Flow Characteristics of Axial Compressor Tandem Cascades at Large Off-Design Incidence Angles

Abstract

In a number of recent and former publications, compressor tandem blade configurations show potential to outperform single blade configurations in terms of turning, loss and operating range at high aerodynamic loading levels. However, very little insight is given into the mechanisms of flow breakdown when comparing tandem blades to single blades at large off-design incidence angles. Single blade cascades tend to fail as a result of either pressure side flow separation for high negative incidence or suction side flow separation for high positive incidence, the latter being mostly accompanied by significant increase of underturning. Tandem blade cascades are expected to show a different behavior due to the aerodynamic interaction in the blade overlapping region. Two different tandem blade configurations are examined together with their respective reference single blades, one being a recently designed and optimized tandem blade for high subsonic inlet Mach numbers, which has also been investigated in cascade wind tunnel testing. The other one is a more generic tandem blade based on NACA65 family, designed for medium inlet Mach numbers using current state-of-the-art understanding of tandem design. The mechanisms of flow breakdown are examined using quasi two-dimensional RANS simulations which are validated with test data for one of the aforementioned tandem configurations. A detailed analysis of the flow structure at heavy off-design conditions gives insight into the characteristics of tandem flow breakdown. In particular, the ability of the tandem configuration to extend the operating range to larger positive incidence is described. The shortcomings of the tandem cascade at large negative incidence are also commented. These and further conclusions can be used to improve tandem blade performance at moderate off-design conditions.

Experimental Investigation of a Transonic Compressor with Variable Stator Vanes in Tandem Arrangement

“…From this it can be concluded that the VSV is more vulnerable to positive incidences, caused by opened VSV angles. This matches the findings of Schneider and Kožulović (2013). Their investigations show a lower susceptibility to positive than to negative incidences at subsonic stator inlet Mach numbers.…”

“…Otherwise the losses of the two vanes in tandem arrangement exceed the losses of the single vane. Schneider and Kožulović (2013), Hertel et al (2013), Hertel et al (2014) and Hertel et al (2016) carried out numerous investigations of a subsonic compressor tandem cascade, focusing on the flow breakdown mechanisms. They found that tandem vanes can outperform single vanes, but are very sensitive to varying incidence flow angles and Mach number variations.…”

Recent Insights into the Flow Topology Around Highly Loaded Tandem Vanes

“…For an incidence variation of i = +5 deg the optimum configuration could be found for a percent pitch of PP = 95 %. This shift towards to a smaller gap size for high incidence angles was also confirmed by Schneider [13]. An increase in inflow angle further accelerates the flow, drawing more fluid through the gap and therefore maintains the positive tandem effect at high incidence angles.…”

“…He points out that if the pitchwise gap is too big, the beneficial and stabilising influence of the gap flow on the boundary layer development on the aft blade is lost. Müller et al [4] as well as Schneider and Kožulović [13] have looked at the off-design performance of tandem configurations. Varying the incidence mainly affects the location of transition on the front blade.…”

Analysis of Flow Characteristic of Transonic Tandem Rotor Airfoil and Its Optimization

“…Ju et al [16] optimized a 2D tandem blade with the multiobjective genetic algorithm and artificial neural network method. A massive amount of simulation results [17][18][19][20] indicated that the tandem blade performance should be estimated in design and off-design conditions comprehensively; for instance, a larger pitchwise blade distance will lead to an increase of the loss and the choke margin simultaneously. All the results stated above show the prescribed velocity distribution design concept has become an effective measure in highly loaded axial compressors optimization.…”