Mixed convection heat transfer of nanofluids inside curved tubes: An experimental study

In this study, convective heat transfer enhancement of Ag-Heat Transfer oil nanofluids, flow inside curved tubes, has been investigated empirically in the thermal entrance region. Tube wall temperature set to be constant (91 °C) as boundary condition. Experiments were carried out for nanofluids and pure fluid flow inside straight and curved tubes to achieve not only the effect of using nanofluids, but also the effect of changing geometry, on the heat transfer. Several parameters such as Reynolds number ranged from 10 to 300, Dean number changed from 5 to 50 and coil to tube diameter ratio (Dc/d) varied from 14.1 to 63.5, have been taken into account. Utilizing nanofluids were prepared with a novel one-step method known as Electrical Explosion of Wire (E.E.W) in three weight fractions of 0.12%, 0.36% and 0.72%. In order to achieve reliable data, all of the rheological properties of applied nanofluids including thermal conductivity, viscosity, density and special heat capacity were measured experimentally. Based on the results, convective heat transfer coefficient and the Nusselt number enhancements were observed to be 57% and 33% respectively. For 0.72 wt%, the maximum thermal conductivity enhancement was measured to be 30% and viscosity enhanced about 28% rather than the pure base fluid. Finally mixed convection heat transfer of working fluids has been assessed due to determining local Nusselt number in the state of Rayleigh number = 1.03.