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.     

Coupled pore network model for the cathode gas diffusion layer in PEM fuel cells

Acta Mechanica Sinica - A pore network model (PNM) is developed for gas diffusion layer (GDL) in the cathode side of polymer electrolyte membrane fuel cells (PEMFCs). The model is coupled to...     

Optimisation of Gas Access Through a Thin Porous Layer with a Partially Occluded Inlet Surface

Transport in Porous Media - The gas access through a thin porous layer is studied in relation to the reactant gas transfer in the gas diffusion layer (GDL) of polymer electrolyte membrane fuel...     

质子交换膜燃料电池扩散层电阻的实验研究

气体扩散层是质子交换膜燃料电池的重要组件,其与极板之间的接触电阻是燃料电池内阻的重要组成部分,也是导致燃料电池功率损失的重要因素。为了得到扩散层与极板间接触电阻以及扩散层本体电阻,首先在扩散层试样表面溅射厚度为50nm的金属铜,再在实验室搭建的燃料电池压力-位移测试平台上用四点法测得试样的总电阻,而后用斜率-截距法分离了气体扩散层本体电阻和其与极板的接触电阻,计算出了相应的电阻率,得到了压力和接触电阻之间的关系。结果表明,接触电阻随着接触面压强的增大而减小,接触电阻率和接触压强关系满足经验公式ρ=43.39P-0.725。     

Integral-based constitutive equation for rubber at high strain rates

High-speed experiments were conducted to characterize the deformation and failure of Styrene Butadiene Rubber at impact rates. Dynamic tensile stress–strain curves of uniaxial strip specimens and force–extension curves of thin sheets were obtained from a Charpy tensile impact apparatus. Results from the uniaxial tension tests indicated that although the rubber became stiffer with increasing strain rates, the stress–strain curves remained virtually the same above 280 s⁻¹. Above this critical strain rate, strength, fracture strain and toughness decreased with increasing strain rates. When strain rates were below 180 s⁻¹, the initial modulus, tensile strength and breaking extension increased as the strain rate increased. Between strain rates of 180 and 280 s⁻¹, the initial modulus and tensile strength increased with increasing strain rates but the extension at break decreased with increasing strain rates. A hyper-viscoelastic constitutive relation of integral form was used to describe the rate-dependent material behavior of the rubber. Two characteristic relaxation times, 5 ms and 0.25 ms, were needed to fit the proposed constitutive equation to the data. The proposed constitutive equation was implemented in ABAQUS Explicit via a user-defined subroutine and used to predict the dynamic response of the rubber sheets in the experiments. Numerical predictions for the transient deformation and failure of the rubber sheet were within 10% of experimental results.     

Effects of particle size and concentration on bubble coalescence and froth formation in a slurry bubble column

A new approach for simulating the formation of a froth layer in a slurry bubble column is proposed. Froth is considered a separate phase, comprised of a mixture of gas, liquid, and solid. The simulation was carried out using commercial flow simulation software (FIRE v2014) for particle sizes of 60–150 μm at solid concentrations of 0–40 vol%, and superficial gas velocities of 0.02–0.034 m/s in a slurry bubble column with a hydraulic diameter of 0.2 m and height of 1.2 m. Modelling calculations were conducted using a Eulerian–Eulerian multiphase approach with k–ε turbulence. The population balance equations for bubble breakup, bubble coalescence rate, and the interfacial exchange of mass and momentum were included in the computational fluid dynamics code by writing subroutines in Fortran to track the number density of different bubble sizes. Flow structure, radial gas holdup, and Sauter mean bubble diameter distributions at different column heights were predicted in the pulp zone, while froth volume fraction and density were predicted in the froth zone. The model was validated using available experimental data, and the predicted and experimental results showed reasonable agreement. To demonstrate the effect of increasing solid concentration on the coalescence rate, a solid-effect multiplier in the coalescence efficiency equation was used. The solid-effect multiplier decreased with increasing slurry concentration, causing an increase in bubble coalescence efficiency. A slight decrease in the coalescence efficiency was also observed owing to increasing particle size, which led to a decrease in Sauter mean bubble diameter. The froth volume fraction increased with solid concentration. These results provide an improved understanding of the dynamics of slurry bubble reactors in the presence of hydrophilic particles.     

