Heat transfer enhancement of nanofluid flow at the entry region of microtubes

Abstract

A numerical investigation on laminar nanofluid flow and convective heat transfer at the entry region of microtubes subjected to constant wall temperature and constant heat flux boundary conditions has been carried out employing a multiphase Eulerian–Lagrangian method. The impacts of the Peclet number (175 ≤ Pe ≤ 3500), nanoparticle volume fraction (0.1 ≤ φ ≤ 1.0%), and nanoparticle diameter (40 ≤ dp ≤ 130 nm) on thermal characteristics of Al2O3-water nanofluid flow through a microtube are analyzed in detail. The results indicate that the influence of the Reynolds number on apparent friction factor and the impact of the axial heat conduction on the Nusselt number for the nanofluids have to take into account in the entry region. Compared with the influences of particle concentration and particle size, the entrance effect dominates pressure drop and thermal performance of the nanofluids near the entrance of the channel. As the dimensionless axial distance increases, the entrance effect is weakened, and the influences of particle concentration and particle size are gradually reflected in flow and heat transfer results. Besides, the particle effects on thermal performance of the nanofluids are earlier and higher than that on flow resistance. Performance evaluation demonstrates that the nanofluids in the entrance region have not only heat transfer enhancement but also a good economy. When Pe = 175, the PEC of Al2O3-water nanofluids with particle concentrations of 0.1, 0.2, 0.5, and 1% increased by 104.0, 103.1, 113.8, and 128.5% under constant heat flux condition, respectively, at the dimensionless axial distance x* = 0.01 compared with deionized water; the PEC improved by 74.6, 77.2, 85.6, and 102.5% under constant wall temperature condition compared with deionized water, respectively.