Experiments on mixed convection heat transfer and performance evaluation of MWCNT-Oil nanofluid flow in horizontal and vertical microfin tubes

The mixed convection heat transfer characteristics of MWCNT (multi walled carbon nano tube) heat transfer oil based nanofluid were experimentally investigated inside smooth and microfin tubes. The tubes were submitted to uniform wall heat flux over outer surface which were produced by an electrical coil heater. Data were acquired for the horizontal and upward vertical laminar flows in the thermal entrance region. Pure heat transfer oil and nanofluids with particle weight concentrations of 0.05%, 0.1% and 0.2% were utilized as the working fluids. The experiments were performed in the range of Grashof number between 102 and 104, Reynolds number between 10 and 150 and Richardson number between 0.1 and 10. The experimental results indicate that the heat transfer coefficient increases slightly with the increase of particle weight concentration from 0% to 0.2% in microfin and smooth tubes under a given Grashof number. Moreover, it is revealed that Nusselt number increases notably with Richardson number while it augments slightly by increasing Reynolds number in both horizontal and vertical tubes. In addition, results show that using microfin tube instead of plain one, causes an enhancement of heat transfer in each particle concentrations. Also, two empirical correlations are developed to predict the Nusselt number in the thermal entrance region in horizontal and vertical microfin tubes for the range of data used in the study. Finally, the overall performance of the tested microfin and smooth tubes were assessed based on the performance index. The performance evaluation of the two enhanced heat transfer techniques studied in this work shows that applying nanofluids instead of the pure oil is a more effective way to enhance the convective heat transfer coefficient compared to using microfin tube.