Effect of variable viscosity on non-Darcy free or mixed convection flow on a vertical surface in a fluid saturated porous medium

Effect of variable viscosity on non-Darcy free or mixed convection flow on a vertical surface in a fluid saturated porous medium

MECHANICS RESEARCH COMMUNICATIONS Mechanics Research Communications 33 (2006) 148–156 www.elsevier.com/locate/mechrescom Effect of variable viscosity...

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MECHANICS RESEARCH COMMUNICATIONS

Mechanics Research Communications 33 (2006) 148–156 www.elsevier.com/locate/mechrescom

Effect of variable viscosity on non-Darcy free or mixed convection flow on a vertical surface in a fluid saturated porous medium S. Jayanthi, M. Kumari

*

Department of Mathematics, Indian Institute of Science, Bangalore 560012, India Available online 7 October 2005

Abstract This paper analyzes the variable viscosity effects on non-Darcy free or mixed convection flow on a vertical surface in a fluid saturated porous medium. The viscosity of the fluid is assumed to be a inverse linear function of temperature. Velocity and heat transfer are found to be significantly affected by the variable viscosity parameter, Ergun number, Peclet number or Rayleigh number. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Variable viscosity; Free or mixed convection flows; Non-Darcy; Vertical surface; Porous medium

1. Introduction Fluid flow and heat transfer through porous medium have been of considerable interest, especially in the past decade. This is primarily because of numerous applications of flow through porous media, such as storage of radioactive nuclear waste materials transfer, separation processes in chemical industries, filtration, transpiration cooling, transport processes in aquifers, ground water pollution etc. The problem of natural convection heat transfer with constant viscosity from a vertical plate in a saturated porous medium are discussed in references (Cheng and Minkowycz, 1977; Kumari et al., 1985; Plumb and Huenefeld, 1981). Mixed convection flow along vertical surfaces in a porous medium has been studied by several investigators (Cheng, 1977; Hsieh et al., 1993a,b; Nakayama and Shenoy, 1993; Wang et al., 1990). The fundamental analysis of convection through porous media with temperature dependent viscosity is driven by several contemporary engineering applications from cooling of electronic devices to porous journal bearings and is important for studying the variations in constitutive property. The effect of variable viscosity for convective heat transfer through porous media are studied by several investigators (Bagai, 2004; Elbashbeshy, 2000; Gray et al., 1982; Horne and Sullivan, 1978; Kassoy and Zebib, 1975; Ling and Dybbs, 1992;

*

Corresponding author. Tel.: +91 80 2293 2314; fax: +91 80 2360 0146. E-mail address: [email protected] (M. Kumari).

0093-6413/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mechrescom.2005.09.001

S. Jayanthi, M. Kumari / Mechanics Research Communications 33 (2006) 148–156

149

Pantokratoras, 2004; Strauss and Schubert, 1977). Lai and Kulacki (1990) considered the variable viscosity effect for mixed convection flow along a vertical plate embedded in saturated porous medium. The variable viscosity effects on non-Darcy, free or mixed convection flow on a horizontal surface in a saturated porous medium are studied by Kumari (2001). Considering the importance of inertia effects for flow in a porous medium, the problem of free or mixed convective heat transfer along a vertical surface in a fluid saturated porous medium is studied in this paper. The viscosity of the fluid is assumed to be an inverse linear function of temperature. The governing equations for the flow are solved using an implicit finite-difference scheme (Cebeci and Bradshaw, 1984). 2. Governing equations Consider the problem of steady, laminar, incompressible, two-dimensional, non-Darcy, free or mixed convection flow along a heated vertical plate embedded in a saturated porous medium. It is assumed that the fluid and the solid matrix are everywhere in local thermal equilibrium, the thermophysical properties of the fluid are homogeneous and isotropic. With these assumptions and the application of the Boussinesq and boundarylayer approximations, the governing system of conservation equations can be written as (Kumari et al., 1985; Plumb and Huenefeld, 1981) ux þ v y ¼ 0 

ð1Þ 2

ðluÞy þ K ðlu Þy ¼ Kq1 gbT y

ð2Þ

uT x þ vT y ¼ aT yy

ð3Þ

The boundary conditions are vðx; 0Þ ¼ 0; T ðx; 0Þ ¼ T w ðxÞ ¼ T 1 þ Axk T ðx; 1Þ ¼ T 1 ; uðx; 1Þ ¼ 0 ðfor free convection flowÞ uðx; 1Þ ¼ U 1

