Adjacent Flow Channels Research Articles

Research on the Relationship between Stall Propagation and Flange Leakage of Mixed-Flow Pumps

In order to explore the internal relationship between stall core propagation and flange leakage flow in the rotating stall of a mixed-flow pump, based on the k-ε turbulence model, a SIMPLEC algorithm and hexahedral structured grid are used to simulate the internal flow field of the mixed-flow pump. By setting the flange clearance as 0.2 mm, 0.5 mm and 0.8 mm, the propagation characteristics of the rotating stall and the unsteady characteristics of flange leakage flow of the mixed-flow pump under the condition of near stall are studied, and the influence of the flange clearance on the pressure fluctuation characteristics of the mixed-flow pump under the condition of near stall is analyzed. The results show that the stall core is located at the outlet of the impeller and propagates from the leading edge of the adjacent blade along the opposite direction of blade rotation to the next flow channel. The pressure gradient in the stall channel and the energy loss are large. When the flange clearance is 0.5 mm and 0.8 mm, the stall core changes from one to two, and the propagation mechanism of the stall core tends to be complex in the two adjacent flow channels. When the flange clearance is 0.8 mm, the propagation period decreases. The variation law of leakage flow is consistent with the propagation law of stall core. When the flow passage changes from stall state to non-stall state, the leakage flow also changes from one state to another, so the leakage flow can be used as a form of apparent stall. Under the condition of near stall, the pressure fluctuation curve of the adjacent monitoring points has a large phase difference consistent with the propagation period of the stall core, and has a strong pressure drop. When the flange clearance is 0.5 mm and 0.8 mm, the time domain curve of pressure fluctuation has two wave troughs in one cycle. In the near stall condition, the main frequency of the pressure fluctuation at the monitoring point is the stall frequency, and the amplitude of the main frequency at the middle of the outlet is the largest. The characteristics of flange leakage flow and pressure fluctuation can better reflect the flow situation in the pump when rotating stall occurs. The research results can provide a basis for judging whether stall occurs in the flow passage of the pump.

A convection-enhanced flow field for aqueous redox flow batteries

In this work, we propose and fabricate a convection-enhanced flow field for aqueous redox flow batteries. Similar to the conventional single serpentine flow field, the new flow field here has an inlet and outlet, but the repatterned flow path induces higher pressure differences between adjacent flow channels over the entire electrode surface, thereby enhancing the under-rib convection between adjacent flow channels. The application of the new flow field to a vanadium redox flow battery enables substantial performance improvements. At 1.67 mL min−1 cm−2, which is a typical flow rate for a kilowatt-class VRFB stack, the energy efficiency, and electrolyte utilization reach 75.3% and 49.6% at a current density of 250 mA cm−2, increasing by 10.0% and 33.3% as opposed to the use of the conventional serpentine flow field, respectively. Three-dimensional numerical modelling shows that the improved performance is ascribed to the more uniform in-plane distribution of reactants and current with the new flow field. Another important finding of this work is that there is a critical flow rate above which energy efficiency tends to stabilize for a given current density.

A numerical analysis for a damage propagation in a plate-type fuel assembly of a research reactor

If several flow channels are completely blocked in a plate-type fuel assembly of research reactors with high power density, a flow instability (FI) can occur in an unblocked channel and can be propagated to the other adjacent flow channels. Such FI propagation can cause a propagation of fuel plate damages during the flow blockage accident. Hence, the thermal hydraulic behaviors of flow channels have been analyzed in order to investigate the FI propagation process by the simulation of CFD code for a multiple flow channel blockage in KiJang Research Reactor (KJRR) which is the model reactor assumed for this paper. Simulation results show that a FI occurs at the first unblocked channel if 8 flow channels are completely blocked and most of fuel plates are significantly damaged in 29 s after a complete blockage of 8 flow channels occurs in a plate type fuel assembly of the model reactor.

Numerical and experimental investigation of baffle plate arrangement on proton exchange membrane fuel cell performance

Reactant distribution and water management are critically important to the performance of proton exchange membrane fuel cell (PEMFC). The application of baffle plate is an effective way to improve reactant transport and water removal in the porous electrode of PEMFC. In this study, a three-dimensional multiphase PEMFC model is developed with Forchheimer's inertial effect in the porous electrode to better simulate the convective flow induced by the baffle plate, which is validated experimentally. Three kinds of flow field design including the conventional parallel flow field, parallel trapezoid baffle plate (PTBP) and staggered trapezoid baffle plate (STBP) flow fields are investigated both numerically and experimentally, on the PEMFC mass transport characteristics and performance. It is found that both the PTBP and STBP flow fields form the over-block-convection around the baffle plate which is beneficial to mass transfer from channel to electrode. The STBP flow field further forms the over-rib-convection (or cross flow) induced by a stable pressure gradient between the adjacent flow channels. The cross flow stem from the STBP arrangement further improves the uniformity of reactant distribution and removes the excess liquid water in the porous electrode, and hence enhances the PEMFC performance in a large range of operating conditions.

