bluuebunny/crossref_metadata_2025 huggingface dataset:
Modification of Turbulence in Boundary Layers by Mean and Fluctuating Pressure Gradients
Authors: Joshi Pranav , Liu Xiaofeng , Katz Joseph
Published: July 2013

In this study we focus on the effect of mean and fluctuating pressure gradients on the structure of boundary layer turbulence. Two dimensional, time-resolved PIV measurements have been performed upstream of and inside an accelerating sink flow for inlet Reynolds number of Reθ = 3071, and acceleration parameter of K=1.1×10−6. The time-resolved data enables us to calculate the planer projection of pressure gradient by integrating the in-plane components of the material acceleration of the fluid (neglecting out-of-plane contribution). We use it to study the effect of boundary layer scale fluctuating pressure gradients ∂p′~/∂x, which are expected to be mostly two-dimensional, on the flow structure. Due to the imposed mean favorable pressure gradient (FPG) within the sink flow, the Reynolds stresses normalized by the local freestream velocity decrease over the entire boundary layer. However, when scaled by the inlet freestream velocity, the stresses increase close to the wall and decrease in the outer part of the boundary layer. This trend is caused by the confinement of the newly generated vortical structures in the near-wall region of the accelerating flow due to combined effects of downward mean flow, and stretching by velocity gradients.

Within both the zero pressure gradient (ZPG) and FPG boundary layers, sweeping motions mostly occur during positive fluctuating pressure gradients ∂p′~/∂x>0 as the fluid moving towards the wall is decelerated by the presence of the wall. Vorticity is depleted in the near-wall region, as the wall absorbs −ω′ from the flow by viscous diffusion. On the other hand, ejections occur mostly during periods of favorable fluctuating pressure gradients ∂p′~/∂x<0. During these periods, there is more viscous flux of vorticity −ω′ into the flow, since ∂−ω′/∂y<0 at the wall. Large scale ejection motions associated with ∂p′~/∂x<0 are more likely to transport smaller scale turbulence to the outer region of the boundary layer, while turbulence remains largely confined close to the wall due to the sweeping motions accompanying ∂p′~/∂x>0. During periods of ∂p′~/∂x>0 in the ZPG boundary layer, sweeps tend to increase the momentum in the near-wall region, whereas the adverse pressure gradient decelerates the fluid. These competing effects result in an unstable ω′<0 shear layer which rolls up into coherent vortical structures and increases ω′ω′ near the wall as compared to periods of ∂p′~/∂x<0. Due to the strong mean acceleration of the flow and weaker sweeps in the FPG boundary layer, the formation of an unstable shear layer, and hence vortical structures, is suppressed, decreasing the enstrophy close to the wall as compared to periods of ∂p′~/∂x<0.
Impact of Flexibility on the Aerodynamics of an Aspect Ratio Two Membrane Wing
Authors: Gordnier Raymond E. , Attar Peter J.
Published: July 2013

Development of an aeroelastic solver with application to flexible membrane wings for micro air vehicles is presented. A high-order (up to 6th order) Navier-Stokes solver is coupled with a geometrically nonlinear p-version Reissner-Mindlin finite element plate model to simulate the highly flexible elastic membrane. An implicit LES approach is employed to compute the mixed laminar/transitional/turbulent flowfields present for the low Reynolds number flows associated with micro air vehicles. Computations are performed for an aspect ratio two membrane wing at angles of attack α = 10°, 16° and 23° for a Reynolds number, Re = 24,300. Comparisons of the computational results with experimental PIV and surface deflection measurements demonstrated reasonable agreement. Reduced separation and enhanced lift are obtained due to favorable interactions between the flexible membrane wing and the unsteady flow over the wing. The impact of flexibility on the aerodynamic performance comes primarily from the development of mean camber with some further effects arising from the interaction between the dynamic motion of the membrane and the unsteady flowfield above. At lower angles of attack this lift enhancement comes at the cost of reduced L/D. The nose-down pitching moment increases with flexibility at the lowest angle of attack but is reduced for the higher two angles of attack. These results suggest that membrane flexibility might provide a means to reduce the impact of strong gust encounter by maintaining lift and reducing the effect of the gust on pitching moment.
Flow Experiments in Microjunction Networks
Authors: Henderson Andrew W. , Yasar Ozlem , Muvadgah Ane Joan , Lemley Evan C.
Published: July 2013

Laminar flow dominates microscale applications in devices such as microvalves, pumps, and turbines and in biomedical applications such as stents and biological flows. Studies of pressure losses in junctions have mostly been focused on turbulent flow conditions that exist in larger scale piping systems. There is a need for laminar flow studies of energy losses in junctions and networks of junctions so that engineers can better predict, design, and analyze flow in microscale systems. Unlike in the turbulent regime, Reynolds number plays a dominant role in energy losses for laminar flow. Studies focused on Reynolds number dependence are needed.

