Flow Field Channels Research Articles

CFD investigation of the effect of flow field channel design based on constriction and enlargement configurations on PEMFC performance

In this study, a three-dimensional CFD, non-isothermal model was developed to identify an optimal layout for key geometrical parameters of PEMFC flow field designs with constriction and enlargement cross-sections located at the middle of the channel. For this purpose, the effect of channel configurations with both constriction and enlargement cross-sections was explored and compared with a conventional constant channel segment, under varying hydraulic diameter and lengths. As results, the constriction configuration channel with a hydraulic diameter reduction of 50% has shown the best performance among the enlargement and straight channels for all considered lengths. The constriction configuration enhances fluid velocity, resulting in improved flow distribution and relatively increased pressure drop across the flow channels. Moreover, a better temperature distribution, higher oxygen consumption, improved membrane humidification, and water removal were observed. This leads to an increased net power output of the PEMFC. However, considering different hydraulic diameters of the constriction geometry, the best performance of the cell was achieved at a hydraulic diameter reduction of 60%, combined with a length of 4/7 of the total channel length. This optimal balance between pressure drops and reactants’ flow has enhanced the net power of the proposed geometry by 26.30 %, compared to the straight channel. The novel proposed single-channel configuration based on a simple constriction, is a low-cost design that has significantly improved cell performance compared to the straight one. Since operating conditions are crucial for cell performance, the effects of operating temperature and relative humidity were investigated, and the optimum conditions were determined.

Modeling and Simulation of a High Temperature Proton Exchange Membrane Fuel Cell

High temperature (HT) (around 120-200°C) PEM FCs are predicted to be the next generation of PEMFCs particularly for hydrogen-powered automobiles and combined heat and power (CHP) systems because the water management can be simplified at such temperatures as only a single phase of water vapor needs to be considered. Additionally, the cooling system can be streamlined due to an increase in temperature gradient between the coolant and the FC stack. This will also allow easy recovery of waste heat that can be used as a practical heat source. Furthermore, the CO tolerance improves dramatically at high temperatures thereby allowing HT PEMFCs to utilize reformed and impure hydrogen.A single phase three-dimensional, steady-state, isothermal model for a single 5 HT PEM fuel cell with serpentine flow channels is implemented in COMSOL to investigate the effect of various operating conditions like temperature, back pressure and cathode flow rate and design parameters like catalyst layer loading, cathode GDL porosity, and flow field channels including parallel and interdigitated channels. Different performance indicators in terms of polarization curve, loss mechanisms, oxygen molar concentration, and anode and cathode pressure are represented for a complete overall analysis. In fact, the breakdown of different overpotential loss mechanism for HT PEMFC presented here is unique that not only quantifies these losses but also provides an accurate comparison with corresponding low temperature counterpart. The result from the computational model follows the experimental result very closely, thus, validating our model. The simulations stipulates that the performance of a HT PEMFC improves with increasing temperature, back pressure, and air flow rate. Increasing catalyst layer loading improves the performance up to a point after which it starts to drop at low voltages because of hindered gas diffusion. Similarly, a comparative study among different flow field channel is also presented indicating improved performance when going from parallel to interdigitated and serpentine channels. Furthermore, this study provides guidelines to optimize HT PEMFC performance through comprehensive parametric study.

Water liquid distribution in a bioinspired PEM fuel cell

Water management is a key factor in the operation of hydrogen fuel cells since its formation may lead to significant mass transport losses, oxygen diffusion limitation and membrane durability issues. In this work, the effect of different operating conditions on the liquid water distribution inside a 50 cm2 active area bio-inspired PEM fuel cell has been studied. Therefore, a set of experiments was designed varying cell pressure, the reactants relative humidity (anode and cathode), temperature, and cell current density. Liquid water distribution for each operating condition was determined using neutron imaging technique as it has been proved to be an excellent technique for this purpose, including quantitative analysis and water profiles in the different areas of the bio-inspired flow field. The results show that high relative humidity of the inlet gas flows, high pressure, low temperatures and low current density favor the accumulation of water in the flow field channels and GDL. Specifically, water accumulates preferentially in the anode side that make contact with the low part of the cathode foams inserted in the flow field, instead of blocking the closest area to the gases outlets points. This is due to permeation from the cathode foams to the anode side.

