Optimized electrode arrangement in solar air heater

Laminar forced convection inside the solar air heater with various wire electrode arrangements are numerically examined for heat transfer enhancement using electrohydrodynamic technique. The electric field is generated by the wire electrodes charged with DC high voltage ranging from 7.5 to 17.5 kV. Reynolds number corresponding to the flow considered is between 100 and 2000. The numerical modeling of computational fluid dynamics includes the interactions among electric field, flow field, and temperature field. It is found that the enhancement of heat transfer coefficient with the presence of electric field increases in relation with the supplied voltage but decreases when the Reynolds number and the distance between electrode and channel surface are augmented. The optimized electrode arrangement, which obtains the best heat transfer enhancement is investigated incorporating with the pressure drop consideration. The heat transfer enhancement is also depended on the number of electrodes per length and the channel dimension.     

An approximate analytical solution to channel flow in a fuel cell with a draft angle

This paper investigates the channel flow with a suction or injection boundary condition to imitate the reactant flow in fuel cells. A two-dimensional model is proposed for a channel with a trapezoidal cross-section, and systematically analyzed using the perturbation method and the Laplace transform technique. The model predicts the species concentration at the electrode of the fuel cell under the constraint of uniform current density along the channel. For convenience of use as a formula, the asymptotic solutions to some results are found for two different limits of x/k.     

EHD enhanced heat transfer in wavy channel

Heat transfer enhancement with electrohydrodynamic (EHD) technique in laminar forced convection inside a wavy channel with different wire electrode arrangements is numerically investigated. The electric field is generated by the wire electrodes charged with DC high voltage. The mathematical modeling includes the interactions among electric field, flow field, and temperature field. The simulation is firstly conducted with the experimental data in case of rectangular flat channel and the results agree very well. Then the modeling is carried out in the case of wavy channel. It is found that the heat transfer coefficient with the presence of electric field increases with the supplied voltage but decreases when the Reynolds number and the distance between the wire electrodes and the wall surface are augmented. The heat transfer enhancement is also dependent on the number of the wire electrodes, the number of wave per length, and the wave aspect ratio.     

Heat transfer enhancement in a grooved channel with curved vanes

We visualize unsteady temperature fields in the grooved channel with curved vanes using holographic interferometry. The heat transfer performance of the investigated channel is compared with that of the basic grooved channel. The addition of curved vanes above the downstream end of the heated block redirects the flow from the main channel into the groove. Heat transfer shows an increase by a factor of 1.5-3.5, when compared to the basic grooved channel, mainly due to increased flow velocities in the groove region. Flow transition from steady to oscillatory occurs around Re=450 and flow oscillations contribute to heat transfer enhancement. The pressure drop is 3-5 times higher than in the basic grooved channel.     

Temperature distribution on the MEA surface of a PEMFC with serpentine channel flow bed

Knowledge of the temperature distribution on the membrane electrode assembly (MEA) surface and heat transfer processes inside a proton exchange membrane fuel cell (PEMFC) is helpful to improvement of cell reliability, durability and performance. The temperature fields on the surface of MEA fixed inside a proton exchange membrane fuel cell with a serpentine channel flow bed were measured by infrared imaging technology under non-humidification conditions. The temperature distributions over the MEA surface under whole channel region were achieved. The experimental results show that the downstream temperatures are higher than the upstream. The hot region on the MEA surface is easy to locate from the infrared temperature image. The mean temperature on the MEA surface and the cell temperature both increase with the current density. Higher current density makes the non-uniformity of temperature distribution on the MEA surface worse. The loading time significantly affects the temperature distribution. Compared with the electrical performance of the cell, the MEA's temperatures need much more time to reach stable. The results indicate that isothermal assumption is not appropriate for a modeling of PEMFCs, and monitoring the temperature of external surface of the flow field plate or end plate cannot supply accurate reference to control the temperatures on MEA surface.     

Direct measurement of lateral current density distribution in a PEM fuel cell with a serpentine flow field

In a proton exchange membrane (PEM) fuel cell, local current density can vary drastically in the lateral direction across the land and channel areas. It is essential to know the lateral current density variations in order to optimize flow field design and fuel cell performance. Thus the objective of this work is to directly measure the lateral current density variations in a PEM fuel cell with a serpentine flow field. Five 1 mm-width partially-catalyzed membrane electrode assemblies (MEA), each corresponding to a different location from the center of the gas channel to the center of the land area are used in the experiments. Current densities for fuel cells with each of the partially-catalyzed MEAs are measured and the results provide the lateral current density distribution. The measurement results show that in the high cell voltage region, local current density is the highest under the center of the land area and decreases toward the center of the channel area; while in the low cell voltage region local current density is the highest under the center of the channel area and decreases toward the center of the land area. Besides, the effects of cathode flow rates on the lateral current density distribution have also been studied. Furthermore, comparisons have also been made by using air and oxygen in the cathode and it is found that when oxygen is used the local current density under the land is significantly enhanced, especially in the low cell voltage region.     

