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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.

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.

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.

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.

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.

The entropy generation in a plane turbulent jet is revisited. This flow is characterized by quasi-periodic lateral oscillations, documented in the literature, due to the instability of the flow. Based on the laws of Thermodynamics, an analysis of the entropy generation has been presented by Bejan [Entropy Generation Minimization. The Method of Thermodynamic Optimization of Finite-Size Systems and Finite-Time Processes, Wiley, New York, 1996, p. 61]. In this paper, a term has been added that takes into account the experimentally observed oscillations. The results are compared for the cases with oscillations and without oscillations.

The fluid mechanics and heat transfer of zero-mean oscillatory flows at low frequencies and large amplitudes in essentially two-dimensional short channels are studied numerically. The results show that intracycle oscillations do also occur, and that such oscillations are different from those for long ducts. In the case of sudden pressure changes as the flow reverses at the ends, the intracycle oscillations are found to occur in the acceleration phase of the oscillatory cycle and are responsible for the heat transfer enhancement. These results compare favorably with previously obtained experimental data.

Double-gated, modulated-pressure moulding provides orientation of molecules or fibres so that the properties of materials can be controlled during processing. A numerical method for estimating the duration of oscillating flow under a preset pressure amplitude based on the balance of viscous heating and loss to the mould wall is described. This first attempt to quantify an extremely complex process allows the effects of moulding parameters on solidification to be deduced and the experimental array reduced. The predictions are compared with experimental results for high and low thermal diffusivity materials.

Resume On presente une etude experimentale des configurations d'ecoulement et du transfert de chaleur et de masse autour du cylindre, pour un ecoulement incident permanent et pulse. On propose un modele et on met en evidence des zones particulierement instables du point de vue dynamique, a partir de mesures locales et instantanees du gradient parietal de vitesse (module et signe). L'effet des pulsations sur le transfert est examine pour des frequences de 1'ordre de grandeur de celle de Strouhal et des taux allant jusqu'a 40 pour cent. Celui de la turbulence est mis en evidence. On montre que l'epaisseur de la zone de convection a une influence considerable sur l'effet de ces perturbations: turbulence a l'amont, pulsations dans le sillage tourbillonnaire. En particulier, les pulsations qui desorganisent le sillage et minimisent la recirculation ont un effet benefique beaucoup plus net sur le transfert thermique que sur le transfert massique.

Etude de l'influence de la dissipation visqueuse, de la presence d'harmoniques autre que le fondamental, de la largeur du canal et de l'epaisseur des parois