Fan Unit with Special Guide Vane Design for Low Hub Ratio
3.4 Fan Design for Improved Efficiency and Extended Operating Range
Low pressure axial fans for refrigeration, climatization and similar applications typically have low hub ratios of about 0.15..0.5. The reason for that is, on the one hand, that high volume flow rates are required at low pressure rise and, on the other hand, that the outer motor contour is used as impeller hub.
It is well known that fans with low hub ratio suffer from the phenomenon of “hub deadwater”, meaning the existence of a near-hub region of detached flow not contributing to the effective fluid transport from inlet to the outlet. This deadwater region is separated from the “sane” flow region by a separation layer within the fluid. The extent of this region grows with growing pressure rise required from the fan. The cross-section, which is effectively available for flow transport, is reduced by the detached flow region.
Classical guide vane design may leads to poor performances in such configurations. The reason for that is that by effect of guide-vanes, the cross-section of the hub deadwater may be increased reducing the effectively available cross section for the desired fluid transport. Consequently, at given volume flow rate, the axial flow velocity component increases. This effect can over-compensate the effect of reduction of the circumferential flow velocity component, resulting in flow acceleration and thus to a low or negative reaction ratio of the guide vanes.
In the present work, a stator fan unit including guide vanes and outer housing contour is designed which has primarily the effect to reduce the diameter of the hub detachment region. The cross-section effectively available for the fluid transport of the fan is additionally increased by using a diffusor contour of the housing. A part of the dynamic pressure available in the circumferential flow component is also reacted to static pressure rise.
The so designed fan unit leads to an important increase of static efficiency of the fan (20%-30%). The stall pressure point is significantly increased. Due to the reduced flow velocities, also the sound emissions are reduced significantly. The reduction of hub deadwater leads to more-compact, more cylindrical flow leaving the fan, reducing the risk of thermal short cuts in critical applications given by more divergent flow coming out of a fan without guide vane. We present in the paper comparisons of theoretical results obtained by simplified 1D-considerations, CFD results and experimental results.