Hydrogen combustion experiments in a vertical semi-confined channel

Experiments in an obstructed semi-confined vertical combustion channel with a height of 6 m (cross-section 0.4 × 0.4 m) inside a safety vessel of the hydrogen test center HYKA at the Karlsruhe Institute of Technology (KIT) are reported. In the work, homogeneous hydrogen-air-mixtures as well as mixtures with different well-defined H₂-concentration gradients were ignited either at the top or at the bottom end of the channel. The combustion characteristics were recorded using pressure sensors and sensors for the detection of the flame front that were distributed along the complete channel length. In the tests slow subsonic and fast sonic deflagrations as well as detonations were observed and the conditions for the flame acceleration (FA) to speed of sound and deflagration-to-detonation transition (DDT) are compared with the results of similar experiments performed earlier in a larger semi-confined horizontal channel.     

A particle-layer model for solid-oxide-full-cell cathodes with different structures

A particle-layer model is developed to quantitatively evaluate the electrochemical parameters of conventional composite cathodes (CCCs) and impregnated composite cathodes (ICCs) for solid oxide full cells (SOFCs). In this model, these cathodes are considered as a construction composed of particle layers. The parameters such as interfacial polarization resistance, three-phase boundary (TPB) length, and effective electrode thickness are formulated as a function of effective TPB resistivity, ionic resistivity, and cathode structure characteristics including electrode composition, porosity, particle size of electrocatalyst and electrolyte, and thickness. In addition, the model can be used as a convenient tool to estimate the effective TPB resistivity when the interfacial polarization resistance is available or experimentally determined. Furthermore, the ICC and CCC electrodes are theoretically compared. It is confirmed that the electrochemical performance can be significantly enhanced and small effective thickness can be reached by using ICC structure, compared with CCC, due to the remarkable enlargement of TPB length. The model also provides some strategies to design a high performance cathode.     

Methanol steam reforming in a planar wash coated microreactor integrated with a micro-combustor

A numerical simulation of methanol steam reforming in a microreactor integrated with a methanol micro-combustor is presented. Typical Cu/ZnO/Al₂O₃ and Pt catalysts are considered for the steam reforming and combustor channels respectively. The channel widths are considered at 700 μm in the baseline case, and the reactor length is taken at 20 mm. Effects of Cu/ZnO catalyst thickness, gas hourly space velocities of both steam reforming and combustion channels, reactor geometry, separating substrate properties, as well as inlet composition of the steam reforming channel are investigated. Results indicate that increasing catalyst thickness will enhance hydrogen production by about 68% when the catalyst thickness is increased from 10 μm to 100 μm. Gas space velocity of the steam reforming channel shows an optimum value of 3000 h⁻¹ for hydrogen yield, and the optimum value for the space velocity of the combustor channel is calculated at 24,000 h⁻¹. Effects of inlet steam to carbon ratio on hydrogen yield, methanol conversion, and CO generation are also examined. In addition, effects of the separating substrate thickness and material are examined. Higher methanol conversion and hydrogen yield are obtained by choosing a thinner substrate, while no significant change is seen by changing the substrate material from steel to aluminum with considerably different thermal conductivities. The produced hydrogen from an assembly of such microreactor at optimal conditions will be sufficient to operate a low-power, portable fuel cell.     

Evaluation of the boiling effect on oxygen evolution reaction using a three-electrode cell

The high initial cost of polymer electrolyte membrane water electrolyzers (PEMWEs) has delayed their widespread commercialization. A possible means to reduce cost is by reducing the overvoltage and increasing the current density to reduce the electrode area. This study proposes a novel method in which boiling is superimposed on the oxygen evolution reaction (OER) to decrease electrolysis voltage. The vapor bubbles formed by boiling are expected to decrease the concentration overvoltage. The boiling effect was experimentally analyzed using a three-electrode cell. Although a general catalyst layer (CL) was formed on a working electrode (WE) bar, the structure of the working electrode (WE) bar was special, in which a 10-W heater was embedded and made boiling on the electrode under 1 bar condition. Increasing the electrode temperature under static OER current density slightly decreased the OER potential. However, an abrupt decrease in potential was observed when the temperature was scaled over the boiling temperature. Moreover, this abrupt decrease substantially intensified when, similar to a practical PEMWE, a porous transfer layer (PTL) and flow channel were assembled on the CL. These experimental results suggest that boiling can reduce the concentration overvoltage by reducing the oxygen concentration on the CL, especially when the mass transport resistance caused by the PTL is considerable. Innovatively and simply utilizing boiling, as proposed here, can enhance the oxygen transfer and contribute to reducing the initial cost of PEMWEs.     