ð4Þ

ðfor mixed and forced convection flowÞ

Here x and y are the distances along and perpendicular to the surface, respectively; u and v are the velocities in the x and y directions, respectively; T is the temperature; g is the acceleration due to gravity; K is the permeability of the porous medium; K is the form drag coefficient in the Ergun equation; a and b are, respectively, the thermal diffusivity and the coefficient of thermal expansion; q1 and l are, respectively, the density and viscosity of the convecting fluid; A is a constant and k is the parameter representing the variation of the wall temperature. The subscripts x and y denote partial derivatives with respect to x and y, respectively. The subscripts w and 1 denote the conditions at the wall and in the free stream region, respectively. It is assumed that the viscosity of the fluid varies inversely as a linear function of temperature and can be written as (Kumari, 2001; Lai and Kulacki, 1990) 1=l ¼ 1=l1 ½1 þ RðT  T 1 Þ ¼ aðT  T e Þ

ð5Þ

where a = R/l1 and Te  T1 = 1/R, a 5 0, R 5 0. Here a and Te are constants, and their values depend on the reference state and thermal property of the fluid, i.e., R. The viscosity of a liquid usually decreases with increasing temperature and it increases for gases. In general for liquids a > 0 and for gases a < 0. On using Eq. (5), Eq. (2) can be rewritten as ðT  T e Þuy þ 2K  q1 aðT  T e Þ2 uuy ¼ uT y þ Kq1 gbaðT  T e Þ2 T y

ð6Þ

(a) Free convection flow On applying the following transformations g ¼ ðy=xÞRax1=2 ; u ¼ ow=oy; v¼

wðx; yÞ ¼ aRax1=2 f ðn; gÞ;

v ¼ ow=ox;

ða=xÞRax1=2 ½ððk

hðn; gÞ ¼ ðT  T 1 Þ=ðT w  T 1 Þ; 0

u ¼ ða=xÞRax f ðn; gÞ

þ 1Þ=2Þf þ knfn þ ðg=2Þðk  1Þf 0 

Tw > T1

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he ¼ ðT e  T 1 Þ=ðT w  T 1 Þ ¼ 1=ðRðT w  T 1 ÞÞ h  he ¼ ðT  T e Þ=ðT w  T 1 Þ;

n ¼ ðx=dÞ

Rax ¼ ðx=aÞ½KgbðT w  T 1 Þ=m1 ;

k

Er ¼ K  a=md;

Rad ¼ ðd=aÞ½Kgbðad k Þ=m1 

ð7Þ

to Eqs. (1), (6), (3) and the boundary conditions (4), we find that Eq. (1) is satisfied identically, Eqs. (6) and (3) are transformed to f 00  ððh  he Þ=he Þn2ErRad f 0 f 00 ¼ ½ð1=ðh  he ÞÞf 0  ððh  he Þ=he Þh0 h00 þ ð1=2Þðk þ 1Þf h0  kf 0 h ¼ kn½f 0 hn  h0 fn 

ð8Þ ð9Þ

and the boundary conditions (4) become At g ¼ 0 : f ¼ 0;

h ¼ 1;

As g ! 1 : f 0 ! 0;

h!0

ð10Þ

Here n and g are the dimensionless variables, f, f 0 and h are the dimensionless stream function, dimensionless velocity and dimensionless temperature, respectively, he is a parameter which defines the effect of variable viscosity of the fluid, d is the pore diameter, Er and Rad are, respectively, the Ergun number and Rayleigh number based on the pore diameter. The value of the parameter he is determined by the viscosity of the fluid in consideration with the operating temperature difference. The effects of variable viscosity can be neglected for large values of he which implies either R or (Tw  T1) are small. On the other hand, for a smaller value of he, either the fluid viscosity changes considerably with temperature or the temperature difference is high. In either case, the effects of variable viscosity is expected to become very important. It may be noted that for Er = k = 0 Eqs. (8) and (9) reduce to equations of free convection Darcy flow over a vertical plate which are studied by Lai and Kulacki (1990). Furthermore, for k = 0, and he ! 1 we obtain the equations of free convection flow over a vertical plate which are considered by Kumari et al. (1985). (b) Mixed convection flow On using the following transformation g ¼ ðy=xÞPex1=2 ; u ¼ ow=oy;

wðx; yÞ ¼ aPe1=2 x f ðn; gÞ;

v ¼ ow=ox;

hðn; gÞ ¼ ðT  T 1 Þ=ðT w  T 1 Þ;

u ¼ ða=xÞPex f ðn; gÞ ð11Þ

v ¼ ða=xÞPex1=2 ½ð1=2Þf þ knfn  ðg=2Þf 0 Þ Pex ¼ U 1 x=a;

