Abstract
The encapsulation technique of phase change materials in the nanodimension is an innovative approach to improve the heat transfer capability and solve the issues of corrosion during the melting process. This new type of nanoparticle is suspended in base fluids call NEPCMs, nanoencapsulated phase change materials. The goal of this work is to analyze the impacts of pertinent parameters on the free convection and entropy generation in an elliptical-shaped enclosure filled with NEPCMs by considering the effect of an inclined magnetic field. To reach the goal, the governing equations (energy, momentum, and mass conservation) are solved numerically by CVFEM. Currently, to overcome the low heat transfer problem of phase change material, the NEPCM suspension is used for industrial applications. Validation of results shows that they are acceptable. The results reveal that the values of descend with ascending Ha while has a maximum at . Also, the value of increases with ascending . The values of and depend on nondimensional fusion temperature where good performance is seen in the range of . Also, increases 19.9% and ECOP increases 28.8% whereas descends 6.9% when ascends from 0 to 0.06 at . decreases 4.95% while increases by 8.65% when increases from 0.2 to 0.7 at .
1. Introduction
Nanofluids can be produced by adding the nanosized particles (such as copper and silver) into the base fluids (i.e., oils, water, and so on). Many researchers scrutinized the effects of conventional nanofluids on fluid flow in different cases [1–14]. Recently, a new type of nanofluids has been constructed using phase change materials (PCMs) which are known as NEPCMs. As we know, PCMs have various applications such as energy-efficient buildings [15], cooling of electronic equipment [16], waste heat recovery [17], ventilation systems [18], and solar energy storage [19]. Although PCMs are used in many industrial applications, the low thermal conductivity of PCMs is a disadvantage for systems with charge and discharge cycles. Some techniques were proposed to overcome this problem, such as inserting the fins in the PCM enclosure [20, 21], carbon nanotubes in the PCM [22], and using the multilayer PCMs [23, 24]. Furthermore, microencapsulated phase change materials (MPCMs) are used for thermal energy storage [25].
In 2004, the melting process of a PCM was analytically investigated by Hamdan and Al-Hinti [26], where a constant heat flux was imposed on the vertical wall. In 2016, the melting of PCM in a cylindrical medium was studied experimentally and numerically by Azad et al. [27]. In 2017, simulation of melting of a PCM for thermal energy storage was performed by Vikas et al. [28]. They modeled the melting of a rectangular PCM domain using ANSYS (Fluent). In 2017, a simplified model for melting of a PCM in the presence of radiation and natural convection was proposed by Souayfane et al. [29]. They found that natural convection has an important role during the PCM melting process. In 2018, Selimefendigil et al. [30] studied the free convection of CuO–water nanofluid in an enclosure where a PCM and a conductive partition were attached to its vertical wall by the finite element method. Hosseini et al. [31, 32] studied heat transfer in a horizontal shell-and-tube heat exchanger with a PCM. In 2018, characterizations of natural convection in vertical cylindrical shell-and-tube latent heat thermal energy storage (LHTES) systems were numerically and experimentally investigated by Seddegh et al. [33]. In 2018, Jmal and Baccar [34] studied the PCM solidification numerically where vertical fins were installed in a rectangular module. In 2019, Ghalambaz et al. [35] concluded that fusion temperature defines a dynamic behavior for NECPMs. In a similar work, Hajjar et al. [36] reached to the previous result in analyzing the transient fluid flow in a cavity filled by NEPCM. In 2020, Hasehmi-Tilehnoee et al. [37] studied natural convection and entropy generation in a complex medium filled with nanofluid and NEPCM suspension. They used ANSYS-Fluent to solve the nondimensional form of the governing equations. Different methods have been employed to solve the governing equations such as finite difference method (FDM) [38–40], finite element method (FEM) [41, 42], control volume finite element method (CVFEM) [43–48], and homotopy perturbation method (HPM) [49–51].
In this work, the entropy generation and natural convection are scrutinized in a medium located between concentric horizontal wavy-circular wall and elliptical enclosure filled by NEPCMs suspension.
Preference of this study regarding the earlier studies can be summarized as follows:(1)A complex porous medium enclosure filled with NEPCM suspension is considered(2)An inclined magnetic field is applied to the fluid flow(3)The effects of decision parameters such as , and (see the Nomenclature) are investigated on heat transfer and entropy generation(4)The ratio of heat transfer to entropy generation is evaluated
2. Problem Definition
2.1. Physical Model
The porous medium shown in Figure 1 consists of a heated horizontal wavy cylinder at that is concentrically placed inside an elliptic enclosure held at . An external uniform magnetic field has an effect on the fluid flow. The wavy wall of the inner cylinder can be implemented with the following equation:

2.2. Mathematical Model
2.2.1. Dimensionless Governing Equations
The fluid flow is supposed to be steady, incompressible, two-dimensional, and laminar with no radiation effect. The fluid properties are constant but Boussinesq approximation is applied to the density in the buoyancy term. The dimensionless conservation equations are as follows [14, 35]:where , , , Ha, and are given as follows:where
The heat capacity ratio can be rewritten in the nondimensional form as follows [35]:
Also, is the nondimensional fusion function defined by the following equation [52]:
The boundary conditions of the system are as follows:
The average Nusselt number can be calculated using the local Nusselt number along the hot wall as follows:
2.2.2. Entropy Generation
The performance of energy systems has been evaluated by equation (11) which is named as the rate of entropy generation as follows:
The local entropy generation in nondimensional form can be rewritten as follows [14]:
The total entropy generation and the entropy generation number can be obtained, respectively, as follows:where denotes the HT, FF, PM, and MF. For more details, see Refs. [13, 14].
Furthermore, the ecological coefficient of performance (ECOP) is calculated by the following [53]:
3. Numerical Procedure
A FORTRAN code based on the control volume finite element method (CVFEM) has been extended to solve the governing equations.
3.1. Grid Test
The results of the grid independence study are provided in Table 1. The calculations have been performed with , , , , , , , , , , , and . Hence, 81 × 971 can be considered as the grid size.
3.2. Validation
In order to validate the results, two validations are performed. Firstly, the isotherms, streamlines, and Cr contour for a square cavity are obtained for at . Figure 2 shows the results of the present code and those of Ghalambaz et al. [35]. Secondly, the values of average Nusselt number are calculated for several cases that Table 2 presents the outcomes. Other parameters are constant such as , , , and . For both validations, the results are acceptable.

(a)

(b)
4. Results and Discussion
In this paper, the second law of thermodynamics and the free convection heat transfer are numerically studied in a medium between concentric horizontal wavy-circular and elliptical cylinders loaded with a dilute suspension by CVFEM. The effects of nondimensional parameters on the characteristics of the flow and entropy generation number are considered. The effect of an external inclined magnetic field on the fluid flow is specified by Hartmann number (Ha) and angle of the magnetic field . Darcy number (Da) is used for modeling porous medium. Some decision variables are employed for modeling of NEPCM such as , , , , , ,, and . Also, the geometry of enclosure is specified by . The default values of the nondimensional parameters for calculations are presented in Table 3.
Figure 3 shows the isotherms, the streamlines, and contours at , and . The power of the flow increases with the Rayleigh number as it is obvious from the maximum value of the absolute stream function, . The values of are 0.44, 3.86, and 16.93 for , and , respectively. The isotherms are concentric circles around the hot wall at . The convective heat transfer is predominant in this case, while the role of convection in heat transfer becomes more significant with increasing the Rayleigh number. The red lines in isotherms go up from the upper section of the inner wall to the outer wall at . The average Nusselt number ascends with ascending the Rayleigh number where the values of are 2.5784, 2.9136, and 5.8065, for , and , respectively.

The last row in this figure presents the contours for each Rayleigh number. According to equations (7) and (8), has a minimum value that is corresponding to (other parameters are assumed to be constant). For example, for , , , and , the minimum value of is 0.97 (when ). Also, has a maximum value that depends on in the correlation. Here, the maximum value of is 5.988 (this value can be obtained by solving the velocity and temperature fields). The values of 0.97 and 5.988 are the same for each value of the Rayleigh number in this figure. Notice that the heat capacity changes since the phase change occurs. Other decision parameters for this figure are , , , , and .
Figure 4 presents the effects of the magnetic field and its angle on the entropy generation number and the average Nusselt number. Figure 4(a) shows that decreases with ascending Ha while has a maximum at . For instance, descends from 5.5496 to 3.6542 (34.15% decreasing) when Ha increases from 0 to 40. Also, increases from 42.60 to 44.06 (3.43% increasing) when Ha increases from 0 to 16 while it decreases from 44.06 to 35.43 (19.59% decreasing) when Ha increases from 16 to 40. This figure indicates that increasing the Hartmann number is desirable after . Figure 4(b) shows that firstly ascends, attains a maximum value, and then descends with ascending the angle of the magnetic field. The maximum value of is 4.9214, which occurs at . On the other hand, decreases continuously from 44.03 to 33.36 (24.23% decreasing) when increases from to . Other decision parameters were assumed to be , , for Figure 4(a) while they are , , , , without porosity for Figure 4(b).

