Effects of microporous layer penetration ratio and substrate carbonization temperature on the performance of proton exchange membrane fuel cells

The gas diffusion layer (GDL) is composed of a microporous layer (MPL) and a substrate; this substrate is generally fabricated from carbon fiber, carbonized resin, and polytetrafluoroethylene. When the MPL penetrates deeper into the substrate, the porosity and pore size of the GDL decrease, and the tortuosity increases; this leads to a reduction in the water discharge capability of the GDL. In this study, the MPL penetration ratio over the total GDL thickness was controlled using three different substrate manufacturing methods. These manufacturing methods for preventing the MPL from penetrating deeper into the substrate were based on the carbon fiber content within the substrate, the amount of carbonized resin coating on the substrate, and the approach used for loading the MPL. Furthermore, the GDLs were manufactured at different carbonization temperatures to investigate the effects of the carbonization temperature of the substrate on the performance of the proton-exchange membrane fuel cell.

     

燃料电池新型流道对气体扩散层表面水去除的影响

质子交换膜燃料电池(Proton exchange membrane fuel cell,PEMFC)中气体扩散层(Gas diffusion layer,GDL)表面液态水的有效去除和输送对PEMFC的水管理非常重要。为了有效去除GDL表面液态水,提出一种新型流道结构,采用流体体积法对流道内液态水的传输过程进行三维数值研究,研究进气速度、表面润湿性和液滴尺寸对流道内液态水的传输过程和GDL表面液态水去除的影响。研究结果表明,新型流道结构可以有效去除GDL表面液态水。随着进气速度的增大,沿流动方向的空气剪切力增大,流道内水去除速率和压降增大,GDL表面水覆盖率降低。表面润湿性对液态水传输影响显著,GDL表面润湿性增强会减缓液滴运输,流道内阻力降低,压降减小,GDL表面水覆盖率增大。管表面润湿性增强,流道内压降和GDL表面水覆盖率降低。新型流道适用于流道内大液滴的去除。当θ_GDL=150°和θ_pipe=30°时,新型流道结构有较好的GDL表面除水性能。本研究工作为流道结构提供了一种新的选择,对GDL表面液态水的去除具有一定的指导意义。     

A parametric study on the deformation of gas diffusion layer in PEM fuel cell

A well-designed proton exchange membrane (PEM) fuel cell requires a robust design of cell components. Therefore, the understanding of the deformation mechanism of each component is essential for a successful design. This study deals with the induced deformation of gas diffusion layer (GDL) due to the deformation of the membrane during fuel cell operation. A parametric study with six cases is constructed and the corresponding responses of a stress variation, distribution of contact pressure and a channel intrusion are selected to assess the effect of design parameters on the deformation of the GDL. The variation in the thickness of the GDL is considered as a parameter of the study in addition to the material model of the membrane. The variation in stress levels in the GDL induced due to the cyclically deformed membrane is calculated with different material models of the membrane. Through-plane stiffness of the GDL represented by the thickness of the GDL affects the three responses of the model considered. However, the material model of the membrane influences the planned responses selectively. An observation of contact loss at the interface is a strong indication of the importance of through-plane stiffness of the GDL on the deformation between the GDL and the membrane. In order to come up with a way that improves the cell’s performance, we numerically show that the manipulation of through-plane stiffness of the GDL is needed by employing an appropriate thickness of the GDL. The results from this study assist in understanding the interrelation between the GDL and the membrane for the deformation of the GDL.

     

Investigation on the liquid water droplet instability in a simulated flow channel of PEM fuel cell

To investigate the characteristics of water droplets on the gas diffusion layer from both top-view and side-view of the flow channel, a rig test apparatus was designed and fabricated with prism attached plate. This experimental device was used to simulate the growth of a single liquid water droplet and its transport process with various air flow velocity and channel height. Not only dry condition but also fully humidified condition was also simulated by using a water absorbing sponge. The detachment height of the water droplet with dry and wet conditions was measured and analyzed. It was found that the droplet tends towards becoming unstable by decreased channel height, increased flow velocity or making a gas diffusion layer (GDL) dryer. Also, peculiar behavior of the water droplet in the channel was presented like attachment to hydrophilic wall or sudden breaking of droplet in case of fully hydrated condition. The simplified force balance model matches with experimental data as well.     