A Parametric Study of the Thermal-Hydraulic Performance of a Zigzag Printed Circuit Heat Exchanger

The effects of geometric parameters on the performance of a printed circuit heat exchanger have been analyzed using three-dimensional Reynolds-averaged Navier–Stokes equations. The shear stress transport model is used for accurate prediction of the turbulent flows. The numerical solutions are validated in comparison with the available experimental data, and different lengths of the calculation domain have been tested to determine the optimum length of the domain. The effects of two design parameters, namely, the channel angle and the semi-ellipse aspect ratio of the cold channel, on the heat transfer and friction performance in the cold channel have been evaluated. The results indicate that the effectiveness of the heat exchanger is maximized when the cold channel angle is similar to the hot channel angle.     

Heat transfer inside a horizontal channel with an open trapezoidal enclosure subjected to a heat source of different lengths

The heat transfer phenomena inside a horizontal channel with an open trapezoidal enclosure subjected to a heat source of different lengths was investigated numerically in the present work. The heat source is considered as a local heating element of varying length, which is embedded at the bottom wall of the enclosure and maintained at a constant temperature. The air flow enters the channel horizontally at a constant cold temperature and a fixed velocity. The other walls of the enclosure and the channel are kept thermally insulated. The flow is assumed laminar, incompressible, and two‐dimensional, whereas the fluid is considered Newtonian. The results are presented in the form of the contours of velocity, isotherms, and Nusselt numbers profiles for various values of the dimensionless heat source lengths (0.16 ≤ ε ≤ 1). while, both Prandtl and Reynolds numbers are kept constant at (Pr = 0.71) and (Re = 100), respectively. The results indicated that the distribution of the isotherms depends significantly on the length of the heat source. Also, it was noted that both the local and the average Nusselt numbers increase as the local heat source length increases. Moreover, the maximum temperature is located near the heat source location.     

Heat transfer enhancement in a channel with a built-in square cylinder

A two dimensional numerical investigation of the unsteady laminar flow pattern and forced convective heat transfer in a channel with a built-in square cylinder is presented. The channel in the entrance region has a length to plate spacing of ten. The computations were made for several Reynolds number and two square cylinder sizes. Hydrodynamic behavior and heat transfer results are obtained by solution of the complete Navier-Stokes and energy equation. The results show that these flow exhibits laminar self-sustained oscillations for Reynolds numbers above the critical one. This study shows that oscillatory separated flows result in a significant heat transfer enhancement but also in a significant pressure drop increase.     

End effects in a MHD channel with diverging electrode walls

End effects phenomena in a Faraday type generator with diverging electrode walls for two types of velocity profiles—one with a source velocity and the other with a fully developed velocity—are discussed. The electric potential is determined numerically using the successive overrelaxation method in polar coordinates. It is found that the viscous forces increase the end losses and create current concentrations on the electrodes even at far distances from the entrance.     

Numerical investigation on the water saturation of proton exchange membrane fuel cells with channel geometry variation

Many factors, such as mole fractions of oxygen and hydrogen, help improve the performance of proton exchange membrane fuel cells. The variation of mole fractions can be achieved by changing the operating pressure and relative humidity of the fuel cells. The changes in operating conditions are directly related to the electrochemical reaction and water generation of the fuel cells. The geometrical shape of the fuel cells also should be considered a factor in predicting performance because this affects the species' reaction speed and distribution. The current study considers four geometrical cell shapes with varied lengths and electrode and gas channel numbers. The variation in inlet pressure is considered in analyzing the current density distribution of the fuel cells and, subsequently, of liquid water generation. A serpentine gas flow channel is assumed, and its two‐dimensional arrangement is considered in the different gas channel numbers and its length. Four inlet pressure variations and four geometrical shape variations also are considered in analyzing the fuel cells' current density and water generation distributions. The results obtained from this research can be utilized in identifying the fuel cells' optimal operating pressure and designing their gas channel number and arrangement. Copyright © 2011 John Wiley & Sons, Ltd.     