Effect of Serpentine Flow Field Channel Dimensions and Electrode Intrusion on Flow Hydrodynamics in an All-Iron Redox Flow Battery

This paper presents a study on flow hydrodynamics for single-channel serpentine flow field (SCSFF) and cross-split serpentine flow field configurations (CSSFF) for different geometric dimensions of channel and rib width ratios with electrode intrusion over varying compression ratios (CRs) in an all-iron redox flow battery. Pressure drops (Δp) measured experimentally across a cell active area of 131 cm2 for different electrolyte flow rates were numerically validated. A computational fluid dynamics study was conducted for detailed flow analyses, velocity magnitude contours, flow distribution, and uniformity index for the intrusion effect of a graphite felt electrode bearing a thickness of 6 mm with a channel compressed to varying percentages of 50%, 60%, and 70%. Experimental pressure drops (Δp) over the numerical value resulted in the maximum error approximated to 4%, showing good agreement. It was also reported that the modified version of the cross-split serpentine flow field, model D, had the lowest pressure drop, Δp, of 2223.4 pa, with a maximum uniformity index at the electrode midplane of 0.827 for CR 50%, across the active cell area. The pressure drop (Δp) was predominantly higher for increased compression ratios, wherein intrusion phenomena led to changes in electrochemical activity; it was found that the velocity distribution was quite uniform for a volumetric uniformity index greater than 80% in the felt.

Physics-Informed Super-Resolution of Turbulent Channel Flows via Three-Dimensional Generative Adversarial Networks

For a few decades, machine learning has been extensively utilized for turbulence research. The goal of this work is to investigate the reconstruction of turbulence from minimal or lower-resolution datasets as inputs using reduced-order models. This work seeks to effectively reconstruct high-resolution 3D turbulent flow fields using unsupervised physics-informed deep learning. The first objective of this study is to reconstruct turbulent channel flow fields and verify these with respect to the statistics. The second objective is to compare the turbulent flow structures generated from a GAN with a DNS. The proposed deep learning algorithm effectively replicated the first- and second-order statistics of turbulent channel flows of Reτ= 180 within a 2% and 5% error, respectively. Additionally, by incorporating physics-based corrections to the loss functions, the proposed algorithm was also able to reconstruct λ2 structures. The results suggest that the proposed algorithm can be useful for reconstructing a range of 3D turbulent flows given computational and experimental efforts.

Measurement of liquid water distribution in GDL under cross-flow-inducing parallel flow field using operando synchrotron X-ray radiography

Baffles in flow field channels for air in a polymer electrolyte fuel cell are known to enhance the performance by inducing convective flow through the gas diffusion layer (GDL) with smaller pressure loss than interdigitated flow fields. This work experimentally correlates performance enhancement with the amount of liquid water in a GDL. A parallel flow field (PFF) that has different inlet and outlet opening sizes is applied to induce cross-flow in the GDL under the rib between adjacent air channels. Operando synchrotron X-ray radiography experiments are conducted to compare the amount of water in the GDL with that for a PFF having the same inlet and outlet sizes. The performance enhancement by application of different opening sizes increases with decreasing relative humidity and increasing air flow rate. A significant performance enhancement is observed when the amount of water in the GDL substrate under the rib becomes almost zero. No performance enhancement is observed under over-humidified conditions, although a decrease in the amount of water in the GDL is still observed, which suggests that the performance becomes insensitive to the difference in the liquid water saturation as the saturation increases.

Review of Flow Field Designs for Polymer Electrolyte Membrane Fuel Cells

The performance of a polymer electrolyte membrane fuel cell (PEMFC) closely depends on internal reactant diffusion and liquid water removal. As one of the key components of PEMFCs, bipolar plates (BPs) provide paths for reactant diffusion and product transport. Therefore, to achieve high fuel cell performance, one key issue is designing BPs with a reasonable flow field. This paper provides a comprehensive review of various modifications of the conventional parallel flow field, interdigitated flow field, and serpentine flow field to improve fuel cells’ overall performance. The main focuses for modifications of conventional flow fields are flow field shape, length, aspect ratio, baffle, trap, auxiliary inlet, and channels, as well as channel numbers. These modifications can partly enhance reactant diffusion and product transport while maintaining an acceptable flow pressure drop. This review also covers the detailed structural description of the newly developed flow fields, including the 3D flow field, metal flow field, and bionic flow field. Moreover, the effects of these flow field designs on the internal physical quantity transport and distribution, as well as the fuel cells’ overall performance, are investigated. This review describes state-of-the-art flow field design, identifies the key research gaps, and provides references and guidance for the design of high-performance flow fields for PEMFCs in the future.