Prediction of flow crossover in the GDL of PEFC using serpentine flow channel

A serpentine flow channel is one of the most common and practical channel layouts for Polymer electrolyte fuel cells (PEFCs) since it ensures the removal of water produced in the cell with an acceptable parasitic load. The operating parameters such as temperature, pressure and flow distribution in the flow channel and gas diffusion layer (GDL) has a great influence on the performance of PEFCs. It is desired to have an optimum pressure drop because a certain pressure drop helps to remove excess liquid water from the fuel cell, too much of pressure drop would increase parasitic power needed for the pumping air through the fuel cell. In order to accurately estimate the pressure drop precise calculation of mass conservation is necessary. Flow crossover in the serpentine channel and GDL of PEFC has been investigated by using a transient, non-isothermal and three-dimensional numerical model. Considerable amount of cross flow through GDL is found and its influence on the pressure variation in the channel is identified. The results obtained by numerical simulation are also compared with the experimental as well as theoretical solution.     

Lubricant Transfer in Disk Drives

For disk drives with Z-tetraol-coated disks, the ingress of airborne solid particulates into the disk drive was found to result in disk-to-head lubricant transfer. In addition, high humidity was found to enhance the transfer process. Water soluble electrolytes such as alkali halides are most ubiquitous airborne solid particulates. Molecular dynamics calculations were performed to examine (a) the condensation process of H₂O, (b) the effect of alkali halide on the process, (c) the difference between the end-groups of Z-dol and Z-tetraol. It was shown that the OH units of Z-tetraol end-groups would embed themselves into facial layer of water?Celectrolyte droplets, thus encapsulating and stabilizing the droplets, while the OH units of Z-dol would not do so. The lubricant transfer observed uniquely for Z-tetraol-coated disks is attributed to inorganic particulates such as NaCl entering the drive interior, landing on the disk surface, attracting water, and forming Z-tetraol encapsulated water?Celectrolyte droplets. These droplets are viscous and are readily picked up by the slider.     

Effects of rib structure and compression on liquid water transport in the gas diffusion layer of a polymer electrolyte membrane fuel cell

Journal of Mechanical Science and Technology - Understanding the dynamic behavior of water transport in a compressed gas diffusion layer (GDL) is essential for enhancing water management strategies...     

Influence of water injection parameters on the performance of a water-lubricated single-screw air compressor

A thermodynamic model of a water-lubricated single-screw air compressor was established to examine the impact of water injection parameters on performance. Heat transfer and leakage between humid air and water was considered by analyzing the impacts of rotation speed, discharge pressure, the rate of water injection, and diameter of droplets on the performance of a compressor. The discharge temperature could be reduced by increasing the rate of water injection, resulting in the compression procedure moving towards an isothermal state. The increase in the rate of water injection under rated conditions from 60 L/min to 80 L/min resulted in a reduction in the compressor discharge temperature, increased the volume efficiency, and increased adiabatic efficiency by 11.1 K, 1.5 %, and 2.8 %, respectively. Water injection atomization increased the area of transfer of heat between humid air and water and improved the performance of compressor.

     

Effects of heat flux on dropwise condensation on a superhydrophobic surface

The condensation heat transfer efficiencies of superhydrophobic surfaces that have ∼160° contact angle under atmospheric conditions were investigated experimentally. The departing diameter and the contact angle hysteresis of droplets were measured by capturing front and tilted side views of condensation phenomena with a high speed camera and an endoscope, respectively. Condensation behaviors on the surface were observed at the micro-scale using an Environmental scanning electron microscope (ESEM). Apparently-spherical droplets formed at very low heat flux q″ ∼20 kW/m² but hemispherical droplets formed at high q″ ∼440 kW/m². At high q″, heat transfer coefficients were lower on the superhydrophobic surface than on a hydrophobic surface although the superhydrophobic surface is water repellent so droplets roll off. The results of contact angle hysteresis and ESEM image revealed that the reduced heat transfer of the surface can be attributed to the large size of departing droplets caused by adhesive condensed droplets at nucleation sites. The results suggest that the effect of q″ or degree of sub-cooling of a condensation wall determine the droplet shape, which is closely related to removal rates of condensates and finally to the heat transfer coefficient.

