Numerical simulations of rotating stall in axial flow compressors

Gas turbine compressor performance may encounter deterioration during service for various reasons such as damage by debris from the casing or foreign objects impacting on the blades, typically near the rotor's tip. Moreover, mal-schedule of Variable Stator Vanes (VSVs) during start-up may also result in performance deterioration and reduction in the surge margin. Ability to assess the effect of compressor deterioration using Computational Fluid Dynamics (CFD) is important at both design stage and in service. Compressor blade damage breaks the cyclic symmetry and the VSVs mal-schecule creates mis-match between stages together with geometric variations, thus computations are desirable to be performed using full annulus assemblies. Furthermore, downstream boundary conditions are also unknown during rotating stall or surge and simulations become difficult. This research presents unsteady time-accurate CFD analyses of compressor performance with tip curl blade damage in a single stage axial flow compressor and VSVs mal-schedule in a 3.5 stage axial flow compressor. Computations were per- formed near stall boundary to predict rotating stall characteristics. The primary objectives are to characterise the overall compressor performance and analyse the detailed flow behaviour. Computations for the nominal blade configurations were also performed for comparison purposes for both compressors. All unsteady simulations were performed at part speeds with a variable nozzle downstream representing an experimental throttle. For the blade damage study, two different degrees of damage for one blade and multiple damaged blades were investigated and compared with the results from the undamaged case. For the VSVs mal-schedule study, the first two stators were assumed to be variable and were used to create mal-schedule vane settings for the investigation. The effects of blade damage and VSVs mal-schedule on the aerodynamics performance and rotating stall characteristics for both compressor assemblies were investigated respectively and discussed in detail

Large Eddy Simulation of flows in industrial compressors: a path from 2015 to 2035

A better understanding of turbulent unsteady flows is a necessary step towards a breakthrough in the design of modern compressors. Due to high Reynolds numbers and very complex geometry, the flow that develops in such industrial machines is extremely hard to predict. At this time, the most popular method to simulate these flows is still based on a Reynolds Averaged Navier-Stokes (RANS) approach. However there is some evidence that this formalism is not accurate for these components, especially when a description of time-dependent turbulent flows is desired. With the increase in computing power, Large Eddy Simulation (LES) emerges as a promising technique to improve both knowledge of complex physics and reliability of flow solver predictions. The objective of the paper is thus to give an overview of the current status of LES for industrial compressor flows as well as to propose future research axes regarding the use of LES for compressor design. While the use of wall-resolved LES for industrial multistage compressors at realistic Reynolds number should not be ready before 2035, some possibilities exist to reduce the cost of LES, such as wall-modelling and the adaptation of the phase lag condition. This paper also points out the necessity to combine LES to techniques able to tackle complex geometries. Indeed LES alone, i.e. without prior knowledge of such flows for grid construction or the prohibitive yet ideal use of fully homogeneous meshes to predict compressor flows, is quite limited today

Generation Mechanisms of Rotating Stall and Surge in Centrifugal Compressors.

Flow instabilities such as Rotating Stall and Surge limit the operating range of centrifugal compressors at low mass-flow rates. Employing compressible Large Eddy Simulations (LES), their generation mechanisms are exposed. Toward low mass-flow rate operating conditions, flow reversal over the blade tips (generated by the back pressure) causes an inflection point of the inlet flow profile. There, a shear-layer induces vortical structures circulating at the compressor inlet. Traces of these flow structures are observed until far downstream in the radial diffuser. The tip leakage flow exhibits angular momentum imparted by the impeller, which deteriorates the incidence angles at the blade tips through an over imposed swirling component to the incoming flow. We show that the impeller is incapable to maintain constant efficiency at surge operating conditions due to the extreme alteration of the incidence angle. This induces unsteady flow momentum transfer downstream, which is reflected as compression wave at the compressor outlet traveling toward the impeller. There, the pressure oscillations govern the tip leakage flow and hence, the incidence angles at the impeller. When these individual self-exited processes occurs in-phase, a surge limit-cycle establishes

Nonlinear Control and Modeling of Rotating Stall in an Axial Flow Compressor

This thesis focuses on understanding the use of air injection as a means of controlling rotating stall in an axial flow compressor, involving modeling, dynamical systems analysis, and experimental investigations. The first step towards this understanding was the development of a low order model for air injection control, the starting point of which was the Moore and Greitzer model for axial flow compressors. The Moore and Greitzer model was extended to include the effects of air injection and bifurcation analysis was performed to determine how the closed loop system dynamics are different from those of the open loop system. This low order model was then used to determine the optimal placement of the air injection actuators. Experimental work focused on verifying that the low order model, developed for air injection actuation, qualitatively captured the behavior of the Caltech compressor rig. Open loop tests were performed to determine how the placement of the air injectors on the rig affected the performance of the compressor. The positioning of the air injectors that provided the greatest control authority were used in the development of air injection controllers for rotating stall. The controllers resulted in complete elimination of the hysteresis associated with rotating stall. The use of a throttle actuator for the control of the surge dynamics was investigated, and then combined with an air injection controller for rotating stall; the resulting controller performed quite well in throttle disturbance rejection tests. A higher order model was developed to qualitatively match the experimental results with a simulation. The results of this modeling effort compared quite well with the experimental results for the open loop behavior of the Caltech rig. The details of how the air injection actuators affect the compressor flow were included in this model, and the simulation predicted the same optimal controller that was developed through experimentation. The development of the higher order model also included the investigation of systematic methods for determining the simulation parameters. Based on experimental measurements of compression system transients, the open loop simulation parameters were identified, including values for the compressor performance characteristic in regions where direct measurements were not possible. These methods also provided information on parameters used in the modeling of the pressure rise delivered by the compressor under unsteady flow conditions