This paper documents laminar flow experiments in networks constructed from microscale junctions (microjunctions). This work builds on previous experience of the authors in computational fluids dynamics simulations of junctions and networks of junctions. The planar junctions studied consist of circular tubes with two outlets and one inlet. A general technique has been developed to produce computer and physical models of junctions in which the inlet tube size is set, but the outlets are allowed to vary in size and angle relative to the inlet tube. A generalized algorithm has been implemented to create three-dimensional models of the junctions for both computational and experimental studies. There have been several techniques used to create the experimental microjunction networks: a “lost wax” method that creates the network in Polydimethylsiloxane (PDMS), stereolithography-based junctions connected by tubing, and photopolymer cured by ultraviolet radiation. Results are presented for pressure losses in various microjunction networks as a function of Reynolds number.
Modeling of Falling Film in Vertical Downward Two-Phase Pipe Flow
Authors: Lopez Jose M. , Mohan Ram , Shoham Ovadia , Gomez Luis , Kouba Gene
Published: July 2013

Falling liquid films in vertical pipes are found in a variety of different industrial applications and industrial equipment, such as downcomers, caisson separators and reactors. The hydrodynamics of the falling film in vertical two-phase pipe flow can affect droplet entrainment, gas entrainment, and pressure drop. Therefore, a mechanistic model for prediction of falling liquid film thickness, falling liquid film velocity and a correlation for liquid droplet entrainment fraction in vertical downward liquid-gas systems has been proposed.

The falling film model developed is based on applying momentum balance on the liquid film. The liquid film is assumed to be in steady-state, incompressible and free of entrained gas. The mechanistic model includes both the developing and the developed regions. The shear effect between the gas core and the falling liquid film is considered. The liquid droplet entrainment fraction traveling in the gas core is considered and a new correlation for its prediction is proposed. Detailed uncertainty analysis is performed for liquid film thickness and liquid film velocity model predictions, including Monte Carlo simulation. Predicted liquid film thickness, liquid film velocity and liquid droplet entrainment fraction are validated against experimental data for different liquid fluid properties, such as water, Conosol mineral oil (light oil) and Drake mineral oil (heavy oil).
Phase-Locked PIV Measurement in the Wake of Model Wind Turbines Under Various Inflow Conditions
Authors: Green David J. , Chamorro Leonardo P. , Arndt Roger E. , Sotiropoulos Fotis , Sheng Jian
Published: July 2013

This paper focuses on understanding correlative interactions between boundary layer flow structures and the resultant unsteady wake of a Horizontal Axis Wind Turbine (HAWT) model. Phase-locked Particle Image Velocimetry (PIV) is employed to measure turbulence statistics such as velocity, turbulence intensity, shear stress, vorticity, and to subsequently identify large-scale coherent flow structures. In the first stage, phase-lock experiments were performed under free-stream flow conditions. Ten consecutive downstream locations up to six rotor diameters from the turbine are captured. Ensemble averaged velocity and vorticity fields reveal that while the identity of tip vortices are maintained over five rotor diameters downstream of the turbine, their strength decays exponentially. When the turbine is placed in the wake of other units, the vortical structures exhibit a rapid decay in both coherence and strength and substantially suppress the wake-vortex and vortex-vortex interactions, playing an important role in the wake recovery. These observations inspire the current investigation using low-speed phase-locked PIV Interactions among the near wall flow structures in a turbulent boundary layer, hub and tip vortices will be investigated in this paper. The model turbine has a 0.108 m hub height, rotor diameter of 0.128 m and tip speed ratio of 4. It is located in a wind tunnel under nearly zero-pressure-gradient and thermally neutrally stratified conditions. A tripped turbulent boundary layer generated by a picket fence located at the inlet has a boundary layer thickness, δ, of 0.55∼0.6 m. Measurements are performed at Re = 3×105, 4×105, and 12 × 105.. To achieve sufficient spatial resolution, two measurement fields are taken at each stream-wise location to cover upper and lower half of the turbines. Measurements locations extend ten diameters downstream. Robust turbulence statistics, such as velocity fluctuations, Reynolds stresses, full budget of turbulent kinetic energy, are computed from large dataset, totaling 400 GBytes.
A Continuity Outlet Boundary Condition for the Lattice Boltzmann Method
Authors: Vural Yasemin , Pakalapati Suryanarayana R. , Celik Ismail B.
Published: July 2013

A continuity outlet boundary condition for the Lattice Boltzmann Method (LBM) is proposed based on the assurance of the mass conservation of the system. The main advantage of the proposed boundary condition over the conventional Computational Fluid Dynamics (CFD) techniques is that the macroscopic properties, e.g. velocity, pressure etc. are not needed to be prescribed at the outlet, these properties are automatically calculated with the imposed boundary condition. This is especially useful in practice where the macroscopic properties at the outlet are difficult or impossible to be measured and described as in the biological flows. In order to test the feasibility of the proposed method, the LBM simulations are first verified for its capability to simulate flow in a symmetrically bifurcated channel. Then asymmetrically bifurcated geometries representing the blood vessels have been designed with different bifurcation angles. The new boundary condition is also tested for multi-component LBM simulations. For these cases, LBM predictions have been compared with the predictions for the commercial CFD software, namely ANSYS FLUENT at different Reynolds numbers. The results show that there is a good agreement between the LBM and FLUENT predictions, and this proves the capability of the proposed boundary condition as a viable method that can be used in practice.
A New Particle Set of Contours Method for Advection-Diffusion Problems Applied to the Modelling of Waste Disposal
Authors: Beaugendre Héloise , Huberson Serge , Mortazavi Iraj
Published: July 2013

