Conventional Straight Channel Research Articles

Dynamic variation of heat and mass transfer in a unitized regenerative fuel cell with a stepped channel during mode switching

Understanding the variation in heat and mass transfer during mode switching of a unitized regenerative fuel cell is essential for designing efficient flow channels and operational strategies. In this study, dynamic variation of current density, temperature, and species distribution during mode switching is investigated in a unitized regenerative fuel cell with a stepped flow channel. This work is conducted by a transient multi-physics model coupled with electrochemical reaction, heat and mass transfer, and phase change. Results indicate that the response time of current and temperature is prolonged as the mode-switching frequency rises. Using stepped channels instead of conventional straight channels can promote mode-switching and shorten the response time. Specifically, the stepped structure promotes airflow disturbance in flow channels, and the induced forced convection promotes heat and mass transfer. Besides, the stepped structure accelerates the stabilization of current density, temperature, and species during mode switching, leading to a more uniform current density distribution. In addition, the stepped structure promotes water discharge and convective heat transfer, improving the temperature uniformity of the catalyst layer. This study offers valuable insights for improving the durability of the membrane electrodes and improving the operational stability of unitized regenerative fuel cells during mode switching.

Research on the effect of connected cable-type channel structure and flow field arrangement on the performance of SOFC

In order to address Solid oxide fuel cells(SOFC) the issues of uneven gas distribution and low fuel utilization rate in conventional straight-channel. In this research, a connected cable-type channel with enhanced connectivity and broader radiation coverage was developed. The research revealed that in a singular flow channel scenario, the distribution of oxygen on the electrode within the connected cable-type channel exhibited greater uniformity compared to the conventional straight-channel design. The gas utilization rate is increased by 2.38% compared with the conventional straight-channel, and the output power density is increased by 8.55%. In the flow field, it is observed that the gas distribution on the cathode side of connected cable-type channel is significantly superior to that of the conventional straight-channel. In the case of a parallel flow field, the output power of the connected cable channel increases by 8.39% compared to the conventional straight channel, while the serpentine channel shows an increase of 11.65% in output power. Then, the impact of the flow field combination on the flow field was investigated. It was discovered that the output power of the cell could reach up to 5158.17 W/m2 when the serpentine-parallel staggered flow field distribution structure was utilized. This output was 11.33% higher than that of the parallel flow field.

Analysis and comparative assessment of charging dynamics in vertical multi-channel latent heat storage system with corrugated wavy channels

Latent heat thermal energy storage (LHTES) is a promising technology, but thermal response limitations caused by the relatively poor thermal conductivity of phase change materials (PCMs) remains an issue. In recent years, computational fluid dynamics (CFD) has proven to be a powerful and reliable tool in design optimization of different systems and problems in the energy sector. Building on that, the present work will use CFD to investigate the thermal performance enhancements achieved using vertical double-tube LHTES systems with multi-channel sinusoidal wavy geometries. The main parameters investigated here are the wavy channel height (H), pitch (P) and the number of channels (N) and the results are assessed in terms of melting rates and heat transfer coefficients. The analysis found that increasing the height from 1 mm to 3 mm, accelerated the phase change by 21 %, while reducing the pitch from 12.5 mm to 5 mm, enhanced the heat storage rates by 28 %. In addition, increasing the number of channels from 3 to 7 improved the charging rate by 64 %. Compared to a conventional straight channel, the enhanced wavy channel (N = 7, H = 3 mm, P = 5 mm) achieved 80 % higher thermal storage rate and 42 % faster charging time. The above findings provide new insight into the potential of utilizing compact vertical LHTES systems via customized wavy channels, with significant impact towards enhancing the existing thermal energy storage systems.

Experimental evaluation of flow field design on open-cathode proton exchange membrane fuel cells (PEMFC) short stack consisting of three cells

The promising nature of proton exchange membrane fuel cell stacks in energy conversion has sparked the need to improve the cell components for improved performance, durability, sustainability, and cost competitiveness. This work presents the pros and cons of different flow field designs concluded from an open-cathode short stack, composed of three cells. The flow fields consist of 3D fine mesh, fine wire mesh, metal foam, and conventional straight channels. The results presented in this study show that the fine wire mesh gives better performance, both when integrated, and not integrated with straight channels. It is followed by the 3D fine mesh, straight channels, and metal foam. The different performance improvements are caused by the various flow fields influencing flow rate, current conductivity, and temperature increment. Water management seems to be a significant factor in the performance variation. This is presented by the IV curves, which show a dramatic discrepancy over the concentration loss region. The straight channels integrated with fine wire mesh cell have the highest degradation rate at a constant voltage of 1.7 V, even though it reached a constant value after 1.4 h of operation.

