Water Removal Characteristics Research Articles

Effect of GDL Structure and Operating Conditions on PEMFC Performance and Liquid Water Removal

Gas diffusion layers (GDLs) in proton exchange membrane fuel cells (PEMFCs) are responsible for diffusion of reactant gases into the catalyst layers, current collection and the removal of produced water. An accumulation of generated liquid water within the GDL, known as flooding, impedes the supply of reactant gas and results in the increase of concentration overpotential. Therefore, understanding of oxygen transport and produced water removal characteristics is required to enhance cell performance. The objective of this study is to investigate the effect of GDL structure and operation conditions on PEMFC performance and liquid water removal, with a particular focus on comparing a novel GDL to a conventional GDL. The results of a simultaneous evaluation of cell performance and liquid water behavior in the GDLs by means of X-ray imaging under operational conditions are presented.

Effect of GDL Structure and Operating Conditions on PEMFC Performance and Liquid Water Removal

Gas diffusion layers (GDLs) in proton exchange membrane fuel cells (PEMFCs) are responsible for diffusion of reactant gases into the catalyst layers, current collection and the removal of produced water. An accumulation of generated liquid water within the GDL, known as flooding, impedes the supply of reactant gas and results in the increase of concentration overpotential. Therefore, understanding of oxygen transport and produced water removal characteristics is required to enhance cell performance. The objective of this study is to investigate the effect of GDL structure and operation conditions on PEMFC performance and liquid water removal, with a particular focus on comparing a novel GDL to a conventional GDL. The results of a simultaneous evaluation of cell performance and liquid water behavior in the GDLs by means of X-ray imaging under operational conditions are presented.

Study on coarse-grained coal water removal characteristics via steam flash drying for low-rank coal upgrading

Low-rank coal drying plays a crucial role in achieving clean and efficient coal utilization. This study introduces steam flash drying, a novel method specifically designed for upgrading low-rank coal. Unlike conventional drying techniques, steam flash drying utilizes steam as the heat medium, generating a certain amount of condensation. This paper presents a comprehensive analysis of the calculated condensate quantity and the experimental measurement of coal particles' water-holding capacity to determine the initial moisture content before drying. Furthermore, the study investigates the steam flash drying effect on different-sized coal particles at a temperature of 497 K. The findings demonstrate that particles above 3 mm have lower water-holding capacity. The initial moisture content is determined by the particles' water-holding capacity and the combined raw coal water content and condensate content. The research reveals a diminishing flash-drying effect with increasing particle size, characterized by a rapid decrease followed by a gradual decline, observed at a critical point of 6 mm. By strategically adjusting the particle size distribution and employing mixing techniques, the coal drying effect can be significantly enhanced by 22%, ultimately leading to a remarkable 17% increase in processing capacity per unit volume. As a result, this study provides a theoretical foundation and optimization strategies for the clean and efficient utilization of coal through steam flash drying.

Three-dimensional simulation of large-scale proton exchange membrane fuel cell considering the liquid water removal characteristics on the cathode side

A 3D (three-dimensional) numerical simulation model is proposed to investigate the output performance and liquid water distribution of large-scale FF (flow field) considering DMGDZ (“dot matrix” gas distribution zone). The PEMFC (proton exchange membrane fuel cell) is affected by gravity, and the simulation results demonstrate that the PEMFC has the best output performance and water removal effect when the length and width of DM (“dot matrix”) are 1.5 mm and 0.8 mm, respectively. The effect of gravity direction on liquid volume fraction in cathode large-scale FF is not obvious, but liquid volume fraction increases with the increase of the contact angle of GDL (gas diffusion layer) and working current density. The variation of water vapor condensation rate and the MPL (micro-porous layer) contact angle in porous electrodes significantly affects the amount of liquid water in the CFF (cathode flow field), and the PEMFC performance is degraded. Due to the enhancement of convection effect, the waveform channel can effectively remove the liquid water from the cathode channel and slightly strengthen the overall PEMFC performance. According to the results of waveform channel analysis, the comparison of waveform and straight FFs with DMGDZ shows that the waveform FF is more conducive to facilitate the liquid water removal on the cathode side. Simultaneously, the reactant gas distribution is uniform and the output performance is higher.