In this work a particle sets of contours method is coupled to a streamline technique in order to obtain accurate approximations of transport problems. A modified streamlines technique is proposed and several bench tests arising in porous media are then simulated to validate the new method. These results are complemented with the resolution of the diffusion operator in a non conservative form where the non-linear diffusion operator is transformed in a velocity-like term and solved using this new particle method. The method is then used to model transport in porous media applied to the waste disposal.
Simulation of Gas – Non-Newtonian Liquid Flow in a Rectangular Bubble Column by Considering Bubbles Interactions
Authors: Niazi Erfan , Shams Mehrzad , Elahi Arash , Ahmadi Goodarz
Published: July 2013

In this article a CFD model of a three-dimensional Eulerian-Lagrangian is developed for a gas - non-Newtonian liquid flow in a rectangular column. The model resolves the time-dependent, three-dimensional motion of gas bubbles in a liquid to simulate the trajectory of bubbles. Our model incorporates drag, gravity, buoyancy, lift, pressure gradient and virtual mass forces acting on a bubble rising in a liquid, and accounts for two-way momentum coupling between the phases. Population balance equation is solved to model bubble coalescence and break up. In bubble coalescence, Prince and Blanch model is used which can consider the effect of fluid rheology. Luo and Svendosen model was selected for bubble break up. The standard k-e turbulence model is selected for calculating turbulent flow properties. Power-law non-Newtonian liquid is selected for analysis of effect of different solutions of carboxy methyl cellulose in water. The effect of changing fluid to non-Newtonian is discussed in terms of velocity profile and gas hold up.
Numerical Study of Liquid-Liquid Vertical Dispersed Flows
Authors: Adane Kofi Freeman K. , Shah Syed Imran A. , Sanders R. Sean
Published: July 2013

Numerical simulations of liquid-liquid dispersed flow in a vertical pipe (38mm) have been carried out using the two-fluid approach implemented in a commercial CFD code, ANSYS CFX. A dispersion of oil in water (where water is the continuous phase) was studied. Both fluids were considered as turbulent flows. The k-ε model was used for the continuous phase, with the eddy viscosity of the dispersed phase estimated from that of the continuous phase. A comparison of the present numerical results with previous experimental and numerical results in terms of volume fraction, mean velocity and turbulent kinetic energy is discussed. In general, good agreement between the simulation results and experimental measurements was observed.
CFD Study of Particle Deposition in Flows in Inclined Ducts
Authors: Majlesara Mohammad , Salmanzadeh Mazyar , Ahmadi Goodarz
Published: July 2013

Deposition of particles on the walls of exhaust ducts of the commercial kitchen hoods has been a concern in buildings maintenance. The slope of duct in its way to the outdoor can affect deposition of grease particles. In the present work, deposition of particles in an inclined channel with cold walls under turbulent flow regime was studied. The cases that the flow is downward and upward, and when the flow is warmer than the walls were analyzed. The V2F turbulence model was used in the computational model so that the intensity of the turbulence velocity fluctuations normal to the wall can be properly evaluated. Accurate evaluation of the airflow and velocity fluctuations close to the walls was required for evaluation of particle deposition rate. The instantaneous velocity field was generated by adding the velocity fluctuations, obtained by using the Kraichnan model to the mean flow field. The Stokes drag, lift, Brownian, gravity and thermophoretic forces were included in the particle equation of motion. Effects of temperature gradients along with turbulence fluctuations on the deposition rate of different size particles were studied. The simulation results were compared with the empirical equation and discusses.
Unsteady Isothermal Flow Through a Staggered Tube Bundle Array
Authors: Ridluan Artit , Arayangkun Surasing , Phayom Coochart
Published: July 2013

Two-dimensional Unsteady simulations of isothermal flow through a staggered tube bundle array at three different Reynolds numbers 54, 72, and 90 were investigated. The Navier-Stokes equations are numerically solved. Based on the CFD simulation results, the unsteady flow patterns were developed behind the rear row of the array, while for the other rows, the steady separated and reattached flow behaviors were observed, small, short, and closed separation bubble behind the rods. At Reynolds Number of 54, the transient flow was perfectly periodic. The complicated patterns of unsteady flow could be observed at Reynolds numbers of 72 and 90. The shedding patterns of vortices from the last rods were different and no longer periodic as found at Reynolds number of 54. The degree of chaos is increased as Reynolds number progressed.
Flow Simulations in Packed Beds by a Coupled DEM and CFD Approach
Authors: Zhao Xiang , Montgomery Trent , Lu Ping , Zhang Sijun
Published: July 2013