DEVELOPMENT OF HEAT EXCHANGER WITH TWO-PASS DIVERGING MICROCHANNELS FOR EVAPORATION COOLING SYSTEMS

This study developed a two-pass diverging microchannel heat exchanger to avoid flow reversal and partial dry out of HFC-245fa flow boiling in the evaporation cooling systems. It is aimed to improve the diverging microchannel heat exchangers designed by previous studies and to effectively accommodate diverging channels on a limited heating base surface. The test results showed that the maximum heat transfer coefficients in a diverging channel heat exchanger were as much as 20% higher than those in straight channel heat exchanger, while the maximum partial dry-out heat flux is 42% higher in diverging channel heat exchanger. There is no significant difference for the pressure drops in two heat exchangers at low heat fluxes before it reached the partial dry-out condition. For higher flux, the pressure drops in straight channels were higher than those in divergent channels while partial dry out. It is concluded that the two-pass diverging channel design provided higher maximum heat transfer coefficients and higher maximum heat fluxes with lower penalties of pressure drops. It could be a good design to replace the conventional straight channel heat exchanger for evaporation cooling systems.

Presentation and Performance of a Planar Solid Oxide Fuel Cell with Chaotic Gas Channels

Flow condition in gas channels determines the distribution and diffusion of reactants, governing electrochemical reactions in the solid oxide fuel cell (SOFC). It is found that the magnitude and direction of fluid velocity change with the geometric position in the channel, which leads to larger secondary flow velocity components and enhances mass transfer between fluids. Laser Doppler velocimetry experiment is conducted to verify the accuracy of numerical results of fluid flow in chaotic flow channels. Herein, a 3D numerical model with coupled multiphysics fields is built to evaluate fluid flow in channels and SOFC performance. The distribution characteristics of reactants in chaotic flow channel and the characteristics of temperature distribution, current density and power density in SOFCs caused by structures of straight and chaotic flow channels are analyzed and discussed. Compared with conventional straight flow channel SOFCs, the average current density of SOFC with chaotic flow channel is improved and the power density is increased by 9.7%. The reactant homogeneity in the electrode is also improved, which reduces the concentration polarization loss of SOFC.

Numerical study of Tesla valve flow field on proton exchange membrane fuel cell performance

Effective flow channels in proton exchange membrane fuel cells (PEMFCs) can provide sufficient reactant and a more uniform reactant distribution, which allows the fuel cells to operate with high performances. The Tesla valve structure is thought to be a feasible structure for channel design due to its one-directional flow characteristics, yielding different performances with different flow directions. Numerical studies were conducted in this work to investigate the impacts of the channel structure, flow direction, and excess air coefficient on the fuel cell performance. Three types of flow channels (straight, symmetric Tesla valve, and asymmetric Tesla valve) and two flow directions (forward and reverse) were studied. The results showed that the maximum power density of the reversed asymmetric Tesla valve increased by 10.08 % compared to that of the conventional straight channel. Detailed research revealed that the Tesla valve resulted in a more intense flow than the conventional straight channel. The symmetric Tesla valve resulted in a greater pressure drop than the asymmetric valve. The Tesla valves with reverse flow could significantly limit the formation of an oxygen dead zone. Hence, enhanced Z-direction mass transfer and the lowest standard deviations of the oxygen and water distributions were observed. The effects of the excess air coefficient on the net power density were also studied. Increasing the excess air mass to the cathode facilitated the enhancement of the electrode voltage but required more parasitic power due to the growth in the pressure drop along the flow channel.

Enhancement of two-phase flow and mass transport by a two-dimensional flow channel with variable cross-sections in proton exchange membrane fuel cells

A novel two-dimensional flow channel (TDFCH) with variable cross-sections geometrically controlled by sinusoidal function is proposed for proton exchange membrane fuel cells (PEMFCs). Two-phase flow simulations and visualisation experiments are combined to reveal the mechanisms for improved water management and enhanced reactant transport. Results show that the centrifugal force generated by the vortex pushes the liquid water away from the central region of the flow channel, forming droplet and film flows to avoid blockage of the flow channel. The reactant gases accumulate on the windward side of the variable-diameter structures, leading to an increase of the local pressure and facilitating fuel diffusion into the porous electrode. The fluid inertia enhances mass transfer of reactants and removal of liquid water inside the porous electrode. The amplitude A of the flow channel based on sinusoidal curve plays a dominant role in improving the performance of the TDFCH. Under the same testing conditions, when amplitude A = 0.2 and period T = 2, the peak power density of the TDFCH-based cell reaches 517.9 mW cm−2, which is 26 % higher than the conventional straight flow channel. The newly designed TDFCH provides an efficient solution to realize performance improvement and long-term stable operation of PEMFCs.

