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Due to the excellent heat transfer performance and low pressure drop, the parallel-plate regenerator (PPR) is very suitable for use in Stirling engines. At present, researches on the application of...

A system for automated investigations of oscillating fluid flows in channels with the use of a special test rig is presented. In this system, a specially developed computer code defines the oscillating flow rate of the fluid and traces whether the task is performed. The test rig is tested with the use of a flowmeter.

Simulative optimization methods often build on an iterative scheme, where a simulation model is solved in each iteration. To reduce the time needed for an optimization, finding the right balance between simulation model quality, and simulation time is essential. This is especially true for transient problems, such as fluid flow within a hydromechanical system. Therefore, we present an approach to building steady-state surrogate models for oscillating flow in a pipe with a local heat source. The main aspect is to model the fluid as a solid with an orthotropic heat transfer coefficient. The values of this coefficient are fitted to reproduce the temperature distribution of the transient case by parametric optimization. It is shown that the presented approach is feasible for different sets of parameters and creates suitable surrogate models for oscillating flow within a pipe with a local heat source. In future works, the presented approach will be transferred from the simplified geometry under investigation to industrial problems.

The aim of this article is to understand numerically the flow and heat transfer characteristics under oscillating flow conditions for periodically corrugated wavy channel. For the same channel, under steady-state flow conditions, experimental and numerical studies were done under steady-state flow conditions by our two previous studies. Three turbulence models are used, namely the k–ω, the Shear Stress Transport (SST), and the transition SST. According to the previous study, the best agreement with experiments was obtained using the SST turbulence model. Therefore, the SST turbulence model is applied in this study on the oscillating flow. The finite volume method is used as the numerical method. Investigations are performed for air flowing through corrugated channel which has sharp wavy peaks with an inclination angle of 30° and 5 mm minimum channel height. Reynolds number is varied within the range 6294–7380, while keeping the Prandtl number constant at 0.70. Four different sinusoidal oscillating flow conditions are used. Variations of the Nusselt number, friction factor, and thermo-hydraulic performance factor with the Reynolds number are studied. © 2019 Taylor & Francis Group, LLC.

Since their introduction in the early 1990s, pulsating heat pipes (PHPs) have received a lot of attention due to their obvious advantages such as their geometrical simplicity, and their potential for high-heat flux applications even without power consumption. Although numerous investigators have studied PHPs over the last three decades, there still exist a few controversial issues on fundamental characteristics and several technical problems in practical applications. To put the finishing touches to the controversial issues and to shed light on the technical problems, recent advances in PHPs are critically reviewed in this paper. The results of this critical review are classified into two categories: (i) fundamental aspects of PHPs and (ii) practical aspects of PHPs. The first category focuses on reviewing the current state-of-the-art fundamental characteristics of PHPs. The second category summarizes the technical problems that are resolved for utilizing PHPs in practical applications. This review paper would help researchers or engineers who are working on or utilizing PHPs.

In this paper, we explore the stability and dynamical relevance of a wide variety of steady, time-periodic, quasiperiodic, and chaotic flows arising between orthogonally stretching parallel plates. We first explore the stability of all the steady flow solution families formerly identified by Ayats et al. [“Flows between orthogonally stretching parallel plates,” Phys. Fluids 33, 024103 (2021)], concluding that only the one that originates from the Stokesian approximation is actually stable. When both plates are shrinking at identical or nearly the same deceleration rates, this Stokesian flow exhibits a Hopf bifurcation that leads to stable time-periodic regimes. The resulting time-periodic orbits or flows are tracked for different Reynolds numbers and stretching rates while monitoring their Floquet exponents to identify secondary instabilities. It is found that these time-periodic flows also exhibit Neimark–Sacker bifurcations, generating stable quasiperiodic flows (tori) that may sometimes give rise to chaotic dynamics through a Ruelle–Takens–Newhouse scenario. However, chaotic dynamics is unusually observed, as the quasiperiodic flows generally become phase-locked through a resonance mechanism before a strange attractor may arise, thus restoring the time-periodicity of the flow. In this work, we have identified and tracked four different resonance regions, also known as Arnold tongues or horns. In particular, the 1 : 4 strong resonance region is explored in great detail, where the identified scenarios are in very good agreement with normal form theory.

The effect of oscillating flow on the performance of a single cell PEM fuel cell is investigated at the cathode side. The oscillating flow is generated with a Spark-plug plasma synthetic jet (SPSJ) actuator. The PEM fuel cell is directly connected to SPSJ actuator. The significant performance improvement of the PEM fuel cell is by this approach. At two different hydrogen and air flow rates, necessary experiments are performed to obtain the polarization curves and current–time history plots at a constant voltage. The results indicate that the peak power output of the PEMFC increases up to 11% compared to the case without a SPSJ actuator. It can be concluded that the plasma synthetic jet actuator producing oscillating flow enhances the oxygen concentration and diffusion rate at the cathode channel of the PEM fuel cell. © 2019 Elsevier Ltd, Türkiye Bilimsel ve Teknolojik Araştirma Kurumu, The authors would like to acknowledge the financial support of this work by the Scientific and Technological Research Council of Turkey (TUBITAK) under the Contract Number of 213M179 . The authors also thank the rest of project team for their assistance in the performing of the study.

Sweeping jet, featuring with temporally continuous and spatially oscillating flow, has attracted a large amount of attention. In the present study, the heat transfer and flow characteristics of a sweeping jet impinging at narrow spacings were investigated. Effects of Reynolds number (i.e., Re = 5000, 10,000, 15,000) and jet-to-wall spacing (i.e., H/D = 0.5, 1.0, 2.0, 3.0) on heat transfer were quantified extensively by using the TSP technique. A high-resolution Particle Image Velocimetry (PIV) system was applied to correlate the heat transfer results with the flow fields pertinent to jet impingements. The heat transfer of sweeping jet was found to increase as the Reynolds number increased and as the jet-to-wall spacings decreased. At Re = 5000 and 10,000, compared to the circular jet, the sweeping jet demonstrated a lower heat transfer in the near-nozzle regions (i.e., x/D

