Flow Field In Diffuser Research Articles

A Cationic Contamination in PEFC Cathode: A Cause and Effect Study

The cationic impurities that get introduced into the PEFC from the ambient air (e.g. roadside contaminants) and from the corrosion of stack and balance of plant components which are known to have a severe negative effect on the durability and performance during the long-term operation. The performance losses are originated from (a) a reduction in proton conductivity, and (b) a reduction of water retention (1). In addition, in our recent studies, it is found that the cationic contaminant also reduces oxygen concentration in the catalyst layer ionomer (2) and the salt precipitation at the gas diffusion layer and flow field channels that eventually block the reactant transport to the catalytic sites (3). In this study, the ex-situ soaking method for cationic contamination in 0.9 mM CaSO4, 29.1 mM H2SO4 and 1% (v/v) IPA for 100 h similar to our previous study (4) and in-situ injection of acidified water (1 mM H2SO4 and 1% v/v IPA) for performance recovery have been adopted. The galvanostatic performance data at 400 mAcm-2 for PEFC cathodes contaminated with and without CaSO4 solution are given in Figure 1. Cell #1 shows the performance of MEA that pre-conditioned for 24 h (Step I) and disassembled from the cell and exposed to CaSO4 solution through soaking test in 0.9 mM CaSO4, 29.1 mM H2SO4 for 100 h (Step II). Then, the cell was subjected galvanostatic polarization with deionized (DI) water injection around 24 h. Surprisingly, there is not much appreciable performance loss during this step (Step III), which could be possibility of hindrance in penetration of cationic solution to the catalyst layer due to hydrophobic nature of GDL . Hence, MEA subjected to second soaking with 0.9 mM CaSO4, 29.1 mM H2SO4 and 1% (v/v) IPA for 100 h (Step IV). Consequently, the cell is exposed to DI water injection for 100 h (Step V). It is found that a steep voltage loss has been observed after second soaking. The well distributed Ca2+ion in the membrane after the soaking cationic solution was gradually moved and accumulated to the cathode side, resulting in performance loss. The performance recovery test was carried out with injecting 1mM H2SO4 with 1% (v/v) of IPA for further 140 h (Step VI). As there was no considerable performance gain observed at step VI, the cell was disassembled and MEA was reprotonated in 1M H2SO4for 100h and a big jump in performance recovery was observed (step VII). In order to compare the effect of contaminant and recovery efforts, the cell data (Cell #1) was compared with performance of MEA without contamination (Cell # 2), during this step in order to maintain the same water level at cathode the DI was injected. The cell #2 has no remarkable performance loss up to 400 h of continuous operation, where as in the case of Cell #1, even after the recovery efforts also it did not matched with Cell #1 data. To analyze the performance loss of Cell #1 during long term operation, a cross-sectional Scanning Electron Microscopic images were taken and compared with Cell #1 (Figure 2). In this, it is observed that huge catalyst layer thickness loss (thinning) for Cell#1 in compared to Cell#2. These studies confirm the cathodic contaminant not only affects the proton conductivity, water transport and reactant transport, it also severely enhances the carbon corrosion rate at the catalyst layer. The further electrochemical, morphological and spectroscopic studies are in good agreement with our conclusions. Acknowledg e ments Financial support from the Department of Energy (DOE)-EERE, DE-EE0000467 (University of Hawaii, prime contractor, Jean St-Pierre, PI) is greatly acknowledged. References T. Okada, in Handbook of Fuel Cells– Fundamentals, Technology and Applications, W. Vielstich, H. A. Gasteiger, A. Lamm and H. Yokokawa, Editors, John Wiley & Sons, Ltd, New York (2010).M. A. Uddin and U. Pasaogullari, J. Electrochem. Soc., 61, F1081 (2014).X. Wang, J. Qi, O. Ozdemir, A. Uddin, U. Pasaogullari, L. Bonville, and T. Molter, J. Electrochem. Soc., 161, F1006 (2014).M. A. Uddin, J. Park, X. Wang, J. Qi, U. Pasaogullari, L. Bonville, and T. Molter, ECS Trans., 64 (3), 537 (2014). Figure 1

CFD investigation and PIV validation of flow field in a compact return diffuser under strong part-load conditions

The internal flow fields in a compact return diffuser under strong part-load conditions are investigated both numerically and experimentally. For numerical simulation, three-dimensional unsteady Reynolds-Averaged Navier-Stokes equations are solved on high-quality structured grids in conjunction with the shear stress transport k-ω turbulence model by employing the computational fluid dynamics (CFD) software ANSYS-Fluent 14.5. For flow field measurements, a special test rig is designed and the two-dimensional particle image velocimetry (PIV) measurements are conducted in the diffuser midplane to capture the complex flow field and for validation of the CFD results. The analysis of the results has been focused on the flow structure in the diffuser, especially under part-load conditions. The detailed comparison between CFD and PIV results is performed. Vortical flow and recirculation flow patterns in the diffuser are captured and analyzed. Large flow separation and backflow appear under the part-load flow conditions. This paper provides a good data set for developing as well as evaluating the accuracy of various CFD models for capturing the complex flow field in a compact return diffuser used with multistage pumps.