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(b)
Figure 5 presents variations of the entropy generation number and the average Nusselt number versus the fusion temperature of the NEPCM core at and . It shows approximately a regular symmetrical behavior for due to the symmetry of both the boundary conditions and geometry. The variations of the average Nusselt number are very small in the range of , and there is a local minimum at . The maximum value of is 3.268 that occurs at and . Besides, the entropy generation number firstly decreases to a minimum value and then goes up as the fusion temperature increases. The minimum value of is 39 that occurs at although it is approximately constant in the range of . This figure discovers that the best range of is from both second law analysis and heat transfer point of view. Other decision parameters were selected as , , and .

Figure 6(a) demonstrates the variations of and versus while the values of and ECOP are calculated and plotted in Figure 6(b) as a function of . It is obvious from the figure that decreases with ascending the values of whilst the values of ascend as the values of go up. Regarding the definition of the Stefan number , which is the ratio of the sensible heat to the latent heat of the PCM, the increasing means descending the PCM core latent heat. The latter leads to a decrease in the heat storage capacity of the NEPCM particles, which concluded a lower rate of heat transfer. For instance, ascends from 4.6998 to 4.4673 (4.95% decreasing) while increases from 41.24 to 44.81 (8.65% increasing) when increases from 0.2 to 0.7 at . Figure 6(b) illustrates that the values of increase while the ECOP descends with ascending . Therefore, the figure discovers that the heat transfer rate decreases although the fluid velocity enhances with increasing the Stefan number. Also, increases 9.13% while the ECOP decreases 12.54% when increases from 0.2 to 0.7. Other decision parameters for this figure are , , , and .

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Figure 7 represents the variations of , , , and ECOP versus . As shown, increasing is desirable since and ECOP raise and decreases with ascending . Albeit, the values of decrease as increases. For example, increases from 5.4591 to 6.5446 (19.9% increasing) and ECOP ascends from 0.1273 to 0.1640 (28.8% amplified) while decreases from 17.24 to 16.34 (5.2% decreasing) and decreases from 42.88 to 39.89 (6.9% decreasing) when increases from 0 to 0.06. Regarding equation (6), decreasing the values of is justifiable since the viscosity of nanofluid increases when the values of go up. Other decision parameters were assumed to be , , , , , , and .

(a)

(b)
Figure 8(a) presents variations of and versus the Darcy number while Figure 8(b) shows variations of and ECOP against the Darcy number. From the figure, , , and ECOP ascend while the values of descend with ascending the Darcy number. For instance, increases from 6.0034 to 6.6509 (10.8% increasing), increases from 15.42 to 19.46 (26.2% increasing), and ECOP ascends from 0.1270 to 0.1428 (12.4% amplified) while decreases from 47.26 to 46.57 (1.5% decreasing) when Darcy number ascends from to at . Other decision parameters are , , , , and .

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(b)
Figure 9 depicts the streamlines, the isotherms, and the contours for different values of amplitude . The streamlines follow the shape of the inner wall, especially near the hot wall. The flow velocity decreases with increasing the amplitude parameter as it can be understood from the values of . It can be due to this fact that the available space for fluid circulation decreases with increasing the values of . The rate of heat transfer can be reflected by the shape of isotherms, where determines the value of it. The values of are 4.7929, 4.1158, and 3.4985 for , respectively. It is obvious from the figure that the values of descend with ascending the values of . Here, it can be stated that increasing the values is similar to increasing the values. Also, it should be noticed that the values of are 41.01, 40.86, and 40.31 for , respectively. Therefore, increasing the values of amplitude is desirable from the second law of thermodynamics viewpoint. The last row of this figure demonstrates the contours where the phase change region is specified by the color region. The value of is 0.964 (outside the fusion region corresponding to ) while it changes inside the fusion region where it has a maximum value of 10.389 in this figure for each value of the amplitude parameter. The contour is similar to a plume for each value of . In this figure, a horizontal magnetic field has been considered.

Figure 10 illustrates the streamlines, the isotherms, and the contours for the various numbers of undulation . As shown by the figure, the values of are 12.83, 12, 65, and 12.21 that indicate the velocity of fluid flow decreases slightly as the values of increase. The reason could be an increase in the number of obstacles in the path. The red lines in the isotherms show higher temperatures that move from the wavy wall to the enclosure wall. The values of are 5.0886, 4.6966, and 4.4593 for , respectively. So, the value of decreases 12.4% when ascends from 4 to 6. The values of are 44.07, 43.46, and 44.80 for , respectively. Thus, a minimum value can be seen for at . As shown by the contours, the width of the ribbon shape is not the same everywhere. The ribbon shape is wider where the temperature gradients are smooth. Here, we have for . Other decision parameters are , , and .

