The aim of the present study was to investigate heat transfer characteristics of turbine oil based nanofluids inside a circular tube in laminar flow under constant heat flux boundary condition. Oil based nanofluids were prepared dispersing less than 1% volume concentrations of CuO, TiO2 and Al2O3 nanoparticles in turbine oil using a two-step method. The primary objective was to evaluate and compare the effect of different volume concentrations and nanoparticle types on convective heat transfer. An experimental apparatus was designed and constructed to measure the heat transfer coefficient and the Nusselt number of the samples. Due to the high prandtl number of the nanofluids (about 350), we concluded that the nanofluids were in the developing region. Experimental results clearly indicated that all of the added nanoparticles improved both the heat transfer coefficient and the Nusselt number of the turbine oil. Nanofluid is more capable than single-phase fluid insofar as removing heat from high heat flux surfaces. The highest values of Nusselt number and Nusselt number ratio (the ratio of the nanofluid Nusselt number to that of the pure turbine oil) belonged to CuO/turbine oil nanofluid. Among the sample nanofluids, the highest Nusselt number ratio belonged to CuO/turbine oil (0.50%), TiO2/turbine oil (0.50%), Al2O3/turbine oil (0.50%) and Reynolds number of about 800 which were 1.38, 1.31 and 1.15 respectively. Moreover, so as to determine the efficiency of nanofluid, we compared the ratio of pressure drop and Nusselt number of 3 nanofluid samples with that of the base fluid .A third parameter (performance index) was evaluated to determine the possibility of practically using such nanofluids. All the obtained performance indexes for CuO/turbine oil and TiO2/turbine oil were more than 1 meaning, the employment of such nanofluids leads to a higher quality turbine oil.

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