In this study, a mathematical model is proposed to predict flow characteristics of Newtonian fluids inside a concentric horizontal annulus. A numerical solution, including pipe rotation, is developed for calculating frictional head losses in concentric annuli for turbulent flow. Navier-Stokes equations are numerically solved using the finite differences technique to obtain the velocity field. Experiments with water are performed in a concentric annulus with and without pipe rotation. Average fluid velocities are varied in the range of 1.1-3.3 m/s at various inner pipe rotations (0120 rpm) in a horizontal concentric annulus. To verify the proposed model, estimated frictional pressure losses are compared with experimental data and the commercial software package ANSYS Workbench 10.0. The numerical model predicts frictional head losses with an error less than +/-10% in most of the test cases.