Oscillatory flow has many applications in micro-scaled devices. The methods of realizing microfluidic oscillators reported so far are typically based on the impinging-jet and Coanda effect, which usually require the flow Reynolds number to be at least at the order of unity. Another approach is to introduce elastomeric membrane into the microfluidic units, however, the manufacturing process is relatively complex, and the membrane will become soft after long-time operation, which leads to deviation from the design condition. From the perspective of the core requirement of a microfluidic circuit, i.e., nonlinearity, the oscillatory microfluidic flow can be realized via the nonlinear characteristics of viscoelastic fluid flow. In this paper, the flow characteristics of viscoelastic fluid (Boger-type) in a T-shaped channel and its modified structures are studied by two-dimensional direct numerical simulation (DNS). The main results obtained from the DNS study are as follows: (1) Both Weissenberg (Wi) number and viscosity ratio need to be within a certain range to achieve a periodic oscillating performance, (2) With the presence of the dynamic evolution of the pair of vortices in the upstream near the intersection, the oscillation intensity increases as the elasticity-dominated area in the junction enlarges, (3) Considering the simplicity of the T-type channel as a potential oscillator, the improved structure should have a groove carved toward the entrance near the upper wall. The maximum oscillation intensity measured by the standard deviation of flow rate at outlet is increased by 129% compared with that of the original standard T-shaped channel under the same condition. To sum up, with Wi number and viscosity ratio within a certain range, the regular periodic oscillation characteristics of Oldroyd-B type viscoelastic fluid flow in standard T-shaped and its modified channels can be obtained. This structure can serve as a passive microfluidic oscillator with great potential value at an extremely low Reynolds number, which has the advantages of simplicity, no moving parts and fan-out of two.

The paper analyzes heat transfer between an oscillating compressible fluid and solid to evaluate the influence of finite thermal capacity on the heat exchanged between these media. This type of problem is often studied in porous media applications by assuming the solid to have its surface at a constant temperature. When this hypothesis fails, in the frequency domain, the analytical solutions of the thermal field involve a complex dimensionless parameter known in the literature as e s . It was introduced in thermo-acoustic applications to model the solid properties of the porous core. In this work, the parameter is revisited by deriving an analytical formulation valid for arbitrarily shaped channels of porous materials, and a physical interpretation of the parameter is proposed in terms of fluid and solid entropy oscillation. CFD-based simulations of the thermal coupling between solid and fluid on three different simplified geometries of porous material have been conducted to confirm this physical interpretation, by comparing numerical and analytical data. Furthermore, from a numerical point of view, the parameter e s has been written as the inverse of a dimensionless Robin boundary condition. In this work, it is verified that the latter can be adopted at the fluid-solid interfaces to reproduce the thermal effects of the solid material, without numerically simulating it, both in frequency and time domain.

Forced harmonic oscillations of nine configurations consisting of horizontal side-by-side plate elements are performed experimentally and numerically. The configurations are oscillated in vertical direction and represent generalized mudmats of subsea structures. The tests are performed for Keulegan–Carpenter (KC) numbers relevant for force estimation during lifting operations. Hydrodynamic added mass and damping coefficients are presented. The coefficients are found to be amplitude dependent for all tested configurations. The interaction effects between the plates increase with increasing amplitude and decreasing distance between the plates. For small oscillation amplitudes, compared with the gap between the plates, the plates behave approximately like individual plates. A study of the relation between the damping force and the added mass force for the tested structures illustrates the importance of applying representative damping coefficients in numerical analysis of marine operations. Numerical results are obtained using a potential flow solver (BEM) and a viscous flow solver (CFD). Low-KC added mass coefficients predicted with the BEM are in accordance with the experiments. There is acceptable agreement between the CFD and the experiments. Best agreement is obtained for small KC numbers. As the KC numbers increase, the differences are, in general, larger. This is possibly due to the CFD being based on the two-dimensional laminar flow.

A transient simulation of a hybrid thermoacoustic system is used to study temporal evolution of flow structures as well as the velocity and temperature variations within the channels of the stack. The numerical results and experimental data showed good agreement with a difference of 4.29 % and 4.2 % respectively between the amplitudes of the pressure and axial velocity over one full cycle and generated identical flow structures. The simulation results further show the presence of a residual vortex layer inside the channel which can accelerate the velocity near the edges and decelerate the velocity near the center of the channel. Velocity peaks and temperature peaks or dips in the profile can be seen near the edges of the stack plates and are more prominent at resonance frequencies. At the same frequency, increasing the mean pressure shifts the peaks closer to the wall. The maximum pressure amplitude and net heat flux also increases with the mean pressure for different working fluids. Ministry of Education (MOE) The authors thank the support of Ministry of Education of Singapore with Tier 1 RG188/17.

Objectives: The principles of cryogenic regenerator operation, its design and optimization procedures are reviewed in this paper. Recent and ongoing developments of these regenerators are highlighted. Due to the non linear behaviour of geometry and operating parameters with performance parameters, design and optimization of regenerator performance has not been possible in conventional mathematical terms. Methods: Methodologies of design to determine the volume having maximum heat transfer ability with minimum losses are listed. The optimization in terms of mathematical, analytical and experimental approaches with their inherent difficulties is discussed. Findings: The regenerator mesh with a lower hydraulic diameter in the cold region and a larger hydraulic diameter in hot region will lead to lower regenerator losses. Thus a regenerator consisting of layers of different matrix geometry would have a better performance than a regenerator with single mesh geometry. Regenerators that incorporate heat transfer components in a parallel orientation with respect to the oscillating flow theoretically provide a better performance than screen mesh regenerators. An optimum regenerator would have a continuous variation in its hydraulic diameter along the length of the tube. Applications: This review is expected to be a catalogue of principles needed for effective simulation, design, and optimization of regenerators for cryogenic refrigerators. The optimum regenerator has the potential to significantly improve the performance of cryogenic refrigerators and make them suitable for more applications.

The structure and dynamics of an oblique shock train in a duct model are investigated experimentally in a hypersonic wind tunnel. Measurements of the pressure distribution in front of and across the oblique shock train have been taken and the dynamics of upstream propagation of the oblique shock train have been analyzed from the synchronized schlieren imaging with the dynamic pressure measurements. The formation and propagation of the oblique shock train are initiated by the throttling device at the downstream end of the duct model. Multiple reflected shocks, expansion fans and separated flow bubbles exist in the unthrottled flow, causing three adverse-pressure-gradient phases and three favorable-pressure-gradient phases upstream the oblique shock train. The leading edge of the oblique shock train propagates upstream, and translates to be asymmetric with the increase of backpressure. The upstream propagation rate of the oblique shock train increases rapidly when the leading edge of the oblique shock train encounters the separation bubble near the shock reflection point and the adverse-pressure-gradient phase, while the oblique shock train slow movement when the leading edge of the oblique shock train is in the favorable-pressure-gradient phase for unthrottled flow. The asymmetric flow pattern and oscillatory nature of the oblique shock train are observed throughout the whole upstream propagation process.