     

Pool boiling heat transfer to zinc oxide-ethylene glycol nano-suspension near the critical heat flux

Experimental study is conducted on the thermal performance of the ZnO nanoparticles dispersed in water-ethylene glycol (water/EG) as the base fluid under the pool boiling condition. Experimental facility provides condition to apply heat fluxes up to the critical heat flux point. Influence of different operating parameters such as heat flux and mass concentration on the boiling heat transfer coefficient and thermal fouling resistance is investigated. Results showed that with an increase in heat flux and mass concentration, the heat transfer coefficient increases; however, at concentration of 0.5 % by weight for ZnO/water/EG nano-fluid. Deposition of particles on the heating surface was a significant disadvantage of the nano-fluid. However, with an increase in mass concentration of nanoparticles, the critical heat flux point is enhanced, which is due to the enhancement in deposition layer of the nanoparticles, resulting in capillary wick and keeping more liquid inside the deposition layer.

     

A study of a nano-sized water droplet’s transition from a flat surface to a pillared surface

The ability to control the hydrophobicity of a surface is of importance to many industries. The dynamic behavior of nano-sized water droplets moving from a flat surface to a pillared surface using molecular dynamics simulations was investigated. Simulations were carried out in two steps. In the first computational step, the initial group of water molecules reached equilibrium on a flat graphite surface. In the second computational step, a constant force was applied to the water droplet and the motion of the water droplet was evaluated as it moved from the flat surface to the pillar-type surface. The movement of the water droplet could be grouped into three different categories and depended on the pillar height and the magnitude of the applied force. The results showed the strongest body force with a pillar height of 6 graphite layers allowed most of the water molecules to move along the top of the pillars. In conclusion, a strong force and pillar height approximately half of the droplet height displayed the best transition from a flat surface to a pillared surface.

     

Synchrotron radiation X‐ray micro‐fluorescence: Protocol to study mesenchymal stem cells

The micro‐X‐ray fluorescence by synchrotron radiation (μ‐XRF) is a method to determine the composition of tissues without destroying the samples. However, this technique has never been used for the analysis of mesenchymal stem cells (MSC). This study compared different protocols for fixing, storing, preserving, and establishing the correct numbers of dental derived MSC submitted to μ‐XRF analysis. Stem cells were obtained from human dental tissue. After cell expansion, and MACS isolation, the samples were fixed and the following quantities of cells 1 × 10⁴ to 1 × 10⁷ were divided in two groups: G1: fixed in 4% paraformaldehyde diluted in phosphate‐buffered saline solution, and G2: fixed in 4% paraformaldehyde diluted in MilliQ water. The G1 cells showed precipitation of chemical components from the solution resulting in the formation of salt crystals while G2 cells were clear and almost transparent in the sample holder. With regards to cells concentration, the best results occurred when four droplets of 1 × 10⁷ cells were analyzed. This work shows that to identify and study the distribution of trace elements in MSC by μ‐XRF, the best protocol is fixation in 4% paraformaldehyde diluted with MilliQ water at 4°C and a concentration of four incremental droplets of 1 × 10⁷ cells. Microsc. Res. Tech. 79:149–154, 2016. © 2016 Wiley Periodicals, Inc.     

Pressure drop and flow distribution characteristics of single and parallel serpentine flow fields for polymer electrolyte membrane fuel cells

This study numerically investigates pressure drop and flow distribution characteristics of serpentine flow fields (SFFs) that are designed for polymer electrolyte membrane fuel cells, which consider the Poiseuille flow with secondary pressure drop in the gas channel (GC) and the Darcy flow in the porous gas diffusion layer (GDL). The numerical results for a conventional SFF agreed well with those obtained via computational fluid dynamics simulations, thus proving the validity of the present flow network model. This model is employed to characterize various single and parallel SFFs, including multi-pass serpentine flow fields (MPSFFs). Findings reveal that under-rib convection (convective flow through GDL under an interconnector rib) is an important transport process for conventional SFFs, with its intensity being significantly enhanced as GDL permeability increases. The results also indicate that under-rib convection can be significantly improved by employing MPSFFs as the reactant flow field, because of the closely interlaced structure of GC regions that have different path-lengths from the inlet. However, reactant flow rate through GCs proportionally decreases as under-rib convection intensity increases, suggesting that proper optimization is required between the flow velocity in GCs and the under-rib convection intensity in GDLs.     

