Study Of Aerodynamic Loss Mechanism In Diffusers And Rotor-Stator Interaction In Centrifugal Compressors

Centrifugal compressors are widely used in petrochemical industry,aeroengines and gas turbines.Due to the expansion of petrochemical industry and development trend of large-sized and energy-saving machines,study of centrifugal compressors with large mass flow coefficient of above 0.15 has attracted much attentions.Up to date,centrifugal compressor model stages with mass flow coefficient of 0.24 have been developed and applied to products internationally,while centrifugal compressor model stages with mass flow coefficient over 0.18 are still unavailable in China.A primary feature of a centrifugal compressor with large mass flow coefficient is highly non-uniform flow at the impeller exit.This is due to large width of impeller exit and leads to enormous difficulties in the aerodynamic design of diffuser.This dissertation intends to design a diffuser applied in a centrifugal compressor model stage with mass flow coefficient of 0.2.Numerical and experimental methods are employed to study aerodynamic loss mechanisms in different diffusers,under the high flow non-uniformity at the impeller exit.Meanwhile,reasons for pressure pulsation in flow field between the impeller and the diffuser are also investigated.This paper consists of the following four parts:1.Steady numerical simulation schemes are studied for a centrifugal compressor model stage with a vaneless diffuser under the condition of simplified clearance structures.Effects of the length of rotating walls,which closes the gap between the impeller and diffuser(no clearance leakage),on the performance prediction of the model stage are analyzed.By constrasting numerical results for the stage with and without clearance leakage to experimental data,a reasonable rotating wall height range is obtained.In addition,the location of the mixing plane between the impeller and the vaneless diffuser is also studied in steady numerical simulations.Through comparing steady numerical results with unsteady numerical results and experimental data,a distortion factor is proposed to determine a reasonable location of the mixing plane.2.Effects of the radial height of a rotating vaneless diffuser on vortex structures at the impeller exit are studied.By comparison with results for a stationary vanelss diffuser,the mechanism of improvement of the uniformity of flow field at the impeller exit and stage performance is revealed for the rotating vaneless diffuser cases..3.Vortex structures and loss mechanism in different types of diffusers are investigated including a vaned diffuser with whole height,a rib vaned diffuser and a tandem vaned diffuser.Features of flow field inside the three diffusers are also revealed.Based on the results obtained,modification of the existing vaned diffuser for the centrifugal compressor stage with the mass flow coefficient of 0.2 studied is carried out.4.Evaluation of unsteady single-passage numerical simulations for the study of rotor-stator interaction is conducted by comparison of predicted pressure fluctuations with measurements for a centrifugal compressor stage with the mass flow coefficient of 0.109.Based on this work,unsteady single-passage numerical simulations are employed to study the rotor-stator interaction of the centrifugal compressor stage with mass flow coefficient of 0.2.Reasons for pressure pulsation and sources of energy loss in the diffusers are analyzed and examined.Based on the above studies,a reasonable steady numerical simulation strategy is obtained for the centrifugal copressor with a vaneless dffuser.The baseline diffuser with tandem vanes of a large mass flow coefficient model stage is modified based upon vortex structures and flow field.Experimental results show that the measured polytropic efficiency of the stage with the modified rib vaned diffuser is 2.5%higher than that of the baseline fiffuser.