Research Article  Open Access
J. Prakash, S. Gouse Mohiddin, S. Vijaya Kumar Varma, "Free Convective MHD Flow Past a Vertical Cone with Variable Heat and Mass Flux", Journal of Fluids, vol. 2013, Article ID 405985, 8 pages, 2013. https://doi.org/10.1155/2013/405985
Free Convective MHD Flow Past a Vertical Cone with Variable Heat and Mass Flux
Abstract
A numerical study of buoyancydriven unsteady natural convection boundary layer flow past a vertical cone embedded in a nonDarcian isotropic porous regime with transverse magnetic field applied normal to the surface is considered. The heat and mass flux at the surface of the cone is modeled as a power law according to and , respectively, where denotes the coordinate along the slant face of the cone. Both Darcian drag and Forchheimer quadratic porous impedance are incorporated into the twodimensional viscous flow model. The transient boundary layer equations are then nondimensionalized and solved by the CrankNicolson implicit difference method. The velocity, temperature, and concentration fields have been studied for the effect of Grashof number, Darcy number, Forchheimer number, Prandtl number, surface heat flux powerlaw exponent (), surface mass flux powerlaw exponent (), Schmidt number, buoyancy ratio parameter, and semivertical angle of the cone. Present results for selected variables for the purely fluid regime are compared with the published results and are found to be in excellent agreement. The local skin friction, Nusselt number, and Sherwood number are also analyzed graphically. The study finds important applications in geophysical heat transfer, industrial manufacturing processes, and hybrid solar energy systems.
1. Introduction
Combined heat and mass transfer in fluidsaturated porous media finds applications in a variety of engineering processes such as heat exchanger devices, petroleum reservoirs, chemical catalytic reactors and processes, and others. A thorough discussion of these and other applications is available in the monographs [1, 2]. Comprehensive reviews of the much of the work communicated in porous media transport phenomena have been presented in [3, 4]. Most studies dealing with porous media have employed the Darcy law. However, for high velocity flow situations, the Darcy law is inapplicable, since it does not account for inertial effects in the porous medium. Such flows can arise, for example, in the nearwellbore region of high capacity gas and condensate petroleum reservoirs and also in highly porous filtration systems under high blowing rates. The most popular approach for simulating highvelocity transport in porous media is the DarcyForchheimer drag force model. This adds a secondorder (quadratic) drag force to the momentum transport equation. This term is related to the geometrical features of the porous medium and is independent of viscosity. A seminal study discussing the influence of Forchheimer inertial effects in porous media convection is presented by Vafai and Tien [5]. The mixed convective boundary layer flow from a vertical surface in a fluidsaturated nonDarcian porous medium including Forchheimer inertial effects is studied by C.H. Chen and C.K. Chen [6] and Chen et al. [7]. Thermal convection boundary layer flow with buoyancy and suction/blowing effects from a cone with nonuniform surface temperature is studied by Hossain and Paul [8]. The study is extended by Hossain and Paul [9] by considering nonuniform surface heat flux, both studies employing numerical methods. Chamkha et al. [10] studied the doublediffusive convection heat and mass transfer over a cone (and wedge) in DarcyForchheimer porous media. Magnetohydrodynamic (MHD) flowandheattransferisofconsiderableinterestbecauseitcanoccurin many geothermal, geophysical, technological, and engineering applications such as nuclear reactors and others. The geothermal gases are electrically conducting and are affected by the presence of a magnetic field. Vajravelu and Nayfeh [11] studied hydromagnetic convection from a cone anda wedge withvariablesurfacetemperatureandinternalheatgenerationorabsorption. Thus far the transient thermal convection flow over a cone in DarcyForchheimer porous media has not been studied in the literature despite important applications in geothermics, geophysics, and materials processing.
