Design High Efficiency Impellers with Splitter Blades

In the design of radial turbomachinery impellers, such as centrifugal compressor or pump impellers, splitter blades are often used to ensure good aerodynamic/ hydrodynamic performance without compromising flow range. In the case of pumps this is necessary to achieve good suction performance.

The conventional design approach is normally based on the use of the same blade profile on the full and splitter blades, with the splitter placed at mid-pitch. There is, however, considerable evidence that such design practice results in poor splitter blade performance. Further optimization of the splitter blade requires iterative modifications that are very time consuming, particularly if a three-dimensional design is to be employed. In TURBOdesign1, the full and splitter blade geometries are determined independently subject to individually specified loading distributions, as shown in Figure 3.

TURBOdesign1 provides considerable freedom in the specification of the loading distribution, and allows the designer to adjust the relative bound circulation on the splitter and full blades for a given specific work. Hence it is possible to increase the relative loading on the splitter blades at the expense of the full blades or vice-versa.

Fig.1: Conventional

Fig.2: TURBOdesign1

Fig.3: Loading distribution specified in TURBOdesign1 for the design of the impeller with splitter.

Fig.4: Comparison of blade angle distribution of the TURBOdesign1 impeller at the hub and shroud.

Fig.5: Comparison of surface pressure of conventional & TURBOdesign1 impellers at the shroud, confirming the elimination of the shock on the splitter blade.

Fig.6: This figure shows a negative incidence at the leading edge of the conventional splitter which has been eliminated by TURBOdesign1.

Fig.7: Comparison of the exit flow radial velocity in the conventional and TURBOdesign1 impellers showing a more uniform exit flow distribution in the case of the TURBOdesign1 impeller.

In Figure 1 the 3D CFD prediction of the flow field in a state-of-the-art conventionally designed impeller with a pressure ratio of 5.5 is presented. The predictions show a strong shock on the full blade and at the leading edge of the splitter blade.

TURBOdesign1 was applied to the design of this impeller by using the loading distribution shown in Figure 3. In order to reduce the shock strength at the leading edge, an aft-loaded distribution was used on the full and splitter blades. The blade angles computed by TURBOdesign1 are shown in Figure 4, where significant differences can be seen between the full and splitter blades, especially at the shroud.

The resulting CFD prediction (Figure 2) for the TURBOdesign1 impeller shows a reduction in shock strength on the full blade, elimination of the shock on the splitter blade and a more uniform exit flow from the impeller (Figure 2 and 7). These results are confirmed by the surface static pressure data presented in Figure 5 and 6, which show clearly the elimination of the shock on the splitter blade of the TURBOdesign1 impeller.