Study of the effect of mechanical uncertainties parameters on performance of PEMFC by coupling a 3D numerical multiphysics model and design of experiment

As the PEMFC is a complex multi-physics device whose reliability and durability depend on the thermal-mechanical-electrical and chemical parameters. In this paper, theoretical and numerical studies is proposed to optimize the fuel cell performance using multiphysics model and design of experiments. 3D finite element analysis including a fully coupling of thermal-electrical-mechanical model is proposed to predict the electrical resistance of fuel cell. As the mechanical parameters (bending radius of the bipolar plate, thickness of the GDL and clamping pressure) remain uncertain, the design of experiments procedure is used to optimize the fuel cell behavior under several conditions.     

Two 3D thermomechanical numerical models of friction stir welding processes with a trigonal pin

ABSTRACT

During the friction stir welding (FSW) process, the behavior of the material is at the interface between solid mechanics and fluid mechanics. This article deals with a comparison of two 3D numerical models of FSW processes with a trigonal pin. The first model is based on a solid formulation and the second one is based on a fluid formulation. Both models use a Norton–Hoff constitutive model with the high temperature sensitivity of the parameters’ value and advanced numerical techniques such as the Arbitrary Lagrangian Eulerian (ALE) formalism. It can be concluded that, basically, these two formulations lead to the same results.     

Safe heating-up of a full scale SOFC system using 3D multiphysics modelling optimisation

Heating-up strategies of full scale solid oxide fuel cell (SOFC) systems still affect the safe operation of the system and incorporation of the technology into the global energy sector. To ensure rapid start-up times whilst retaining the structural reliability of the SOFC system components, requires a safe heating-up operation. To master a controlled heating-up stage, detailed understanding of the component interaction and multiphysics within a fuel cell system is required. State of the art dynamic fuel cell system modelling comprises sub-models of the assembly, or is based on empirical nature. However, invaluable information of the multiphysics inside the system is lost. Therefore, it is of paramount importance to understand and improve the knowledge of the detailed processes, occurring within the interacting components. The effect of integrating different electrical heater cartridges at different locations has been thoroughly investigated to optimise the heating-up of the system. The study utilises a previously developed and experimentally validated full scale three dimensional planar type SOFC system model to mitigate experimental costs and shed light on the details, occurring within the system. A comparison to a simplified variant of the model has been added to shed light on its effect on the results.     

Hierarchical 3D multiphysics modelling in the design and optimisation of SOFC system components

This paper presents a novel bottom-up modelling approach to aid in the design and optimisation of the Research Centre Jülich type integrated module components. The approach is demonstrated by employing the air pre-heater component. A feasibility study considering the thermo fluid, thermo mechanical behaviour of the physical air pre-heater design compared to a simplified design is introduced. Analogue design simplifications are performed to the afterburner and pre-reformer components. The results reveal that a simplified design can be feasible for thermo fluid flow analyses, but are not representative for mechanical analyses. The integrated module considering the simplified system components is simulated in 3D, considering the multiphysics that occur within each component. The predictions of the air pre-heater component obtained from the integrated module analysis are compared to the stand-alone air pre-heater simulation results. The results are in very good agreement. The approach is proven to be useful for the optimisation of the integrated module. Moreover, the investigation of local processes, critical regions can be visualised. The methodology demonstrates the effective simulation of large scale systems in 3D.     

A numerical analysis of PEMFC stack assembly through a 3D finite element model

A finite element model is developed to investigate the influence of the assembly phase of proton exchange membrane fuel cell (PEMFC) stacks on the mechanical state of the active layer (MEAs). Validated by experimental measurements, this model offers the possibility to analyze the influence of different parameters through the use of a complete parametric set, such as the number of cells and their position in the stack. The simulations show that a better uniformity of the MEA compression is obtained with the greatest number of cells, and at the center of the stack. The finite element analysis (FEA) is finally found to be an effective tool to show the influence of the assembly phase on the performance of PEMFCs, and will help the designer to adapt the future generations of stack to ensure the uniformity of the MEA mechanical strain.     

Sensitivity analysis for solid oxide fuel cells using a three-dimensional numerical model

A three-dimensional numerical solver is developed to model complex transport processes inside all components of a solid oxide fuel cell (SOFC). An initial assessment of the accuracy of the model is made by comparing a numerically generated polarization curve with experimental results. Sensitivity derivatives of objective functions representing the cell voltage and the concentration polarization are obtained with respect to the material properties of the anode and the cathode using discrete adjoint method. Implementation of the discrete adjoint method is validated by comparing sensitivity derivatives obtained using the adjoint technique with results obtained using direct-differentiation and finite-difference methods.     