Tw > T1

0

Ped ¼ U 1 d=a;

n ¼ ðx=dÞ

k

Eqs. (6) and (3) reduce to f 00  ððh  he Þ=he Þ2ErPed f 0 f 00 ¼ ½f 0 =ðh  he Þ  nðRad =Ped Þððh  he Þ=he Þh0

ð12Þ

h00 þ ð1=2Þf h0  kf 0 h ¼ kn½f 0 hn  h0 fn 

ð13Þ

and the corresponding boundary conditions (4) become At g ¼ 0 : f ¼ 0;

h ¼ 1;

As g ! 1 : f 0 ! 1;

h!0

ð14Þ

Here Ped is the Peclet number based on the pore diameter. It may be noted that Eqs. (12) and (13) under boundary conditions (14) for k = Er = 0 reduce to the equations of mixed convection Darcy flow over a vertical plate considered by Lai and Kulacki (1990). Furthermore, for he ! 1, Eqs. (12) and (13) reduce to the equations of mixed convection flow which are studied by Hsieh et al. (1993b) for Er = 0 and the corresponding equations of Wang et al. (1990), are obtained by putting k = Er = 0. (c) Forced convection flow The dimensionless equations for the forced convection flow are obtained by putting Rad/Ped = 0 in Eqs. (12) and (13). These are given by

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151

f 00  ððh  he Þ=he Þ2ErPed f 0 f 00 ¼ f 0 h0 =ðh  he Þ h00 þ ð1=2Þf h0  kf 0 h ¼ kn½f 0 hn  h0 fn 

ð15Þ ð16Þ

and the corresponding boundary conditions are At g ¼ 0 : f ¼ 0;

As g ! 1 : f 0 ! 1;

h ¼ 1;

h!0

ð17Þ

For Er = k = 0 Eqs. (15) and (16) reduce to those of forced convection flow over a vertical plate which is investigated by Lai and Kulacki (1990). It may noted that the equations of constant fluid viscosity for free, mixed and forced convection flows can be obtained by putting he ! 1 in the corresponding dimensionless equations. The heat transfer coefficient in terms of the Nusselt number Nux is expressed as Nux Rax1=2 ¼ ½h0 ðn; 0Þnc Nux Pex1=2

for free convection

ð18Þ

¼ ½h ðn; 0Þmc

for mixed convection

ð19Þ

¼ ½h0 ðn; 0Þfc

for forced convection

ð20Þ

0

Here the subscripts nc, mc and fc correspond to free, mixed and forced convection respectively. The free convection asymptotes can be obtained by rewriting Eq. (18) as Nux Pex1=2 ¼ ðRad =Ped Þ

1=2

n½h0 ðn; 0Þnc

3. Results and discussion The dimensionless differential equations for free convection, mixed convection and forced convection are solved numerically using Keller-box method. The details of the method is omitted as it is available in Cebeci and Bradshaw (1984). In order to check the accuracy of the method, the results for mixed convection flow over a vertical plate with constant viscosity in a porous medium are compared with the results of Hsieh et al. (1993b) and Cheng (1977). The velocity and heat transfer at the wall (f 0 (n, 0), h 0 (n, 0)) for various values of ErRad for the case of free convection flow are compared with those obtained by Plumb and Huenefeld (1981) and Kumari et al. (1985). These results are not presented, in order to conserve the space. For

10

1.5

x

Er = 0, n = 1 λ=0

f (ξ,0 )

5

Er = 0, n = 1, λ=0

Presents results x Lai & Kulacki (1990)

x

1

x x xx x x x

x

-

−θ (0 )

x x

x x x x x x x x

x x x x x x x x x

0

-5 -10

0.5

x

-5

0

θe

5

10

x x

x x x x x x x x x x x x x x x x x

0 -10

-5

Fig. 1. Effect of he on f 0 (n, 0) and h 0 (n, 0).

Present results Forced convection Present results Free convection

x Lai & Kulacki (1990)

x x x x x x x x x x x x x x x x x x

0

θe

5

10

152

S. Jayanthi, M. Kumari / Mechanics Research Communications 33 (2006) 148–156 3

30

Er = 0, Ped = 1, Rad/Ped = 1, n = 1 λ=0

Mixed convection

x

20 x x

f (ξ,0 )

10

x x x x x x

-

10

0

x x

x

x

x x xx 0.1 xx x xx 1 x x x x x x x x x x x x x x x x x

x

Lai & Kulacki (1990)

x

x x x x x x

x

0.1

x x xx x x x x xx x x x x x

10

x x x x x x x

x

-

1

1

1 x x x x x x x x x x x x x x x x x

x x x x x x x xxxx x x x x x x

-5

0

θe

5

0 -10

10

-5

x

10 x x x x x x x

x x

1 x x x x x x x x x x x x x x x x x 0.1 xx

0.1

-10 -10

Present results x Lai & Kulacki (1990)

x

x x

x

2

10

x

Er = 0, Ped = 1, Rad/Ped = 1, n =1 λ=0

x

Rax/Pex

Present results

−θ (ξ,0)

Rax/Pex

Mixed convection

0

θe

5

10

Fig. 2. Effect of he on f 0 (n, 0) and h 0 (n, 0).

