Application of Casing Circumferential Grooves to Counteract the Influence of Tip Clearance

Abstract

Widening of surge margin of a transonic compressor stage is the main objective of the paper. This stage is a typical middle stage of a modern high pressure compressor (HPC) with decreased number of stages. Hot tip clearance of the stage being integrated into a six-stage HPC providing total pressure ratio π* HPC ≥ 12 and mass flow-rate < 16 kg/sec is estimated at 2.5 – 3% of blade height and is classified as a large tip clearance. In this paper experimental and 3D viscous numerical performances of the stage are obtained for two values of rotor tip clearance — equal to 0.76% (small size) and 2.66% (large size) of blade height. In doing so, tip clearance enlargement from 0.76% to 2.66% has been made by increase of casing (shroud) radius. This increase is manufactured as a circumferential trench (recess) with axial width 30% larger than rotor axial chord. Below this tip clearance is called “recessed” tip clearance. A distinguishing feature of leakage flow in case of large tip clearance is a formation of reversed flow near rotor casing. This backflow being intensified by throttling causes increase of incidence at the rotor leading edge and development of rotor stall. Casing treatments are intended to inhibit and delay the process. Among them circumferential grooves is the simplest casing treatment. Investigated in this paper casing circumferential grooves cover 82% of rotor axial chord. Numerical visualization of the near-casing streamlines demonstrates that tip leakage flow drains into the casing grooves giving rise to extended domains of positive axial velocity. Calculated mass flow-rate through groove’s cross-section demonstrates maximum over the rotor blade tip (flow into the groove) and minimum at mid-pitch (flow out of the groove). Amplitude of this variation depends on the groove location and stage throttling.

Circumferential groove on flow field and pressure fluctuation intensity of a semi-open centrifugal pump under design condition

“…30 In addition, the interface between the TLF and main flow moves toward the blade trailing edge, because of the circumferential groove. 31 Mileshin et al 32 investigated a transonic compressor with a circumferential groove. The interface was still in the flow channel, because the positive circumferential velocity in the tip region increased.…”

Numerical and Experimental Study of the Effect of Circumferential Grooves on HPC Last Stage Performance

“…В работе [9] исследовалось влияние НРУ лабиринтного типа на характеристики типовой средней ступени КВД при увеличенном радиальном зазоре. Показано, что из-за увеличения зазора над рабочим колесом падают запасы ГДУ, и применение НРУ лабиринтного типа позволяет устранить этот негативный эффект.…”

“…На рис. 12, б в варианте НРУ с четырьмя бороздками интенсивность этого вихря заметно ниже и его траектория располагается ближе к стороне разрежения лопатки, что хорошо согласуется с предыдущими исследованиями[9].На рис. 13 показано, как из-за перетекания через радиальный зазор возникает и изменяется застойная зона у стороны давления лопатки на периферии.…”