Literature search of publications concerning the prediction of dynamic inlet flow distortion and related topics

Publications prior to March 1981 were surveyed to determine inlet flow dynamic distortion prediction methods and to catalog experimental and analytical information concerning inlet flow dynamic distortion prediction methods and to catalog experimental and analytical information concerning inlet flow dynamics at the engine-inlet interface of conventional aircraft (excluding V/STOL). The sixty-five publications found are briefly summarized and tabulated according to topic and are cross-referenced according to content and nature of the investigation (e.g., predictive, experimental, analytical and types of tests). Three appendices include lists of references, authors, organizations and agencies conducting the studies. Also, selected materials summaries, introductions and conclusions - from the reports are included. Few reports were found covering methods for predicting the probable maximum distortion. The three predictive methods found are those of Melick, Jacox and Motycka. The latter two require extensive high response pressure measurements at the compressor face, while the Melick Technique can function with as few as one or two measurements

Analysis of the Unsteady Flow Field in a Centrifugal Compressor from Peak Efficiency to Near Stall with Full-Annulus Simulations

This study concerns a 2.5 pressure ratio centrifugal compressor stage consisting of a splittered unshrouded impeller and a vaned diffuser. The aim of this paper is to investigate the modifications of the flow structure when the operating point moves from peak efficiency to near stall. The investigations are based on the results of unsteady three-dimensional simulations, in a calculation domain comprising all the blade. A detailed analysis is given in the impeller inducer and in the vaned diffuser entry region through time-averaged and unsteady flow field. In the impeller inducer, this study demonstrates that the mass flow reduction from peak efficiency to near stall leads to intensification of the secondary flow effects. The low momentum fluid accumulated near the shroud interacts with the main flow through a shear layer zone. At near stall condition, the interface between the two flow structures becomes unstable leading to vortices development. In the diffuser entry region, by reducing the mass flow, the high incidence angle from the impeller exit induces a separation on the diffuser vane suction side. At near stall operating point, vorticity from the separation is shed into vortex cores which are periodically formed and convected downstream along the suction side

Comparison of two- and three-dimensional flow computations with laser anemometer measurements in a transonic compressor rotor

Two and three dimensional inviscid solutions for the flow in a transonic axial compressor rotor at design speed are compared with probe and laser anemometers measurements at near-stall and maximum-flow operating points. Experimental details of the laser anemometer system and computational details of the two dimensional axisymmetric code and three dimensional Euler code are described. Comparisons are made between relative Mach number and flow angle contours, shock location, and shock strength. A procedure for using an efficient axisymmetric code to generate downstream pressure input for computationally expensive Euler codes is discussed. A film supplement shows the calculations of the two operating points with the time-marching Euler code

Computational fluid dynamics simulations of blade damage effect on the performance of a transonic axial compressor near stall

Gas turbine axial compressor blades may encounter damage during service for various reasons such as damage by debris from casing or foreign objects impacting the blades, typically near the rotor’s tip. This may lead to deterioration of performance and reduction in the surge margin. The damage breaks the cyclic symmetry of the rotor assembly; thus, computational fluid dynamics simulations have to be performed using full annulus compressor assembly. Moreover, downstream boundary conditions are unknown during rotating stall or surge, and simulations become difficult. This paper presents unsteady computational fluid dynamics analyses of compressor performance with tip curl damage. Computations were performed near the stall boundary. The primary objectives are to understand the effect of the damage on the flow behaviour and compressor stability. Computations for the undamaged rotor assembly were also performed as a reference case. A transonic axial compressor rotor was used for the time-accurate numerical unsteady flow simulations, with a variable area nozzle downstream simulating an experimental throttle. Computations were performed at 60% of the rotor design speed. Two different degrees of damage for one blade and multiple damaged blades were investigated. Rotating stall characteristics differ including the number of stall cells, propagation speed and rotating stall cell characteristics. Contrary to expectations, damaged blades with typical degrees of damage do not show noticeable effects on the global compressor performance near stall

Active Flow Control in an Axial Compressor: Effect of the Injection Angle

This Thesis work has dealt with the problem of Rotating Stall in axial flow compressor. Particularly, the work was focused on active control to delay Rotating Stall appearance; this was performed by means of air injectors placed on the compressor's shroud. The study was concerning the optimal injection angle layout able to reach the maximum Stall Margin Improvement. Studies were done on "Laboratoire de Mà©canique des Fluides de Lille" in ENSAM university.ope