Time Dependent MHD Nano-Second Grade Fluid Flow Induced by Permeable Vertical Sheet with Mixed Convection and Thermal Radiation

The aim of present paper is to study the series solution of time dependent MHD second grade incompressible nanofluid towards a stretching sheet. The effects of mixed convection and thermal radiation are also taken into account. Because of nanofluid model, effects Brownian motion and thermophoresis are encountered. The resulting nonlinear momentum, heat and concentration equations are simplified using appropriate transformations. Series solutions have been obtained for velocity, temperature and nanoparticle fraction profiles using Homotopy Analysis Method (HAM). Convergence of the acquired solution is discussed critically. Behavior of velocity, temperature and concentration profiles on the prominent parameters is depicted and argued graphically. It is observed that temperature and concentration profiles show similar behavior for thermophoresis parameter Νt but opposite tendency is noted in case of Brownian motion parameter Νb. It is further analyzed that suction parameter S and Hartman number Μ depict decreasing behavior on velocity profile.


Introduction
Many engineering and industrial applications involve a working fluid that may be active or inactive in its own capacity. The role of this fluid is to transfer energy/heat from one location to other. For a long period, the performance of adequate heat transfer has been a major problem. The introduction of nanofluid as a working fluid has opened the gates of new era in the area of heat transfer. With thermal conductivity more than base fluid and a size of 1-100 nm, nanoparticles are utilized to attain the maximum enhancement in the thermal characteristics under minimum concentrations. The pioneering work of Choi [1] with the declaration that thermal conductivity of base fluid will be doubled by adding the nanoparticles into the base fluid revolutionized the related engineering applications in a variety of directions. These include coolants of nuclear reactors, cancer therapy, safer surgeries and in safety problems related to nuclear reactors. In designing the waste heat removal equipment, nanoparticles play an important role [2]. With both liquid and magnetic properties, magneto nanofluid with its varied biomedical applications like sterilized devices, wound treatment, gastric medications, asthma treatment and elimination of tumors has a vital role in daily life. Some recent studies on nanofluids and magneto nanofluids may be found in the references [3][4][5][6][7][8][9][10][11][12] and many therein.
Comprehensive knowledge of non-Newtonian fluids' flow characteristics is the need of the day because of their vital role in growing industrial and engineering applications. These may include shampoos, soaps, apple sauce, polymeric liquids, tomato paste, ketchup, paints, blood at low shear rate etc. The Navier-Stokes equations are not sufficient to explore the true behavior of such materials. Different types of non-Newtonian fluid models are developed in the past to describe the actual behavior of these liquids. The fluid model which is used in the present investigation is a subclass of differential type non-Newtonian fluids and known as second grade fluid. This fluid model is capable to explore the shear thinning and shear thickening effects. Fetecau et al. [13] studied the unsteady flow of second grade fluid induced due to the time-dependent motion of wall. They provided the exact solutions of this flow analysis by employing Fourier sine transform. Helical flows of second grade fluid between two coaxial cylinders are investigated by Jamil et al. [14]. Here the flow generation is due to inner cylinder motion. Hayat et al. [15] reported two dimensional boundary layer flow of second grade fluid with convective boundary condition via homotopy analysis method. Turkyilmazoglu [16] discussed the dual and triple solutions of MHD second grade non-Newtonian fluid in the presence of slip condition. Heat transfer analysis in viscoelastic non-Newtonian fluid flow is discussed by Ashorynejad et al. [17]. Heat source effect in second grade fluid in the presence of power law heat flux condition is explored by Hayat et al. [18]. Hayat et. al [19] discussed the stratifications and mixed convection radiative flow of Jeffrey fluid over a stretching sheet. But very less approaches have been reported in the presence of nanofluids.
To bridge this gap, we have studied the thermal radiation effects in MHD unsteady flow of second grade nanofluid in the presence of mixed convection. The flow is induced due to the vertical stretching sheet. We developed series solutions of velocity, temperature and nanoparticle concentration via homotopy analysis method (HAM) [20][21][22][23][24][25][26]. Graphs are plotted to examine the effects of various physical parameters on the dimensionless temperature and nanoparticle concentration fields. Values of skin-friction coefficient, local Nusselt and Sherwood numbers are computed and discussed. From the literature survey, it is revealed that no such analysis is reported yet.

