Nanofluids are a class of heat transport fluids created by suspending nano-scaled metallic or nonmetallic particles into a base fluid. Some experimental investigations have revealed that the nanofluids have remarkably higher thermal conductivities than those of conventional pure fluids and are more suited for practical application than the existing techniques of heat transfer enhancement using millimeter and/or micrometer-sized particles in fluids. Use of nanoparticles reduces pressure drop, system wear, and overall mass of the system leading to a reduction in costs over existing enhancement techniques. The focus of this study is to determine the role of nanoparticle motion in the enhancement of the overall heat transfer coefficient of a nanofluid at different nanoparticle loadings. The enhancement of the heat transfer coefficient is determined experimentally by dispersing CuO nanoparticles (40 nm) with different particle loadings (0.25 wt% and 1 wt%) into water and then flowing the resulting nanofluid through a heated copper tube. The experimental results illustrated that numerous factors including Reynolds number and particle concentration are all capable of impacting the enhancement ratio. To further explain the impact of these variables on the hydrodynamic and thermal parameters of a nanofluid, we developed a CFD model using a Eulerian-Lagrangian approach to study the nature of both the laminar and turbulent flow fields of the fluid phase as well as kinematic and dynamic motion of the dispersed nanoparticles. The main goal is to provide additional information about the fluid and particle dynamics to explain the observed behavior in the experimentally observed trends of the heat transfer coefficient enhancement relative to both nanoparticle concentrations and fluid flow behavior. Our results indicate that heat transfer enhancement significantly depends on particle motion within the system and is highly dependent upon the position of nanoparticles relative to the tube wall. (C) 2017 Elsevier Ltd. All rights reserved.