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.     

Boundary layer flow and stability analysis of forced convection over a diverging channel with variable properties of fluids

The objective of the current study is to investigate the forced convection laminar boundary layer flow over a flat plate in a diverging channel with variable viscosity. The physical governing equations are converted to nondimensional partial differential equations (PDEs) using similarity transformation. The coupled PDEs with boundary constrains are solved numerically using quasilinearization technique. Computational results are given in terms of flow parameter $ϵ\left(0<ϵ<1\right)$, suction or injection A, and viscous dissipation parameter Ec. Stability analysis was conducted and the solutions were found to be stable for real values of $\gamma$. We found that variable Prandtl number with quasilinearization technique method gives smoothness of solution compared to fixed Prandtl number. This is shown graphically for different fluids in Section 5. Also, the significant effect of the suction/injection parameter $\left(A\right)$ on velocity, temperature profiles, skin friction, and heat transfer is observed.     

Gas–liquid two-phase flow patterns in a miniature square channel with a gas permeable sidewall

Characteristics of air–water two-phase flow patterns in a miniature square channel having a gas permeable sidewall were investigated experimentally. Water was fed into the channel from its entrance, while air was injected uniformly into the channel along the permeable sidewall. This configured two-phase flow problem is encountered in direct feed methanol fuel cells. Flow patterns in both vertical upward and horizontal flows were identified using a high-speed motion analyzer. The visualization shows that the typical flow pattern encountered in the conventional co-current gas–liquid two-phase flow, such as bubbly flow, plug flow, slug flow and annular flow were also observed in the present work. However, unlike the conventional co-current gas–liquid two-phase flow in a channel with gas and liquid uniformly entering from one of its ends, for the flow configuration considered in this work, the stratified flow and wavy flow were not found in horizontal flow. And a so-called “single layer bubbly flow” was found in vertical upward flow, which is characterized by a mono small-gas-bubble layer existing adjacent to the surface of the permeable sidewall with the reminding space occupied by the liquid phase. Four transitional flow patterns such as bubbly-plug flow, bubbly-slug flow, plug–slug flow, and slug-annular flow, were found to exist between the distinct flow patterns. Finally, the flow regime maps for various liquid volumetric fluxes are presented in terms of mass quality versus the volumetric flux of gas phase.     

Investigation of gas flow in micro-filters and modification of scaling law

In this study, important micro gas flow features including slip velocity, compressibility, and rarefaction effects for flow micro-filters are investigated. In this regard the compressible Navier–Stokes equations with wall slip and temperature jump boundary conditions are solved for a proper range of Knudsen numbers. For the filters, the ratio of the open orifice area to the total area is selected as 0.6 in order to compare the results with the literature. Considering that the filter holes repeat in a periodic fashion, gas flow is simulated through only one hole by imposing periodicity conditions. The simulation results are compared with the empirical scaling laws developed in the last works and a modified relation for the scaling law is presented. This modified relation predicts high-accuracy pressure drop through the micro-filters in the slip regime.     

Thermodynamic based environmental and sustainability assessments of gas flow in a curved annular channel

The main objective of this research paper is to perform a parametric comparison of gas flow (air and hydrogen) through a curved channel in terms of thermodynamic based environmental and sustainability analysis. For this analysis, the following sustainability indicators which are i) exergetic efficiency (ee), ii) waste exergy ratio (wer), iii) environmental effect factor (eef) and iv) exergetic sustainability index (esi) are defined and estimated in terms of the channel aspect ratio (AR), Dean Number (De) and reference temperature (T₀). Consequently, it is found that ee and esi rise with the increment of De and AR of the curved channel and with the decrease of T₀. However, wer and eef show the opposite behavior. As an important conclusion, air flow through the channel is found to be more exergetic than that of hydrogen under the boundary conditions assumed for the problem. The ee and esi increases with the rise of De while rising with the decrement of T₀. For air, maximum ee and esi values are obtained to be 99.9% and 751.69 incase De is 207.1 and T₀ is 243 K. Adding that, for hydrogen, the maximum ee and esi values have been estimated to be 99.8% and 710.5 while De is 202.3 and T₀ is 243 K. The ee and esi increase with the rise of AR. For air, the maximum ee and esi values are found to be 97.9% and 45.8 while De is equal to 207.1 and AR is 5.5. Also, for hydrogen, the maximum ee and esi values have been calculated to be 97.5% and 38.6 while De is 202.3 and AR is 5.5.     

