Rotating Inlet Distortion Research Articles

Analysis on stall mechanism of axial flow compressor with rotating inlet distortion by dynamic mode decomposition

To study the impact of rotating inlet distortion on stall mechanism of an axial compressor, the full-annulus unsteady simulation results are analysed using dynamic mode decomposition (DMD). Firstly, the frequency and energy ratio of each mode at near stall condition for the compressor with uniform inflow are obtained. A dominating frequency band 0.3–0.6 BPF regarded as the characteristic frequency of unsteady tip leakage flow is observed. The distribution of eigenvalues further confirms that there are a few critical unstable modes in the flow field. With further throttling, four large-scale disturbance structures are captured by the second-order mode, whose frequency is twice as the rotor frequency. The circumferential propagation velocity of the stall cell is thus deduced to be about 50% of the rotor speed. Further, when the compressor is subjected to rotating inlet distortion, the change of stalling process is related to the relative position of distortion plate and stall cell according to the DMD mode frequency and shape. The addition of rotating distortion does not change the stall type of the compressor, but the unstable modes indicate that the number and shape of the stall cells have been varied.

Resolvent Analysis for Subsonic Compressor With Impedance Boundary Condition Casing Treatment

Abstract A resolvent analysis framework for the axial compressor is established, and the impedance optimization is achieved based on the resolvent analysis framework for the impedance boundary condition casing treatment. This framework is derived by treating the nonlinearity in the perturbation equations as an unknown forcing, the linear relationship between wall impedance and energy gains is obtained. The validity of this framework is confirmed through experiments on rotating inlet distortion, which captures the most susceptible frequency for the stall points of the compressor. The resolvent analysis framework further confirms that the first-order circumferential mode exhibits the highest energy in the given cases. Subsequently, the proposed impedance optimization method is tested under throttling operating conditions, especially focusing on the first-order circumferential mode. The introduction of a favorable impedance boundary condition notably reduces energy gain within the low-order circumferential mode and in the range of the rotor rotating frequency, particularly in near-stall operating conditions. The energy suppression mechanism of the impedance boundary condition casing treatment is investigated, demonstrating that the impedance boundary condition, with an optimal impedance value, significantly suppresses perturbations compared to the case with the solid wall boundary condition. Lastly, a design method for the impedance boundary condition casing treatment is discussed, offering a reliable theoretical design tool for enhancing the stall margin of axial compressors.

Effect of Rotating Inlet Distortion with Multi-distorted Regions on Compressor Stability

Effect of Rotating Inlet Distortion with Multi-distorted Regions on Compressor Stability

Effects of rotating inlet distortion on the stall mechanism of an axial flow compressor

To study the influence of rotating inlet distortion on a single-stage axial compressor, both experiment and full-annulus unsteady simulation methods are carried out to reveal the stall mechanism under different inflows. First, the experimental results show that compared with static distortion, rotating distortion reduces the compressor stall margin more seriously. The wavelet analysis of dynamic pressure data shows that although the addition of inlet distortions has changed the stalling process, the compressor still stalls through spike-type. The full-annulus unsteady simulations indicate that the low-velocity zone first appears at the hub under uniform inflow condition. When the compressor is subjected to rotating distortion, the low-velocity zone first appears at the tip region. Time-averaged solutions of unsteady simulations are further studied to reveal the underlying mechanisms. Compared with the flow field under uniform inflow, the addition of rotating distortion can produce co-swirl, but it also causes the change of axial velocity distribution along the radial direction. Under the combined effect of both, the blade loading of rotor tip increases significantly, altering the position where the low-velocity zone first appears.

