Effect of placing different obstacles in flow fields on performance of a PEM fuel cell: numerical investigation and experimental comparison

In this study a complete two-dimensional model for proton exchange membrane (PEM) fuel cells was used to investigate the effect of using different obstacles on the performances, current density and gas concentration for different aspect ratios (ARs). The proposed model is a full cell model, which includes all the parts of the PEM fuel cell, flow channels, gas diffusion electrodes, catalyst layers and the membrane. Also a series of tests are carried out to investigate and validate the numerical results of the polarization curve under the normal conditions. A PEM fuel cell with 25 cm² active area and Nafion 117 membrane with 4 mg Pt/cm² for the anode and cathode is employed as a membrane electrode assembly. The results show that the predicted polarization curves by using this model are in good agreement with the experimental results. Also the results show that the local current density reduces more obviously at a higher overpotential than at a lower overpotential because of the more obvious reflection phenomena in the downstream region. At lower operating voltage conditions, the overall cell performance decreases as the AR decreases.     

Investigation of bifurcation structure flow field for bipolar plates in PEMFC

Uniform flow field in fuel cells is important to the performance. Even distribution of reactant gases over the electrode surfaces is a key to the good performance of fuel cells since this enables them to operate as close as possible to maximum capability and electrochemical reactions. In this paper, the bifurcation principle can be used to design the flow field structure for bipolar plate in proton exchange membrane fuel cells (PEMFC). It is demonstrated by numerical simulation that branch flow field structure can provide substantially flow-field distribution, current density and heat transfer when compared to the traditional structure. Then a kind of excellent flow field structure for bipolar plate in PEMFC can be achieved.     

Modeling development on the meso-scale reacting transport phenomena in proton exchange membrane fuel cells

The catalyst layer (CL) of proton exchange mem-brane fuel cell (PEMFC) involves various particles and pores in meso-scale, which has an important effect on the mass, charge (proton and electron) and heat transport coupled with the electrochemical reactions. The coarse-grained molecular dynamics (CG-MD) method is employed as a meso-scale structure reconstruction technique to mimic the self-organization phenomena in the fabrication steps of a CL. The meso-scale structure obtained at the equilibrium state is further analyzed by molecular dynamic (MD) software to provide the necessary microscopic parameters for understanding of multi-scale and-physics processes in CLs. The primary pore size distribution (PSD) and active platinum (Pt) surface areas are also calculated and then compared with the experiments. In addition, we also highlight the implementation method to combine microscopic elementary kinetic reaction schemes with the CG-MD approaches to provide insight into the reactions in CLs. The concepts from CG modeling with particle hydrodynamics (SPH) and the problems on coupling of SPH with finite element modeling (FEM) methods are further outlined and discussed to understand the effects of the meso-scale transport phenomena in fuel cells.     

Two-phase annular flow and evaporative heat transfer in a microchannel

A physical and mathematical model has been developed to predict the two-phase flow and heat transfer in a microchannel with evaporative heat transfer. Sample solutions to the model were obtained for both constant wall temperature and constant wall heat flux conditions. Results are provided for evaporation rate, liquid film thickness, liquid and vapor phase pressure and temperature distributions. In addition to the sample calculations that were used to illustrate the transport characteristics, computations based on the current model were performed to generate results for comparisons with the experimental results of Qu and Mudawar (2004) where two different mass flow rates of the working fluid were used in the experiment. The comparisons of total pressure drops with the experimental data of Qu and Mudawar (2004) cover the wall heat flux range of 142.71-240 W/cm² with a total channel mass flux of 400.1 kg/m² s and also the wall heat flu range of 99.54-204.39 W/cm² with total channel mass flux of 401.9 kg/m² s. The calculated results from the current model match closely with those of Qu and Mudawar (2004).     