3D transient multiphysics modelling of a complete high temperature fuel cell system using coupled CFD and FEM

Full commercialisation of the solid oxide fuel cell (SOFC) technology faces many technological challenges that prevent the incorporation of the technology into the global energy sector. The effort to increase the transient thermomechanical reliability of the interacting fuel cell components and the associated fuel cell system requires a comprehensive understanding of the complex multiphysics, occurring within the system. State of the art dynamic fuel cell system modelling comprises sub-models of the assembly, or is based on empirical nature. The present study introduces a transient, coupled 3D computational fluid dynamics/computational solid mechanics model of a complete solid oxide fuel cell system and its experimental validation. The model includes all system components; namely the fuel cell stack, afterburner, pre-reformer, air pre-heater and the auxiliary components. All components are presented in their real geometrical resolution. The capabilities of the 3D system level model are demonstrated by simulating the heating-up process and the critical system locations susceptible to thermomechanically induced stress, over time.     

1D and 3D numerical simulations in PEM fuel cells

The potential of fuel cells for clean and efficient energy conversion is generally recognized.The proton-exchange membrane (PEM) fuel cells are one of the most promising types of fuel cells. Models play an important role in fuel cell development since they enable the understanding of the influence of different parameters on the cell performance allowing a systematic simulation, design and optimization of fuel cells systems. In the present work, one-dimensional and three-dimensional numerical simulations were performed and compared with experimental data obtained in a PEM fuel cell. The 1D model, coupling heat and mass transfer effects, was previously developed and validated by the same authors [1] and [2]. The 3D numerical simulations were obtained using the commercial code FLUENT - PEMFC module.The results show that 1D and 3D model simulations considering just one phase for the water flow are similar, with a slightly better accordance for the 1D model exhibiting a substantially lower CPU time. However both numerical results over predict the fuel cell performance while the 3D simulations reproduce very well the experimental data. The effect of the relative humidity of gases and operation temperature on fuel cell performance was also studied both through the comparison of the polarization curves for the 1D and 3D simulations and experimental data and through the analysis of relevant physical parameters such as the water membrane content and the proton conductivity. A polarization curve with the 1D model is obtained with a CPU time around 5 min, while the 3D computing time is around 24 h. The results show that the 1D model can be used to predict optimal operating conditions in PEMFCs and the general trends of the impact on fuel cell performance of several important physical parameters (such as those related to the water management). The use of the 3D numerical simulations is indicated if more detailed predictions are needed namely the spatial distribution and visualization of various relevant parameters.An important conclusion of this work is the demonstration that a simpler model using low CPU has potential to be used in real-time PEMFC simulations.     

3D numerical study on syngas production in a structured packed bed with connected pellets

Fuel-rich partial combustion in porous media is numerically studied at the pore scale with 3D staggered arrangement of connected pellets. The chemistry is treated with detailed chemical mechanism and the solid-to-solid radiation is taken into account. The numerical results are validated against experimental results and indicates that 3D pore scale numerical modeling gives reasonable syngas components. Considerable heat recirculation in the preheating zone is predicted by 3D pore-scale model. Root mean squares (RMS) is used to quantitatively estimate the local spatial variations of the velocity and gas temperature. It is shown that the distribution of RMS of gas temperature along flow direction is basically wave-like shape and the spatial variation of gas velocity reaches 78% of the average velocity.     

Comparison of a small-scale 3D PCM thermal control model with a validated 2D PCM thermal control model

A three-dimensional (3D) numerical model was developed to simulate the use of a phase change material linked to a photovoltaic (PV) system to control the temperature rise of the PV cells. The model can be used to predict temperatures, velocity fields and vortex formation within the system. The 3D predictions have been compared with those from a previously developed experimental validated two-dimensional (2D) finite-volume heat transfer model conjugated hydro-dynamically to solve the Navier–Stokes and energy equations. It was found that for the systems simulated with appropriate boundary conditions, the 2D model predictions compare well with those of the 3D model. The 3D model was used to predict the temperature distributions when the heat transfer to the phase change material was enhanced by high thermal conductivity pin fins.     

3D numerical optimization of a heat sink base for electronics cooling

In this paper, the possible optimal thickness of a heat sink base has been explored numerically with different convective heat transfer boundary conditions in a dimensionless three dimensional heat transfer model. From the numerical results, relations among different heat transfer mechanisms (natural or forced, air or liquid), different area ratios of a heat sink to a heating source, and the lowest thermal resistance have been obtained and discussed. Also a simple correlation for these three parameters from data fitting is given for guiding a heat sink design.     

