An experimental investigation was carried out to determine the average heat transfer coefficients and friction factors in the transitional flow regime of a horizontal concentric annular passage. The flow was in the mixed convection flow regime and was both hydrodynamic and thermally developing. The annular diameter ratio was 0.483 with an inner passage wall diameter of 15.90 mm. The test facility, which consisted of a counter-flow heat exchanger having a heat transfer length of 5.08 m, was operated at different degrees of longitudinal wall temperature uniformity on the inner wall of the annular passage. Both heated and cooled flow applications with water as fluid (cold fluid and hot fluid respectively) were investigated using a conventional annular inlet geometry type. It was found that the degree of temperature uniformity on the inner surface of the annular passage had an influence on the transitional Reynolds number span, the heat transfer coefficients, and friction factors. Depending on the wall temperature uniformity it was found that the critical Reynolds number, based on the Nusselt number results, was approximately between 350 and 500 for a cooled annulus and between 430 and 510 for a heated annulus. The critical Reynolds numbers based on the friction factor results were different from those based on the Nusselt number and was found to be approximately 800 for isothermal flow, between 1000 and 1030 for a cooled annulus, and approximately 1000 for a heated annulus. Correlations for the prediction of the transitional regime Nusselt number and friction factor as a function of among others, the wall temperature uniformity are proposed. The Nusselt number correlation predicts all the experimental data within a +/- 6% error band for the heated and cooled annulus cases. The friction factor correlation also predicts all the data points, within a +/- 6% error band. (C) 2017 Elsevier Ltd. All rights reserved.