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Lie Group Method for Studying the Heat Generation Effect on Freeconvection Laminar Boundary-layer Flow Over a Vertical Flat Plate | OMICS International
ISSN: 1736-4337
Journal of Generalized Lie Theory and Applications
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Lie Group Method for Studying the Heat Generation Effect on Freeconvection Laminar Boundary-layer Flow Over a Vertical Flat Plate

Abd-el-Malek MB1,3*, Badran NA1, Hassan HS2 and Abbas HH2

1Department of Engineering Mathematics and Physics, Faculty of Engineering, Alexandria University, Alexandria 21544, Egypt

2Department of Basic and Applied Science, Arab Academy for Science, Technology and Maritime Transport, P.O. BOX 1029 Alexandria, Egypt

3Department of Mathematics and Actuarial Science, The American University in Cairo, New Cairo 11835, Egypt

*Corresponding Author:
Abd-el-Malek MB
Department of Engineering Mathematics and Physics
Faculty of Engineering, Alexandria University, Alexandria 21544, Egypt
Tel: 01000066496
E-mail: [email protected]

Received date: August 10, 2015; Accepted date: October 06, 2015; Published date: October 12, 2015

Citation: Abd-el-Malek MB, Badran NA, Hassan HS, Abbas HH (2015) Lie Group Method for Studying the Heat Generation Effect on Free-convection Laminar Boundary-layer Flow Over a Vertical Flat Plate. J Generalized Lie Theory Appl 9:232. doi:10.4172/1736-4337.1000232

Copyright: © 2015 Abd-el-Malek MB, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Abstract

The nonlinear equations of heat and mass transfer in two-dimensional free-convection, laminar, boundary layer flow of a viscous incompressible fluid over a vertical plate with thermophoresis and heat generation effect have been considered. We apply Lie-group method for determining symmetry reductions of partial differential equations. Liegroup method starts out with a general infinitesimal group of transformations under which the given partial differential equations are invariant. The determining equations are a set of linear differential equations, the solution of which gives the transformation function or the infinitesimals of the dependent and independent variables. After the group has been determined, a solution to the given partial differential equations may be found from the invariant surface condition such that its solution leads to similarity variables that reduce the number of independent variables of the system. The effect of the heat generation parameter He, the Prandtl number Pr, the Schimted number Sc, the thermophoretic parameter τ, the solutal Grashof number Gc and the thermal Grashof number Gr on velocity, concentration and temperature have been studied and the results are plotted.

Keywords

Lie-group; Similarity solutions; Thermophoresis; Heat generation; Free convection

Nomenclature

a, b : Constants

image : Concentration of the substance in the fluid inside the boundary layer

C : Concentration of the fluid in the free stream air

Cw: Concentration of the fluid beside the plate

cρ : Specific heat of the fluid

D : Diffusion coefficient

Gc : Solutal Grashof number

Gr : Thermal Grashof number

g : Acceleration due to gravity

He : Heat generation parameter

k : Thermal conductivity of fluid

Pr : Prandtl number

Q : Heat generation constant

Sc : Schimted number

image: Temperature of the fluid inside the boundary layer

T : Temperature of the fluid in the free stream air

Tr: Some reference temperature

Tw: Temperature of the fluid in the plate

image : Velocity component along image direction

image : Thermophoretic velocity

image : Velocity component along image direction

image : Space coordinates

Greek symbols

β * : Coefficient of expansion with concentration

β : Coefficient of thermal expansion

ρ : Density of the fluid

λ : Small parameter

v : Kinematic viscosity of the fluid

k : Thermophoretic coefficient

kv : Thermophoretic diffusivity

τ : Thermophoretic parameter

Introduction

Thermophoresis is a phenomenon which causes small particles to be driven away from a hot surface and towards a cold one. Small particles, such as dust, when suspended in a gas with a temperature gradient, experience a force in the direction opposite to the temperature gradient [1].

The velocity acquired by the particles is called the thermophoretic velocity and the force experienced by the suspended particles due to the temperature gradient is known as the thermophoretic force. The magnitudes of the thermophoretic force and velocity are proportional to the temperature gradient, thermal conductivity of aerosol particles and the carrier gas, thermophoretic coefficient and the heat capacity of the gas. This phenomenon has many practical applications in removing small particles from gas streams, in determining exhaust gas particle trajectories from combustion devices, prevention of fouling and corrosion in heat exchangers and turbines, semiconductor manufacture and ceramic powder production.