An experimental and numerical investigation on the cross flow through gas diffusion layer in a PEM fuel cell with a serpentine flow channel

A serpentine flow channel is one of the most common and practical channel layouts for a polymer electrolyte membrane (PEM) fuel cell since it ensures the removal of water produced in a cell with acceptable parasitic load. During the reactant flows along the flow channel, it can also leak or cross to neighboring channel via the porous gas diffusion layer due to the high pressure gradient caused by the short distance. Such a cross flow leads to a larger effective flow area altering reactant flow in the flow channel so that the resultant pressure and flow distributions are substantially different from that without considering cross flow, even though this cross flow has largely been ignored in previous studies. In this work, a numerical and experimental study has been carried out to investigate the cross flow in a PEM fuel cell. Experimental measurements revealed that the pressure drop in a PEM fuel cell is significantly lower than that without cross flow. Three-dimensional numerical simulation has been performed for wide ranges of flow rate, permeability and thickness of gas diffusion layer to analyze the effects of those parameters on the resultant cross flow and the pressure drop of the reactant streams. Considerable amount of cross flow through gas diffusion layer has been found in flow simulation and its effect on pressure drop becomes more significant as the permeability and the thickness of gas diffusion layer are increased. The effects of this phenomenon are also crucial for effective water removal from the porous electrode structure and for estimating pumping energy requirement in a PEM fuel cell, it cannot be neglected for the analysis, simulation, design, operation and performance optimization of practical PEM fuel cells.     

Three dimensional numerical simulation of gas-liquid two-phase flow patterns in a polymer-electrolyte membrane fuel cells gas flow channel

Water management in polymer-electrolyte membrane fuel cells (PEMFCs) has a major impact on fuel cell performance and durability. To investigate the two-phase flow patterns in PEMFC gas flow channels, the volume of fluid (VOF) method was employed to simulate the air-water flow in a 3D cuboid channel with a 1.0 mm × 1.0 mm square cross section and a 100 mm in length. The microstructure of gas diffusion layers (GDLs) was simplified by a number of representative opening pores on the 2D GDL surface. Water was injected from those pores to simulate water generation by the electrochemical reaction at the cathode side. Operating conditions and material properties were selected according to realistic fuel cell operating conditions. The water injection rate was also amplified 10 times, 100 times and 1000 times to study the flow pattern formation and transition in the channel. Simulation results show that, as the flow develops, the flow pattern evolves from corner droplet flow to top wall film flow, then annular flow, and finally slug flow. The total pressure drop increases exponentially with the increase in water volume fraction, which suggests that water accumulation should be avoided to reduce parasitic energy loss. The effect of material wettability was also studied by changing the contact angle of the GDL surface and channel walls, separately. It is shown that using a more hydrophobic GDL surface is helpful to expel water from the GDL surface, but increases the pressure drop. Using a more hydrophilic channel wall reduces the pressure drop, but increases the water residence time and water coverage of the GDL surface.     

MHD flow and heat transfer of a rarefied gas in a rotating channel between conducting plates

An exact analysis of the MHD flow of a rarefied gas in a rotating flat-plate channel is presented. The channel is assumed to rotate about an axis perpendicular to the plane of the plates, which are assumed to be finitely electrically conducting. Velocity profiles, magnetic field lines, and the function affecting the temperature field are shown on graphs. The numerical values of the axial and transverse skin friction, axial and transverse components of the mass flux, and the function affecting the rate of heat transfer are entered in Tables. The results are discussed in terms of the parameters M (Hartmann number), α (Ekman number), λ (slip-parameter), Γ (temperature jump coefficient), ?₁, ?₂ (electrical conductance ratios of the upper and lower plates, respectively).     

Modeling of liquid water transport in a proton exchange membrane fuel cell gas flow channel with dynamic wettability

Liquid water transport in the gas flow channel is significantly important for the water removal and management in proton exchange membrane fuel cells. Previous numerical studies consider a single and constant static contact angle for the liquid water transport on the channel surface, which is insufficient to account for the dynamic wettability behavior of the flow. In this study, a dynamic wettability model is developed that incorporates the sliding angle and dynamic contact angles for the simulation of water transport in the flow channel. It is found that both the sliding and dynamic contact angles have significant impact on the characteristics of the water transport and dynamics in the flow channel. Water spreading on the channel surface is elliptic, and its minor and major axes oscillate out of phase with the droplet height. The pressure loss for the 2‐phase flow in the channel is directly related to this oscillation and deformation of the droplet shape. Flow channel surface with a small sliding angle facilitates the water transport and removal and reduces the associated pressure loss in the channel. The conventional static wettability model would overpredict droplet deformation and breakup as well as the pressure loss in the channel.     

