Second Law Analysis for Couple Stress Fluid Flow through a Porous Medium with Constant Heat Flux

In the present work, entropy generation in the flow and heat transfer of couple stress fluid through an infinite inclined channel embedded in a saturated porous medium is presented. Due to the channel geometry, the asymmetrical slip conditions are imposed on the channel walls. The upper wall of the channel is subjected to a constant heat flux while the lower wall is insulated. The equations governing the fluid flow are formulated, non-dimensionalized and solved by using the Adomian decomposition method. The Adomian series solutions for the velocity and temperature fields are then used to compute the entropy generation rate and inherent heat irreversibility in the flow domain. The effects of various fluid parameters are presented graphically and discussed extensively.


Introduction
Flow and heat transfer with constant heat flux through a porous medium has been a major challenge in several geological, medical, thermal problems and engineering applications.For instance, in the adsorption/water treatment processes, ground water flows, breathing filters, oil recovery, constructions of roads and buildings, as dryers and many more metallurgical and geothermal utilizations.Given the broad applications, several works have been reported in this active area of research.For example, Cimpean et al. [1] reported the convective fluid flow down an inclined plane subjected to constant heat flux.Hajipour and Dehkordi [2] examined the nanofluid channel flow that is partially filled with porous material.Mahmoudi [3] analyzed the forced convective flow subjected to constant heat flux and flowing through a nanochannel that is immersed in a porous medium with velocity slip and temperature jump conditions.In the work of Mahdavi et al. [4], the entropy generation in pipe flow with partial porous materials exposed to constant heat flux was investigated.Cimpean and Pop [5] discussed the steady flow of nanofluid in an inclined channel with a porous material and exposed to constant heat flux.Torabi et al. [6] presented a robust analysis of bifurcation problem associated with entropy minimization in a two-phase channel flow with partial porous medium and constant heat flux.For more results of flows in porous media, kindly see the following books [7][8][9][10][11].
From a practical applications point of view, fluid viscosity is highly sensitive to changes in temperature, especially when subjected to uniform heat flux.As reported by Sahin [12], by increasing the temperature of engine oil from 20 • C to 80 • C, its viscosity decreases by 24 times; water viscosity drops by 2.7 times while that of air reduces by 1.4 times.Therefore, for optimal performance in a thermo-fluid setup, there is need to introduce size-dependent polymer additives of mechanical significance that could sustain the rheological properties of these fluids needed at extremely high temperature.The classical Navier-Stokes theory does not support the inclusion of the microstructures or additives.However, following Stokes [13] couple stress theory, he elaborated the presence of body couples, couple stresses and non-symmetric stress tensor in a moving base fluid.Common examples of couple stress fluid are blood, lubricants, pharmaceutical mixtures, paints, synovial fluids and many more applications that contain tiny microstructures.By using the couple stress model, Srinivasacharya and Kaladhar [14] explained the hydromagnetic couple stress fluid flow through parallel disk taking the Hall and ion-slip effects into consideration.Ahmed et al. [15] applied the couple stress model to explain the magnetohydrodynamics (MHD) oscillatory flow in a rotating inclined channel.Akhtar and Shah [16] obtained an exact solution for unsteady flow of couple stress fluid in a channel.Recently, Aksoy [17] examined the couple stress fluid through a channel subjected to constant heat flux.More related studies on the theory and wide range of applications on the couple stress fluid model are documented in studies by Srinivasacharya and his collaborators in [15][16][17][18][19][20][21][22], Bég and his cohorts [23,24], and Hayat et al. [25,26] and references therein.
From an energy management perspective, it is well known that the performance of any thermo-fluid engineering set-up depends on the design and the operating temperature of the working fluid since heat transfer is an irreversible process.Therefore, for optimal performance, it is imperative to monitor the entropy generation in the flow channel.Recent findings have shown that the second law analysis approach is a reliable and efficient method for minimising entropy generation in a moving fluid.For instance, Ting et al. [27] utilized this method to describe the inherent irreversibility in Al 2 O 3 nanofluid flow through heated leaky micro-channel.Anand [28] discussed the entropy production nanofluids through a heated tube.In [29], Chen et al. monitored the entropy generation rate in a buoyancy-induced nano fluid flow with frictional effects.López et al. [30] studied the entropy generation in radiative MHD nanofluid in a leaky vertical microchannel.In [31], Havzali et al. investigated the inherent irreversibility in a Newtonian gravity-driven flow through an inclined channel.Other related works on the second law analysis approach for minimizing entropy generation includes [32][33][34][35][36]. Interestingly, there is a thin line between the second law analysis approach and that described by the material cost flow accounting perspective [37][38][39] regarding waste management and achieving a cleaner production.Motivated by Hayat et al. [31,[40][41][42][43][44][45][46], the present study addresses the second law analysis for a shear-induced asymmetrical slip couple stress fluid flow through saturated porous medium subjected to constant heat flux studied.To the best of our author's knowledge, the thermal analysis reported here has not been investigated despite the enormity of the work done in this area of study.In the following section, the problem will be formulated and nondimensionalized.In Section 3, solutions of the dimensionless boundary-valued-problems will be obtained by using the rapidly convergent computational Adomian decomposition method [47][48][49] implemented in Mathematica 10.0.In Section 4, results are presented and discussed extensively, and the contributions to knowledge are presented in Section 5.

