Investigation of Counterflow Microchannel Heat Exchanger with Hybrid Nanoparticles and PCM Suspension as a Coolant

Mechanical Engineering Department, College of Engineering, University of i-Qar, Nasiriyah, Iraq Department of Mechanical Engineering, Sharif University of Technology, Tehran, Iran Center for Renewable Energy and Environment Development, Department of Mechanical Engineering, Birla Institute of Technology & Science, Pilani Campus, Vidya Vihar, Pilani, Rajasthan 333 031, India Faculty of Mechanics and Mathematics, Department of Mathematical and Computer Modelling, Al-Farabi Kazakh National University, Almaty, Kazakhstan


Introduction
e need for cooling devices in various industries such as building [1], electronic devices [2], and solar applications [3][4][5][6] has increased so that various researches have been conducted on this field. us, enhancing the performance of heat transfer and thermal storage systems is very important and has gained lots of interest [7]. e geometry and working flow are the most noteworthy methods for improvement. Using metal and carbon nanofluids, due to the high thermal conductivity of these materials, has intensified their application in modern heat transfer systems. Phase change materials (PCMs) could improve the heat storage capacity of the fluid and can enhance the heat storage and heat transfer potential of the working fluid by absorbing and releasing heat at a constant temperature. A point to be noted is that the weak thermal conductivity of PCM has limited severely the heat transfer of these materials during both discharging and charging processes in thermal storage systems. A wide range of investigations was conducted to enhance the thermal conductivity of PCMs. From these methods, the use of nanoparticles to increase the heat transfer rates is of high significance [8]. Kashani et al. [9] carried out a numerical investigation into the discharging of Nanoenhanced Phase Change Material (NEPCM) inside an enclosure. ey used different types of nanofluids at concentrations of ϕ � 0%, 2.5%, and 5% in a cavity in which the left wall is under constant heat flux, and the other sides were thermally insulated. It was illustrated that the heat transfer rate of the enclosure improves, as the nanoparticles are added to the system. Gujarathi [10] studied the characteristics of PCM enriched by nanoparticles to be used in a data center cooling system. us, wide ranges of concentration of copper were added to the paraffin as the PCM. e dispersion of nanoparticles into the paraffin increased its thermal conductivity when compared with conventional PCM. As a result, the heat transfer and melting rate were improved in the presence of nanoparticles. A point to be mentioned is that since concentration is associated with viscosity increment, the lower concentrations of nanoparticles showed better heat transfer properties in comparison with higher concentrations. Using high latent heat capacity of PCM along with high thermal conductivity of nanoparticles was stated as a promising potential in data center cooling applications. Farsani et al. [11] compared the impacts using nanoalumina at the concentration of 0.01, 0.02, and 0.03 by volume on the melting and solidification of PCM.
ey employed the enthalpy-porosity method to predict the PCM behavior. It was found that the overall performance of the phase change exceeds at the concentration of 0.02, which is due to the dominance of conduction improvement over the convection weakening.
In fact, dispersing carbon and metal nanoparticles into the base fluid could improve the thermal conductivity, and dispersing microencapsulated PCM (MEPCM) could enhance the heat storage capacity of the base fluid.