Single serpentine flow fields design and sub-rib convection analysis for a PEM fuel cell

In a polymer electrolyte membrane fuel cell (PEMFC), transport of reactant gases through a bypass convection mechanism established between two adjacent channels along the gas diffusion layer (GDL) under the serpentine type flow field of a bipolar plate can enhance the fuel cell performance by reducing the power losses. In this paper, three different serpentine type flow field models for bipolar plates of a PEM fuel cell unit have been designed, with different channel width/rib width ratios of 1, 1.05 and 1.2, and different pressure loses have been evaluated. An analytical model for bypass sub-rib convection, based on two adjacent flow channels and a serpentine in contact with a GDL layer was used here. This model established the influence of bypass convection in the form of Peclet number Pe for the flow field models, by modifying the GDL thickness t (mm) and active channel length L (mm). The optimal combination of thickness t, porosity ε and permeability k for a series of commercial GDL materials from the perspective of sub-rib convection was established for SigracetTM GDL 35 BC, with highest Pe number modified along the channel.

Investigating the Dehumidification Characteristics of Turbine Stator Cascades with Parallel Channels

Steam turbines play a vital role in the power industry. The efficiency of the steam turbine can be improved by reforming the flow structure. For the White cascade, a parallel channel which is an additional steam passage was designed to connect adjacent steam flow channels. In view of the steepening and sensitive characteristics of the steam condensation parameters in the White cascade, the two-fluid model was applied to study the condensation and aerodynamic characteristics of the prototype cascade and the modified cascade. Wetness losses were regarded as the evaluation standard, and the optimum design of the parallel channel was obtained. The influence of channel width was also studied. The location of the suction surface (D point) has great influence on the dehumidification performance of the cascade. On the contrary, the location of the pressure surface (C point) has no significant effect on dehumidification performance. Moreover, it is necessary to select the width of the parallel channel in combination with the cascade performance. It is suggested that the parallel channel width should be between 3 and 5 mm. The research results can provide a reference for the optimal design of a turbine stator cascade.

Altered biofilm formation at plasma bonded surfaces in microchannels studied by attenuated total reflection infrared spectroscopy

This paper is dedicated to Professor P.R. Norton on the occasion of his 75th birthday, in honor of his profound contributions to Surface Science. In this work, we investigate how plasma bonding of a germanium ATR crystal to a microfluidic device can affect biofilm growth and development. Using attenuated total reflection Fourier transform infrared spectroscopy, individual measurements were made at the attachment surface of growing Pseudomonas fluorescens biofilms in adjacent flow channels during parallel experiments. Biofilm growth in channels with ATR surfaces exposed to air plasma exhibited a faster accumulation of a biomolecular conditioning layer compared to unexposed channels. As well, in-line microscopy revealed enhanced bulk biofilm growth in plasma-treated channels. Since the surface chemistry the Ge ATR was only partially recovered to its original state during the three day experiments, it is believed that the enhanced biofilm growth was ultimately due effects of plasma exposure. It is proposed that observations are transferable to microfluidic devices with sealing layers from other hard surfaces such as glass, silicon and plastic due to their ability to retain surface functionalization after plasma exposure. Plasma treatment could, therefore, offer a route to faster start up times for bioreactors, but could also result in unexpected artifacts in other studies.

A tapered serpentine flow field for the anode of micro direct methanol fuel cells

We develop a self-breathing micro direct methanol fuel cell (μDMFC) characterized by a new anode structure with tapered single serpentine flow fields to improve cell performance. Compared with the conventional single serpentine flow field, this new design enhances the methanol mass transport efficiency and the exhaust resultant (CO2) rate due to the increasing pressure difference between adjacent flow channels. The μDMFCs with two single serpentine flow fields are fabricated using silicon-based micro-electro-mechanical systems (MEMS) technologies and are tested at room temperature. The experimental results reveal that the new tapered single serpentine flow field exhibits a significantly higher peak power density than that of the conventional flow field, demonstrating a substantial increase of 17.9% in mass transport coefficients.

