An experimental investigation has been carried out to simulate the onset of gas entrainment phenomenon from a stratified region through branches located on a semi-circular wall configuration, in close dimensional resemblance with a CANDU header-feeder system. New experimental data for the onset of gas entrainment was developed during single and multiple discharge from an air/water stratified region over a wide range of Froude numbers (0 to 100), in order to thoroughly understand the onset of gas entrainment phenomenon. The present data is necessary in validating the analytical and numerical models of the onset of gas entrainment, particularly at low Froude numbers. Part I of this paper, Andaleeb et al. (2005), provides the theoretical investigation of the single discharge case.