ere are numerous studies regarding the performance of microchannel heat exchangers by using MEPCM suspension, nanofluids, and hybrid suspensions. Hasan et al. [12] numerically studied the performance of a counterflow microchannel heat exchanger (CFMCHE) charged with nanofluid as a coolant fluid. Two types of nanofluids including Al 2 O 3 -water and Cu-water were studied at concentrations of 1% to 5% by volume. It was inferred that using the nanofluids as a coolant medium results in the enhancement of the thermal performance of CFMCHE, with no rise in pressure drop because of low concentrations and also the ultrafine solid particles. e effectiveness of nanoparticles was higher in lower concentrations than in high concentrations. In other words, the effectiveness decreased with increasing the concentration. ey also concluded that the best nanoparticle to be used is the one with higher thermal conductivity. Hasan [13] numerically investigated a CFMCHE with MEPCM suspension as the working fluid. e MEPCM suspension used in his research was made of microcapsules constructed from n-octadecane and polymethylmethacrylate as the core and shell materials, respectively. ese capsules were dispersed in pure water at a concentration range of 0 to 20%. From their results, it was found that using MEPCM suspension as a cooling medium results in improvement of thermal performance of the CFMCHE but increasing the pressure drop significantly. As well, in order to obtain the benefits of melting of phase change material (PCM) and releasing the latent heat, it is favorable to use the MEPCM suspension with low concentration. Ho et al. [14] employed a hybrid suspension as a working fluid, consisting of Al 2 O 3 nanoparticles and MEPCM particles and water as the base fluid. e thermal properties of hybrid suspension, including the thermal conductivity, specific heat, dynamic viscosity, latent heat of fusion, and density, were experimentally examined. It was concluded that increasing the concentration of Al 2 O 3 nanoparticles increases the thermal conductivity of the PCM suspension. Ho et al. [15] experimentally investigated the effect of using a hybrid nanofluid on the natural convection in a circular tube. ey employed pure water, PCM suspension (wPCM � 2, 5, and 10 wt.%), and nanofluids (wnp � 2, 6, and 10 wt.%) as cooling fluids and investigated the convection in the tube. e nanofluid decreased the specific heat and increased the thermal conductivity while PCM suspension weakened the thermal conductivity and increased the specific heat. e dominance of the thermal conductivity over specific heat in nanofluid and also specific heat over thermal conductivity in PCM suspension led to the improvement of the heat transfer of the working fluid. Ho et al. [16] experimentally used a hybrid nanofluid containing Al 2 O 3 nanoparticles along with MEPCM particles to investigate their effects on the laminar convective cooling performance in a circular tube. A significant enhancement in cooling was obtained by using the hybrid nanofluid compared with the pure PCM suspension, Al 2 O 3 nanofluid, and water. However, as the concentration rose, because of viscosity increment, the pressure drop increased, and thus, the effect of utilizing hybrid suspension decreased. Meanwhile, the dependency of pressure drop on concentration was higher in case of hybrid suspension than the nanofluid or pure PCM suspension. Elbahjaoui and El Qarnia [17] studied a shell and tube thermal storage system. ey used water as the heat transfer fluid and n-octadecane embedded in shell space as the PCM. ey investigated two cases of stationary and pulsating heat transfer fluid (HTF) on the storage characteristics and, thus, used nanoparticles with concentrations ranging from 0 to 7% by volume and HTF with dimensionless frequency from 0.01 to 3. e best performance was obtained in a volume fraction of 7% and pulsating frequency of 1. In the optimum case, the melting time was reduced up to 14.4%.
Augmenting convective heat transfer by using a swirl generator is one of the most popular techniques in the HVAC&R industry [18][19][20][21][22][23]. Hao and Tao [24] numerically investigated the mixed convective heat transfer. A square cavity with partial slip-filled kerosene-cobalt nanofluid is used for the study. e horizontal walls of the cavity are kept insulated while the vertical wall is partially heated. It was reported that heat flow was affected by the change in volume concentration of ferrofluid.
Goel et al. [25] numerically investigated the free convective heat transfer and flow of MHD Alumina-Cu/water nanofluid filled in a square cavity. e cavity contains a corrugated conductive cylinder. e left wall of the cavity is given constant heat. Corcione correlation is used for determining the thermal conductivity and viscosity of the 2 Mathematical Problems in Engineering hybrid nanofluid. It was reported from the results that the presence of a wavy block in the cavity influences the flow and thermal characteristics. Local ermal Nonequilibrium model has been utilized by Tsai and Chein [26] to investigate the heat transport through convection in a porous medium filled with Ag-MgO/H 2 O nanofluid. e influencing parameters investigated in the study are Rayleigh number, porosity, the volume fraction of nanoparticles, interface convective heat transfer coefficient, and thermal conductivity ratio.