2. Mathematical Model
An axisymmetric unsteady natural convection boundary layer flow past a vertical cone with transverse magnetic field applied normal to the surface with variable heat and mass flux in a DarcyForchheimer fluid saturated porous medium in a cartesian () coordinate system is formulated mathematically in this section. Initially, it is assumed that the cone surface and the surrounding fluid which are at rest possess the same temperature and concentration level everywhere in the fluid. At time , heat supplied from the cone surface to the fluid and concentration level near the cone surface are raised at a rate of and, respectively, and they are maintained at the same level. It is assumed that the concentration of the diffusing species in the binary mixture is very less in comparison to the other chemical species, which are present and hence the Soret and Dufour effects are negligible. We consider viscous flow where pressure work, viscous dissipation, and thermal dispersion effects are neglected. The coordinate system chosen (as shown in Figure 1) is such that the direction is measured along the cone surface from the leading edge , and the direction is normal to the cone generator. The cone apex is located at the origin ().
Here designates the semivertical angle of the cone, and is the local radius of the cone.
Then under the previous assumptions, the governing boundary layer equations with Boussinesq’s approximation are where all terms are defined in the nomenclature. Under the Boussinesq approximation, buoyancy effects are simulated only in the momentum equation, which is coupled to the energy equation, constituting a free convection regime. The corresponding spatial and temporal initial and boundary conditions at the surface and far from the cone take the following form: where all the parameters are defined in the nomenclature.
Equation (1) is highly coupled, parabolic, and nonlinear. An analytical solution is clearly intractable, and in order to facilitate a numerical solution we nondimensionalize the model. Proceeding with the analysis, we now introduce the following transformations:
The transport (1) is thereby reduced to the following dimensionless form:
The corresponding nondimensional initial and boundary conditions are given by where again all the parameters are given in the nomenclature. The dimensionless local values of the skin friction, Nusselt number, and the Sherwood number are given by the following expressions:
3. Numerical Solution
In order to solve the unsteady, nonlinear, coupled (4) under condition (5), an implicit finite difference scheme of CrankNicolson type has been employed which is discussed by many researchers [12–15]. The finite difference scheme of dimensionless governing equations is reduced to tridiagonal system of equations and is solved by Thomas algorithm as discussed elsewhere [16]. The region of integration is considered as a rectangle with and where and corresponds to which lies very well outside both the momentum and thermal boundary layers. The maximum of was chosen as 22, after some preliminary investigation so that the last two boundary conditions of (5) are satisfied within the tolerance limit . The mesh sizes have been fixed as , with time step . The computations are carried out first by reducing the spatial mesh sizes by 50% in one direction, and later in both directions by 50%. The results are compared. It is observed in all cases that the results differ only in the fifth decimal place. Hence, the choice of the mesh sizes seems to be appropriate. The scheme is unconditionally stable. The local truncation error is , and it tends to zero as , and tend to zero. Hence, the scheme is compatible. Stability and compatibility ensure the convergence. The derivatives involved in (6) are evaluated using five point approximation formula.
4. Results and Discussion
Only selective figures have been reproduced here for brevity. In the numerical computations, the following values for the dimensionless thermophysical parameters are prescribed: Grashof number () = 1.0, Darcy number (Da) = 0.1 (high permeability), Forchheimer number (Fs) = 0.1 (weak quadratic drag), Prandtl number (Pr) = 7.0 (water), Schmidt number (Sc) = 0.6 (oxygen diffusing in air), surface heat flux power law exponent , surface mass flux power law exponent , buoyancy ratio parameter , and semivertical angle of the cone . All graphs therefore correspond to these values unless otherwise indicated. To test the accuracy of the computations, the local shear stress and local Nusselt number computations for the nonporous case are compared with those of Hossain and Paul [9] for a heat flux gradient of and in the steady state, in Tables 1 and 2, respectively, and are found to be in good agreement.


In Figures 2(a) and 2(b), the influence of Grashof number () on steady state velocity () and temperature () distributions with coordinate are shown. Free convection, that is, thermal buoyancy effects are analyzed via the Grashof number. For an increasing from 0.1 through 1.0, 10.0, 50.0 to 100.0 cooling of the cone by free convection occurs; that is, heat is conducted away from the cone to the surrounding regime.