A numerical study on the fluid flow and thermal characteristics inside the scrubber with water injection

New technologies have been adopted by the marine industry to control the air emission from ship. Wet scrubbers have found widespread use in cleaning contaminated gas streams by the application of scrubbing of marine engine exhaust gases by spraying sea-water and thus develop their ability to remove particulates like SO_X for pollution free environment. In real case problems, to control the emission of SO_X in scrubber, the temperature of the flue gases should be dropped down by spraying sea-water droplets which brings down the lifetime of harmful pollutants. In order to examine the flow characteristics inside the scrubber, the computer simulation methodology is carried out in this paper with the boundary conditions of hot air at ideal gas state and water droplets injection using lagrangian method. During the impact of injecting water droplets over the hot air, the nature invites evaporation of hot air which is the obvious benefit of scrubber in diminishing the domain temperature. After the numerical analysis, if the model is observed with reasonable reduction in domain temperature, the optimum scrubber design can be elected for pollutant emission control in real case problems. From the assessment of possible impacts of pollutants from marine industry, the temperature of the system are in need of investigation facilitating the evaporation rate of water droplets and the computation of droplets concentration over the entire domain to bring out the competent modeling. The numerical analysis using CFD facilitates in understanding the problem better over its entire operating envelope.

     

A numerical study on the performance of polymer electrolyte membrane fuel cells due to the variation in gas diffusion layer permeability

Convective flow in the under-rib regions of gas diffusion layers (GDLs) is a non-negligible transport process that can enhance the performance of polymer electrolyte membrane fuel cells (PEMFCs) by facilitating efficient utilization of catalyst layers (CLs) in those regions. The permeability of GDLs has been recognized as a dominant factor influencing the intensity of the under-rib convection in PEMFCs. In this study, the correlation between the permeability of GDLs and the performance of PEMFCs was numerically investigated through a detailed simulation of the transport and electrochemical processes in PEMFCs using a computational fluid dynamics (CFD) tool. Three serpentine flow fields with one, three, or five parallel paths were considered as reactant flow channels for an active cell area of 3 cm × 3 cm, while the permeability of GDLs was varied from 1×10⁻¹² m² to 1×10⁻¹⁰ m². The effects of the flow field design and the GDL permeability on the performance of PEMFCs were presented, along with their impacts on the local distribution of current density, water content, and reactant concentration.     

Droplet generation with integrated 3D pneumatic actuator for orifice control

A droplet generator with an embedded 3D balloon actuator around a flow-focusing junction is realized to adjust the size of the droplets as required with only one element. The 3D actuator encircling the orifice causes a geometric deformation due to pneumatic pressure, which controls the orifice and thus the sizes of the droplets. The orifice and actuator are designed to have triangular cross-sections with the largest possible reduction in hydraulic diameters under the same pneumatic pressure. We empirically demonstrated that, compared with typical fixed orifice structures, droplet generators with variable orifices reduce the droplet size without changing the flow rate and can even be adjusted to a wider range of 259.3 %. The devices were fabricated by the 3D-printed soluble mold technique to achieve a fully 3D structure within a single body of polydimethylsiloxane that is unattainable by conventional standard microfabrications as well as a non-bonding structure without any leakage.

     

General morphology and ultrastructure of the female reproductive apparatus of Trichomalopsis shirakii crawford (Hymenoptera,Pteromalidae)

The morphology and ultrastructure of the female reproductive system were examined for a larval–pupal parasitoid Trichomalopsis shirakii Crawford of Oulema oryzae Kuwayama using light and electron microscopes. The reproductive system includes two ovaries, two pairs of accessory glands, an unbranched venom gland, a large venom reservoir and a Dufour gland. Each ovariole contains follicles and oocytes at different stages of maturation. A fibrous layer covers the surface of mature egg. The accessory glands are made up of a layer of secretory cells surrounded by muscle fibers. In these secretory cells, numerous mitochondria, electron‐dense secretory granules and vesicles filled with dense granular particles are present. These granular particles appear as virus‐like particles (VLPs). The venom gland consists of a single layer of secretory cells which are organelle rich with abundant rough endoplasmic reticulum, mitochondria and vesicular organelles, a layer of duct cells and an inner intima. The reservoir consists of a muscular sheath, epidermal cells with few organelles and an intima layer. The Dufour gland has a relatively large lumen surrounded by a single layer of columnar epithelial cells which are characterized by clusters of smooth endoplasmic reticulum and lipid droplets. Aside from the venom, the fibrous layer coating the egg and the granular particles which may be VLPs have been discovered in our study. They may serve as one of the parasitoid‐associated factors in their host–parasitoid relationship and play a role in host immune suppression. Microsc. Res. Tech. 79:625–636, 2016. © 2016 Wiley Periodicals, Inc.     