e study concluded that vortex strength is increased with an increase in Ra while the dispersion of combined Ag-MgO nanoparticles in the fluid reduces the flow and thermal transport in the cavity as compared with Ag and MgO nanoparticles individually. Lee and Mudawar [27] numerically inspected the effect of MHD flow of second-grade fluid on the convective heat transfer in a vertical channel filled with porous medium. e results are analyzed with respect to velocity, temperature, volume concentration, skin friction, and heat and mass flow rate. It was revealed that increasing the permeability results in an increase in skin friction.
Manikandan and Rajan [28] experimentally investigated the double pipe HE, shell and tube HE, and plate HE for the same surface area under the same condition with the same nanofluids for thermal and flow characteristics. e results obtained from the experiments showed that minimum pressure drop was obtained with plate-type HE at 27% enhancement while maximum pressure drop was observed with double pipe HE with 85% enhancement in thermal transport. Sadeghi et al. [29] numerically revealed the benefits of using multilayer PCMs. 15 sets of experiments have been conducted to investigate the effect of various arrangements and thickness of the layer of phase change materials. It was revealed that the single layer saves 23% of inlet energy while three layers of PCM save 41% of inlet energy. Dogonchi et al. [30] utilize controlled volume-based FEM for investigating the presence of nanoparticles in a square cavity fitted with an elliptical cylinder for free convective heat transfer. e results obtained from the simulation show that, at a specific aspect ratio, enhancement in heat transfer and Nu was observed.
Ghalambaz et al. [31] numerically investigated the impact of Ag-MgO/water hybrid nanofluid filled in a square cavity fitted with a solid conductive layer at a hot wall for free convection. e parameters used for the study are volume concentration of nanoparticles, Rayleigh number, and thermal conductivity ratio. e study concluded that adding the hybrid nanoparticles in the base fluid and increasing the Ra and thermal conductivity ratio lead to the enhancement of heat transport. Ishak et al. [32] investigated the effect of wall thickness on heat transfer and entropy production in a square cavity filled with Al 2 O 3 nanofluid. A moving heat source is present at the lower wall, which allows partial isothermal heating while other walls remain insulated. It was revealed from the obtained results that thermal conductivity and thickness of the wall are key parameters to control the optimal heat transfer and pressure drop.
Rejvani et al. [33] experimentally reveal the effect of the addition of SiO 2, and MWCNT in 10 W 40 engine oil. e viscosity of the resulting hybrid nanofluid was tested for various concentrations at different temperature ranges. e results show a 35% increase in the viscosity of the hybrid nanofluid comparative to base fluid.
Alsabery et al. [34] computationally studied the convective heat transfer and entropy production in a trapezoidal cavity filled with nanohomogenous Al 2 O 3 -water nanofluid under the influence of a magnetic field. e governing PDEs are solved using Galerkin weighted residual FEM. e parameters studied are Rayleigh No., the concentration of nanoparticles, Hartmann No., thermal conductivity, and height of the trapezoidal body. It was revealed that streamlines show more sensitive behavior for Hartmann as compared with the change in volume fraction.