Coupled annealing temperature and layer thickness effect on strengthening mechanisms of Ti/Ni multilayer thin films

A systematic study was performed on mechanical and microstructural properties of Ti/Ni multilayers with layer thickness from 200 nm to 6 nm and annealing temperature from room temperature to 500 °C. Based on the observed hardness evolution, a coupled layer-thickness and annealing-temperature dependent strengthening mechanism map is proposed. For as-deposited films, the deformation behavior follows the traditional trend of dislocation mediated strengthening to grain boundary mediated softening with decreasing layer thickness. For annealed films, grain boundary relaxation is considered to be the initial strengthening mechanism with higher activation temperature required for thicker layers. Under further annealing, solid solution hardening, intermetallic precipitation hardening, and fully intermixed alloy structure continue to strengthen the thin layered films, while recrystallization and grain-growth lead to the eventual softening of thick layered films. For the films with intermediate layer thickness, a strong orientation dependent hardness behavior is exhibited under high temperature annealing due to mechanism switch from grain growth softening to intermetallic precipitation hardening when changing the loading orientation from perpendicular to parallel to the layer interfaces.     

CFD simulation of a gas–solid fluidized bed with two vertical jets

A computational fluid dynamics (CFD) model is used to investigate the hydrodynamics of a gas–solid fluidized bed with two vertical jets. Sand particles with a density of 2660 kg/m³ and a diameter of 5.0 × 10^?4 m are employed as the solid phase. Numerical computation is carried out in a 0.57 m × 1.00 m two-dimensional bed using a commercial CFD code, CFX 4.4, together with user-defined Fortran subroutines. The applicability of the CFD model is validated by predicting the bed pressure drop in a bubbling fluidized bed, and the jet detachment time and equivalent bubble diameter in a fluidized bed with a single jet. Subsequently, the model is used to explore the hydrodynamics of two vertical jets in a fluidized bed. The computational results reveal three flow patterns, isolated, merged and transitional jets, depending on the nozzle separation distance and jet gas velocity and influencing significantly the solid circulation pattern. The jet penetration depth is found to increase with increasing jet gas velocity, and can be predicted reasonably well by the correlations of Hong et al. (2003) for isolated jets and of Yang and Keairns (1979) for interacting jets.     

Modeling and measuring visco-elastic properties: From collagen molecules to collagen fibrils

Collagen is the main structural protein in vertebrate biology, determining the mechanical behavior of connective tissues such as tendon, bone and skin. Although extensive efforts in the study of the origin of collagen exceptional mechanical properties, a deep knowledge of the relationship between molecular structure and mechanical properties remains elusive, hindered by the complex hierarchical structure of collagen-based tissues. Understanding the viscoelastic behavior of collagenous tissues requires knowledge of the properties at each structural level. Whole tissues have been studied extensively, but less is known about the mechanical behavior at the submicron, fibrillar and molecular level. Hence, we investigate the viscoelastic properties at the molecular level by using an atomistic modeling approach, performing in silico creep tests of a collagen-like peptide. The results are compared with creep and relaxation tests at the level of isolated collagen fibrils performed previously using a micro-electro-mechanical systems platform. Individual collagen molecules present a non-linear viscoelastic behavior, with a Young's modulus increasing from 6 to 16 GPa (for strains up to 20%), a viscosity of 3.84±0.38 Pa s, and a relaxation time in the range of 0.24–0.64 ns. At the fibrils level, stress–strain–time data indicate that isolated fibrils exhibit viscoelastic behavior that could be fitted using the Maxwell–Weichert model. The fibrils showed an elastic modulus of 123±46 MPa. The time-dependent behavior was well fit using the two-time-constant Maxwell–Weichert model with a fast time response of 7±2 s and a slow time response of 102±5 s.     

Hydrodynamics and energy consumption studies in a three-phase liquid circulating three-phase fluid bed contactor

The hydrodynamics and energy consumption have been studied in a cold flow, bubbling and turbulent, pressurized gas–liquid–solid three-phase fluidized bed (0.15 m ID × 1 m height) with concurrent gas–liquid up flow is proposed with the intention of increasing the gas hold up. The hydrodynamic behaviour is described and characterised by some specific gas and liquid velocities. Particles are easily fluidized and can be uniformly distributed over the whole height of the column. The effect of parameters like liquid flow rate, gas flow rate, particle loading, particle size, and solid density on gas hold up and effect of gas flow rate, solid density and particle size on solid hold up, energy consumption and minimum fluidization velocity has been studied. At the elevated pressures a superior method for better prediction of minimum fluidization velocity and terminal settling velocities has been adopted. The results have been interpreted with Bernoulli’s theorem and Richardson–Zaki equation. Based on the assumption of the gas and liquid as a pretend fluid, a simplification has been made to predict the particle terminal settling velocities. The Richardson–Zaki parameter n′ was compared with Renzo’s results. A correlation has been proposed with the experimental results for the three-phase fluidization.     

