Abstract: An improved vaneless fluid impeller of the friction type including a plurality of co-rotating axisymmetrical aligned spaced annular disks mounted for rotation about a common axis. Depending on whether a low kinematic viscosity or a high kinematic viscosity fluid is being utilized, the spacing between adjacent side surfaces of the disks decreases with increasing radial distance from the axis of rotation or increases with increasing radial distance from the axis of rotation, respectively, to provide increased energy transfer efficiency. The impeller with variable inter-disk spacing may be used either for radial flow directed outwardly or radial flow directed inwardly.