Numerical Investigation On The Aerodynamic Effect Of The Removal Of Seal Coating On Axial Compressor Rotors

Seal coating is often thermal-sprayed on the compressor casing of aero-engines in order to avoid the rubbing between the rotating blade and casing,and reducing the tip clearance to improve the efficiency of the compressor.Seal coating is doomed to be removed during service because of rubbing,high temperature,high pressure and so on,thus the geometry of tip gap,and consequently the performance of the compressor can be affected.It is of great scientific significance to study the effect of the removal of seal coating on the performance of aero-engine compressors for the development of the theory of tip leakage flow.Meanwhile,it has a high engineering reference value for the airworthiness certification of domestic aero-engines.In this paper,the removal of the seal coating is divided into two categories:circumferentially uniform and circumferentially non-uniform removal,and the effect of them on the performance of compressors are studied by numerical simulation respectively.Taking the 12^th stage rotor of a high-pressure compressor of an aero-engine as the research object,the effect of circumferentially uniform removal of seal coating on the performance of compressors is studied.The blade geometry is obtained by inverse engineering,and the single-passage grid is generated for numerical simulation.The results show that the efficiency and total pressure ratio gradually decrease,and the surge margin decreases exponentially as seal coating falls off.When the removal thickness is4 times the designed tip gap（4τ）,the compressor reaches its near-stall point at the working flow rate.The main reasons for the stall are the periodicity and self-induced unsteadiness of tip leakage flow,as well as the complex flow phenomena such as the vortex breakdown and the low-velocity blockage region induced by the interaction between the tip leakage vortex,the secondary leakage flow and the main flow.Taking a transonic axial compressor rotor as the research object,the effect of circumferentially non-uniform romoval of seal coating is studied.The circumferentially non-uniform romval is reasonably simplified as circumferential deep groove,and the corresponding full-annulus grid is generated for numerical simulation.The results show that as the circumferential length of the romaval of seal coating increases,the total pressure ratio and isentropic efficiency decrease,and the surge margin decreases at first but then increases.After seal coating is removed,the areas where the flow loss changes are mainly in the vicinity of the tip passage and the suction side of blades.The former is caused by the low-velocity blockage due to tip leakage flow,and the effect of the removal on the blockage,which play a dominant role in the flow loss and instability.The latter is closely related to the interaction between the leading edge shock/in-passage shock and the boundary layer.The change in surge margin is the result of two competing factors:the mass flow rate and the strength of tip leakage flow,and the surge margin drops to the minimum when the removal length is 9 passages.The full-annulus simulation was carried out on the case with lowest surge margin（9-passage-remval case）at stall condition,according to previous study.The results show that in the circumferentially deep groove formed by the removal of seal coating,the static pressure fluctuation frequency is consistent with the blade passing frequency,so the flow structure is closely related to the rotation of the blade.During the process of the blade entering and exiting the removal range,the static pressure at tip region oscillates violently,and the blockage effect at tip passage is strengthened and weakened respectively,which is attributable to the intensity,trajectory change of the tip leakage vortex and the vortex breakdown.When the blade just exits the removal range,the tip leakage vortex strongly mixes with the surrounding fluid,and the leakage vortex breaks down,resulting in the unsteadiness of the flow.