Impact of variable stator vane schedule on nonsynchronized vibration

The internal losses in the tip clearance region strongly influence the compressor performance and its operational range. Previous research proved that passive wall treatments with circumferential grooves in axial compressors effectively increase the compressor stall margin. The vortex generated inside the circumferential grooves creates a resistance to the flow that leaks into the tip clearance region of the compressor. However, most works found in the literature on circumferential grooves in axial compressors deal only with high-performance single-stage axial compressors. Therefore, there is a need to investigate and analyze the behavior of circumferential grooves in a multi-stage environment. In the present work, a passive wall treatment with circumferential grooves was implemented in a multi-stage axial compressor. Different configurations of circumferential grooves were created at the casing of the first and second rotor rows used in a four-stage axial flow compressor. Numerical simulations were performed to evaluate the influence of the circumferential grooves on the performance of a multi-stage axial compressor. The results obtained after the simulations for the different circumferential groove configurations were compared with the results obtained for the compressor without casing treatment (smooth wall) for different rotational speeds. Furthermore, the complete compressor map characteristics were simulated for the different casing treatment configurations, and the results were compared with the compressor characteristics of the smooth wall case. The passive wall treatment with circumferential grooves produced changes in the multi-stage axial compressor flow field, especially in the tip clearance region, improving the compressor stability mainly for part load speeds.

A Multi-Stage Axial Compressor Characteristic Calibration Method based on One-Dimensional Average Streamline Method

Tip clearance has great influence on the performance of multistage axial compressors including efficiency, pressure rise, mass flow, as well as matching. This paper reports a study into the influence of tip clearance on the performance and matching of a 5-stage axial compressor by a numerical method. Different tip clearances from 0% to 5.0% span which represents the typical range of tip clearance in modern multistage axial compressors were simulated and analyzed. The results show that as tip clearance increases from 0% to 5.0% span, the choked mass flow decreases by about 21.8%, the peak pressure ratio decreases by about 43.1% and the peak efficiency decreases about 14.3 percents. As tip clearance increases, the efficiency of the whole compressor decreases in a parabolic manner not linearly as previous suggested, which is partially attributable to the cantilevered stators considered in this paper and primarily due to the mismatching of different stages. It is of great importance to control the tip clearance. When tip clearance increases, the front stage tends to work near surge condition and the rear stage tends to work near choke condition, which leads to lower efficiency than in the middle stages. A weight was defined to evaluate each stage’s contribution to the whole compressor’s efficiency deficit caused by the increase of tip clearance. Front and rear stages contribute more to the efficiency deficit than the middle stages, which indicates that more attention should be paid on front and rear stages to improve the performance of multistage axial compressors. In order to evaluate the matching of multistage axial compressors with a quantified method, a new parameter named “Peak Efficiency Deviation (PED)” was defined based on the difference between each stage’s operating efficiency and its peak efficiency. The mass flow of multistage axial compressors should be well considered to make the PED parameter to be close to zero as possible. In the most commonly used range of tip clearance from 0.5% to 3.0% span, the PED varies little within 0.4 percent, which is only about 8.4% of the peak efficiency deficit at 1.5% span tip clearance. So, the PED could be small within a wide range of tip clearances if the matching of the compressor is perfect at design tip clearance.

Multistage compressor is the most important constituent of gas turbines used in land, naval and aeronautical applications. Overall performance of such machinery depends mainly on the axial compressor performance. Due to the relative motion between rotor and stator blades, the flow field in this machinery is highly unsteady. Furthermore several technological effects like tip clearances, complexity of the blade shapes, variation of axial distance between stator and rotor, seal leakages and cooling holes among others complicate the machine. Therefore the study of a complicated, strongly three-dimensional flow field inside a compressor is considered to be one of the most difficult tasks to be performed by a CFD expert. The present work is the extensive numerical study of the effect of: (1) tip clearance of rotor blades and (2) the axial gap between rotor and stator on the overall performance of a multistage axial compressor. A commercial software package is used for this study. Reynolds-averaged Navier–Stokes equations are solved using Spalart–Allmaras model. A number of steady-state viscous flow simulations were run for both the tip clearance effect and different axial gaps between stator and rotor. All simulations were performed for the first stage, i.e. Stator–Rotor–Stator. Simulations were carried out with coarse, medium and fine meshes to find an optimum, mesh-independent solution. It has been found that larger tip clearance has a detrimental effect on the stage pressure ratio and efficiency of a multistage axial compressor. Similarly there exists a certain distance ratio between the stator1–rotor and rotor–stator2, where stage performance is optimum. Overall performance characteristics obtained through simulation for both the tip clearance and axial gap variation were also compared with the experimental studies and found to be in good agreement.