Sensitivity analysis of a biomass gasification model in fixed bed downdraft reactors: Effect of model and process parameters on reaction front

A detailed sensitivity analysis is performed on a one-dimensional fixed bed downdraft biomass gasification model. The aim of this work is to analyze how the heat transfer mechanisms and rates are affected as reaction front progresses along the bed with its main reactive stages (drying, pyrolysis, combustion and reduction) under auto-thermal conditions. To this end, a batch type fixed-bed gasifier was simulated and used to study process propagation velocity of biomass gasification. The previously proposed model was validated with experimental data as a function of particle size. The model was capable of predicting coherently the physicochemical processes of gasification allowing an agreement between experimental and calculated data with an average error of 8%. Model sensitivity to parametric changes in several model and process parameters was evaluated by analyzing their effect on heat transfer mechanisms of reaction front (solid–gas, bed–wall and radiative in the solid phase) and key response variables (temperature field, maximum solid and gas temperatures inside the bed, flame front velocity, biomass consumption and fuel/air ratio). The model coefficients analyzed were the solid–gas heat transfer, radiation absorption, bed–wall heat transfer, pyrolysis kinetic rates and reactor-environment heat transfer. On the other hand, particle size, bed void fraction, air intake temperature, gasifying agent composition and gasifier wall material were analyzed as process parameters. The solid–gas heat transfer coefficient (0.02 < correction factor < 1.0) and particle size (4 < diameter < 30 mm) were the most significant parameters affecting process behavior. They led to variations of 88% and 68% in process velocity, respectively.     

3D heat transfer model of hybrid laser Nd:Yag-MAG welding of S355 steel and experimental validation

A three-dimensional heat transfer model was developed to predict the temperature fields, the weld geometry and the shape of the solidified weld reinforcement surface during hybrid laser-MAG arc welding of fillet joints. Melt pool deformation due to arc pressure was calculated by minimizing the total surface energy. A series of hybrid welding experiments was conducted on S355 steel for different welding speeds and wire feeding rates. A high speed video camera was used to measure weld pool depression and surface weld pool geometry. Visualization of the weld pool during welding has also allowed for a better understanding of the interaction between the keyhole and droplets. The various weld bead shapes were explained through these observations. The arc pressure, the surface energy distribution, and arc efficiency were evaluated by comparing experimental data and numerical results for a wide range of welding operating parameters. Good correlation was found between the calculated and experimental weld bead shapes obtained for the hybrid laser-MAG arc welding process as well as for laser or MAG alone.     

Sensitivity analysis of the uncertainties of the mechanical design parameters: Stochastic study performed via a numerical design of experiment

As the GDL (Gas diffusion layer) is the most sensitive component in the fuel cell, any change in its structure causes a change in its porosity, which strongly influences the contact between the components of the fuel cell. Note that the state of contact depends on the applied clamping pressure, the thickness and the porosity of the GDL, and the geometry of the rib (bending radius) of the BPP (Bipolar plates). These components can be subject to variations coming from very high compression, so it is necessary to consider the reliability of their dimension via modeling/simulation by the integration of uncertainties. In this article, we will study the influence on the contact pressure of the uncertainties of the mechanical design parameters. A probabilistic approach (Gauss's law) is applied to evaluate the effect of the mechanical uncertainties parameter on the contact pressure between GDL/MEA and GDL/BPP.     

Development of a numerical model for natural gas steam reforming and coupling with a furnace model

This paper presents a numerical model (SRP) that was developed to describe the steam reforming process within tubes or channels. This model was implemented in C language and is used as a User-Defined Function (UDF) in the commercial program Fluent. The SRP model is one-dimensional representing mass and energy balances along the tubes/channels assuming uniform conditions in the cross section, except for temperature within porous regions. The model calculates the gas species concentrations and temperature profiles along the tubes/channels and since it is coupled with the Fluent furnace calculation, the boundary temperature is continuously updated and is a model result.     

Sensitivity of structural analyses to material parameters and application to elastoplastic constitutive equations through a case study

Structural failure is often related to cyclic loading. The applied loading can have a mechanical, thermal or combined nature. Even where cyclic loadings are only thermal, non-compensated deformation can appear from one cycle to another. This will lead to an accumulation of deformation with time until the structure fails by excessive deformation. The prediction of the structural response is directly dependent on the smoothness of the chosen model and its capacity to include the governing phenomena.     

