Mahnoor
Sarfraz
,
Muhammad
Yasir
* and
Masood
Khan
Department of Mathematics, Quaid-i-Azam University, Islamabad 44000, Pakistan. E-mail: myasir@math.qau.edu.pk
First published on 10th November 2023
Hybrid nanofluids show great potential for heat transport applications such as solar thermal systems, car cooling systems, heat sinks, and thermal energy storage. They possess better thermal stability and properties compared to standard nanofluids. In this study, a base fluid, methanol, is injected into an electrically conducting heat-generating/absorbing disk of permeable boundary, and dual solutions are obtained. Two alternative models, Xue and Hamilton–Crosser are considered, and their thermal conductivities are contrasted. Furthermore, thermal radiation and ohmic heating are also considered, and convective boundary conditions are utilized to simulate overall heat gains or losses resulting from conduction, forced or natural convection between nearby objects of nearly constant temperature. Using a similarity transform, the governing equations are obtained and numerically solved via bvp4c, a finite difference method. It is observed that the presence of a magnetic field and the shrinking of the disk elevate the energy transport rate and wall stress. Additionally, the skin friction coefficient and thermal distribution rate increase with wall transmission constraint while fluid flow and energy transport diminish. Furthermore, particle clustering and nano-layer creation suggest that the Hamilton–Crosser model exhibits better thermal conductivity than the Xue model.
Convective heat transfer models can present non-linearity issues and dual solutions, which can result in complex behaviors. Therefore, it is essential to understand both stable and unstable states. To address this, several studies have been conducted on different scenarios. For instance, Zheng et al.13 explored the radiation effect on velocity and temperature fields in a quiescent micropolar fluid with nonlinear power-law surface velocity and temperature distributions. In a similar study, Mahapatra et al.14 investigated dual solutions in magnetohydrodynamic stagnation-point flow and heat transfer over a shrinking surface with partial slip, while Rostami et al.15 developed an analytical solution for the steady laminar MHD mixed convection boundary layer flow of a SiO2–Al2O3/water hybrid nanofluid near the stagnation point on a vertical flat plate. Moreover, Mousavi et al.16 studied the dual solutions for Casson hybrid nanofluid flow due to a stretching/shrinking sheet with suction, radiation, and convective boundary condition effects. Asjad et al.17 introduced a new fractional operator to model memory effects and solved analytically for temperature and velocity fields using the Laplace transform approach. Studies of heat transport are discussed in further studies, as reported in ref. 18–28.
Drawing inspiration from the aforementioned studies, it is formulated that the premise that the base fluid, methanol (CH3OH), is comprised of a blend of silica (SiO2) and alumina (Al2O3), forming a disk with a porous boundary capable of generating and absorbing heat. The problem's coordinates take the form of a cylinder (r,θ,z), with the thermal conductivities of the two alternative models: Xue and Hamilton–Crosser, compared. The surface is seen to radially expand and contract with time, with convective boundary conditions provided. A uniform magnetic field is applied perpendicularly to the z-axis, with minimal interference from the electric field. Furthermore, both thermal radiation and ohmic heating effects are taken into consideration. The dimensionless equations are obtained through similarity profiles and numerically solved using bvp4c. The novelty of this study is in using these nanomaterials and the effects taken. Methanol-based hybrid nanofluids incorporating silica and alumina nanoparticles have demonstrated a significant increase in heat transport efficiency, attributed to their high thermal conductivities and extensive surface area. These hybrid nanomaterials have garnered attention for superior rheological, thermal, and economic performance compared to monotype nanofluids. They enhance convective heat transfer, reduce boundary layer thickness, and provide thermal stability, making them indispensable for diverse applications such as solar thermal systems, car cooling, and heat sinks.
The article is structured into four distinct sections. In Section 2, the mathematical problem at hand is comprehensively addressed, providing a detailed insight into the complex computations and analytical techniques employed to obtain the desired results. Section 3 is devoted to graphical illustrations, accompanied by their corresponding physical significance, which helps readers visualize the intricate details of the problem and facilitates better understanding. Finally, Section 4 presents a concise summary of the findings, highlighting the key takeaways from the study and providing insights for future research in this area.
To assist in understanding, Fig. 1 displays the geometry of the problem, while Table 1 presents the thermophysical characteristics of both the methanol and alumina–silica nanoparticles.