Brock [2] explained the disagreement between experimental results and aerosol thermal force theories for regimes from continuum toward free-molecule through the application of first-order slip-flow boundary conditions. In his work, the influence of the convective flow on the thermal force is explored through a perturbation technique. He developed an expression for the thermophoretic force for continuum regime.

Brock [3] reviewed the theoretical and experimental results for the thermal force in the free molecule region. He developed a theory for the thermal force for the transition region. Experimental results appear to support his developed theory.

A laser-Doppler velocimeter (LDV) has been used by Talbot et al. [4] to study of velocity profiles in the laminar boundary layer adjacent to a heated flat plate revealed that the seed particles used for the LDV measurements were driven away from the plate surface by thermophoretic forces, causing a particle-free region within the boundary layer of approximately one half the boundary-layer thickness. Measurements of the thickness of this region were compared with particle trajectories calculated according to several theories for the thermophoretic force.

Goren [5] considered theoretically the thermophoresis of aerosol particles in the laminar compressible boundary layer on a flat plate is. The particles are attracted by a plate colder than the gas and the dust concentration drops monotonically as the wall is approached; in the absence of Brownian diffusion a nonzero concentration at the wall is found. The particles are repelled by a plate hotter than the gas and, in the absence of Brownian diffusion, the aerosol concentration profile exhibits a singularity at which the concentration rapidly approaches zero or infinity. The location of this singularity is taken to be the boundary of the dust-free layer found on heated objects.

Chiou [6] analyzed the sub-micron deposition from a laminar free convection boundary layer developing on a heated isothermal vertical cylinder. In his work, a finite-difference method has been used to theoretically calculate the local cumulative particle deposition with the thermophoretic velocity. It was demonstrated that the thermophoresis has a pronounced effect on the particle transfer but only a slight effect on the very smaller particles and that the effect increases with particle size and distance along the cylinder. Thermophoresis in natural convection for a cold vertical surface has been studied by Epstein et al. [7].

The interaction between radiation and thermophoresis in forced convection laminar boundary-layer flow over an impermeable flat plate is investigated by Jia et al. [8]. The fluid is a radiatively nonparticipating constant-property gas containing emitting, absorbing, and isotropically scattering gray aerosol particles.

Sub-micron deposition from a laminar forced convection boundary layer developing on a heated isothermal vertical cylinder has been investigated by Chiou and Cleaver [9]. Pseudo-similarity solutions have been used to calculate the local cumulative particle deposition theoretically; with the thermophoretic velocity.

Selim et al. [10] investigated the effect of surface mass flux on mixed convective flow past a heated vertical flat permeable plate with thermophoresis. A non-uniform surface mass flux through the permeable surface has been considered. The governing equations, reduced to local non-similarity boundary layer equations using suitable transformations, have been integrated employing an implicit finite difference method together with the Keller-box technique.

Alam et al. [11] studied the problem of steady, two-dimensional, laminar, hydromagnetic flow with heat and mass transfer over a semiinfinite, permeable inclined flat plate in the presence of thermophoresis and heat generation is numerically. A similarity transformation is used to reduce the governing non-linear partial differential equations into ordinary ones. The obtained locally similar equations are then solved numerically by applying Nachtsheim-Swigert shooting iteration technique with sixth-order Runge-Kutta integration scheme.

This work is concerned with the study of the effect of thermophoresis and heat generation on free-convection laminar boundary-layer flow over a vertical flat plate. Lie-group method is applied to the equations of motion, energy and diffusion for determining symmetry reductions of partial differential equations, [12-24]. Exact solutions for the temperature have been obtained and we used them to get the velocity and the concentration using MATLAP package.

Mathematical Formulation of the Problem

Consider the steady, laminar, two-dimensional free-convection boundary layer flow of a viscous incompressible fluid over a vertical plate placed in the region image > 0 of a Cartesian system of coordinates image. The plate has a uniform temperature Tw and the free stream temperature is T with Tw > T . The species concentration beside the plate is maintained uniform at Cw and that of the ambient fluid is assumed to be C. A heat source is placed within the flow to allow for possible heat generation effects. The fluid is assumed to be Newtonian and heat generating. The effects of thermophoresis are being taken into account (Figure 1).

generalized-theory-applications-Physical-model-coordinate

Figure 1: Physical model and coordinate system.