Thermohydraulic performance of ammonia, isopropanol, water and nanofluids as cooling fluid for lithium-ion 1C and 3C rating batteries

Cooling Lithium-ion batteries of different C ratings receive excellent attention amongst researchers in thermal management. The present study proposes to investigate the thermohydraulic performance of a wavy channel and compare the finding with the conventional straight channel. The full Navier Stokes equations and the energy equation were solved numerically using the finite element technique. Water is the cooling liquid used in the simulation. The flow is laminar and steady state. It is found that the average Nusselt number is higher as the waviness of the channel wall increases. The average Nusselt number for the wavy channel case was increased by up to 31% compared to the straight channel configuration. Also, the performance evaluation criterion increases by 23% compared to straight channel configuration. Thus, allowing the fluid to absorb more heat. However, at the expense of the pressure drop, the performance evaluation criterion is higher for the wavy wall channel mode than the straight channel wall. Amongst all fluids studied in this paper, including water, ammonia, isopropanol, Ammonia binary mixture and TiO2 nanofluid at different concentrations, ammonia is the most suitable liquid for cooling.

A novel cathode flow field for PEMFC and its performance analysis

A good flow field design is important to the proton exchange membrane fuel cell (PEMFC) performance, especially under a high current density region, which is dominated by concentration polarization. Motivated by the variable cross-section channel idea, in this study, a novel flow field containing a converging-diverging (C-D) pattern is proposed. A three-dimensional multiphase model is established to study its performance. The numerical results show that it outperforms the conventional straight channel and only depth-variant channel. In the novel flow field the enhanced under land cross flow and higher effective mass transfer coefficient both improve the reactant transport. The effects of operating conditions, like stoichiometric ratio and operating pressure, on cell output performance are studied. It is found that a higher promotion rate can be obtained by increasing the stoichiometric ratio, but increasing the operating pressure has little effect. The droplet dynamic behavior in the C-D channel and straight channel are studied, and the results prove the better drainage capability of the novel flow field.

Design and optimization of a novel sinusoidal corrugated channel for microfluidic fuel cell with gas-liquid two-phase flow model

Membraneless microfluidic fuel cell (MMFC) has potential application prospects in micro-devices due to the characteristics of considerable output performance, superior cleanliness and easy miniaturization. Whereas, the performance of MMFC is restricted by the mass transfer of two-phase flow, which manifests as the inefficiency transportation in liquid phase and obstruction of gas phase. This paper proposes a novel sinusoidal corrugated channel (SCC) for MMFC though developing a three-dimensional two-phase model by coupling the multi-physics field to improve the cell performance in mass transfer, and further studies the two-phase flow characteristics of geometric parameters of SCC, including the amplitude A and angular frequency ω. Based on the optimal configuration in A of 0.5 mm and ω of 3 rad s−1, compared with the conventional straight channel, the increase ratios of maximum current density, power density, fuel utilization are 13.69%, 8.39% and 13.72%, respectively. Additionally, the gas volume fraction at the maximum power density decreases by 12.62%. This paper provides new insights and theoretical foundations for the study of MMFC flow field configurations, and furnishes references for the mass transfer intensification.

Numerical investigation on heat transfer and fluid flow in a multi-minichannel heat sink: Effect of channel configurations

Three dimensional numerical simulation has been performed to explore the hydro-thermal performance of novel configurations of multichannel (MMCHS) heat sink. The study included the effect of three channel configurations matched with chosen inlet/outlet locations of the cooling fluid on the flow distribution between channels. The three new modified channel configurations were considered in this work, namely, arc-shaped, S-shaped and wave-shaped channels. The study is conducted under laminar flow condition with single phase flow using water as a coolant of mass flow rate range (0.002–0.005 kg s−1). The results revealed that the new modified channels are capable of enhancing heat transfer comparative with conventional straight channel. Among the all configurations, the wave-shaped channels showed the highest enhancement in Nusselt number of 30.5% followed by S-shaped channels with 18.7% and arc-shaped channels with 10.8%. Meanwhile, the arc-shaped channels achieved lowest pressure drop with a ratio of 16% less than the traditional channel. Furthermore, it's found that temperature uniformity for new channel configurations is better than traditional channel. The maximum difference in temperature distribution showed a reduction about 2.64 K in wave-shaped channels. The best performance factor of MMCHS has been accomplished by wave-shaped channels with a value of 1.3.

