**1. Introduction**

The nanofluids are novel working fluids which show significant improvement in thermophysical properties due to dispersion of nanoparticles into the base fluid. The nanofluids enable the superior thermal conductivity and convective heat transfer rate compared to conventional working fluids. Therefore, the research trend on applicability of nanofluids in thermal systems is a growing body of work since the last few years. The nanoparticle shapes have considerable effect on the thermal and hydraulic performance characteristics of thermal systems incorporated with nanofluids. Numerous research studies elaborate the influence of nanoparticle shapes on hydrothermal performance of single-particle and hybrid nanofluid flow in various thermal systems. The introduction is arranged as, the first paragraph summarizes various research studies on thermophysical properties of

**Citation:** Garud, K.S.; Hwang, S.-G.; Lim, T.-K.; Kim, N.; Lee, M.-Y. First and Second Law Thermodynamic Analyses of Hybrid Nanofluid with Different Particle Shapes in a Microplate Heat Exchanger. *Symmetry* **2021**, *13*, 1466. https://doi.org/ 10.3390/sym13081466

Academic Editor: Toshio Tagawa

Received: 12 July 2021 Accepted: 4 August 2021 Published: 10 August 2021

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nanofluid with different-shaped nanoparticles, the second paragraph discusses various research studies on behavior of heat transfer characteristics of single-particle nanofluid with different-shaped nanoparticles, the third paragraph summarizes various research studies to present the effect of different-shaped nanoparticles on heat transfer characteristics of hybrid nanofluid and at the end, the key research gaps and objectives of the present study are highlighted in the last paragraph.

This paragraph presents the effect of nanoparticle shapes on the thermophysical properties of nanofluids. Kim et al. have presented improvement in the thermal conductivity by 16%, 23% and 28% for BL-, PL- and BR-shaped nanoparticles, respectively, compared to Sp-shaped nanoparticles for alumina nanofluid [1]. Xie et al. have shown enhancements of 22.9% and 18.5% in thermal conductivity of CY- and Sp-shaped silicon carbide nanoparticles, respectively [2]. Similarly, Murshed et al. have also presented the superiority in thermal conductivity of CY nanoparticles compared to Sp-shaped nanoparticles [3]. Timofeeva et al. and Maheshwary et al. have proved that the Al2O<sup>3</sup> and TiO<sup>2</sup> nanofluids with CY-shaped nanoparticles present higher thermal conductivity and that those with Sp-shaped nanoparticles present lower viscosity [4,5]. Singh et al. have reported maximum enhancement in thermal conductivity for PL-shaped silicon carbide nanoparticles [6]. Jeong et al. have concluded that the ZnO nanofluid with rectangular-shaped nanoparticles present higher thermal conductivity and viscosity compared to that with Sp-shaped nanoparticles [7]. Zhang et al. have reported the thermal conductivities of 0.2619 W/m-K and 0.2843 W/m-K for silver nanofluid with Sp and nanowire nanoparticles, respectively [8]. Nithiyanantham et al. have presented enhancement in thermal conductivity by 16% and 12%, and in viscosity by 25% and 37% for Sp- and CY-shaped alumina nanoparticles [9].

This paragraph presents the summary of single-particle nanofluid with various nanoparticle shapes in heat transfer applications. Vanaki et al. have analyzed the heat transfer and flow characteristics of SiO<sup>2</sup> nanofluid flow in a wavy channel for various nanoparticle shapes and concentrations. The SiO<sup>2</sup> nanofluid with a PL shape presents the highest enhancement in heat transfer characteristics [10]. Mahian et al. have presented the first law analysis in terms of heat transfer coefficient and Nusselt number, and the second law analysis in terms of entropy generation for alumina nanofluid with nanoparticle shapes of BL, PL, CY and BR [11]. Akbar et al. have concluded that the PL nanoparticle shape presents maximum velocity, and the BR nanoparticle shape presents maximum enhancement in thermal conductivity for nanofluid flow in non-uniform channel [12]. Bahiraei et al. have proved that the Sp, BR, BL, CY and PL nanoparticle shapes present the descending order of entropy generation for alumina nanofluid flow in microchannel heat sink [13]. Sheikholeslami et al. have presented that the PL nanoparticle shape has the highest Nusselt number compared to Sp, CY and BR nanoparticle shapes for Fe3O<sup>4</sup> nanofluid in a porous curved enclosure, as well as in a porous cavity [14,15]. Nguyen et al. have concluded that the PL-shaped nanoparticles show more than 55% enhancement in heat transfer rates compared to Sp-shaped nanoparticles for CuO nanofluid flow in a wavy channel with obstacles [16]. Hatami et al. have presented that the TiO<sup>2</sup> nanofluid with a PL nanoparticle shape results in superior engine cooling or heat recovery performance at the higher volume fraction [17]. Kim et al. have concluded that the acetone-based Al2O<sup>3</sup> nanofluid with CY-, BR- and Sp-shaped nanoparticles present lower thermal resistance by 16%, 29% and 33%, respectively, compared with pure acetone [18]. Bahiraei et al. have investigated the thermal and hydraulic characteristics of alumina nanofluid in micro plate heat exchangers, considering nanoparticle shapes of CY, OS, BR, BL and PL [19]. Vo et al. have reported that the PL-shaped nanoparticles present the highest heat transfer rate and the best performance evaluation criteria, whereas BR-shaped nanoparticles show the lowest pressure drop [20]. Khan et al. have concluded that the nanoparticle shapes of CY, PL and BR have a significant effect on temperature distribution compared to velocity distribution for copper nanofluid flow in parallel channels [21]. Raza et al. have shown that the Sp-shaped nanoparticles have a higher heat transfer rate compared with CY- and lamina-shaped nanoparticles [22].