Under the above assumptions, the governing equations of the mass, momentum, energy and concentration for such flow can be written as

Continuity Equation: image (2.1)

Momentum equation:

image (2.2)

Energy equation:

image (2.3)

Diffusion equation:

image (2.4)

where,

image (2.5)

The boundary conditions are

(i) image (2.6)

(ii) image (2.7)

The variables in equations (2.1) - (2.4) are dimensionless according to

image (2.8)

Substitution from (5) and (8) into (1) - (4) yields

image (2.9)

image (2.10)

image (2.11)

image (2.12)

where subscripts denote partial derivatives with respect to the indicated variables.

The boundary conditions (2.6) and (2.7) will be

(i) image (2.13)

(ii) image (2.14)

From the Continuity Equation (2.9), there exists a stream function ψ (x, y) such that

image (2.15)

which satisfies equation (2.9) identically.

Substitution from (2.15) into (2.10) - (2.12) yields

image (2.16)

image (2.17)

image (2.18)

where,

image

The boundary conditions (2.13) and (2.14) will be

(i) image (2.19)

(ii) image (2.20)

Solution of the Problem

Firstly, we derive the similarity solutions using Lie-group method under which (2.16) - (2.18) and the boundary conditions (2.19) - (2.20) are invariant, and then we use these symmetries to determine the similarity variables.

Lie point symmetries

Consider the one-parameter (ε) Lie group of infinitesimal transformations in (x,y;ψ,T,C) given by

image (3.1)

where “ε” is a small parameter. The partial differential equations (2.16) - (2.18) are said to admit a symmetry generated by the vector field

image (3.2)

if it is left invariant by the transformation (x,y;ψ,T,C) → (x*,y*;ψ*,T*,C*). The solutions ψ=ψ (x,y), T=T (x,y) and C=C(x,y), are invariant under the symmetry (3.2) if

image (3.3)

image (3.4)

and

image (3.5)

Assume,

image (3.6)

image (3.7)

image (3.8)

A vector Γ given by (3.2), is said to be a Lie point symmetry vector field for (2.16)-(2.18) if

image (3.9)

where,

image (3.10)

is the third prolongation of Γ.

To calculate the prolongation of the given transformation, we need to differentiate (3.1) with respect to each of the variables x and y. To do this, we introduce the following total derivatives

image (3.11)

Equation (3.9) gives the following system of linear partial differential equations

image (3.12)

image (3.13)

image (3.14)

The components image can be determined from the following expressions

image (3.15)

where S,L and J are standing for x,y. By substituting from (3.15) into (3.13) will lead to a large expression, then, equating to zero the coefficients of image gives

image (3.16)

By substituting (36) into (33) and equating to zero the coefficients of image we get

image (3.17)

image (3.18)

image (3.19)

By substituting (3.16)-(3.19) into (3.12) and (3.14) will lead to a system of determining equations. By solving the system of resulting determining equations in view of the invariance of the boundary conditions (2.19)-(2.20) yields

image (3.20)

So, the nonlinear equations (16)-(18) have the three-parameter Lie group of point symmetries generated by

image (3.21)

The one-parameter group generated by Γ1, Γ2 and Γ3 consists of translation. There aren’t any non-zero commutation relations for these symmetries, where the commutation relation is given by [Γij]=ΓiΓj− ΓjΓi. The finite transformations corresponding to the symmetries Γ1, Γ2 and Γ3 are respectively

image (3.22)

where ε1, ε2 and ε3 are the group parameters.

One-dimensional optimal system of subalgebras of the symmetry group

Since the symmetry Lie algebra is three-dimensional which given by the operators (3.21), we look for solutions invariant under the linear combination of these operators. All the possible invariant solutions can be obtained by determine the optimal system of one-dimensional subalgebras of the given system of partial differential equations. This is the most systematic procedure presented by Olver [16]. Following Olver’s approach, we compute first the commutators of the symmetry Lie algebra (3.17), which is obtained in section 3.1, and then find the adjoint representations. The adjoint action on Lie algebras is defined by the adjoint operator given by

image (3.23)

where, λ is a small parameter.

