The focus of the paper is on the evaluation of the correlations for predicting single-phase friction, single and two-phase forced convection heat transfer coefficients in rectangular narrow channels, where the wall heat flux and the coolant flow can reach relatively high values. For this purpose, several correlations are reviewed and assessed against the SULTAN-JHR experiments. These tests were performed at CEA-Grenoble with upward water flow in two vertical uniformly heated narrow rectangular channels with gap of 1.51 and 2.16 mm. The experimental conditions range between 0.2 and 0.9 MPa for the pressure; 0.5-18 m/s for the coolant velocity and between 0.5 and 7.5 MW/m(2) for the heat flux. The use of an appropriate turbulent friction factor leads to good comparison with the experimental data. The analysis of the single-phase turbulent heat transfer coefficient shows that the standard correlations (e.g. Dittus-Boelter) significantly under-estimate the heat transfer coefficient, especially at high Reynolds number. Therefore, new best-fitting correlations are derived. It is also observed that a reduction in gap size may lead to the enhancement of the heat transfer. The heat transfer is also under-estimated in two-phase flow if standard correlations (e.g. Jens-Lottes) are employed; however, good comparison with the experimental data are obtained with more appropriate models for fully developed boiling, such as the Forster-Greif correlation. The global accuracy associated to these correlations is also quantified in a rigorous manner. (C) 2016 Elsevier Ltd. All rights reserved.