Dynamic Operation of HT-PEFC: In-Operando Imaging of Phosphoric Acid Profiles and (Re)distribution

Synchrotron based X-ray tomographic microscopy (XTM) is used for imaging and quantifying the redistribution of phosphoric acid (PA) in high temperature polymer electrolyte fuel cells (HT-PEFC) in-operando. The main focus of this work is the redistribution of phosphoric acid under dynamic load conditions. Therefore, two different load cycling protocols were applied and the transient redistribution within the fuel cell components was imaged. XTM, for the first time, revealed that the examined PBI based membrane system exhibits extensive electrolyte migration from cathode to anode under high current density operation. PA flooding of anode gas diffusion layer (GDL) and flow field channels occurred. Implications for technical applications and fuel cell degradation are discussed. Quantification of the migrated electrolyte is made by correlating in-operando grayscale values to ex-situ reference samples.

Particle Image Velocimetry Measurements and Performance Experiments in a Compact Return Diffuser Under Different Rotating Speed

Variable speed is the most efficient way to increase the pumping system efficiency and expand the pump application range. The detailed internal flow field in a compact return diffuser is investigated experimentally under different rotating speed. A special test rig is designed and the two-dimensional (2D) particle image velocimetry (PIV) measurements are conducted in the diffuser midplane to capture the complex flow field. The analysis of the results has been focused on the flow structure in the diffuser, especially under part-loading conditions. The pump head and design flow rate under two rotating speed is compared and analyzed. The detailed comparison between different rotating speed is performed. The strong vortical flow and recirculation flow patterns in the diffuser are captured and analyzed. Large flow separation and back flow appear under the part-loading flow conditions, and the position of the vortex core is compared. This paper provides a good data set for understanding the complex flow field in a compact return diffuser used with multi-stage pumps.

PIV measurements in a compact return diffuser under multi-conditions

Due to the complex three-dimensional geometries of impellers and diffusers, their design is a delicate and difficult task. Slight change could lead to significant changes in hydraulic performance and internal flow structure. Conversely, the grasp of the pump's internal flow pattern could benefit from pump design improvement. The internal flow fields in a compact return diffuser have been investigated experimentally under multi-conditions. A special Particle Image Velocimetry (PIV) test rig is designed, and the two-dimensional PIV measurements are successfully conducted in the diffuser mid-plane to capture the complex flow patterns. The analysis of the obtained results has been focused on the flow structure in diffuser, especially under part-load conditions. The vortex and recirculation flow patterns in diffuser are captured and analysed accordingly. Strong flow separation and back flow appeared at the part-load flow rates. Under the design and over-load conditions, the flow fields in diffuser are uniform, and the flow separation and back flow appear at the part-load flow rates, strong back flow is captured at one diffuser passage under 0.2Qdes.

Experimental study of centrifugal compressor tip clearance and vaneless diffuser flow fields

Three vaneless diffuser designs, varying in diffuser width, for centrifugal compressor were studied experimentally. Along with the diffuser width, the tip clearance was altered. The compressor overall performance and diffuser flow fields were studied for each of the three diffusers at four different tip clearances. For the diffuser flow fields, the total pressures were measured with probes traversed over the diffuser width both at the diffuser inlet and outlet. Along with the total pressures, the static pressures were measured adjacent to the probes. This enabled the axial flow angle and velocity distributions to be studied. The pinches tested improved the stage efficiency mainly by suppressing the secondary flow region present near the shroud at the impeller outlet. This leads to a lower strain rate, resulting in lower losses. The efficiency decrease due to the increased tip clearance was similar with and without pinch present. This indicates that in the diffuser, the main source of the tip clearance associated losses is the tip jet, and the tip clearance vortices mix out already in the impeller.

