It is common practice to design heat exchangers that are operated in either the laminar or the turbulent flow regime and not in the transition flow regime. This could mainly be due to a lack of knowledge on the behaviour in the transition flow regime for several reasons. However, due to factors such as design constraints, heat exchangers may indeed operate in the transition flow regime. An experimental study was conducted to determine the lower and upper Reynolds number limits of the transition flow regime, and the characteristics of the heat transfer coefficients and friction factors for annular passages with different geometric dimensions. The inner wall of the annular passage had different degrees of non-uniform temperature, while the outer wall was insulated. Both heated and cooled flow applications with water as fluid (cold fluid and hot fluid respectively) were investigated. The isothermal condition investigation was also conducted for pressure drop. Four horizontal concentric counter-flow tube-in-tube heat exchangers with conventional inlet geometries were considered to obtain the required data. The hydraulic diameters of the test sections were 26.2 mm, 23 mm, 20.2 mm and 17 mm, their respective annular diameter ratios were 0.327, 0.409, 0.386 and 0.483 and their length-to-hydraulic ratios were 193, 221, 251 and 299 respectively. The flow was both hydrodynamic and thermally developing. Test data of laminar, transition and turbulent flow regimes was collected. However, the transition flow regime was the main area of interest for this study. The transition flow regime was found to exhibit either mixed or forced convection types. Average heat transfer coefficients were obtained for both heating and cooling cases, while friction factors were obtained for heating, cooling and isothermal conditions. Uncertainties in the friction factor and Nusselt number were on average below 5.6% and 10.5%, respectively. The geometric size of the annular passage, degree of wall temperature uniformity and direction of the heat flux (heating and cooling cases of annular fluid) had a significant influence on the heat transfer coefficients, friction factors and Reynolds number span of the transition flow regime. New correlations for predicting the transition flow regime Reynolds number spans, Nusselt numbers and friction factors were developed for the transition flow regime and predicted most of the data to within �10%.