Three-Dimensional Rotating Stall Model For Vaneless Diffusers In Centrifugal Compression System

Vaneless diffuser stall is one of the most commonly seen unsteady flow phenomena in centrifugal compression system, which limits the steady operating range of the compression system as well as of the diffuser itself, and impacts operation reliability. In order to extend steady operation range and to improve the performance of the vaneless diffusers as well as the whole compression system, it is important to investigate the unsteady flow and to predict the stall limit of vaneless diffusers. Also, a further study and better understanding of the flow field in vaneless diffusers will have instructional value in optimizing diffuser geometries and matching of diffusers with other parts of the compression system.In this thesis, three three-dimensional vaneless diffuser stall models in centrifugal compression system were developed. Unsteady flow and stall characteristics in vaneless diffusers were studied. A three-dimensional vaneless diffuser stall model for incompressible flow was first proposed by solving linearized three-dimensional Euler equations for incompressible flow, and the effects of axial distribution of inlet radial velocity of undisturbed flow on disturbed flow and on stall characteristics were studied. Also studied were the effects of wave number, impeller backswept angle and diffuser width on diffuser stability. The results showed that axial velocity component of the disturbed flow would be provoked by inlet distortion of undisturbed radial velocity. And study of three different types of inlet distribution showed that the one which meet the conditions of general unsteady flow proposed by Fj0rtoft was least stable. The study of wave number showed that the least stable wave number consisted with the stall cell number found in most experimental measurements, which is 2 to 4. Impeller backswept angle stabilized vaneless diffusers and the stabilizing effect was more obvious for long diffusers. The multiple resonances found in diffusers with impeller backswept angle and inlet distortion provided a theoretical explanation of the experimental findings that a low-speed rotating stall cell could happen just after a high-speed rotating stall cell in vaneless diffusers. If the undisturbed inlet radial velocity distribution slop was kept, stability would be improved for long diffusers when the diffuser width increases.A three-dimensional vaneless diffuser stall model for compressible flow was then established based on the previous model for incompressible flow by solving linearized three-dimensional Euler equations for compressible flow. The effects of axial distribution of inlet radial velocity of undisturbed flow , wave number, impeller backswept angle and diffuser width on stall characteristics under different inlet Mach number were studied, and critical inlet mass flow rate and rotating speed of stall cells were predicted. The results showed that the amplitudes of disturbed flow parameters increased with inlet Mach number. For short diffusers, axial distribution of inlet radial velocity had more obvious effects on diffuser stability, while for long diffusers inlet Mach number had more obvious effects on diffuser stability. The effects of wave number on diffuser stability changed little with inlet Mach number, so did the relative stability extension brought by impeller backswept angle. However, the stabilizing effects of increasing diffuser width but keeping distribution slop increased with inlet Mach number. Under higher inlet Mach number, multiple resonances of which critical inlet mass flow rates were close to each other but rotating speed of stall cells were of difference were found not only in short diffusers but also in diffusers of medium length.Based on the developed stall models for vaneless diffusers with parallel walls, a three-dimensional stall model for vaneless diffusers with unparallel walls was then established via solving linearized three-dimensional Euler equations for compressible flow in non-orthogonal coordinates. The effects of different types of shroud wall on diffuser stability were studied. Also studied were the effects of wave number, impeller backswept angle and contraction of diffuser inlet part on diffuser stability in unparallel wall diffusers, and the critical inlet mass flow rate and rotating speed of stall cells were predicted. The results showed the stabilizing effects of shroud walls with contractions, and the effects increased with contraction ratio and inlet Mach number. For unparallel wall diffusers, the least stable wave number was smaller than that for parallel wall diffusers, but still higher than one. Impeller backswept angle showed stabilizing effect in unparallel wall diffusers. However, multiple resonances found in parallel wall diffusers were restrained in unparallel wall diffusers with short length. The study also found that contraction of diffuser inlet part had stabilizing effect on diffusers and the effect increased with contraction ratio even the inlet and outlet width of these diffusers were not changed.At last, the predicted stall characteristics from the developed three-dimensional vaneless diffuser stall model for incompressible flow and for compressible flow, and from the three-dimensional model for diffusers with unparallel walls were compared with the results from open literatures and the test rig established in our lab of low speed centrifugal compression system, the results from Honeywell CFD simulation of high speed centrifugal compression system, and the results from open literature of experimental measurements in an unparallel wall diffuser respectively. The comparison showed that for wide diffusers the predictions from the developed models based on core flow theory were satisfying and three-dimensional models gave better predictions than two-dimensional ones.