초음속 디퓨져 내부 역압력 구배에 대한 수치적 연구

초음속 디퓨져에서 천이구간에 대해서 수치적 기법에 의한 분석을 수행하였다. 수치기법으로는 초음속 디퓨져의 내부유동해석을 위하여 2차원 축대칭 Navier-Stokes equation와 <TEX>$k-{\epsilon}$</TEX> 난류모델을 사용하였으며, 로켓엔진 연소실의 천이 구간의 압력변화에 따라서 디퓨져 내부의 마하수 및 진공챔버의 온도 분포를 비교 검토하였다. 초음속 디퓨져의 작동과정에서 진공챔버 내부에 연소가스가 유입되어지고 이러한 현상에 따라서 진공챔버 내부의 압력 및 온도가 상승하는 결과를 확인하였다. 이러한 유동현상에 따라서 천이과정에서 압력 및 온도 상승을 방지하는 시스템이 필요하다. A study is analyzed on the adverse pressure gradient and the transient regime of supersonic diffuser with Computational Fluid Dynamic. The flow field of supersonic diffuser is calculated using Axisymmetric two-dimensional Navier-Stokes equation with <TEX>$k-{\epsilon}$</TEX> turbulence model. The transient simulation is compared in terms of mach number and static temperature of vacuum chamber according to pressure variation of rocket engine combustion chamber. Combustion gas flow into the vacuum chamber during operation of the supersonic diffuser. According to this phenomenon, the pressure and the temperature rise in the vacuum chamber were observed. Thus, the protection system will be necessary to prevent the pressure and temperature rise in the transition process during operation of the subsonic diffuser.

J055032 遷音速遠心圧縮機の羽根なしディフューザにおける非定常流れ構造(〔J055-03〕流体機械の研究開発におけるEFD/CFD(3))

Transonic centrifugal compressors for automotive turbochargers operate from near surge limit to choke limit. In order to improve the performance, it is indispensable to understand unsteady flow phenomena in the centrifugal compressors at off-design point. In this study, the flow fields in the vaneless diffuser of a transonic centrifugal compressor at off-design points have been investigated by DES(Detached Eddy Simulation). The numerical simulation results show that the jet-wake structure discharged from the impeller becomes larger with a decrease in the flow rate. At the disturbance onset point, the jet-wake structure flows into the scroll, so that non-uniform pressure distribution appears in the circumferential direction in the scroll. The high-pressure region in the scroll rotates in the impeller rotational direction with a speed close to the impeller rotating speed. At the developed disturbance point, the high-pressure region in the scroll causes steep adverse pressure gradient in the diffuser.

Numerical modeling on air quality in an urban environment with changes of the aspect ratio and wind direction

Due to heavy traffic emissions within an urban environment, air quality during the last decade becomes worse year by year and hazard to public health. In the present work, numerical modeling of flow and dispersion of gaseous emissions from vehicle exhaust in a street canyon were investigated under changes of the aspect ratio and wind direction. The three-dimensional flow and dispersion of gaseous pollutants were modeled using a computational fluid dynamics (CFD) model which was numerically solved using Reynolds-averaged Navier-Stokes (RANS) equations. The diffusion flow field in the atmospheric boundary layer within the street canyon was studied for different aspect ratios (W/H=1/2, 3/4, and 1) and wind directions (θ=90°, 112.5°, 135°, and 157.5°). The numerical models were validated against wind tunnel results to optimize the turbulence model. The numerical results agreed well with the wind tunnel results. The simulation demonstrated that the minimum concentration at the human respiration height within the street canyon was on the windward side for aspect ratios W/H=1/2 and 1 and wind directions θ=112.5°, 135°, and 157.5°. The pollutant concentration level decreases as the wind direction and aspect ratio increase. The wind velocity and turbulence intensity increase as the aspect ratio and wind direction increase.

Numerical and experimental analysis of instability phenomena in pump turbines

Today, utilities operating pump turbines require fast and frequent changes between pumping and generating modes as well as extended operation at off-design conditions. Operation of the units in unstable areas of the machine characteristic is not acceptable and may lead to self-excited vibration of the hydraulic system. Two main features of unstable behaviour of pump turbines are known. One sometimes occurs in generating mode at low load off-design operation close to runaway conditions (S-shape of the turbine characteristic) and the other one shows in pump operation as a drop in head as the flow is reduced (saddle-type pump instability of head curve). If the stability characteristics of a pump turbine need to be known already at the design stage of the runner, numerical flow simulation (computational fluid dynamics, CFD) is the most promising tool. As the characteristics of the flow near the stability limit are highly unsteady, steady state CFD, as it is usually applied for the runner design, does not deliver the necessary insight into the flow field. In order to analyse the flow field in runner and diffuser of a pump turbine in the unstable areas of operation, ANDRITZ HYDRO has evaluated several CFD procedures with different approaches for the calculation of the described instabilities in pump and turbine operation. The results of the unsteady flow calculation are compared with model test results.