In this paper, an unsteady tip leakage flow instability is identified and investigated for an axial compressor at near surge conditions. We describe the results of experimental verification of a new compressor developed by improving the blade geometry based on the criterion for the occurrence of this unsteady phenomenon. In a high-speed multistage axial flow compressor having a subsonic high stagger rotor blade, a surge test was carried out by changing the tip clearance. Under a condition of large tip clearance, a drastic decrease in static pressure rise coefficient near the surge point was observed. At this operating condition, large, unsteady pressure fluctuation at the blade tip was confirmed, and the occurrence of tip leakage vortex breakdown that was clarified by unsteady multipoint pressure measurement and detailed unsteady numerical simulations. Due to the blockage effect caused by vortex breakdown of the tip leakage, double leakage and axially reversed flow near the trailing edge was observed. It is found that the vortex breakdown region of the tip leakage vortex propagated in the circumferential direction and caused the rotating instability. In order to investigate the relationship between this unsteady flow phenomenon, tip clearance size and flow pattern, unsteady calculation was conducted by changing the blade tip stagger and tip clearance size. A new concept of tip clearance of staggered pitch reference is proposed, which makes it possible to include the effect of blade loading on the clearance and clarifies that there exists a threshold at which vortex breakdown occurs/does not occur. On the basis of the above results, a high-speed multi-stage improved compressor was designed and manufactured to prevent tip leakage vortex breakdown. A clearance change test using active clearance control technology was conducted and increase in the static pressure rise coefficient near the surge point was confirmed for each clearance. The design concept of the improved blade which suppressed the unsteady tip leakage flow instability was tested and verified, and the effectiveness of the design guideline in actual gas turbines for power generation was confirmed.

The results of mathematical modeling processes in the turboshaft gas turbine engine (GTE) are presented. The using calculation method based on a high-level GTE mathematical model, which is founded on a multi-stage axial compressor blade-to-blade description. The model was developed at the Aviation Theory Chair of National Aerospace University “KhAI”. The model is based on a multistage axial compressor thermodynamic parameters calculations using a 1D and 2D approaches to analyzing of the flow. The model named above allows one to take into account air intakes from of the compressor blade gaps, as well as adjusting the angles of installation of the rotary stator vanes depending on the rotational speed. The GTE model has a modular structure. To determine the compressor parameters the modules for 1D or 2D flow calculation can be connected. As the initial data, besides the data traditionally specified in the 1st level GTE models it is necessary to set the geometrical parameters of the compressor flow path and blades on the medium radius (for the 2nd level GTE model) or along with the blade height (for the 3rd level). Both calculating compressor parameters methods are verified and have a fairly wide experience of practical use. The article presents the results of calculating the maps of the GTE multi-stage compressor using one- and two-dimensional approaches. Comparison of the compressor performances achieved by using of these two methods among themselves and with the experimental data has shown their good agreement. The approach used to simulate the flow in compressors makes it possible to estimate, by calculation, the surge margin, to consider the incidence angles and other flow parameters in the blade gaps in a wide range of GTE operation modes. Such results, as well as a comparison with experimental data, are presented in the article. The article also demonstrates the results of applying the described above model to the gas turbine engine performances calculation. The engine has the 12-stage axial compressor with the stator blades position of the first stages regulation. The calculated line of joint operation modes of the gas generator units, the dependence of the power and specific fuel consumption on the rotational speed. Presented are the processes in GTE on stationary modes analyzing results given in the article showed the used model advantage, reliability and expediency of its practical application.

