Axial compressors have inherently unsteady flow fields because of relative motion between rotor and stator airfoils. This relative motion leads to viscous and inviscid (potential) interactions between blade rows. As the number of stages increases in a turbomachine, the buildup of convected wakes can lead to progressively more complex wake/wake and wake/airfoil interactions. Variations in the relative circumferential positions of stators or rotors can change these interactions, leading to different unsteady forcing functions on airfoils and different compressor efficiencies. In addition, as the Mach number increases the interaction between blade rows can be intensified due to potential effects.It has been shown, both experimentally and computationally, that airfoil clocking can be used to improve the efficiency and reduce the unsteadiness in multiple-stage axial turbomachines with equal blade counts in alternate blade rows. While previous investigations have provided an improved understanding of the physics associated with airfoil clocking, more research is needed to determine if airfoil clocking is viable for use in modern gas-turbine compressors. This paper presents the results of a combined experimental/computational research effort to study the physics of airfoil clocking in a high-speed axial compressor. Computational simulations have been performed for eight different clocking positions of the stator airfoils in a 1-1/2 stage high-speed compressor. To accurately model the experimental compressor, full-annulus simulations were conducted using 34 IGV, 35 rotor and 34 stator airfoils. It is common practice to modify blade counts to reduce the computational work required to perform turbomachinery simulations, and this approximation has been made in all computational clocking studies performed to date. A simulation was also performed in the present study with 1 inlet guide vane, 1 rotor airfoil, and 1 stator airfoil to model blade rows with 34 airfoils each in order to examine the effects of this approximation. Time-averaged and unsteady data (including performance and boundary layer quantities) were examined. The predicted results indicate that simulating the full annulus gives better qualitative agreement with the experimental data, as well as more accurately modeling the interaction between adjacent blade rows.
It is ascertained that optimization of bladed rows clocking positions is an effective tool for control of bladed rows unsteady interaction in a multistage turbo machine and could be used equally in refinement and in compressor design stages [1–3]. Up to now the clocking effect issue of highly loaded rotors in compressor stages was not investigated due to growing design complexity of the experimental facility. The issue concerning tip clearances value influence on rotor and stator clocking effects was not studied as well. In the frames of this work a two stage compressor (HPC2 with the designed pressure ratio π* = 3.7 [4]) with a unique design was developed to investigate influence of tip clearance values on rotor and stator clocking effect. In order to realize the clocking effect the HPC2 compressor has the following parameters: numbers of stator blades are identical for all stators and equal to ZIGV = ZS1 = ZS2 = 68; number of R1 blades is two times less than of R2: ZR2 = 56, ZR1 = 28. This work studies HPC2 compressor performances at 3 R1 and R2 tip clearances — dtip = 0.5mm – nominal clearance, 0.75mm, and 1.0 mm – increased clearance. Clocking effects of stators and rotors are tested at 0.5mm and 0.75mm tip clearances for two values of corrected rotational speeds — n = 0.7 and 0.88. As shown, variations in max. efficiency from maximum to minimum when changing the clocking position both the stator and the rotor reach Δη*ad≈0.008÷0.012 at dtip = 0.5mm or Δη*ad≈0.007÷0.008 at dtip = 0.75mm. For more detailed analysis of the tip clearance influence on rotor and stator clocking effect a mathematical model of HPC2 was developed on base of through flow 3D viscous unsteady flow computations in the HPC2 compressor rows [3–4]. In full unsteady statement the calculation domain includes the following number of blade rows: IGV = 2, R1 = 1, S1 = 2, R2 = 2; S2 = 2.
In recent years, a number of studies in Russia and abroad was completed with the aim of decreasing pressure fluctuations and losses in blade cascades by controlling the unsteady interactions of blade rows (known as “clocking effect”) [1–4]. Tests of individual stages demonstrated that the clocking effect is responsible for 1.5–2.0% in efficiency and 50% in pressure fluctuations [5]. This paper presents the results of experimental and theoretical studies of the clocking effect on gas-dynamic characteristics of a high-loaded two-stage compressor simulating the first two stages of HPC for an advanced engine. The compressor is designed with the help of up-to-date 1D, 2D and 3D direct and inverse problem solutions and distinguished by high aerodynamic loads of stages with πk=3.7 total pressure ratio, 17% stall margin and 88% adiabatic efficiency at Ncor=88% rotational speed that was demonstrated experimentally [6]. The compressor was tested at CIAM’s C-3 test facility in the assembly with d=0.5, 0.75, 1.0-mm tip clearance in both rotors (relative clearance in first stage 4.6·10−3; 6.9·10−3; 9.2·10−3 and relative clearance in second stage 9.1·10−3; 13.7·10−3; 18.3·10−3). When tested, clocking effects were checked up for separate and simultaneous changes in clocking positions of stator and rotor blade rows. Indications of a blade tip-timing system and pressure pulsation sensors were used as experimental data. Earlier, it was shown that physics of the rotor clocking is a wake interaction which modifies the behavior of a boundary layer in Rotor 2 blades. This work studies the mechanism of rotor clocking in combination with changes in angular position of Rotor 2 blades due to interactions with Rotor 1 wakes. Tests showed that changes in the clocking position of the rotor with a multiple number of Rotor 1 and Rotor 2 blades affected the static position of Rotor 2 blades causing re-position of the blades depending on the rotor clocking-position. To confirm this result, 3D unsteady aerodynamic calculation was completed with the help of NUMECA software package simulating one of the test points. This work presents the calculated and experimental data showing that vortex wakes from Rotor 1 blades extend downstream, reach Rotor 2 and cause a variable aerodynamic load and a variable blade pitch.
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