Choked flow and heat transfer of low density gas in a narrow parallel-plate channel with uniformly heating walls

The discharge and heat transfer characteristics of the continuum and slip choked gas flows through a narrow parallel-plate channel with uniform heat flux walls are studied by experimental means, numerical simulation, and analytical approximate solution. The numerical results of the discharge coefficient and the wall surface temperature distributions agree relatively well with the experimental results. The effects of the heat transfer at the walls on the discharge coefficient can be correlated with the dimensionless heat input at the walls. Three kinds of Nusselt numbers which are defined by adiabatic wall, bulk mean, and total temperatures as a reference temperature, respectively, are proposed and the effects of the viscous heating on these Nusselt numbers are clarified.     

Analysis of unsteady force convection flows on irregular boundary over a diverging channel

The purpose of this study report is to examine the unsteady force convection flow on irregular boundary over a diverging channel with effect of viscous dissipation. The fundamental equations of fluid, continuity, momentum, and energy equations are converted into dimensionless form. The resulting coupled partial differential equations are linearized by quasilinearization technique, the backward, and central difference scheme for nonuniform mesh used to solve the system of linear equations. We found that finite difference scheme for nonuniform mesh gives smoothness of solution compared to finite difference scheme for uniform mesh on irregular boundary, this is evident from Figure 2. Near the wall region, as viscous dissipation $\left(Ec\right)$ increases from 0.2 to 0.5, the temperature profiles overshoots 20% to 112%. Also the effects of Reynolds number (Re) on heat transfer coefficient is comparatively less than skin-friction coefficient.     

Second law analysis for mixed convection nanofluid flow in an inclined channel with convectively heated walls

In this study, entropy generation analysis for Cu–water nanofluid mixed convective flow in an inclined channel occupied with a saturated porous media with Navier slip and convective boundary conditions is explored. The governing equations composed of equations of velocity and temperature are nondimensionalized and then solved utilizing the technique of homotopy analysis. Temperature and velocity profile expressions are acquired, which are then used to calculate the entropy produced in the scheme. The impacts of the corresponding fluid parameters are addressed in‐depth on velocity, temperature, entropy generation, Bejan number, Nusselt number, skin friction, volume flow rate, and heat carried out by the fluid for nanofluid concentration. Entropy has been observed to be minimal in all cases just above the channel center and maximum at the channel's bottom wall. Fluid friction‐generated entropy has been discovered to have a higher influence on entropy generation. We also provide a comparative study with existing literature to validate our current results.     

Rarefied gas flow in a planar channel caused by arbitrary pressure and temperature drops

The paper presents a numerical analysis of the nonlinear rarefied gas flow through a long planar channel of finite length. The solution is constructed for the arbitrary pressure and temperature drops, including flow into vacuum. The obtained results for the flow rates are compared with the linearized solutions in the large range of degree of gas rarefaction. The boundaries of the validity of the linearized approximation are established.     

Numerical study of two-phase flow patterns in the gas channel of PEM fuel cells with tapered flow field design

In this study the air–water two-phase flow in a tapered channel of a PEMFC was numerically simulated using the volume of fluid (VOF) method. In particular, a 3D mathematical model of the fuel cell flow channel was used to obtain a reliable evaluation of the fuel cell performance for different taper angles and different temperatures and to calculate the total amount of water produced. This information was then used as boundary conditions to simulate the two-phase flow in the cell channel through a 2D VOF model. Typical operating conditions were assigned and the numerical mesh was constructed to represent the real fuel cell configuration. The results show that tapering the channel downstream enhances the water removal due to increased airflow velocity. In the rectangular channel no film formation is noted with a marked predominance of slug flow. In contrast, as the taper angle is increased the predominant two-phase flow pattern is film flow. Finally many contact angles have been used to simulate the effect of the hydrophobicity of a GDL surface on the motion of the water. As the hydrophobicity of a GDL surface is decreased the presence of film is more evident even for less tapered channels.     

