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This paper presents the optimization of the driving strategy of a high efficiency fuel cell based power train. This power train is developed to equip a light duty urban-concept vehicle that runs energetic races. The objective is to go the furthest with the lowest quantity of fuel. A comprehensive dynamical model is presented, including the mechanical requirement, the thermal behavior of the fuel cell stack and the various losses and consumptions of the power train devices. This model is next integrated into a global optimization algorithm, to determine the best race strategy to be adopted. These results are validated on experimental measurements, obtained during a real race at the Shell Eco-Marathon, in 2015.

In this work, we report the existence of a stoichiometric regime where the performances of operating polymer electrolyte membrane (PEM) fuel cells are entirely governed by the mass transport in the cathode channels. An analytical model of the fuel cell stoichiometric regime is derived and evidenced experimentally: from the cell spectral signature at low frequency based on electrochemical impedance spectroscopy, and from the current density distribution measured using a segmented current collector. The existence of such regime provides a simple way to characterize, model and predict PEM fuel cell performances.

International audience; The thermal design of an industrial shell-and-tube condenser requires the use of heat transfer coefficients, usually obtained from tables or correlations. Willing to develop a numerical model for design purposes, the present authors noticed the surprising diversity of correlations for the shellside heat transfer coefficient in the case of pure vapour condensation outside of horizontal smooth tubes. In order to shed light on this specific topic, a bibliographic study was therefore initiated. This comprehensive review is meant to provide the designers with means to understand how each correlation was obtained, from the assumptions to the resolution method. Thus two main phenomena are well accounted for in this paper: vapour shear stress and condensate inundation. Indeed, the review lists the most important contributions to this field and details their interconnections. Consequently, the present authors conclude this paper with their recommendations.

The motivation for the present work was to demonstrate clearly the effects of surface wettability on the two-phase flow in channels for PEMFC applications. Actually, these wettability effects are still controversial in the literature. The experimental investigation was conducted in isothermal conditions with ex situ dedicated experimental apparatus. Two channel materials with two different wettabilities were tested. In both hydrophobic and hydrophilic configurations, starting from a dry channel, the first emitted droplets left a residual liquid film in the channel corners and/or on the channel walls. These liquid films that spread all along the channel made the two-phase flow unstable. Finally, no distinction could be made between the two-phase flow regimes in hydrophilic and hydrophobic configurations. Our conclusions explain why the effects of surface wettability on fuel cells have been so unclear in the literature. For flow rates representative of a fuel cell application, it was impossible to draw flow pattern maps because of a lack of reproducibility. Based on the present results, the strict limit between slugs and droplets or droplets and film does not seem justified.

Within an emulsified combustible drop, micro-explosion is defined as the sudden vaporization of water drops inside the continuous phase, i.e. oil. Past studies have shown a relationship between the superheat temperature reached by the emulsion and the radii of inner water droplets. This raises the question of the heating history and the observed water coalescence prior to micro-explosion. The experimental set-up consists in a Differential Scanning Calorimetry (DSC) measurement device. To investigate water coalescence, the present study compares previous DSC micro-explosions of emulsion drops to oil drops containing a unique, bigger water droplet. It results that the unique water drop and the allegedly coalesced water drops exhibit the same behavior concerning superheat temperature. This thermal behavior is in agreement with the water coalescence just before micro-explosion, that was visualized in other studies.

This paper presents a topology optimization algorithm based on the lattice Boltzmann method coupled with a level-set method for increasing the efficiency of reactive fluid flows. The multi-relaxation time model is considered for the lattice Boltzmann collision operator, allowing higher Reynolds numbers flow simulations compared to the ordinary single-relaxation time model. The cost function gradient is obtained with the derivation of the adjoint-state formulation for the fully coupled problem. The proposed method is tested successfully on several numerical applications involving Reynolds numbers from 10 up to 1,000, as well as with different Damkohler and Peclet numbers. A limitation of the maximal pressure drop is also applied. The obtained results demonstrate that the proposed numerical method is robust and efficient for solving topology optimization problems of reactive fluid flows, in different operating conditions.