A third-order numerical model based on one-dimensional computational fluid dynamics is developed for kinetic Stirling engines. Various loss mechanisms in Stirling engines, including gas spring hysteresis loss, shuttle loss, appendix displacer gap loss, gas leakage loss, finite speed loss, piston friction loss, pressure drop loss, heat conduction loss, mechanical loss and imperfect heat transfer, are considered and embedded into the basic control equations. The non-equilibrium thermal model is adopted for the regenerator to capture the oscillating features of the gas and solid temperatures. To improve the numerical stability and accuracy, the implicit second-order time difference scheme and the second-order upwind scheme are adopted for discretizing the time differential terms and convective terms, respectively. Experimental validations are then conducted on a beta-type Stirling engine with the extensive experimental data for diverse working conditions. The results show that the developed model has better accuracies than the previous second-order models. Good agreements are achieved for predicting various critical system parameters, including pressure-volume diagram, indicated power, brake power, indicated efficiency, brake efficiency and mechanical efficiency. In particular, both the experiments and simulations show that the Stirling engine charged with helium tends to have much lower optimal working frequencies and poorer performances compared to the hydrogen system. Based on the analyses of the losses, it reveals that the pressure drop in the flow channels plays a critical role in shaping the different behaviors. The pressure drop in the helium system is much larger and more sensitive to the frequency increase due to the much larger viscosity of gaseous helium. Hydrogen is a superior working gas for a Stirling engine. The transient characteristics of the oscillating flow and the associated thermal interactions between gas and solid in the regenerator are finally analyzed in order to have an insight of the complex thermodynamic process. The study provides a promising numerical approach in simulating Stirling engines for further understandings of their operating characteristics and the underling mechanisms.

The open-cell metal foam is a porous medium which possesses a true metal skeletal structure. The fully inter-connected pore and ligament structures provide the extreme large fluid-to-solid contact surface area and tortuous coolant flow path inside the metal foam, which could increase dramatically the overall heat transfer rate. The high specific surface area, low density and open-celled nature of a metal foam possesses a combination of properties ideally suited for applications in high heat flux thermal management where conventional materials and products are not adequate. In this research, forced convection in a heated channel filled with open-cell metal foam subjected to oscillating flow has been investigated experimentally and numerically. DOCTOR OF PHILOSOPHY (MAE)

This article deals with the possibility of pulse turbine utilization in compressor technology. The ways of acoustic energy utilization in silencing systems within energy plants and compressors with the generation of additional energy were described. Experimental unit for determination of optimal resonator length depending on gas flow oscillation frequency was created. The experimental research results for the pulse turbine under the oscillating flow influence are presented.

The heat transfer and pressure drop for oscillating flow in helically coiled tube heat-exchanger were numerically investigated based on the Navier–Stokes equations. The correlation of the average Nussel number and average friction factor were proposed considering the frequency and the inlet velocity. The oscillating flow heat transfer problems are influenced by many factors. Hence we need an easy way to reduce the numbers of simulation or experiment. Therefore, the method of uniform design was adopted and the feasibility of this method was verified. The field synergy principle was used to explain the heat transfer enhancement of oscillating flow in helically coiled tube heat-exchanger. The result shows that the smaller the volume average field synergy angle in the helically coiled tube, the better the rate of heat transfer.

The current work analyses the onset characteristics of Rayleigh–Bénard convection in confined two-dimensional two-layer systems. Owing to the interfacial interactions and the possibilities of convection onset in the individual layers, the two-layer systems typically exhibit diverse excitation modes. While the attributes of these modes range from the non-oscillatory mechanical/thermal couplings to the oscillatory standing/travelling waves, their regimes of occurrence are determined by the numerous system parameters and property ratios. In this regard, the current work aims at characterising their respective influence via methodical linear and fully nonlinear analyses, carried out on fluid systems that have been selected using the concept of balanced contrasts. Consequently, the occurrence of oscillatory modes is found to be associated with certain favourable fluid combinations and interfacial heights. The further branching of oscillatory modes into standing and travelling waves seems to additionally rely on the aspect ratio of the confined cavity. Specifically, the modulated travelling waves have been observed to occur (amidst standing wave modes) at discrete aspect ratios for which the onset of oscillatory convection happens at unequal fluid heights. This behaviour corresponds to the typical $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}m$:$n$ resonance where the critical wavenumbers of convection onset in the layers are dissimilar. Based on all of these observations, an attempt has been made in the present work to identify the oscillatory excitation modes with a reduced number of non-dimensional parameters.

Stirling engine heaters are characterized by oscillating flows which affect heat transfer coefficients greatly. A 36-tube Stirling engine heater with a piston-link drive machine is used to study heat transfer characteristics of oscillating flows. The influences of the overall heating power, oscillating frequency and gas pressure on the heat transfer characteristics are investigated. To raise the working gas pressure is positive to decrease the wall temperature and to improve the heat transfer. When the pressure varies from 0.1 to 0.4 MPa, the wall temperature reduces 17 °C and the heat input increases 10 W. The time-averaged heat transfer coefficients reach the maximum value of 78.0 W/(m 2 K) among the testing conditions when the working gas pressure is 0.4 MPa and the revolution is 420 rpm. An oscillating heat transfer correlation is derived based on the experimental data which are obtained under situations close to an actual Stirling engine’s flow region, where Re and Re ω are in the ranges of 740–4110 and 12–71, respectively. The estimated uncertainty for the heat transfer coefficient is usually within 7.69%, and the determination coefficient of a regression fitted correlation is 0.97. The proposed correlation is respected to predict the heat transfer coefficients of oscillating flows for practical design of tubular heaters, while the classical unidirectional steady correlations are not suitable, especially at high Re conditions.

Almost all prior models for respiratory aerosol deposition have utilised rigid analogues of airways, which do not account for the effects of lung motion on particle deposition. This work initially examines particle deposition in a Weibull-type geometry, comparing traditional computational fluid dynamics approaches with a novel moving mesh method. A distinct difference was found to exist between the results obtained using a stationary geometry (with either constant or oscillating flow) and a moving mesh. The second part of the work applies the moving mesh method to a Sprague-Dawley rat airway. It was found that a hybrid moving mesh and oscillating flow method was required to produce optimal results. The new method agrees well with in vivo experimental data for local and global deposition in rat airways.