Optimization of a Single-Cell Solid-Oxide Fuel Cell Using Computational Fluid Dynamics

To determine the effects of various parameters on the performance of a solid-oxide fuel cell (SOFC), a series of simulations was performed using computational fluid dynamics (CFD). The first step in this process was to create a three-dimensional CFD model of a specific single-cell SOFC for which experimental performance data had been published. The CFD simulation results developed using this baseline model were validated by comparing them to the experimental data. Numerous CFD simulations were then performed with various thermal conditions at the cell’s boundaries and with different fuel and air inlet temperatures. Simulations were also conducted with fuel utilization factors from 30% to 90% and air ratios from 2 to 6. As predicted by theory, conditions that resulted in higher cell temperatures or in lower air and fuel concentrations resulted in lower thermodynamically reversible voltages. However, the higher temperatures also reduced Ohmic losses and, when operating with low to moderate current densities, activation losses, which often caused the voltages actually being produced by the cell to increase. Additional simulations were performed during which air and fuel supply pressures were varied from 1 atm to 15 atm. Although the increased pressure resulted in higher cell voltages, this benefit was significantly reduced or eliminated when air- and fuel-compressor electrical loads were included. CFD simulations were also performed with counterflow, crossflow, and parallel-flow fuel-channel to air-channel configurations and with various flow-channel dimensions. The counterflow arrangement produced cell voltages that were equal to or slightly higher than the other configurations, and it resulted in a differential temperature across the electrolyte that was significantly less than that of the parallel-flow cell and was close to the maximum value in the crossflow cell, which limits stress caused by uneven thermal expansion. The use of wider ribs separating adjacent flow channels reduced the resistance to the electrical current conducted through the ribs. However, it also reduced the area over which incoming fuel and oxygen were in contact with the electrode surfaces and, consequently, impeded diffusion through the electrodes. Reducing flow-channel height reduced electrical resistance but increased the pressure drop within the channels. Plots of voltage versus current density, together with temperature and species distributions, were developed for the various simulations. Using these data, the effect of each change was determined and an optimum cell configuration was established. This process could be used by fuel cell designers to better predict the effect of various changes on fuel cell performance, thereby facilitating the design of more efficient cells.

A numerical study of flow crossover between adjacent flow channels in a proton exchange membrane fuel cell with serpentine flow field

The focus of this paper is to study the flow crossover between two adjacent flow channels in a proton exchange membrane (PEM) fuel cell with serpentine flow field design in bipolar plates. The effect of gas diffusion layer (GDL) deformation on the flow crossover due to the compression in a fuel cell assembly process is particularly investigated. A three-dimensional structural mechanics model is created to study the GDL deformation under the assembly compression. A three-dimensional PEM fuel cell numerical model is developed in the aforementioned deformed domain to study the flow crossover between the adjacent channels in the presence of the GDL intrusion. The models are solved in COMSOL Multiphysics—a finite element-based commercial software package. The pressure, velocity, oxygen mass fraction and local current density distribution are presented. A parametric study is conducted to quantitatively investigate the effect of the GDL’s transport related parameters such as porosity and permeability on the flow crossover between the adjacent flow channels. The polarization curves are also examined with and without the assembly compression considered. It is found that the compression effect is evident in the high current density region. Without considering the assembly compression, the fuel cell model tends to over-predict the fuel cell’s performance. The proposed method to simulate the crossover with the deformed computational domain is more accurate in predicting the overall performance.

Neutron imaging investigation of liquid water distribution in and the performance of a PEM fuel cell

In this study neutron radiography is applied to investigate the performance of a polymer electrolyte membrane (PEM) fuel cell based on the effect of liquid water accumulation in the cell. Dynamic performance tests have been carried out on a PEM fuel cell with a specially designed serpentine flow channel under various operating conditions and simultaneous measurements of accumulated liquid water with neutron imaging. Liquid water tends to accumulate in the gas diffusion layer (GDL) adjacent to the flow channel area while the liquid water formed in the GDL next to the channel land area seems to be effectively removed by the cross leakage flow through the porous GDL between the adjacent flow channels. The amount of liquid water accumulation in the cell is dependant on the cell operating temperature, the pressure drop in the flow channel and the current density under the present test conditions of fixed stoichiometry. It is shown that the cell performance is strongly affected by the presence and accumulation of liquid water, especially at high current densities. This phenomenon results in performance hysteresis for load variations. The rate of liquid water production is also mathematically modeled to analyze the effect of the cell operating temperature and pressure drop on the liquid water formation in a cell. The model result shows good agreement with experimental measurements. The history of liquid water accumulation is also analyzed.