In this paper, a new dimensionless pseudo 2D steady state current density distribution model along the channel of a polymer electrolyte membrane (PEM) fuel cell is presented. This model includes four fundamental phenomena observed in PEM fuel cell cathode such as the air concentration depletion along the channel, the mass transport through the gas diffusion layer (GDL), the charge transport through the membrane and the electrochemical transfer in the catalyst layer (CL). A dimensional analysis is performed, and three dimensionless parameters are found to govern the current density distribution along the channel: a Peclet number at the channel/GDL interface, a Damkholer number at the GDL/CL interface, and a Wagner number at the CL/PEM interface. Four regimes of operation are defined based on the values of these dimensionless numbers, and for each of them new analytical solutions are developed. A good agreement with the experimental measurements of current density distribution reported in literature is found, concluding that the macroscopic PEM fuel cell physic is well described by our model. The specific operating regime described with relatively simple equations of current density distributions paves the way for a better control of fuel cell operation and performance, and in situ characterisation of fuel cell material properties.

International audience; This paper presents an adjoint Lattice Boltzmann Method (LBM) coupled with the Level-Set Method (LSM) for topology optimization of thermal fluid flows. The adjoint-state formulation implies discrete velocity directions in order to take into account the LBM boundary conditions. These boundary conditions are introduced at the beginning of the adjoint-state method as the LBM residuals, so that the adjoint-state boundary conditions can appear directly during the adjoint-state equation formulation. The proposed method is tested with 3 numerical examples concerning thermal fluid flows, but with different objectives: minimization of the mean temperature in the domain, maximization of the heat evacuated by the fluid, and maximization of the heat exchange with heated solid parts. This latter example, treated in several articles, is used to validate our method. In these optimization problems, a limitation of the maximal pressure drop and of the porosity (number of fluid elements) is also applied. The obtained results demonstrate that the method is robust and effective for solving topology optimization of thermal fluid flows.

In this article, the liquid water eruptive transport occurring during the water breakthrough from the Gas Diffusion Layer (GDL) of a PEMFC into the gas channel is investigated. A dedicated experimental set-up is used enabling the simultaneous measurement of pressure inside the water system and visualization of droplet formation. A single-drop capillary model is proposed to explain the eruptive nature of droplet formation. The model is built on four physical equations involving Laplace's law, a water system compressibility law, the pressure losses occurring during droplet formation, and a spherical droplet geometrical function. Two numerical resolution methods are implemented and compared. For this model, three parameters have to be identified from the experimental data. The identification procedure is explained in the paper and the comparison with experimental results shows the ability of the present model to reproduce the eruptive nature of liquid water breakthrough from the GDL.

The present paper deals with Heat Exchanger sizing methods and offers a comparison between two of them: 1D global method and CFD porous media method. Following Prithiviraj et al. work [1], new developments are based on recent knowledge acquired on porous media, using a coupling strategy of a three-dimensional commercial code with an in-house code library. The distributed hydraulic resistance concept and the numerical model are briefly described and confronted with pressure drop measurements from an experimental E-type STHE setup (shell-and-tube heat exchanger) from the literature. The present paper will put into perspective capabilities and limits of each method with needs for heat exchanger rating. Flow rate repartition is calculated with CFD-porous media using Tinker’s current approach. This new analysis provides a complete comparison with 1D global method. It also reveals the major impact of leakage flow rate between baffle and tubes. The numerical estimation of pressure losses, consistent with experimental measurements of Halle et al. [2], implies that our future work will include thermal performance characterization and geometrical optimization.

