J. P. Longley

Active control of rotating stall in a low-speed axial compressor

The onset of rotating stall has been delayed in a low-speed, single-stage, axial research compressor using active feedback control. Control was implemented using a circumferential array of hot wires to sense propagating waves of axial velocity upstream of the compressor. Using this information, additional circumferentially traveling waves were then generated with appropriate phase and amplitude by «wiggling» inlet guide vanes driven by individual actuators. The control scheme considered the wave pattern in terms of the individual spatial Fourier components. A simple proportional control law was implemented for each harmonic. Control of the first spatial harmonic yielded an 11 percent decrease in the stalling mass flow, while control of the first, second, and third harmonics together reduced the stalling mass flow by 23 percent

A Review of Nonsteady Flow Models for Compressor Stability

This paper presents a review of the different approaches to modeling the nonsteady fluid dynamics associated with two-dimensional compressor flow fields. These models are used to predict the time development of flow field disturbances and have been found useful in both the study of rotating stall and the development of active control. The opportunity to digest the earlier investigations has now made it possible to express the modeling ideas using only a very simple mathematical treatment. Here, the emphasis is on the underlying physical processes that the models simulate and how the assumptions within the models affect predictions

Effects of rotating inlet distortion on multistage compressor stability

In multispool engines, rotating stall in an upstream compressor will impose a rotating distortion on the downstream compressor, thereby affecting its stability margin. In this paper experiments are described in which this effect was simulated by a rotating screen upstream of several multistage low-speed compressors. The measurements are complemented by, and compared with, a theoretical model of multistage compressor response to speed and direction of rotation of an inlet distortion. For corotating distortions (i.e., distortions rotating in the same direction as rotor rotation), experiments show that the compressors exhibited significant loss in stability margin and that they could be divided into two groups according to their response. The first group exhibited a single peak in stall margin degradation when the distortion speed corresponded to roughly 50 percent of rotor speed. The second group showed two peaks in stall margin degradation corresponding to distortion speeds of approximately 25-35 percent and 70-75 percent of rotor speed. These new results demonstrate that multistage compressors can have more than a single resonant response. Detailed measurements suggest that the two types of behavior are linked to differences between the stall inception processes observed for the two groups of compressors and that a direct connection thus exists between the observed forced response and the unsteady flow phenomena at stall onset. For counterrotational distortions, all the compressors tested showed minimal loss of stability margin. The results imply that counterrotation of the fan and core compressor, or LP and HP compressors, could be a worthwhile design choice. Calculations based on the two-dimensional theoretical model show excellent agreement for the compressors, which had a single peak for stall margin degradation. We take this first-of-a-kind comparison as showing that the model, though simplified, captures the essential fluid dynamic features of the phenomena. Agreement is not good for compressors that had two peaks in the curve of stall margin shift versus distortion rotation speed. The discrepancy is attributed to the three-dimensional and short length scale nature of the stall inception process in these machines; this includes phenomena that have not yet been addressed in any model.

The interaction of turbine inter-platform leakage flow with the mainstream flow

Individual nozzle guide vanes (NGV???s) in modern aeroengines are often cast as a single piece with integral hub and casing endwalls. When in operation, there is a leakage flow through the chord-wise interplatform gaps. An investigation into the effect of this leakage flow on turbine performance is presented. Efficiency measurements and NGV exit area traverse data from a low-speed research turbine are reported. Tests show that this leakage flow can have a significant impact on turbine performance, but that below a threshold leakage fraction this penalty does not rise with increasing leakage flow rate. The effect of various seal clearances are also investigated. Results from steady-state simulations using a three-dimensional multiblock Reynolds-averaged Navier-Stokes solver are presented with particular emphasis paid to the physics of the mainstream/leakage interaction and the loss generation.

ANALYSIS OF ROTATING STALL ONSET IN HIGH SPEED AXIAL FLOW COMPRESSORS

Ahstrart A computational and theoretical procedure is described for the analysis of flow instability in high speed, muhistage COInpreSWTS. Specifically, lhe paper presenu the first rigorous analysis of the type of twedirnensianal, long wavelength, small amplitude, compressible flow perturbations which have been experimentally observed to develop into rotating stall. The analysis shows that compressibility has a stabilizing influence for both singlestage and multistage machines. A much more impwtant conclusion, however, is that the axial structure of the perturbations, as well as the behavior of the least stable eigenmode (which determines stability for the machine), is similar for the high speed situation and for incompressible flow. The rotating stall onset behaviors of high speed and low speed machines are therefore predicted to be similar in many respecrs. One key feature of this similarity is &at it is the overall slope of the (futl) compressor pressure rise characteristic, rather than the slope of any single stage, that determines instability onset. Numerical resulb are given to illusbate these points, as well as to show the specific influence of parameters such as blade tip Mach number and compessot length.

