This experimental study focuses on the investigation of a flow boiling that occurs during the cooling process of a high temperature heated horizontal channel. Unpressurized water is used as working fluid. An intrusive thermally instrumented cell enables the phase change to be detected at different positions inside the channel. Alongside, welded thermocouples on the external wall of the channel give information about the influence of the phase change. Several cooling flowrates are tested in order to study their influence on the phase change duration. Finally, experimental results are compared to numerical simulations to determine some characteristic parameters as bulk temperature and heat transfer coefficient.