Physical properties | Base fluid | Nanoparticles | |
---|---|---|---|
CH3OH | SiO2 | Al2O3 | |
ϱ f | 792 | 2650 | 3970 |
c p | 2545 | 730 | 765 |
k | 0.2035 | 1.5 | 40 |
Pr at 25 °C | 6.83 | — | — |
s | 3.7 (spherical) |
The governing equations are (see ref. 16)
![]() | (1) |
![]() | (2) |
![]() | (3) |
The thermophysical properties are given as (see ref. 7)
![]() | (4) |
For Hamilton–Crosser model, the thermal conductivity is given as
![]() | (5) |
The value of s vary for different shapes, such as for spherical s = 3.0. For Xue model, the thermal conductivity is given as
![]() | (6) |
The boundary conditions (BCs) are (see ref. 16)
![]() | (7) |
![]() | (8) |
Introducing stream function where
and local Reynolds number
with the following similarity ansatz
![]() | (9) |
The negative sign in eqn (9) indicates that the fluid at the far-field is being pulled towards the surface, rather than being pushed away from it. Using eqn (11)
![]() | (10) |
![]() | (11) |
![]() | (12) |
![]() | (13) |
The dimensionless parameters are
![]() | (14) |
The quantities of engineering interest, f and Nur are given as
![]() | (15) |
![]() | (16) |
Fig. 2(a) and (b) elucidate the impact of the magnetic field parameter M on the key parameters, namely f and Nur. Both figures represent the first and second solutions for control parameters χ and M = 0.5, 1.0, 1.5. Fig. 2(a) demonstrates that both solutions' values of
f escalate with an increase in M. The critical values, χc = −3.73484, −3.84431, −3.95486, are observed at M = 0.5, 1.0, 1.5, respectively. This enhancement in the behavior of
f can be attributed to the linear increment in
f with the magnetic field's increase. The magnetic field induces Lorentz forces that alter the velocity profile of the fluid, resulting in changes to the skin friction coefficient. Specifically, the skin friction coefficient decreases with an increasing magnetic field due to the development of a boundary layer with lower momentum. The Lorentz force causes the flow field to decline, magnifying the frictional forces, thus augmenting
f. On the other hand, Fig. 2(b) demonstrates the impact of M on Nur, which indicates that heat energy gains result from an increase in magnetic force, leading to chaotic particle motion. The system temperature increases due to the fluid molecules' physical energy and their greater intermolecular vibrations caused by increased kinetic energy. The magnetic field enhances fluid mixing and promote heat transfer, leading to an increase in the Nusselt number. It is noteworthy that the behavior of Xue and Hamilton–Crosser models is depicted here. The results indicate that the magnetic parameter has improved the outcomes of Nur. Moreover, the values for the Hamilton–Crosser models are more pronounced than the Xue model, indicating that they have better thermal conductivity than the Xue model. The critical values, χc = −3.73484, −3.84431, −3.95486, are observed at M = 0.5, 1.0, 1.5, respectively.
Fig. 3(a) and (b) demonstrate the influence of unsteadiness parameter λ on f and Nur, respectively. Critical values χc = −4.097, −3.925155, −3.73484 at λ = 0.5, 0.71, 0.9, respectively, are observed. As depicted in Fig. 3(a), an increase in λ leads to an enhancement in
f (for both solutions), resulting in a linear increment over time. Physically, unsteadiness can cause a delay in fluid flow, leading to an increase in frictional forces. Conversely, Fig. 3(b) illustrates the impact λ on Nur, with critical values of χc = −4.097, −3.925155 at λ = 0.5, 0.71, respectively. Augmentation in thermal transport rate occurs as a result of an increase in unsteadiness, which leads to chaotic particle motion, greater intermolecular vibrations, and increased kinetic energy, resulting in elevated system temperatures. The behavior of Xue and Hamilton–Crosser models is depicted for first and second solutions, with Hamilton–Crosser models exhibiting better thermal conductivity than the Xue model.
Fig. 4(a) and (b) depict the influence of Eckert number and Biot number on Nur. It can be observed that Nur increases with an increase in Ec, which represents the self-heating of the fluid due to dissipation effects and temperature gradients. Notably, the critical value remains the same for different values of Ec, i.e., χc = −3.73484 for Ec = 0.01,0.05. On the other hand, the Biot number βi relates the rate of heat transfer inside a solid to the rate of heat transfer at the solid's surface. It is defined as the ratio of the internal thermal resistance to the external thermal resistance. Fig. 4(b) demonstrates that an increase in βi leads to an increase in thermal transport rate. The critical value is χc = −3.079495 for βi = 0.01, 0.05. In general, the Nur increases with increasing Biot number. This is because as the βi increases, the thermal resistance of the solid becomes smaller compared to the thermal resistance of the fluid, and the temperature gradient in the fluid becomes more uniform. This leads to increased heat transfer from the solid to the fluid, and therefore a higher convective heat transfer coefficient, resulting in a higher Nur.
