Effect of Upstream Rotor Vortical Disturbances on the Time-Average Performance of Axial Compressor Stators: Part 2 — Rotor Tip Vortex/Streamwise Vortex-Stator Blade Interactions

Abstract

In a two-part paper, key computed results from a set of first-of-a-kind numerical simulations on the unsteady interaction of axial compressor stator with upstream rotor wakes and tip leakage vortices are employed to elucidate their impact on the time-average performance of stator. Detailed interrogation of the computed flowfield showed that for both wakes and tip leakage vortices, the impact of these mechanisms can be described on the same physical basis. Specifically there are two generic mechanisms with significant influence on performance: reversible recovery of the energy in the wakes/tip vortices (beneficial) and the associated non-transitional boundary layer response (detrimental). In the presence of flow unsteadiness associated with rotor wakes and tip vortices, the efficiency of the stator under consideration is higher than that obtained using a mixed-out steady flow approximation. The effects of tip vortices and wakes are of comparable importance. The impact of stator interaction with upstream wakes and vortices depends on the following parameters: axial spacing, loading, and the frequency of wake fluctuations in the rotor frame. At reduced spacing, this impact becomes significant. The most important aspect of the tip vortex is the relative velocity defect and the associated relative total pressure defect, which is perceived by the stator in the same manner as a wake. In Part 2, the focus will be on the interaction of stator with the moving upstream rotor tip and streamwise vortices, the controlling parametric trends, and implications on design.

Effects of Rotor Tip Blade Loading Variation on Compressor Stage Performance

“…Valkov proposed that tip leakage recovery process occurs in a similar manner to wake recovery process as tip leakage flow has a wake-like structure [12]. As such, one would expect inviscid-dominated flow behavior to apply for the situation of tip leakage flow recovery.…”

“…Sakulkaew, on the other hand, delineated the changes of the flow field in the tip region but has not demonstrated performance improvement conclusively [2]. Furthermore, the unsteady reversible tip leakage recovery process has been shown to be beneficial by Valkov [12]. Thus, it can be stated that there is a need to quantify the effects of aft-loading a rotor blade tip followed by subsequent tip leakage recovery process in a downstream stator with the presence of unsteady tip leakage flow and the rotor-stator interaction (and vice versa i.e.…”

Unsteady Flows in Turbines

“…4a), and with it the viscous mixing loss. Besides, the tip leakage vortex has the same effect mechanism as a wake (see Valkov & Tan, 1999b). That is why, in compressors, mixing out the wake within the blade row will generate less loss overall than if we had mixed it out upstream of the blade row.…”

“…These streamwise vortices are shown to have a major influence on the secondary flow and viscous flow behavior of the downstream blade row. It has been shown, just like the wake, that the transport of streamwise vorticity can have an impact on the mixing losses in the next blade rows (Valkov & Tan, 1999b;Pullan, 2004). At the leading edge of the blade, the streamwise vortex is compressed so that, also by Kelvin's circulation theorem, the velocity deficit must decrease, and with it the rate of entropy production due to mixing.…”

Blade Row Interaction Effects on the Performance of a Moderately Loaded NASA Transonic Compressor Stage

“…Valkov and Tan [11] examined the influence of rotor wakes and the rotor tip clearance flow on the performance of a downstream stator. They found two opposing mechanisms at work in the stator passage: 1) beneficial 'recovery' of the disturbance energy of both the wake and clearance vortex due to vortex stretching in the stator and 2) an increase in loss due to distortion of the stator boundary layers because of the disturbance velocity field created by the upstream blade row.…”