In the first part of this article on the design of a Sirocco fan stage, we showed that there is scope for improvement in the baseline impeller especially in terms of achieving the stage total pressure rise requirement.
In this second part, we show how the performance of the baseline stage can be enhanced to achieve the target through the application of manual redesign on the baseline impeller and volute.
A parametric study was started on the baseline fan and in the first redesign, we looked at the blade loading on the baseline design, and we started doing a parametric study. The first redesign was to increase the leading edge loading as shown in Figure 1. By increasing the leading-edge loading in TURBOdesign1, we design with positive incidence as shown in the geometry comparison of the two designs. This is also reflected in the relative velocity plots from the inviscid flow analysis in TURBOdesign1 where there is now a gap at the leading edge throughout the span.
By doing this we see a slight reduction in diffusion along with a total elimination of flow separation, indicating that the blade is now well-aligned with the incoming flow. There is also a corresponding jump in the total-to-static efficiency and pressure rise levels in the CFD results, but still far from the target of 500 Pa.
Figure 1: Cordier-diagram for axial, diagonal and radial fans
In the second redesign shown in Figure 2, we change the blade loading from mid-loaded to fore-loaded, as can be seen in the geometry comparison. The relative velocity plots clearly show that the loading is increased in the front portion and reduced in the aft-portion because of this change in the blade loading. In fact, this has quite a major effect: In addition to a significant reduction in the diffusion ratio, this early turning helps to better guide the flow through the blade passage, and as a result it improves the efficiency as well as the pressure rise levels significantly. Consequently, the total-to-total pressure rise is even higher than the target value of 500 Pa.
Figure 2: Meanline design of Sirocco fan stage in TURBOdesign Pre
Figure 3 shows the last step of manual redesign, where we extract the wheel exit velocity from the stage CFD results which is highly non-uniform around the circumference, and not a constant value that was used for the original volute. Basically, we take the average of the circumferential plots of radial and tangential velocity at the hub, midspan and shroud, and then use it in TURBOdesign Volute to optimize the volute geometry and its cross-section area variation for the given wheel exit flow. It is interesting to note that the resulting area variation obtained using the non-uniform velocities is quite different when compared with the results obtained with constant velocity.
Figure 3: Sirocco fan volute redesign based on impeller wheel exit velocity
Finally, the stage CFD results with the redesigned volute are presented in Figure 4, and it is interesting to observe that there is a further increase in the total-to-static efficiency and pressure rise as a result of the volute redesign, with the total-to-total pressure rise still maintained above the target value. The reason can be understood from the static pressure comparison on a cross-section between the two volute designs, where we end up with a higher static pressure recovery in the redesigned volute that has a positive impact on the total-to-static efficiency.
Figure 4: Redesigned Sirocco fan volute showing higher static pressure recovery
The parametric study of the manual redesign clearly demonstrates that the blade loading inputs to the inverse design method very directly influence the efficiency of Sirocco fans, and that there are two main criteria for producing high performance:
- Positive leading-edge loading should be applied
- Streamwise loading distribution should be fore-loaded at both hub and shroud
If these guidelines are followed, high performance Sirocco fans can be reliably and quickly produced using manual redesign in TURBOdesign1. In fact, there is a lot of generality about this type of loading because it works regardless of the fan speed or size, and therefore can be considered best practice of sorts. Furthermore, the importance of considering the highly non-uniform impeller exit flow when designing the volute is confirmed, which can be very easily done using the volute design code. The overall redesign process is very quick and the fan performance improvement is verified with CFD results, bolstering the importance of adapting the volute design to the impeller exit flow.