This paper presents flow simulations in packed beds by a coupled discrete element method (DEM) and computational fluid dynamics (CFD) approach. The realistic packing structure in packed beds is generated by DEM. Then the packing structure is imported into the CFD preprocessor to generate a mesh for flow simulations in packed beds. The subsequent CFD simulations are carried out. The predicted results reveal that not only the local behavior but also macroscopic quantities like the pressure drop depend remarkably on the local packing structural parameters, which is unable to be taken into account when using correlations with averaged values.
A Bio-Inspired Aircraft Design Concept
Authors: Fisher Emily , Bauhofer Anton , Beauchene Christine , Dress Brian , Marshall Stephen , McCraw Cory , Mehrvarzi Christopher , Ng Colin , Simcic Jeff , Bayandor Javid , Kurdila Andrew
Published: July 2013

The goal of the research has been to design a robotic bird that utilizes principles of nature to optimize flight. Seagulls were the preferred model for the robot because they have a large wing span that provides a more steady flight and sheds a continuous wake vortex, creating lift on both the upstroke and down-stroke of flight [1]. Research has been done on the architecture of a seagull’s wing as well as the aerodynamic features of its comprising airfoils. The robotic wings developed will capture the architecture of the seagull wings with a variety of airfoils that improve lift and reduce drag and joints that enable bending on the upstroke in the flapping motion. A main focus of this research was to study how the seagull uses air flow to improve its flight performance. The fluid mechanics of the wing was analyzed for steady and unsteady flight using Fluent code to see how seagull adapts to different flow conditions. Using the developed robotic model of the wing attempts were made to achieve the necessary wing positioning that fully complied with that of the seagull during flight. Actuation of the wings was achieved using servo motors. Fabrication of the robotic prototype involved material selection for the fuselage, wing surface and skeletal structure. At completion of the prototyping, trials were performed using stereovision to study the complex effects of unsteady flow, and to verify the computational analyses undertaken.
Experimental Study on Drag-Reduction Effect of a New Sinusoidal Riblet
Authors: Sasamori Monami , Iwamoto Kaoru , Murata Akira
Published: July 2013

An experimental study of a new three-dimensional (3-D) riblet has been carried out. The lateral spacing of our 3-D riblet surface is sinusoidally varied in the streamwise direction (see Fig. 3). In the comparison of the optimal two-dimensional (2-D) blade riblet which shows 9.9% drag reduction rate [1], the riblet height, thickness and averaged lateral spacing are respectively 0.83, 5 and 2.5 times larger than those of the optimal 2-D riblet in wall units. The net drag reduction rate of 11.7% has been confirmed in a low-speed wind channel at the bulk Reynolds number of 3400. The flow structure over the 3-D riblet mounted a wall was also analyzed in the velocity field by using 2-D Particle Image Velocimetry and was compared with the corresponding flow over the flat surface in an attempt to identify the physical mechanisms for the drag reduction. The normal turbulent intensities on the present riblet are almost same as those of the flat surface, whereas the Reynolds shear stress is much decreased, and especially becomes negative near the riblet height. These are different phenomena from those of all the previous riblets [1–7].
Progress in the Modeling and Simulation of Flow Boiling Heat Transfer of Binary Fluids Using Advanced Multi-Fluid Modeling Techniques
Authors: Srinivasan Vedanth , Kopun Rok
Published: July 2013

In this paper, we discuss the implementation and testing of a novel boiling mass transfer model to simulate the thermal and phase transformation behavior, generated due to boiling of binary mixtures, using the commercial CFD code AVL FIRE® v2011. The phase change model, based on detailed bubble dynamics effects, is solved in conjunction with incompressible phasic momentum, turbulence and energy equations in a segregated fashion, to study the flow boiling process inside a rectangular duct. Full three dimensional validation studies including the effect of flow velocity and exit pressure conditions, acting on a wide range of operating wall (superheat) temperatures, clearly shows the suppression of heat and mass transfer coefficients with enhancement in flow convection. Competing mechanisms such as phase change process and turbulent convection are identified to influence the heat transfer characteristics. In particular, the varying influence of the mass transfer effects on the heat flux characteristics with alteration in wall temperature is well demonstrated. Comparisons of the predicted total heat flux, computed as the sum of the convection and phase change components, indicate a very good agreement with experimental data, wherever available. Description of the flow field inclusive of phasic fraction, temperature and velocity field provides extensive details of the multiphase behavior of the boiling flow. Some preliminary results on the phase change work flow to model heat transfer in cooling jackets, for automotive applications, is also discussed.
Application of Deterministic Correlations in the Analysis of Rotor-Stator Interactions in Axial Flow Fans
Authors: Galdo Vega Mónica , Rodriguez Lastra María , Argüelles Diaz Katia María , Fernández Oro Jesús Manuel
Published: July 2013

Deterministic stresses account for the time-averaged contribution of the periodic unsteadiness in single-stage turbomachinery. An accurate modelling of these stresses allows for the development of steady simulations with full contribution of deterministic unsteady sources. Moreover, introducing both relative and absolute reference frames, the deterministic tensor can be split into a set of correlations which addresses the influence of the stator and rotor non-uniformities separately. As a consequence, a pure unsteady term showing rotor-stator interaction is rediscovered and can be established as an essential indicator of the strength of viscous, non-linear effects in multistage turbomachinery environments.

