length decrease, resulting in shorter distances for the merging and combining points. In addition, the inner vortices between the two jets are deformed and cancelled due to the jet interaction, which leads to the inner and outer vortices merging into a single vortex. The vortex interactions and merging are delayed as the orifice spacing increases, while the advection speed of vortices is increased without change of flow structure as the dimensionless stroke length increases.합성 제트 장치는 내부의 작동 유체를 오리피스를 통해 내보내면서 와류 고리를 포함한 펄스 제트를 생성한다. 이는 총 질량 변화 없이 모멘텀을 발생시킬 수 있어 유체 공급을 위한 파이프 등이 불필요하며, 연속 제트에 비해 높은 확산율을 가지기 때문에 능동 유동 제어와 제트 충돌 냉각 등 다양한 응용분야에 쓰이고 있다. 최근에는 다중 합성 제트를 사용해 모멘텀 생성과 혼합을 증대시킴으로써 응용분야에서의 효율이 향상됨이 보고되고 있다. 따라서 본 연구에서는 두 합성제트 간 유동 상호작용 연구를 통해 합성 제트의 응용 가능성을 극대화하고자 한다. 실험 유동은 듀얼 오리피스 - 단일 공동 합성 제트에서 생성하였으며, 고정된 레이놀즈 수 조건에서 다양한 제트간 거리와 무차원화된 스트로크 길이에 대해 수행했다. 평균 유동장과 위상 분해 유동장의 계측은 위상 동기 PIV를 사용해 수행되었다. 제트간 거리가 감소하고 무차원화된 스트로크 길이가 감소함에 따라 두 합성제트간 상호작용이 증대되면서, 시간 평균 된 제트 중심속도의 빠른 감소와 확산이 발생하였다. 동시에 오리피스 사이 대칭축 상의 유속이 빠르게 증가하여 두 제트의 접촉 위치와 결합 위치가 가까워짐이 관찰되었다. 구동 주기 동안 오리피스에서 방출된 내부의 와류는 서로 상호작용하면서 유동방향으로의 변형을 거쳐 대부분 상쇄되며 이윽고 외부의 와류를 추월하면서 소산하였고, 남아있는 각 제트의 외부 와류는 새로운 와류 쌍을 형성하여 진행함이 관찰되었다. 와류간 상호작용은 제트간 거리가 가까워짐에 따라 더 이른 시점에 오리피스 가까이에서 발생하였으며, 스트로크 길이가 증가함에 따라 와류 상호작용의 유동 형태는 변하지 않았으나, 강화된 후행 제트의 영향으로 전체적인 거동이 빠르게 진행됨을 보였다.Chapter 1. Introduction 1 1.1 Research background 1 1.2 Purpose of research 3 Chapter 2. Experimental method 5 2.1 Experimental chamber and dual synthetic jet actuator 5 2.2 Phase-locked particle image velocimetry 6 2.3 Data reduction 7 Chapter 3. Time-averaged flow fields 12 3.1 Streamwise mean velocity distributions 12 3.2 TKE and PKE contours 15 Chapter 4. Phase-averaged flow fields 29 4.1 Phase-averaged vorticity contours 29 4.2 Vortex advection 30 Chapter 5. Conclusion 37 Bibliography 39 Abstract in Korean 41Maste

    Enhancement of synthetic jets by means of an integrated valve-less pump Part II. Numerical and experimental studies

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    The paper studies the performance of the new fluid jet actuator based on the novel principle of the generation of fluid jet, which has been presented in [Z. Travnicek, A.I. Fedorchenko, A.-B. Wang, Enhancement of synthetic jets by means of an integrated valve-less fluid pump. Part I. Design of the actuator, Sens. Actuators A, 120 (2005) 232-240]. The fluid jet actuator consists of a synthetic jet actuator and a valve-less pump. The resulting periodical fluid jet is intrinsically non-zero-net-mass-flux, in contrast to the traditional synthetic jet. The numerical results have been compared with the laboratory experiments comprising phase-locked smoke visualization and time-mean velocity measurements. The results have confirmed the satisfactory performance of the actuator
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