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.

Effects of diverging channel design cooling plate with oblique fins for battery thermal management

In this study, the thermal and hydraulic performances of a battery thermal management system are investigated in terms of the design of the cooling plate. A diverging channel with oblique fins (DCOF) cooling plate is newly proposed to overcome the limitations of the conventional straight channel cooling plate. The diverging design is applied to control the coolant velocity and heat transfer area, with oblique fins inserted to enhance the heat transfer efficiency. A simulation model of the battery and cooling system is developed and experimentally validated. When the DCOF design is applied to the battery module, the temperature uniformity is improved in terms of the temperature difference and standard deviation. As the investigation is extended from the module to the battery pack, the temperature difference and temperature standard deviation in the battery module are improved by 19.07% and 27.14%, respectively. Considering the power consumption of the cooling system, as the thermal performance of the DCOF design and baseline design is maintained, the power consumption of the coolant pump of the DCOF design is significantly reduced by 67.8%.

Fluid dynamics and mass transfer in curved reactors: A CFD study on Dean flow effects

The effect of Dean flow on the performance of a curved channel reactor in terms of fluid flow and mass transfer characteristics was numerically evaluated. Laminar multi-component gas flow considering the photocatalytic NOx degradation as model reaction was investigated using a validated three-dimensional computational fluid dynamics (CFD) model. The curved channel device was compared with a conventional straight channel reactor, considering the same catalytic area and cross-section. The results showed that the velocity profile was distorted from its original symmetrical shape, and the maximum velocities shifted towards the outer-side wall for De > 120.1, due to the influence of centrifugal acceleration in the radial direction. The curved channel reactor resulted in higher mass transfer coefficients compared to the straight channel device. The higher mass transfer enhancement factor (γ ~ 1.8) was obtained for AR = 1 and Re = 725.5. However, this condition imposed a pressure drop 3 times higher the straight channel device. The reactor’s performance was also represented by an association of ideal reactors (PFR and CSTR). The NO conversion in the curved channel device was equivalent to the behavior of a CSTR (n = 1.5) due to the Dean flow contributions favoring the mixing. Additional analyzes regarding the influence of the radius of curvature were carried out in a U-shaped reactor. Finally, an analysis of dimensionless numbers (P é clet and Damköhler) corroborated the results found numerically. Therefore, this study can provide insights into the design and analysis guidelines for curved channel reactors.

Enhanced oxygen discharge with structured mesh channel in proton exchange membrane electrolysis cell

The oxygen discharge in the proton exchange membrane electrolysis cell (PEMEC) has a great influence on the performance because it hinders the liquid water supply from channel to the catalyst layer (CL). In this study, a novel structured mesh channel is proposed to enhance the oxygen discharge capacity in PEMEC, in which the solid skeleton structure facilitates oxygen bubble splitting, and the structural capillary force effect promotes the oxygen detach the liquid/gas diffusion layer (L/GDL) surface. The gas and liquid two-phase flow characteristics in structured mesh channel and conventional straight hollow channel are compared in detail using the volume of fluid (VOF) model, which is subsequently integrated with a three-dimensional (3D) multiphase full-cell model through data exchange to simulate the influence of oxygen in channel on the PEMEC performance. Note that the reliability of this integration method is validated against the experimental oxygen distribution in parallel and serpentine flow fields and the electrochemical performance simultaneously. It is also found that reducing the skeleton surface hydrophobicity (i.e. contact angle) gradually from the channel top to L/GDL surface helps improve the oxygen discharge capacity and hence the PEMEC performance.

Numerical study of optimized three-dimension novel Key-shaped flow field design for proton exchange membrane fuel cell

Key-shaped three-dimension (3D) flow field channel is designed to improve the performance and mass transfer of proton exchange membrane fuel cell (PEMFC). This study comprehensively analyses the impacts on the performance and mass transfer of the flow channel from multiple dimensions such as the size, shape, and placement of the blocks. In comparison with the conventional straight single flow field channel, the new channel with rectangular blocks can effectively improve performance by 30%. Semi-elliptical and quarter-elliptical blocks are designed to make forced convection and increase the diffusion area of oxygen. The results indicate that the flow velocity in the Z-axis direction can be increased to 0.08–0.2 m/s due to the narrow space formed by variable cross-sections. In conclusion, the Key-shaped design has a potential to improve the performance of mass transfer in the cathode channel, providing a new strategy for the development of flow field design in PEMFC field.