(a)
(b)
Figures 3(a) and 3(b) show the effect of Darcy number (Da) on dimensionless velocity () and temperature () with transformed radial coordinate () close to the leading edge (i.e., cone apex) at . To study the influence of regime permeability from sparsely packed media to densely packed materials, the following values , 0.1, 0.01, 0.001 are considered. for a fixed value of the reference length () is directly proportional to permeability () of the porous regime. Increasing Da increases the porous medium permeability and simultaneously decreases the Darcian impedance since progressively less solid fibers are present in the regime. The flow is therefore accelerated for higher Da values causing an increase in the velocity as shown in Figure 3(a). Maximum effect of rising Darcy number is observed at intermediate distance from the cone surface around . Conversely, temperature depicted in Figure 3(b) is opposed by increasing Darcy number. The presence of fewer solid fibers in the regime with increasing Da inhibits the thermal conduction in the medium which reduces distribution of thermal energy. The regime is therefore cooled when more fluid is present, and values in the thermal boundary layer are decreased. Profiles for both velocity and temperature are smoothly asymptotic decays to the free stream indicating that excellent convergence (and stability) is obtained with the numerical method. Velocity boundary layer thickness will be increased with a rise in Da and thermal boundary layer thickness reduced. The effect of the Forchheimer inertial drag parameter (Fs) on dimensionless temperature () profiles is shown in Figure 4. The Forchheimer drag force is a secondorder retarding force simulated in the momentum conservation equation. Increasing Fs values from 0.0 through 0.1, 1.0, 5.0, 10.0, 20.0, and 50.0 causes a strong increase in Forchheimer drag which decelerates the flow, that is, reduces velocities. For higher values of Fs, it is expected that the porous medium flow becomes increasingly chaotic. Temperature () however is slightly increased with a rise in Forchheimer parameter. The effects of the Prandtl number (Pr) on velocity profiles are depicted in Figure 5. Pr encapsulates the ratio of momentum diffusivity to thermal diffusivity. Larger Pr values imply a thinner thermal boundary layer thickness and more uniform temperature distributions across the boundary layer. Hence, thermal boundary layer will be much less thick than the hydrodynamic (translational velocity) boundary layer. Smaller Pr fluids have higher thermal conductivities, so that heat can diffuse away from the cone surface faster than for higher Pr fluids (thicker boundary layers). Physically the lower values of Pr correspond to liquid metals (Pr 0.02, 0.05), Pr = 0.7 is accurate for air or hydrogen and Pr = 7.0 for water. The computations show that translational velocity is therefore reduced as Pr rises from 0.72 through 1.0, 2.0, 5.0, 7.0 and 10.0 since the fluid is increasingly viscous as Pr rises.
(a)
(b)
Figure 6 shows the effect of the Schmidt number (Sc) on the dimensionless concentration ().We note that the Schmidt number (Sc) embodies the ratio of the momentum to the mass diffusivity. Sc therefore quantifies the relative effectiveness of momentum and mass transport by diffusion in the hydrodynamic (velocity) and concentration (species) boundary layers. Smaller Sc values can represent for example hydrogen gas as the species diffusing in air, Sc = 2.0 implies hydrocarbon diffusing in air, and higher values to petroleum derivatives diffusing in fluids (e.g. ethyl benzene) as indicated elsewhere [17]. As Sc increases, Figure 6 shows that values are strongly decreased as larger values of Sc correspond to a decrease in the chemical molecular diffusing that is, less diffusion therefore takes place by mass transport. The dimensionless concentration profiles all decay from a maximum concentration to zero in the freestream. Greater Sc values correspond to lower chemical molecular diffusivity of the parent fluid so that less diffusion of the species occurs in the regime. Concentration boundary layer thickness will therefore be reduced. For low Sc fluid greater species diffusion occurs and concentration boundary layer thickness increased. For Sc = 1, the Concentration and velocity boundary layers will have approximately the same thickness that is, species and momentum will be diffused at the same rates. With lower Sc values the decay of concentration from the cone surface is more controlled, for increasing values of Sc the profiles descend more and more steeply and concentration falls faster from the surface to a short distance into the boundary layer regime.
The effect of surface heat flux power exponent () on the steady state temperature () is shown in Figure 7. An increase in the value of reduces the temperature. It is also seen that the time required to reach the steady state temperature is more at lower values of . Figure 8 depict the distribution of concentration () with radial coordinate () for various values of the surface mass flux power law exponent (). The concentration reduces with the increasing values from 0.0 through 0.25, 0.50, 0.75 and 1.0. Increasing Fs clearly reduces the local Nusselt number as shown in Figure 9.
