Effect of Unsteady Stator Wake—Rotor Double-Leakage Tip Clearance Flow Interaction on Time-Average Compressor Performance

Abstract

A study has been conducted, using unsteady three-dimensional Reynolds-averaged Navier-Stokes simulations to determine the impact on rotor performance of the interaction between upstream (steady defect and time-varying defect) stator wakes and rotor tip clearance flow. The key effects of the interaction between steady stator wakes and rotor tip clearance flow are: 1) a decrease in loss and blockage associated with tip clearance flow; 2) an increase in passage static pressure rise. Performance benefit is seen in the operability range from near design to high loading. The benefit is modest near design and increases with loading. Significant beneficial changes due to the stator-rotor interaction occur when the phenomenon of tip clearance flow double-leakage is present. Double-leakage occurs when the tip clearance flow passes through the tip gap of the adjacent blade. It is detrimental for compressor performance. The effect of strong stator-rotor interaction is to suppress double-leakage on a time-average basis. Double-leakage typically takes place at high loading but can be present at design condition as well, for modern highly loaded compressor. A benefit due to unsteady interaction is also observed in the operability range of the rotor. A new generic causal mechanism is proposed to explain the observed changes in performance. It identifies the interaction between the tip clearance flow and the pressure pulses, induced on the rotor blade pressure surface by the upstream wakes, as the cause for the observed effects. The direct effect of the interaction is a decrease in the time-average double-leakage flow through the tip clearance gap so that the stream-wise defect of the exiting tip flow is lower with respect to the main flow. A lower defect leads to a decrease in loss and blockage generation and hence an enhanced performance compared to that in the steady situation. The performance benefits increase monotonically with loading and scale linearly with upstream wake velocity defect. With oscillating defect stator wakes, rotor performance shows dependence on oscillation frequency. Changes in the tip region occur at a particular reduced frequency leading to (1) decrease in blockage, and (2) increase in passage loss. The changes in rotor performance at a particular reduced frequency are hypothesized to be associated with the inherent unsteadiness of the tip clearance vortex and its resonance behavior excited by the oscillating wakes.

Study of Sweep and Induced Dihedral Effects in Subsonic Axial Flow Compressor Passages—Part II: Detailed Study of the Effects on Tip Leakage Phenomena

“…In Figure 7, clear cut lines of the relative velocity contour demarcating the higher flow reversal regions from the main flow can be observed. It is seen that for this UNS rotor case at φ = 0.50 and τ/ch = 2.7%, where the leakage vortex penetration and blockage are the highest among the various clearance and rotor configurations, the relative velocity deficit region is observed to penetrate close to the TE of the adjacent blade, without crossing the tip again to cause what is referred to as "double leakage" [10]. Double leakage refers to tip clearance flow leaking across the adjacent blade tip again but with a lower streamwise velocity component.…”

An improved model for tip clearance loss in transonic axial compressors

“…Khalid stated that this reduction of total pressure in tip leakage flow changes the leakage flow angle and thus leads to variation in endwall blockage. Sirakov 32 also proved the assumption of Storer and Cumpsty in the original model that On the other hand, the static pressure distribution on the suction side of blade tip is very different from the static pressure well away from the endwall, and is affected by the leakage vortex (TLV) 8 and the shockinduced boundary layer separation (SIBLS). 33 Although previous studies 3,8 have revealed that the clearance jet flow has the same static pressure as blade suction surface at blade tip, the effects of static pressure distribution at blade tip suction side on tip clearance loss have seldom been investigated.…”

Numerical Study on Unsteadiness of Tip Clearance Flow and Performance Prediction of Axial Compressor

“…It seems that the mixing-plane assumption at the interfaces between the rotating and stationary domains causes this discrepancy. The beneficial effects of blade row interaction on rotor performance were studied by Valkov and Tan [12], Sirakov and Tan [13], and Graf et al [14]. Valkov and Tan [12] showed that the upstream wake recovery increased the pressure rise in the downstream passage by 1 to 3%.…”

“…Valkov and Tan [12] showed that the upstream wake recovery increased the pressure rise in the downstream passage by 1 to 3%. Sirakov and Tan [13] explained that the upstream wake suppressed double leakage of the tip clearance flow, thereby enhancing the pressure rise by about 2%. The effects of the downstream stator on the rotor are shown in Graf et al [14].…”