Gas-liquid two-phase slug in a concentric annulus with a rotating inner cylinder has been experimentally investigated. The flow patterns identified on the basis of visual observations are classified into four categories, i.e., slug flow, spiral-slug flow, peripherally elongated slug flow and ring-form slug flow. Such a flow regime map is expressed as a function of the total volumetric flux and the rotational speed. The rising velocity of a large bubble approximately coincides with Nicklin et al.\u27s correlation at relatively low rotational speed, while the rising velocity is lower than that of Nicklin et al.\u27s correlation at high rotational speed. This is closely related to the void fraction distribution along the radial direction in the annulus. The ratio of the kinetic energy loss to the potential one in the slug flow regime is well correlated by a linear function of the volumetric flux ratio of two phases as in the case of a bubbly flow regime. This ratio of energy losses is well expressed using the drift-flux model modified by taking into account the large bubble size in the annulus