This work provides a comprehensive description of these correlations in the interrow region of a single stage axial flow fan. Two configurations are analyzed here: the upstream stator acting as a non-uniform inflow for the rotor and the complementary stator placed downstream of the rotor for pressure recovery at the exit. To illustrate the driven mechanisms in the tensor, a three-dimensional numerical simulation at the midspan section of a low-speed axial fan is employed in the present investigation. A deep analysis of the interaction between the rows using the deterministic framework is shown as a useful tool to devise deterministic models for steady computations.

The commercial code FLUENT is used to resolve the full unsteady 3-D Navier-Stokes equations using LES schemes. Nominal and off-design flow conditions have been considered to observe the relevance of the different deterministic correlations in the establishment of the unsteady sources.

The final objective pursues a deep understanding of the correlations behavior under variable flow conditions and different stage configurations. Thus, physical insight will be gained and more efficient and reliable deterministic models could be proposed, available for researchers and experts in the field.
Multiphase Flow Concentration Characterization in Slurries During Pulse Jet Mixing
Authors: Bamberger Judith Ann
Published: July 2013

Obtaining real-time, in situ slurry concentration measurements during unsteady mixing can provide increased understanding into mixer performance. During tests of an operating pulse jet mixing system, an ultrasonic attenuation sensor was inserted into a mixing vessel to measure the slurry concentration during unsteady mixing in real time. Pulse jet mixing tests to suspend noncohesive solids in Newtonian liquid were conducted at three geometric scales. To understand the solids suspension process and resulting solids distribution, the concentration of solids in the cloud was measured at various elevations and radial positions during the pulse jet mixer cycle. This paper presents transient concentration data obtained at three scales at the tank center to provide insight into pulse jet mixer performance.
Free Vortex Turbulent Swirling Flow Reconstruction Using a Neural Network
Authors: Ahmed Saad A. , Raghavan Bharath V.
Published: July 2013

Experimental data of the continuous evolution of fluid flow characteristics in a dump combustor is very useful and essential for better and optimum designs of gas turbine combustors and ramjet engines. Unfortunately, experimental techniques such as 2D LDV measurements provide only limited discrete information at given points; especially, for the cases of complex flows such as free vortex dump combustor swirling flows. For this type of flows, usual numerical interpolating schemes appear to be unsuitable. Recently, neural networks have emerged as viable means of expanding a finite data set of experimental measurements to enhance better understanding of a particular complex phenomenon. This study showed that artificial neural networks are suitable for the prediction of free vortex turbulent swirling flow characteristics in a model dump combustor. These techniques are proposed for optimum designs of dump combustors and ramjet engines.
Estimation of Uncertainty Bounds From Cross Correlation Peak Ratio for Individual PIV Measurements
Authors: Charonko John J. , Vlachos Pavlos P.
Published: July 2013

Numerous studies have established firmly that particle image velocimetry (PIV) is a robust method for non-invasive, quantitative measurements of fluid velocity, and that when carefully conducted, typical measurements can accurately detect displacements in digital images with a resolution well below a single pixel (in some cases well below a hundredth of a pixel). However, previously these estimates have only been able to provide guidance on the expected error for an average measurement under specific image quality and flow conditions. This paper demonstrates a new method for estimating the uncertainty bounds to within a given confidence interval for a specific, individual measurement. Here, the ratio of primary to secondary peak heights in a phase-only generalized cross-correlation is shown to correlate strongly with the range of observed error values for a given measurement, regardless of flow condition or image quality. Using an analytical model of the relationship derived from synthetic data sets, the uncertainty bounds at a 95% confidence interval are then computed for several artificial and experimental flow fields, and the resulting errors are shown to match closely to the predicted uncertainties. While this method stops short of being able to predict the true error for a given measurement, knowledge of the uncertainty level for a PIV experiment should provide great benefits when applying the results of PIV analysis to engineering design studies and CFD (computational fluid dynamics) validation efforts.
Optimization of the Porosity Distribution in Transonic Wind Tunnel
Authors: Marino Antonello , Bonfiglioli Aldo
Published: July 2013

During past years, to improve the quality of wind tunnel data in transonic configurations, researchers first designed new wind tunnel geometries (as porous and/or slotted wind tunnels), then developed more accurate correction laws giving acceptable results in certain conditions but absolutely not sufficient to satisfy the increasing aeronautical requirements. Recent studies showed that the quality of wind tunnel data can be improved by using test sections provided with variable streamwise porosity distributions instead of the typical uniform ones. Some authors identified this new concept of variable porosity distribution as the third generation of porous wind tunnels.

In order to improve knowledge about effects of the porosity distribution on the wall interference in subsonic/transonic conditions, an experimental investigation was carried out in the PT-1 CIRA transonic wind tunnel in the Mach range between 0.3 and 0.9 (over 400 test points were measured on different models and wall porosity configurations). At this aim, a dedicated experimental setup consisting in five plates positioned on the top and bottom walls of the PT-1 porous test section, has been designed and realized. Setting independently each plate, it is possible to obtain practically unlimited combinations of porosity distributions along the streamwise direction.