The effect of stator-rotor hub sealing flow on the mainstream aerodynamics of a turbine

An investigation into the effect of stator-rotor hub gap sealing flow on turbine performance is presented. Efficiency measurements and rotor exit area traverse data from a low speed research turbine are reported. Tests carried out over a range of sealing flow conditions show that the turbine efficiency decreases with increasing sealant flow rate but that this penalty is reduced by swirling the sealant flow. Results from time-accurate and steady-state simulations using a three-dimensional multi-block RANS solver are presented with particular emphasis paid to the mechanisms of loss production. The contributions toward entropy generation of the mixing of the sealant fluid with the mainstream flow and of the perturbed rotor secondary flows are assessed. The importance of unsteady stator wake/sealant flow interactions is also highlighted.Copyright

Calculating the Flowfield Behaviour of High-Speed Multi-Stage Compressors

This paper describes the development of a computational model for the moderate to long lengthscale flowfield behaviour of high-speed multi-stage compressors. The simulation is a time accurate solution of the fully compressible non-linear equations for fluid flow through an entire compressor. Flow in the non-bladed regions is calculated using a two-dimensional Euler solver whilst the individual blade rows are modelled using multiple one-dimensional flowfields with body forces. The body forces are chosen to give the required steady state blade row performance and a physically based circulation model is used to give the correct non-steady flow dynamics. The capabilities of the simulation are demonstrated in this paper by calculating how the flowfield breakdown of a four stage compressor is affected by operating speed and a range of different inlet flow distortions.Copyright

Effect of length on compressor inter-stage duct performance

This paper describes the effect of reducing the length of a plain S-shaped compressor inter-stage duct on its performance. The investigation is aimed at understanding the physical mechanisms that determine the flow within these ducts and to assess the effect of these mechanisms in the overall duct performance. An experimental and computational investigation on the development of the boundary layers and loss generation within a datum duct (100% length) was performed. These results were used to design two S-shaped ducts with 74%, and 64% duct length of those currently used as inter-stage ducts. The duct designs were optimised for minimum stagnation pressure loss using a one degree-of-freedom design methodology. Using the duct with lengths 100%, 74% and 64% of current inter-stage ducts, an experimental investigation on the effect of duct length on the development of the boundary layers and loss generated was performed. As the duct length was reduced the loss increases mainly due to boundary layer separation. The size of the separations was found to be strongly dependant on Reynolds number. The results show that at the Reynolds numbers tested relaminarization of the boundary layer may have occurred upstream of separation.Copyright

An Experimental Study of the Inception of Rotating Stall in a Single-Stage Low-Speed Axial Compressor

There are two established mechanisms, spike and modal inception, by which rotating stall is initiated in an axial flow compressor. Whilst the “Critical incidence hypothesis” and the “Zero slope criterion” are useful ideas in explaining the different stability boundaries for spikes and modes they do not provide the designer with a predictive tool. A detailed experimental investigation utilising a single-stage low-speed compressor is presented in which the aerodynamic environment of a rotor blade row is changed (rotor geometry is held fixed) so that it exhibited both spike and modal inception upon throttling into stall. The dominant mechanism of stall inception was found to be dependent on both the inlet flowfield and the downstream stator. The measurements are analysed and show that the meridional acceleration across the tip region of the rotor influences the mechanism by which rotating stall is incepted. This research is presented as a contribution towards the prediction of the stall inception mechanism.Copyright

Calculating Stall and Surge Transients

Many aspects of the aerodynamic operation and the mechanical integrity of an aeroengine are determined by the stall and surge transients of the compression system. A, physically based, blockage-mixing method and a novel blockage-transport equation are presented that can be used to estimate blade row operation at severe off-design conditions. The blockage-mixing method is proposed based on an analysis of the large separations that are present within a blade passage during reverse-flow (negative axial velocity) conditions. A time accurate computational simulation has been developed, based on the blockage-mixing method, which solves for the moderate to long lengthscale compressible unsteady flowfield within a compression system. The flow through each blade row is resolved in the axial, circumferential and radial directions. Calculated stall and surge events are compared with experimental data and the ability to calculate an aeroengine transient is demonstrated. The simulation is presented as a computational tool for estimating the aerodynamic loads during compression system instability events.Copyright