An experimental system with a transparent flash evaporation chamber was set up in this work to simulate the single-phase seawater flow in the flash stage in the multi-stage flash (MSF) desalination process. The flash evaporation chamber is 300 mm in height x 823.8 mm in length x 40 mm in width and was designed to be able to run under flash evaporation flow conditions. The whole field fluid flow velocity vectors in the chamber were measured using a particle image velocimetry (PIV) system to study the effects of main flow parameters such as water level and flow rate on flow patterns. The fluid flow in the chamber was also numerically simulated using a two-dimensional k-epsilon turbulent flow model. The measured free surface profiles and inlet gate velocity distributions were used as the boundary conditions. The streamlines were generated directly from the velocity vectors using the TECPLOT software package. The simulated results were compared with the measured data. The results show that a large recirculation region with several vortices embedded would be generated at a higher water level or at a larger flow rate.