Boundary layer flows with internal heat generation past a horizontal plate continues to receive considerable attention because of its practical appli-cations in a broad spectrum of engineering systems like cooling of nuclear reactors, thermal insulation, combustion chamber and geothermal reservoirs.
Many principal past studies concerning natural convection flows over a semi-infinite vertical plate immersed in an ambient fluid have been found in the literature ([18], [27]). In many cases, these problems may admit similarity solutions.
The idea of using a convective boundary condition was recently intro-duced by Aziz [18], while Magyari [141] revisited this work, and obtained an exact solution for the temperature boundary layer in a compact integral form.
Bataller [23] investigated the same problem by considering radiation effects on Blasius and Sakiadis flows ([5], [30], [69], [102], [103], [128]). The effects of suction and injection have been studied by the similarity analysis by Ishak [115] and a couple of recent papers have been devoted to the subject of bound-ary layer flow with convective boundbound-ary conditions (see e.g., [11], [45], [97], [100], [101], [144], [162], [169], [193], [194], [217], [218]). The similarity solu-tions to the convective heat transfer problems for Newtonian fluids have been studied by Aziz [18] and Magyari [141] for impermeable plate and by Ishak [115] for permeable plate. Motivated by the above mentioned investigations,
we consider the heat transfer characteristics of a viscous and incompressible power-law non-Newtonian fluid over a permeable moving sheet in a uniform shear flow with a convective surface boundary condition ([45], [50]).
5.2.1 Basic equations
Fig. 5.13 Representation of the boundary layer velocity
We consider a uniform laminar flow of an incompressible viscous fluid with constant velocity U∞ at high Reynolds number, past a parallel porous semi-infinite plate moving with a constant velocity Uw in the direction opposite to the main stream (see Fig. 5.13). The fluid temperature is T∞ over the top surface of the flat plate. It is assumed that the bottom surface of the plate is heated by convection from a hot fluid of temperature Tf.
Within the framework of the above-noted assumptions, the governing equations of motion and heat transfer for non-Newtonian power-law flow ne-glecting pressure gradient and body forces can be described by the equations (1.1), (1.6) and (1.7) [225]:
The applicable boundary conditions for the present model are :
i.) on the plate surface y = 0 (no slip, permeable surface and convective surface heat flux)
where hf is the heat transfer coefficient, and k denotes the thermal conduc-tivity; vw(x) is the mass transfer velocity at the surface, and vw(x)>0 for injection (blowing), vw(x) < 0 for suction and vw(x) = 0 for impermeable surface. As indicated in [199], a similarity solution is possible only if the injection/suction velocity vw has an xvariation of the form xn+1−n.
ii.) matching with the free stream asy → ∞ u(x,∞) = U∞, (5.29)
T(x,∞) = T∞. (5.30)
For the uniform temperatureTw over the top surface of the plate we have the relations: Tf > Tw > T∞.
Introducing the stream function, the equation of continuity (1.1) is satisfied identically. On the other hand, we have
(5.31) ∂ψ and conditions (5.26), (5.27), (5.29) can be written as
(5.32) ψy(x,0) = 0, ψx(x,0) =vw(x), ψy(x,∞) =U∞.
Equation (5.31) with the transformed boundary conditions has the form (see Section 2.2 and [45]):
determines the transpiration rate at the surface. Then fw > 0 corresponds to suction, fw < 0 to injection and fw = 0 to impermeable surface. The dimensionless velocity components have the form
u(x, y) = U∞f0(η),
The thermal diffusivity can be defined as
(5.35) αt =ω al. [225]). Hence, from equation (5.25) we have
(5.36) u∂w Defining the non-dimensional temperature by
Θ(η) = w(x, y),
where Pr = K/ρω is the Prandtl number. The transformed boundary con-ditions for the energy equation (5.37) is
Θ0(0) =−
one can obtain
(5.39) Θ0(0) =−¯a(1−Θ(0))
under the assumption that the heat transfer coefficient hf =cx−1/(n+1).