Polymer Electrolyte Membrane (PEM) fuel cells are considered as promising clean sources for automotive applications. A key challenge to reduce the cost of this technology is to increase the power density by operating PEM fuel cells at high current densities. Part of the research are focused in developing and designing new materials for this technology such as advanced catalyst layers (CL), new gas diffusion layers (GDL) structure, or innovative cell assembly conditions (clamping pressure, channel design…)[1]. To assess the performance of new materials, in situ characterisation techniques are required. Electrochemical impedance spectroscopy (EIS) is one of the most used technique to characterise the material impact onto the fuel cell mass/charge transfer, ohmic resistance and mass transport. However, due to the large number of parameters which has to be identified, inaccuracy in the identification process may arise leading to incorrect fuel cell material properties characterisation [2]. In authors’ recent work [3], it was shown that current density distributions can be used as a useful information to characterise fuel cell material properties. Thus, in this communication we will present a segmented cell designed and built to measure current density distribution along the channel while allowing optical access inside the channel to visualise the presence of liquid water. A range of fuel cell GDLs is characterised in situ in this cell for a range of operating conditions. The effective diffusivity of the GDLs is directly obtained from the current density distribution without any knowledge of the other fuel cell parameter (membrane ohmic resistance or CL kinetics). The values of effective diffusivity obtained using our methodology are compared to the ones obtained through EIS measurements, and their respective accuracy will be discussed. In addition, examples on the use of this characterisation cell to assess the performance of channel designs and fuel cell assembly conditions will be given. The results presented in this communication will introduce a novel fuel cell characterisation methodology which can strongly improve the development of more efficient fuel cell systems. References [1] T. Yoshida, K. Kojima, Toyota MIRAI Fuel Cell Vehicle and Progress Toward a Future Hydrogen Society, Interface Mag. 24 (2015) 45–49. [2] S. Chevalier, D. Trichet, B. Auvity, J.C. Olivier, C. Josset, M. Machmoum, Multiphysics DC and AC models of a PEMFC for the detection of degraded cell parameters, Int. J. Hydrogen Energy. 38 (2013) 11609–11618. [3] S. Chevalier, C. Josset, B. Auvity, Analytical solutions and dimensional analysis of pseudo 2D current density distribution model in PEM fuel cells, Renew. Energy. 125 (2018) 738–746.

This paper presents a pseudo 2D multiphysics A.C. and D.C. model that has been successfully used with Electrochemical impedance spectroscopy (EIS) measurements to detect cells state-of-health in a PEMFC stack. The model is able to reproduce the gas concentration oscillations that develop along the cell channel while doing EIS measurements. This feature makes it possible to explicitly take into account the air stoichiometry effects on cells state-of health. The model being both static and harmonic, the identification of model parameters is based on static cell data (current and voltage) and on measured cells impedance spectra. The cells state-of-health is presented in terms of repartition of losses due to four limiting phenomena: the membrane ionic resistance, the mass and charge transport limitations in the electrode, the mass transport limitations through GDLs and along the cell channels and the kinetic irreversibilities. When coupled with pressure drop measurements across the stack, it has been possible to prove that flooding conditions for the stack leads to inhomogeneous cells state-of-health: on one hand, most of the cells get flooded while, on the other hand, some of them remain in the nominal conditions.

The present paper proposes a new 2D modelling of ac impedance spectra of polymer electrolyte fuel cells (PEMFC). The computational domain includes the Membrane Electrode Assembly, the Gas Diffusion Layers and the channels on both the anode and cathode sides. The model takes into account the main fuel cell phenomena, i.e. reactants, charges transport and transfer and electrochemical reactions. First, the partial differential equations are solved in the steady state regime, then in the frequency domain in order to obtain the cell dynamic behaviour at different potentials. Experimental PEMFC impedance spectra are satisfactory reproduced over a relative large potentials range using only one set of model parameters. Numerical analysis of the key model parameters linked to the cell flooding state has been done. It is concluded that at least two impedance spectra at low and high potential are needed in order to discriminate the nature and the location of the cell degradations (anode or cathode, electrode or GDL). Based on a least square criterion, the model inversion is presented and several cell flooding scenarios have been precisely identified.

