Abstract
In the present paper, three centrifugal stages of high volume flow coefficient are compared to each-other regarding their aerodynamic performance in design point and off-design point conditions at different speed and IGV-setting angle: two stages with full-blade design (no splitter blades) have been numerically designed with different design geometry methodology. One geometry is based on a classical ruling surface design with a linear leading edge, the second geometry based on a fully-3d surface including a blade bow at the trailing edge and a barreled sweep at the leading edge. According to impeller test rig measurements and CFD-calculation, the classical ruling surface designed impeller outperforms the more sophisticated centrifugal stage with fully-3D-blade at fully axially guided IGV-flow. In the contrary, at closing IGV-off-design setting angles, towards surge operation, the fully-3D-blade-impeller performs with higher efficiency and steeper negative pressure slope. On the search of the geometrical causes for the different aerodynamic performance (especially at IGV-off-design conditions), focus is set on the analysis of IGV-flow-interaction with the inducer flow, and impeller diffusion. The one-dimensional -analysis of the span-wise flow at the impeller leading edge reveals that, compared with the ruling surface impeller, the fully 3D-blade performs with lower flow incidence losses in favor to IGV-off-design operation than at IGV-neutral position. The stream-wise flow analysis confirms the improved flow incidence characteristics of the 3D-blade impeller due to reduction of aerodynamic blockage and entropy production in the vicinity of the impeller leading edge. Based on CFD-calculations, a new correlation of secondary flow and flow incidence is proposed, to be used for one-dimensional modelling.