Euler-Euler CFD modeling of fluidized bed＂ Influence of specularity coefficient on hydrodynamic behavior

Euler-Euler two-fluid model is used to simulate the hydrodynamics of gas-solid flow in a bubbling flu- idized bed with Geldert B particles where the solid property is calculated by applying the kinetic theory of granular flow （KTGF）. Johnson and Jackson wall boundary condition is used for the particle phase, and different amount of slip between particle and wall is given by varying the specularity coefficient （φ） from 0 to 1. The simulated particle velocity, granular temperature and particle volume fraction are compared to investigate the effect of different wall boundary conditions on the hydrodynamic behavior, Some of the results are also compared with the available experimental data from the literature. It was found that the model predictions are sensitive to the specularity coefficient. The hydrodynamic behavior deviated sig- nificantly for φ = 0 and φ = 0.01 with maximum deviation found at φ = 0 i.e. free-slip condition. However, the overall bed height predicted by all the conditions is similar.     

Numerical analysis of turbulent convection heat transfer from an array of perforated fins

Numerical investigation is made for three-dimensional fluid flow and convective heat transfer from an array of solid and perforated fins that are mounted on a flat plate. Incompressible air as working fluid is modeled using Navier–Stokes equations and RNG based k ? ? turbulent model is used to predict turbulent flow parameters. Temperature field inside the fins is obtained by solving Fourier’s conduction equation. The conjugate differential equations for both solid and gas phase are solved simultaneously by finite volume procedure using SIMPLE algorithm. Perforations such as small channels with square cross section are arranged streamwise along the fin’s length and their numbers varied from 1 to 3. Flow and heat transfer characteristics are presented for Reynolds numbers from 2 × 10⁴ to 4 × 10⁴ based on the fin length and Prandtl number is taken Pr = 0.71. Numerical computations are validated with experimental studies of the previous investigators and good agreements were observed. Results show that fins with longitudinal pores, have remarkable heat transfer enhancement in addition to the considerable reduction in weight by comparison with solid fins.     

Effect of Gas Diffusion Layer Properties on Breakthrough Time and Pressure

The gas diffusion layer (GDL) plays an important role in the removal of product water from the catalyst layer to the flow plate in a fuel cell. Numerous studies have reported water management, especially in the GDL, as the limiting factor hindering convective and diffusive transport of reactants which results in lowering power density. In this paper, an experimental technique is presented to study the GDL water transport properties associated with the breakthrough conditions which are critical to overall water management. Fluorescence microscopy technique is used to measure the pressure and time required for water to penetrate and break through the surface of the GDL. The results obtained for GDLs produced by different manufacturers confirm that the breakthrough time and pressure are larger for PTFE treated hydrophobic GDLs. The results are analyzed in terms of the contact angle, thickness, and SEM images to see the effects of different structural properties. The changes in morphology due to compression are also presented. In addition, the changes in breakthrough conditions when samples are reused are presented. The results provide basic insights into the water transport properties of the GDL, leading to the design of new materials with enhanced water management.     

Experimental investigation of turbulence modulation in particle-laden coaxial jets by Phase Doppler Anemometry

The effect of solid particles on the flow characteristics of axisymmetric turbulent coaxial jets for two flow conditions was studied. Simultaneous measurements of size and velocity distributions of continuous and dispersed phases in a two-phase flow are presented using a Phase Doppler Anemometry (PDA) technique. Spherical glass particles with a particle diameter range from 102 to 212 μm were used in this two-phase flow, the experimental results indicate a significant influence of the solid particles and the Re on the flow characteristics. The data show that the gas phase has lower mean velocity in the near-injector region and a higher mean velocity at the developed region. Near the injector at low Reynolds number (Re = 2839) the presence of the particles dampens the gas-phase turbulence, while at higher Reynolds number (Re = 11 893) the gas-phase turbulence and the velocity fluctuation of particle-laden jets are increased. The particle velocity at higher Reynolds number (Re = 11 893) and is lower at lower Reynolds number (Re = 2839). The slip velocity between particles and gas phase existed over the flow domain was examined. More importantly, the present experiment results suggest that, consideration of the gas characteristic length scales is insufficient to predict gas-phase turbulence modulation in gas-particle flows.     

Thermo-viscoelastic and viscoplastic behavior of high-density polyethylene

Observations are reported on high-density polyethylene in uniaxial tensile tests with constant strain rates and relaxation tests at various temperatures ranging from 25 to 90 °C. A constitutive model is derived for the nonlinear viscoelastic and viscoplastic behavior of semi-crystalline polymers at three-dimensional deformations. Adjustable parameters in the stress–strain relations are found by fitting the experimental data. It is demonstrated that (i) the model correctly approximates the observations and (ii) material parameters are independent of strain rate and change consistently with temperature.     

