On the Phenomenon of Pressure Pulses Reflecting Between Blades of Adjacent Blade Rows of Turbo-Machines

The main emphasis of the paper is on the work done by the writer between the years 1957 and 1992 on the phenomenon of pressure waves in form of pulses which reflect from blades of adjacent blade rows of turbo-machines. The recently revived interest in “acoustic resonances”, whose details are still not well defined or understood, points to a realization that a new look at some previously unrecognized findings is needed to explain problems encountered in operation of compressors and turbines. The purpose of this paper is to call attention of the turbo-machinery community to an important physical phenomenon discovered more than forty years ago about whose existence and consequences there is little awareness today. The turbine test results which led the writer in 1957 to hypothesize the existence of the phenomenon of reflecting pressure pulses are described. Subsequently, his 1966 ASME paper is discussed. In it the writer reported on the photographed observations of pressure pulses reflecting between stationary nozzles and moving blades of a water-table turbine at Lehigh University, on the description of the various types of such waves, and on an explanation of some of the resonant blade excitation frequencies observed by NACA in a turbine of turbojet engine. This is followed by a description of his 1984 ASME paper in which more general formulae were derived for the blade excitation frequencies caused by reflections of pressure pulses between the rotor blades and both upstream and downstream stator vanes. These equations were subsequently used to explain blade excitation frequencies measured in an axial compressor stage. Finally, his 1992 AIAA paper is discussed in which additional formulae relating to the reflecting pressure pulses were derived and the process of formation of a pressure pulse explained. To put this work in perspective, the writer provided, in mostly chronological order, excerpts from reports on operational problems encountered with turbo-machines in service and brief descriptions, from selected publications, of pertinent research work.