Due to the generally high stage and blade count, the current standard industrially adopted to perform numerical simulations on multistage axial compressors is the steady-state analysis based on the Reynolds-averaged Navier-Stokes approach (RANS), where the coupling between adjacent rows is handled by means of mixing planes. In addition to the well-known limitations of a steady-state picture of the flow, namely its inherent inability to capture the potential interaction and the wakes from the upstream blades, there is another flow feature which is lost through a mixing-plane, and which is believed to be a major accountable for the radial mixing: the transport of stream-wise vorticity. Streamwise vorticity arises throughout a compressor for various reasons, mainly associated with secondary and tip-clearance flows. A strong link does exist between the strain field associated with the transported vortices and the mixing augmentation: the strain field increases both the area available for mixing and the local gradients in fluid properties, which provide the driving potential for mixing itself. Especially for the rear stages of a multistage axial compressor, due to high clearances and low aspect ratios, only accounting for the development along the meridional path of secondary and clearance flow structures it is possible to properly predict the spanwise mixing. In this work, the results of steady and unsteady RANS simulations on the high-pressure section of an industrial heavy-duty axial compressor are presented and compared with experimental data acquired during a test campaign. Adopting an unsteady full-annulus URANS approach, the enhanced radial mixing in the rear stages of the compressor is properly captured, obtaining a really good agreement with experimental data both in terms of total temperature and pressure outlet radial distributions. On the contrary, with a steady-state modelling, the radial transport is strongly underestimated, leading to results with marked departures from experiments. Examining what occurs across the inter-row interfaces for RANS and URANS solutions, a possible explanation for this underestimation is provided. In particular, as the stream-wise vorticity associated with clearance flows is one of the main drivers of radial mixing, restraining it by pitch-averaging the flow at mixing planes of a steady-state analysis is the reason why this simplified approach is not able to properly predict the radial transport of fluid properties in the rear part of the axial compressor.

This study presents experimental investigations into blade suction surface boundary layer flow in a multistage axial flow compressor. The experiments were focused on the third stage of the 4-stage Low Speed Research Compressor (LSRC) at Cranfield University. Measurements within the boundary layer were obtained using a hot wire probe. This was traversed normal to the blade surface at small increments, capturing the unsteady velocity profile within the boundary layer. Detailed boundary layer flow measurements covering most of the stator suction surface were taken and are described using time mean and ensemble averaged velocity profiles. Turbulence intensity in the boundary layer flow on the blade suction surface is also discussed. A strong wake-induced strip zone due to passing wake disturbances are generated at midspan near the blade leading edge at rotor blade passing frequency. Corner separation was observed at the tip region near the trailing edge. Normalized velocity profiles in this region show no variation in time.

The operational envelope of gas turbine engines such as employed in the Army Blackhawk helicopter is constrained by the stability limit of the compression system. Stall control technologies developed at the ARL Vehicle Technology Directorate and NASA Glenn Research Center to improve the stable operating range of compressors lack fundamental understanding beneficial to design guidance. Improved understanding of the stall inception process and how stall control technologies mitigate such will provide compressors with increased tolerance to stall, thereby expanding the operational envelope of military gas turbine engines. To provide improved understanding, time accurate CFD simulations are being computed at full-scale actual engine conditions using the parallel solver, TURBO, developed at Mississippi State University (MSU) with support from NASA, DoD, and industry. As a compressor nears stall the flow field is no longer periodic from passage to passage, therefore full-annulus (i.e., all blade passages of every blade row) must be computed. For a typical multistage compressor this becomes a formidable computational challenge requiring access to massively parallel machines in order to meet the computational and memory demands of the problem. Unsteady full-annulus simulations of both a single stage axial and centrifugal compressor, for which detailed experimental data are available, provide validation of the TURBO code for subsequent use in predicting the stall inception processes in an axicentrifugal multistage compressor employed in the Army Blackhawk helicopter. The simulations show the inception of stall and subsequent evolution into a fully developed stall cell. Also, shown are the predictions of range extension with stall control and comparison to measurements. Finally, a status of the unsteady fullannulus simulations of the compression system components of the U.S. Army Helicopter Gas Turbine Engine is presented. All military and commercial gas turbine engine systems can benefit from the proposed work. In the future, this work could lead to new gas turbine engine designs with resistance to compressor stall and thus improved combat capability. This work will provide the first-ever 3-dimensional viscous time-accurate simulation of the stall inception process in a multistage compressor, providing insight into the causal link between compressor blade design parameters and the stable operating limit, which will be used to guide new design practices leading to compressor designs with increased tolerance to stall. This paper presents final-year progress on this challenge project.

Variability in stator vanes is a widely used technique to improve the stability and efficiency of axial flow compressor in gas turbine engines. Most of the modern aircraft jet engines use variable stator vanes in both low pressure and high pressure compressors primarily for off-design performance. This study discusses in detail about the effect of stator variability in a three stage low pressure axial compressor at design and off-design conditions. Computational flow analysis were carried out for the three stage low pressure compressor with variability in inlet guide vane and first stage stator blade. Detailed investigation on flow physics was carried out in rotor blade passages with stator variability. At off-design speeds, the reduction in flow velocity is lower than the reduction in blade tip speed. This leads to mismatch in flow angles and inlet blade angles causing high incidence and large flow separation in blade passage. This results in poor aerodynamic stability of the axial compressor at off-design speeds. In this study, aerodynamic performance of compressor is evaluated from 70% to 100% design speeds with different stagger angle setting of inlet guide vane at each speed. Further, to improve 2nd stage rotor performance, variability was introduced in 1st stage stator blade and performance was evaluated. Compressor test results are compared with CFD data for design and off-design speeds.