Three-dimensional performance simulation of PEMFC of metal foam flow plate reconstructed with improved full morphology

Recently, Proton Exchange Membrane Fuel Cell (PEMFC) with metal foam flow field has attracted extensive attention from scholars. In the present work, the full morphological reconstruction method of metal foam is improved and is successfully applied to a three-dimensional simulation of a metal foam PEMFC. The numerical predictions of metal foam PEMFC before and after reconstruction improvement are compared. The numerical results of the traditional channel flow field and two metal foam flow fields with pore sizes (40PPI and 100PPI) are compared. It is found that the application of metal foam can greatly improve the performance of PEMFC at higher current densities, and the smaller pore size (100PPI) of the metal foam makes the performance better. In addition, the numerical results of the oxygen concentration, ohmic loss, intra-membrane ionic current density and pressure drop of the cathode component are elaborated to explain the phenomenon.

Experimental study on Velocity Distribution in Rod Bundle Channels under Flow Fluctuation

Thermal-hydraulic behaviors within a PWR fuel assembly is very complex under accident conditions and marine conditions, which has a significant impact on reactor safety. It is crucial to carry out studies on thermal-hydraulic properties under transient conditions. A telecentric lens and a pulse signal generator are applied to improve the performance of PIV system, a high temporal and spatial resolution measurement for flow fields in rod bundle channels is achieved, the velocity distribution in rod bundles are obtained under accelerating flows and decelerating flows. The steady-state flow field is measured to use as a reference, the effect of flow fluctuations on the flow fields in the rod bundle is analyzed. The results present, the accelerating flows improve the velocity gradient between adjacent flow layers in rod bundles and weaken the lateral velocity. In the contrary, the decelerating flows weaken the velocity gradient between flow layers in rod bundles and improve lateral velocity. Conversely, the velocity gradient attenuates in decelerating flows between the rod beam layers and increases the lateral velocity. During the accident, the thermal-hydraulic behavior in the reactor has the characteristics of transient complexity and uncertainty. Studying the flow and heat transfer characteristics of the reactor core fuel assembly under accident conditions can improve a understanding for the accident development process.

Double Parameters Generalization of Water-Blocking Effect of Submerged Vegetation

Submerged vegetation has strong vitality, and the root system is highly developed. Because this vegetation has a good bank-solidifying-and-beautifying effect, it is widely used in ecological river construction. However, the open channel flow field and water-blocking mechanism of submerged vegetation are complicated. It is not convenient to use this kind of original model directly in engineering calculation, but it can be much more convenient if the original model is generalized into a simple model. However, there are not many generalization models, so it is necessary to propose a simple generalization model of the water-blocking effect of submerged vegetation to facilitate engineering calculation. Upon theoretical analysis, numerical calculation and experiment data analysis, the following conclusions are obtained: As the basis of generalization, in order to make up for the deficiency of experimental results, a new numerical simulation model for the flow field of submerged vegetation open channel flow was firstly proposed. For the purpose of this research, a simple generalization model of the water-blocking effect of submerged vegetation was proposed. Finally, two parameters of generalized roughness coefficient and virtual channel elevation were obtained to reflect the water-blocking effect. They can be substituted directly into a planar two-dimensional model in engineering. It achieves the ultimate goal of convenient engineering calculation.

Effect of non-uniform heating on temperature and velocity profiles of buoyancy driven flow in vertical channel of prismatic modular reactor core

To overcome the lack of experimental results of buoyancy-driven flow fields in vertical core channels of prismatic modular reactors, hot wire anemometry; micro foil sensors; and T-type thermocouples were integrated simultaneously at six axial positions along an electrically heated stainless steel channel of a test facility designed and developed with a representative geometry of prismatic modular gas-cooled reactor. At each axial position, nine radial measurements were obtained from the wall to the centerline of the channel. Fluid measurements display typical temperature and velocity profiles throughout the inlet- and mid-section of the channel. However, wall-to-fluid temperature differences along with axial variation of velocity fields indicate flow instabilities at the top section of the channel. This could be attributed to heat losses by conduction through the thickness of channel’s wall in addition to interactions occurring between heated air jets exiting the core channel and cooled air plumes cooled in the upper plenum which is anticipated to take place more rigorously in real prismatic facilities. Additionally, a new definition of the characteristic temperature difference is proposed to calculate a modified Grashof number that can be used to indicate such flow instabilities. Distortion factors between the developed facility and reference modular gas-cooled reactor are found to range from 18% to 24%. Current results are crucial for guiding and validating the commercial computational fluid dynamics codes.