High flow-rate ultrasonic seeder

Application of optical techniques to gaseous flows usually requires seeding the fluid with tracers sufficiently small to follow the flow, added in suitable concentrations. This work describes the design and manufacture of a seeder based on ultrasonic atomization of aqueous solutions or other fluids of similar viscosity, capable of operating in a continuous way with high atomization rates. It includes a dozen piezoceramic disks that oscillate at 1.65 MHz, generating droplets with Sauter mean diameter in the range of 4–5 μm and rates over 0.6 g/s when working with water. To test its performance, the device has been used to seed a simple free air jet issuing from a 3.5 cm diameter pipe. Initially the seeding density is very satisfactory, but if only water is nebulized the droplets evaporate in a short time and the concentration becomes too low when moving downstream. The situation can be greatly improved mixing the water with a less volatile liquid. Here, a small percentage of glycerol (5% in volume) has been added to the water to extend the droplet lifetime, although in general, atomization rate strongly decreases when increasing the liquid viscosity. The seeded flow has been visualized, and the 3D velocity field has been successfully measured using stereoscopic particle image velocimetry.     

Dynamic analysis of multilayer ceramic capacitor for vibration reduction of printed circuit board

Owing to their high permittivity and volumetric efficiency, the demand for multilayer ceramic capacitors (MLCCs) has increased rapidly in recent times. Because of the electromechanical characteristics of BaTiO₃, MLCC vibrates, resulting in printed circuit boards (PCBs) generating acoustic noise. To construct an accurate finite element model of an MLCC, piezoelectric and electrostrictive coefficients were extracted and verified through experiments. The top cover layer thickness and bandwidth were chosen as design parameters to reduce the vibration of PCB. The simulation results indicate that the bandwidth and top cover layer thickness are highly related to the vibration in the top direction and the rotational moment generated from the head surface, respectively. Based on the analysis results, a novel MLCC was suggested and it exhibited reduced vibrational characteristics of PCB about 75 % compared with that of commercial MLCCs.

     

Determination of the water droplet size distribution of fat spreads using confocal scanning laser microscopy

Knowledge of the water droplet size distribution of fat spreads is necessary for the development and production of high quality microbiological safe products. Fat spreads are water‐in‐oil emulsions. The water droplet size distribution can be determined by confocal scanning laser microscopy (CSLM) after staining the fat with Nile Red. The profiles of the non‐fluorescent water droplets in the 2D images are identified and measured using image analysis. The ‘true’ water droplet size distribution is calculated from the distribution of the measured profile diameters using a Wicksell transformation of log‐normal distributions. The influence of the fluorescent staining and CSLM parameters on the information were studied. The CSLM method was tested on fat spreads with a fat content ranging from 40% to 80%. The results were compared with those obtained by nuclear magnetic resonance spectroscopy (NMR). The distribution parameters [volume weighed geometric mean diameter (D?_3,3) and the standard deviation (σ) of the logarithm of the droplet diameter] calculated for 80% fat spreads are in good agreement with those obtained by NMR (within ± 7% relative). Small differences were found for 65% fat spreads and large differences were identified for 40% fat spreads. The precision for the determination of the D?_3,3 value by CSLM is worse than that of NMR, even when three images were used to calculate this parameter [3–11% and 1–6% relative standard deviation (RSD), respectively]. The precision for the determination of exp(σ) by CSLM is comparable or better than that of NMR (1–5% and 3–6% RSD, respectively). CSLM proved to be a reliable method for the determination of the water droplet size distribution of margarines (80% fat). The advantage of CSLM compared to NMR is that visual information is given about the water droplet size distribution in the sample.