Investigation Of Design Strategies For Axial Slots Casing Treatment In Axial Compressors

Axial slot casing treatment is an effective passive flow control method in axial compressors.However,there is not a universal design method for various compressors with different features.For this purpose,a design strategy is proposed based on momentum transportation analysis at rotor tip.The momentum transportation between flow fileds of compressor rotor tip leakage flow and casing treatment slot openings is calculated and analyzed.Based on deamands of stall margin and efficiency,momentum transportation is translated into specific descriptions of design structures and parameters.Thus design criteria of slot casing treatment are customized on account of tip flow fields.Unsteady simulation methods for slots casing treatment are primarily validated by detailed measurement data in a transonic compressor with semi-circular slots and an in-house low speed compressor with arc-curved slots.Dynamic pressure transducers and Time-Resolved Particle Image Velocimetry(TR-PIV)are successfully applied in the low-speed compressor to acquire unsteady tip flow fields.Thus,the measurement database for this low speed compressor with axial slot casing treatment is established.On the basis of validated simulation methods,an analysis method of Rotor tip/Slots Momentum Transportation is proposed,taking the transonic compressor rotor as an example.The three-dimensional velocity fields are able to be deconstructed by defining momentum components at slots openings.Then,three typical compressors with different tip loading and design features,including a transonic compressor rotor,a high subsonic 1.5 compressor stage and a low speed compressor,are analyzed in depth with four kinds of slots casing treatment such as semi-circular slots,axial skewed slots,arc-curved slots and bend skewed slots.The stability enhancement and loss mechanisms of slots casing treatment are revealed.Consequently,the universal design strategies for low loss slots casing treatment in axial compressors are proposed.Primary conclusions are as follows:(1)For the low speed and 1.5 stage high subsonic compressors,rotor tip stall is respectively induced by tip clearance backflow at trailing edge and spillage at leading edge.Near stall flow fields are improved by rational-designed slots casing treatment and thus the stall margin is enhanced.For the transonic compressor rotor,spiral-type breakdown of tip leakage vortex is caused by shock wave/tip leakage vortex interaction at near-stall operating condition.Consequently,a mass of blockage emerges and incoming flow/tip leakage flow interface moves upstream,which inducing the rotor tip stall finally.Semi-circular slots extend the stall margin by improving tip leakage flow distributions and suppressing shock wave/tip leakage vortex interaction.(2)Two approaches of peak efficiency improvement by slot-type casing treatment have been illuminated.Firstly,reduction of circumferential absolute velocity at rotor tip results in a “negative pre-whirl” effect.This impact brings about additional work input at rotor tip.Secondly,increase of chord-wise component and decrease of normal component of tip leakage velocity contribute to reduction of mainstream/tip leakage flow mixing loss.(3)Based on the analysis method of Rotor tip/Slots casing treatment Momentum Transportation,design strategies of slot-type casing treatment considering both stability extension and efficiency are proposed.The rotor tip exposure and length of slots front part are two key parameters for stability enhancement and efficiency loss.The optimal design methods of these two parameters are,firstly,maximizing effective axial momentum at slots openings at near stall condition;secondly,reducing injection mass flow to lower mixing loss at peak efficiency condition,as well as generating “negative pre-swirl” at rotor inlet by circumferential momentum at front part of slots.