Polymer Electrolyte Membrane (PEM) fuel cells are considered as promising clean sources for automotive applications. A key challenge to reduce the cost of this technology is to increase the power density by operating PEM fuel cells at high current densities. One strategy is to improve the 3D structure of the gas diffusion layer (GDL) located between the channel and the catalyst layer (CL). At the cathode side, this GDL is used to evacuate the liquid water toward the channel while providing access to the oxygen to the CL [1]. Although innovative GDL structures have been proposed, a better understanding of the liquid water would help to design better GDL. In particular, few works have investigated the liquid water transport after several hours of steady state fuel cell operating conditions although changes in liquid water preferential pathway have been observed [2]. Such changes are not predicted in theory, and a physical explanation of these mechanisms is still required. In this presentation, we investigate ex situ the change of liquid water preferential pathways through PEM fuel cell GDLs. Liquid water breakthough from the GDL to the fuel cell channels is mimicked using a microfluidic PDMS device with embedded GDLs. Liquid water is injected through the GDL to the microchannel, and changes in liquid water pathways are observed after several hours of operations. These changes in liquid water pathway are attributed to the dynamic of droplet eruption in fuel cell microchannel. To validate this assumption, a small capillary network with is built. It is shown that after the liquid water breakthrough, the pressure variation due to the growth of the water droplets in the microchannel enables the liquid water to invade smaller capillaries which at the end can change the preferential path. The experimental observations reported in this work show that the invasion percolation theory can be used to predict the preferential pathway at the onset of the liquid water breakthrough, but pressure-induced dynamic transport [3] due the water droplet eruption in the channel has also to be taken into account to predict the preferential pathway after long time of fuel cell operation. References [1] T. Yoshida, K. Kojima, Toyota MIRAI Fuel Cell Vehicle and Progress Toward a Future Hydrogen Society, Interface Mag. 24 (2015) 45–49. [2] Z. Lu, M.M. Daino, C. Rath, S.G. Kandlikar, Water management studies in PEM fuel cells, part III: Dynamic breakthrough and intermittent drainage characteristics from GDLs with and without MPLs, Int. J. Hydrogen Energy. 35 (2010) 4222–4233. [3] S. Chevalier, C. Josset, B. Auvity, Fluid dynamic breakthrough in two connected capillaries: From stationary to oscillating state, Phys. Fluids. 29 (2017) 102102.

This paper presents a time-domain numerical simulator of oscillating water column (OWC) wave power plants. The whole problem (the simulation of the movements of the inner free surface due to the excitation of the outer sea-waves) is split into two subproblems: an outer one, dealing with the incident, diffracted and radiated waves, solved once for all, and an inner one, concerning the inner water volume behavior. The kernel of this simulator is a hybrid numerical method [Josset C, Clement AH, Duclos G. A hybrid method for time-domain simulation of oscillating water column wave power plants. Ocean Engineering, to appear.] which allows for the coupling of these two problems. After the validation of each part of this simulator, it was applied to the European wave energy power plant seated on Pico Island, Azores, first to estimate the annual performance of the plant, then to simulate different alternative configurations, in a way to improve the productivity.

This paper describes a numerical wave-to-wire model of the second-generation wave energy converter called SEAREV. Governing equations are given in the time domain for the motion of the masses involved in the device and for the hydraulic power take-off (PTO) used to convert the motion into electricity. The hydrodynamic forces are derived using the standard linear potential theory. The memory term in the radiation force is replaced by additional states using the Prony method in order to change the equation of motion into the ordinary differential equation form. The PTO is composed of hydraulic rams, an accumulator, and a hydraulic generator, which delivers electricity when there is enough energy stored in the accumulator. Using the MATLAB Simulink tool, the equation of motion is solved to simulate the full device (including the power take-off) from the incident wave to the electricity delivered to the grid. Simulation results are presented in the paper and comparisons are made with a simpler PTO: a linear damper. They show that the torque applied to the hydraulic PTO must exceed a threshold to start absorbing energy, unlike the linear damping model. They also show that the power production can be very discontinuous, depending on the level of the incident wave power. This is due to the fact that the generator considered can transform the energy stored in the accumulator faster than the energy transmitted by the rams into the accumulator. It could therefore be interesting to use several generators to adapt the electrical energy production to the level of incident wave power, or a generator that could work efficiently at part load in order to achieve continuous energy production.

International audience; This paper reports the results of an experimental investigation into the transport of fluids composed of water and polypropylene particles, in order to study the transport of floating particles. The determination of the pressure drop and the delivered concentration, for a wide combination of flow rates (0-0.5 kg s À1), solid loads (0-25 %vol) and particles mean sizes (341-756 lm), enabled the transport efficiency for this kind of slurry in a circular horizontal pipe (30 mm diameter) to be measured. The results are displayed in relation to flow patterns; they show that the size of the particles has a little impact on the transport efficiency. On the contrary, the flow rate and the volumetric concentration have a strong impact on efficiency, because of their large contribution to the flow pattern. The best operating conditions are obtained close to the limit deposition velocity, when the fully suspended pattern is reached. The solids are then well transported with no waste of energy due to a too high velocity. The optimum concentrations, for a wide range of high efficiency are around 20-25 %vol.

