Photochemical reactors tend to exhibit turbulent flow even with low Reynolds numbers. The k-epsilon model is not always appropriate in this situation. An annular photoreactor was designed with tangential inlet and outlet tubes to investigate this. The fluid flow was characterized by residence time distribution (RTD) experiments, which were reproduced by computational fluid dynamics considering four relevant turbulence models: the k-epsilon, the k-omega, the shear stress transport, and the Reynolds stress models. Inlet effects induced helical flow throughout the reactor, switching to plug flow depending on the flow rate and the turbulence model. The k-omega model properly deals with viscous effects and reproduces the experimental RTD curves with correlation coefficients greater than 0.9566, against 0.8705 from the k-epsilon model.