We note that for Newtonian case it was shown in [18], [23], [115] and [210] that similarity solutions exist if hf is proportional tox−1/2. For a uniform surface temperature Θ(0) = 1 holds and from (5.39) Θ0(0) = 0. This adiabatic case has been analyzed by Magyari for Newtonian fluid [141].
Boundary condition (5.30) can be formulated as
(5.40) Θ(∞) = lim
η→∞Θ (η) = 0.
There is no exact solution to (5.33), (5.34) and (5.37), (5.39), (5.40), therefore we solve the boundary value problems associated with the similarity problems numerically to examine the behavior of the solutions.
5.2.2 Numerical results
The symbolic algebra software Maple 12 was used to solve the nonlinear ordinary differential equation (5.33) subject to the boundary conditions in (5.34) by applying the Runge-Kutta-Felhberg fourth-fifth method. On the flow and thermal fields the influence of the governing parameters, the Prandtl number, the power-law indexn, the convective parameter ¯aand the constant value fw characterizing the transpiration rate at the surface is discussed.
Fig. 5.15 shows the Maple generated numerical solution to the velocity profiles for different values of n. The velocity gradient at the surface, which represents the skin friction coefficient, increases with increasing n (see [38]) and also with increasing fw (see Fig. 5.14 and [115]). Suction thins the boundary layer and increases the wall slope. Hence, the wall shear stress is higher for suction compared to injection. Blowing thickens the boundary layer and make the profile S-shaped.
For fixed Prandtl numbers 0.72 and 50, for selected values of the power index n, for a range of parameters ¯a, for fw =−1; 0; 2 the numerical data for −Θ0(0) and Θ(0) were calculated [45]. Fig. 5.16 shows the temperature profiles for different Prandtl numbers and Fig. 5.17 for different values of n.
Fig. 5.16 represents that the heat transfer rate at the surface is higher as Pr is increasing. Moreover, the heat transfer rate at the surface is higher for dilatant fluids (n >1) than for pseudoplastics (n < 1).
Fig. 5.14 The profiles of f0(η) for different values of fw
Fig. 5.15 The profiles of f0(η) = u(x, y)/U∞ for different values of n
Fig. 5.16 Temperature profiles for different values of P r when n= 0.5, fw = 0 and ¯a=1
Fig. 5.17 Temperature profiles for different values of n when P r= 10, fw = 0 and ¯a=1
In case of Newtonian fluid (n = 1), the numerical values show a good agreement with those reported by Aziz [18] and Ishak [115].
Fig. 5.19 exhibits that that the heat transfer rate at the surface is higher for suction and smaller for injection. This is due to the fact that the surface shear stress increases for suction. Fig. 5.18 shows the numerical solutions for different values of ¯a when P r= 0.72 for a pseudoplastic fluid with n= 0.5.
We see that the surface temperature increases as ¯a increases.
Fig. 5.18 Temperature profiles for different values of ¯a when P r= 0.72, fw = 0 and n = 0.5
Fig. 5.19 Temperature profiles for different values of fw when n= 0.5, Pr = 1 and ¯a= 0.2
6 Conclusions
Due to the practical necessity, it is important to study the influence of the non-Newtonian behavior on the lubrication velocity and temperature fields.
Within the thin boundary layer, the wall shear stress and the friction drag of the surface can also be estimated.
This dissertation is concerned to derive useful information in the bound-ary layer by calculating the velocity and the temperature distributions, and predict the drag coefficients for non-Newtonian power-law fluids.