Numerical Simulation on the Inner Flow Field of Ventilator Diffuser and Research on Energy-Saving

Ventilator diffuser with appropriate structure can improve the character of the diffuser . Building models of the log spiral diffusers of ventilator were classified and combined according to the length of post catheter, the condition whether the entrance bucket has been installed or not, and the length of the entrance bucket. Based on EFA software for hydrokinetics, the inner flow field of all combined diffusers were simulated and the related data were analyzed. In this way, graph of the static pressure coefficient (Cp), the atmospheric pressure coefficient (Cpa) and the diffuser efficiency (η) will be concluded under the work condition of every combination. By numerical analysis， an issue that the length of postpositive catheter, the condition whether the entrance bucket has been installed or not, and the length of the entrance bucket influent performance of the diffuser will be concluded. The issue studied would be a helpful reference for design to the log spiral diffuser of ventilator and for research on energy -saving.

Experimental simulation of air quality in street canyon under changes of building orientation and aspect ratio

To assist validation of numerical simulations of urban pollution, air quality in a street canyon was investigated using a wind tunnel as a research tool under neutral atmospheric conditions. We used tracer gas techniques from a line source without buoyancy. Ethylene (C(2)H(4)) was used as the tracer gas. The street canyon model was formed of six parallel building rows of the same length. The flow and dispersion field was analyzed and measured using a hot-wire anemometer with split fiber probe and fast flame ionization detector. The diffusion flow field in the boundary layer within the street canyon was examined at different locations, with varying building orientations (θ=90°, 112.5°, 135° and 157.5°) and street canyon aspect ratios (W/H=1/2, 3/4 and 1) downwind of the leeward side of the street canyon model. Results show that velocity increases with aspect ratio, and with θ>90°. Pollutant concentration increases as aspect ratio decreases. This concentration decreases exponentially in the vertical direction, and decreases as θ increases from 90°. Measured pollutant concentration distributions indicate that variability of building orientation and aspect ratio in the street canyon are important for estimating air quality in the canyon. The data presented here can be used as a comprehensive database for validation of numerical models.

Low Field in the Vaned Diffuser of a Centrifugal Pump at Different Vane Setting Angles

Particle image velocimetry (2D-PIV) has been used for the investigation of the time and phase-averaged flow field inside the vaned diffuser of a centrifugal pump operating at the best efficiency point (BEP). Pump performances and the diffuser flow field were analyzed for three different vane setting angles. It was evidenced that flow coefficient at the best efficiency point decreases with the diffuser setting angle reduction. From 2D-PIV measurements in five different blade to blade planes, spanwise averaged flow angles and velocity distribution were computed. It was evidenced an influence of the diffuser setting angle on the spanwise distribution of the flow rate and of the absolute mass averaged flow angle at impeller discharge. An overturning at the diffuser discharge was also evidenced for the two highest setting angles. 2D-PIV flow fields are presented in three different pseudosecondary planes in order to enlighten the cross flows affecting the diffuser flow field.

Experimental study of the impact of structural geometry and wind direction on vehicle emissions in urban environment

Vehicle emissions inside an urban environment are investigated using a wind-tunnel under neutral atmospheric conditions. The urban environment was formed as street canyon model. The diffusion flow field in the boundary layer inside the street canyon was examined at different locations of varying geometry of the street and wind directions in the downwind distance of the leeward side of the street canyon model. The results show that the vertical velocity increases as the aspect ratio increases and with wind direction increases from θ = 90°. The pollutant concentration increases as the aspect ratio decreases. The pollutant concentration decreases as the wind direction increases from θ = 90°. The pollutant concentration distributions indicate that the variability of the structure, geometry and wind direction inside the street canyon are important parameters for estimating air quality in the urban street canyon.

2차원 원심 압축기의 깃 없는 디퓨저에서의 불안정 유동

This study investigated on details of flow characteristics in a vaneless diffuser of a compressor with 2-dimensional impeller at various flow rates. Experiment for a low speed compressor model in a water reservoir was performed to analyze the flow field in the vaneless diffuser and volute casing, which was done by PIV measurement. It was also focused on the periodic flow patterns occurring at low flow rate near unstable operating region of the compressor. At low flow rate condition, the flow visualization clearly shows that the flow energy from impeller is highly accumulated at the compressor exit by the blockage effect of a flow damper and consequently the reverse flow occurs in the diffuser.