Mehryan et al. [35] revealed the impact of MWCNT-Fe 2 O 3 /H 2 O nanofluid on the convective heat transfer under the influence of an inclined magnetic field. e parameters that influence the thermal and flow characteristics are studied. An increase in average Nusselt's number was up to Ra � 10 4 while an increase in viscosity due to the addition of nanoparticles decreases the average Nu. Hoseinzadeh et al. [36] numerically investigated porous rectangular fin and run the simulation for analyzing the thermal transport. e results obtained from Runge-Kutta numerical analysis are compared with results obtained through collocation method (CM), homotopy perturbation method (HPM), and homotopy analysis method (HAM). It was revealed from the study that on increasing the porosity, convection, and radiation; temperature gradient and heat flow increase along the fin length. Taamneh et al. [37] computationally investigated the thermal and flow behavior of Al 2 O 3 -H 2 O nanofluid in a triangular duct. e Reynolds number varied between 4000 and 10000. 20% increase in friction factor was observed with the addition of nanoparticles in comparison to base fluid while pressure drop was increased by 85% for Re � 10000. Besides this, a 40% increase in the production of entropy was also observed when varying the concentration of nanoparticles from 0.05 to 0.1%. Raza et al. [38] investigated the effect of MoS 2 -H 2 O nanofluid in a channel with different shapes of nanoparticles. A magnetic field influenced the flow. e results obtained from the analysis show that increasing the concentration of nanoparticles in the base fluid increases the Nu while increasing the value of wall expansion ratio results in augmentation of velocity profile between bottom wall and center.
Giwa et al. [39] investigated the impact of ferrous hybrid nanofluid on the convective thermal and flow performance in the rectangular cavity under the influence of a magnetic field. e thermal properties of hybrid nanofluid are also analyzed for different particle concentrations. e magnetic field enhances the average Nu by 4.9% while increasing the strength of the magnetic field further enhances the thermal transport.
e authors also noticed that the use of hybrid nanofluid shows better results in comparison to mono nanofluid. Giwa et al. [40] in another work investigated the thermal performance of Alumina-MWCNT/water nanofluid filled in a square enclosure. e nanofluid has been investigated for the different weight ratios of nanoparticles. It was revealed that nanofluid with a 60-40 weight ratio of Al 2 O 3 -MWCNT  [43] optimized the concentration of TiO 2 -water nanofluid for maximum convective heat transfer in a square cavity. e upper and lower walls of the cavity are insulated while a temperature gradient exists between the left wall and right wall. It was found that the addition of TiO 2 nanoparticles enhances the heat transport for all concentrations while the maximum heat transfer of 8.2% was observed at 0.05% concentration. Sharifpur et al. [44] experimentally investigate the impact of the size of Al 2 O 3 nanoparticles on the thermal conductivity of Al 2 O 3 -glycerol nanofluid. e nanofluid prepared with 3 different sizes of nanoparticles for concentration varied between 0.5 and 4%. It was revealed from the experiments that maximum augmentation of thermal conductivity was observed at 4% concentration for 31 nm nanoparticles. It was also found that thermal conductivity is influenced by volume concentration and size of nanoparticles, while temperature variation shows negligible impact.
After a wide review of previous research, the researcher did not find a previous study regarding the usage of hybrid suspension in the heat exchanger of microchannels. us, it is a source of modernity for this paper. In fact, by using the hybrid suspension, both advantages of nanoparticles and microcapsules are obtained. MEPCM can improve the heat capacity of the cooling fluids, and on the other hand, nanofluid can improve the thermal conductivity; thereupon, using a mixture of these materials may lead to a significant increase in heat transfer of microchannel heat exchanger. Figure 1 represents the schematic structure of CFMCHE used in this study with square channels carrying cold and hot fluids. To study the fully CFMCHE, numerically, a huge amount of time is required due to its complication. As shown in Figure 2, given that there is a symmetry between the cold and hot channel rows, geometrically and thermally, assuming a part of the geometry as the model studied is acceptable, since it gives an adequate indication for the whole heat exchanger performance. Hence, two channels in the cold and hot fluids flow, and the separating wall between them is considered as the studied cases and modeled numerically [18,19].

