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Droplet Dynamics in Shearing Gas Flow
Droplet Dynamics in Shearing Gas Flow
Sessile droplets exposed to shearing gas flows resist depinning owing to surface tension and contact angle hysteresis. It is known that contact line depinning occurs when the shearing gas flow is large enough to deform the droplet beyond its contact angle hysteresis. This work explores the contact line depinning process by visualizing growing droplets on a porous layer in laminar shear gas flows. High-speed imaging of droplets revealed an oscillatory motion in droplets, which is speculated to originate from an interaction between the drag force and surface tension effects. This oscillatory motion creates an inertial force within the droplet which combines with the drag force when droplet acceleration is in the stream-wise direction. The combined effect competes against the droplet adhesion force, setting the depinning criteria. Analyzing droplet images revealed that droplet local velocity and acceleration (i.e., sessile droplet dynamics prior to detachment from the substrate) increase with the superficial gas velocity. At the same time, the contact line depinning occurs at a smaller droplet size for higher superficial gas velocities.
Related publications
M. Mortazavi*,1 and S.Y. Jung1, “The Role of Droplet Dynamics on Contact Line Depinning in Shearing Gas Flow”, Langmuir, 39, 10301-10311 (2023) https://doi.org/10.1021/acs.langmuir.3c00065.
Water behavior in GDL pores of PEMFC
Water behavior in GDL pores of PEMFC
Understanding about water transport in gas diffusion layer (GDL) is important for the water management of proton exchange membrane fuel cells (PEMFC). In this study, the in-plane water transport in a commercial GDL was visualized to investigate the transient characteristics using an optical imaging system providing a high spatial resolution. Previously proposed non-dimensional scaling model and specific interfacial area were obtained from measured wetted area, perimeter, and percolation pressure. In addition, the wetted area with time is graphically presented by spatiotemporal maps. It is newly found that the transition of flow characteristics including flow pattern and transport mechanism occurs. Results indicate that at higher compressions, the transition occurs earlier under lower saturation, and water invades along a more complex path through pores that are altered by compression.
Related publications
S.Y. Jung and M. Mortazavi*, “Transient characteristics of in-plane water transport in gas diffusion layers of PEM fuel cells”, International Journal of Hydrogen Energy, in-press, https://doi.org/10.1016/j.ijhydene.2023.10.055.
J. Park, H. Oh, H. Park, J.W. Moon, S.J. Lee SJ and S.Y. Jung*, “Water transport in polymer electrolyte membrane fuel cell: Degradation effect of gas diffusion layer”, International Journal of Energy Research, 46(7), 9058-9070 (2022) doi:10.1002/er.7782
In situ two-phase flow in gas channels of PEMFC
In situ two-phase flow in gas channels of PEMFC
Performance reduction of polymer electrolyte membrane fuel cell caused by the degradation of gas diffusion layer (GDL) is hydrodynamically analyzed by comparing visualized two-phase flows in flow channels. GDL was chemically degraded using hydrogen peroxide, and the degree of GDL degradation was controlled by changing soaking time. The I-V curves for fresh and aged GDLs were compared using a commercial single cell, and the behavior of water in the cathode flow channel was visualized using a specially designed single cell having a transparent window. Two-phase flow characteristics were quantitatively compared after applying image processing technique to captured images. As a result of the experiment, it was confirmed that the hydrophobicity of GDL decreases as it degrades, absorbing the water generated inside, and reducing the efficiency by blocking the supply of reactants.
Related publications
J.W.Moon, S.K.Kim and S.Y. Jung*, “In-situ visualization of cathode flow channel in polymer electrolyte membrane fuel cell: Effect of GDL degradation”, International Journal of Hydrogen Energy, in-press, https://doi.org/10.1016/j.ijhydene.2023.03.342
Analysis of PEMFC performance by superimposing acoustic waves
Analysis of PEMFC performance by superimposing acoustic waves
Preventing flooding that disturbs oxygen transport to the catalyst layer is important to prevent performance losses in Polymer electrolyte membrane fuel cells (PEMFC). In this study, acoustic pressure waves were supplied on the oxygen gas flow through the cathode to solve the flooding problem by controlling the water inside the channel. The effect of external acoustic pressure waves on the cell performance and characteristics of two-phase flows in the channel was investigated. Water generated during PEMFC operation was discharged more frequently when the external excitation was applied. This enhanced water removal ability prevented internal flooding which led to the improvement of PEMFC performance.
