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Numerical Investigation of Rotating-Stall in a stage of an Axial Compressor with Two Different Approaches
Abstract and Figures
Rotating-Stall (RS) through a cascade of an axial compressor has been nu-merically investigated by employing an unsteady two-dimensional density based computer code. To validate the computer code, three test cases were p r epared, and the good a g r eement resulting from comparison of the results has given adequate assurance of the code. The cascade problem was studied through diierent geometric approaches, which were classiied in two main categories. The was the semi-stage (single rotor-cascade), and the second was the complete stage. For each geometric approach, the number of blades was varied f r om a minimum number to a maximum value (variable cascade length). The RS modal characteristics and its development for all geometric cases were observed. The RS was incepted with a 40% reduction in coeecient and a 0.4% increase in the load coeecient from their normal operating values. In all cases, the captured m o dal characteristics of RS varies with the variation of the cascade length to a maximum value, and this triggers the same modes of RS. The variation of the modal characteristics of the RS, in the two main geometric approaches seems to be similar, but at diierent levels.
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During a major overhaul of an 85 MW gas turbine unit in Iran-Rey power plant, 39 cracks were detected with different lengths and locations on the compressor disk of stage 11. All of the cracks initiated from the dovetail regions. Preliminary visual inspections and further micro-fractography using the scanning electron microscope demonstrated that the fretting fatigue phenomenon was the main cause of failure. Four repair methods were suggested to restart the unit. The first one was to remove all of the cracks from the disk by machining, or the so-called blending. The second, third, and fourth ways were to remove the entire rotor blades of stage 11, to remove the entire rotor and stator blades of the stage 11 simultaneously, and to remove those rotor blades of stage 11 corresponding to the damaged dovetails, respectively. Although the first way of solution was initially carried out on the damaged disk, the first author offered that restarting the unit with the blended disk is not reliable enough because of the presence of a large number of repair points on the disk. Using the numerical investigations based on the computational fluid dynamics, it was found that only the second suggestion (i.e., removing the entire rotor blades of the stage 11) might be applicable. Ultimately, the entire stage 11 rotor blades were removed from the blended disk, and the gas turbine unit was successfully restarted without encountering abnormal operation. Although the performed process resulted in approximately 20% output power loss compared with the unit's power before the blades' removal, the unit was quickly restored to be ready to restart, and the electric power could be generated during the period of peak consumption.
The quality of theoretical flow prediction in turbomachines is strongly influenced by the modeling of the turbulent fluctuations. These fluctuations are generated by various secondary flow mechanisms, the swirl of the flow occuring in blade channels and the curvature of the streamlines. Furthermore, the character of turbulence can become highly anisotropic indicating that the well known algebraic and two-equation k-ε type models might become unserviceable.
The present paper deals with the validation of a fully implicit 2-D steady-state Navier-Stokes algorithm and the comparison of the applicability of various turbulence models for predicting the flow along the blades of a turbine nozzle.
In the first section a short review is given on the basic numerical assumptions of the used computer code and on the employed two layer algebraic, one- and two-equation turbulence models. The influence of the fully implicitly formulated transport equations for the turbulent quantities, especially the influence of the discretization of the production and dissipation terms on the stability of the numerical scheme is pointed out. The validation of the numerical algorithm is done for an inviscid flow through the Hobson 2 impulse cascade, for the turbulent flow over a flat plate and for the flow through a turbine stator channel.
In a last section results obtained from calculations for a subsonic turbine stator flow inside and downstream of the blade passage with several turbulence models are presented. The apparent differences, especially in the distribution of the total pressure loss downstream of the trailing edge are discussed.
Copyright © 1995 by ASME Country-Specific Mortality and Growth Failure in Infancy and Yound Children and Association With Material Stature
Use interactive graphics and maps to view and sort country-specific infant and early dhildhood mortality and growth failure data and their association with maternal
As part of a European collaborative project, four high-speed compressors were tested to investigate the generic features of stall inception in aero-engine type compressors. Tests were run over the full speed range to identify the design and operating parameters that influence the stalling process. A study of data analysis techniques was also conducted in the hope of establishing early warning of stall. The work presented here is intended to relate the physical happenings in the compressor to the signals that would be received by an active stall control system. The measurements show a surprising range of stall-related disturbances and suggest that spike-type stall inception is a feature of low-speed operation while modal activity is dearest in the midspeed range. High-frequency disturbances were detected at both ends of the speed range and nonrotating stall, a new phenomenon, was detected in three out of the four compressors. The variety of the smiling patterns. and the ineffectiveness of the stall warning procedures, suggests that the ultimate goal of a flightworthy active control system remains some way off.
Stall characteristics and stall cell behavior in a linear cascades system of a single stage axial compressor are presented by applying a numerical analysis of compressible N-S equations. The system consists of a rotor and a stator, where flow is assumed to be periodic over six blades and ten vanes respectively. Although the number of blades and vanes per period is much less than that in the real-rig, and the analysis is conducted in a laminar viscosity mode, computed stage performance from normal operation to deep stall agrees fairly well with experimental data. In deep stall, rotor cell propagates in linkage with stator cell at a computed speed of 65 % of the rotor speed. This speed is considerably close to the measured value of 55 %. Stall cell propagation is discussed on flow patterns showing stall development, stall induction in the follower blade and interaction between the rotor and stator.