Experimental study of the flow field in compound channels with converging and inclined floodplains

ABSTRACT This paper presents experimental results of overbank flow in compound channels with converging and inclined floodplains. Velocity and boundary shear stress distributions at different selected sections along the narrowing part of the flume for two convergence angles of 3.81° and 11.31° were measured. The study indicated that the velocity and boundary shear stress increase along the flume, especially in the second half of the converging part of the flume. It was shown that the maximum velocity at the end of the converging transition can be over than twice as much as at the beginning section. By using the experimental data and force–momentum balance, the apparent shear forces at the vertical interface between the main channel and floodplains were investigated. The apparent shear forces at the vertical interface of the flume subsections increased with the increase of the convergence angle and relative depth, reached a peak at the relative depth of 0.4, and then decreased.

Subgrid Variational Optimized Optical Flow Estimation Algorithm for Image Velocimetry

The variational optical flow model is used in this work to investigate a subgrid-scale optimization approach for modeling complex fluid flows in image sequences and estimating their two-dimensional velocity fields. To solve the problem of lack of sub-grid small-scale structure information in variational optical flow estimation, we combine the motion laws of incompressible fluids. Introducing the idea of large eddy simulation, the instantaneous motion can be decomposed into large-scale motion and a small-scale turbulence in the data term. The Smagorinsky model is used to model and solve the small-scale turbulence. The improved subgrid scale Horn-Schunck (SGS-HS) optical flow algorithm provides better results in velocity field estimation of turbulent image sequences than the traditional Farneback dense optical flow algorithm. To make the SGS-HS algorithm equally competent for the open channel flow measurement task, a velocity gradient constraint is chosen for the canonical term of the model, which is used to improve the accuracy of the SGS-HS algorithm in velocimetric experiments in the case of the relatively uniform flow direction of the open channel flow field. The experimental results show that our algorithm has better performance in open channel velocimetry compared with the conventional algorithm.

An improved zigzag-type printed circuit heat exchanger for supercritical CO2 Brayton cycles

Printed Circuit Heat Exchanger (PCHE) is a kind of high performance compact heat exchanger. In this study, a zigzag-type PCHE with improved flow passage arrangement, i.e., cellular arrangement, is proposed considering the fin efficiency enhancement. And the new PCHE with channels cellular arrangement coupled with supercritical CO2 (sCO2) Brayton cycle is investigated numerically. Flow and heat transfer characteristics in cellular zigzag-type PCHE is compared with the traditional flat-plate zigzag-type PCHE. Results show that the structure of the flow field in zigzag channels promotes the mixing between the fluid near the wall and the bulk fluid. The process of mixing is beneficial to intensify the convective heat transfer capacity. A pronounced increase of frictional pressure drop is also observed in zigzag channels when compared with straight channel. The overall heat transfer coefficient of cellular zigzag-type PCHE is about 8.6% higher than that of traditional flat-plate zigzag-type PCHE with the same operating condition in this study. The reason for heat transfer enhancement in cellular zigzag-type PCHE is that the secondary heat transfer surface in flat-plate zigzag-type PCHE is transferred to be the primary heat transfer surface, which is highly efficient heat transfer surface. As just alternating the hot and cold channels in cellular arrangement when compared with traditional arrangement, the pressure drop does not increase much while heat transfer is increased, so thermal performance is improved in cellular zigzag-type PCHE. As a result, the fixed investment/operation cost of cellular zigzag-type PCHE can be decreased. In this paper, a possible manufacture design of the arrangement improved PCHE is also achieved with turn-over channels.

Performance Comparison of Proton Exchange Membrane Water Electrolysis Cell Using Channel and PTL Flow Fields through Three-Dimensional Two-Phase Flow Simulation.