This operator can be rewritten in terms of Lie brackets using Campbell–Baker–Hausdorff theorem [25] as

image (3.24)

For our problem, Ω=<Γ123> is the Lie algebra associated with the symmetry group. The calculations of the adjoint action are summarized in Table 1.

Ad Γ1 Γ2 Γ3
Γ1 Γ1 Γ2 Γ3
Γ2 Γ1 Γ2 Γ3
Γ3 Γ1 Γ2 Γ3

Table 1: Table of adjoint representations.

From Table 1, it is clear that, there are not any nontrivial adjoint actions for the Lie symmetry algebra Ω . To construct the onedimensional optimal system of Ω , consider a general element of Ω given by

E=a1Γ1 + a2Γ2 + a3Γ3, (3.25)

for some constants a1, a2 and a3 and probe whether E can be transformed to a new element E’ under the general adjoint action, where E′ takes a simpler form than E, [26]. Let,

image (3.26)

We make appropriate choice of a such that the a’i’s can be made 0 or 1. We end up with simpler forms of E that will constitute the one-dimensional optimal system. By substituting Γi= Γ1 in (3.26) and dropping the primes, we get

E’ =a1Γ1 + a2Γ2 + a3Γ3. (3.27)

Now, equation (3.27) allows the consideration of the cases a1 ≠ 0 and a1= 0.

Case (I): a1 ≠ 0

Scaling the resulting operator by a1, equation (3.27) will be

E’ =Γ1 + aΓ2 + bΓ3. (3.28)

We can further consider the subclasses (i) a=0, b ≠ 0, (ii) a ≠ 0, b=0 and (iii) a=b=0. Therefore, an optimal system of one-dimensional subalgebra for this case is given by:

image , where, a∈R and b∈R.

Case (II): a1=0

Using repeatedly the adjoint operation to simplify E, an optimal system of one-dimensional subalgebra for this case is given by{Γ23, Γ2 + bΓ3}. In summary, the optimal system of one-dimensional subalgebras of the symmetry Lie algebra is

image (3.29)

Table 2 shows the solution of the invariant surface conditions (3.3) – (3.5) associated with the optimal system.

  Generator  Characteristic image  Solutions of the invariant surface conditions
image image image
image image image
image image  No solution
image image image
image image image
image image image
image image image

Table 2: Solutions of the invariant surface conditions associated with the optimal system.

(i) For Γ1, the characteristic

image (3.30)

has the components

image (3.31)

Therefore, the general solutions of the invariant surface condition (3.3) - (3.5) are

image (3.32)

Substitution from (3.32) into (2.15) yields

image (3.33)

This solution is not a particularly interesting one since it contradicts the boundary conditions. So, no solutions are invariant under the group generated by Γ1.

(ii) For Γ2, the characteristic (3.30) has the components

image (3.34)

Therefore, the general solutions of the invariant surface condition (3.3) - (3.5) are

image (3.35)

Substitution from (3.35) into (2.15) yields

image (3.36)

This solution is not a particularly interesting one since it contradicts the boundary conditions. So, no solutions are invariant under the group generated byΓ2.

(iii) For Γ3, the characteristic (3.30) has the components

image (3.37)

So, no solutions are invariant under the group generated by Γ3.

(iv) For Γ1+ aΓ2, the characteristic (3.30) has the components

image (3.38)

Therefore, the general solutions of the invariant surface condition (3.3) - (3.5) are

image (3.39)

Substitution from (3.39) into (2.15) yields

image (3.40)

Practically, Eqatation. (3.39) is a solution of the equations (2.9) - (2.12), even though it is not a particularly interesting one since it contradicts the boundary conditions. So, no solutions are invariant under the group generated by Γ1+aΓ2.

(v) For Γ1+bΓ3, the characteristic (50) has the components

image (3.41)

Therefore, the general solutions of the invariant surface conditions (3.3)-(3.5) are

image (3.42)

Substitution from (3.42) into (2.15) yields

image (3.43)

Substitution from (3.42) into (2.17) yields to two cases.

Case (1): image

image (3.44)

where, image . Substitution from (3.44) into (2.18) and (2.16), respectively, yields

image (3.45)

image (3.46)

Case (2): image

image (3.47)

where, image.