Model Formulation
Consider the steady flow of couple stress fluid through a parallel inclined channel of distance 2h apart as shown in Figure 1 below.The channel is further assumed to be filled up with porous materials of constant porous permeability.The upper wall of the channel is subjected to constant heat flux while the lower wall is insulated.Therefore, the equation governing the gravity-driven steady fully developed couple stress fluid flow can be written as [17]: where , and the additional term in (1) is due to the inclusion of porous materials that tend to restrict the flow.The slip boundary conditions along the channel walls are given by Devakar et al. [46] as: For the thermal analysis, the lower wall of the channel is taken to be insulated while the upper plate is subjected to constant heat flux w q .Following Aksoy [17], the energy conservation equation for the chemically-inert porous medium can then be written as: Together with the constant heat flux condition and the insulated wall condition: The upper wall of the channel is subjected to constant heat flux while the lower wall is insulated.Therefore, the equation governing the gravity-driven steady fully developed couple stress fluid flow can be written as [17]: where 0 < ϑ < 90 0 , and the additional term in ( 1) is due to the inclusion of porous materials that tend to restrict the flow.The slip boundary conditions along the channel walls are given by Devakar et al. [46] as: For the thermal analysis, the lower wall of the channel is taken to be insulated while the upper plate is subjected to constant heat flux q w .Following Aksoy [17], the energy conservation equation for the chemically-inert porous medium can then be written as: Together with the constant heat flux condition and the insulated wall condition: respectively.In the fully developed flow situation, the momentum equation is independent of x as a result,

∂T(x,y) ∂x
= constant.Thus, there is need to evaluate the mass flow rate by integrating (3) along the y-direction in the form: By using (4), Equation ( 5) becomes With ( 6) in (3), we get an energy equation for the couple stress fluid flow as follows: together with the non-uniform boundary conditions Introducing the following dimensionless variables and parameters we get the following dimensionless equations: subject to shear-induced and non-uniform wall boundary conditions

Adomian Method of Solution
To obtain the solution of the dimensionless Equations ( 10)-( 12), we first write the differential equations in the integral forms as follows: Entropy 2017, 19, 498 Observe that, to arrive at the integral equations in ( 13) and ( 14) above, the following boundary conditions have been used; and we now form an infinite series of the form: Evidently, by substituting ( 15) in ( 13) and ( 14), we get the following recurrence relations: Obtaining a few terms in ( 16), the partial sum provides the approximate solution and is used to obtain expressions for the undetermined constants dY 3   regarding the other boundary conditions remaining in (5).The expressions for the constants above are obtained by using the following boundary conditions: while the condition θ(1) = 1 was used to derive the unknown constant in (14).The convergent series solution is then used to determine the solution of ( 14) together with the thermal boundary conditions.Next, we code the scheme in a computer algebra package known as Mathematica 10.0 for easy iteration of the successive approximant ( 16) and (17), and the graphical results are presented as Figures 2-5.
The skin friction and the heat transfer rate are determined by

Entropy Generation Analysis
Due to the heat irreversibility in the flow channel, the expression for the entropy generation rate due to heat transfer and particles frictional interactions in the inclined channel can be written as: In dimensionless form, we get Entropy 2017, 19, 498 The first term of Equation ( 21) represents the heat irreversibility due to heat transfer while other terms described the thermal inefficiency are due to fluid friction irreversibility.In other words, let Then, Bejan number is defined as the ratio of irreversibility due to heat transfer and the total entropy generated within the flow channel, i.e., Observe that Bejan number is bounded between zero and unity.In an exceptional case, when both heat transfer and frictional irreversibilities have equal contributions, Be attains 0.5.