Copyright © 1994 by ASME Country-Specific Mortality and Growth Failure in Infancy and Yound Children and Association With Material Stature
Use interactive graphics and maps to view and sort country-specific infant and early dhildhood mortality and growth failure data and their association with maternal
Rotating-stall inception in axial-flow compressors has been studied computationally using a quasi-three-dimensional time-marching Navier-Stokes method with a mixing-length turbulence model, Of particular interest was the effect of external circumferential disturbances corresponding to inlet stagnation pressure distortions and rotor/stator blade interactions, The present results show that rotating stall onset patterns in terms of number of stall cells and rotating speeds were influenced by small external circumferential stationary or rotational disturbances, First mode circumferential disturbances had the most destabilizing effect, resulting in a single-cell pattern rotating in the absolute frame at about 50% rotor speed, as Is observed in most experiments, Short-scale multiple-cell patterns rotating at a higher absolute speed could also be excited by a disturbance with the same circumferential length scale, Short-scale multiple-cell patterns tended to be more persistent in an isolated blade row than in a stage, In the latter case, a short-scale pattern initiated by a rotor-stator interaction would quickly change into a long-scale single-cell pattern, associated with a distinct change of the rotating speed.
The paper presents a numerical and experimental study of stall propagation in axial compressors. In the numerical study, the compressor blades are represented as an isolated linear cascade of airfoils, and the stall propagation is simulated using a vortex tracking method. This method involves the use of periodic vortex arrays to simulate infinite cascades, the use of a vortex merging algorithm to allow computations for large times, the imposition of the no-slip and the impenetrability boundary conditions in an integral sense, the use of integral boundary-layer methods to locate the separation points, and the use of Fourier decomposition to compute the velocities of the propagating stall. Detailed parametric studies have been performed to analyze the influence of flows parameters such as the inflow angle and stall wavelength and geometric parameters such as cascade solidity, blade camber, and stagger. The experimental investigations were conducted in a single-stage axial compressor test rig. The occurrence of rotating stall in this research compressor was demonstrated, and some experimental studies on the effect of various flow parameters on the stall propagation were conducted. The computational and experimental results are compared and are shown to be in qualitative agreement. © 1990 American Institute of Aeronautics and Astronautics, Inc., All rights reserved.
Evolution and structure of multiple stall cells with short length-scale in an axial compressor rotor have been investigated experimentally. In a low-speed research compressor rotor tested, a short length-scale stall cell appeared at first, but did not grow rapidly in size unlike a so-called “spike-type stall inception” observed in many multi-stage compressors. Alternatively, the number of cells increased to a certain stable state (a mild stall state) under a fixed throttle condition. In the mild stall state the multiple stall cells, size of which was on the same order of the inception cell (a few blade spacings), were rotating at 72% of rotor speed and at intervals of 4.8 blade spacings. With further throttling, a long length-scale wave appeared overlapping the multiple short length-scale waves, then developed to a deep stall state with a big cell.
In order to capture the short length-scale cells in the mild stall state, a so-called ‘double phase-locked averaging technique’ has been developed, by which the flow field can be measured phase locked to both of the rotor and the stall cell rotation. Then, time-dependent ensemble averages of the 3D velocity components upstream and downstream of the rotor have been obtained with a slanted hot-wire, and the pressure distributions on the casing wall with high response pressure transducers.
By a physically plausible explanation for the experimental results, a model for the flow mechanism of the short length-scale stall cell has been presented. The distinctive feature of the stall cell structure is on the separation vortex bubble with a leg traveling ahead of the rotor, with changing the blade in turn on which the vortex leg stands.
As part of a European collaborative project, four high-speed compressors were tested to investigate the generic features of stall inception in aero-engine type compressors. Tests were run over the full speed range to identify the design and operating parameters that influence the stalling process. A study of data analysis techniques was also conducted in the hope of establishing early warning of stall. The work presented here is intended to relate the physical happenings in the compressor to the signals that would be received by an active stall control system. The measurements show a surprising range of stall-related disturbances and suggest that spike-type stall inception is a feature of low-speed operation while modal activity is clearest in the midspeed range. High-frequency disturbances were detected at both ends of the speed range and nonrotating stall, a new phenomenon, was detected in three out of the four compressors. The variety of the stalling patterns, and the ineffectiveness of the stall warning procedures, suggests that the ultimate goal of a flightworthy active control system remains some way off.
This paper presents a parallel numerical and experimental study of rotating stall cells in an axial compressor. Based on previous theoretical and experimental studies stressing the importance of fluid inertia and momentum exchange mechanisms in rotating stall, a numerical simulation using the Euler equations is conducted. Unsteady 2-D solutions of rotating stall behavior are obtained in a one-stage low subsonic axial compressor. The structure and speed of propagation of one fully developed rotating stall cell together with its associated unsteady static pressure and throughflow field distributions are presented. The numerical capture of a stalled flow region starting from a stable high-flow operating point with an axisymmetric flow distribution and evolving at a reduced mass flow operating point into a rotating stall pattern is also discussed.
The experimental data (flow visualization, time-averaged and unsteady row-by-row static pressure measurements) acquired in a four-stage water model of a subsonic axial compressor covers a complete characteristic line ranging from high mass flow in the stable regime to zero throughflow. Stall inception is presented together with clearly marked different operating zones within the unstable regime.
For one operating point in the unstable regime, the speed of propagation of the cell as well as the static pressure spikes at the front and rear boundaries of the rotating stall cell are compared between computations, measurements and an idealized theory based on momentum exchange between blade rows entering and leaving the stalled cell. In addition, the time-evolution of the pressure trace at the rotor/stator interface is presented.
This study seems to support the assumption that the cell structure and general mechanism of full-span rotating stall propagation are essentially governed by inertial effects and momentum exchange between the sound and stalled flow at the cell edges.