Novel serpentine-baffle flow field design for proton exchange membrane fuel cells

An appropriate flow field in the bipolar plates of a fuel cell can effectively enhance the reactant transport rates and liquid water removal efficiency, improving cell performance. This paper proposes a novel serpentine-baffle flow field (SBFF) design to improve the cell performance compared to that for a conventional serpentine flow field (SFF). A three-dimensional model is used to analyze the reactant and product transport and the electrochemical reactions in the cell. The results show that at high operating voltages, the conventional design and the baffled design have the same performance, because the electrochemical rate is low and only a small amount of oxygen is consumed, so the oxygen transport rates for both designs are sufficient to maintain the reaction rates. However, at low operating voltages, the baffled design shows better performance than the conventional design. Analyses of the local transport phenomena in the cell indicate that the baffled design induces larger pressure differences between adjacent flow channels over the entire electrode surface than does the conventional design, enhancing under-rib convection through the electrode porous layer. The under-rib convection increases the mass transport rates of the reactants and products to and from the catalyst layer and reduces the amount of liquid water trapped in the porous electrode. The baffled design increases the limiting current density and improves the cell performance relative to conventional design.

A new flow field design for polymer electrolyte-based fuel cells

We present a new flow field design, termed convection-enhanced serpentine flow field (CESFF), for polymer electrolyte-based fuel cells, which was obtained by re-patterning conventional single serpentine flow fields. We show theoretically that the CESFF induces larger pressure differences between adjacent flow channels over the entire electrode surface than does the conventional flow field, thereby enhancing in-plane forced flow through the electrode porous layer. This characteristic increases mass transport rates of reactants and products to and from the catalyst layer and reduces the amount of liquid water that is entrapped in the porous electrode, thereby minimizing electrode flooding over the entire electrode surface. We applied this new flow field to a single direct methanol fuel cell and demonstrated experimentally that the new flow field resulted in substantial improvements in both cell performance and operating stability as opposed to the conventional serpentine flow field design.

Cross-leakage flow between adjacent flow channels in PEM fuel cells

In polymer electrolyte membrane (PEM) fuel cells, serpentine flow channels are used conventionally for effective water removal. The reactant flows along the flow channel with pressure decrease due to the frictional and minor losses as well as the reactant depletion because of electrochemical reactions in the cells. Because of the short distance between the adjacent flow channels, often in the order of 1 mm or smaller, the pressure gradient between the adjacent flow channels is very large, driving part of reactant to flow through the porous electrode backing layer (or the so-called gas diffusion layer)—this cross-leakage flow between adjacent flow channels in PEM fuel cells has been largely ignored in previous studies. In this study, the effect of cross-flow in an electrode backing layer has been investigated numerically by considering bipolar plates with single-channel serpentine flow field for both the anode and cathode side. It is found that a significant amount of reactant gas flows through the porous electrode structure, due to the pressure difference, and enters the next flow channel, in addition to a portion entering the catalyst layer for reaction. Therefore, mixing occurs between the relatively high concentration reactant stream following the flow channel and the relatively low reactant concentration stream going through the electrode. It is observed that the cross-leakage flow influences the reactant concentration at the interface between the electrode and the catalyst layer, hence the distribution of reaction rate or current density generated. In practice, this cross-leakage flow in the cathode helps drive the liquid water out of the electrode structure for effective water management, partially responsible for the good PEM fuel cell performance using the serpentine flow channels.

CHANNEL-TO-CHANNEL PRESSURE DIFFERENCES IN SERPENTINE MINICHANNEL FLOW SYSTEMS

One of the interconnected factors that can lead to failures in the flow plates of PEM fuel cells is the pressure differences that exist between adjacent flow channels. These pressure differences lead to stresses in the channel supports—i.e., the ribs—which can be important in the presence of stresses arising due to other factors such as temperature gradients in the flow plates. In order to investigate the magnitudes of the pressure differences across the supports and the places where the maximum pressure differences occur, the flow and pressure variations in various forms of serpentine channels, these channels having a rectangular cross-sectional shape, have been numerically calculated. The presence of the diffusion layer has been ignored and the flow has been calculated using a commercial finite-element software package using the governing equations written in dimensionless form. Solutions have been obtained for various values of the Reynolds number for each of the flow geometries considered for two channel height-to-width ratios (one and three). Except for the flow in the vicinity of the bends in channels, the pressure has been found, as is to be expected, not to vary significantly across the channel cross section. The difference between the center point dimensionless pressure in a given channel with those at the same longitudinal position in the adjacent channels has been determined. The dependence of the highest dimensionless pressure difference between channels on the input parameters has been examined. 273pt