Water electrolysis technology is required to overcome the intermittency of renewable energy sources. Among various water electrolysis methods, the proton exchange membrane water electrolysis (PEMWE) cell has the advantages of a fast response and high current density. However, high capital costs have hindered the commercialization of PEMWE; therefore, it is important to lower the price of bipolar plates, which make PEMWE expensive. In addition, since the flow field inscribed in the bipolar plate significantly influences the performance, it is necessary to design the enhanced pattern. A three-dimensional two-phase flow model was used to analyze the two-phase flow and electrochemical reactions of the PEMWE anode. In order to compare the experimental results with the simulation, experiments were conducted according to the flow rate, and the results were in good agreement. First, as a result of comparing the performance of the channel and PTL (porous transport layer) flow fields, the channel flow field showed better performance than the PTL flow field. For the channel flow field, the higher the ratio of the channel width-to-rib width and the permeability of PTL, the performance got better. In the case of the PTL flow field, with the increased capillary pressure, the performance improved even if the PTL permeability decreased. Next, the direction of gravity affected the performance only when the channel flow field was used, and the X+ and Z+ directions were optimal for the performance. Finally, increasing the inlet flow rate could reduce the difference in performance between the channel and PTL flow fields, but the pressure drop gradually increased.

Enhanced mass transfer and water discharge in a proton exchange membrane fuel cell with a raccoon channel flow field

The cathode flow channel plays an important role in the water management of proton exchange membrane (PEM) fuel cells. In order to improve mass transfer and water management in traditional channels, this paper presents a new wavy flow field to increase the fuel cell performance and overall efficiency. A raccoon channel with various waviness parameters is numerically studied by a three-dimensional two-phase PEM fuel cell model. The electrochemical current-voltage performance results confirm that the PEM fuel cell's performance with the raccoon design is higher than a conventional straight channel at high current densities. The effects of uniform gas distribution, removal of the accumulated water, and sufficient water content in the membrane are crucial factors for improving cell performance. A parametric analysis is also performed by considering the amplitude and wavelength of the raccoon pattern on the fuel cell performance. The results show that the output power improves in the raccoon channel by increasing the amplitude or increasing the wavelength. The raccoon channel's output power with amplitude of 0.5 mm and a wavelength of 2 mm is enhanced by about 20%. The increase of performance in the raccoon channel is significantly higher than in the past reported studies.

Flow and Species Distribution Characteristics in a 15-cells Proton Ceramic Fuel Cell Stack with the Inter-Parallel Flow Field Channel by 3D Modeling

Flow and Species Distribution Characteristics in a 15-cells Proton Ceramic Fuel Cell Stack with the Inter-Parallel Flow Field Channel by 3D Modeling

(Digital Presentation) Concentration Admittance Spectroscopy for Oxygen Transport Diagnostics in Polymer Electrolyte Fuel Cells

Polymer electrolyte fuel cells will be crucial as efficient and environmentally benign energy conversion devices in a sustainable hydrogen economy. The further development and deployment of PEFCs require powerful diagnostic tools to assess the interplay of transport and reaction processes in an operating cell, extract the relevant parameters, and perform causal analyses of deviations from healthy cell operation. The diagnostic capabilities of electrochemical impedance spectroscopy (EIS) are widely known and extensively exploited in disentangling the transport and reaction processes in electrochemical cells. The kinetics of the oxygen reduction reaction (ORR) and thus the current density produced by a PEFC at a given cell voltage are not only sensitive to modulations in the electrode potential (or cell voltage), as used in EIS, but they are also affected by modulations in oxygen concentration. The latter effect gives rise to another impedance-type response referred to as concentration or pressure impedance. An oxygen concentration impedance (ζ = δE/δc, where δE and δc are the small-amplitude harmonic perturbations of cell voltage and oxygen concentration) could provide useful complementary capabilities to scrutinize oxygen transport processes. Various experimental works have explored the possibility of probing the response of the PEFC cell voltage with small-amplitude periodic perturbations in oxygen concentration or gas pressure1-6 and numerical models have been developed to rationalize these response functions.4,7-9 The presented work builds on a recently developed analytical model for the oxygen concentration/pressure impedance.10,11 In that work, the limit of large air flow stoichiometry and large oxygen transport loss in the catalyst layer was considered. The present work relaxes these assumptions and it focuses on the case of the so-called concentration admittance spectroscopy, which is based on the hitherto unexplored idea of measuring the response in the oxygen concentration variation to a voltage perturbation. We will present a newly developed quasi-2D model for the cathode side concentration admittance of a PEFC that accounts for oxygen transport in the flow-field channel, in the gas diffusion layer, and in the cathode catalyst layer. An analytical expression for the concentration admittance will be presented and parametric dependencies of the static admittance will be discussed. We will demonstrate how information on the oxygen transport coefficients in the flow field channel, gas diffusion layer, and catalyst layer can be drawn from the admittance at the air channel outlet.