The final purpose of the present activity was to evaluate the optimal porosity distribution able to minimize wind tunnel wall interferences in the considered Mach range. The huge number of factors (Mach number and the positions of the five plates setting the porosity distribution) made practically impossible to study the porosity distribution effects by using a traditional One Factor At a Time (OFAT) approach. Therefore, the optimum porosity distribution has been achieved through an experiment designed with a Modern Design of Experiment (MDOE) approach. Within the MDOE approach, the RSM (Response Surface Modeling) has been selected. The objective of the experiment, designed with the RSM approach, is to acquire a sufficient number of data to create one or more response surface models to be used to predict the response variable of interest (within a specified uncertainty) as function of the factors which can affect the selected response variables.

The best porosity distribution able to improve the quality of wind tunnel data has been found for the PT-1 Wind Tunnel (but results and/or the procedure are applicable to all similar Wind Tunnels).

In the present paper, to contextualize the activity, after a short summary of the historical wind tunnel development, the stat of art of the variable streamwise porosity distribution is discussed. Then, the experimental setup to simulate in wind tunnel several streamwise variable porosity distribution and the design of Experiment are described. Finally, the main experimental results are reported and critically analyzed.
Surface-to-Surface Radiation Exchange Effects in a 3D SOFC Stack Unit Cell
Authors: DiGiuseppe Gianfranco
Published: July 2013

This paper reports a new study where radiation effects are studied in details in an SOFC stack. The 3D model used includes and couples fluid dynamics, electrochemistry, electrical conduction, diffusion, and heat transfer physics. The model was built using in-house experimental voltage-current density data for validation purposes. The objective of this study is to understand the effects of radiation in the flow channels of SOFC stacks. Both gas radiation and surface-to-surface heat exchange are considered. This study indicates that gas radiation is negligible when compared to surface-to-surface heat exchange. It is also found that surface-to-surface heat exchange cannot be neglected and actually provides a more uniform temperature distribution along the SOFC stack. Heat transfer via convection is also significant and should be included when modeling similar situations. Finally, the model indicates that viscous dissipation is a negligible source of heat generation.
Recent Advances in High Temperature Electrolysis at Idaho National Laboratory: Single Cell Tests
Authors: Zhang Xiaoyu , O’Brien James E. , O’Brien Robert C.
Published: July 2013

An experimental investigation on the performance and durability of single solid oxide electrolysis cells (SOECs) is under way at the Idaho National Laboratory. In order to understand and mitigate the degradation issues in high temperature electrolysis, single SOECs with different configurations from several manufacturers have been evaluated for initial performance and long-term durability. A new test apparatus has been developed for single cell and small stack tests from different vendors. Single cells from Ceramatec Inc. show improved durability compared to our previous stack tests. Single cells from Materials and Systems Research Inc. (MSRI) demonstrate low degradation both in fuel cell and electrolysis modes. Single cells from Saint Gobain Advanced Materials (St. Gobain) show stable performance in fuel cell mode, but rapid degradation in the electrolysis mode. Electrolyte-electrode delamination is found to have significant impact on degradation in some cases. Enhanced bonding between electrolyte and electrode and modification of the microstructure help to mitigate degradation. Polarization scans and AC impedance measurements are performed during the tests to characterize the cell performance and degradation.
Mitigation of Carbon Formation During Autothermal Reforming of Biodiesel for Solid Oxide Fuel Cell Applications
Authors: Lin J. , Walluk M. R. , Smith D. F. , Trabold T. A.
Published: July 2013

Biodiesel is considered as a renewable hydrogen source for solid oxide fuel cells (SOFCs). This study contributes to a fundamental understanding of biodiesel auto-thermal reforming (ATR), which has not yet been widely explored in the open literature. Ultra-lower sulfur diesel (ULSD) ATR is established as a baseline for this analysis. Solid carbon formation during AT R has been recognized as a primary degradation mode in solid oxide fuel cell-based auxiliary power unit systems in transportation applications, but is difficult to detect and control. To overcome these challenges, this work applies a direct photo-acoustic method to analyze carbon dynamic evolutions and quantify the carbon formation in a single-tube reformer under various operating conditions (temperature, steam/carbon ratio, and oxygen/carbon ratio). The key objective is to locate the optimum operating environment for biodiesel ATR with carbon-free deposition and peak hydrogen yield. Thermodynamic analysis based on the method of total Gibbs free energy minimization is used to evaluate the equilibrium reformate compositions. The experimental investigations complimented with the theoretical analysis of biodiesel ATR helps effectively optimize the onboard reforming conditions.
SOFC Modeling at the Cell Scale Including Hydrogen and Carbon Monoxide as Electrochemically Active Fuels
Authors: Andersson Martin , Navasa Maria , Yuan Jinliang , Sundén Bengt
Published: July 2013

Fuel cells are promising for future energy systems, because they are energy efficient and able to use renewable fuels. A fully coupled computational fluid dynamics (CFD) approach based on the finite element method (with the software COMSOL Multiphysics) in two-dimensions is developed to describe an intermediate temperature solid oxide fuel cell (SOFC) single cell. Governing equations covering heat, gas-phase species, momentum, ion and electron transport are implemented and coupled to kinetics describing internal reforming and electrochemical reactions. Both hydrogen and carbon monoxide are considered as electrochemically active fuels within the anode.