Three-dimensional visualization and characterization of paper machine felts and their relationship to their properties and dewatering performance

Polymeric felts are commonly used in the papermaking process on the paper machine wet end, in the press section, and in the dryer section. They provide an important function during paper manufacturing, including as a carrier or support; as a filter media assisting with water removal on the paper machine; in retention of fibers, fines, and fillers; and in some applications, such as tissue and towel, to impart key structural features to the web. These felts can have highly interwoven complex internal structures comprised of machine direction and cross-machine direction yarns of varying sizes and chemical compositions. Here, we present a non-intrusive three-dimensional (3D) image visualization method using advanced X-ray computed tomography (XRCT). This method was used to characterize the complex 3D felt structure and determine the water removal characteristics of some commonly used paper machine felts. The structural features analyzed include porosity; specific pore-yarn interfacial surface area; 3D pore size distribution; 3D fiber or yarn-size distribution; and their variations through the thickness direction. The top, middle, and bottom layers of the felt have very different structures to assist with water removal and impart paper properties. The size distribution of the yarns, as well as the pores in the different layers of the felt, are also inherently different. These structural features were non-intrusively quantified. In addition, variation in the structural characteristics through the thickness of the felts and its potential role in papermaking is explored. In addition to the 3D structural characteristics, permeability characteristics and water removal characteristics, including rewetting of select felt samples, have also been experimentally determined. It is interesting to observe the relationship between key structural features and permeability and water removal characteristics. These relationships can provide additional insights into press felt design, as well as ways to improve product properties and the dewatering efficiency and productivity of the paper machine.

Facile Airbrush Fabrication of Gas Diffusion Layers Comprising Fine-Patterned Hydrophobic Double-Layer and Hydrophilic Channel for Improved Water Removal in Polymer Electrolyte Membrane Fuel Cells

We develop a facile and eco-friendly airbrush-based method for the fabrication of gas diffusion layers (GDLs) with improved water removal characteristics for high-performance polymer electrolyte membrane fuel cells (PEMFCs). The GDL architecture airbrushed on a porous carbon paper uniquely comprises hydrophobic double-layer coating and fine-patterned hydrophilic channels, which facilitate the outbound penetration of water generated from inside the membrane electrode assembly without flooding or undesirable mingling with oxygen inflow. We first prepare the colloids and solutions suitable for airbrushing, optimize the airbrush coating structure based on the water contact angle measurements, and finally confirm the improved water removal characteristics of the fabricated GDL samples by measuring the water transuding times. Our GDL fabrication procedure based on simple and fast airbrushing does not require costly vacuum process or toxic chemical treatment, thus may benefit practical and scalable manufacturing of high-performance PEMFCs towards the next-generation green transportation.

Numerical Investigation of Gas-Liquid Two-Phase Flow inside PEMFC Gas Channels with Rectangular and Trapezoidal Cross Sections

The dynamics of liquid water in the gas channels with rectangular sections (REC), trapezoidal sections with open angles of 60 degrees (T60), and trapezoidal sections with open angles of 72 degrees (T72) are numerically investigated via the volume of fluid method. The effects of the contact angle of the top and side walls (CATS), the water inlet configuration, and the air inlet velocity are studied based on the temporal evolution of gas-liquid interface, the water volume fraction (WVF), the water coverage ratio of the gas diffusion layer (GDL) surface (GWCR), and the pressure drop between the air inlet and the outlet. For the hydrophobic GDL surface and the hydrophilic top and side walls, the T72 provides the lowest WVF and GWCR of around 7 percent due to periodic pressure spikes. The REC and T60 show a higher WVF and a lower GWCR as most of liquid water moves along the channel while attached to the top wall. As the CATS increases from 60 to 120 degrees, the behaviors of liquid water become similar for the three cross-sectional shapes. The T72 shows especially similar results irrespective of the CATS. When the liquid water emergence is concentrated along the side wall, the T72 shows the best water removal characteristics. For all the three channel cross-sectional shapes, water slugs move faster and have smaller sizes as the air inlet velocity increases.

Three‐dimensional Modeling of Gas Purge in a Polymer Electrolyte Membrane Fuel Cell with Co‐flow and Counter‐flow Pattern

AbstractGas purge is commonly utilized to minimize residual water after shutdown of proton exchange membrane fuel cell (PEMFC) in cold weather, aiming to reduce damage of ice formation on cell performance and durability. In this paper, a three‐dimensional multiphase gas purge model of proton exchange membrane fuel cell with co‐flow and counter‐flow pattern is established to investigate water removal characteristics using two‐fluid model. The present model mainly includes water transport in membrane, mass transfer between dissolved water and water vapor in catalyst layer (CL), phase change between liquid water and water vapor in porous media. Several cases with co‐flow and counter‐flow pattern have been investigated numerically. In the last, gas purge time comparison between a fresh cell and degraded cell is conducted. The numerical results show that counter‐flow pattern is better in keeping even water content distribution and avoiding over‐drying of membrane. Time constant for gas purge is different in terms of different final target value: water vapor, liquid water saturation, membrane water content. Degraded cells have 2 more seconds than fresh cells when cell temperature is 80 °C and velocity of purge gas 1m s−1.

