The velocity field inside a new concept of heat exchanger, which is a component of a high protons linear accelerator, is investigated experimentally in order to validate the design. A full scale facility with optical accesses is used for the measurements by particle image velocimetry. The choice of the technique is set by the three-dimensional and strongly unsteady structure of the flow. A filtering procedure is applied to the recorded images before processing the velocity field with an optical flow algorithm using dynamical programming. The distribution of the velocity between the different tubes of the heat exchanger shows a large scatter of flow rate between these tubes. In addition, the turbulence characteristics are presented.