## 1 Introduction  
In recent years, nanotechnology has attracted the research community through which an innovative technique is developed to enhance heat transfer by adding nanoparticles to pure fluid. The term 'nanofluid' was originally coined by [PERSON] [1] to refer to the fluids with nanoparticle suspensions. The heat transfer characteristic of a nanofluid depends mainly on thermo-physical properties of the base fluid and the nanoparticles, along with nanoparticle's shape, size and solid volume fraction [2; 3; 4]. It is clear that the addition of high thermal conductive nanoparticles to base fluid enhances thermal performance of the resultant mixtures. However, experimental investigations reveal that heat transfer is deteriorated with high nanoparticle volume fraction. [PERSON] et al. [5] demonstrated experimentally that the Nusselt number reduces by enhancing a nanoparticle's volume fraction for Al\({}_{2}\)O\({}_{3}\)/water and CuO/water. [PERSON] and [PERSON] [6; 7] and [PERSON] et al. [8] confirmed similar results toPutra et al. [5]. On the other hand, the majority of computational studies reported an enhancement in heat transfer by adding nanoparticles. [PERSON] et al. [9] critically analyzed the diverse models of nanofluids by measuring density, thermal expansion coefficients and viscosity. His findings establish that the heat transfer rate is amplified due to suspended nanoparticles. [PERSON] and [PERSON] [10] worked on [PERSON]'s model and remarked that as the nanoparticle's volume fraction increases, the average heat transfer coefficient enhances. [PERSON] and [PERSON] [11] numerically investigated the natural convection in nanofluids by using FVM inpartially heated enclosures with different aspect ratios and established that the Nusselt numberwas augmented with an increase inthe nanoparticle's volume fraction and Rayleigh number. [PERSON] et al. [12] also observed the heat transfer rate enhancement with the addition of nanoparticles. [PERSON] and [PERSON] [13] carried out numerical investigation of natural convection in CuO-Water nanofluid within rectangular enclosures and ascertained that addition of CuO nanoparticles enhanced the heat transfer. [PERSON] and [PERSON] [14] used FVM to employ numerical investigation of natural convection in a truncated cone and found an heat transfer characteristics of cooling device by adding Cu-water nanofluid. In addition, the heat source length and inclined sidewalls' angle influence the maximum temperature and the heat transfer rate. Most researchers predicted an enhancement in heat transfer by the addition of nanoparticles. On the contrary, some researchers reported the deterioration in heat transfer. The mitigation or enhancement in heat transfer of nanofluid may be because of the hypothesis employed in the theoretical models used for their thermal properties. Hence, most of the numerical works assume that nanoparticles are in thermal equilibrium with the fluid and there is no slip between the nanoparticles and fluid molecules. [PERSON] et al. [15] given theoretical prediction for the experimental results of natural convection in an Al\({}_{2}\)O\({}_{3}\)-water nanofluid an external boundary layer and concluded that the models used for nanofluid viscosity and effective thermal conductivity model influenced the heat transfer rate. [PERSON] [16] claimed from his experimental works that the augmentation in heat transfer may be due to the rotation of nanoparticles about their own axis due to the shear stress effect, and subsequently a three-dimensional hydrodynamic boundary layer was also observed. The rotating micro-constituents' effects in nanofluids should be considered to understand the fluid flow behavior in a better way and hence the micropolar theory explains the difference between experimental and numerical observations. This theory was originally proposed by [PERSON] [17] to elucidate fluid particles' micro-motions which cannot be explained by the classical models. The aforementioned subject has been recently reviewed by [PERSON] et al. [18] and [PERSON] [19].