Mathematical Model
e assumptions used to solve this model are 3D, laminar, steady-state, incompressible fluid, continuum flow, constant properties, and thermally isolated from the ambient.

Governing Equations
e governing equations are continuity, momentum, and energy used for the flow of pure fluids, nanofluid, MEPCM suspension, and hybrid suspension as follows [13,20,21].

Governing Equations for Pure Water and Nanofluids
e continuity equation is e momentum equations are where j stands for h for the hot and c for the cold fluids, respectively. e energy equation for fluids in heat exchanger is

Governing Equations for MEPCM and Hybrid
Suspensions. e governing equations used for pure fluid (equations 1 to 4 and 6) are also used MEPCM and hybrid suspension.
e enthalpy-based energy equation is written as follows: It also can be defined as e enthalpy of the slurry (H e ) is shown by equation (9) and computed as follows: e sensible heat is calculated by equation (10), in which h ref is the reference enthalpy at T ref [21].
e slurry's latent heat (ΔH) is calculated by equation (11), where (β) is the mass ratio of molten to the total PCM in the slurry, (Φ) is the MEPCM mass fraction, and (L) is the fusion latent heat. e PCM begins to melt at T solidus where the liquid fraction is zero and melting ends at T lquidus . At this point, the liquid fraction reaches one. where

Model Boundary Conditions.
e working fluids enter the channels at a defined velocity and temperature. Pure water is the hot fluid used in hot channels while the cold fluid in the cold channels is tested (pure water, Nanofluid, MEPCM suspension, and hybrid suspension). e boundary conditions of the model are given in Tables 1-3. In the present study, to solve the boundary conditions and also governing equations, the finite volume method (FVM) is employed. Computational fluid dynamics (CFD) is employed to solve the model and also determine the profile of the fluid flow, temperature, and pressure fields in the CFMCHE. Meanwhile, the dimensions of the square channel used in this model are W � 100 μm, H � 100 μm, L � 10 mm, and t � 50 μm. W, H, L, and t are width, channel height, length, and wall thickness, respectively.
Performance parameters have been calculated as follows. e ratio of the experimental heat transfer to the maximum heat that is possible to be transferred is defined as heat exchanger effectiveness (Ɛ).
where C c � mCp c and C h � mCp h. In the heat exchange cell, the total pressure drop is In order to investigate the overall performance of the CFMCHE, both hydrodynamic and thermal performances must be taken into account.
us, performance index is defined as the effectiveness of CFMCHE to the total pressure drop of heat exchanger [19]: e input power required for the pump to circulate fluids in the microchannel heat exchanger is Flow rate (m 3 /s) is defined as follows:  Mathematical Problems in Engineering 5 ere is another factor called performance factor defined as the heat transfer rate to the pumping power ratio and used to investigate the overall performance of the CFMCHE and also verify the obtained results of the performance index [19]:

Properties of MEPCM Suspension.
MEPCM particles are made of a polymer shell surrounding a core of PCM, preventing PCM from leakage, and keeping the form during phase change. Figure 3 represents a schematic of a microcapsule during the phase change process [13]. Herein, the average diameter of MEPCM particles is 5 μm. e selection of core and shell materials of the microcapsule corresponds to the transporter fluid. N-octadecane with a melting temperature of about 301 K is used as PCM in the core of the capsule, and the shell is made up of polymethylmethacrylate (PMMA) [22][23][24]. Given that the physical properties of the wall material and PCM affect the properties of the MEPCM particle, different components must be taken into account while calculating the properties of MPCM. e densities of solid and liquid PCM were assumed to be the same as the density of n-octadecane. Moreover, energy and mass balances were utilized to calculate the density and specific heat of the microcapsules, respectively [25].
e thermal conductivity of microcapsules is also represented as follows: PCM stands for the whole (capsule � wall + core) and the wall is the wall of the capsule (polymer). C is the core material (PCM) and d is the diameter. e suspension properties are a function of the properties of water (base fluid) and properties of the microcapsules. Hence, the specific heat and density are determined using a mass and energy balance [22][23][24][25].
e viscosity of the suspension is also determined using e thermal conductivity of the suspension is calculated as

Location Boundary condition Comments
Hot fluid outflow (fully developed flow, end of the channel) At y � 0 No-slip, adiabatic wall Table 2: Top channel (cold fluid) (Hh + t ≤ y ≤ Hh + t + Hc).

Location Boundary condition
Comments No-slip, adiabatic wall Table 3: Solid wall separating two channels (Hh ≤ y ≤ Hh + t).