The activation polarization in the electrodes and the ohmic polarization due to ion transport in the YSZ material are found to be the major part of the potential losses. The activation polarization is the most significant and it is smaller within the cathode compared to the anode for this study. The ion current density and the activation polarization are the highest at the electrolyte-electrode interface and decrease rapidly within the electrodes as the distance from the interface increases. However, the ohmic polarization by ion transfer increases for the positions away from the interface. The addition of the electrochemical reaction with CO as fuel increases the current density. It is concluded that the temperature and current density are strongly integrated and when any of them is changed, the other follows, and the change is accelerated.
An Innovative Three Dimensional Numerical Model for Bipolar Plates to Enhance the Efficiency of PEM Fuel Cells
Authors: Arbabi Faraz , Roshandel Ramin
Published: July 2013

The efficiency of proton exchange membrane (PEM) fuel cell is straightly correlated to the bipolar plate design and fluid channel arrangements. Higher produced energy can be attained by optimal design of type, size, or patterns of the channels. Previous researches showed that the bipolar plate channel design has a considerable effect on reactant distribution uniformity as well as humidity control in PEM fuel cells. This paper concentrates on enhancements in the fuel cell performance by optimization of bipolar plate design and channels configurations. A numerical model of flow distribution based on Navier-Stokes equations using individual computer code is presented. The results gained from this three dimensional, multi-component simulation showed excellent agreement with the existed experimental data in the previous publications. In this paper, a new flow field design inspired from the nature is presented and analyzed. In this work, two mostly used flow channels design — serpentine and parallel — have been studied and compared to the newly introduced bio inspired bipolar plate design. To compare, velocity distributions of fluid, mass fraction of reactant gases and polarization curves for different bipolar plate designs have been analyzed. The key design criteria in this study are based on more homogenous molar spreading of species and more uniform velocity distribution along the flow channels and also higher voltage and power density output in different current densities. By developing a numerical code it was concluded that the bio inspired bipolar plate can enhance the PEM fuel cell performance especially at middle current densities, where the losses caused by mass transport limitations are not significant.
A Statistically Based Thermal Conductivity Model for PEMFC Gas Diffusion Layers
Authors: Sadeghifar Hamidreza , Bahrami Majid , Djilali Ned
Published: July 2013

Little attention has been paid to the geometrical parameters of Gas Diffusion Layers (GDLs), as many researchers have reported the GDL transport properties, like thermal conductivity, as a function of only one parameter; porosity. In this paper, a statistical unit cell approach is presented for estimation of the thermal conductivity on the basis of measured geometrical data such as the angles and also the distance between fibers (aspect ratio) and fiber diameter. The dependency of the thermal conductivity on each of these parameters is considered to obtain the best GDL structure. It is discussed, for the first time in this paper, that the parameter of the aspect ratio can usually be as important as porosity, and the traditional thought that a porous media with higher porosity has a lower thermal conductivity cannot always be correct. It is also concluded that the maximum thermal conductivity of a GDL is not a function of the aspect ratio and fiber diameter and only dependent on porosity and fiber angle. In general, the geometrical concepts and the optically measured data presented in this paper can be employed for estimation of other GDL transport properties such as permeability.
Experimental Validation of Methanol Crossover in a Three-Dimensional, Two-Fluid Model of a Direct Methanol Fuel Cell
Authors: Olesen Anders C. , Berning Torsten , Kær Søren Knudsen
Published: July 2013

A fully coupled three-dimensional, steady-state, two-fluid, multi-component and non-isothermal DMFC model has been developed in the commercial CFD package CFX 13 (ANSYS inc.). It accounts for the presence of micro porous layers, non-equilibrium phase change, and methanol and water uptake in the ionomer phase of the catalytic layer, and detailed membrane transport of methanol and water. In order to verify the models ability to predict methanol crossover, simulation results need to be compared with experimental measurements under different current densities along with air and methanol stoichiometries. Methanol crossover be indirectly measured based on the combined anode and cathode exhaust CO2 mole fraction and by accounting for the CO2 production at the anode as a function of current density. This approach is simple and assumes that all crossed over methanol is oxidized. Moreover, it takes CO2 crossover into account.
Manufacturing Process Modeling of 100-400 kWe Combined Heat and Power Stationary Fuel Cell Systems
Authors: Warren Joshua , Das Sujit , Zhang Wei
Published: July 2013

Despite significant cost reductions and reliability improvements in recent decades, combined heat and power (CHP) stationary fuel cells are expected to remain relatively limited in their commercial acceptance until additional advancements are realized.

