Hall effects on magnetohydrodynamic rotating flow through porous medium in a parallel plate channel with various oscillations of pressure gradient

We explored the unsteady flow of an incompressible electrically conducting viscous fluid in a gyratory porous medium with a changeable pressure gradient by taking Hall currents into account. The governing equations are then solved analytically with the help of the Laplace transforms methodology. It is regarded as three dissimilar cases, namely, an impulsive change, cosine as well as sine oscillations of the pressure gradient. The physical significances of different dimensionless parameters on velocity distributions are explored analytically and computationally. It is observed that a thin boundary layer is formed near the plate of the channel and the thicknesses of the layer increase with the increase in either the Hall parameter or Reynolds number while it decreases with an increase in Hartmann number. It is interesting to note that the rotation and Lorentz forces are having noteworthy effects on velocity profiles with pressure gradient and Hall currents.     

Simulating the effect of subsurface stresses and transient pore pressure on wellbore stability in subsea horizontal wells

The most prominent challenge associated with offshore horizontal drilling is wellbore stability. In this paper, simulation of in-situ stresses around the wellbore is conducted to study the effects of transient pore pressure on the stability of horizontal wells. The rock mechanical analysis based on finite element technique lead to investigate a unique behavior found in subsea horizontal wells known as transient pore pressure behavior and near wellbore pressure gradients. The results demonstrate that near wellbore pore pressure gradient is only active in rock formations which possess transient pore pressure behavior; therefore, simulated solutions require adjustment to achieve accurate results.     

Influence of slip condition on transient laminar flow over an infinite vertical plate with ramped temperature in the presence of chemical reaction and thermal radiation

An analysis was made using the numerical approach of a transient laminar slip flow over an infinite vertical plate with ramped and constant temperatures in which chemical reaction is involved and thermal radiation had to be considered. Slip conditions have caused much concern because of their broad applicability in industry and chemical engineering. By following the finite element technique, the equation of momentum together with the equations of energy and species was numerically solved. The expressions for skin friction, Nusselt number, and Sherwood number are also derived. The variations in fluid velocity, fluid temperature, and species concentration are displayed graphically whereas numerical values of skin friction, Nusselt number, and Sherwood number are presented in tabular form for various values of the pertinent flow parameters. The findings indicate that the radiation has a noticeable impact to a minor intensity of R and is more apparent in the constant condition than in the ramped condition. Radiation and buoyancy effects produce a strong flow near the plate, which is accelerated by slip. Finally, it is shown logically and mathematically that when two buoyancies are opposite and equal in magnitude with equal solutal and thermal diffusions, the flow should be taken as stationary flow in the absence of radiation and the presence/absence of slip.     

Time-dependent natural convection fluid flow of stably stratified fluid in an asymmetrically heated vertical channel filled with an anisotropic porous material

This paper presents a theoretical analysis of the combined effects of anisotropic porous material and thermal stratification on the transient natural convection fluid flow in an asymmetrically heated vertical parallel channel. The solutions of the governing equations for the temperature and velocity fields are obtained using Laplace transform technique, Riemann sum approximation, and the D'Alembert method. The choice of the D'Alembert method is to provide a simple decoupling procedure for the coupled governing equations while still retaining their original orders. The research established that owing to the layering effect induced by the thermal stratification $(S)$, the temperature and the velocity distributions of the fluid are found to be attenuated with an increase in thermal stratification. It is also observed that the inclusion of anisotropic parameters in the transport equations aids in regulating the fluid velocity, temperature, Nusselt number, skin friction, and mass flow rate. In addition, by neglecting the anisotropic parameter and taking into account the adiabatic stratification of the fluid, the numerical values for the mass flow rate of the present research favorably compared with the numerical results obtained by Singh et al.     

Horizontal pressure gradient and Soret effects on the onset of thermosolutal porous convection

The intricacies of a constant horizontal pressure gradient on the onset of Soret-driven thermosolutal porous convection have been investigated. The resulting generalized eigenvalue problem is solved numerically using the Galerkin method and also the condition for the onset is obtained in a closed-form using a single-term Galerkin method with trigonometric trial function. The results obtained from both methods are found to be in good agreement. The effect of increasing horizontal pressure gradient, Lewis number, Soret parameter, and the Vadasz number is to hasten, while the increase in the solute Darcy–Rayleigh number is to delay the onset of oscillatory convection. The presence of the horizontal pressure gradient is found to decrease the threshold value of solute Darcy–Rayleigh number beyond which the instability sets in as oscillatory. Moreover, the horizontal pressure gradient imparts a conflicting behavior on the critical wave number and critical frequency of oscillations. The numerical results attained under the limiting cases are shown to be in excellent agreement with the published ones.     

