Nanofluid is a new type of engineering fluid synthesized by suspending nanoparticles with a size less than 100 nm such as and in common liquids. Nanofluid terms were first introduced by Choi (1995)  in the National Laboratory of Argonne, U.S.A. Today, nanofluids occupy a very important place in many technology fields such as solar energy [2-4], Biomedical application [5, 6]. Especially, heat exchangers , Cooling of electronics and Diesel combustion, Nuclear systems cooling, Defense and Space cooling nuclear reactors . The nanoparticles shapes may have a considerable effect on the dynamic viscosity and thermal conductivity of the nanofluids, which in turn, may influence the rate of heat transfer enhancement.
Several experimental and numerical research studies have dealt with the nanoparticle shape effect. Timofeeva et al.  studied experimentally the alumina nanoparticles shape effect on thermophysical properties of , they demonstrated that the evaluation of nanofluids for a particular application requires a proper understanding of all the characteristics and thermophysical properties of nanoparticle suspensions. Jeong et al.  realized an experimental study for the determination of the thermal conductivity of nanofluids with rectangular and spherical shaped nanoparticles for different nanoparticle volume fractions.
The particles shape was found to play a significant role in the thermal conductivity enhancement. Murshed et al.  developed the thermal conductivity of rod and spherical shaped nanoparticles suspended in deionized water. They observed that the particle size and shape affect the thermal conductivity enhancement of the nanofluid. Vanaki et al.  studied numerically the effect of nanoparticle shapes on the heat transfer enhancement in a wavy channel with different phase shifts, they found that the nanofluid with platelets nanoparticle shape gives the highest heat transfer improvement compared with other nanoparticle shapes. They reported that the nanoparticle shape plays a very important role in the thermal and hydrodynamic behavior, which influences the improvement of the rate of heat transfer. Akbar et al. realized a theoretical and numerical study of heat transfer in the peristaltic propulsion of magnetic (electrically conducting) nanofluids using different nanoparticles shape (cylindrical, platelet and brick). The results show that at higher Hartmann number, the flow intensity is enhanced for platelet nanoparticles whereas it is decreased for brick particles. Higher thermal conductivity is attained with brick-shaped nanoparticles in the fluid. Hatami et al.  modeled numerically a flat tube of an engine radiator for improving the cooling process or heat recovery of the engine using nanofluids. They analyzed the effect of the nanoparticles shape. Ellahi et al.  studied numerically the natural convection boundary layer flow along an inverted cone. The shape of nanoparticles on entropy generation with based fluid is considered. They made a correlation of Nusselt number and skin friction corresponding to active parameters. Elias et al.  studied numerically the effect of different nanoparticle shapes (such as cylindrical, bricks, blades, platelets, and spherical) on the performance of a shell and tube heat exchanger. They found that cylindrical shaped nanoparticles showed better performance in terms of thermal conductivity, heat transfer coefficients, and the heat transfer rate.Hybrid nanofluid presents a new generation of technologically advanced fluid in engineering and industries with applications in almost all fields of heat transfer. The Hybrid nanofluid is a mixture of two different types of nanoparticles dispersed in a base fluid. The choice of these nanoparticles materials fairly is very important. Metallic nanoparticles as Ag, Cu, Al, and gold possess high thermal conductivity, but the use of these nanoparticles are limited due to his low stabilities and high reactivity into base fluid. The uses of metallic oxide nanoparticles such as and have many favorable properties as more stability and chemical inertness [17-19]. Experimentally, some studies available concerning the synthesis and measurement the physical properties of hybrid nanofluid. Suresh et al. used the thermo-chemical synthesis technique for the preparation of hybrid nanoparticles using (copper nitrate) and (aluminum nitrate) reagent grade chemicals. Nine et al. synthesized Cu-Cu2O/water hybrid nanofluid using the wet ball milling process. Batmunkh et al. use the ball milling technique for the synthesis of nanoparticles. Hemmat et al.  prepared and measured the thermal conductivity of hybrid nanofluid. Two correlations for predicting the thermal conductivity in terms of nanoparticles volume fraction and temperature are proposed using ANN and experimental data. Toghraie et al.  conducted an experimental investigation on the effects of temperature and nanoparticles concentration on the thermal conductivity of ZnO”TiO2/pure ethylene glycol based hybrid nanofluids. Baby and Ramaprabhu  prepared copper oxide decorated graphene dispersed in deionized water and ethylene glycol mixture and studied the thermal transport properties of those hybrid nanofluids. For numerical studies, Takabi et al.  studied numerically laminar forced convection of different working fluids including pure water, various volume concentrations of a nanofluid ( nanoparticles dispersed in water), and a hybrid water-based suspension of and Cu nanoparticles in a uniformly heated circular tube. They found that the convective heat transfer coefficient increases in accordance with the Reynolds number and the nanofluid volume concentration. Abdul Rahman et al. investigated numerically forced convective heat transfer on hybrid nano‚uid. It was found that the dominant nanoparticle in the hybrid nano‚uid strongly in‚uence the thermal behavior of the hybrid nano‚uid. Balla et al.  investigated hybrid nano‚uid ‚owing in a circular pipe and found that the heat transfer rate enhanced in a range of 30-35% in comparison to the pure water. Labib et al.  investigated forced convective heat transfer of hybrid nanoparticles in the ethylene glycol base ‚uid and the water base ‚uid using the multiphase Mixture model. They observed a significant enhancement of the convective heat transfer coefficient when nanoparticle combines with nanoparticle in the ethylene glycol base ‚uid. Tayebi and Chamkha  numerically investigated natural convection enhancement in a square cavity filled with hybrid nanofluid. In another studies, Tayebi and Chamkha [18 and 19] examined numerically free convection enhancement of a hybrid nanofluid in an annulus . In their studies, they found that employing a hybrid nanofluid provides a better thermal and dynamic performance compared to the similar nanofluid. Recently, Benkhedda et al. investigated the three dienssional laminar mixed convection of hybrid nano‚uid ‚ow through an annular space with heat flux imposed in outer cylinder they found that increasing the number of Grashof increases the effect of the buoyancy force which increases the heat transfer rate.The present study is a first attempt to use different nanoparticles shapes for a laminar flow of hybrid water-based suspension of and Ag nanoparticles where the two nanoparticles formed a hybrid nanofluid having the same nanoparticle shape, passing through a uniformly heated horizontal circular tube. Four different nanoparticle shapes are investigated; spherical, platelet, cylindrical and blade. Heat transfer and hydrodynamic characteristics such as the Nusselt number, friction factor and bulk temperature profile are reported. The effects of related parameters such as Reynolds number (750 -1775) and solid volume fraction (0% – 8%) and nanoparticle shape of both nanofluid and hybrid nanofluid are examined.