We investigate the rheograms of concentrated suspensions of magnetic particles obtained under imposed shear rate in parallel plate geometry. We show that under magnetic field application the usual trend of the rheogram, i.e., increasing shear stress for the whole range of shear rates, is altered by the appearance of a region in which the shear stress decreases as the shear rate is increased. The existence of this region gives to the rheograms an N-like shape. The two initial regions (preyield regime) of these N-like rheograms present unstable flow, characterized by the oscillation of the shear stress with time for each imposed value of shear rate. We also show that rheograms obtained at different sample thicknesses approximately overlap in the developed flow regime, whereas there is a tendency of the shear stress to increase as the thickness is decreased in the preyield regime. This tendency is likely due to the strengthening of pre-existing particle structures by compression as the gap thickness is decreased. Finally, we analyze the effect of the applied magnetic field strength, H, and demonstrate that the rheograms scale with H^1.5 to a single master curve, for the range of applied magnetic fields under study., This work was supported by project FIS2013-41821-R, MINECO, Spain (Co-funded by ERDF, European Union); by project 3.12.2014/K, Program of Ministry of Science and Education of the Russian Federation; by the Act 211 Government of the Russian Federation, Contract No. 02.A03.21.0006; by grants of RFFI 13-02-91052, 13-01-96047 (Ural), and 14-08-00283; and by project PICS 6102 CNRS/Ural Federal University. L.R.-A acknowledges financial support from University of Granada.

International audience; Nonlinear phenomena in oscillating flow devices cause the appearance of a relatively minor secondary flow known as acoustic streaming, which is superimposed on the primary oscillating flow. Knowledge of control parameters, such as the time-averaged second-order velocity and pressure, would elucidate the non-linear phenomena responsible for this part of the decrease in the system�s energetic efficiency.This paper focuses on the characterization of a travelling wave oscillating flow engine by measuring the time-averaged secondorder pressure and velocity. Laser Doppler velocimetry technique was used to measure the time-averaged second-order velocity. As streaming is a second-order phenomenon, its measurement requires specific settings especially in a pressurized device. Difficulties in obtaining the proper settings are highlighted in this study. The experiments were performed for mean pressures varying from 10 bars to 22 bars. Non-linear effect does not constantly increase with pressure.

A numerical investigation of three-dimensional sinusoidally oscillating flow around an infinitely long cylinder was conducted to examine the onset of the Honji instability and to gain insight into the mechanism that causes the Honji instability to arise. An attempt is made to quantify when the instability occurs using the dimensionless flow parameters of the Keulegan–Carpenter number (KC) and the Sarpkaya number (β). Through numerical analysis and an explanation of physics, it is shown that the Honji instability occurs through the mechanisms described by Lord Rayleigh, but is significantly different from the Taylor, Dean, and Gortler instabilities.

Hall effects on the viscous incompressible fluid due to non-coaxial rotations of an oscillating porous disk and a fluid at infinity are studied. The velocity field, shear stresses and temperature distribution are obtained in closed form. It is found that with increase in frequency parameter, the primary velocity increases near the disk and becomes almost stationary away from the disk. The secondary velocity also increases with increase in frequency parameter. It is seen that with increase in Hall parameter, the primary velocity increases near the disk and decreases away from the disk. The reversed effect is observed for the secondary velocity. The shear stresses at the disk are also obtained. It is found that the shear stresses due to the primary and the secondary velocities decrease with increase in Hall parameter. The heat transfer characteristic is also studied on taking viscous dissipation into account. It is found that the mean temperature at the disk decreases with increase in Hall parameter.

Key Words: Oscillating Flow(진동유동), Flow Sensor(유동 센서), Artificial Sensory Hair(인공 감각모),Modulus of Elasticity(탄성계수), Response Characteristics(응답특성)초록: 주위매질의움직임에반응하는섬유상감각모는대부분의생물체에존재하여먹이, 침입자, 또는동족여부를감지하는역할을한다. 이기능을모방한인공감각모를설계제작하기위하여유연감각모를기초로한수학모델을제안하고, 작동영역에서의응답특성을파악하기위하여매개변수해석을수행하고각인자들의영향을고찰하였다. 유동감각모의변위를일반화좌표및고유진동모드로나타낸결과, 탄성계수가비교적큰Cytop 감각모의경우고유진동의기본모드가지배적인것으로나타났다. 주어진유동중에있는감각모의동적거동은형상에크게의존하였다. 또한탄성계수가큰인공감각모에서는생물학적으로중요한주파수범위내에서공진현상은나타나지않았다.Abstract: Filiform hairs that respond to movements of the surrounding medium are the mechanoreceptors commonlyfound in arthropods and vertebrates. The hairs function as a sensory system for perceiving information produced byprey, predators, or conspecifics. A mathematical model is proposed, and the parametric analyses for the response ofartificial filiform hair are conducted to design and predict the performance of a microfabricated device. The resultsfor the Cytop hair, one of the most popular polymer optical fibers (POFs), show that the fundamental mode has adominant effect on the hair behavior in an oscillating medium flow. The dynamic behavior of sensory hair is alsodependent on the physical dimensions such as length and diameter. It is found that the artificial hair with a highelastic modulus does not show a resonance in the biologically important frequency range.

A new mathematical analysis of a laminar Burke-Schumann type of spray diffusion flame in an oscillating flow field is presented within a framework in which mild slip is permitted between the droplets and their host surroundings. A perturbation analysis using a small Stokes number is used for solving the liquid phase governing equations. The effect of droplet grouping in the oscillatory flow field is accounted for by constructing an appropriate model for the vaporization Damkohler number. A formal analytical solution is developed for the Schwab-Zeldovitch parameter through which instantaneous flame front shapes are found. Computed results based on the solution expose the strong impact that the phenomenon of droplet grouping can have on flame characteristics such as flame height, shape and type (over- or under-ventilated). Despite the models simplicity its predictions offer an opening insight into the mechanisms prevalent in more complex spray-combustion situations in which droplet grouping may occur.