A numerical study to investigate the effect of turbulators on thermal flow and heat performance of a 3D pipe

To reduce the heat exchanger's costs in a highly competitive industry, thermal performance enhancement of the heat exchangers has successfully gained attention in the last few decades. Among different engineering approaches, the application of the enhanced pipes provides a key solution to improve heat performance. In this paper, the investigation develops a numerical study based on the commercially available computational fluid dynamics codes on the turbulent flow in three-dimensional tubular pipes. Various concavity (dimple) diameters with corrugation and twisted tape configurations are investigated. The study has shown that perforated geometrical parameters lead to a high fluid mixing and flow perturbation between the pipe core region and the walls, hence better thermal efficiency. Moreover, a model of concavity (dimple) with a 4 mm diameter allows the highest heat transfer enhancement among other designs. In addition, the study shows that due to the disturbance between the pipe core region and the pipe wall, the transverse vortices and swirl flow generated are forceful, which leads to better heat transfer enhancement compared with the conventional (smooth) pipes. As the Reynolds number (Re) rises, the mixing flow, secondary, and separation flow extend to become higher than the values in a smooth pipe, allowing a higher value of performance evaluation factor to be achieved for a dimple diameter of 1mm at the low Re values. This study, therefore, shows the promising potential of the enhanced pipes in the heat transfer enhancement of heat exchangers that is crucial in industrial applications to save more energy.     

3D numerical simulation of transient temperature field for lens mold embedded with heaters

In injection mold, cooling process has important effects on parts quality and production cost. Traditional cooling simulation is conducted in a decoupled way by a cycle-averaged approach plus one-dimensional transient variation for computational efficiency concern. In the present study, fully 3D simulations of mold temperature variation based on FEM and FVM implemented in parallel computation scheme were executed for lens mold embedded with heaters. With the fully 3D simulation, mold temperature and melt temperature are solved in coupled manner and obtained simultaneously. Also the time when mold temperature distribution reaches the cyclic-average steady state can be determined. The simulation results were verified with measured cavity temperatures using infrared thermal image system at interest locations. The numerical results for cyclic, transient with heat source problem show reasonable consistency with measured values. The computing time with multi-CPU and parallel scheme is much less than that of single CPU.     

3D CFD model of a multi-cell high-temperature electrolysis stack

A three-dimensional (3D) computational fluid dynamics (CFDs) electrochemical model has been created to model high-temperature electrolysis stack performance and steam electrolysis in the Idaho National Laboratory (INL) Integrated Lab Scale (ILS) experiment. The model is made of 60 planar cells stacked on top of each other operated as solid oxide electrolysis cells (SOECs). Details of the model geometry are specific to a stack that was fabricated by Ceramatec, Inc. [References herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the U.S. Government, any agency thereof, or any company affiliated with the Idaho National Laboratory]. and tested at INL. Inlet and outlet plenum flow and distribution are considered. Mass, momentum, energy, and species conservation and transport are provided via the core features of the commercial CFD code FLUENT. [References herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the U.S. Government, any agency thereof, or any company affiliated with the Idaho National Laboratory]. A solid oxide fuel cell (SOFC) model adds the electrochemical reactions and loss mechanisms and computation of the electric field throughout the cell. The FLUENT SOFC user-defined subroutine was modified for this work to allow for operation in the SOEC mode. Model results provide detailed profiles of temperature, Nernst potential, operating potential, activation over potential, anode-side gas composition, cathode-side gas composition, current density, and hydrogen production over a range of stack operating conditions. Variations in flow distribution and species concentration are discussed. End effects of flow and per-cell voltage are also considered.     

3D numerical modeling of non-isotropic turbulent buoyant helicoidal flow and heat transfer in a curved open channel

A 3D non-isotropic algebraic stress/flux turbulence model is employed to simulate turbulent buoyant helicoidal flow and heat transfer in a rectangular curved open channel. The prediction shows that, unlike the isothermal flow, there are two major and one minor secondary flow eddies in a cross section of thermally stratified turbulent buoyant helicoidal flow in a curved open channel. The results compare favorably with available experimental data. The thermocline in a curved channel is thicker than that in a straight channel. All of these is the result of complex interaction between the buoyant force, the centrifugal force and the Reynolds stresses. The turbulent flow in a curved channel is obviously non-isotropic: the turbulence fluctuations in vertical and radial directions are lower in magnitude than that in the axial direction, which illustrates the suppression of turbulence due to buoyant and centrifugal forces. The results are of significant practical value to engineering works such as the choice of sites for intake and pollutant-discharge structures in a curved river.