Discussion
In this section, graphical results are plotted and discussed based on the fluid physics for dimensionless velocity, temperature distribution, entropy generation and heat irreversibility.Table 1 attests to the rapid convergence of the series solutions.In achieving this, the boundary conditions are used to evaluate the undetermined coefficient a i s , and it is observed that convergence is reached with just a few terms of the series.Another significant result from the table is the convergence of the Nusselt number and Skin friction.Table 2 shows the comparison of the present results with previously obtained results in a particular case when β = 0, and the result shows a perfect agreement in the absence of the porous permeability parameter.Figure 2a shows the effect of variation in the lower wall slip parameter.It is observed that an increase in the slip parameter enhances the fluid flow at the lower wall of the channel while an increase in the upper wall slip is seen to elevate the flow velocity at the upper wall of the channel as presented in Figure 2b.In Figure 2c, an increase in the porous permeability parameter is observed to reduce the fluid flow velocity; this is physically true since an increase in the porous permeability parameter implies a reduction in the porous permeability of the medium.Figure 2d depicts the influence of the couple stress parameter on the fluid flow: it is observed that an increase in the parameter represents a decrease in the fluid viscosity, hence, as the fluid viscosity increases, the fluid flow velocity decreases.Figure 2a shows the effect of variation in the lower wall slip parameter.It is observed that an increase in the slip parameter enhances the fluid flow at the lower wall of the channel while an increase in the upper wall slip is seen to elevate the flow velocity at the upper wall of the channel as presented in Figure 2b.In Figure 2c, an increase in the porous permeability parameter is observed to reduce the fluid flow velocity; this is physically true since an increase in the porous permeability parameter implies a reduction in the porous permeability of the medium.Figure 2d depicts the influence of the couple stress parameter on the fluid flow: it is observed that an increase in the parameter represents a decrease in the fluid viscosity, hence, as the fluid viscosity increases, the fluid flow velocity decreases.Figure 3 shows the effect of variation of parameters on the temperature distribution in the flow channel.In Figure 3a, the effect of the rise in the lower slip parameter is presented.As observed from the figure, an increase in the lower slip is seen to decrease the fluid temperature in the region closer to the insulated plate, while it enhances the temperature of the fluid particles closer to the upper wall exposed to constant heat flux.The reverse phenomenon is experienced in Figure 3b as the parameter of higher slip increases.One observes an asymmetrical thermal structure in the two cases.The effect of the couple stress inverse parameter is shown in Figure 3c.From the plot, it is noticed that an increase in the couple stress inverse parameter enhances the fluid temperature due to a reduction in the dynamic viscosity of the fluid.Finally, as the porous permeability parameter increases, a decrease in the fluid temperature is observed.This is due to an increase in the permeability of the porous bed. Figure 3 shows the effect of variation of parameters on the temperature distribution in the flow channel.In Figure 3a, the effect of the rise in the lower slip parameter is presented.As observed from the figure, an increase in the lower slip is seen to decrease the fluid temperature in the region closer to the insulated plate, while it enhances the temperature of the fluid particles closer to the upper wall exposed to constant heat flux.The reverse phenomenon is experienced in Figure 3b as the parameter of higher slip increases.One observes an asymmetrical thermal structure in the two cases.The effect of the couple stress inverse parameter is shown in Figure 3c.From the plot, it is noticed that an increase in the couple stress inverse parameter enhances the fluid temperature due to a reduction in the dynamic viscosity of the fluid.Finally, as the porous permeability parameter increases, a decrease in the fluid temperature is observed.This is due to an increase in the permeability of the porous bed.
Figure 4 described the influence of fluid parameters on the entropy generation in the flow channel.As observed in Figures 2a and 3a, the lower slip parameter at the insulated wall enhances the velocity profile, and this increase resulted in a decreased fluid temperature in Figure 3a.The net effect of the increased lower slip parameter on flow and heat transfer is seen on the entropy generation rate that is presented as Figure 4a.As observed, entropy generation is slightly higher in the entire flow region except at the upper region with constant heat flux where it falls.The reverse phenomenon is seen as the upper slip parameter is varied.channel.In Figure 3a, the effect of the rise in the lower slip parameter is presented.As observed from the figure, an increase in the lower slip is seen to decrease the fluid temperature in the region closer to the insulated plate, while it enhances the temperature of the fluid particles closer to the upper wall exposed to constant heat flux.The reverse phenomenon is experienced in Figure 3b as the parameter of higher slip increases.One observes an asymmetrical thermal structure in the two cases.The effect of the couple stress inverse parameter is shown in Figure 3c.From the plot, it is noticed that an increase in the couple stress inverse parameter enhances the fluid temperature due to a reduction in the dynamic viscosity of the fluid.Finally, as the porous permeability parameter increases, a decrease in the fluid temperature is observed.This is due to an increase in the permeability of the porous bed.effect of the increased lower slip parameter on flow and heat transfer is seen on the entropy generation rate that is presented as Figure 4a.As observed, entropy generation is slightly higher in the entire flow region except at the upper region with constant heat flux where it falls.The reverse phenomenon is seen as the upper slip parameter is varied.Figure 5a,b represent the effect of the slip parameter on the heat irreversibility ratio.In Figure 5a, as the lower slip parameter increases, there is a rise in heat generated by fluid friction, hence fluid friction irreversibility dominates over heat transfer irreversibility as the lower slip parameter increases.A similar explanation holds for the results presented in Figure 5b.In Figure 5c, we observed that, as the couple stress inverse increases, the fluid viscosity decreases, and both fluid friction and  Figure 5a,b represent the effect of the slip parameter on the heat irreversibility ratio.In Figure 5a, as the lower slip parameter increases, there is a rise in heat generated by fluid friction, hence fluid friction irreversibility dominates over heat transfer irreversibility as the lower slip parameter increases.A similar explanation holds for the results presented in Figure 5b.In Figure 5c, we observed that, as the couple stress inverse increases, the fluid viscosity decreases, and both fluid friction and heat transfer contribute to the heat irreversibility.However, as the fluid viscosity increases, fluid friction decreases and heat transfer irreversibility dominates over irreversibility due to fluid friction.Finally, as the porous permeability parameter increases, the fluid velocity and frictional interaction decreases; therefore, heat transfer irreversibility dominates over fictional irreversibility, as seen in Figure 5d.

