• Energy Research

Purpose This is a 3D numerical study of convective heat transfer through a micro concentric annulus governing non-uniform heat flux boundary conditions employing water-Al2O3 nanofluid. The nanofluid is modeled using two-phase mixture model, as it has a good agreement to experimental results. Design/methodology/approach Half of the inner pipe surface area of the annulus section of a double pipe heat exchanger is exposed to a constant heat flux which two models are considered to divide the exposing surface area to smaller ones considering the fact that in all cases half of the inner pipe surface area has to be exposed to the heat flux: in model (A), the exposing surface area is divided radially to two parts (A1), four parts (A2) and eight parts (A3) by covering the whole length of the annulus and in model (B) the exposing surface area is divided axially to two parts (B1), four parts (B2) and eight parts (B3) by covering half of the annulus radially. Findings The results reveal that model (B) leads to higher Nusselt numbers compared to model (A); however, at Reynolds number 10, model (A3) exceeds model (B3). The average Nusselt number is increased up to 142 and 83 per cent at models (A3) with Reynolds number 10 and model (B3) with Reynolds number 1000, respectively. Originality/value This paper is a two-phase investigation of water-Al2O3 nanofluid in a micro concentric annulus under non-uniform heat flux boundary conditions.

In this experimental research, the laminar forced convection of water–magnetite nanofluid (NF) in a horizontal twisted tube (TT) is examined under a rotating magnetic field (MF). The findings are compared with those of the plain tube (PT). The influence of nanoadditive concentration ( $$\varphi$$ ), Reynolds number (Re), twist pitch (P) and MF arrangement on the heat transfer, friction factor and overall thermohydraulic features of NF is assessed. The MF consists of two magnets that rotate around the tube. For each of the magnets, three modes of clockwise rotation, counterclockwise rotation and without rotation are considered. The findings showed that the combined use of TT and rotating MF entails an increase in the overall thermohydraulic features of water–magnetite NF. In addition, it was found that the overall thermohydraulic features of NF augment with boosting $$\varphi$$ , while they decline with boosting P and Re. Moreover, it was revealed that the best thermohydraulic features of the water–magnetite NF belonged to the case of $$\varphi$$ = 2%, Re = 500 and P = 10 mm in the presence of a rotating magnetic field resulting from the clockwise rotation of the first magnet and the counterclockwise rotation of the second magnet.

The impetus of this experimental investigation is to analyze the laminar forced convection of water-based nanofluid (NF) including Fe3O4 nanoparticles inside a twisted tube. The impacts of NF concentration (0% < $$ \varphi $$ < 2%), Reynolds number (500 < $$ {\text{Re}} $$ < 2000) and twist pitch (10–100 mm) on the average Nusselt number ( $$ \overline{\text{Nu}} $$ ), friction factor, and overall hydrothermal performance indicator are assessed, and the results are compared with those of the plain tube. It was found that the $$ \overline{\text{Nu}} $$ of NF rises with boosting $$ \varphi $$ and $$ {\text{Re}} $$ , while it declines with boosting twist pitch. In addition, it was found that the rise of $$ \varphi $$ causes a rise in the friction factor, while it diminishes with the rise of $$ {\text{Re}} $$ and twist pitch. Moreover, the results depicted that the overall hydrothermal performance of NF in the twisted tube is superior to that of the water in the plain tube. The best overall hydrothermal performance of the NF occurred at $$ \varphi $$ = 2%, $$ {\text{Re}} $$ = 2000 and twist pitch = 10 mm.

Abstract The effects of various surface roughness geometrical properties including roughness height (5%, 10%, 15%), number (3, 6), and shape (rectangular and triangular) on the flow and heat transfer of slip-flow in trapezoidal microchannels were investigated. The effects of mentioned parameters on the heat transfer coefficient through the microchannel, average Nusselt number and pressure drop for Reynolds number of 5, 10, 15 and 20 were examined. The obtained results showed that increasing the roughness height and number increases the pressure drop due to higher stagnation effects before and after roughness elements and decreases the Nusselt number due to higher recirculation zones effects than obstruction effects. The most reduction in Nusselt number and the most increment in pressure drop occur at the roughness height of 15%, roughness number of 6 and Reynolds number of 20 by about 10.6% and 52.8% than the smooth microchannel respectively.

Abstract In recent years, numerous studies have been done on the rheological behavior and heat transfer of nanofluids. In this paper, an experimental study on the rheological behavior of hybrid WO3-MWCNTs/Engine Oil hybrid Newtonian nanofluid has been carried out. Tungsten oxide (WO3) nanoparticles have diameter of 23–65 nm and MWCNTs nanoparticles have diameter of 20–30 nm. Experiments were carried out at 6 vol fraction of nanoparticles of φ = 0, 0.05, 0.1, 0.2, 0.4 and 0.6% at the temperatures of 20, 30, 40, 50 and 60 °C. The results show that the relationship between the shear rate and shear stress of the base fluid and the hybrid nanofluid of WO3-MWCNTs/Engine Oil is linear at all temperatures and volume fraction of nanoparticles. So, the base fluid and the hybrid nanofluids are Newtonian. Also, with increasing temperature, the viscosity of nanofluid decreases and with increasing volume fraction of nanoparticles, the viscosity increases. Finally, a mathematical model has been proposed to estimate the nanofluid viscosity. The results obtained from the model show good compatibility with experimental values.