Entry length correlations for alumina-water nanofluid in laminar pipe flow

Numerical simulations were performed on the hydrodynamic and heat transfer characteristics of the three-dimensional flow of Al2O3-water nanofluid for horizontal circular tube flow. The flow is incompressible, steady-state, with a constant and uniform heat flux along the tube surface under boundary conditions. The nanofluid is considered with both constant and temperature-dependent thermophysical properties with volume fractions between 0.1 % and 5 %. The simulations were carried out using the single-phase Newtonian model with Reynolds numbers between 310 and 1950. The Nusselt number and the heat transfer coefficient in the tube are investigated simultaneously in the developing and hydrodynamically developed regions. Our results showed that higher nanoparticle concentrations improved the heat transfer performance. We compared the accuracy of simulation results using temperature-dependent and constant thermophysical properties. The results for the temperature-dependent thermophysical properties were found to be more accurate than those for the constant properties. Novel correlations were proposed to calculate the hydrodynamic entry length and the thermal entry length for Al2O3-water nanofluid flow using the temperature-dependent thermophysical properties. The results show that nanofluids require lower flow velocity and pumping power compared to the base fluid to provide the same level of heat transfer coefficient. Our study found that nanofluids with a volume fraction below 0.6 % have a favourable Performance Efficiency Index (PEI). However, higher volume fractions showed reduced PEI regardless of the inlet velocities. The highest performance efficiency index was observed at a nanoparticle volume fraction of 0.1 % for Al2O3-water nanofluid.