Swirling air flow is a key feature in many types of combustors. Tangential flow component is generated in an aerodynamic element called swirler (swirl generator, flame holder), which is often designed in the form of axial guide vanes. Such design is typical in low-NOx diffusion burners with staged gas and/or air supply. The swirler is a key burner design component that significantly influences the flow pattern in combustion chambers. Current industrial practice in the CFD modelling of swirling flow combustors tends to include swirler into the computational domain since detailed measured data of inlet velocity profiles for swirling combustion air are generally unavailable. However, including swirler into computational domain has not been verified or deeply discussed for standard turbulence models. Therefore there is a need for validation of RANS-based industry-standard codes in the prediction of flow through swirl generators. This work compares predicted velocity profiles and swirl numbers to the measured data. Measurement was carried out at the water tunnel facility with guide vane swirler placed before a sudden expansion chamber. Several drawbacks of standard turbulence models are revealed. Results show problematic predictions near the axis of water tunnel and near the wall.