Experimental and numerical investigation of turbulent nanofluid flow in helically coiled tubes under constant wall heat flux using Eulerian–Lagrangian approach

• Experimental and numerical investigation of turbulent flow in helical pipes
• Tendency of particles to increase mean axial velocity and suppress turbulence
• Remarkable enhancement of heat transfer rate using helical tube and nanofluid
• More accurate predictions of two-phase approach compared to the single-phase one
• More performance index of the use of helical tube compared to the use of nanofluid

In the present study, turbulent flow in helically coiled tubes under constant wall heat flux is numerically and experimentally investigated. Pressure drop and convective heat transfer behavior of water and water–silver nanofluid are studied. In experimental section, the pressure drop measurements as well as the average convective heat transfer coefficient calculations are carried out. The numerical computations are performed by Eulerian–Lagrangian two-phase approach in connection with an RNG k–ε turbulence model accounting for four-way coupling collisions using ANSYS CFX software. The Brownian motion of nanoparticles is taken into account. Single-phase approach (homogeneous model with constant effective properties) is also used. Two-phase approach predicted much more accurate results than the homogeneous model. More enhanced heat transfer was observed for tubes with greater curvature ratio. The results showed that the nanoparticles did not change the axial velocity and turbulent kinetic energy significantly, while the micrometer-sized particles increased mean axial velocity and suppressed turbulence. It was found that the utilization of the base fluid in helical pipe with greater curvature ratio compared to the use of nanofluid in straight tubes increased heat transfer more effectively. Based on the numerical results validated by the experimental ones, two correlations were developed to predict the ratio of the mean heat transfer coefficient and the pressure drop of nanofluid to water in helical tubes.

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