A radial inflow expander is a turbine. It extracts energy from the working fluid (gas), and supplies power to a rotating shaft. In this type of machine, the flow of the gas is (mostly) radial, and (mostly) inward from a large radius to a smaller radius, and generally exits the expander in a (mostly) axial direction near the shaft diameter. There is some means of flow distribution at the outer diameter, to control for uniformity and inlet angle of the flow. Multiple wheels (stages) can be assembled on a shaft, with return channels redirecting the flow from the hub back out to the OD of the succeeding stage.
Think of it as the opposite of a centrifugal compressor.
They can offer a more compact axial configuration than an axial flow steam turbine, but are not as efficient as axial flow turbines, nor can they handle such large volume flows as axial turbines.