Piezoelectric fans are thin elastic beams whose vibratory motion is actuated by means of a piezoelectric material bonded to the beam. These fans have found use as a means to enhance convective heat transfer while requiring only small amounts of power. The objective of the present work is to quantify the influence of each operational parameter and its relative impact on thermal performance. Of particular interest are the vibration frequency and amplitude as well as the geometry of the vibrating cantilever beam. The experimental setup consists of a piezoelectric fan mounted normal to a constant heat flux surface. Temperature contours on this surface captured via an infrared camera are used to extract the forced convection coefficient due to the fluid motion generated from the fan. Different fans, with fundamental resonance frequencies ranging from 60 to 250 Hz, are considered. Results show that the performance of the fans is maximized at a particular value of the gap between the fan tip and the heated surface. It is found that when a fan operates at this optimum gap, the heat transfer rate is dependent only on the frequency and amplitude of oscillation. Correlations based on appropriately defined dimensionless parameters are developed and found to successfully predict the thermal performance across the entire range of fan dimensions, vibration frequency and amplitude. An understanding of the dependence of thermal performance on the governing variables allows for improved design of piezoelectric fans as a method of enhancing heat transfer. (C) 2009 Elsevier Ltd. All rights reserved.

Oscillatory flow about a pair of circular cylinders is considered. The distance between the cylinders can be varied as can the angle that the undisturbed oscillatory flow makes with the line joining the cylinder centres. In common with other fluid flows dominated by oscillatory flow, a time-independent, or steady streaming, motion develops. Attention is focused on the case of high streaming Reynolds numbers and the resulting jets that erupt from the surfaces of the cylinders.

The wall friction induced by the oscillating flow of the fractional derivative Maxwell viscoelastic fluid in the pipe is investigated. The velocity and the shear stress solutions of the flow are solved. The friction is derived from the shear stress expression and analysed by numerical simulation. From analysis, it is found that the friction amplitude exhibits resonance-like phenomena. Moreover, the number of the resonance-like peaks, the enhancement magnitude, and the resonance-like frequency of the same order vary with the pipe radius and rheological parameters of fluids. When the radius overtakes the critical value, the friction curve monotonously decreases, and when the radius is big enough, the enhancement disappears.

A valveless micropump was realized with a diffuser/nozzle shaped channel and a variable volume actuator which produces an oscillating flow. One-way flow may be realized in the nozzle direction since the pressure loss in a nozzle channel is lower than that in a diffuser channel. Pump characteristics were measured for various angles of a diffuser/nozzle element and positions of the actuator to investigate the effect of pump geometry on characteristics. The experimental results showed an optimal diffuser/nozzle angle for pump efficiency, and the optimal actuator position. The frequency characteristics and the pump characteristics were measured. Dimensionless variables were introduced to rearrange the measured data and to understand the physical mechanisms of the micropump.

