Nowadays，more and more low aspect ratio（AR） blades are used in fans，how does the aspect ratio influence the performance of compressors?What's the difference between low AR blades and high AR blades?
Dear Xiang
The optimum stage efficiency is determined in terms of several factors. Among these factor is AR. The blade loss is dependent on a numerous parameters which include tip clearance, blade aspect ratio, and AR.
AR = (blade height length / blade chord length)
The cascade profile is convergence in compressor and is divergence in turbine, so to maintain the axial velocity thoroughly constant during compression and expansion processes both the blade height and chord lengths are varied. For compressor blade at the end of the blade cascade will be very short and thus have higher losses and more susceptible to mechanical stresses.
The design maintains the AR fixed along the cascaded stages. Hence to maintain the rotor blade and stator vane chords to be quite small if the aspect ratio is to be within the limit of designer experience. However, as the flow path height is contracted through each blade row to maintain acceptable flow, the aspect ratio is no longer under control of the designer. Generally, for compressor low aspect ratio is preferable, however it is limit by blade stress in turbine.
For compressor study in the attached file, it was reported that the mean aspect ratio is dependent on the blade loading. The efficiency of lightly loaded stage decreases as the rotor aspect ratio reduces from 1.0 to 0.67. On contrary, for moderate loads the efficiency increases as the rotor aspect ratio reduces from 1.2 to 0.7. See also in table 1, the effect of, AR, on surge margin for low and moderate blade loadings. Increasing the, AR, for low blade loading decreases the margin, however it increases for moderate loading.
Dear Xiang
The optimum stage efficiency is determined in terms of several factors. Among these factor is AR. The blade loss is dependent on a numerous parameters which include tip clearance, blade aspect ratio, and AR.
AR = (blade height length / blade chord length)
The cascade profile is convergence in compressor and is divergence in turbine, so to maintain the axial velocity thoroughly constant during compression and expansion processes both the blade height and chord lengths are varied. For compressor blade at the end of the blade cascade will be very short and thus have higher losses and more susceptible to mechanical stresses.
The design maintains the AR fixed along the cascaded stages. Hence to maintain the rotor blade and stator vane chords to be quite small if the aspect ratio is to be within the limit of designer experience. However, as the flow path height is contracted through each blade row to maintain acceptable flow, the aspect ratio is no longer under control of the designer. Generally, for compressor low aspect ratio is preferable, however it is limit by blade stress in turbine.
For compressor study in the attached file, it was reported that the mean aspect ratio is dependent on the blade loading. The efficiency of lightly loaded stage decreases as the rotor aspect ratio reduces from 1.0 to 0.67. On contrary, for moderate loads the efficiency increases as the rotor aspect ratio reduces from 1.2 to 0.7. See also in table 1, the effect of, AR, on surge margin for low and moderate blade loadings. Increasing the, AR, for low blade loading decreases the margin, however it increases for moderate loading.
In my experience there are two important factor to watch out for the aspect ratio, one is the end-wall(s) secondary flows and (also for a rotor tip leakages) – the aforementioned effects become more dominant for low aspect ratio blades, e.g. corner separation and so on. The second general rule is to do with loading, obviously more chord will allow you to increase the loading, hence turning, But at the expense of larger loss (skin friction) and so on, hence need to design the sections correctly to account for this.
In a nutshell, lower aspect ratio compressor blades are more challenging from an aerodynamic design point of view because of the stronger influence of secondary flows, i.e. there is a smaller region with "near 2D flow". They do carry several advantages, though:
- Since the rotor tip clearance is usually a fixed absolute dimension dictated by the thermo-mechanical design of the compressor, having the longer tip chord of the low aspect ratio row means a lower tip gap relative to the chord. This is known to increase the stall margin considerably.
- At same space/chord ratio, you need fewer blades
- The blade becomes more stiff, which may be desired from a mechanical strength point of view
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