In multistage axial compressors used for gas turbine engines, it is import ant to clarify unsteady phenomena such as surge and rotating stall. The purpose of this study is to develope a practical analysis method for both the surge and rotating stall in axial compressors. In this paper, firstly, some characteristics of the unsteady phenomena in axial compressors are summarized, derived from the measurments in test compressors. Secondary, a numerical method for the unsteady phenomena assuming incompressible flow is described. A feature of the method is solving the equations by Galerkin's method with the circumferential flow distortion expressed in the form of a high-order Fourier series. Finally, the usefulness of the method is examined by some results from a parameter-survey computation and by a comparison with the measured results in a 3-stage test compressor.

The inlet attack angle of rotor blade reasonably can be adjusted with the change of the stagger angle of inlet guide vane (IGV); so the efficiency of each condition will be affected. For the purpose to improve the efficiency, the DSP (Digital Signal Processor) controller is designed to adjust the stagger angle of IGV automatically in order to optimize the efficiency at any operating condition. The A/D signal collection includes inlet static pressure, outlet static pressure, outlet total pressure, rotor speed and torque signal, the efficiency can be calculated in the DSP, and the angle signal for the stepping motor which control the IGV will be sent out from the D/A. Experimental investigations are performed in a three-stage, low-speed axial compressor with variable inlet guide vanes. It is demonstrated that the DSP designed can well adjust the stagger angle of IGV online, the efficiency under different conditions can be optimized. This establishment of DSP online adjustment scheme may provide a practical solution for improving performance of multi-stage axial flow compressor when its operating condition is varied.

Experimental study of rotor blades vibration and noise in multistage high pressure compressor and their relevance

The following numerical investigations are performed in the frame of a research project that aims at a better understanding of the flow unsteadiness that develops in a multistage high-speed axial compressor. First, the paper presents a new version of the 3.5 stages high-speed axial compressor CREATE (Compresseur de Recherche pour l’Etude des effets Aérodynamiques et TEchnologiques), which has been designed by Snecma and is based at the LMFA (Laboratory for Fluid Mechanics and Acoustics) on a 2MW test rig. This paper is based on numerical results obtained with 3D steady and unsteady RANS computations using the CREATE configuration. The unsteady RANS simulations are carried out over the whole spatial and temporal periodicity of the compressor. The main numerical setup has been fixed according to the state of the art. Second, the effect of three different time discretizations on the flow field in CREATE is discussed. The global performance of the compressor is not significantly affected. However the change in the time discretization impacts the structure of the flow at specific locations. The main focus of this study lies on the transport of flow structures and the analysis of their interactions. A double modal decomposition method, which highlights the specific contribution of the interactions on the overall flow field, is applied for the study of the highly complex and unsteady flow field. It allows identifying which interactions are more sensitive to the change in the time discretization.

The general effect of Reynolds Number on axial flow compressors operating over a sufficiently wide range is described and illustrated by experimental data for four multistage axial compressors. The wide operating range of military aircraft engines leads in the back stages of high pressure ratio compression systems to three distinctly different regimes of operation, characterized by the boundary layer conditions of the cascade flow: • laminar separation, • turbulent attached flow with hydraulically smooth blade surface, • turbulent attached flow with hydraulically rough blade surface. Two “critical” Reynolds Numbers are defined, the “lower critical Reynolds Number” below which laminar separation occurs with a definite steepening of the efficiency/Reynolds Number relation and an “upper critical Reynolds Number” above which the blade surface behaves hydraulically rough, resulting in an efficiency independant of Reynolds Number. The permissible blade surface roughness for hydraulically smooth boundary layer conditions in modern high pressure ratio compression systems is derived from experimental data achieved with blades produced by grinding, electrochemical machining and forging. A correlation between the effect of technical roughness and sand type roughness is given. The potential loss of efficiency in the back end of compression systems due to excessive blade roughness is derived from experimental results. The repeatedly experienced different sensitivity of front and back stages towards laminar separation in the low Reynolds Number regime is explained by boundary layer calculations as a Mach Number effect on blade pressure distribution, i.e. transonic versus subsonic flow.