Effect of Gas Channel Wall Contact Angle on a Liquid Water Slug Dynamics in a Straight Gas Channel of a Trapezoidal PEMFC

The volume of fluid method is used to investigate the behavior of a liquid water slug in a PEMFC trapezoidal gas channel(GC) with a open angle of 60 degrees. To evaluate the effect of the contact angle of the top and side walls, the gas diffusion layer water coverage ratio(GWCR) and water volume fraction(WVF) in a inspection control volume are analyzed. As the contact angle increases, GWCR increases and WVF decreases. The cases with the GC contact angle of 60 and 80 degrees show the more favorable water removal characteristics compared to the other cases in a GC flooding condition.

Comparison of Liquid Water Dynamics in Bent Gas Channels of a Polymer Electrolyte Membrane Fuel Cell with Different Channel Cross Sections in a Channel Flooding Situation

The transport characteristics of water slugs in a bent gas channel of a polymer electrolyte membrane (PEM) fuel cell are numerically studied using the volume of fluid (VOF) method. To investigate the effects of channel cross-sectional shape in a channel flooding situation, the gas channels (GCs) with one rectangular and two trapezoidal cross sections are compared. Parametric studies are also conducted to evaluate the effects of the contact angle of the top and side walls, the contact angle of the gas diffusion layer (GDL) surface, and the air inlet velocity. Considering both of the water volume fraction (WVF) and GDL water coverage ratio (WCR), the trapezoidal channel with open angles of 60 degrees provides the most favorable performance in a channel flooding condition. Among all the top and side wall contact angles considered, the hydrophobic contact angle of 120 degrees shows the best results. Among the three GDL contact angles of 90, 110 and 140 degrees, the hydrophobic GDL contact angle of 140 degrees provides the most favorable water removal characteristics in a channel flooding situation. For all cross-sectional shapes, the water removal rate increases and the liquid water interface shows more complex patterns as the air inlet velocity increases.

Effect of Air and Water Inlet Velocities on Liquid Water Droplet Behaviors in a PEMFC Trapezoidal Gas Channel

In a PEMFC gas channel with a trapezoidal cross-section, the effect of air and water inlet velocities on water removal characteristics is numerically studied via the volume of fluid(VOF) method. When the channel wall contact angle is 60 degrees, the air inlet velocities higher than 2.5 m/s are advantageous to obtain lower GDL surface water coverage ratio(WCR). The WCR increases as the wall contact angle increases to 90 or 120 degrees due to the relatively lower surface tension force. In overall, WCR decreases as the air inlet velocity increases and WCR increases as the water inlet velocity increases.

Numerical Analysis of Liquid Water Removal Characteristics in a PEMFC Gas Channel with Trapezoidal Cross-section

The water removal characteristics in a PEMFC trapezoidal gas channel are investigated with the volume of fluid (VOF) method. For the case of wall contact angle of 60 degree, liquid water attaches on the top wall and moves toward the exit. In contrast, liquid water moves along the channel side corner or GDL surface irregularly for the higher wall contact angles. The hydrophillic wall contact angle of 60 degrees provides more favorable diffusion of reactants to cathode reaction sites as the GDL surface water coverage ratio approaches zero even if the water flow rate increases.

Gas Channel Optimisation for PEM Fuel Cell using the Lattice Boltzmann Method

A scheme for two-phase flow with large density differences using the lattice Boltzmann method (LBM) was applied to investigate the optimum gas channel design for water drainage. Three different channel geometries were used: rectangular, trapezoidal, and triangular with constant gas flow rate and a droplet initially placed at the corner. The results show that the rectangular channel offers the best water removal characteristics under a moderate pressure drop. When using hydrophilic gas channels, a larger droplet is drawn up, which may induce better fuel cell performance. The cases of both channels with the same cross-section area, height and width are discussed.

J0802-1-2 高分子膜における水とプロトンの移動に着目した物質輸送現象の有限要素法による数値解析([J0802-1]PEFC)

Computational analysis for water management in polymer electrolyte membrane fuel cells (PEFCs) contributes to the effective development of fuel cells. Recently, an improvement in the quality of the computational analysis concerning water management was made possible due to development and modification in both the software and the hardware. In this research, focus is on the water proton synchronized transfer and an experimental study on the relationship between capillary pressure and water saturation. The relationship between the liquid and proton transfer is refined through experimental study and mathematical remodeling, and results show the influence on water removal characteristics.

