Magnetohydrodynamics flow in a tube with centrally placed electrodes

We look at the problem of unsteady magnetohydrodynamic flow of an electrically conducting, laminar, incompressible, Newtonian, viscous fluid through a tube exposed to screen electrodes placed centrally in the flow field and at the two ends of the tube. The governing differential equations which comprise such effects as the electric field, the magnetic and flow fields are non-dimensionalized and solved numerically. The rotational electromagnetic forces arising from the combined influences of both the magnetic and the flow fields, are not balanced by the fluids pressure gradient forces and impact on the hydrodynamics of the fluid flow significantly. Graphical representation of numerical results for the steady state flow and electromagnetic fields are discussed. For cases involving no net flow through the tube, symmetric flow is observed for the symmetrical geometric configuration. This is however not the case for through flow, where the flow decelerates as it moves towards the centrally placed electrodes. A comprehensive parametric study is conducted to show the influence of certain dimensionless variables on the flow and magnetic fields, and in all cases they help to confirm both the numerical formulation and the overriding physics.     

Numerical simulation of two-fluid electroosmotic flow in microchannels

This paper presents a numerical scheme for stratified two-liquid electroosmotic flows. The simulation results highlight that using the electroosmotic effects can control the interface location of a pressure-driven two-liquid flow. A finite volume method is used to solve the coupled electric potential equation and Navier–Stokes equation together.The validity of the numerical scheme is evaluated by comparing its predictions with the results of the analytical solutions in the fully developed regions. The liquid–liquid interface developments due to the favorably and adversely applied electric field are examined.     

Current distribution in polymer electrolyte membrane fuel cell with active water management

Air delivery is typically the greatest parasitic power loss in polymer electrolyte membrane fuel cell (PEMFC) systems. We here present a detailed study of an active water management system for PEMFCs, which uses a hydrophilic, porous cathode flow field, and an electroosmotic (EO) pump for water removal. This active pumping of liquid water allows for stable operation with relatively low air flow rates and low air pressure and parallel cathode channel architectures. We characterize in-plane transport issues and power distributions using a three by three segmented PEMFC design. Our transient and steady state data provide insight into the dynamics and spatial distribution of flooding and flood-recovery processes. Segment-specific polarization curves reveal that the combination of a wick and an EO pump can effect a steady state, uniform current distribution for a parallel channel cathode flow field, even at low air stoichiometries (α_air = 1.5). The segmented cell measurements also reveal the mechanisms and dynamics associated with EO pump based recovery from catastrophic flooding. For most operating regimes, the EO pump requires less than 1% of the fuel cell power to recover from near-catastrophic flooding, prevent flooding, and extend the current density operation range.     

An analytical solution for thermally fully developed combined pressure – electroosmotically driven flow in microchannels

An analytical solution is presented to study the heat transfer characteristics of the combined pressure – electroosmotically driven flow in planar microchannels. The physical model includes the Joule heating effect to predict the convective heat transfer coefficient in two dimensional microchannels. The velocity field, which is a function of external electrical field, electroosmotic mobility, fluid viscosity and the pressure gradient, is obtained by solving the hydrodynamically fully-developed laminar Navier–Stokes equations considering the electrokinetic body force for low wall zeta potentials. Then, assuming a thermally fully-developed flow, the temperature distribution and the Nusselt number is obtained for a constant wall heat flux boundary condition. The fully-developed temperature profile and the Nusselt number depend on velocity field, channel height, solid/liquid interface properties and the imposed wall heat flux. A parametric study is presented to evaluate the significance of various parameters and in each case, the maximum heat transfer rate is obtained.     

Microfluidic velocimetry with near-wall resolution

A combined experimental and numerical analysis technique for velocity profiles with near-wall resolution in liquid microchannel flows is described. The working fluids employed are aqueous solutions with caged fluorescent dyes. A sheet of fluorescent dye is photo-injected by briefly exposing a cross-section of the fluid to ultraviolet light. The transport of the resulting ‘band’ of fluorescent dye is imaged onto a CCD camera using an epi-illumination fluorescent microscope system. The velocity profile is calculated from images acquired and processed after each uncaging event. Results are shown to compare favorably to analytical solutions in both pressure-driven and electroosmotic flow in circular cross-section capillaries. Near-wall resolution is verified through application to electroosmotic flows with thin electrical double layers. The ability of the technique to collect velocimetry data up to 2.5 μm from the wall is demonstrated.     