Numerical investigation of water dynamics in a novel wettability gradient anode flow channel for proton exchange membrane fuel cells

The effective removal and transport of water in flow channels play an important role in the water management of proton exchange membrane fuel cells (PEMFCs). In this paper, a novel design of anode serpentine flow channel with the wettability gradient wall is discussed and numerically investigated by utilizing the volume-of-fluid (VOF) method. The effects of the contact angle and the wettability gradient of channel walls, as well as hydrogen flow velocity and water droplet size, on the droplet dynamic behavior are studied. The results indicate that compared with the conventional flow channel, the water droplet can be more effectively removed from the turning part in the wettability gradient flow channel. And the water removal ability in the turning part is improved with the increase of the wettability gradient. Moreover, the wettability gradient flow channel can also improve the water removal performance for the cases with different hydrogen flow velocities and water droplet sizes. This study provides ideas for guiding the design of flow channel to effectively enhance anode water management.     

Capillary pressure of a continuous gas–oil–water flow in a horizontal micron capillary

Capillary pressure is one of the most important factors in characterizing the fluid behavior in a wide variety of processes with environmental and energy concerns. In this paper, Washburn equation is extended to describe the displacement kinetics of the continuous gas–oil–water flow in a single horizontal tube. Experimental investigations of the continuous two- and three-phase flows in capillaries with diameters of 2–10 μm were performed to study the effect of the capillary pressure on their fluid behavior. The results indicate that the total capillary pressure of a continuous three-phase flow is affected by the combined action of the two fluid interfaces presented: gas–liquid and liquid–liquid interfaces.     

Bioconvection in a squeezing flow of a micropolar fluid in a horizontal channel

The bioconvection flow of an incompressible micropolar fluid containing microorganisms between two infinite stretchable parallel plates is considered. A mathematical model, with a fully coupled nonlinear system of equations describing the total mass, momentum, thermal energy, mass diffusion, and microorganisms is presented. The governing equations are reduced to a set of nonlinear ordinary differential equations with the help of suitable transformations. The resulting nonlinear ordinary differential equations are linearized using successive linearization method, and the resulting system of linear equations is solved using the Chebyshev collocation method. The detailed analysis illustrating the influences of various physical parameters, such as the micropolar coupling number, squeezing parameter, the bioconvection Schmidt number, Prandtl numbers, Lewis number, and bioconvection Peclet number on the velocity, microrotation, temperature, concentration and motile microorganism distributions, skin friction coefficient, Nusselt number, Sherwood number, and density number of motile microorganism, is examined. The influence of the squeezing parameter is to increase the dimensionless velocities and temperature and to decrease the local Nusselt number and local Sherwood number. The density number of motile microorganism is decreasing with squeezing parameter, bioconvection Lewis number, bioconvection Peclet number, and bioconvection Schmidt number.     

Time-dependent pressure-driven flow in a horizontal cylinder filled with a bidisperse porous material

Some properties of time-dependent that modify Brinkman equations for fluid flow in a cylindrical tube filled with Bidisperse Porous Material are discussed in this article. The fluid velocities through the fracture and porous phases of the Bidisperse Porous Medium (BDPM) resulting from the application of pressure gradient are described by two coupled second-order partial differential equations. Laplace transform technique, D'Alembert and Riemann-Sum Approximation Methods are used to obtain a semianalytical solution for the model. The choice of the D'Alembert is made to systematically decouple the coupled governing equations without altering their initial orders. The role of the coupling parameter: The coefficient of momentum transfer $(\eta )$ in the flow formation is considered. Accordingly, three cases are analyzed: (a) weak coupling $(\eta =0)$ which described the fluid flow in the absence of the coupling parameter, (b) the strong coupling resulting from a large value of the coupling parameter $(\eta \to \infty )$, and (c) fluid momentum for any arbitrary value of $\eta$. It is observed that fluid stability is attained when ${\mathit{Da}}_f$ and ${\mathit{Da}}_p$ are decreased; a finding that agrees with the findings of Nield and Kuznetsov and Magyari. Also, the maximum velocity in the fracture phase of the BDPM is attained when the coefficient of momentum transfer is neglected $(\eta =0)$ while an opposing flow formation is demonstrated in the fracture and porous phases of BDPM as $\eta$ is increased.     