Tez (Doktora) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2015, Thesis (PhD) -- İstanbul Technical University, Institute of Science and Technology, 2015, Gözenekli ortamda salınımlı akış ve ısı geçişine nükleer santrallerin soğutma üniteleri, pistonlu içten yanmalı motorlar, ısı boruları, rejeneratörler, Stirling motorları gibi birçok mühendislik uygulamasında rastlanmaktadır. Yüksek ısı kayıp oranları sebebiyle, yüksek güç yoğunluklu, yüksek hızlı elektronik bileşenleri soğutmak için gözenekli ortamda salınımlı akışın kullanılmasına ilgi artmıştır. Gözenekli ortamda salınımlı akışla ısı geçişi konusu, örneğin bilyalı yataklarla, daha önce çalışılmıştır. Metal (Alüminyum, bakır, nikel vb.) köpükler nispeten yeni nesil gözenekli ortamlardır. 10000 m2/m3’lere ulaşan, oldukça yüksek yüzey alan yoğunluklarına ve %90 mertebelerinde yüksek gözenekliliklere sahiptirler. Metal köpüklerin içyapılarını oluşturan, kirişlerle çevrili hücrelerin şekilleri tetradekahedrona benzetilebilir. Metal köpüklerde havanın salınımlı akışı da akademik çalışmalara daha önce konu olmuştur. Fakat suyun bu tür çalışmalarda kullanıldığı görülmemiştir. Gözenekli ortamlarda akış ve ısı geçişi bakımından, hava ve su birbirinden oldukça farklıdır: Momentum ve enerji mekanizmalarında kaynak terimi olarak bulunan dispersiyon su için önemli, hava için ise ihmal edilebilirdir. Doymuş gözenekli ortamlarda hava ve su için etkin ısı geçiş katsayıları birbirilerinden yine oldukça farklıdır. Bunların yanı sıra, Prandtl sayıları ve diğer termofiziksel özellikleri farklılık göstermektedir. Suyun sıkıştırılabilirliği ve titreşimli akıştaki rastgele davranışı farklı incelenebilir. Bu gibi ayrılıkların akış alanı ve sıcaklık dağılımlarında da kendilerini belli etmesi beklenmektedir. Bu çalışmada, 20 ppi (inç başına gözenek) gözenek yoğunluğuna ve %87 gözenekliliğe sahip, silindirik şekilli ticari bir alüminyum köpük, suyun düz ve salınımlı akış durumlarında incelenmiştir. Silindirin malzemesi 6101-T6 kodlu alüminyum alaşımıdır ve 6061-T6 kodlu yine bir alüminyum alaşımından üretilmiş bir borunun içine sert lehimle kaynatılmıştır. Bu birleştirme yönteminin seçilmesindeki amaç ısıl temas direncinin minimum değerde tutulması gerekliliği idi. Metal köpüğü bir test bölmesi olarak, daha önceki çalışmalarda kullanılmış, var olan düzeneğe yerleştirmeden önce düzenek hidrodinamik olarak denendi. Bu aşama için kullanılan gözenekli ortamlar 1 ve 3 mm çapındaki çelik kürelerin oluşturduğu, boyut olarak da metal köpüğün brüt hacmiyle benzeşen bilyalı yataklardı. Literatürde rastlanan tüm gözenekli ortam rejimleri bu denemede bulundu. Ayrıca daha önceki bilyalı yataklarda elde edilen sonuçlar tekrarlanabildi. Böylece düzeneğin sürünen akıştan türbülanslı akışa uzanan geniş bir yelpazedeki kullanılabilirliği gösterilmiş oldu. Bilyalı yataklar denemeden sonra metal köpük ile değiştirildi, fiber contalar ve teflon bantlar ile sızdırmazlık sağlandı. Sabit hızlı, tek yönlü akış deneyleri için 3.5 m yüksekliğinde bir kule yapıldı ve üzerine 50 litrelik bir su tankı yerleştirildi. Tankın üst noktasına yakın, aynı seviyede 4 noktaya 1.9 cm çapında delik delindi. Bu deliklerden su çıkışı sağlanarak sabit su yüksekliği ve dolayısıyla düşük debiler için 0.38 bar değerinde sabit bir basınç kaynağı elde edildi. Yüksek debiler için musluktan gelen su doğrudan kullanıldı. Test düzeneği açık sistem olduğu için metal köpüğü terk eden su çevreye bırakıldı. Basınç kaybı ölçümü için değiştirilen basınç aralıklarına sahip fark basınç ölçüm cihazları kullanıldı. Debi için ise faklı kapasite ve hassasiyetlerde teraziler ve bir kronometre kullanıldı. Yaklaşık 100 lineer basınç gradyeni-hız çifti ölçüldü, tabloya işlendi ve grafikte gösterildi. Gözenekli ortamların bir karakteristiği olan kuadratik eğrinin oluştuğu gözlemlendi. Basınç gradyeni hız ile bölünerek hızın yine yatay eksende olduğu grafikte gösterildi ve farklı bölgelerde farklı eğimlere sahip olduğu görüldü. Bu bölümlerin her birinin ayrı akış bölgelerinin temsil ettiği kabul edildi. Aralardaki geçiş bölgelerinin yanı sıra 4 akış rejimi tespit edildi: Darcy öncesi, Darcy, Forchheimer ve türbülanslı bölgeler. Geçirgenlik ve şekil direnç katsayısı isimli iki önemli gözenekli ortam parametresi Forchheimer denklemi vasıtasıyla her rejim için ayrı ayrı hesaplandı ve bulunan değerlerin birbirinden farklı olduğu gözlemlendi. Hıza bölerek indirgenmiş basınç gradyenleri ve Forchheimer denklemindeki diğer terimler tekrar değiştirilerek boyutsuz hale getirildi ve sürtünme katsayısı Reynolds sayısı cinsinden bir fonksiyon olarak gösterildi. Bu aşamada Reynolds sayısı ve sürtünme katsayısı için geçerli ve doğru olan karakterisik uzunluğun Darcy rejiminde bulunan geçirgenliğin karekökü olduğu gösterildi. Bu fikrin çıkış noktası, Darcy rejiminde akışkanın sürünen akış halinde ilerlemesi ve metal köpüğün iç yapısına dair bilginin ancak bu akış tarafından gösterilebileceğinin düşünülmesidir. Salınımlı akış deneylerinde ileri ve geri periyodik hareket yapan pistonlu bir mekanizma kullanıldı. Çift yönlü çalışan piston ile sistem kapalı hale geldi. Akış, doğası gereği zamana bağlı olduğu için hız ölçümleri zamana bağlı olarak kaydedildi. Atalet etkilerine bağlı olarak ölçüm cihazı ve system arasındaki mesafeden kaynaklanabilecek hataları yok edebilmek için fark basınçölçerler kullanılmadı. Bunun yerine statik basıncı yerinde ölçebilecek basınç transmiterleri kullanıldı. Deneyler 0.116 Hz ve 0.696 Hz arasında eşit aralıklı olarak değişen frekanslar ve üç farklı akış yer değiştirme mesafesinin kombinasyonları ile tamamlandı. Basınç değerleri dijital multimetre ile zamana bağlı olarak kaydedildi, tabloya işlenip grafiklerde gösterildi. İki farklı frekans bölgesi ortaya çıkarıldı. Düşük frekans bölgesinde köpüğün iki ucundaki basınç değerleri birbirilerine zıt olarak değişti. Faka yüksek basınç bölgesinde davranışları paraleldi fakat yine de her durumda harekete sebep olacak bir basınç gradyeni mevcuttu. Salınımlı akışta elde edilen sürtünme faktörleri düz akışa kıyasla daha yüksek bulundu fakat bilyalı yataklara göre daha düşüktü. Düz akışta ısı geçişi deneyleri için köpük duvarı üzerine eksen boyunca 33 adet 4 mm derinliğinde 1 mm çapında delikler delindi. Bu deliklere ısıl yapıştırıcılar ile sabitlenen termoeleman telleri ile duvar sıcaklığı ölçüldü. Debi için ise 20 l/d ölçüm kapasiteli manyetik debimetre kullanıldı. Duvarda sabit ısı akısı sağlamak amacıyla tüm boru yüzeyini kaplayacak bir kelepçe ısıtıcı ve birbirilerine seri bağlanmış iki adet doğru akım üreteci kullanıldı. Köpük giriş ve çıkışında suyun sıcaklığını ölçmek için iki sıcaklık probu üretildi. Bu problar sayesinde akışa dik eksende eşit aralıklı beş noktada sıcaklık ölçümü yapıldı. Suyun yığın sıcaklık dağılımı bu iki probdan alınan ortalama sıcaklıklar ve duvar sıcaklığı dağılımı yardımıyla ölçüldü. Ölçüm yapılan akış hızları Darcy, Forchheimer ve geçiş bölgelerinden seçildi. Her hız için Nusselt sayısı dağılımı duvar sıcaklıkları ve ilgili noktalardaki yığın sıcaklıkları sayesinde bulundu. Sıcaklık ve Nusselt sayısı dağılımları duvar boyunca tabloya işlendi ve grafikte gösterildi. Isıl giriş bölgelerinin yanısıra çıkış bölgeleri de tespit edildi. Isıl giriş uzunlukları ve Nusselt sayıları literatürdeki değerlerle karşılaştırıldı. Darcy rejimindeki ısı geçişlerinde tam gelişmiş bölgedeki Nusselt sayısının, ısıl dengesizlik kabulü altında, literatürdeki değerlere yakın olduğu bulundu. Düz ve kararlı akışta ısı geçişi için hazırlanmış açık düzenek aynı salınım üretecinin bağlanması ile yine kapalı hale getirildi. Benzer frekans ve yer değiştirme değerleri kullanıldı. Çevrim ortalaması alınmış duvar sıcaklığı değerlerinin eksen boyunca simetrik dağılıma sahip olduğu ortaya çıkarıldı. Böylece metal köpük duvarının yalnızca bir yarısının incelenmesinin yeterli olduğuna karar verildi. Nusselt sayısının hesabında, literatürde de karşılaşıldığı gibi giriş sıcaklıkları ve duvar sıcaklıkları kullanıldı. Çevrim ortalaması alınmış duvar sıcaklığı ve Nusselt sayısı dağılımı tablolara işlendi ve grafiklerde gösterildi. Her frekans-yer değiştirme çifti için azami sıcaklık tespit edildi ve yüksek frekans ve değiştirme değerlerinde bu değerin daha düşük olduğu tespit edildi. Ayrıca bu yüksek değerlerde sıcaklıkların duvar boyunca daha düzgün dağılımlı olduğu görüldü. Nusselt sayılarının her konfigürasyon için duvar boyunca da ortalamaları alınarak kinetik Reynolds sayısı cinsinden, yine literatürde olduğu gibi üstel fonksiyon olarak ifade edilebildiği gösterildi. Metal köpük içinde suyun düz akışı ve ısı geçişi, gerçek hacmin dörtte birinin 3 boyutlu olarak modellendi ve ANSYS Fluent isimli ticari akış