Thermal performance of mixed electroosmotic-pressure driven flows in microtubes with constant wall temperature

A numerical analysis is performed to explore the heat transfer characteristics of mixed electroosmotic and pressure-driven flows in a microtube with constant wall temperature. Thermally fully-developed flow with Joule heating is considered. The Joule heating is generated by imposed voltage gradient and can be regarded as volumetric heat source. The analysis combines energy equation with overall energy balance equation for a control element to generate a nondimensional governing equation. Of interest are the effects of the relative duct radius a (ratio of the duct radius to Debye length), the pressure gradient parameter P (ratio of pressure gradient to electroosmotic forces) and the Joule number S (ratio of heat generation due to Joule heating to heat transfer at the wall) on the temperature distribution and the local heat transfer rate. The results indicate that the Nusselt number increases with an increase in the relative duct radius and the Joule number or a decrease in the pressure gradient parameter. The resulting solution due to the conditions of energy unbalance in the flow is also discussed.     

Reticulated porous and parallel channel cathode flow fields in real scale polymer electrolyte membrane fuel cells: A comparative experimental study

This paper focuses on understanding the effect of reticulated porous cathode flow fields in real scale close and open cathode polymer electrolyte membrane fuel cells (PEMFCs) in terms of their thermo-electrical performance. This research contributes to addressing challenges with PEMFCs linked to oxygen supply to the cathode and proper mixing of gasses as well as water removal issues. Parallel channel and porous cathode flow fields in both open and closed cathode PEMFCs of medium scale (active area of 15 × 15 cm²) have been investigated. The porous material consisted of 20 pores per inch with a porosity level of 80%. The cells’ polarisation and impedance characteristics have been analysed. The porous flow field has been found providing better electrical performance in closed cathode PEMFC compared to the open cathode. Improvements in gas diffusion and temperature uniformity were observed with porous flow field; however, water removal has been observed challenging, which need to be addressed before the benefits of using porous flow field are fully realised.     

Scale-up of high power density redox flow batteries by introducing interdigitated flow fields

Lab-scale redox flow batteries (RFBs) employing thinner electrodes have achieved outstandingly high power densities. When these high-performance thinner electrodes are scaled up to larger sizes required for kW-scale stacks, adding interdigitated flow fields is a simple solution in maintaining low pressure drops. A 3-D model of a half-battery with an active area of 900 cm² was developed to explore the design rules of flow fields. Optimizing the number and size of channels is essentially striking a balance between the pressure drop and the electrolyte velocity in the electrode, which have important effects on the pumping loss and mass transport loss respectively. In addition to the magnitude of the average velocity, the uniformity of velocity distribution should also be paid attention to in designing flow fields, which is determined by the ratio of flow resistance in the electrode to that in the channels. Acceptably thicker channels are recommended to improve uniformity of velocity distribution.     

Joule heating effect on electroosmotic flow and mass species transport in a microcapillary

This study presents a numerical analysis of Joule heating effect on the electroosmotic flow and mass species transport, which has a direct application in the capillary electrophoresis based BioChip technology. A rigorous mathematic model for describing the Joule heating in an electroosmotic flow including the Poisson-Boltzmann equation, the modified Navier-Stokes equations and the energy equation is developed. All these equations are coupled through the temperature-dependent liquid dielectric constant, viscosity, and thermal conductivity. By numerically solving the aforementioned equations simultaneously, the double layer potential profile, the electroosmotic flow field, and the temperature distribution in a cylindrical microcapillary are computed. A systematic study is carried out to evaluate the Joule heating and its effects under the influences of the capillary radius, the buffer solution concentration, the applied electric field strength, and the heat transfer coefficient. In addition, the Joule heating effect on sample species transport in a microcapillary is also investigated by numerically solving the mass transfer equation with consideration of temperature-dependent diffusion coefficient and electrophoresis mobility. The simulations reveal that the presence of the Joule heating could have a great impact on the electroosmotic flow and mass species transport.     

