Numerical investigation of turbulent heat transfer enhancement using combined propeller-type turbulator and nanofluid in a circular tube

Heat transfer enhancement is an important issue because of environmental effects and also cost reduction in any thermal installation. A combined propeller-type turbulator and Al2O3 nanofluid are considered for heat transfer enhancement in a horizontal circular tube. Three-dimensional governing equations using the two-phase mixture model with SST k-epsilon turbulent model are numerically solved to predict the behavior of different thermo-fluid parameters in a horizontal circular tube. In addition to studying the effects of these, both means on the heat transfer enhancement, an important discussion on their simultaneous effects are also presented. The effects of turbulator on the generation of swirl flow and also on the turbulent kinetic energy are shown and discussed for different nanoparticle volume fractions and different nanoparticle mean diameters. On one hand, the effect of inserting a turbulator on the thermal performance significantly dominates the effect of using a nanofluid. For instance, at the Z/D = 13.5, using combined nanofluid and turbulator the heat transfer coefficient 2.68 times augments while using only a turbulator it is 2.53 times enhanced. On the other hand, turbulator generates a radial gradient on the momentum forces. The latter significantly deteriorates the uniformity of the nanoparticles concentration. A higher nanoparticle concentration at the near-wall region where the sedimentation problem is taken placed is seen. Thus, it is recommended that nanofluid is not a suitable choice for simultaneous consideration with a propeller-type turbulator in such an application particularly when considering its physicochemical stability drawback. However, further experimental investigations must be carried out to verify such a conclusion and recommendation. It should be mentioned that agglomeration and sedimentation of nanoparticles on the wall surface have not been considered on the modeling of nanofluid physical properties.