This paper is dedicated to the optimization of the driving strategy of a high efficiency fuel cell based power train. This power train is developed to equip an urban-concept vehicle that runs energetic races where the objectives are to go the furthest with the lowest quantity of fuel (Shell Eco Marathon). Through a comprehensive dynamical model, including the mechanical requirement, the thermal behavior of the fuel cell stack and the various losses and consumption of the power train devices. This model is then integrated into a global optimization algorithm, to determine the best race strategy to be adopted (velocity profile, motor current, gearbox ratio).

In this paper, the wave transmission, reflection, and energy dissipation characteristics as well as the hydrodynamic forces of two partially submerged breakwaters are compared. The breakwaters under consideration are designed to be installed in coastal areas where the water depth may be important (typically 50 m), making traditional bottom seated configurations unsuitable. They are composed of a free-surface piercing caisson mounted on vertical piles. The first breakwater considered in this study, as a matter of reference for comparison, is a rectangular caisson. The second one is a new profile proposed by Colmard and called BYBOP by the writer. The seaside face, fronting the ocean, is an elliptic profile while the harbor side is a plane inclined 55° to the vertical. A numerical simulation on both these shapes, at model scale, was carried out in two numerical wave tanks. The first one, CANAL, is based on fully nonlinear potential flow theory while the second one, ICARE, is a Reynolds averaged Navier–Stoke...

We investigate experimentally the formation of bedforms caused by the sustained flow of water and solid particles in a circular pipe ($\varnothing =30~\text{mm}$). The special feature of the tests carried out was the use of floating particles ($d=756~{\rm\mu}\text{m}$, ${\it\rho}_{s}=907~\text{kg}~\text{m}^{-3}$) whereas bedforms are usually studied with sedimental materials. A closed loop was used, so that the solid flux could be maintained for an infinite time. The finite size of the tube led to the saturation of the growth of the vortex ripples produced. For the set of parameters studied, the threshold of motion was obtained within a range of laminar to low turbulent flow. The saturated state was studied to characterise it for different flow rates and solid loads. The frequency, wavelength and propagating velocity of ripples were determined using different methodologies based on image analysis and pressure analysis. The frequency and propagating velocity show a clear linear dependence on the initial Shields number, while the wavelength seems to be constant in our experiments.

Chronic heart failure is linked to high rate of death and hospitalization. Some studies have highlighted the beneficial effect of heart failure clinics on morbidity and mortality. We have developed this type of structure at CHR Dubos since 3 years and we have recently created an heart failure clinic (10 beds). It's based on a concept including an experienced medical and nurse team, patient's and patient's family education and evaluation of the structure.

In this paper, we investigate the pore structure and the impacts of Haines jumps on the change in preferential pathways (called the dynamic breakthrough) during fluid percolation through thin porous media. Two capillaries connected in parallel are used to represent a thin porous medium, and Haines jumps are observed through the formation of droplets. Using a droplet growth model and experimental visualisations, the change in preferential pathways is shown to be strongly influenced by the pore lengths, pore radii ratios, and droplet detachment volumes. This work provides a better understanding of the redevelopment of continuous fluid paths observed through thin porous media in electrochemical systems.

The micro-explosion phenomenon is involved in emulsified fuel droplets placed in a hot atmosphere, such as spray combustion. Droplets of water-in-sunflower oil emulsion are used, since they are representative of a class of emulsions used in practical applications of biofuels. Once the micro-explosion is triggered after a short delay, the rapid (

International audience; This paper presents a study of the estimation of potential energy saving in bread baking process using the jet impingement technology. This new technology is developed to increase the heat transfer efficiency during the baking process. Based on a mechanistic heat exchange model identified in the past work, a non convex optimization problem is formulated taking account of a non zero energy cost related to the new technology. The simulation result shows that one can expect to obtain up to 12% of energy saving under some reasonable assumptions.

