Computational analysis of heat transfer enhancement in tubes using capsule dimpled surfaces and AL2O3-water nanofluid

This study aims to numerically investigate the enhancement of heat transfer by utilizing geometrical configurations as capsule dimples on tube surfaces. Two working fluids, water and Al2O3 water nanofluid, are used under uniform heat flux applied to the tube surface. The originality of this work lies in combining two passive heat transfer enhancement methods, using geometrical improvements and nanofluids together. Capsule dimples are used at different depths, such as 1 mm, 1.5 mm, 2 mm and 2.5 mm for each case. Al2O3 water nanofluid modeled as a single-phase fluid at three different nanoparticle concentrations of 1%, 2% and 3%. The effects of dimple depth and nanofluids in different nanoparticle concentrations on Nusselt number, friction factor and performance evaluation criteria (PEC) which are widely used in heat transfer enhancement evaluation, were studied. Numerical studies were performed using ANSYS 20R1 Fluent commercial software in the 2000 14000 Reynolds number range and inlet fluid temperature of 293°K. The turbulence is modelled using the realizable k  model. It was found that for laminar, transient and fully developed turbulent flows, as the dimple depth increases, the Nusselt number as well as the friction factor increases for both cases of water and Al2O3 Water nanofluid as working fluids. The Nusselt number for dimpled tubes is enhanced by 41.3% and 46.3% compared to plain tubes for water and nanofluid with 3% concentration, respectively. Furthermore, the variation of PEC for capsule dimpled tubes are dependent on flow regions and dimple depths. Generally, increasing the nanofluid volume concentration and dimple depth in laminar flows increases PEC significantly. Among all simulated cases, for laminar flow PEC would vary up 5.5 when dimple depth is 2.5 mm and volume concentration is 3%. In transition flows, PEC = 1.2 for 2 mm depth and 3% volume concentration. Lastly, for fully turbulent flows considered the highest PEC is obtained as 1.057 in the case of 1 mm depth and 1% volume concentration.