Response and stabilization of a two-stage axial flow compressor restricted by rotating inlet distortion

This paper considers effects of Rotating Inlet Distortion (RID) on a two-stage axial compressor and the stabilization effects of a casing treatment subjected to the considered RID. In multi-spool engines, the downstream compressor suffers a RID when the upstream fan/compressor is in rotating stall. A controlled rotating sectional screen upstream of the compressor was adopted to generate the RID in different intensities, and co- or counter spinning direction compared with the rotor. The response of the compressor to inlet distortion was interpreted as the changes of stall margin. Results show that there are two peaks for the stall margin degradation where the distortion screen rotates near 20% and 70% of the rotor’s rotational frequency, respectively. Based on the measurements taken during the pre-stall process, it is suggested that the two frequencies are associated with the eigen frequencies of the compressor. Specifically, the first is associated with a stall precursor that is suspected as an initial disturbance existing in compressor and the second is related to a spike-type stall precursor. At last, a kind of Stall Precursor-Suppressed (SPS) casing treatment was applied to enhance the compressor stability in RID conditions. The result indicates that the stall margin is improved by 6% at an average level and there is nearly no efficiency loss caused by the application of such casing treatment.

Performance analysis of the two-stage axial compressor with 90-degree bend inlet conditions

The 90-degree bend is one of the basic connection components applied in industrial flowing systems. This bend influences the aerodynamic performance of the downstream connecting equipment. In this study, the performance of a two-stage axial compressor with a 90-degree bend inlet was numerically and experimentally analyzed under low rotating speed. The testing results show that the outflow of the bend was numerically non-uniform in the circumferential and radial directions. To analyze the performance influence of the bend on the downstream compressor, a full passage compressor model with and without the inlet bend was simulated. The size of the distortion region gradually spread to the entire cascade, and the intensity of the distortion obviously dropped after the first stage as the compressor ran with the bend. The deterioration of the compressor performance, especially in the first stage, was verified numerically and experimentally. The total pressure coefficient and isentropic efficiency decreased by 2.6 % and 1.13 %, respectively. To save simulation cost, a model with a downstream single blade passage and cylindrical inlet was proposed, and the distorted flow was set as the inlet boundary condition. In addition, the unsteady numerical simulation was performed with the rotating inlet distortion region. Compared with the full blade passage model, the unsteady single blade passage model obtained the downstream spread characteristic of the distortion and performance deterioration. The latter can therefore be suggested as a compromised approach for obtaining the propagation characteristics of the inlet distortion with acceptable accuracy and low computational cost.

Effects of Rotating Inlet Distortion on Compressor Stability With Stall Precursor-Suppressed Casing Treatment

This paper conducts an experimental research of rotating inlet distortion on a low-speed large size test compressor with emphasis on the stability problem of axial fan/compressors, and the stall margin enhancement with a kind of stall precursor-suppressed (SPS) casing treatment. Some results on compressor stall margin and prestall behavior under the restriction of rotating inlet distortion are presented. The experimental results show that whether the inlet distortion is co-rotating or counter-rotating, the SPS casing treatment can still improve the stall margin without leading to additional efficiency loss caused by such configuration. The experiment results also show that the mechanism of the stall margin improvement with such casing treatment is associated with delaying the nonlinear development of the stall precursor waves and weakening the unsteady flow disturbances in a compression system.

The role of tip leakage flow in triggering rotating stall of a low speed compressor under inlet distortion

The stall behavior in a single-stage low-speed axial compressor under rotating inlet distortion (RID) is investigated in the first half of this paper. The tests demonstrate that the tip leakage flow (TLF) plays an important role in triggering rotating stall. The tracking of the spike-like disturbances caused by the spillage of TLV indicates that most of such spike-like disturbances will be smeared by non-distorted sector and the growth of the spike-like disturbances actually relate closely to how and how often the path of the propagating disturbances come across the path of the rotating distorted sector. In the second half of this paper, micro air injections are applied to test the effect behavior of TLF on stall inception. Contrasts to without micro air injections, the spike-like disturbances are much fewer, so the possibilities that spike-like disturbances may trigger rotating stall are fewer too. As a result, the compressor gets a lower mass flow rate at stall for both co-rotating inlet distortion and counter-rotating inlet distortion.