Conclusions
Entropy generation in the slip flow and heat transfer in the couple stress fluid through a parallel plate subjected to constant heat flux and saturated with porous materials are investigated.A rapidly convergent Adomian decomposition method is applied to obtain the approximate solution of the coupled problem.The influence of lower and upper slip parameters, couple stress inverse parameter and porous permeability parameter on the velocity, temperature, entropy generation production and the Bejan number are examined and discussed physically.The results show the influence of the couple stresses and porous permeability in minimizing entropy within the flow channel.It is important to note that the present analysis is limited to micro-scale thermal analysis, and subsequent analysis will incorporate a macroscale thermal description as presented in [50].

Entropy 2017, 19 , 498 8 of 13 Figure 2 .
Figure 2. Velocity profile: (a) effect of lower slip parameter; (b) effect of upper slip parameter; (c) effect of porous permeability parameter; (d) effect of the couple stress inverse parameter.

Figure 2 .
Figure 2. Velocity profile: (a) effect of lower slip parameter; (b) effect of upper slip parameter; (c) effect of porous permeability parameter; (d) effect of the couple stress inverse parameter.

Figure 3 .
Figure 3. Temperature profile (a) effect of lower slip parameter; (b) effect of upper slip parameter; (c) effect of couple stress inverse parameter; (d) effect of porous permeability parameter.

Figure 4 Figure 3 .
Figure 4 described the influence of fluid parameters on the entropy generation in the flow channel.As observed in Figures 2a and 3a, the lower slip parameter at the insulated wall enhances the velocity profile, and this increase resulted in a decreased fluid temperature in Figure 3a.The net Figure 3. Temperature profile (a) effect of lower slip parameter; (b) effect of upper slip parameter; (c) effect of couple stress inverse parameter; (d) effect of porous permeability parameter.

Figure 4 .
Figure 4. Entropy generation (a) effect of lower slip parameter; (b) effect of upper slip parameter; (c) effect of porous permeability parameter; (d) effect of couple stress inverse parameter.

Figure 4 .
Figure 4. Entropy generation (a) effect of lower slip parameter; (b) effect of upper slip parameter; (c) effect of porous permeability parameter; (d) effect of couple stress inverse parameter.