The burning of water in oil emulsions is considered as an effective alternative to bring out waste oil because of a significant reduction in carbon monoxide, nitrogen oxides and particulates in the exhaust. These advantages have different origins, an important contribution is provided by the phenomenon of micro-explosion. In this work, the influence of the size of the dispersed water droplets in the micro-explosion phenomenon is studied by the hot plate technique. The variation of the temperature and the evolution of the phenomenon have been investigated using a synchronized thermocouple/high speed imaging system. Three emulsions with the same amount of water (30% mass ) but with different distribution of the size of the dispersed water droplets have been tested. The results show that the size distribution of the dispersed water droplets plays an important role in the phenomenon of micro-explosion. Moreover, some internal phenomenon as the separation process between water and oil seems to affect significantly the phenomenon.

International audience; This paper illustrates the benefits of two energy optimization strategies to improve the overall process efficiency of a food defrosting system. First, an off-line energy analysis, including both the effects of the refrigeration cycle and the fan used to control the cooling air temperature and speed, is carried-out. This first approach puts on display an optimal running point of the process for a specific cooling air temperature value, which leads to an optimization of the overall energy consumption. Second, an on-line energy optimization approach, based on a nonlinear model-based predictive control strategy, is developed. This second approach takes simultaneously into account the expected thawing time, the highest temperature accepted and above all an energetic cost. Simulation results show the benefits of this on-line energy optimization to significantly increase the overall process efficiency. Indeed, this strategy leads to an optimization of the overall energy consumption whatever the expected thawing time and the inlet air temperature.

In this paper, an analytic tank to wheel model of a light-duty fuel-cell vehicle is presented. This model takes into account all the elements of the vehicle along with their interactions. It is used to optimize the velocity profile of the vehicle in order to minimize the energy consumption per kilometer.

This paper presents a fuel cell to wheel model that has been developed to design and optimize a high efficiency fuel cell based vehicle. This prototype car runs energetic races where the main objective is to go the furthest with the lowest energy. The main subsystems of the vehicle are the fuel-cell, the power converter, the motor and the mechanical train. Each subsystem has a best operating point which can not be obtained when all subsystems are linked together. The fuel-cell to wheel model is then used to reach the best efficiency and the results are compared with experimental measurements.

International audience; Abstract—This paper will begin by presenting two power takeoff (PTO) technologies for the SEAREV wave energy converter (WEC) followed by the design methodology applied to electromagnetic generator cycles for the all-electric solution. The operating principle associated with the SEAREVWEC will be described before discussing the two conversion technologies intended to transform wave energy into electricity. The types of systems are twofold: hydroelectric and all-electric. The strong coupling between the hydrodynamic, mechanical, and electrical phenomena heavily influences the behavior of the recovery (PTO) system and leads to a complex system design that requires a full-scale modeling description. A unique design methodology for the all-electric conversion chain has been developed around several distinct control modes, including one featuring power leveling.

Most crystallization models for ice slurries are based on the equilibrium thermodynamic approach, an assumption that is not made a priori here. We present results of simulations grounded on classical nucleation theory and crystal growth included in global Nakamura-type kinetics coupled with the one-dimensional nonlinear heat equation, another way to model the phase change. The work focuses on ice slurry crystallization using kinetic tools integrating the temperature glide for mixtures (i.e., the solid liquid equilibrium temperature T-f depends on the residual fluid's solute concentration) without any equilibrium assumption. The heat-transfer phenomena in the experimental device are first determined, and then the parameters of kinetic function of crystallization are identified for pure water samples and then for 15 wt% monopropylene glycol (MPG) - water mixtures. An inverse method is implemented with a genetic algorithm to determine the kinetic parameters. (C) 2010 Elsevier Ltd and IIR. All rights reserved.

International audience

Chronic heart failure is linked to high rate of death and hospitalization. Some studies have highlighted the beneficial effect of heart failure clinics on morbidity and mortality. We have developed this type of structure at CHR Dubos since 3 years and we have recently created an heart failure clinic (10 beds). It's based on a concept including an experienced medical and nurse team, patient's and patient's family education and evaluation of the structure.