Gireesha et al. have concluded that the BL-shaped nanoparticles have a superior heat transfer rate and Sp-shaped nanoparticles have the highest entropy generation rate compared to BR-, PL- and CY-shaped nanoparticles [23]. Elias et al. have concluded that the CY-shaped nanoparticles show better heat transfer and entropy generation characteristics for shell and tube heat exchangers with and without baffles [24,25]. The PL-shaped nanoparticles show the maximum heat transfer rate and Sp-shaped nanoparticles show the minimum pumping power as concluded by Shahsavar et al. for laminar flow and that by Alsarraf et al. for turbulent flow in a mini channel heat exchanger [26,27]. Al-Rashed et al. have reported that the PL-shaped nanoparticles show the maximum entropy generation rate and Bejan number, and Sp-shaped nanoparticles show the minimum entropy generation rate for laminar flow, and reverse results are reported by Monfared et al. for turbulent flow in a mini channel heat exchanger [28,29]. Sadripour and Chamkha have presented the heat transfer and entropy generation comparison of various shapes of metallic and non-metallic nanoparticles for different nanofluids flow in a solar collector [30]. The heat flow path, heat transfer and entropy generation of CuO nanofluid with Sp-, CY-, BR- and PL-shaped nanoparticles are simulated by Liu et al. [31].

The open literature on nanofluid in heat transfer application presents that research is trending towards the hybrid nanofluids due to their improved thermophysical properties compared to single-particle nanofluids. Therefore, there are few studies which attempted to demonstrate the improvement in heat transfer performance of hybrid nanofluids under the consideration of different nanoparticle shapes. This paragraph presents the summary of various research studies on hybrid nanofluids with various nanoparticle shapes in heat transfer applications. Ghadikolaei et al. have compared the TiO2/Cu nanofluid with CY-, BR- and PL-shaped nanoparticles and reported that the PL-shaped nanoparticles show the highest heat transfer rate [32]. Ghadikolaei et al. have also proved that the PL-shaped nanoparticles of Fe3O4/Ag nanofluid show the maximum heat transfer rate due to an increase in the shape factor [33]. Dinarvand et al. have investigated the heat transfer and fluid flow characteristics of TiO2/CuO nanofluid with Sp-, CY-, PL- and BR-shaped nanoparticles and the shown maximum Nusselt number for PL-shaped nanoparticles [34]. Bhattad and Sarkar have proved that the BR- and PL-shaped nanoparticles present the best and worst hydrothermal performances, respectively, for the hybrid nanofluid with combinations of alumina, titania and copper oxide or copper with silica nanoparticles [35]. Benkhedda et al. have reported the highest heat transfer rate for BL-shaped nanoparticles and the highest friction factor for PL-shaped nanoparticles when the TiO2/Ag nanofluid flows through a tube [36]. Ghobadi et al. have compared the magnetohydrodynamic heat transfer of hexahedron- and lamina-shaped nanoparticles for the Al2O3/TiO<sup>2</sup> nanofluid and reported that the Nusselt number is affected mostly by lamina-shaped nanoparticles [37]. Aziz et al. have proposed an inverse relation between the shape factor of the nanoparticle and heat transfer for Cu and Fe3O4/Cu nanofluids [38]. Ghadikolaei et al. have reported the highest heat transfer rate and Nusselt number for TiO2/CuO and MoS2/Ag nanofluids with BL-shaped nanoparticles because of an increase in the shape factor at a higher temperature [39,40]. Similar results are deducted for the GO/MoS<sup>2</sup> nanofluid by Ghadikolaei and Gholinia [41]. Maraj et al. have investigated the shape effect of nanoparticles on magnetohydrodynamic heat transfer and flow characteristics [42]. Sahu et al. have presented the energy and exergy analyses of various hybrid nanofluids with Sp-, CY- and PL-shaped nanoparticles [43].

The comprehensive literature review reveals that there is no concrete comparative study on the first and second law analyses of single-particle and hybrid nanofluids with different particle shapes in heat transfer applications. The objective of this study is to investigate the first and second law characteristics of the microplate heat exchanger incorporated with single-particle and hybrid nanofluids with different nanoparticle shapes under various volume fractions, temperatures and mass flow rates. The computational fluid dynamics approach with symmetrical heat transfer and fluid flow concept is adopted to evaluate the first and second law characteristics of the microplate heat exchanger. The

NTU, effectiveness, performance index, thermal entropy generation rate, friction entropy generation rate, total entropy generation rate and Bejan number are compared for Al2O<sup>3</sup> and Al2O3/Cu nanofluids with Sp-, OS-, PS1-, PS2-, PS3-, PS4-, BL-, PL-, CY- and BRshaped nanoparticles. In addition, the best combination of nanofluid with nanoparticle shape to archive an optimum heat transfer performance in the microplate heat exchanger is suggested based on the first and second law characteristics.
