Low-speed Axial-flow Compressor Research Articles

Measurement of unsteady force on rotor blade surfaces in axial flow compressor

To assess the aerodynamic performance and vibration characteristics of rotor blades during rotation, a study of unsteady blade surface forces is conducted in a low-speed axial flow compressor under a rotating coordinate system. The capture, modulation, and acquisition of unsteady blade surface forces are achieved by using pressure sensors and strain gauges attached to the rotor blades, in conjunction with a wireless telemetry system. Based on the measurement reliability verification, this approach allows for the determination of the static pressure distribution on rotor blade surfaces, enabling the quantitative description of loadability at different spanwise positions along the blade chord. Effects caused by the factors such as Tip Leakage Flow (TLF) and flow separation can be perceived and reflected in the trends of static pressure on the blade surfaces. Simultaneously, the dynamic characteristics of unsteady pressure and stress on the blade surfaces are analyzed. The results indicate that only the pressure signals measured at the mid-chord of the blade tip can distinctly detect the unsteady frequency of TLF due to the oscillation of the low-pressure spot on the pressure surface. Subsequently, with the help of one-dimensional continuous wavelet analysis method, it can be inferred that as the compressor enters stall, the sensors are capable of capturing stall cell frequency under a rotating coordinate system. Furthermore, the stress at the blade root is higher than that at the blade tip, and the frequency band of the vibration can also be measured by the pressure sensors fixed on the casing wall in a stationary frame. While the compressor stalls, the stress at the blade root can be higher, which can provide valuable guidance for monitoring the lifecycle of compressor blades.

Simulation and experiment of the aerodynamic performance of a vane compressor damaged by a bird strike

Based on the purpose of studying the impact of bird strike on compressor aerodynamic performance, this paper takes a single-stage low-speed axial flow compressor as an object to establish a blade model damaged by bird strike, and on this basis, it carries out a three-dimensional full-loop numerical simulation and experiments on the compressor model with damaged blades. Through comparison with the experimental data of undamaged compressors, the results show that after the rotor blades are damaged, the rotor blades are damaged. The aerodynamic performance and stability of the compressor have significantly decreased, which accords with our understanding. The reliability of the numerical simulation method is verified by analyzing the detailed flow field structure of typical working conditions and comparing the experimental data of the compressor, which lays a foundation for the relevant calculation of damaged blades in the future.

Effect of wire mesh casing treatment on axial compressor performance and stability

In this paper, a kind of Wire Mesh Casing Treatment (WMCT) is proposed to improve the stable operating range of the compressor. In contrast to the traditional circumferential groove, as for WMCT, a layer of wire mesh is laid on the surface of the circumferential groove. Parametric studies were conducted on the low-speed axial flow compressor, including the groove width, axial location, and mesh count. The optimum axial location for WMCT is related to its groove width. A higher wire mesh count results in a smaller compressor stall margin improvement. Steady simulations were carried out to study the effect of WMCT on the flow structure of the compressor. The wire mesh in the WMCT has a certain flow resistance, which restricts the flow into and out of the groove. Due to the WMCT, the flow parameter in the tip region of the rotor is less sensitive to changes in the operating conditions of the compressor. The WMCT causes the rotor tip blade loading to shift backward, inhibiting the formation of spill forward of the leakage flow, and thus improving the stability of the compressor. The flow resistance on the groove surface is a new degree-of-freedom for the casing treatment designer.

