speed of the rotor in the present centrifugal reverse flow disk turbine above which the centrifugal effect becomes dominant is provided by solving the following equation 1 u03b4p pressure difference between at tip clearance and inlet near rotor shaft a u03b2 u03c6 parameters determined by given geometry of flow channel in the previous equation 1 centrifugal driving is initiated when the tesla saitoh number exceeds a value 2 915 in case that flow channel geometry obeys a logarithmic spiral if typical physical properties were given in this example one of hydro fluoro carbons hfc is selected as a working fluid and r 0 05 m u03b2 2 u03c6 60 u00b0 a u221a 1 p u2032 2 p u2032 3 333 and n 40 the critical speed of rotation n can be calculated to be if the speed of rotation exceeds this critical value the present centrifugal disk turbine will be operated under a centrifugal mode the optimal speed of rotation is approximately 12000 rpm for previously given conditions on the other hand the optimal speed of rotation for a pressure effect mode is about 6100 rpm which is much lower than that for a centrifugal mode in the present centrifugal driving mode.