Design and development of low solidity vaned diffusers for centrifugal compressors

Interest has recently increased in the use of Low Solidity Vaned Diffusers. Typically downstream of a centrifugal compressor impeller, three types of radial diffusers are used: (1) vaneless diffuser (VNL), (2) conventional vaned diffuser (CVND), and (3) Low Solidity Vaned Diffuser (LSVD). It is well known that a low solidity vaned diffuser is a good compromise between the vaneless diffuser and the conventional vaned diffuser.; Four low solidity vaned diffusers (LSVD5 through LSVD8) were designed for experimental analysis. Experimental testing of a centrifugal compressor with the low solidity vaned diffusers along with two vaneless diffusers and one vaned diffuser has been carried out downstream of the same impeller to understand the pressure recovery phenomena in each of the three types of diffusers, and the effect of design parameters on performance and the results were compared with the results from numerical simulation. The design parameters include the solidity, turning angle, vane setting angle angle, and the number of vanes. The testing was performed at three different rotational speeds (M{dollar}rmsb{lcub}u{rcub}{dollar} = 0.69, 0.88, 1.02). The experimental results proved the superior merits of LSVDs relative to the vaneless and vaned diffuser. The LSVD fulfilled the high expectations since they seemed to combine the advantages of the conventional diffuser systems by providing a good pressure recovery over a wide flow range.; Besides steady performance analysis, pressure fluctuations were measured to understand the instability of a compressor stage such as stall and surge using fast response dynamic pressure transducers installed in various locations of the compressor stage. A rotating stall was found only with the vaneless diffuser configurations. It is presumed the progressive impeller stall is due to the destabilization of the impeller flow. This fluctuation was added to the destabilization of the vaneless diffuser and triggered the stage surge with high fluctuation.; Some of the experimentally tested low solidity vaned diffusers were compared with 3-D viscous calculations to understand the flow phenomena inside the diffuser. The effect of geometric parameters of the low solidity vaned diffuser on the flow characteristics was also investigated by numerical calculation. The results showed that low solidity vaned diffuser stalls when the vane suction surface separation was accompanied by the end wall separation, and this qualitatively compared well with the experiments.