Turbulent flow of a shear-thinning non-Newtonian CMC-based nanofluid in twisted finned tubes with an elliptical profile

Abstract

In this article, the forced convective turbulent flow of a shear-thinning non-Newtonian nanofluid through twisted tubes with elliptical cross sections is numerically studied and analyzed at various Reynolds numbers. The base fluid of the studied nanofluid is 0.5% carboxymethyl cellulose solution in water (which has non-Newtonian behavior of shear-thinning), and it contains copper oxide nanoparticles. Two types of twisted tubes are considered: finned tubes and unfinned ones. The finite volume method is utilized, and the two-equation model of k-ε has been applied to analyze the flow characteristics of the turbulent problem. The computational results demonstrate an excellent agreement with respect to already published reports, which is a testament to our reasonable and correct procedure. The effects of pipe wall twisting on the average Nusselt number have been investigated. The achievements demonstrate a powerful secondary and rotating flow is formed with the tube surface twisting. This secondary flow causes better mixing and increases the average Nusselt number. The increase is up to 45% for the unfinned case and 62% for the finned case in comparison with the tube without projections. In addition, the coefficient of friction in these pipes increases to 62% and 104% for the unfinned and finned tubes, respectively. It is also observed that by increasing the length of the fin and adding nanoparticles up to 1.5%, the highest value of the performance evaluation criterion (i.e. 1.78) in the twisted finned tube could be achieved, which occurred at the Reynolds number of 4000. The combination of present geometry and the working fluid of shear-thinning non-Newtonian CMC-based nanofluid is the main novelty of this work, and the results can be applicable and reliable in the relevant industries.