Substitution from (3.47) into (2.18) and (2.16), respectively, yields

image (3.48)

image (3.49)

(vi) For Γ2+bΓ3, the characteristic (3.30) has the components

image (3.50)

Therefore, the general solutions of the invariant surface condition (3.3) - (3.5) are

image (3.51)

Substitution from (3.51) into (2.15) yields

image (3.52)

This solution is not a particularly interesting one since it contradicts the boundary conditions. So, no solutions are invariant under the group generated by Γ2+bΓ3.

(vii) For Γ1+aΓ2+bΓ3, the characteristic (3.30) has the components

image (3.53)

Therefore, the general solutions of the invariant surface condition (3.3) - (35) are

image (3.54)

Substitution from (3.54) into (2.15) yields

image (3.55)

Practically, Equatation. (3.54) is a solution of the equations (2.9) - (2.12), even though it is not a particularly interesting one since it contradicts the boundary conditions. So, no solutions are invariant under the group generated by Γ1+aΓ2+bΓ3.

Results and Discussion

The effect of the heat generation parameter He

Figures 2 and 3 illustrate the horizontal velocity u, temperature T and concentration C profiles for Gc=1, Gr=1, Pr=1, Sc=0.22 and τ=1 over a range of the heat generation parameter He, for cases (1) and (2), respectively. For case (1), it is observed that the fluid temperature increases with the increase in the heat generation parameter He. Also the velocity boundary layer thickness increases with the increase in the thermal boundary layer, therefore the fluid velocity increases (Figure 2).

generalized-theory-applications-heat-generation-parameter

Figure 2: Effect of the heat generation parameter He on: (a) Horizontal velocity, (b) Temperature and (c) Concentration, for Gc=1, Gr=1, Pr=1, Sc=0.22, and t =1for case (1).

generalized-theory-applications-heat-generation-parameter

Figure 3: Effect of the heat generation parameter He on: (a) Horizontal velocity, (b) Temperature and (c) Concentration, for Gc=1, Gr=1, Pr=1, Sc=0.22, b=2, and τ =1 for case (2).

For case (2), the fluid temperature decreases with the increase in the heat generation parameter He. The velocity boundary layer thickness decreases with the decrease in the thermal boundary layer which result in less induced flow along the plate therefore the fluid velocity decreases (Figure 3). A slight variation in the rate of increase of the concentration appears in both cases, as the heat generation parameter increases.

The effect of Prandtl number Pr

Figures 4 and 5 illustrate the horizontal velocity, temperature and concentration profiles for Gc=1, Gr=1, Sc=0.22 and τ=1 over a range of the Prandtl number Pr, case (1) for He=0.2 and case (2) for He=1.5, respectively. As seen, the temperature decreases as Pr increases. Physically Pr characterizes the ratio of thicknesses of the viscous and thermal boundary layers. The thermal boundary layer thickness decreases sharply with the increase in Pr which causes the fluid temperature to decrease. Also the velocity boundary layer thickness decreases with the decrease in the thermal boundary layer which result in less induced flow along the plate therefore the fluid velocity decrease. In both cases, a slight variation in the rate of decrease of the concentration appears as the Prandtl number Pr increases.

generalized-theory-applications-Horizontal-velocity-Temperature

Figure 4: Effect of the Prandtl number Pr on: (a) Horizontal velocity, (b) Temperature and (c) Concentration, for Gc=1, Gr=1, Pr=1, Sc=0.22, b=3 and τ =1 for case (1) at He=0.2.

generalized-theory-applications-Prandtl-number-Pron

Figure 5: Effect of the Prandtl number Pron: (a) Horizontal velocity, (b) Temperature and (c) Concentration, for Gc=1, Gr=1,Sc=0.22, b=2 and τ =1 for case (2) at He=1.5.

The effect of Schimted number Sc

Figures 6 and 7 illustrate the horizontal velocity and the concentration profiles for Gc=1, Gr=1, Pr=1 and τ=1 over a range of the Schmited number Sc, case (1) for He=0.2 and case (2) for He=1.5, respectively. The concentration decreases as Sc increases. Physically Sc characterizes the ratio of thicknesses of the viscous and concentration boundary layers. The concentration boundary layer thickness decreases as the Sc increases therefore the fluid velocity decreases.

generalized-theory-applications-Schmited-number-Scon

Figure 6: Effect of the Schmited number Scon: (a) Horizontal velocity and (b) Concentration, for Gc=1, Gr=1, Pr=1, b=3, and τ =1 for case (1) at He=0.2.

generalized-theory-applications-Schmited-number-Scon

Figure 7: Effect of the Schmited number Scon: (a) Horizontal velocity and (b) Concentration, for Gc=1, Gr=1, Pr=1, Sc=0.22 and τ =1 for case (2) at He=1.5.