analizi koduyla çözümlendi. Hidrodinamik analiz sonuçları deney sonuçları ile uyuşuyordu. Bu sebeple, deneysel olarak bulunması oldukça güç olan hidrodinamik giriş uzunluğunun kestirimi yapıldı. Diğer taraftan ısı geçişi sonuçları düşük hızlarda deneysel sonuçlarla benzeşirken yüksek hızlarda deneysel sonuçlardan ıraksamaya başladı. Bu sonucun sebebi Fluent içindeki gözenekli ortam modelinin geleneksel gözenekli ortamlar düşünülerek hazırlanmış olması olarak kabul edilebilir. Salınımlı akış ve ısı geçişi 2 boyutlu olarak modellendi. Bunun sebebi, zamana bağlılık sebebiyle akış alanı hesabı yakınsasa bile sıcaklığın yayılması ve sanki-dengeli hale gelmesi için yüksek işlemci zamanlarının gerekmesiydi. Hidrodinamik sonuçlar deneyle yine benzerlik gösterse de ısı geçişi sonuçlarında deneydeki belirsizlik değerlerini de aşan belirgin farklar vardı. Bu sonuç yukarıdaki sebeple birlikte, salınımdan ötürü oluşması gereken karışmanın mevcut türbülans modelinin yetersizliği sebebiyle bulunamamasına bağlandı., Oscillating flow and heat transfer in porous media is encountered in many engineered systems such as heat pipes, regenerators, Stirling engines, cooling units of nuclear power plants and reciprocating internal combustion engines. Due to substantial heat removal rates, there has been interest in using oscillatory flow in porous media for cooling high-power-density high-speed electronic components, as well. Heat transfer due to oscillating flow in traditional porous media (e.g. packed spheres) has been studied before. Metal (aluminum, copper, etc.) foams are relatively new class of porous materials. They have extremely large surface area density, up to 10000 m2/m3, and very high porosity, around 90%. The shape of cells of metal foams can be regarded as tetrakaidecahedra. Oscillating flow and heat transfer of air in metal foam has also been studied. However, heat transfer due to oscillating flow of water in metal foam has never been studied. In terms of flow and heat transfer in porous media, air and water, as working fluids, are very different: in water, an added momentum and heat transport mechanism called dispersion is important, while it is negligible for air flow. There is also a big difference in the effective thermal conductivity when the porous medium is saturated with air compared to water. This is in addition to the difference in Prandtl number and other thermophysical properties for the two fluids. There is a difference in compressibility between the two fluids and an expected splashing for the case of oscillating water flow. These differences are expected to produce vastly different flow field and temperature distribution in metal foam. In the current study, a 20-ppi (pores per inch) cylinder-shaped aluminum metal foam core with a porosity of 87% was tested under the conditions of steady and oscillating water flow. The core was made of 6101-T6-aluminum alloy and it was brazed into an aluminum tube with the designation code of 6061-T6. The test section was deliberately brazed in order to avoid high thermal contact resistance. Before installing the foam as a test section, the test setup was tested hydrodynamically as a qualification study. The porous media used for this step were packed beds of 1- and 3-mm steel spheres with a cylindrical bulk volume similar to that of the metal foam. All of the porous-media flow regimes reported in literature were found. Therefore the system was proven to reveal not only turbulent flow regime in porous media, but also regimes observed at flow conditions even with the slightest fluid motion, namely pre-Darcy and Darcy regimes. The packed bed was replaced by the metal foam and care was taken so as to prevent any leakage. For steady-state flow experiments, water inlet from an elevated tank and directly from the network were used as constant pressure sources for low and high flow rates, respectively. The system was set to be open rather than a closed loop. For measuring pressure loss, differential pressure sensors with changeable ranges were used. For the flow rate measurements, mass scales with different ranges and accuracies along with a stopwatch were used. A quadratic relation between pressure loss and velocity was observed as expected. The pressure gradients were modified obtain linear curves with slopes varying from zone to zone. These zones with different slopes denoted different regimes. Four different regimes, namely pre-Darcy, Darcy, Forchheimer and turbulent regimes, were identified along with the transitions among them. Two important foam parameters, permeability and form drag coefficient were calculated, and proven to have different values, for each regime. Finally, the already-reduced pressure gradient-velocity couples were modified with the purpose of displaying the relationship between non-dimensional quantities, which were the friction factor and Reynolds number. This step ensured that the square root of the permeability calculated in the Darcy regime was a viable characteristic length for Reynolds number. This idea had been originated from the fact that the flow crept encapsulating the ligaments of the foam and was related well to internal morphology of the foam. The oscillating flow experiments involved the use of a reciprocating mechanism, which also resulted in a closed system. Because the flow was transient in nature, the revolutions of the oscillation-generating mechanism were recorded with respect to time for velocity calculations. Besides, rather than a differential pressure sensor, two pressure transmitters located at both ends of the foam were used to avoid inertia-induced errors. The runs were completed with combinations multiple frequencies and flow displacements. The data was acquired using a data logger. Two frequency zones were identified. In the low frequency zone, the pressures had counteracting behaviors, whereas in the high frequency zone, those values were in parallel, still with a certain pressure difference. In both of the zones, the friction factors were higher than those of the steady-state flows, and lower than those found previously oscillating water flow experiments in packed beds of spheres. For the steady-state heat transfer experiments, holes were drilled along the wall of the foam to measure wall temperatures. Constant heat flux was introduced through the wall. In addition, inlet and outlet water temperatures were measured. The bulk temperatures were calculated using averaged temperatures from these two ends and the wall temperature distribution. The velocities were in Darcy, Forchheimer and transition regimes. Nusselt number for each velocity case was calculated using the corresponding wall and bulk temperatures. In addition to thermal entry lengths, exit lengths were also found. The thermal entry lengths were contrasted to their counterparts in literature as well as the Nusselt numbers. The Nusselt numbers in the fully developed region for Darcy flow matched the analytical solutions extremely well for the thermal non-equilibrium approach. The steady-state heat transfer mechanism was turned into a closed system using the same oscillation generator for the oscillating heat transfer test. The cycle-averaged wall temperature distribution was observed to be symmetric under oscillation. Therefore, only one half of the axial domain was studied. The cycle-averaged Nusselt number and temperature distribution were obtained. It was observed that the temperatures were lower for higher displacements and frequencies. The distribution was also more uniform. Nusselt number was correlated with respect to the kinetic Reynolds number. The steady-state flow and heat transfer were modeled in three dimensions and simulated using ANSYS finite volume tools. The flow results were in good agreement with their experimental counterparts. Therefore numerical approximations were made for hydrodynamic entry length since this value is extremely hard to determine experimentally. The heat transfer results on the other hand exhibited divergence from experimental findings at high velocities. This result was attributed to the fact that the current built-in model was originally for traditional porous media, not high-porosity foam. The oscillating flow and heat transfer were modeled in two dimensions because the propagation of the temperature required an extensive CPU time. The flow results matched the experimental values well. However, there was a significant mismatch of heat transfer results, exceeding the experimental uncertainty. This was a result of the same problem reported above and inadequate mixing due to the current turbulence model in the numerical tools., Doktora, PhD