A slight increase in local Nusselt number accompanies the increment in Pr as shown in Figure 10. The influence of the concentration to thermal buoyancy ratio parameter (), on dimensionless temperature () with radial coordinate () is shown in Figure 11. indicates that thermal and species buoyancy forces are both absent. For , thermal and species buoyancy forces aid each other. implies that both buoyancy forces are of the same order of magnitude. A rise in from 0.0 through 1.0, 2.0, 3.0 and 5.0 induces a retarding effect on the flow in the porous regime that is, velocities are decreased.Increasing (thermal and concentration buoyancy forces assisting each other) decreases temperatures in the regime that is, cools the boundary layer regime. The effect of semivertical angle of the cone () on dimensionless temperature () with coordinate is shown in Figure 12. It is observed that a rise in substantially increases the temperature in the boundary layer regime. And more time is required to reach the steady state. Figure 13 the influence of magnetic parameter () versus spanwise spatial distributions of velocity are depicted. Application of magnetic field normal to the flow of an electrically conducting fluid gives rise to a resistive force that acts in the direction opposite to that of the flow. This force is called the Lorentz force. This resistive force tends to slow down the motion of the fluid along the cone and causes an increase in its temperature and a decrease in velocity as increases. An increase in from 1 though 2, 3, 4 clearly reduces streamwise velocity both in the nearwall regime and farfield regime of the boundary layer.
5. Conclusions
Numerical solutions have been presented for the buoyancydriven unsteady natural convection boundary layer flow past a vertical cone embedded in a nonDarcian isotropic porous regime. Present results are compared with those of [8] and found to be in excellent agreement. The following conclusions are drawn.(i)Increasing Grashof number boosts the translational velocity in the cone surface regime and decreases temperature throughout the flow regime.(ii)Increasing Darcy number accelerates the flow, that is, increases translational velocities. However, the temperature is reduced with a rise in Darcy number.(iii)An increase in the Forchheimer inertial drag parameter is observed to slightly increase the temperature but reduces both velocity and local Nusselt number.(iv)An increase in Prandtl number is observed to decrease both temperature and velocity, but the concentration is slightly increased. A slight increase in local Nusselt number accompanies the increment in Pr.(v)The concentration is observed to significantly decrease with an increase in Schmidt number.(vi)The temperature is observed to decrease with an increase in buoyancy ratio parameter but decrease with an increase in semivertical angle of the cone. The time taken to reach the steady state increases with increasing .
Nomenclature
:  Coordinates along the cone generator and normal to the generator 
:  Velocity components along the  and directions 
:  Gravitational acceleration 
:  Local radius of cone 
Time  
:  Dimensionless time 
:  Temperature 
:  Dimensionless temperature 
:  Concentration 
:  Dimensionless concentration 
:  Mass diffusion coefficient 
:  Permeability of porous medium 
:  Heat flux (i.e., heat transfer rate per unit area) 
:  Mass flux (i.e., mass transfer rate per unit area) 
:  Thermal conductivity of fluid 
:  Reference length 
:  Dimensionless coordinates along the cone generator and normal to the generator 
:  Dimensionless velocity components along the x and ydirections 
:  Forchheimer geometrical constant 
Da:  Darcy number 
Fs:  Forchheimer number 
:  Grashof number 
:  Magnetic parameter 
:  Magnetic field strength 
Pr:  Prandtl number 
:  Buoyancy ratio parameter 
Sc:  Schmidt number 
:  Powerlaw index for surface heat flux relation 
:  Powerlaw index for surface mass flux relation 
:  Local Nusselt number 
:  Dimensionless local Nusselt number 
:  Local Sherwood number 
:  Nondimensional local Sherwood number 
:  Dimensionless local radius of cone. 
:  Dynamic viscosity of fluid 
:  Kinematic viscosity of fluid 
:  Semivertical cone angle 
:  Thermal diffusivity 
:  Volumetric thermal expansion coefficient 
:  Dimensionless temperature function 
:  Dimensionless time 
:  Dimensionless local shear stress function (skin friction). 
:  Condition on the wall 
∞:  Free stream condition. 
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Copyright
Copyright © 2013 J. Prakash et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.