Location Boundary condition
Comments e mass fraction can be calculated from

Properties of Nanofluids.
According to the previous studies [26,27], the thermophysical properties of the nanofluids are a function of the properties of solid particles, the base fluid, particles shape, and volume fraction of the solid particles in the suspension. e properties of nanofluids are determined using the following relations: ermal conductivity: Viscosity: Density: Specific heat: In equation (27), SH indicates the solid particle shape factor.
ψ represents the ratio of the surface area of a sphere (with the same volume of the particle) to the surface area of the particle. us, for the spherical particles, SH is 3.
k p , k f , and k nf are thermal conductivities of the solid particles, base fluid, and nanofluid, respectively.

Properties of Hybrid Suspensions.
To determine the viscosity and thermal conductivity properties of hybrid suspension, the following relations are used [28]: ermal conductivity: Viscosity: To calculate the specific heat and density of the suspension, we use the following relations [14].
Specific heat:  Mathematical Problems in Engineering 7 Density: (35) where hs, np, MEPCM, and bf refer to hybrid suspension, nanoparticle, microencapsulated phase change material, and base fluid, respectively. In Table 4, the rheological and thermal properties of the materials employed are listed.

Numerical Model
Governing equations were transformed into algebraic equations using the finite volume method. e SIMPLE algorithm was employed to handle the mass conservation equation and get the pressure field. Computational fluid dynamic software (FLUENT 19.1) was used to determine the fields of velocity, pressure, and temperature distribution along the CFMCHE. Afterward, all the computational domain (two channels and the separating wall) was meshed with suitable size, and then using a mesh refinement process, the generated mesh was refined.