The effect of a tidally oscillating flow in dispersing brine waste discharge into the sea is investigated using a two-dimensional advection-diffusion equation. As a seawater desalination plant is continuously disposing of brines at a constant rate, unacceptably high salinity in coastal water on both sides of the outfall are created due to the flow reversals. It is found that by building a longer outfall, such potential environmental impacts may be reduced. Other discharge strategies that minimize the salt concentration levels at the beach are discussed.

Time-accurate three-dimensional Reynolds-averaged Navier-Stokes simulations have been performed to explore the flow unsteadiness of the subscale J-2S thrust optimized contour rocket nozzle in over-expansion at prescribed air supply conditions in the chamber. The results show the development of periodic low-frequency, high-amplitude, self-sustained oscillations at the simulated nozzle pressure ratios (NPRs). The cap-shock pattern is responsible for the two-coaxial jet topology that appears. The complex three-dimensional evolution of the flowfield is characterized by a periodical jet flapping, axial oscillations, and azimuthal fluctuations with or without swirl, depending on the NPR. According to the simulations, jet flapping quasi-restricted to a preferred plane may occur during the firings, being supported by the reported thrust and side-load measurements. The computed dominant frequencies corresponding to the longitudinal and transverse oscillation modes show an excellent agreement with the experimental counterpart. The analysis of the forces spectra and flowfields has identified that synchronization between side-load (transverse) and thrust (longitudinal) oscillations develops in consonance with the observations. Besides, their strong coupling is favoured by the response of the shocks to the pulsating pressure near the large recirculation bubble that arises behind the cap-shock. A discussion on the self-oscillating, high-coherent flow phenomenon is addressed. The present three-dimensional study has captured for the first time the phenomenon of cyclic transition between reattached flow separation mode and non-reattached mode, where the alternating bursting and formation of a separation bubble is accompanied by the large amplitude axial displacement of the shock pattern. Furthermore, the role of the separation bubble dynamics on triggering high side-load peaks at fixed NPRs is numerically confirmed.

The characteristics of longitudinal dispersion enhancement by the flow oscillation are numerically studied according to various groove geometries in a 2-D channel in the present study. The length of expanded section l/h is varied from 0 to 8.75. The oscillating flow condition is given at both side ends, i.e., u

The output of Stirling engine is influenced by the regenerator effectiveness. The regenerator effectiveness is influenced by heat transfer and flow friction loss of the regenerator matrix. In this paper, in order to provide a basic data for the design of regenerator matrix, characteristics of heat transfer and flow friction loss were investigated by a packed method of matrix in the oscillating flow as the same condition of operation in a Stirling engine. As matrices, 6 kinds of steel wires, 4 kinds of combined steel wires, 8 kinds of combined steel wires with screen meshes were used. The results are summarized as follows; Among 6 kinds of steel wires the two steel wires showed the highest in effectiveness. Among 4 kinds of combined steel wires showed the highest in effectiveness. Among 8 kinds of combined steel wires with screen meshes showed the highest in effectiveness.

The unsteady stagnation point flow of the Walters B′ fluid is examined and solutions are obtained. It is assumed that the infinite plate at y=0 is oscillating and the fluid impinges obliquely on the plate.

This study focuses on developing reduced-order models for unsteady aerodynamic flows past a cascade of two-dimensional airfoils. A reduction method known as System Equivalent Reduction Expansion Process (SEREP) is used. The computational efficiency of the SEREP reduced-order model is compared with a reduced-order model formed using the Proper Orthogonal Decomposition (POD) technique. The present study shows that the SEREP is computationally more efficient than POD.

Heat transfer characteristics of a circular cylinder exposed to a slowly oscillating flow with zero-mean velocity were investigated. The flow oscillation amplitude and frequency were changed in the range where the flow remains laminar and fluid particle travels back and forth over much larger distance compared to the cylinder diameter. The time- and space-averaged Nusselt number was measured by transient method, while two-dimensional numerical simulation was conducted to discuss the instantaneous flow and thermal fields around the cylinder. It was found that the time- and space-averaged Nusselt number can be correlated with the oscillating Reynolds number and Richardson number. Unique heat transfer characteristics under oscillating flow condition can be seen at the phases when the cross-sectional mean velocity is small or increasing from small value. During such period, heat transfer can be enhanced due to the local fluid motion induced by the vortices around the cylinder, which once moved away but returned back by the reversed flow. This heat transfer enhancement, however, is countered by the local warming effect of the hot vortices clinging around the cylinder at such phases.

In the present work, for both a COSMOS-HP (Counter-Stream-Mode Oscillating-Flow Heat Pipe) and a dream pipe, the optimum conditions yielding the highest effective thermal conductivity and/or the highest operating coefficient are analyzed for oscillating flows of a given amplitude S. The parameters used in the optimization are the thermophysical properties of the operating liquid, the channel size and the frequency of oscillating flow. Based on the analytical results of the optimum conditions, both the optimum liquid and the optimum oscillating flow conditions are discussed. The highest effective thermal conductivity of COSMOS-HP is compared with that of a dream pipe, and it is found that the former is much higher than the latter.