Transient heating effects in high pressure Diesel injector nozzles

The tendency of today’s fuel injection systems to reach injection pressures up to 3000 bar in order to meet forthcoming emission regulations may significantly increase liquid temperatures due to friction heating; this paper identifies numerically the importance of fuel pressurization, phase-change due to cavitation, wall heat transfer and needle valve motion on the fluid heating induced in high pressure Diesel fuel injectors. These parameters affect the nozzle discharge coefficient (C_d), fuel exit temperature, cavitation volume fraction and temperature distribution within the nozzle. Variable fuel properties, being a function of the local pressure and temperature are found necessary in order to simulate accurately the effects of depressurization and heating induced by friction forces. Comparison of CFD predictions against a 0-D thermodynamic model, indicates that although the mean exit temperature increase relative to the initial fuel temperature is proportional to (1 − C_d²) at fixed needle positions, it can significantly deviate from this value when the motion of the needle valve, controlling the opening and closing of the injection process, is taken into consideration. Increasing the inlet pressure from 2000 bar, which is the pressure utilized in today’s fuel systems to 3000 bar, results to significantly increased fluid temperatures above the boiling point of the Diesel fuel components and therefore regions of potential heterogeneous fuel boiling are identified.     

Numerical simulation of a supersonic gas–solid flow over a blunt body: The role of inter-particle collisions and two-way coupling effects

A supersonic dusty gas flow over a blunt body is considered. The mathematical model of the two-phase gas–particle flow takes into account the inter-particle collisions and the two-way coupling effects. The carrier gas is treated as a continuum, the averaged flow field of which is described by the complete Navier–Stokes equations with additional source terms modeling the reverse action of the dispersed phase. The dispersed phase is treated as a discrete set of solid particles, and its behavior is described by a kinetic Boltzmann-type equation. Particles impinging on the body surface are assumed to bounce from it. Numerical analysis is carried out for the cross-wise flow over a cylinder. The method of computational simulation represents a combination of a CFD-method for the carrier gas and a Monte Carlo method for the “gas” of particles. The dependence of the fine flow structure of the continuous and dispersed phases upon the free stream particle volume fraction α_p∞ and the particle radius r_p is investigated, particularly in the shock layer and in the boundary layer at the body surface. The particle volume fraction α_p∞ is varied from a negligibly low value to the value α_p∞ = 3 × 10⁵ at which inter-particle collisions and two-way coupling effects are simultaneously essential. Particular attention has been given to the particles of radii close to the critical value r_p1, because in this range of particle size the behavior of the particles and their effect on the carrier gas flow are not yet completely understood. An estimate of the turbulent kinetic energy produced by the particles in the shock layer is obtained.     

Methanol diffusion rates through the anode diffusion layer in direct methanol fuel cells from limiting current measurements

Metal foams may be used in direct methanol fuel cells to feed reactants to the catalyst layer and to collect current from the resulting electrochemical reaction. Although the mass transfer from the metal foam to the underlying gas diffusion layer (GDL) is diffusion-dominated, it is found that at a fixed methanol concentration, the limiting current density increases with increasing methanol flow rates. This unexpected result is attributed to the more efficient removal of product CO₂ from the GDL. A methodology is developed to estimate the effective diffusion coefficient of methanol in the anode diffusion layer from limiting current density measurements, and to extract the fraction of GDL volume occupied by CO₂.     

Computational investigation of hydrodynamics and cracking reaction in a heavy oil riser reactor

This paper presents a computational investigation of hydrodynamics, heat transfer and cracking reaction in a heavy oil riser operated in a novel operating mode of low temperature contact and high catalyst-to-oil ratio. Through incorporating feedstock vaporization and a 12-lump cracking kinetics model, a validated gas–solid flow model has been extended to the analysis of the hydrodynamic and reaction behavior in an industrial riser. The results indicate that the hydrodynamics, temperature and species concentration exhibit significantly nonuniform behavior inside the riser, especially in the atomization nozzle region. The lump concentration profiles along the riser height provide useful information for riser optimization. Compared to conventional fluid catalytic cracking (FCC) process, feedstock conversion and gasoline yield are respectively increased by 1.9 units and 1.0 unit in the new FCC process, the yield of liquefied petroleum gas is increased by about 1.0 unit while dry gas yield is reduced by about 0.3 unit.     

Phase field simulation of heterogeneous cubic → tetragonal martensite nucleation

The mechanisms for heterogeneous cubic → tetragonal martensite nucleation due to different types of microscopic defects (voids, stress-concentration site, inertial inclusion and pre-existing nucleus) and the temporal evolution of martensite morphology are monitored with finite element simulation of phase filed model. The results demonstrate that the nucleation prefers to occur around void and stress-concentration site initially; high residual stress exists around inertial inclusion; pre-existing nucleus promotes nearby martensite phase to develop on it. The effects of various defects on heterogeneous nucleation are different, and stress relaxation behavior is the dominant factor which characterizes the whole microstructure evolution process.