Enhancement of heat transfer from hot water by co-flowing it with mercury in a mini-channel

The present study is a numerical investigation on the flow and heat transfer in a mini-channel where both hot liquid water and mercury co-flow together in a direct contact manner. Results show that the presence of a high thermal conductivity liquid metal such as mercury enables the hot water to lose much more of its initial thermal energy content, than when only water alone flows in the channel. However, too unjustified excessive mercury co-flowing with the hot water can lead to adverse effects in regards to the heat loss from the hot water. The reason behind the enhanced heat transfer between the two liquids is due to the initiation of high temperature gradients sites inside the channel, especially in the region of the interface between the two liquids, in addition to the high thermal conductivity of mercury, as compared to water thermal conductivity. These two effects lead to effective conduction cooling in the transverse direction over the whole length of the channel. These aspects are quantified in this study.     

Optimum geometrical design for improved fuel utilization in membraneless micro fuel cell

In membraneless micro fuel cells, the mixing and depletion widths are major factors that determine the cell performance. Cells in which these widths are too large exhibit severely reduced fuel utilization and, hence, less electrochemical reaction especially in the downstream region of the channel. For cells with conventional rectangular geometry, increasing the aspect ratio reduces the mixing width but reduces the effective electrode area. This work proposes a trident-shaped geometrical design for membraneless micro fuel cells in which the anode fluid, cathode fluid and proton-conducting fluid are introduced through three distinct inlets. The anode and cathode fluids are interconnected by the proton-conducting fluid channel. In addition, the anode fluid and proton-conducting fluid are connected by a small narrow passage, and the cathode fluid and proton-conducting fluid channel are also connected by a small narrow passage. Numerical simulations, including the effects of electrochemical reaction and fluid flow, are carried out to investigate reactant distributions in the downstream region of the channel and to study fuel utilization. A fuel utilization of around 51% is achieved when the two opposite walls are used as reaction surfaces and the inlet velocity is set at 0.01 m s⁻¹. By varying the cell length and expanding the reaction surface areas by including additional surfaces within the cell, simulations show that the fuel utilization can be improved to around 86%, which is much higher than has been achieved in previous studies. The present numerical results are validated by comparison with available literature data.     

Evaluation of a cathode gas channel with a water absorption layer/waste channel in a PEFC by using visualization technique

The polymer electrolyte fuel cell (PEFC) cathode is a performance-limiting component due to the slower oxygen reduction kinetics and mass transport limitations imposed by water generated in an electrochemical reaction. This water assists the performance of the PEFC by preventing drying of the polymer electrolyte. Conversely, the water hinders the transport of the reactant species by blocking the pores in the gas diffusion layer. Moreover, the effective electrode area is decreased, causing the cathode channel to become clogged with supersaturated water from the gas diffusion layer. This problem is overcome by separating the gas channel and the waste channel, and installing a water absorption layer (WAL). The new “WAL type” gas channel has an installed WAL in which the designed waste channel is compared with the gas flow characteristics of a conventional cathode gas channel by using the visualization technique. Gas flowing into the WAL type separator is barely blocked before the WAL absorbs water condensed in the cathode gas channel. Therefore, the WAL type separator effectively improves the PEFC performance.     

A comprehensive,consistent and systematic mathematical model of PEM fuel cells

This paper presents a comprehensive, consistent and systematic mathematical model for PEM fuel cells that can be used as the general formulation for the simulation and analysis of PEM fuel cells. As an illustration, the model is applied to an isothermal, steady state, two-dimensional PEM fuel cell. Water is assumed to be in either the gas phase or as a liquid phase in the pores of the polymer electrolyte. The model includes the transport of gas in the gas flow channels, electrode backing and catalyst layers; the transport of water and hydronium in the polymer electrolyte of the catalyst and polymer electrolyte layers; and the transport of electrical current in the solid phase. Water and ion transport in the polymer electrolyte was modeled using the generalized Stefan–Maxwell equations, based on non-equilibrium thermodynamics. Model simulations show that the bulk, convective gas velocity facilitates hydrogen transport from the gas flow channels to the anode catalyst layers, but inhibits oxygen transport. While some of the water required by the anode is supplied by the water produced in the cathode, the majority of water must be supplied by the anode gas phase, making operation with fully humidified reactants necessary. The length of the gas flow channel has a significant effect on the current production of the PEM fuel cell, with a longer channel length having a lower performance relative to a shorter channel length. This lower performance is caused by a greater variation in water content within the longer channel length.     

Natural convection in a periodically finned vertical channel

This paper presents the experimental results of natural convection in a rectangular cross-sectional vertical channel. Along the channel fins connected to both plates are placed, periodically. The channel walls are maintained at uniform heat flux. Rayleight number and Nusselt number are obtained based on channel width. The ratio of the channel length to the channel width, L/b, is 66. The experiments are performed for modified Rayleigh number, (b/L)RA, ranges from 20 to 90. Results shows that Nusselt number for finned channel are less than those of the smooth channel.