Mathematical analysis of SOFC based on co-ionic conducting electrolyte

In co-ionic conducting solid oxide fuel cell (SOFC), both oxygen ion (O2) and proton (H+) can transport through the electrolyte, generating steam in both the an-ode and cathode. Thus the mass transport phenomenon in the electrodes is quite different from that in conventional SOFC with oxygen ion conducting electrolyte (O-SOFC) or with proton conducting electrolyte (H-SOFC). The generation of steam in both electrodes also affects the concentration over-potential loss and further the SOFC performance. However, no detailed modeling study on SOFCs with co-ionic electrolyte has been reported yet. In this paper, a new mathematical model for SOFC based on co-ionic electrolyte was developed to predict its actual performance considering three major kinds of overpotentials. Ohm’s law and the Butler-Volmer formula were used to model the ion conduction and electrochemical reactions, respectively. The dusty gas model (DGM) was employed to simulate the mass transport processes in the porous electrodes. Parametric simulations were performed to investigate the effects of proton transfer number (tH) and current density (jtotal) on the cell performance. It is interesting to find that the co-ionic conducting SOFC could perform better than O-SOFC and H-SOFC by choosing an appropriate proton transfer number. In addition, the co-ionic SOFC shows smaller difference between the anode and cathode concentration overpotentials than O-SOFC and H-SOFC at certain t H values. The results could help material selection for enhancing SOFC performance.     

Flow patterns and pressure drop of ionic liquid–water two-phase flows in microchannels

The two-phase flow of a hydrophobic ionic liquid and water was studied in capillaries made of three different materials (two types of Teflon, FEP and Tefzel, and glass) with sizes between 200 μm and 270 μm. The ionic liquid was 1-butyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}amide, with density and viscosity of 1420 kg m⁻³ and 0.041 kg m⁻¹ s⁻¹, respectively. Flow patterns and pressure drop were measured for two inlet configurations (T- and Y-junction), for total flow rates of 0.065–214.9 cm³ h⁻¹ and ionic liquid volume fractions from 0.05 to 0.8. The continuous phase in the glass capillary depended on the fluid that initially filled the channel. When water was introduced first, it became the continuous phase with the ionic liquid forming plugs or a mixture of plugs and drops within it. In the Teflon microchannels, the order that fluids were introduced did not affect the results and the ionic liquid was always the continuous phase. The main patterns observed were annular, plug, and drop flow. Pressure drop in the Teflon microchannels at a constant ionic liquid flow rate, was found to increase as the ionic liquid volume fraction decreased, and was always higher than the single phase ionic liquid value at the same flow rate as in the two-phase mixture. However, in the glass microchannel during plug flow with water as the continuous phase, pressure drop for a constant ionic liquid flow rate was always lower than the single phase ionic liquid value. A modified plug flow pressure drop model using a correlation for film thickness derived for the current fluids pair showed very good agreement with the experimental data.     

Behaviors of flow mechanisms and heat transfer of non-Newtonian fluids through a spinneret

This investigation examines non-Newtonian flow mechanisms and heat transfer characteristics for a micro spinneret. The working fluid, Polyethylene terephthalate (PET), is the raw material of micro fiber, and a large-scale experimental test model was designed to visualize the complex viscous flow system in the micro spinneret. To visualize the complex convective flow system, an experimental test model was constructed, using glycerin instead of PET. The related parameters of PET were compared with those of glycerin. The power law correlates the shear strain with PET viscosity at various temperatures. The pressure distribution along the flow direction was measured and the flow pattern was visualized using polyethylene (PE) powder of 20–40 m. Similar configurations were calculated for micro spinneret physical parameters to determine the thermal flow characteristics. The Reynolds number in the test model is not less than 10^–2. In the non-Newtonian PET working fluid of practical micro spinneret, flows with Re = 10⁴ to 10^–2 are in the same low Reynolds number flow regime. Therefore, the working fluid is expected to have the same flow characteristic. A numerical solution covering the range of approximately Re = 10^–4 at PET confirms that the flow characteristics of glycerin are constant for Re = 1.228 × 10^–2. The Peclet number in the test model can be adjusted to a value similar to that in the micro spinneret. The flow visualization was compared with that of the numerical solution, and the friction factor and Nusselt number in the micro spinneret were analyzed. Finally, numerical results and friction factor with various exit angles of micro spinneret in a triangular zone flow system were also summarized.     

