Heat transfer and entropy generation analysis of turbulent flow of TiO2-water nanofluid inside annuli with different radius ratios using two-phase mixture model

Abstract

Turbulent fully developed flow of TiO2-water nanofluid through annuli with different radius ratios (RR) are numerically simulated using k-ε turbulence model and two-phase mixture model. Heat transfer and entropy generation studies are conducted to scrutinize the performance of systems in order to find the optimal working condition to reach the minimum irreversibility. Two cases for thermal boundaries are considered in which a constant and uniform heat flux is exerted on one of annulus walls, while the other wall is insulted. To confirm the validity of numerical solver, results are compared against those of other numerical and experimental data. The effects of Reynolds number (Re), nanoparticles concentration and the radius ratio of annuli – with the same cross sectional area – on heat transfer and entropy generation rates are studied. It is revealed that Re and RR have optimal values to maximize Nusselt and also the heat transfer enhancement due to addition of nanoparticles. In addition, it has been shown that RR has a high impact on entropy generation rate and for each nanofluid flowing inside an annulus, there is an optimal Re value to minimize entropy generation. Finally, by definition of the evaluation parameter of PE, which is a trade-off between the first and the second laws of thermodynamics, the optimum Re is found to reach the best PE of the nanofluid flowing inside annuli with different RRs.