Design strategy for a polymer electrolyte membrane fuel cell flow-field capable of switching between parallel and interdigitated configurations

Novel water management strategies are important to the development of next generation polymer electrolyte membrane fuel cell systems (PEMFCs). Parallel and interdigitated flow fields are two common types of PEMFC designs that have benefits and draw backs depending upon operating conditions. Parallel flow fields rely predominately on diffusion to deliver reactants and remove byproduct water. Interdigitated flow fields induce convective transport, known as cross flow, through the porous gas diffusion layer (GDL) and therefore are superior at water removal beneath land areas which can lead to higher cell performance. Unfortunately, forcing flow through the GDL results in higher pumping losses as the inlet pressure for interdigitated flow fields can be up to an order of magnitude greater than that for a parallel flow field. In this study a flow field capable of switching between parallel and interdigitated configurations was designed and tested. Results show, taking into account pumping losses, that using constant stoichiometry the parallel flow field results in a higher system power under low current density operation compared to the interdigitated configuration. The interdigitated flow-field configuration was observed to have lower overvoltage at elevated current densities resulting in a higher maximum power and a higher limiting current density. An optimal system power curve was produced by switching from parallel to interdigitated configuration based on which produces a higher system power at a given current density. This design method can be easily implemented with current PEMFC technology and requires minimal hardware. Some of the consequences this design has on system components are discussed.     

Large eddy simulation of passive scalar in complex turbulence with flow impingement and flow separation

In order to reveal unknown characteristics of complex turbulent passive scalar fields, large eddy simulations in forced convection regimes have been performed under several strain conditions, including flow impingement and flow separation. By using the simulation results, relations between the dynamic and scalar fields are carefully examined. It is then confirmed that the scalar is transported by a large vortex structure near the examined regions wherever the mean shear vanishes, although in the high‐shear regions, the scalar transport is governed by a coherent structure due to the high shear strain. In addition, a priori explorations are attempted by processing the data, focusing on the derivation of a possible direction for modeling algebraically the passive scalar transport in a complex strain field. The a priori tests suggest that an expanded form of the GGDH model introducing a quadratic product of the Reynolds stresses is promising for general flow cases. © 2001 Scripta Technica, Heat Trans Asian Res, 30(5): 402–418, 2001     

Fluid flow characteristics and convective heat transfer improvement on a gas turbine blade

The flow field features and heat transfer enhancement are investigated on a gas turbine blade by applying the jet impingement cooling method. The distribution of the flow field and the Nusselt number (Nu) was determined on the targeted surface in the cooling channel. The injection holes of different shapes, such as circular, square, and rectangular were considered. The Reynolds numbers (Re) of the airflow in the range of 2000–5000 and aspect ratios of 0.5–2 were particularly focused. The flow vortices and recirculation in the cooling channel and their influence on the heat transfer enhancement were analyzed in detail under different airflow and geometric conditions. Decreasing the ratio of the distance between jet-to-target plate to the diameter of the jet orifice (H/d) increased the heat transfer rate and produced high-intensity vortices and recirculation zones. It was noticed that the formation and generation of vortices and recirculation have important effects on the convective heat transfer rate at the impingement surface. Local Nusselt number, formation of complex vortices, and airflow recirculation in the cooling channel decreased with the increase in the distance between the jet hole and the targeted surface. It was found that with the increase in the Reynolds number of the jet, heat transfer between cold airflow and the targeted surface increased. Moreover, it was observed that the cooling performance of the round and square jet holes was better than the rectangular holes.     

Experimental investigation of the effect of channel length on performance and water accumulation in a PEMFC parallel flow field

Longer channels within serpentine flow fields are highly effective at removing liquid water slugs and have little water accumulation; however, the long flow path causes a large pressure drop across the cell. This results in both a significant concentration gradient between the inlet and outlet, and high pumping losses. Parallel flow fields have a shorter flow path and smaller pressure drop between the inlet and outlet. This low pressure drop and multiple routes for reactants in parallel channels allows for significant formation of liquid water slugs and water accumulation. To investigate these differences, a polymer electrolyte membrane fuel cell parallel flow field with the ability to modify the length of the channels was designed, fabricated, and tested. Polarization curves and the performance, water accumulation, and pressure drop were measured during 15 min of 0.5 A cm⁻² steady-state operation. An analysis of variance was performed to determine if the channel length had a significant effect on performance. It was found that the longer 25 cm channels had significantly higher and more stable performance than the shorter 5 cm channels with an 18% and an 87% higher maximum power density and maximum current density, respectively. Channel lengths which result in a pressure drop, across the flow field, slightly larger than that required to expel liquid water slugs were found to have minimal water accumulation and high performance, while requiring minimal parasitic pumping power.     