Results and Discussion
In order to investigate the effectiveness of the nanofluids, the model was firstly operated with pure water. e inlet temperatures of cold and hot fluids flowing in the channels were set as the boundary conditions with values of T ci � 293 K and T hi � 373 K. en, we repeated operating the model by using nanofluid and MEPCM suspensions with volume fractions of (2%, 4%, 6%, and 8%). Noting that a core material of PCM is n-octadecane which has T solidus � 297 K and T liquidus � 302 K; also, latent heat ΔH � 245,000 (J/kg). And then, we repeated again operating the model by adding nanoparticles of (Cu and Al 2 O 3 ) with volume fractions of (2% and 4%) to the MEPCM suspension to form the hybrid suspension.
To examine the validity of the present model, the model presented by Kashani et al. [9] was solved, and a comparison was made between the obtained results and those presented by them. e numerical model that used them is a microchannel heat exchanger composed of rectangular microchannels with hydraulic diameter D h � 100 μm, channel height H � 100 μm, channel width W � 100 μm, and length L � 10 mm. A silicon wall with a thickness of 50 μm separates the channels, and the inlet velocity of the fluid was chosen to be Vi � 1 m/s. Figure 4 illustrates the heat transfer rate distribution in terms of volume concentration for two cases of the present study and the numerical model of [9]. According to the figure, it is evident that the two models are in good agreement with each other, and the average error is 1.16% which is attributed to the end effect. ereupon, the present model has acceptable accuracy and could be used as a reliable model to examine the impact of different nanofluids, including MEPCM and hybrid suspension on the performance of CFMCHE. e heat transfer rate versus inlet velocity for pure MEPCM suspension at a concentration of 4% and enhanced (hybrid) suspension with the addition of 2% and 4% of Cu and Al 2 O 3 nanoparticles is shown in Figure 5.
It is inferred that, for all cases, the heat transfer rate rises with increasing the velocity, which is because of the increase in flow rate. Also, results reveal that the heat transfer rises with adding nanoparticles, due to enhancing the thermal properties of suspension, especially thermal conductivity which increases with the increment of the amount of nanoparticles volume concentration. Furthermore, it could be noted that the enhancement in heat transfer is higher in case of adding Cu particles compared with Al 2 O 3 particles because of the higher value of thermal conductivity of Cu compared with that of Al 2 O 3 . e variation of pressure drop with inlet velocity for pure MEPCM suspension at a concentration of 4% and enhanced (hybrid) suspension with adding 2% and 4% of Cu and Al 2 O 3 nanoparticles is portrayed in Figure 6.
From Figure 6, it is found that the pressure drop increases with the increment of inlet velocity for all cases, which is attributed to the increase in the frictional and dynamic losses. Also, it is shown that adding nanoparticles to the fluid leads to a higher pressure drop due to an increase in the dynamic  viscosity. is trend increased with the increment of concentration. A slight difference was discovered between pressure drop in case of adding Cu particles compared with Al 2 O 3 particles resulting from the difference between the densities. Figure 7 shows the effectiveness versus inlet velocity for pure MEPCM suspension (at a concentration of 4%) and enhanced (hybrid) suspension by adding 2% and 4% of Cu and Al 2 O 3 nanoparticles, respectively.
As depicted in Figure 7, for all cases, as the inlet velocity increased, the effectiveness decreased. In fact, in high velocities, there is not enough time for all the particles to melt completely. As a result, the thermal energy was absorbed during melting, and also the effectiveness of MEPCM decreases. It is also evident that the effectiveness rises slightly with adding nanoparticles due to the enhancement of the thermal properties of suspension, especially thermal conductivity. A point to be mentioned is that, in high flow rates, the impact of nanoparticles on the development of the boundary layer decreases, resulting in better heat transfer of the suspension in higher velocities. In fact, in high velocities, the volume flow rate is dominated, and the effect of nanoparticles is weakened. Figure 8 represents the performance index in terms of inlet velocity for pure MEPCM suspension (at a concentration of 4%) and enhanced (hybrid) suspension by adding 2% and 4% of Cu and Al 2 O 3 nanoparticles, respectively.
As shown in Figure 8, the performance index (η) for all cases decreases with an increment of inlet velocity. It is due to the higher increment of pressure drop than effectiveness. Also, the increase in pressure drop for hybrid suspensions is higher in comparison with that of MEPCM suspension resulting in a higher performance index of MEPCM suspension than that of hybrid suspensions. Figure 9 indicates the variation of pumping power with inlet velocity for pure MEPCM suspension at a concentration of 4% and enhanced (hybrid) suspension by adding 2% and 4% of Cu and Al 2 O 3 nanoparticles, respectively.
From Figure 9, it is found that as the inlet velocity increases, pressure drop increases and thus pumping power rises. Also, the pumping power increases with adding nanoparticles because of the increase in dynamic viscosity that results in an increment of pressure drop. Moreover, the more the number of nanoparticles, the more pumping power. Since there is no noticeable difference between the density values of Cu and Al 2 O 3 , a slight difference was seen between pumping power in case of adding Cu particles compared with Al 2 O 3 particles. e variation of performance factor in terms of inlet velocity for pure MEPCM suspension (at a concentration of 4%) and enhanced (hybrid) suspension with adding 2% and 4% of Cu and Al 2 O 3 nanoparticles is depicted in Figure 10, respectively.
It is inferred that the performance factor ( ) for all cases decreases with the increment of inlet velocity since the effect    of inlet velocity on the increment of pumping power is higher in comparison with that on heat transfer rate. Also, the performance factor of MEPCM suspension is higher than that of the hybrid suspensions. It is due to the higher increment of pumping power for hybrid suspensions than MEPCM suspensions.   200  300  400  500  600  700  800  900  1000  1100  1200  1300  1400  1500  1600  1700  1800  1900  2000   He: Enthalpy of suspension (W) he:

Conclusions
Sensible heat (W) K: ermal conductivity (W/m K) L: Heat exchanger length (m) M: Mass flow rate (kg/s) P: Total pressure (Pa) Q: Heat transfer rate (W) t: Separating wall thickness (m) T: Temperature ( Total.

Data Availability
No data availability statement is assigned.

Conflicts of Interest
e authors declare that they have no conflicts of interest.