Mathematical formulation
We consider the magnetohydrodynamics (MHD) and time dependent flow of an incompressible second grade nanofluid over a porous stretching surface. The electrically conducting fluid under the influence of a unsteady magnetic field B(t) which is applied in a direction normal to the stretching surface. Under the assumption of a small magnetic Reynolds number, the induced magnetic field is negligible. Moreover, heat transfer process is also taken into account. The geometrical configuration of the present flow is shown in Fig 1. The governing boundary layer equations using above mentioned suppositions and Boussineq's approximation can be written as: rC P @T @t þ u @T @x þ v @T @y ¼ a 1 @ 2 u @y@t @u @y þ u @ 2 u @x@y @u @y þ v @ 2 u @y 2 @u @y þ k @ 2 T @y 2 þ m @u @y By using Rosseland approximatoin for radiation we have in which q r the radiative heat flux in the y-direction, g the gravitational acceleration, T the fluid temperature, σ Ã the Stefan-Boltzmann constant, ν the kinematic viscosity, σ the electrical conductivity, ρ the fluid density, β C and β T are the concentration and thermal expansion coefficients respectively, D B and D T are the Brownian diffusion coefficient and thermophoretic diffusion coefficient, respectively, k Ã is the mean absorption coefficient, α 1 the second grade Unsteady MHD Flow of Nano-Second Grade Fluid parameter and C p the specific heat and. Since the fluid abide by the second law of thermodynamics and the assumption that the specific Helmholtz free energy is least when the fluid is at a constant temperature then we have μ ! 0, α 1 ! 0, α 1 + α 2 = 0. Expanding T 4 in Taylor series about T 1 and neglecting higher terms, we found By making use of Eqs (5) and (6), Eq (3) has the following form @u @y þ u @ 2 u @x@y @u @y þ v @ 2 u @y 2 @u @y þ m @u @y 2 À @ @y The imposed boundary conditions are given below where is the mass transfer at surface with V w < 0 for suction and V w > 0 for injection. Moreover, the surface temperature T w (x, t), stretching velocity U w (x, t) and the value of nanoparticle volume fraction C w (x, t) are given by: with a and c are the constants with a ! 0 and c > 0 (with ct < 1), and time −1 is the dimension for both a and c. We select unsteady magnetic field of the form Similarity transformation for the present case is given below ð12Þ and the velocity components identically satisfies Eq (1) with stream function ψ while Eqs (2)-(4) and (7)-(9) are converted into the following form Here A = a/c is the unsteadiness parameter, is the Lewis number. The Skin friction coefficient, local Nusselt and local Sherwood numbers are given by the expressions where the skin friction τ w and wall heat flux q w and the concentration flux j w are defined as Dimensionless forms of skin friction coefficient, local Nusselt and local Sherwood numbers are where Re z = w e z/ν is the Reynolds number.

Homotopic solutions
The initial guesses and the auxiliary linear operators are essential for the homotopic solutions. The initial guesses and the auxiliary linear operators for the present flow problems are The auxiliary linear operators have the following properties where C i (i = 1−7) are the arbitrary constants. The zeroth and mth order deformation problems are stated below.

Convergence of solution
To find the meaningful series solutions of momentum, energy and concentration equations, the convergence region is essential to determine. Convergence region of the series solutions depend upon the auxiliary parameter ℏ. Therefore we have plotted the ℏ-curves in the Fig 2. The     the temperature and thermal boundary layer thickness but a decrease is seen for the nanoparticle concentration profiles. Figs 9 and 10 depicts the variation in temperature and nanoparticle concentration profiles for different values of suction parameter S. An increase in suction parameter corresponds to a lower temperature and nanoparticle concentration profiles. Here suction parameter works as an agent which leads to a reduction in both temperature and nanoparticle concentration profiles. From Figs 11 and 12, we observe that both temperature   Prandtl number shows thinner thermal and nanoparticle concentration boundary layer thickness. Larger Prandtl number fluids have lower thermal diffusivity. Due to the lower thermal diffusivity, thinner thermal and nanoparticle concentration boundary layer thicknesses are observed. From Figs 15 and 16, it is seen that the larger values of Eckert number Ec corresponds to higher temperature and nanoparticle concentration. With an enhancement in value of Eckert number, we see an increase in kinetic energy due to which the temperature and nanoparticle    Table 1 gives the convergent values of f@(0), θ 0 (0) and ϕ 0 (0) at different order of HAM deformations. Here it is seen that the values of f@(0) converges from 10-th order of deformations while the values of θ 0 (0) and ϕ 0 (0) repeats from 13-th and 16-th order of computations. Hence the 16-th order of HAM computations is essentials for the convergent homotopic solutions. Table 2 Table 3. The numerical values of skin-friction coefficient, local Nusselt and Sherwood numbers are enhanced with an increase in the value of Pr. The Nomenclature of all parameters used is depicted in Table 4.     • An increase in second grade parameter results in an increase in temperature and thermal boundary layer thickness but a decrease is seen for the nanoparticle concentration profiles • Thermal boundary layer thickness and temperature θ (η) decrease by increasing buoyancy parameter λ.
• Nt and Nb are increasing functions of the temperature θ (η) whereas they depict an opposite behavior in case of Concentration ϕ.
• Pr decreases with an increase in values of temperature θ and concentration ϕ.
• For increasing values of λ, Skin friction coefficient and local Nusselt number increase whereas sherwood number decreases.
• Le show an opposite behavior for temperature θ and concentration ϕ.