Figures 11(a)–11(d) present the contribution of the heat transfer , the fluid friction , and the magnetic field in the total entropy generation number for an enclosure without porous medium. Comparison between Figures (a) and (b) discovers that increases from 10% to 43% while decreases from 74% to 15% as the Rayleigh number attains from to . Its reason is increasing fluid flow velocity (ascending the values of ) with ascending the Rayleigh number. As we know, the friction losses enhance with enhancement of fluid flow velocity. Also, a comparison between Figures (c) and (d) indicates that ascends from 15% to 19% as ascends from 0.01 to 0.06. Regarding the first term in the right-hand side of equation (12), increasing of is justifiable. Figures 11(e)–11(h) show the contribution of the heat transfer , the fluid friction , the magnetic field , and the porous medium in . Comparison between Figures (e) and (f) reveals that increases from 22% to 49% while other type of irreversibilities decreases when the Hartmann number increases from 12 to 24. The value of increases with ascending as shown by the last term in equation (12).

5. Conclusion
The entropy generation and heat transfer analysis were investigated in a complex cavity filled with NEPCMs suspension. The nondimensional governing equations were solved by CVFEM. Here, significant outcomes are remembered as follows:(i)The values of , , and rise with ascending Ra. The values of are 0.44, 3.86, and 16.93, the values of are 2.5784, 2.9136, and 5.8065, and the values of are 3.03, 4.61, and 45.91, for , and , respectively (Figure 3). These values indicate that fluid velocity and rate of heat transfer increase with increasing the Rayleigh number.(ii)The values of descend with ascending Ha while has a maximum at . Also, there is a maximum value for at (Figure 4).(iii)Symmetrical behavior was seen for and with respect to . The maximum value of is 3.268 that occurs at and while there is a minimum value for that equals 39 and happens at (Figure 5).(iv)Lower values of are desirable because descends and ascends with (Figure 6). Indeed, as the latent heat of the PCM cores rises, the Stefan number decreases.(v) increases 19.9%, and ECOP increases 28.8% while decreases by 6.9% when enhances from 0 to 0.06 (Figure 7).(vi)Higher values of are suitable since the values of ECOP and increase while descends with ascending (Figure 8). It must be recalled that the ECOP represents the performance of the cavity.(vii)The effects of and on the streamlines, the isotherms, and the contours show that lower values of these parameters are proper (Figures 9 and 10).(viii)The contribution of ascends from 22% to 49% as goes up from 12 to 24. Also, the value of increases while the value of descends with ascending Ra (Figures 11).
Nomenclature
A: | Amplitude |
: | Magnetic field strength (T) |
: | Specific heat at constant pressure (J kg−1 K−1) |
: | The heat capacity ratio defined by equation (7) |
f: | The nondimensional fusion function defined by equation (8) |
: | Darcy number |
ECOP: | Ecological coefficient of performance |
: | Gravitational acceleration (ms−2) |
Ha: | Hartmann number |
: | Thermal conductivity (Wm−1K−1) |
: | Permeability of the medium (m2) |
L: | Gap between the inner and outer walls of the enclosure (m) |
N: | Number of undulations |
: | Entropy generation number |
: | Thermal conductivity parameter |
: | Electrical conductivity parameter |
Nu: | Nusselt number |
: | Viscosity parameter |
Pr: | Prandtl number |
Ra: | Rayleigh number |
Ste: | Stefan number |
: | Rate of entropy generation per unit volume (J s−1 K−1 m−3) |
T: | Temperature |
: | Mean temperature (K) |
: | Components of velocity (m s−1) |
X, Y: | Dimensionless coordinates |
: | Thermal diffusivity (m2s−1) |
: | Angle of magnetic field |
: | Thermal expansion coefficient (K−1) |
: | Nondimensional parameter of fusion range |
: | Dimensionless temperature |
: | Rotation angle (°) |
: | Ratio of the heat capacity of the NEPCM nanoparticles to the base fluid |
: | Dynamic viscosity (N s m−2) |
: | Kinematic viscosity (m2s−1) |
: | Density (kg m−3) |
: | Electrical conductivity (Ω/m) |
: | Temperature difference (K) |
: | Volume fraction of NEPCM nanoparticles |
: | Irreversibility distribution ratio |
: | Dimensionless stream function |
: | Dimensionless vorticity |
ave: | Average |
b: | Bulk properties of the suspension |
c: | Cold |
f: | Fluid |
FF: | Fluid friction |
gen: | Generation |
h: | Hot |
HT: | Heat transfer |
L: | Local |
max: | Maximum |
MF: | Magnetic field |
PM: | Porous medium |
s: | Solid. |
Data Availability
The data used to support this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.