Time-dependent model of mixed electroosmotic/pressure-driven three immiscible fluids in a rectangular microchannel

The time-dependent aspects of pressure-driven three-liquid flow under the effect of electroosmotic flow (EOF) are analytically studied, in which non-conducting liquid is delivered by the pressure gradient and the interfacial viscous forces of two conducting liquids, the two conducting liquids are driven by electroosmosis and pressure gradient. The flow of the three liquids depends on the coupling effects between them, which involve the electrokinetic effect. The surface charges at the liquid-liquid interface are accounted in this model. At the interface, the shear stress is not continuous because of the presence of the surface charges. With Laplace transform method, an analytical solution of such time-dependent three-liquid flow in rectangular channel is presented.     

Viscous dissipation effects on thermal transport characteristics of combined pressure and electroosmotically driven flow in microchannels

This study investigates the influence of viscous dissipation on thermal transport characteristics of the fully developed combined pressure and electroosmotically driven flow in parallel plate microchannels subject to uniform wall heat flux. Closed form expressions are obtained for the transverse distributions of electrical potential, velocity and temperature and also for Nusselt number. From the results it is realized that the Brinkman number has a significant effect on Nusselt number. Generally speaking, to increase Brinkman number is to decrease Nusselt number. Although the magnitude of Joule heating can affect Brinkman number dependency of Nusselt number, however the general trend remains unchanged. Depending on the value of flow parameters, a singularity may occur in Nusselt number values even in the absence of viscous heating, especially at great values of dimensionless Joule heating term. For a given value of Brinkman number, as dimensionless Debye–Huckel parameter increases, the effect of viscous heating increases. In this condition, as dimensionless Debye–Huckel parameter goes to infinity, the Nusselt number approaches zero, regardless of the magnitude of Joule heating. Furthermore, it is realized that the effect of Brinkman number on Nusselt number for pressure opposed flow is more notable than purely electroosmotic flow, while the opposite is true for pressure assisted flow.     

Investigation of mass transport and cell performance on μDMFC with different anode flow fields

The study systematically analyzes the performance of micro direct methanol fuel cell (μDMFC) with different flow fields. A two‐phase three‐dimensional model is developed to evaluate the mass transport accurately. The transport of methanol and air, the pressure distribution, the anode saturation, and the methanol crossover are numerically observed, the under‐rib convection is also investigated numerically. The flow fields with an active area of 0.64 cm² are fabricated on silicon wafers by micro electromechanical system technology. Performance of μDMFCs with different flow fields is sorted as: double‐serpentine flow field (DSFF) > single‐serpentine flow field (SSFF) > triple‐serpentine flow field (TSFF), and the dynamic test results indicate the cell with DSFF takes the shortest time to reach a stable power output when compared with other cells. Copyright © 2013 John Wiley & Sons, Ltd.     

Experimental and numerical investigation of circularly lobed ejector with bend mixers

The vent pipe for a turbo shaft engine may be required to have a lobed nozzle installed and to bend for the purpose of infrared stealth. The experimental setup was a circular 12‐lobed nozzle with bend mixers to study the effects on the pumping performance of the lobed bend mixer parameters. The experimental results show that the pumping ratio of the secondary mass flow to the primary mass flow for a mixer bend angle equal to 40^° is the same as that for the same lobed nozzle with the same diameter cylindrical mixer that was used in the author's previous papers. There is a great decrease of the pumping ratio for a mixer bend angle larger than 40^°. The higher the bend angles, the lower the pumping ratios. The pumping ratios initially increase and then decrease with the increasing of the cross‐area ratio of mixer to lobe. The optimal cross‐area ratio that corresponds to the maximum pumping ratio is strangely nearly equal to the optimal cross‐area ratio of a cylindrical mixer, although the maximum pumping ratio is less than that for a cylindrical mixer. The pumping ratios increase approximately linearly with the cross‐area ratio of the secondary inlet to the lobed nozzle. To investigate the flow characteristics and the pumping ratio changing mechanism, the flow field inside the ejector is numerically simulated. The numerical results show that the main reason for the great decrease of the pumping ratio when the mixer bend angle is larger than 40^° is due to the great static pressure around the bend part, which is caused by the primary flow jet. The great static pressure around the bend section chokes back the exhausted secondary fluid flow. There is a good agreement between the calculated and the measured wall static pressure coefficients in the mixer wall along the lobe crest symmetry plane of the lobed ejector. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(7): 387–397, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20175     

