Heat transfer enhancement of Water-Al2O3 nanofluid in an oval channel equipped with two rows of twisted conical strip inserts in various directions: A two-phase approach

This is a numerical study of convective heat transfer of the Water-Al2O3 nanofluid in an oval channel using two-phase mixture model. The channel is fitted with two rows of twisted conical strip inserts with various directions relative to each other leading to three different combinations of the mentioned inserts, namely inward Co-Conical inserts (CCI-inward), Counter-Conical inserts (CoCI), and outward Co-Conical inserts (CCI-outward) in which its lower wall is exposed to a constant heat flux. The effect of Reynolds number ranging from 250 to 1000, nanofluid volume fraction ranging from 1 to 3 % and conical strip insert combinations are examined on the fluid flow patterns and heat transfer characteristics. The results showed that among the three combinations of the twisted conical strip insert, CCI-inward locally presents the highest values of heat transfer coefficient, as about 17% higher than plain tube, considering the nature of the secondary flow created in this case. It is also found that the effect of increasing nanofluid concentration on the channel thermal performance is more significant at higher values of Re number; however, the pressure drop difference between the three models is subtle. (C) 2019 Elsevier Ltd. All rights reserved.