Experimental investigation of irreversibility of a proton exchange membrane fuel cell at different channel geometry

The geometry of the channels of a fuel cell is very important for performance and efficiency of it. For this reason, a thermodynamic analysis is performed for a PEM fuel cell at different channel geometry that three different fuel cells with rectangular, elliptical and triangular serpentine channels have constructed. The active area of each cell is 25?cm² that its weight is 1300?g. The material of the gas diffusion layer is carbon clothes, the membrane is Nafion 112 and the catalyst layer is a plane with 0.004?g/cm² Platinum. Also a test bench designed and constructed for testing the cell and a series of experiments are carried out to investigate the influence of the geometry of the cell on irreversibility under the normal conditions. The results show that the performance of the cell at $ T_{{{\text{O}}_{2} }} $ ?=?55?°C, $ T_{{{\text{H}}_{2} }} $ ?=?55?°C, $ T_{\text{cell}} $ ?= 60?°C, oxygen flow rate?=?0.5?L/min, hydrogen flow rate?=?0.3?L/min and P?=?2.905 bar is higher about 12?% and 18?% when the geometry of the channels is rectangular in comparison of elliptical and triangular channels and the irreversibility is lower about 17?% and 33?% when the geometry of the channels is rectangular in comparison of elliptical and triangular channels.     

Measurement and prediction of droplet size in annular gas–liquid flows in aero-engine oil systems

Two-phase gas–liquid annular flows are encountered in ventless aero-engine oil system pipes. The droplet size in the flow has an important impact on the performance of downstream equipment as breathers and de-aerators. However, literature studies present semi-empirical models that are not in the range of operating conditions of the oil system. To investigate the effect of the use of lubrication oil on the droplet sizes, this paper presents experimental results of annular flow with oil flow rates from 160 to 640 l/h and air flow rates from 60 to 120 Nm³/h. Comparison of the Sauter–Mean Diameter predicted by existing correlation show an error of minimum 30% compared to experimental values for higher oil flow rates, which are the most important in oil systems. To address this issue, correlations were adapted to fit experimental results. With the new set of parameters, the Sauter-Mean Diameter is estimated with an error of maximum 18% for higher oil flow rates. Results analysis illustrate that the main difference between existing and new correlations could be due do the surface tension and viscosity of lubrication oil, which are very different from water at low temperature. The results are also consistent with the transition between bag and ligament break-up droplet generation mechanism at a flow rate of 80 Nm³/h.     

The effect of material properties on the performance of a new geometry PEM fuel cell

In this paper a computational dynamics model for duct-shaped geometry proton exchange membrane (PEM) fuel cell was used to investigate the effect of changing gas diffusion layer and membrane properties on the performances, current density and gas concentration. The proposed model is a full cell model, which includes all the parts of the PEM fuel cell, flow channels, gas diffusion electrodes, catalyst layers and the membrane. Coupled transport and electrochemical kinetics equations are solved in a single domain; therefore no interfacial boundary condition is required at the internal boundaries between cell components. This computational fluid dynamics code is used as the direct problem solver, which is used to simulate the 2-dimensional mass, momentum and species transport phenomena as well as the electron- and proton-transfer process taking place in a PEMFC that cannot be investigated experimentally. The results show that by increasing the thickness and decreasing the porosity of GDL the performance of the cell enhances that it is different with planner PEM fuel cell. Also the results show that by increasing the thermal conductivity of the GDL and membrane, the overall cell performance increases.     

Dual-tracer fluorescence thermometry measurements in a heated channel

The exponential growth of component density in microelectronics has renewed interest in compact and high heat flux thermal management technologies that can handle local heat fluxes exceeding 1 kW/cm². Accurate and spatially resolved thermometry techniques that can measure liquid-phase temperatures without disturbing the coolant flow are important in developing new heat exchangers employing forced-liquid and evaporative cooling. This paper describes water temperature measurements using dual-tracer fluorescence thermometry (DFT) with fluorescein and sulforhodamine B in laminar Poiseuille flow through polydimethyl siloxane-glass channels heated on one side. The major advantage of using the ratio of the signals from these two fluorophores is their temperature sensitivity of 4.0–12% per °C—a significant improvement over previous DFT studies at these spatial resolutions. For an in-plane spatial resolution of 30 μm, the average experimental uncertainties in the temperature data are estimated to be 0.3°C.     

