The paper presents comprehensive experimental and numerical studies on water flow through rectangular S-shaped diffusers. The experimental setup was designed and constructed to perform the measurements which have been carried out on twelve models of Sshaped diffusers. The measurements of pressure distributions along the outer and inner walls
of the S-shaped diffuser were performed for different parameters including area ratios, curvature ratios, inflow Reynolds numbers, turning angles (45/45, 60/60, 90/90), and flow paths (45/45, 60/30, 30/60). The energy-loss coefficients for each model are based on detailed measurements of the wall pressure distributions along walls of the S-shaped - diffuser models including long upstream and downstream tangents. The energy-loss coefficient data were plotted as a function of Reynolds number for the tested models at different geometrical parameters. The experimental data for different S-diffuser configurations have been used for assessing credibility of the numerical code using different turbulence models. High performances of computations with turbulence closure provided by the standard k-ε model have been carried out for prediction of the performance of S-diffusers at different inflow and geometrical conditions. The results showed that the diffuser energyloss coefficient is strongly affected by the geometrical parameters of S-shaped diffuser and inflow Reynolds number. Increasing area ratio, curvature ratio, and inflow Reynolds number increase the diffuser performance. The turning angle plays an important role in improving the S-shaped diffuser performance. But, the diffuser performance decreases, when the turning angle is larger than 60° and the flow path changes far than the symmetric path (45°/45°).