Modelling and optimization of reactant gas transport in a PEM fuel cell with a transverse pin fin insert in channel flow

A proton exchange membrane (PEM) fuel cell has many distinctive features which make it an attractive alternative clean energy source. Some of those features are low start-up, high power density, high efficiency and remote applications. In the present study, a numerical investigation was conducted to analyse the flow field and reactant gas distribution in a PEM fuel cell channel with transversely inserted pin fins in the channel flow aimed at improving reactant gas distribution. A fin configuration of small hydraulic diameter was employed to minimise the additional pressure drop. The influence of the pin fin parameters, the flow Reynolds number, the gas diffusion layer (GDL) porosity on the reactant gas transport and the pressure drop across the channel length were explored. The parameters examined were optimized using a mathematical optimization code integrated with a commercial computational fluid dynamics code. The results obtained indicate that a pin fin insert in the channel flow considerably improves fuel cell performance and that optimal pin fin geometries exist for minimized pressure drop along the fuel channel for the fuel cell model considered. The results obtained provide a novel approach for improving the design of fuel cells for optimal performance.     

Effect of surfactant addition on boiling heat transfer in a liquid film flowing in a diverging open channel

An experimental study was conducted to investigate how the addition of small amounts of a surfactant influences the heat transfer characteristics in a thin boiling liquid film flowing in a diverging open channel. Heat transfer experiments were conducted with fluid inlet temperatures from 40 °C to 92 °C. The flow field on the plate included thin film supercritical flow upstream of a hydraulic jump and thick film subcritical flow downstream of a hydraulic jump. Nusselt numbers for the non-boiling heat transfer without surfactant addition scaled linearly with the film Reynolds number. The boiling heat transfer produced higher Nusselt numbers with a weaker dependence on the Reynolds number. Experimental results showed that a boiling surfactant solution created a thick foam layer with high heat transfer rates and Nusselt numbers that are very weakly dependent on the inlet flow rate or the inlet Reynolds number.     

Turbulent flow in a composite channel

Numerical solutions for turbulent flow in a composite channel are presented. Here, a channel with a centered porous material is considered. The interface between the porous medium and the clear flow was assumed to have different transversal positions and the porous matrix was simulated with distinct permeabilities. Governing equations were discretized and solved for both domains making use of one unique numerical methodology. Increasing the size of the porous material pushes the flow outwards, increasing the levels of turbulent kinetic energy at the macroscopic interface. For high permeability media, a large amount of mechanical energy is converted into turbulence inside the porous structure.     

Dynamic behaviour of liquid water emerging from a GDL pore into a PEMFC gas flow channel

A numerical investigation of the dynamic behaviour of liquid water entering a polymer electrolyte membrane fuel cell (PEMFC) channel through a GDL pore is reported. Two-dimensional, transient simulations employing the volume of fluid (VOF) method are performed to explicitly track the liquid–gas interface, and to gain understanding into the dynamics of a water droplet subjected to air flow in the bulk of the gas channel. The modeled domain consists of a straight channel with air flowing from one side and water entering the domain from a pore at the bottom wall of the channel. The channel dimensions, flow conditions and surface properties are chosen to be representative of typical conditions in a PEMFC. A series of parametric studies, including the effects of channel size, pore size, and the coalescence of droplets are performed with a particular focus on the effect of geometrical structure. The simulation results and analysis of the time evolution of flow patterns show that the height of the channel as well as the width of the pore have significant impacts on the deformation and detachment of the water droplet. Simulations performed for droplets emerging from two pores with the same size into the channel show that coalescence of two water droplets can accelerate the deformation rate and motion of the droplets in the microchannel. Accounting for the initial connection of a droplet to a pore was found to yield critical air inlet velocities for droplet detachment that are significantly different from previous studies that considered an initially stagnant droplet sitting on the surface. The predicted critical air velocity is found to be sensitive to the geometry of the pore, with higher values obtained when the curvature associated with the GDL fibres is taken into account. The critical velocity is also found to decrease with increasing droplet size and decreasing GDL pore diameter.     

Micropolar flow in a porous channel with high mass transfer

In this research the injective micropolar flow in a porous channel is investigated. The flow is driven by suction or injection on the channel walls, and the micropolar model is used to describe the working fluid. This problem is mapped into the system of nonlinear coupled differential equations by using Berman's similarity transformation. These are solved for large mass transfer via Optimal Homotopy Asymptotic Method (OHAM). Also the numerical method is used for the validity of this analytical method and excellent agreement is observed between the solutions obtained from OHAM and numerical results. Trusting this validity, effects of some other parameters are discussed.     

The developing flow in a channel filled with porous media

The developing velocity profiles of fluids flowing into a porous media were solved analytically for a two-dimensional channel bounded by impermeable walls. Three inlet velocity distributions were considered. The effect of the inlet velocity and the Darcy number on the variation of the velocity distribution was examined and the entrance lengths were determined.