The effect of thermo-phoretic parameter τ

Figures 8 and 9 illustrate the horizontal velocity and the concentration profiles for Gc=1, Gr=1, Pr=1 and Sc=0.22 over a range of the thermo-phoretic parameter τ, case (1) for He=0.2 and case (2) for He=1.5, respectively. It is observed that the velocity and concentration of the fluid decrease with increase of τ. In particular, the effect of increasing the parameter is limited to be increasing slightly the wall slope of the concentration profiles but decreasing the concentration.

generalized-theory-applications-thermo-phoretic-parameter

Figure 8: Effect of the thermo-phoretic parameter τ on: (a) Horizontal velocity and (b) Concentration, for Gc=1, Gr=1, Pr=1, Sc=0.22 and b =3 for case (1) at He=0.2.

generalized-theory-applications-thermo-phoretic-parameter

Figure 9: Effect of the thermo-phoretic parameter τ on: (a) Horizontal velocity and (b) Concentration, for Gc=1, Gr=1, Pr=1, Sc=0.22 and b =2 for case (2) at He=1.5.

The effect of the solutal Grashof number Gc

Figure 10 illustrates the horizontal velocity profiles for Gr=1, Pr=1, Sc=0.22 and τ=1, over a range of the solutal Grashof number Gc, case (1) for He=0.2 and case (2) for He=1.5, respectively. As seen from both cases, the horizontal velocity increases as Gc increases. The Solutal Grashof number represents the relative importance of the buoyancy force to the viscous force. The increasing value of Gc implies the increasing strength of the flow which increase the velocity.

generalized-theory-applications-horizontal-velocity

Figure 10: Effect of the solutal Grashof number Gc on the horizontal velocity, for Gr=1, Pr=1, Sc=0.22, and τ =1 , (a) case (1) for He=0.2 and b=3 (b) case (2) for He=1.5 and b=2.

The effect of thermal Grashof number Gr

Figure 11 illustrates the horizontal velocity profiles for Gc=1, Pr=1, Sc=0.22 and τ=1, over a range of the thermal Grashof number Gr, case (1) for He=0.2 and case (2) for He=1.5, respectively. We observe that, the velocity increases as Gr increases.

generalized-theory-applications-horizontal-velocity

Figure 11: Effect of the thermal Grashof number Gr on the horizontal velocity, for Gc=1, Pr=1, Sc=0.22, and τ =1, (a) case (1) for He=0.2 and b=3 (b) case (2) for He=1.5 and b=2.

Conclusion

The steady, laminar, two-dimensional free-convection boundary layer flow of a viscous incompressible fluid over a vertical plate with the effect of thermophoresis and heat generation has been investigated. The system of non-linear partial differential equations is solved using Liegroup method. The resulting ordinary differential equation from the energy equation is solved analytically and we have got exact solutions for the temperature of the fluid for two cases. The resulting ordinary differential equations obtained from the momentum and diffusion equations were solved numerically using using MATLAP package. The product of the manufacturing processes is affected by increasing or decreasing the physical parameters which appear in the governing equations that describe the physical phenomenon, which gives the significance of our results. It was found that the horizontal velocity decreased due to increases in the Prandtl number, Schimted number and thermo-phoretic parameter and it increased due to increases in the solutal Grashof number, thermal Grashof number. As increases in the heat generation parameter, the horizontal velocity increased in case (1) and it decreased in case (2). In addition, the temperature decreased due to increases in the Prandtl number. As increases in the heat generation parameter, the temperature increased in case (1) and it decreased in case (2). It was found that the particle concentration level as well as the concentration boundary layer thickness decreased due to increases Prandtl number, Schimted number and thermo-phoretic parameter. A slight variation in the rate of increase of the concentration appears as the heat generation parameter increases.

Acknowledgements

The authors are very thankful to the reviewers for their constructive and valuable comments.

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