Drag reduction system in shrouded turbo machine

A rotary machine to effect a useful energy exchange by changes in pressure of a working gas passing through the machine, comprising a rotor rotatably mounted in a housing having passages therethrough arranged in flow connecting relation with passages extending through the rotor for the passage of said working gas in energy exchanging relation with the machine rotor, said rotor comprising a rotatable shroud having blade members extending from a radially inner surface of the shroud, the shroud having a first, radially inner boundary and second, radially outer boundary defining an annular surface therebetween, a first gas seal means adjacent said first boundary and a second gas seal means adjacent said second boundary to substantially isolate an annular zone surrounding the shroud, displacer gas supply means connected to the zone to provide low density displacer gas thereto, selected from the group consisting of hydrogen and helium, to reduce parasitic gas drag acting on the shroud, a gas annulus within the zone located adjacent the first seal means having gas supply means connected thereto including inlet flow control valve means to regulate the gas supply, a gas collection gallery within the zone adjacent said second seal to collect said displacer gas together with gas entering through said second seal and gas exhaust passage means connected to said gallery having an exhaust flow control valve connected therewith to regulate the passage of gas including said displacer gas from said zone.