A study on entropy generation and heat transfer in a magnetohydrodynamic flow of a couple-stress fluid through a thermal nonequilibrium vertical porous channel

The effect of local thermal nonequilibrium (LTNE) on the entropy generation and heat transfer characteristics in the magnetohydrodynamic flow of a couple-stress fluid through a high-porosity vertical channel is studied numerically using the higher-order Galerkin technique. The Boussinesq approximation is assumed to be valid and the porous medium is considered to be isotropic and homogeneous. Two energy equations are considered one each for solid and fluid phases. The term involving the heat transfer coefficient in both equations renders them mutually coupled. Thermal radiation and an internal heat source are considered only in the fluid phase. The influence of inverse Darcy number, Hartmann number, couple-stress fluid parameter, Grashof number, thermal radiation parameter, and interphase heat transfer coefficient on velocity and temperature profiles is depicted graphically and discussed. The entropy generation, friction factor, and Nusselt number are determined, and outcomes are presented via plots. The effect of LTNE on the temperature profile is found to cease when the value of the interphase heat transfer coefficient is high, and in this case, we get the temperature profiles of fluid and solid phases are uniform. The physical significance of LTNE is discussed in detail for different parameters' values. It is found that heat transport and friction drag are maximum in the case of LTNE and minimum in the case of local thermal equilibrium. We observe that LTNE opposes the irreversibility of the system. The corresponding results of a fluid-saturated densely packed porous medium can be obtained as a limiting case of the current study.     

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.     

Effect of Pressure Gradient on Heat Transfer of Impinging Submerged Circular Jets

An experimental and numerical study have been carried out to investigate the distribution of radial local heat transfer coefficients of impinging submerged circular jets. Good agreement is achieved between the experimental results and the predicted value. Results show that the outer peak usually occurs at the radial location of r/d= 1.8~2.0, in which transition from laminar to turbulence happens resulting from disappeared pressure gradient abruptly, and that the inner peak appears rigidly at r/d=0.5, where the boundary layer has a minimum thickness because of elevating pressure gradient.     

水平PDMS微通道内气-液弹状流特性观察

对以空气、甘油为工作流体在水平聚二甲基硅氧烷(polydimethylsiloxane,PDMS)微通道内产生的弹状流进行了可视化实验研究.实验结果表明,微通道中弹状流的气柱速度随着体积含气率增大而阶梯状减小.气柱长度在体积含气率小于0.06时几乎不发生变化;当体积含气率大于0.06时,随体积含气率的增大而呈线性增大.气柱间距随体积含气率的变化与气柱长度相反.气柱和微通道壁面之间存在稳定的薄液膜,当地薄液膜压力稳定,气柱流经当地时所造成的压力波动很小;当地静压随体积含气率的增大而减小.     

An improved numerical scheme to evaluate the pressure gradient on unstructured meshes for two-phase flow analysis

In multi-dimensional two-phase flows, a local pressure is very important because it directly influences the phase change and it may lead to a great change in the flow field. This in turn puts emphasis on the accurate evaluation of local pressure gradient. This paper presents a new numerical scheme to evaluate the pressure gradient at cell centers on unstructured meshes for a three-dimensional thermal-hydraulic code, named CUPID. The results of the new scheme for a simple test function, a gravity-driven cavity, and a wall boiling two-phase flow are compared with those of the previous schemes in the CUPID code.     

The onset of vortex instability in laminar forced convection flow through a horizontal porous channel

The onset of convective instability in a fluid-saturated porous layer between the two horizontal plates heated isothermally from below has been analyzed theoretically by using propagation theory. In the analysis the thermal dispersion coefficient is assumed to be proportional to the streamwise velocity. The results show that both inertia and thermal dispersion stabilize the system.     