The impression of w is noted in Fig. 5(a) and (b) on
f and Nur, respectively. The mass transmission constraint shows the effect of suction (
w > 0) and injection (
w < 0). When
w > 0, fluid suction at the surface lowers the viscous effects closer to the wall. Suction has the potential to lower fluid velocity
′(ζ), whereas blowing or injection has the opposite effects. As a result, greater suction velocities yield higher entertainment velocities. Thus, suction causes the fluid's velocity to decrease, while blowing causes it to increase, as shown in Fig. 6(a). Therefore,
f improves for
w > 0, as shown in Fig. 5(a). On the other hand, energy of the system is declined as given in Fig. 6(b). However, Fig. 5(b) depicts that Nur is an increasing function of
w as well. It is seen that the thermal conductivity of Hamilton–Crosser's model is more than as compared to Xue's model. The critical values for Fig. 5(a) and (b) are χc = −3.73484, −4.35887, −5.05495 at
w = 1, 2, 3, respectively.
The behavior of shrinking/stretching parameter χ is noted in Fig. 7(a) and (b) on flow and energy distribution. The parameter shows the effect of stretching (χ > 0) and shrinking (χ < 0). It is illustrated that shrinking causes the flow field to decline for both solutions, as shown in Fig. 7(a). However, the temperature profile is boosted significantly for both models, as given in Fig. 7(b). Whereas it is seen that due to particle clustering and nano-layer creation, the thermal conductivity of Hamilton–Crosser is better than Xue model.
Fig. 8(a) and (b) provide an insight into the impact of Prandtl number Pr and heat generation/absorption parameter δ* on the system's behavior. The Prandtl number is a ratio of momentum to thermal diffusivity, indicating the dominance of thermal diffusion mechanism for a given fluid. The value of Pr for methanol at 25 °C is 6.83, much higher than that of air (Pr = 0.71). A lower value of Pr signifies the prevalence of heat conduction over convection, where heat diffuses faster than fluid velocity. In general, Pr for gases ranges around 0.7, while it varies between 1 and 10 for fluids. Fig. 8(a) indicates that the thermal transport decreases with increasing Pr, implying that a higher Pr value leads to a significant reduction in temperature/energy transport. The presence of a heat source or sink has a significant effect on the temperature distribution in a system. A heat source increases the temperature in its vicinity, while a heat sink reduces it. This effect propagates through the system via heat transfer mechanisms such as conduction, convection, and radiation. The magnitude of the effect depends on the strength and location of the heat source or sink, as well as the thermal properties and boundary conditions of the system. The effect of a heat source or sink can be used to control or optimize temperature distribution in a system (ζ), as shown in Fig. 8(b).
By contrasting the existing numerical values f with the prior results, the current model in Table 2 is validated. The variation of χ < 0 is presented in the absence of nanoparticles and magnetic field. It is seen that the coefficient of skin friction enhanced significantly due from χ = −0.25 to −0.95.
❖ Due to particle clustering and nano-layer creation, the thermal conductivity of the Hamilton–Crosser was better than the Xue model.
❖ Energy transport rate and wall stress were elevated due to shrinking and the presence of the magnetic field.
❖ Unsteadiness declined the fluid motion, which caused the frictional forces to incline.
❖ Due to the increment in Biot number, the surface heat resistance declined, which dominated the convection mechanism resulting in a higher thermal transport rate.
❖ The critical value was the same for variation of Eckert and Biot numbers, i.e., χc = −3.73484 and −3.079495, respectively.
❖ Coefficient of skin friction and thermal distribution rate was an increasing function of wall transmission constraint, whereas the fluid flow and energy transport diminished.
❖ Incrementing Prandtl number reduced the temperature and energy transport significantly.
❖ The thermal transport was dominant due to the heat generation/absorption parameter.
(ũ,ṽ,![]() | Velocity components (m s−1) |
k | Thermal conductivity (W m−1 K−1) |
ν | Kinematic viscosity (m2 s−1) |
ϱ | Density (kg m−3) |
![]() | Pressure (N m−2) |
α | Unsteadiness rate (s−1) |
μ | Absolute viscosity (kg m−1 s−1) |
σ | Electric conductivity (Ω m) |
B 0 | Magnetic field strength (N s C−1 m−1) |
Rer | Reynolds number |
σ* | Stefan–Boltzmann coefficient |
h w | Heat transfer coefficient |
τ w | Wall-shear stress |
![]() | Temperature (K) |
![]() ![]() | Dimensionless velocity and temperature |
![]() | Wall mass transfer coefficient |
λ | Unsteadiness parameter |
![]() | Dimensionless pressure |
M | Magnetic field parameter |
γ 1 | Biot number |
R d | Radiation parameter |
ϕ | Nanoparticle volume fraction |
ζ | Dimensionless variable |
k* | Mean absorption coefficient |
Pr | Prandtl number |
Nur | Nusselt number |
r,z | Derivative w.r.t r and z |
∞ | Far-field condition |
w | Wall boundary condition |
f | Base fluid |
n | Nanofluid |
hn | Hybrid nanofluid |
′ | Derivative w.r.t ζ |
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