The main results of the dissertation are listed below:
1. Applying a similarity transformation, the boundary layer governing equations (2.17) and (2.18) for the two-dimensional steady flow of an incom-pressible, non-Newtonian power-law fluid flow along a stationary, horizontal plate situated in a fluid stream moving with constant velocityU∞have been re-duced to an ordinary differential equation called generalized Blasius equation (2.25). For non-Newtonian fluid flows using a modified version of T¨opfer’s method, instead of the boundary value problem (2.25) (2.26) an initial value problem (2.31)-(2.33) has been solved to determine the non-dimensional ve-locity gradient f00(η). The influence of the power exponent n on the velocity components has been examined. From the velocity profilesf0(η) =u(x, y)/U∞
andv(x, y)/v∗(x) = ηf0(η)−f(η), we have concluded that the boundary layer thickness decreases as n increases (Figs. 2.2-2.3). The non-dimensional ve-locity gradient f00(η) is decreasing from a positive f00(0) = γ at the wall to zero outside the viscous boundary layer (Fig.2.4). It was observed that the rate of decrease is greater with increasing the value n (Fig.2.4). I found that the effect of power n on f00(0) is significant (Fig.2.5); it is decreasing up to n ≈0.7 and after it is monotonically increasing [38], [51], [47].
2. It was shown that there exists a series solution of the form f(η) = η2
∞
P
k=0
akη3k to the generalized Blasius problem (2.25), (2.26), where the first three coefficients are given by
a0 = γ
2, a1 =− γ3−n
5!n(n+ 1), a2 = γ5−2n(21−10n) 8!n2(n+ 1)2 ,
and for the further coefficients the recursive formula (2.43) was given. The radius of convergence of the power series can be calculated by (2.45). The numerical simulations exhibit that the radius of convergence is significantly increasing with increasing power exponent n [38].
3. From the continuity equation (2.17) and momentum equation (2.18) for a non-Newtonian power-law fluid flow with fluid velocity U∞ = ˜Byσ, a
boundary value problem has been derived applying the similarity transforma-tion method. The basic equatransforma-tions are subjected to the boundary conditransforma-tions in (2.46) and are transformed to (2.56), (2.57). If n 6= 2, the velocity com-ponents are expressed with similarity variables in (2.60) and (2.61). The similarity solutions are determined in power series form and the recursive formula (2.67) has been obtained for the determination of the coefficients.
Numerical calculations were obtained for some values of n (0.5; 1; 1.5) and for different values of σ (−1/2; −1/3; 0) (see Figs.2.7-2.12). On the base of simulations, it was observed that with increasing the power exponent n, the boundary layer thickness and the parameter[f00(0)]ninvolved in the wall shear stress are decreasing both for σ = 0 and σ =−1/2. For the non-Newtonian power-law fluids when n6= 2, my results [42], [46] generalize Cossali’s results obtained for the Newtonian case [70].
4. For permeable and non-permeable surface moving with velocity Uw(x) = Axκ in an otherwise quiescent fluid medium, the Crane’s solution [72], and Gupta and Gupta’s solutions [96] are generalized into the exponen-tial series form f(η) =α(A0+P∞
i=1Ai ai e−αiη), where α >0, and A0 = 1, Ai (i = 1,2, ...) denote the coefficients. A method was presented for the de-termination of the coefficients when the surface is impermeable or permeable.
The values of f00(0) involved in the wall shear stress
(6.1) τw = have been calculated for each case (see Tables 3.1-2). [43]
5. The fluid flow properties over an impermeable flat plate moving with a constant velocity Uw in an otherwise quiescent fluid medium are examined.
The boundary layer equations (2.17), (2.18) are considered with the bound-ary conditions given in (3.15). The similarity solutions satisfy the equation (2.25) with boundary conditions
f(0) = 0, f0(0) = 1, lim
η→∞f0(η) = 0.
The simulations were carried out for pseudoplastic media. It was observed that the skin friction parameter in absolute value, the value of [−f00(0)]n and the boundary layer thickness decrease as the power exponentnincreases [39].
6. According to our simulations of the flow characteristics in a uniform mainstream U∞ over a surface moving with velocity Uw in the direction op-posite to that of main stream, it was observed that similarity solution exists
only if the velocity ratio λ = Uw/U∞ < λc. An iterative method was de-termined for the solution of the boundary value problem (2.25), (4.10) to evaluate the skin friction parameter f00(0) for different values of n and λ. It was shown that the upper bound λc increases asnincreases (see Fig.4.3). On the base of numerical simulations, we represented how [f00(0)]n changes with λ for different power exponents n (see Fig.4.2) [53]. For some values of λ, it was observed that f00 is strictly monotonically decreasing for negative values of λ while for positive λ it takes its maximum in the boundary layer. Upper bounds were given for λc[44], thus generalizing the results of Hussaini, Lakin and Nachman [111].