Assessment of different bio-inspired flow fields for direct methanol fuel cells through 3D modeling and experimental studies

The performance impact of using bio-inspired interdigitated and non-interdigitated flow fields (I-FF and NI-FF, respectively) within a DMFC is investigated. These two flow fields, as well as a conventional serpentine flow field (S-FF, used as a reference), were examined as possible anode and cathode flow field candidates. To examine the performance of each of these candidates, each flow field was manufactured and experimentally tested under different anode and cathode flow rate combinations (1.3 mL/min [methanol] and 400 mL/min [oxygen], as well as 2 and 3 times these flow rates), and different methanol concentrations (0.50 M, 0.75 M, and 1.00 M). To help understand the experimental results and the underlying physics, a three dimensional numerical model was developed. Of the examined flow fields, the S-FF and the I-FF yielded the best performance on the anode and cathode, respectively. This finding was mainly due to the enhanced under-rib convection of both of these flow fields. Although the I-FF provided a higher mean methanol concentration on the anode catalyst layer surface, its distribution was less uniform than that of the S-FF. This caused the rate of methanol permeation to the cathode to increase (for the anode I-FF configuration), along with the anode and cathode activation polarizations, deteriorating the fuel cell performance. The NI-FF provided the lowest pressure drops of the examined configurations. However, the hydrodynamics within the flow field made the reactants susceptible to traveling directly from inlet to outlet, leading to several low concentration pockets. This significantly decreased the reactant uniformity across its respective catalyst layer, and caused this FFs performance to be the lowest of the examined configurations.     

不同型式流场的PEMFC内部传质分析

对采用不同型式流场的PEMFC进行建模,并用控制容积法对控制方程进行离散,求解得到PEMFC内部各物理量的分布以及综合水拖带系数、质子交换膜平均电导率等。分析了采用交趾型流场和常规流场时PEMFC的内部传质以及阴极性能、电池性能和膜性能,结果认为采用交趾型流场时,PEMFC阴极性能高于采用常规流场的PEMFC阴极性能,但质子交换膜的平均电导率低于采用常规流场时。在没有液态水产生时常规流场PEMFC性能高于交趾型流场PEMFC。     

Heat transfer and hydromagnetic electroosmotic Von Kármán swirling flow from a rotating porous disc to a permeable medium with viscous heating and Joule dissipation

Magnetohydrodynamic flow and heat transfer in an ionic viscous fluid in a porous medium induced by a stretching spinning disc and modulated by electroosmosis under an axial magnetic field and radial electrical field is presented in this study. The effects of convective wall boundary conditions, Joule heating and viscous dissipation are incorporated. The governing partial differential conservation equations are transformed into a system of self-similar coupled, nonlinear ordinary differential equations with associated boundary conditions. The Matlab bvp4c solver featuring a shooting technique and the fourth-order Runge–Kutta–Fehlberg method are used to numerically solve the governing dimensionless boundary value problem. Multivariate analysis is also performed to examine the thermal characteristics. An increase in rotation parameter induces a reduction in the radial velocity, whereas it elevates the tangential velocity. Greater electrical field parameter strongly damps the radial velocity whereas it slightly decreases the tangential velocity. Increasing magnetic parameter also damps both the radial and tangential velocities. An increment in electroosmotic parameter substantially decelerates the radial flow but has a weak effect on the tangential velocity field. Increasing permeability parameter (inversely proportional to permeability) markedly damps both radial and tangential velocities. The pressure gradient is initially enhanced near the disk surface but reduced further from the disk surface with increasing magnetic parameter and electrical field parameter, whereas the opposite effect is produced with increasing Joule dissipation. Increasing magnetic and rotational parameters generate a strong heating effect and boost temperature and thermal boundary layer thickness. Nusselt number is boosted with increasing Brinkman number (viscous heating effect) and Reynolds number. The simulations are relevant to electromagnetic coating flows, bioreactors and electrochemical sensing technologies in medicine.