AIAA 2000-2233 Control of Tip Clearance Flows in Axial Compressors

A compressor tip clearance flow control scheme is explored using fluidic actuators, the so-called " synthetic jets " , on the casing wall acting over the clearance region. The analysis of flow visualization images show the largest increase in mixing level when forced at the optimal frequency. A reduction in the tip clearance-related blockage is demonstrated using an array of synthetic jet actuators in a linear cascade with tip clearance. The change in the blockage has a clear frequency response that is similar to that observed in the mixing enhancement of model two-dimensional leakage flow. However, the net mass-average total pressure loss remains more or less intact with the actuation. NOMENCLATURE A b blocked area due to blockage A b,2D blocked area related to boundary layers on blade surfaces A b,ew endwall blocked area A b,tip tip clearance related blocked area A ex blade passage exit area = hS C blade chord C D discharge coefficient =) / 2 /(ρ P A Q ∆ C P static pressure coefficient = ∞ ∞ − q P P /) (f frequency h span (blade height) I intensity of smoke image in gray scale (white = 0, black = 1 in images) L effective acoustic length of neck in actuator Q flow rate = AU q ∞ upstream dynamic head = 2 5. 0 ∞ U ρ Re C Reynolds number = U ∞ C / ν S pitch (spacing between blades) U L core leakage velocity leaving clearance gap U J peak center velocity of synthetic jet actuator leaving actuator slit (or hole) U ∞ flow velocity far upstream V voltage β τ reduced frequency = 2πfτ/U L or 2πfτ/U ∞ (in cascade) δ actuator slit size θ momentum thickness of shear layer µ momentum ratio =) /() (2 2 τ ρ δ ρ L J U U or) /() (2 2 τ ρ δ ρ ∞ U U J (in cascade) τ clearance size ω total pressure loss coefficient = ∞ ∞ − q P P t t /) (, M ew ω mass-average total pressure loss coefficient in endwall region M tip ω tip clearance related mass-average total pressure loss coefficient X 0 baseline quantity of X (usually means case without actuation)