An Essential Cause of Stall Stagnations in Compressor Surges

The author has proposed in the preceding study on the behaviors of infinitesimal perturbations of surge in simplified compressor systems that the occurrences of stagnations could be closely related with the travelling wave conditions of the surge oscillations through the phase-differences and the propagations along the flowpaths. The concept was applied to real situations of systems of compressors and flowpaths in the present study. It resulted in a non-dimensional parameter named flowpath-average reduced resonance frequency, f Rav, which is an equivalent of the phase-propagation parameter proposed in the preceding study. It is finally a product of the resonance frequency of the system and the time required for the fluid particles to pass through the whole flowpath, namely, the total number of excitations that the fluid particles suffer in the time. A number of numerical experiments on compressor surges have demonstrated that the f Rav parameter could explain the cause of surge stagnations very simply and clearly. It has values of 1.5 – 3 at the stagnation boundaries for variations in flowpath configurations and compressors having a single-stage to nine stages. It means that the stagnations occur for the total number of the excitation cycles less than 1.5-3. It could be the most essential cause of the stagnations. In other words, deep surges could develop only after the fluid particles have experienced more number of cycles of the excitations in the passing time. In further, the f Rav parameter is made more comprehensive by multiplication of the compressor tip Mach number Mt, i.e., f Rav×M t, which gives a nearly constant value of 1.5 over a wide range of flowpath configurations and tip speeds of compressors, for relatively small number of stages. Those parameters not only suggest the essential cause of surge stagnations, but also are the simplest and clearest measurers of stagnation boundaries. It is much easier to apply in real situations of the systems than any of the parameters proposed so far. However, for multi-stage compressors in long delivery flowpaths and in off-design operations, the parameters appear to deviate somewhat from the constant value. KCI Citation Count: 0