Investigation of possibility of flow field diagnostics by laser induced biacetyl phosphorescence

Experimental results of laser induced phosphorescence of biacetyl triplet³A _u in a mixture of biacetyl and N₂ show that: the lifetime of phosphorescence is a function of temperature and independent of density and concentration; the initial phosphorescence intensity is a linear function of density and insensitive to the variation of temperature. The temperature and density distribution of gas flow could be measured by observing the phosphorescence lifetime and initial intensity of biacetyl mixed with N₂ respectively. The velocity distribution could be measured by observing the time-of-flight of the gaseous phosphorescent spot under pulsed laser excitation. Compared with Doppler anemometer, it, to a great extent, avoids the particle lag problem in flow field with large velocity gradient. The phosphorescence decay mechanism is also analysed and the analytical results agree with experimental ones. The project supported by National Natural Science Foundation of China and The Third World Academy.     

Flow resistance and heat transfer characteristics in micro-cylinders-group

De-ionized water flows through in-line and staggered arrays micro-cylinders-group plates with different distances among micro-cylinders, and the resistance characteristic and the Nusselt numbers of micro plates are experimentally obtained. The investigations show that the distances among cylinders (S values) have slight influence on flow resistance and the experimental Nusselt number and they are far less than predictions of classical correlations at extremely low Reynolds number due to the appearance of the sluggish regions in micro cylinders-group plate. With the increase of Reynolds number, the influence of the S values on flow resistance becomes more apparent and the flow resistance and the experimental Nusselt number of micro-cylinders-group rapidly increases due to the decrease of sluggish regions in the micro-cylinders-group plate and the weakness of the variation of the thermophysical properties with temperature and the endwall effects.     

Numerical modeling and simulation of PEM fuel cells: Progress and perspective

This paper provides a comprehensive review on the research and development in multi-scale numerical modeling and simulation of PEM fuel cells. An overview of recent progress in PEM fuel cell modeling has been provided. Fundamental transport phenomena in PEM fuel cells and the corresponding mathematical formulation of macroscale models are analyzed. Various important issues in PEM fuel cell modeling and simulation are examined in detail, including fluid flow and species transport, electron and proton transport, heat transfer and thermal management, liquid water transport and water management, transient response behaviors, and cold-start processes. Key areas for further improvements have also been discussed.     

Energy dissipation and contour integral characterizing fracture behavior of incremental plasticity

Jep -integral is derived for characterizing the frac- ture behavior of elastic-plastic materials. The J ep -integral differs from Rice’s J-integral in that the free energy density rather than the stress working density is employed to define energy-momentum tensor. The J ep -integral is proved to be path-dependent regardless of incremental plasticity and deformation plasticity. The J epintegral possesses clearly clear physical meaning: (1) the value J ep tip evaluated on the infinitely small contour surrounding the crack tip represents the crack tip energy dissipation; (2) when the global steadystate crack growth condition is approached, the value of J ep farss calculated along the boundary contour equals to the sum of crack tip dissipation and bulk dissipation of plastic zone. The theoretical results are verified by simulating mode I crack problems.     

Numerical simulation of pedestrian flow past a circular obstruction

In this paper, a revisiting Hughes' dynamic continuum model is used to investigate and predict the essential macroscopic characteristics of pedestrian flow, such as flow, density and average speed, in a two dimensional continuous walking facility scattered with a circular obstruction. It is assumed that pedestrians prefer to walk a path with the lowest instantaneous travel cost from origin to destination, under the consideration of the current traffic conditions and the tendency to avoid a high-density region and an obstruction. An algorithm for the pedestrian flow model is based on a cellcentered finite volume method for a scalar conservation law equation, a fast sweeping method for an Eikonal-type equation and a second-order TVD Runge-Kutta method for the time integration on unstructured meshes. Numerical results demonstrate the effectiveness of the algorithm. It is verified that density distribution of pedestrian flow is influenced by the position of the obstruction and the path-choice behavior of pedestrians.     