Related publications
J.Y.Kim, M. Mortazavi, and S.Y.Jung*, "Improving polymer electrolyte membrane fuel cell performance and preventing flooding by exciting gas flow" Journal of Power Sources 617, 235181 (2024) https://doi.org/10.1016/j.jpowsour.2024.235181.
Mechanical degradation in PEMFC
Mechanical degradation in PEMFC
It is important to understand that it occurs when operating under certain conditions to prevent and improve the performance and durability deterioration of polymer electrolyte membrane fuel cell (PEMFC). This study investigated the effect of mechanical degradation on gas diffusion layer (GDL) and mass transport loss due to repetitive relative humidity (RH) variations. An accelerated wet/dry cycle was used to cause repetitive RH variations. The surface characteristics and water removal ability of GDL were identified using a scanning electron microscope and monitoring penetration pressure. After cycles, PTFE agglomeration in the substrate layer of GDL was observed. The water removal ability of GDL has deteriorated with the decrease of breakthrough pressure and the increase of the range of pressure fluctuation. In addition, mass transport loss calculated using the Nernst equation significantly increased.
Related publications
S. Lee, C. Kim, Y.Y. Choi, S.Y. Jung, Y.J. Shon and H. Oh*, “Effects of Gas-Diffusion Layers and Water Management on the Carbon Corrosion of a Catalyst Layer in Proton-Exchange Membrane Fuel Cells”, International Journal of Energy Research, Article ID 7961519 (2024) https://doi.org/10.1155/2024/7961519.
In situ two-phase flow in gas channels of PEMWE
In situ two-phase flow in gas channels of PEMWE
PEMWE (proton exchange membrane water electrolyzer) can operate at a high current density with a small size to obtain a large mount of hydrogen. However, when PEMWE operate under high current density conditions, lots of air bubbles are generated which causes the reduction of water electrolysis efficiency due to the increase of mass transport loss. In this study, three types of flow channels (single serpentine, double serpentine, and parallel) were designed to reduce mass transport loss and PEMWE performances were compared. Among tested channels, the parallel channel had the worst performance because slug flows with Tayler bubbles, that could disturb the water supply, were mainly occurred. Whereas, the single serpentine channel had the best performance because annular flows with a thin liquid film could reduce mass transport loss were dominant.
Related publications
S.K. Kim and S.Y. Jung*, “The effect of two-phase flows on PEM water electrolysis cell performance”, Journal of Mechanical Science and Technology 38(8) 3933~3939 (2024) http://doi.org/10.1007/s12206-024-2106-5.
In situ two-phase flow in pulsating heat pipe
In situ two-phase flow in pulsating heat pipe
The importance of cooling devices for solving the heat generation problems of electronic devices is increasing, and a heat pipe can provide the highest thermal performance. PHPs using water are commonly made of copper and silicon; however, aluminum is more sustainable than copper, and more economical than silicon (in addition to weight and durability advantages). However, aluminum has not been employed as a material for PHPs with water as the working fluid. This is because non-condensable hydrogen gas is generated from the reaction of aluminum and water at high temperature, leading to the stoppage of flows inside the PHPs and resulting in the worst thermal performance. In this study, a micro-nano surface treatment was applied to the surface of the aluminum. This prevented hydrogen gas formation by forming aluminum hydroxide, thereby removing possible reaction sites on the surface. The proposed method improved both the performance and durability of water-aluminum PHPs.
Related publications
J.Y.Kim, H.H.Cho, and S.Y.Jung*, "Improvements in performance and durability through surface modification of aluminum micro-pulsating heat pipe using water as working fluid." International Journal of Heat and Mass Transfer 214 (2023): 124445.
H. Ahmad, S.K. Kim, J. H. Park and S.Y. Jung*, “Development of two-phase flow regime map for thermally stimulated flows using deep learning and image segmentation technique”, International Journal of Multiphase Flow, 146, 103869 (2022).
H. Ahmad and S.Y. Jung*, “Effect of active and passive cooling on the thermo-hydrodynamic behaviors of the closed-loop pulsating heat pipes”, International Journal of Heat and Mass Transfer, 156, 119814 (2020)
H. Ahmad, S.K. Kim and S.Y. Jung*, “Analysis of thermally driven flow behaviors for two-turn closed-loop pulsating heat pipe in ambient conditions: An experimental approach”, International Journal of Heat and Mass Transfer, 150, 119245 (2020).
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