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The convective heat transfer and entropy generation of diamond-Fe3O4/water hybrid nanofluid through a rectangular minichannel is numerically investigated under laminar flow conditions. Nanoparticle volume fractions for diamond-Fe3O4/water hybrid nanofluid are in the range 0.05-0.20% and Reynolds number varies from 100 to 1000. The finite volume method is used in the numerical computation. The results obtained for diamond-Fe3O4/water hybrid nanofluid are compared with those of diamond/water and Fe3O4/water conventional nanofluids. It is found that 0.2% diamond-Fe3O4 hybrid nanoparticle addition to pure water provides convective heat transfer coefficient enhancement of 29.96%, at Re=1000. The results show that diamond-Fe3O4/water hybrid nanofluid has higher convective heat transfer coefficient and Nusselt number when compared with diamond/water and Fe3O4/water conventional nanofluids. For diamond-Fe3O4/water hybrid nanofluid, until Re=600, the lowest total entropy generation rate values are obtained for 0.20% nanoparticle volume fraction. However, after Re=800, diamond-Fe3O4/water hybrid nanofluid with 0.20% nanoparticle volume fraction has the highest total entropy generation rate compared to other nanoparticle volume fractions. A similar pattern emerges from the comparison with diamond/water and Fe3O4/water conventional nanofluids. For 0.2% nanoparticle volume fraction, diamond-Fe3O4/water hybrid nanofluid and diamond/water nanofluid have their minimum entropy generation rate at Re=500 and at Re=900, respectively. Moreover, this minimum entropy generation rate point changes with nanoparticle volume fraction values of nanofluids.

The convective heat transfer and entropy generation characteristics of Ag-MgO/water hybrid nanofluid flow through rectangular minichannel were numerically investigated. The Reynolds number was in the range of 200 to 2000 and different nanoparticle volume fractions were varied between  = 0.005 and 0.02. In addition, Artificial Neural Network was used to create a model for estimating of entropy generation of Ag-MgO/water hybrid nanofluid flow. As a result, it was found that the convective heat transfer coefficient for  = 0.02 Ag-MgO/water hybrid nanofluid is 21.29% higher than that of pure water, at Re=2000. Total entropy generation of Ag-MgO/water hybrid nanofluid increased with increasing nanoparticle volume fraction. The results obtained by ANN showed good agreement with the numerical results obtained in this study.