Flow Structure of Short-Length-Scale Disturbance in an Axial-Flow Compressor

Short-length-scale disturbances, also called spikes, are often responsible in triggering rotating stall in axial-flow compressors. One hypothesis suggests that spikes can be the consequence of dynamic interaction among forward-spilled tip-leakage flow, the main throughflow, and the reversed flow. However, the transit process of such a dynamic interaction in the vicinity of the rotor tip clearance and, thus, the physical images of the flow structure of a spike are still unknown. In this paper, we present a numerical study with a novel scheme for a low-speed axial-How compressor by incorporating rotating inlet distortion. Because the inlet distortion will overload a portion of the blades while keeping the rest working normally, the short-length-scale disturbances can be observed without advocating the computational difficulty of simulating a fully stalled compressor. Two unsteady simulations using a commercial, three-dimensional, time-accurate, Reynolds-averaged Navier-Stokes solver are performed: one for a one-fourth rotor annulus with finer grids and the other for the entire rotor annulus with coarser grids. After the code is validated by comparing experimental results with the simulation for the entire rotor annulus, the one-fourth-annulus simulation is used to unveil the flow physics. As elucidated from the computational results, the complete birth-to-decay process of the short-length-scale disturbances is captured for the first time. The corresponding 3-D flow structure is also revealed. It is shown that the effects of dynamic How interaction at the tip extend beyond the tip region and deeply into the blade span. A horn-shaped vortex with one end at about 30% of the blade span and the other end at the casing is formed, which generates a low-pressure dip in the casing-pressure measurement. The spike, as identified from casing-pressure measurement, corresponds to a flow image in which the vortex rotates around the annulus.

The response of a low speed compressor on rotating inlet distortion

In multi-spool engines, the downstream compressor experiences a rotating inlet distortion if rotating stall occurs in upstream compressor, which may induce the instability of the whole compressor. In this paper, the compressor dynamic behavior is the major focus. The experiment was carried out on a single-stage low-speed axial-flow compressor. A rotating distortion generator equipped with different distortion sector(s) was designed to produce different rotating inlet distortion clockwise or counterclockwise with up to 100% of the compressor design speed. The distortion sector can be installed single or in some combination such as four sectors together. Three types of distortion sector/combination are used in the research work, which are single 30 degree sector, four 30 degree sectors and single 120 degree sector. It is found that the total pressure loss caused by rotating sector(s) increases when the distortion speed rises. For co-rotating distortions, all the three types of inlet distortion exhibited a peak in stall margin degradation when the distortion speed corresponded to roughly 50% of rotor speed. The two-dimensional numerical simulations of the compressor flow field clearly show the propagation of the disturbances trigged by the distortion sector(s).

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.

Unsteady Aerodynamics and Gust Response in Compressors and Turbines

A comprehensive series of experiments and analyses was performed on compressor and turbine blading to evaluate the ability of current, practical, engineering/analysis models to predict unsteady aerodynamic loading of modern gas turbine blading. This is part of an ongoing effort to improve methods for preventing blading failure. The experiments were conducted in low-speed research facilities capable of simulating the relevant aerodynamic features of turbomachinery. Unsteady loading on compressor and turbine blading was generated by upstream wakes and, additionally for compressors, by a rotating inlet distortion. Fast-response hot-wire anemometry and pressure transducers embedded in the airfoil surfaces were used to determine the aerodynamic gusts and resulting unsteady pressure responses acting on the airfoils. This is the first time that gust response measurements for turbines have been reported in the literature. Several different analyses were used to predict the unsteady component of the blade loading: (1) a classical flat-plate analysis, (2) a two-dimensional linearized flow analysis with a ‘frozen gust’ model, (3) a two-dimensional linearized flow analysis with a “distorted gust” model, (4) a two-dimensional linearized Euler analysis, and (5) a two-dimensional nonlinear Euler analysis. Also for the first time, a detailed comparison of these analyses methods is made and the importance of properly accounting for both vortical and potential disturbances is demonstrated. The predictions are compared with experiment and their abilities assessed to help guide designers in using these prediction schemes.