From the view point of energy saving and the development of new energy resources, it is important to utilize geothermal resources and waste heat from factories. As one of the energy conversion expanders, there is a radial outflow reaction turbine (that is, Hero's turbine). Performance characteristics of Hero's turbine using subcooled hot water as a working fluid are clarified analytically and experimentally. It is found that : (a) there is an optimum rotational speed at which maximum turbine efficiency can be obtained, (b) Hero's turbine internal efficiency can be expressed by an algebraic equation (7), and (c) nozzle loss amounts to almost 90 percent of the total turbine loss.