Er = k = 0 and n = 1 the velocity and heat transfer at the wall (f 0 (n, 0), h 0 (n, 0)) for the case of free, forced and mixed convection flows compared with those of Lai and Kulacki (1990). These are presented in Figs. 1 and 2. In all the cases the results are found to be in good agreement. The velocity and heat transfer parameter asymptotically approaches to the case of constant viscosity case as he ! 1, as seen from Fig. 3. This implies either R or (Tw  T1) are very small. This means the variation of fluid viscosity is negligible. Figs. 4–6 depict the effect of the Ergun number Er, Rayleigh number Rad and Peclet number Ped on the velocity and heat transfer at the wall (f 0 (n, 0), h 0 (n, 0)), with the variable viscosity parameter he for the case of free, forced and mixed convection respectively. It is observed that the velocity and heat transfer at the wall, decrease as the Ergun number Er increases. This is true for both liquids and gases. Also, for any particular value of the Ergun number Er, the velocity and heat transfer at the wall, for the case of mixed convection flows are greater than that of forced and free convection flows. It is also observed that the effect of the Rayleigh 20

5

Er = 0.01, Ped (Rad) = 1, Rad/Ped = 10 λ = 0.5, ξ = 1

Constant viscosity Variable viscosity

4

Er = 0.01, Pedor Rad = 1 Rad/Ped = 10.0, λ = 0.5, ξ = 1.0.

Constant viscosity Variable viscosity

15

-

Mixed convection

Mixed convection

−θ (ξ,0 )

f (ξ ,0 )

Mixed convection

10

3

Mixed convection

-

2

Forced convection

Forced convection Forced convection

5

Free convection

Free convection

Forced convection

1

Free convection 0 -10

-5

0

θe

5

10

0 -10

-5

Fig. 3. Effect of he on f 0 (n, 0) and h 0 (n, 0).

Free convection 0

θe

5

10

S. Jayanthi, M. Kumari / Mechanics Research Communications 33 (2006) 148–156 2

2.5

Rad/Ped = 10 λ = 0.5, ξ = 1 Er = 0.001 Er = 0.01 Er = 0.1

f (ξ ,0 )

2

10

5 Rad = 1

1

1

-

Er = 0.001 Er = 0.01 Er = 0.1

λ = 0.5, Rad = 1

1.5

Rad = 1

1.5

Rad = 10, ξ = 1

Free convection

−θ (ξ,0 )

Free convection

-

153

5

λ = 1.0 λ = 0.8

λ = 0.5, λ = 0.8 λ = 1.0 Rad = 1

5

10

0.5

λ = 0.0

10

5 λ = 0.0

10

0.5

0 -10

-5

0

θe

5

0 -10

10

-5

0

θe

5

10

Fig. 4. Effect of he on f 0 (n, 0) and h 0 (n, 0).

2

2.5

Rad/Ped = 10 λ = 0.5, ξ = 1

Forced convection Ped = 1 2 5

Er = 0.001 Er = 0.01 Er = 0.1

1.5

10

λ = 1.0

10

1.5

Er = 0.001 Er = 0.01 Er = 0.1

Ped = 1

−θ (ξ,0 )

f (ξ ,0 )

2

Forced convection

-

-

1

5

2 Ped = 1,2,5,10 λ = 1.0

λ = 0.5

λ = 0.5

1

λ = 0.0

λ = 0.0

0.5 0.5

0 -10

Ped = 1,2,5,10

-5

0

5

10

0 -10

-5

θe

0

5

10

θe Fig. 5. Effect of he on f 0 (n, 0) and h 0 (n, 0).

number Rad on velocity and heat transfer at the wall for free convection and the effect of the Peclet number on the velocity and heat transfer at the wall in the case of mixed and forced convection is analogous to the effect of the Ergun number Er on the velocity and heat transfer at the wall (f 0 (n, 0), h 0 (n, 0)). The effect of the parameter k representing the variation of the wall temperature on the heat transfer at the wall h 0 (n, 0) with he is also shown in Figs. 4–6. It is observed that the heat transfer at the wall h 0 (n, 0) increases with the parameter k for free, forced and mixed convection flows. It is also observed that the parameter k has no significant effect on the velocity at the wall f 0 (n, 0) and so it is not shown in the figures. The reason for such an effect is that the parameter k does not explicitly occur in the equation of motion. The effect of the parameter Rad/Ped on the velocity and heat transfer (f 0 (n, 0), h 0 (n, 0)) at the wall for mixed convection flows with he is shown in Fig. 7. The velocity and heat transfer at the wall increase as Rad/Ped increases. This is true for all values of he.