Numerical investigation of water droplet removal characteristics in novel block channels of PEMFC using dynamic wettability model

Liquid water transport and removal is one of the critical issues in the proton exchange membrane fuel cell (PEMFC) for achieving good performance and durability. In this study, two novel channels with different blocks are designed to study their effects on water removal using the volume of fluid (VOF) model considering the dynamic contact angle effect. It is found that compared with the conventional straight channel, both the one-block and two-block channels can promote liquid water removal. The one-block channel leads to faster water movement and removal on the gas diffusion layer (GDL) surface, but results in a much higher pressure drop. The separated two-block channel directly drags water away from the GDL surface by the capillary wicking effect of the block surface, achieving both faster water removal and smaller pressure drop. Effects of the droplet size, air velocity and static contact angle of GDL surface on water removal are investigated comprehensively in both the novel channels, as well as the conventional straight channel, with particular attention on the variations of water removal time, water coverage ratio and pressure drop.

Thermo-hydraulic and entropy generation analysis for non-Newtonian fluid flow through sinusoidal wavy wall channel

The present investigation deals with numerical simulation of thermo-hydraulic transport characteristics of non-Newtonian fluid flowing through a wavy channel subjected to isothermal condition. The power-law constitutive model is adopted to illustrate the rheological behavior of fluid. The influence of various parameters like index of power-law (n), Reynolds number (Re), amplitude of sinusoidal profile (A), etc. on flow, thermal field, and entropy generation characteristics are delineated in detail. Furthermore, the outcome of the sinusoidal channel is compared with that of conventional straight channel in order to check the performance of wavy-wall channels. Results divulge that reliance of the thermo-hydraulic characteristics on wall geometry is unequivocally affected due to the amplitude of sinusoidal profile. Augmentation in heat transfer rate for lower values of amplitude of sinusoidal profile over the conventional channel is inconsiderable although difference upraises considerably owing to two factors namely: higher amplitude of sinusoidal profile and Re. The heat transfer rate diminishes with increment in power-law index. Consequently, heat transfer rate is progressively conspicuous for shear thinning fluids compared to shear thickening fluids. For comparison of pressure drop in the wavy and equivalent straight channel taking into account heat transfer and corresponding enhancement of pressure drop, we have defined a performance parameter. The comparison of performance parameter illustrates that shear-thickening fluids are more appropriate for engineering applications. Two nondimensional numbers are defined to elucidate the behavior of entropy generation with various parameters like Reynolds number, amplitude of sinusoidal profile, and power-law index. The entropy generation increases as the amplitude of waviness increases. Furthermore, entropy generation also intensify with increment in index of power-law for a distinct Reynolds number and amplitude of waviness. Results obtained from present study can be used as a tool to select wall geometry for design of compact and economical energy exchange devices dealing with non-Newtonian power-law fluids.

Performance enhancement of a beam and slot interconnector for anode-supported SOFC stack

This study proposes a novel interconnector, termed beam and slot interconnector (BSI), for the anode-supported planar solid oxide fuel cell (SOFC). A detailed comparative investigation is conducted on various transport characteristics and electrical performance of the SOFC stacks with conventional straight channel interconnectors (SCIs) or with novel interconnectors. Results show that the peak power density of a SOFC stack with BSIs is 24.8% higher than it with SCIs at 700 °C and a fuel–air flow rate of 16–40 Nml/(min·cm2). Moreover, BSI can reduce the fuel–air feeding and enhance the fuel–air utilization while maintaining high output power density. Compared with SCI, BSI promotes the gas disturbance, significantly increases the gas velocity and vorticity, and leads the gas to flow in the direction perpendicular to the channel. BSI eliminates the limitation of SCIs on gas diffusion in the electrode and transfers sufficient reactant gas into the electrode function layers for electrochemical reactions. BSI shortens the charge transfer path in SOFC and almost avoids the adverse effects of the electrode-interconnector contact resistance. Compared with the conventional SOFC stack, the novel SOFC stack with BSIs significantly reduces various overpotentials. At 700 °C and 0.5 A/cm2, the activation, concentration, and contact overpotentials are reduced by 8.5%, 47.4%, and 96.4% respectively, and the total overpotential finally drops by 20.0%. Overall, the electrical performance of the SOFC stack with novel interconnectors significantly exceeds the one with conventional interconnectors under the same operating conditions.