Links between Main Frequencies of Established Rotating Stall and Rotational Frequencies and/or Blade Passing Frequencies

The ratios between the main frequency of rotating stall and rotational frequency may be considered in the form of exact ratios of small natural numbers if the pressure signals in compressors during rotating stall include the rotor rotation frequency component. During rotating stall in compressors with good rotor balancing (with absence of the rotational frequency component in the frequency characteristics of pressure signals), these ratios between the main frequency of rotating stall and rotational frequency are or are not in the form of ratios of small natural numbers. The experimentally received characteristics of power spectral density of pressure signals also show the presence of components with combinations of blade passing frequency and different harmonics of main rotating stall frequency. 1. Introduction Rotating stall is an undesirable and even dangerous oscillatory process in a compressor. Its important characteristic is that the phase of fluctuations of pressure on a compressor circle changes in any cross-section depending on the angle of sensor positions (see, e.g., Moore and Greitzer, 1986 [1], Greitzer and Moore, 1986, [2], Longley, 1994 and 2007, [3, 4], Day and Freeman, 1994 [5], Camp and Day, 1998, [6], Day et al., 1999, [7], Bright et al., 1999, [8], Inoue et al., 2002, [9], Bergner et al., 2006, [10], etc.). Three basic frequencies can be found in a compressor during an established rotating stall process and at its inception: the main frequency of rotating stall, the frequency of rotor rotation, and the blade passing frequency. Besides these, there can be their harmonics and also frequencies representing combinations of these three frequencies and their harmonics. The physical nature of all these frequencies is absolutely different. (i)Main (basic) frequency of rotating stall is caused by dynamic characteristics of the compressor and the gas system in which the compressor is included. (ii)Existence of components with rotational frequency is caused by mechanical reasons (nonideal geometrical sizes of blades, non-ideal balance of disks and blades of the rotor, shift of the axis of rotor rotation concerning the stator axis, etc.). (iii)Occurrence of pressure fluctuations with blade passing frequency is caused by the gas flow crossing the compressor rotor blades during rotation. These components exist even when compressor balance is ideal. The two first frequencies are frequencies of about one order; they are usually considered and analyzed in common. Blade passing frequency (being a product of the rotor rotation frequency and the number