In the present study, three stationary CHP fuel cell technologies — phosphoric acid (PAFC), molten carbonate (MCFC), and solid oxide (SOFC) — operating in the 100–400 kWe range were examined with process based cost models to estimate manufacturing costs and to highlight high cost steps where alternative production methods may have the greatest potential for high impact cost reduction. PAFC manufacturing costs, estimated to be $3049/kWe for a baseline manufacturing volume of 20 MWe/yr, may benefit most from reduced catalyst loading, lower cost substrates, reduced fuel processing balance of plant (BOP) costs, and near net shape bipolar plate manufacturing. MCFC manufacturing costs, while not reported in detail due to the proprietary nature of inputs employed in the model, may be reduced through advances in fuel desulfurization and stainless steel stack component manufacturing. SOFC manufacturing costs, estimated to be $1242/kWe, approach a commercially acceptable level, but significant advancements in seal and bipolar plate reliability are likely required before a commercially acceptable stack life of 40,000 hours is realized.
Solid Oxide Fuel Cell Auxiliary Power Units for Heavy Duty Trucks
Authors: Alhelfi Ali , Yuan Jinliang , Sunden Bengt
Published: July 2013

This paper explores the potentials of solid oxide fuel cells (SOFCs) as 3 kW auxiliary power units for trucks and military vehicles operating on diesel fuel. Various issues are discussed, e.g., the requirements and specifications for the unit as well as the advantages, challenges, and development issues for SOFCs in such applications. System design and analysis are carried out. The major component of the system is the fuel cell stack. The calculations are performed for two voltage systems 12/14 V and 36/42 V, respectively. The influences of others parameters like the mass flow rates of air and gases, water production and heat production are also evaluated for the two voltage systems.
Optimising the Performance of a PEM Fuel Cell With Transverse Fins Insert in the Channel Flow Using Mathematical Algorithm
Authors: Obayopo Surajudeen O. , Bello-Ochende Tunde , Meyer Josua P.
Published: July 2013

The performance of proton exchange membrane (PEM) fuel cell is greatly hinged on effective distribution of reactant species concentration in the system which subsequently determines distribution in local current density, temperature and water over the area of PEM fuel cell. Flow-field geometry optimization can effectively insure proper distribution of reactants to eliminate the build-up stresses within different regions of the fuel cells that leads to effects such as flooding or drying of the membrane. In this work, a numerical investigation was conducted to analyse the flow field and reactant gas distribution in a PEM fuel cell channel with transversely inserted pin fins in the channel flow. The impact of the fin disturbances on reactant gas transport and fuel distribution on the GDL was investigated. A prerequisite for an efficient fuel cell system is a uniform distribution of reactants to every cell in the fuel cell stack. A fin configuration of small hydraulic diameter and high volume porosity (ratio of volume not occupied by fins) is employed to reduce the additional pressure drop. The influence of the pin fin parameters, fuel flow Reynolds number and the gas diffusion layer (GDL) porosity on the reactant gas transport through the GDL and the pressure drop across the channel length are explored. The parameters examined were optimized using a mathematical optimization code integrated with the commercial computational fluid dynamics code. The results obtain indicate that for a consideration of high performance and considerable pressure drop in a PEM fuel cell, pin fin insert in the channel flow greatly enhances fuel cell performance when compared with channel without pin fin and optimal pin fin geometries exist for minimized pressure drop along the PEM fuel cell channel studied. The results obtained will complement the generic information needed for improving the PEM fuel cell design.
Development of Radial Flow Field for Improved Water and Gas Management in PEMFC
Authors: Friess Brooks R. , Hoorfar Mina
Published: July 2013

This paper presents an innovative radial flow field design for PEMFC cathode flow plates. This new design, which is in the form of a radial field, replaces the standard rectangular flow channels in exchange for a set of flow control rings. The control rings allow for better flow distribution and use of the active area. The radial flow field was constructed with aluminum and plated with gold for superior surface and conductive properties. These materials were selected based on the results obtained from the performance of the standard flow channels of serpentine and parallel constructed of hydrophilic gold and hydrophobic graphite materials. The innovative radial flow field produces the largest limiting current density compared to other channel designs, even the serpentine flow field. The water removal and mass transport capacity of the radial flow field was proven to be better than parallel and serpentine. This performance increase was achieved while maintaining the pressure drop nearly half of the pressure drop measured in the serpentine flow fields.
A Three-Dimensional Two-Phase Model for Simulating PEM Fuel Cell Performance
Authors: Chen Ken S. , Carnes Brian , Hao Liang , Luo Gang , Wang Chao-Yang
Published: July 2013

For the last couple of years, we have been working on developing and validating a three-dimensional, two-phase, comprehensive PEM (polymer electrolyte membrane) fuel cell model, and our efforts were funded by the US Department of Energy. In this paper, we provide an up-to-date progress report on our team efforts. Specifically, we present comparisons of simulation results (liquid-water saturation distribution) computed by our improved partially two-phase and fully two-phase models. We also present sample model-validation results by comparing model prediction with experimental data.