Tailoring Flow Field Channel Aspect Ratio for Efficient Mass Transport and Compression in Fuel Cells

The adverse effects of global warming have made it critical to transition our reliance on fossil fuels to more sustainable energy sources. Polymer electrolyte membrane fuel cells (PEMFCs) can facilitate this transition by providing on-demand power with zero local carbon emissions. However, the high cost and poor durability of fuel cells hinder their widespread adoption. Particularly, PEMFC performance is strongly dependent on the flow fields which should be designed to optimize the transport of reactants and byproducts while maintaining uniform compression with subsequent layers. An important flow field design parameter is the channel aspect ratio which directly influences compression and the transport of reactants and products. Although novel flow field configurations have been studied previously, a comprehensive investigation on the effects of channel aspect ratio on cell performance has yet to be performed. In this study, we compared the electrochemical performance across varying flow field channel aspect ratios (channel width by height) from 0.5-2.0 with a fixed active area. The ohmic and mass transport resistances were quantified using electrochemical impedance spectroscopy. Operando X-ray imaging was performed to spatially resolve water saturation under the land and channel regions of the flow fields. We observed that lower channel aspect ratios (or higher number of channels for the given active area) led to lower ohmic resistance but resulted in higher mass transport losses at higher current densities due to significant water saturation under the hydrophilic ribs of the flow field. Notably, from these results we elucidated that there exists an ideal channel to rib width ratio to facilitate efficient mass transport and effective contact between the porous microstructures.

Flooding Characteristics and Countermeasures in a PEM Fuel Cell System

PEM fuel cell systems in automotive application face challenging power requirements, as they must provide full load as well as part load efficiently. Especially in systems without an external humidifier, relatively low cathode stoichiometries and increased operating pressures are the key to prevent membrane dry-out by increasing the local relative humidity. During low load operation at increased temperature and pressure, low gas velocities within the flow field occur. Too low gas velocities are known to result in a transition of the two-phase flow from film flow to slug flow regime [1]. This transition within the flow field channel structure can result in a local or global blockage of the reactant paths by liquid water droplets.While the flow regime transition within simple channel geometries is well known in literature, there is a lack of experimental data for full stack situations in fuel cell systems. Some work regarding the stationary minimal flow limitation exists [2], but countermeasures enabled by dynamic operation of the whole system are seldom in focus of systematic studies. However, well guided dynamic operation enables robust low load capability. The negative impact of flooding on stack power and lifetime can be minimized and a possible dry-out can be prevented, while maintaining a relatively low required compressor energy consumption.In the work to be presented, targeted experiments conducted with an automotive fuel cell system confirm the existence of a minimum cathode gas velocity for a stable operation and its relevance in application-like operation. It is shown that during low load operation of a non-humidified system, the targets of sufficient membrane humidification and liquid water drainage are incompatible. Because of a response time of multiple seconds until flooding or dry-out occurs, an intermittent operation with periodic switching between two operating points becomes feasible. During the first phase, the membrane is humidified due to a relatively low cathode stoichiometry and an increased cathode inlet pressure. As the liquid water accumulation becomes critical, the pressure is decreased and the stoichiometry is increased for the second phase, where droplets are blown out by utilizing a high gas velocity, as shown in figure 1.The exemplary results suggest that dynamic water management analysis can widen the operating range of a fuel cell system towards lower loads due to dynamic control. It is expected that a deeper understanding of the underlying processes will allow the development of optimized control strategies in the future.[1] M. Grimm, E.J. See, S.G. Kandlikar. Modeling gas flow in PEMFC channels: Part I – Flow pattern transitions and pressure drop in a simulated ex situ channel with uniform water injection through the GDL. Int. Journal of Hydrogen Energy 37, 12489-12503 (2012).[2] D. Jenssen, O. Berger, U. Krewer. Anode flooding characteristics as design boundary for a hydrogen supply system for automotive polymer electrolyte membrane fuel cells. Journal of Power Sources 298, 249-258 (2015).Figure 1: Normalized current, high frequency resistance, cell voltages, cathode stoichiometry, cathode inlet pressure and cathode inlet velocity, measured on a system test rig during intermittent low load operation. The operating point is periodically switched between membrane humidification (green) and liquid water drainage (orange). Figure 1