Design Specification of an Axial Flow Fan
CFD analysis of the existing fan design, under ducted fan configuration, showed flow separations both at the hub and at the tip regions of the fan blade (see Fig. 2a). Additionally, the CFD analysis of the cooling tower and the evaluation of the inflow to the fan showed flow distortion at the fan inlet. Considering these factors, the design strategy was set as follows:
- Suppress flow separations: Flow separations will cause not only the deterioration in fan efficiency but also have negative effects on the overall noise level due to the pressure fluctuation created by the flow separations
- Reduce loading at the hub section: The flow in the hub section can be disturbed by the downstream structure of the cooling tower such as a motor and a reduction pulley. Because of this, excessive blade loading at the hub will deteriorate the fan performance in reality
- Optimize the blade loading: Spanwise vortex pattern and the lean angle of the blade, considering the inlet distortion caused by the cooling tower structure
Following this design strategy, several different axial-flow fan blades were designed and evaluated by CFD. The final design exhibits well-suppressed flow separation at the tip as well as the improved flow pattern at the hub (see Figure 2b). The new axial-flow fan designed by TURBOdesign1 was installed into the cooling tower and tested under actual operating conditions. A significant improvement was gained across the whole flow range in the fan efficiency, with the efficiency increase more than 20% (see Fig. 3). Additionally, the noise level was also improved, particularly at higher frequency range.