Synchrotron X-ray tomography for investigations of water distribution in polymer electrolyte membrane fuel cells

Synchrotron X-ray tomography is used to visualize the water distribution in gas diffusion layers (GDL) and flow field channels of a polymer electrolyte membrane fuel cell (PEMFC) subsequent to operation. An experimental setup with a high spatial resolution of down to 10 μm is applied to investigate fundamental aspects of liquid water formations in the GDL substrate as well as the formation of water agglomerates in the flow field channels. Detailed analyses of water distribution regarding the GDL depth profile and the dependence of current density on the water amount in the GDL substrate are addressed. Visualizations of water droplets and wetting layer formations in the flow field channels are shown. The three-dimensional insight by means of this quasi in situ tomography allows for a better understanding of PEMFC water management at steady state operation conditions. The effect of membrane swelling as function of current density is pointed out. Results can serve as an essential input to create and verify flow field simulation outputs and single-phase models.

An experimental investigation of the flow field between two radial plates

The flow field in a radial diffuser downstream of a radial impeller is highly complex, as the flow is turbulent, unsteady, viscous, and three-dimensional. Depending on the initial conditions at the diffuser inlet, the flow may separate from one or both walls of the diffuser. The objective of this experimental investigation of flow characteristics in a radial vaneless diffuser of a centrifugal blower is to enhance the performance of centrifugal compressors over a wide operating range. Numerical and experimental investigations of rotating stall effects on compressors and blowers have been conducted over the past four decades to achieve this objective. The flow characteristics in a radial diffuser were measured while changing the impeller speed and keeping other parameters such as diffuser radius and width ratio constant. The velocity and pressure distributions in the diffuser were measured along a radial path using a miniature X-wire probe and dynamic as well as static pressure transducers. The axisymmetrical radial and tangential mean velocity distributions and their statistics, as well as pressure distributions, are reported. Some of the results presented in this paper agree with those from earlier work on this subject. For example, the flow angle α (measured at half the diffuser width and from the diffuser inlet to its outlet) did not exceed 65°. This means that α < αc, although a recirculation region was present on the diffuser shroud wall. The critical flow angle αc required to initiate self-excited flow oscillations has been generally reported to be about 78°. Current data also indicate that the flow exiting the impeller is skewed, as revealed by the triple velocity product correlations. The current data are valuable for vaneless diffuser numerical model developers, since there are very limited similar data available in the open literature. The current data support the opinion that the flow separation is not enough to trigger or initiate stall at the diffuser.

Particle Image Velocity Investigation of a High Speed Centrifugal Compressor Diffuser: Spanwise and Loading Variations

An efficient diffuser is essential to a modern compressor stage due to its significance in stage performance, durability, and operability. To address the need for data that describe the complex, unsteady flow field in a vaned diffuser, particle image velocity is utilized to characterize the spanwise and circumferential variations in the flow features in the vaned diffuser passage of a transonic centrifugal compressor. The spanwise variation in the diffuser flow field is further investigated by the comparison of three different operating conditions representative of low, nominal, and high loading. These data demonstrate that not only the diffuser flow field is highly dependent on the operation conditions, e.g., hub-to-shroud variation increases with loading, but also the circumferential periodicity, created by the highly three dimensional impeller discharge flow, generates a larger unsteadiness toward the hub region of the vaned diffuser.

Analysis of the Rotor-Stator Interaction in a Radial Flow Pump

The paper presents the analysis of the interactions between the impeller and the vaned diffuser of a radial flow pump. Experimental data were obtained within one blade passage of a vaned diffuser using the 2D/2C PIV technique. The diffuser flow field was analyzed in various measuring planes in the hub to shroud direction, for seven relative impeller positions in the diffuser frame and in four different operating conditions. A post-processing procedure, based on statistical tools, was applied to the experimental results so as to validate their meaningfulness. The experimental results werecompared to numerical data obtained with the help of CFX computer code.

Experimental and numerical investigation of separated flows in subsonic diffusers

The results of a special investigation of the diffuser flowfield are presented for two models of curvilinear diffuser channels with annular and rectangular cross-sections. The flow is visualized and the total pressure fields are measured by means of low-inertia transducers. At the same time, the flows are numerically calculated using commercial programs, together with codes developed by the authors. In these calculations the stationary and time-dependent Reynolds equations closed by different turbulence models, as well as the time-dependent Navier-Stokes equations, were integrated. A considerable difference between the measured data and the results of the numerical calculations in the stationary formulation is found to exist. At the same time, it has been possible to describe the occurrence of spatial inhomogeneities, the flow pattern, and the level of the experimentally observed aerodynamic losses on the basis of the solution of time-dependent problems.