Effect of horizontal pressure gradient on Rayleigh–Bénard convection of a Newtonian nanoliquid in a high porosity medium using a local thermal non-equilibrium model

A study of linear and weakly nonlinear stability analyses of Darcy–Brinkman convection in a water–alumina, nanoliquid-saturated porous layer for stress-free isothermal boundaries, when the solid and nanoliquid phases are in local thermal nonequilibrium, is conducted. The critical eigenvalue is found using the Galerkin approach. The effect of the pressure gradient, thermal conductivity ratio, interphase heat transfer coefficient, inverse Darcy number, and Brinkman number on the heat transport and onset of convection is examined and represented graphically. The critical values of wavenumber and nanoliquid Rayleigh number are found for different problem parameter values. The effect of increasing the porosity-modified ratio of thermal conductivity advances the onset of convection and increases the amount of heat transport, whereas the remaining parameters have the opposite impact on the onset of convection and amount of heat transport. The classical results of the local thermal equilibrium case and Darcy–Bénard convection in the presence of pressure gradient are obtained as a limiting case of the present problem.     

往复式多孔介质燃烧器流动特性的试验研究

研究了往复式多孔介质燃烧器在冷态条件下气流在其中的流动特性规律。在多孔介质的类型和运行温度不变的情况下，往复式多孔介质燃烧器的压降与空截面流速的平方成正比，与多孔介质的厚度成正比；燃烧器的稳定时间基本不受多孔介质厚度和空截面流速的影响。建立了计算压降的简单数学模型，理论模型的计算结果与实验数据符合较好。     

The flow of a Jeffrey fluid in a composite horizontal channel partially filled with a deformable porous material

The flow of a Jeffrey liquid in a composite deformable porous layer is examined. The Jeffrey model governs fluid flow in the free-flow zone, while the theory of mixtures describes the fluid flow in the deformable porous region. The governing equations are transformed to dimensionless by using appropriate nondimensional quantities. The velocity field is achieved in both the clear flow and deformable porous regions. The displacement expression in the porous zone is also established. The results are discussed for various substantial parameters in detail. It is scrutinized that the flow rate accelerates with a rise in the Jeffrey parameter ${\lambda }_1$. Also, it is perceived that the flow rate is less for a Newtonian liquid than the non-Newtonian Jeffrey liquid. The fluid momentum enhances by increasing the volume fraction of the fluid phase. It is found that the magnitude of skin friction at the clear flow region and porous region grows with an increase in the Jeffrey parameter ${\lambda }_1$. Furthermore, it is noticed that the wall shear stress is less for a Newtonian liquid when compared with non-Newtonian Jeffrey liquid both in the free-flow region as well as in the porous regions.     

Enhancement of mixed convection heat transfer in a three-dimensional horizontal channel flow by insertion of a moving block

Enhancement of mixed convection heat transfer rate of a heat surface in a three dimensional horizontal channel with insertion of a moving block is studied numerically. Bossinique assumption is not adopted, then methods of Roe scheme, preconditioning, and dual time stepping are needed to solve the governing equations. Contributions of important parameters of Gr/Re² and moving block velocity to the heat transfer rate are validated. Due to the consideration of natural convection, under situations of large magnitude of Gr/Re² a counter-effect for promotion of heat transfer rate is observed. Oppositely, under situations of small magnitude of Gr/Re², the enhancement of heat transfer rate is remarkably achieved.     

Investigation of the influence of various numbers of impeller blades on internal flow field analysis and the pressure pulsation of an axial pump based on transient flow behavior

In the current study, the flow behavior in an axial pump through changing the number of impeller blades is analyzed. Due to the number of blades being very important geometrical parameters in the pump, the study of the influence of various numbers of blades on flow and pressure pulsation in the pump is carried out using the computational fluid dynamics technique. The sliding mesh with the standard turbulence k‐ε model is used to investigate the unsteady flow with several flows and impeller blades. Pump performance prediction results with available experimental data indicate reasonable and good agreement with each other. Static pressure, shear stress, and different velocity compounds are qualitatively analyzed. Moreover, the fluctuation pressure and average pressure under different operating conditions and impeller blades are quantitatively investigated. The numerical results show that the impeller blade has a high impact on pressure, shear stress, magnitude velocity, axial velocity, radial velocity, tangential velocity, and average pressure. Furthermore, this numerical study provides good and useful information for the hydraulic design of axial pumps.