Investigation of centrifugal compressor vaneless diffuser stability via a local flow angle approach

A local flow angle approach to centrifugal compressors vaneless diffuser stability is proposed. For this investigation, 12 compressors of varying geometries were simulated. This approach employs computational fluid dynamic modeling software to model each geometry. The results revealed that the width ratio of the diffuser matters. For the four geometries with b₄/R₄ < 0.05, the numerical tests showed that the local flow profiles were not able to breach the Senoo line. For the two geometries with 0.05 < b₄/R₄ < 0.06 the local flow angle breached the Senoo line 50% of the time. For the six cases with b₄/R₄ > 0.06, the local flow angle crossed the Senoo line for all geometries except one. There were four cases with b₄/R₄ > 0.075. For three of the four cases, the local flow angle approach found flow reversal at the diffuser inlet. These results show that the stability of the diffuser is dependent upon the width ratio. Also, the results demonstrated that the ability of the local flow approach is limited by the width of the diffuser. A range of applicability for the local flow approach determining the stability was found. Lastly, for geometries with comparable width ratios the radius ratio and the impeller blade number influence the stability. This leads to the conclusion that as the width ratio shrinks the flow will breakdown in the circumferential direction as opposed to the spanwise direction.