Dominant frequency of pressure drop signal as a novel diagnostic tool for the water removal in proton exchange membrane fuel cell flow channel

In this work, a transparent assembly was self-designed and manufactured to perform ex situ experimental study on the liquid water removal characteristics in PEM fuel cell parallel flow channels. It was found that the dominant frequency of the pressure drop across the flow channels may be utilized as an effective diagnostic tool for water removal. Peaks higher than 1 Hz in dominant frequency profile indicated water droplet removals at the outlet, whereas relatively lower peaks (between 0.3 and 0.8 Hz) corresponded to water stream removals. The pressure drop signal, although correlated with the water removal at the outlet, was readily influenced by the two phase flow transport in channel, particularly at high air flow rates. The real-time visualization images were presented to show a typical water droplet removal process. The findings suggest that dominant frequency of pressure drop signal may substitute pressure drop as a more effective and reliable diagnostic tool for water removal in PEM fuel cell flow channels.

Method to Characterize the Air Flow and Water Removal Characteristics During Vacuum Dewatering. Part II—Analysis and Characterization

This is part II of a study reported earlier on a method to characterize the air flow and water removal characteristics during vacuum dewatering. This article presents experimental data and analysis of results from the use of a cyclically actuated vacuum dewatering device for removing moisture from wetted porous materials such as paper with the intermittent application of vacuum and accompanying air flow though the material. Results presented include sheet moisture content as a function of residence time and hence water removal rate under a variety of process conditions. Also, experimental results on air flow through the wet porous structure and hence the role and importance of air flow during vacuum dewatering are presented. Vacuum dewatering process conditions include exit solids content between 11 and 20% solid under applied vacuum conditions of 13.5 to 67.7 kPa (4 to 20 in. Hg). Regression analysis indicated that the exit sheet moisture content exhibited a nonlinear relationship with residence time with exit solids reaching a plateau after a certain residence time. Final moisture content correlated linearly with the average overall flow rate of air through the paper sample and the basis weight of the material.

Method to Characterize the Air Flow and Water Removal Characteristics during Vacuum Dewatering. Part I—Experimental Method

An experimental method using a novel design to characterize the air flow and water removal during vacuum dewatering in paper manufacturing is discussed. The experimental setup involves the intermittent application of vacuum, similar to commercial systems, using a rotating disk with slot opening arrangement. The system is capable of commercially realistic residence times of the order of milliseconds. The intermittent application of vacuum simulates vacuum dewatering on commercial paper machines. The air flow rate is calculated from changes in pressure and temperature in the vacuum tank underneath the sample. The role and importance of air flow during vacuum dewatering is studied by accurately measuring the air flow, properly taking into account the leaks during vacuum dewatering. The method described here provides for the first time accurate air flow and water removal data during vacuum dewatering. Methods of analysis of the experimental data are also presented. This information can be used to better understand the water removal process as well the role and importance of air flow during vacuum dewatering.

Water removal characteristics of proton exchange membrane fuel cells using a dry gas purging method

Water removal from proton exchange membrane fuel cells (PEMFC) is of great importance to improve start-up ability and mitigate cell degradation when the fuel cell operates at subfreezing temperatures. In this study, we report water removal characteristics under various shut down conditions including a dry gas-purging step. In order to estimate the dehydration level of the electrolyte membrane, the high frequency resistance of the fuel cell stack was observed. Also, a novel method for measuring the amount of residual water in the fuel cell was developed to determine the amount of water removal. The method used the phase change of liquid water and was successfully applied to examine the water removal characteristics. Based on these works, the effects of several parameters such as purging time, flow rate of purging gas, operation current, and stack temperature on the amount of residual water were investigated.

Innovative gas diffusion layers and their water removal characteristics in PEM fuel cell cathode

Liquid water transport is one of the key challenges regarding the water management in a proton exchange membrane (PEM) fuel cell. Conventional gas diffusion layers (GDLs) do not allow a well-organized liquid water flow from catalyst layer to gas flow channels. In this paper, three innovative GDLs with different micro-flow channels were proposed to solve liquid water flooding problems that conventional GDLs have. This paper also presents numerical investigations of air–water flow across the proposed innovative GDLs together with a serpentine gas flow channel on PEM fuel cell cathode by use of a commercial computational fluid dynamics (CFD) software package FLUENT. The results showed that different designs of GDLs will affect the liquid water flow patterns significantly, thus influencing the performance of PEM fuel cells. The detailed flow patterns of liquid water were shown. Several gas flow problems for the proposed different kinds of innovative GDLs were observed, and some useful suggestions were given through investigating the flow patterns inside the proposed GDLs.