7. The similarity solutions are compared with numerical simulations ob-tained by using the commercial code ANSYS FLUENT when a flat surface is moving parallel to an ambient stream of a power-law fluid media. Instead of the boundary layer equations (1.1), (1.6), the system of full equations (1.1), (1.4), (1.5) is considered, where the apparent viscosity is calculated by the re-lationship (1.12). In our computations, the coupled scheme for the pressure and the velocity is applied. Comparing the theoretical (similar) velocity solu-tion u/U∞ with the numerical solutions obtained by ANSYS FLUENT, satis-factory agreement has been found. Therefore, the similarity solutions verify the numerical simulations calculated by ANSYS FLUENT. Moreover, the numerical pressure and velocity distributions prove the validity of Prandtl’s boundary layer assumptions.
8. Assuming that the solid surface is impermeable, the surface tension varies linearly with the temperature and the interface temperature is a power-law function of the distance along the surface, the Marangoni effect has been investigated for Newtonian fluid flow. The power in the temperature gradient was denoted by m with minimum value -1, which corresponds to no tempera-ture variation on the surface and no Marangoni induced flow. The similarity solution has been determined in exponential series form. For m = 1, our solution is the same as Crane’s solution [72]. Applying solutions of f, the temperature profiles were generated in series form and the influence of mand the Prandtl number Pr was investigated. It was observed that f0 decreases with increasing m. The thermal boundary layer thickness increases with in-creasing m, or Pr. From the temperature profiles, it is observed that for low Prandtl number the temperature decreases as Pr increases and for high Prandtl numbers the influence of Pr is opposite [48].
9. The boundary layer flow with internal heat generation past a hori-zontal surface has been investigated. The heat transfer characteristics of a
viscous and incompressible power-law non-Newtonian fluid over a permeable moving sheet in a uniform shear flow with a convective surface boundary condition were examined using the similarity method ([45], [50]). Both the hydrodynamic and thermal boundary layer thickness increase as λ increases, or Pr decreases, or n decreases. Our calculations indicate that the velocity gradient at the surface, which is involved in the wall shear stress and in the drag coefficient, increases with increasing nand also with increasingfw which characterizes the transpiration rate at the surface. Suction thins the thermal boundary layer and increases the wall slope. Blowing thickens the boundary layer and make the profile S-shaped. The heat transfer rate at the surface is higher for suction and smaller for injection. Moreover, the heat transfer rate at the surface is higher for dilatant fluids than for pseudoplastics. For a Newtonian fluid, our numerical results are in good agreement with those reported by Aziz [18] and Ishak [115].
7 Acknowledgement
My kindest thanks belongs to my co-authors Ondrej Dosly (Masary Univer-sity, Plzen, Czech Republic), Pavel Dr´abek (University of West Bohemia, Brno, Czech Republic), Siavash Sohrab (Northwestern University, Evanston IL, USA), Mikl´os Ront´o (University of Miskolc), K´alm´an Marossy (Univer-sity of Miskolc), Erika Rozgonyi (Univer(Univer-sity of Miskolc), Imre Gombk¨ot˝o (University of Miskolc), J´anos Kov´acs (Institute of Materials and Environ-mental Chemistry, Chemical Research Center, Hungarian Academy of Sci-ences), Kriszti´an Hricz´o and Zolt´an Cs´ati (University of Miskolc), for their essential contribution to our joint papers, and their helpful advice in my field of research. I wish to express my deepest gratitude to ´Arpad Elbert and Mikl´os Farkas, for his inspiring attitude, never-ending optimism and continuous encouragement.
I would also like to thank Ibolya Hap´ak for the careful revision of the language of this work.
I owe my most sincere thanks to my family for their patience, and con-tinuous support that they have always shown to me.
This research was (partially) carried out in the framework of the Center of Excellence of Mechatronics and Logistics at the University of Miskolc.
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