154

S. Jayanthi, M. Kumari / Mechanics Research Communications 33 (2006) 148–156 5

30

Rad/Ped = 10, ξ = 1

Mixed convection

Er = 0.001 Er = 0.01 Er = 0.1

4

20

−θ (ξ ,0 )

f (ξ ,0 )

λ = 1.0

-

-

Ped = 1 2 10

5 10

Ped = 1

2

5

Rad/Ped = 10, ξ = 1 Er = 0.001 Er = 0.01 Er = 0.1

Mixed convection

3

10

Ped = 1 5

.0

λ=1

2

10 5

10 λ = 0.5

2

-5

0

5

1 -10

10

Ped = 1

λ = 0.5

λ = 0.0 0 -10

2

λ = 0.0 -5

0

5

10

θe

θe Fig. 6. Effect of he on f 0 (n, 0) and h 0 (n, 0).

4

20

Er = 0.01, Ped = 10 λ = 0.5, ξ = 1.0.

Mixed convection

Mixed convection

Er = 0.01, Ped = 10.0, λ = 0.5, ξ = 1

3

Rad/Ped = 10

-

10

−θ (ξ ,0 )

f (ξ ,0 )

Rad/Ped = 10 Rad/Ped = 10

Rad/Ped = 10

-

5.0 2

5.0

5.0 5.0 1.0 0 -10

1.0

1.0

1.0

1

0.1

0.1 0.1

0.1 -5

0

θe

5

10

-10

-5

0

5

10

θe Fig. 7. Effect of he on f 0 (n, 0) and h 0 (n, 0).

The variation of the velocity and heat transfer at the wall (f 0 (n, 0), h 0 (n, 0)) with n for different values of he for free, forced and mixed convection flows is displayed in Fig. 8. The velocity and heat transfer at the wall decrease as he increases. Also for any value of he the velocity and heat transfer at the wall are less affected with n for the case of free and forced convection flows. For mixed convection flows, for any fixed value of he, the velocity and heat transfer at the wall increase with n. The heat-transfer at the wall (h 0 (n, 0)) as a function of the mixed convection parameter Rad/Ped is presented in Fig. 9. This figure also shows the limiting cases of free and forced convection flows. It is observed that the heat transfer is more for variable viscosity case, than the constant viscosity case for liquids (he < 0) and it has the opposite trend for gases (he > 0). Similar behaviour has also been observed for the case of vertical plate by Lai and Kulacki (1990) and for the case of a horizontal plate by Kumari (2001).

S. Jayanthi, M. Kumari / Mechanics Research Communications 33 (2006) 148–156 35

2

Er = 0.01, Rad or Ped = 10, Rad/Ped = 0 or 10 f’(ξ,0) λ=0.5,ξ = 1 −θ (ξ,0)

Er = 0.01, Ped = 10, Rad/Ped = 10 λ = 0.5

-

30

f (ξ ,0 ),−θ (ξ ,0)

(f (ξ ,0 ), −θ’ (ξ ,0 ))

Forced convection Free convection



θe = −2

1

-2 2 2

-



25

20

θe = −2

15

-

2

0.5

2

10

Free convection Forced convection

0

f (ξ,0) −θ (ξ,0) -

θe = −2

1.5

0

155

1

2

5

0

3

-2 2 0

1

ξ

ξ

2

3

Fig. 8. Effect of n on f 0 (n, 0) and h 0 (n, 0).

10

Er = 0.01, Ped =1 λ = 0.5, ξ = 1 Mixed convection Forced convection asympotote Free convection asympotote

−θ (ξ ,0 )

5

-

θe = −2



2

10

-3

10

-2

10

-1

0

10

1

10

2

10

Rad/Ped Fig. 9. Heat transfer results as a function of Rad/Ped.

4. Conclusions Velocity and heat transfer are significantly affected by the variable viscosity parameter, Ergun number, Peclet number or Rayleigh number. In the case of variable viscosity it is seen that for liquids, the heat transfer is more than the constant viscosity, whereas for gases, it has the opposite trend. Acknowledgement One of the authors (MK) is thankful to the University Grants Commission, India, for the financial support under the Research Scientist Scheme.

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