Precursor ionization ahead of strong shock waves in argon

The mechanism of precursor ionization ahead of strong shock waves has been studied in a low density shock tube. The experimental results are illustrated with Arrhenius plots with kink points dividing them into two parts with apparent activation energy ratio 1:2, namely with the values 7.7 eV and 15.3 eV, and varying with first and third power of the density respectively. A model is proposed to interpret the facts where the process taking place in the precursor region, is a two step photo ionization accompanied with the drift flow effect of the gas relative to the shock wave or the ionization recombination effect according to whether the shock speed and initial density are low enough. The product of the A-A collision excitation cross section coefficientS ^* multiplied by the radiation cross sectionQ ^* of ArgonS ^*×Q ^*=1×10⁻³⁶ (cm⁴eV⁻¹) and the three body recombination coefficient of Argon at room temperaturek _ra =1×10⁻²⁴ (cm⁻⁶s⁻¹). The project supported by the National Natural Science Foundation of China     

Shock tube study of n-decane ignition at low pressures

Ignition delay times for n-decane/O 2 /Ar mixtures were measured behind reflected shock waves using endwall pressure and CH* emission measurements in a heated shock tube. The initial postshock conditions cover pressures of 0.09-0.26 MPa, temperatures of 1 227-1 536 K, and oxygen mole fractions of 3.9%-20.7% with an equivalence ratio of 1.0. The correlation formula of ignition delay dependence on pressure, temperature, and oxygen mole fraction was obtained. The current data are in good agreement with available low-pressure experimental data, and they are then compared with the prediction of a kinetic mechanism. The current measurements extend the kinetic modeling targets for the n-decane combustion at low pressures.     

Experimental investigation of heat transfer in vertical upward and downward supercritical CO2 flow in a circular tube

An experimental investigation of turbulent heat transfer in vertical upward and downward supercritical CO₂ flow was conducted in a circular tube with an inner diameter of 4.5 mm. The experiments were performed for bulk fluid temperatures from 29 to 115 °C, pressures from 74.6 to 102.6 bar, local wall heat fluxes from 38 to 234 kW/m², and mass fluxes from 208 to 874 kg/m² s. At a moderate wall heat flux and low mass flux, the wall temperature had a noticeable peak value for vertical upward flow, but increased monotonically along the flow direction without a peak value for downward flow. The ratios of the experimental Nusselt number to the value obtained from a reference correlation were compared with Bo^* and q⁺ distributions to observe the buoyancy and flow-acceleration effects on heat transfer. In the experimental range of this study, the flow acceleration predominantly affected the heat-transfer phenomena. Based on an analysis of the shear-stress distribution in the turbulent boundary layer and the significant variation of the specific heat across the turbulent boundary layer, a new heat-transfer correlation for vertical upward and downward flow of supercritical pressurized fluid was developed; this correlation agreed with various experimental datasets within ±30%.     

Model of Formation of the Electric Current in Aircraft Jet Engine Ducts

A possible mechanism of formation of electric currents (engine currents) in aircraft engine jets (with subsequent charging of the aircraft) is investigated. This mechanism is a result of the presence of electrons and ions at concentrations of 10⁷–10¹⁰cm^-3 in the gas flow in the engine duct. The electrons and ions are formed in the fuel combustion chamber as a result of chemo- ionization reactions. The wall flow zones in which the electrical quasi-neutrality of the medium is violated are considered. In these zones a nonzero normal component of the electric current is formed on the wall surface and a streamwise electric current develops in the duct. A general functional dependence of the engine current on the basic dimensionless parameters is obtained on the basis of similarity theory and dimensional analysis. Within the framework of electrical diffusion boundary layer theory a model problem of formation of the maximum possible engine current is formulated. A universal system of equations and boundary conditions, which contains no dimensionless parameters, is obtained and investigated. The engine current is qualitatively estimated for real engines and the calculation results are compared with the experimental data obtained under airfield conditions.