Investigation of Vaned-Recessed Casing Treatment in a Low-Speed Axial Flow Compressor, Part I: Time-Averaged Results

This paper investigates the effects of two modifications to a vaned recessed casing treatment. First, the shape of a circular curve was used in the top of the treated casing. Second, a fully curved guide vane was also applied. The goals of the modifications are to enhance the flow recirculation as well as to relieve the low-speed flow, which is normally accumulated within the corners of the vaned recessed casing treatment. The solid casing in addition to the two vaned recessed configurations with 23.2% and 53.5% rotor blade tip axial chord exposure have been studied numerically. The results indicated that two mechanisms are involved in the stall margin enhancement. First, the circumferential pressure gradient is reduced for both configurations. The reduction in pressure gradient largely reduces the development of tip leakage vortex and, thus, the generation of low-speed fluid is diminished. Second, the main flow/tip leakage interface moves toward downstream and the movement of interface toward the leading edge is delayed. The second configuration with a greater rotor blade tip exposure enables extra flow recirculation due to decreasing surface area and, therefore, could be superior to the application of the first casing treatment configuration. The major streamlines within the casing treatment are also discussed. The time-averaged results are presented in this paper, while the unsteady results including instantaneous flow fields, origins of the unsteadiness and frequency analysis are discussed in part II.

Investigation of Vaned-Recessed Casing Treatment in a Low-Speed Axial-Flow Compressor, Part II: Unsteady Results

In this paper, unsteady characteristics of a modified vaned-recessed casing treatment with 23.2% rotor blade tip axial chord exposure were studied numerically. The modifications to the traditional vaned-recessed casing treatments were composed of geometrical amendments to the casing treatment’s guide vanes and the top of the treated casing. The solid casing and the casing treatment configurations were simulated using the Unsteady Reynolds-Averaged Navier–Stokes equations (URANS), and the results were validated by experimental results. Firstly, standard deviation and frequency analysis were performed to find the sources of unsteadiness. Secondly, velocity components analysis, including velocity triangles, was presented instantaneously to clarify their effects on rotor tip flow fields as well as stall margin improvement. Thirdly, unsteady interactions between the rotor and casing treatment flow fields, including flow structure and pressure distributions, were discussed. In the end, flow streamline patterns, in addition to the physical mechanism of the vaned-recessed casing treatment, were also discussed. The results indicated that unsteadiness plays an important role in the flow mechanism and cannot be ignored. The unsteadiness increases as the mass flow is reduced toward the stall/surge condition. Moreover, the analysis of velocity components demonstrated that the casing treatment has distinct behavior at the last operating points before the onset of the stall for solid casing and casing treatment configurations in terms of axial velocity change.

Characteristics of a Fluidic Oscillator with Low Frequency and Low Speed and Its Application to Stall Margin Improvement

Active flow control methods are commonly used in expanding the operating range of compressors. Indeed, unsteady active control methods are the main focus of researchers due to their effectiveness. For constructing an unsteady active control system, reliable actuators are significant. To compare with conventional actuators such as synthetic jet actuators and rotating valves, fluidic oscillators have structurally robust characteristics and can generate self-excited and self-sustained oscillating jets, which leads to its higher applicability in compressors under severe working conditions. Thus, to explore the feasibility of unsteady active control systems by the usage of fluidic oscillators, a low-frequency and low-speed oscillator is first designed and experimentally studied for improving the stability of a low-speed axial flow compressor. During the experiments, a special casing is designed to install 15 uniformly distributed oscillators in the tip region of compressor. Based on the unsteady micro injections of the rotor tip with rotor rotation frequency, the results indicate that the frequency/period of oscillators are flexible, in which the values are decoupled with the variation of inlet pressure. When the inlet-to-outlet pressure ratio of the oscillator is in the range of 1.1~2.0, the maximum velocity ranges from 30 m/s to 80 m/s. Moreover, the mass flow rate of the single oscillator only varies from 0.017‰ to 0.059‰ from the designed compressor mass flow rate. For the improvement of the compressor stall margin, the value is 3.45% when the total mass flow of oscillators is 0.08% of the designed compressor mass flow.

Stability Enhancement and Noise Reduction of an Axial Compressor with Foam Metal Casing Treatment

Foam metal is a foam-like substance with a high porosity; it has been used in flow control, vibration abatement, and acoustic absorption, mainly based on its physical properties. The aim of the current paper is to investigate the effect of foam metal casing treatments on the stability and acoustic level of a low-speed axial flow compressor. The experimental results show that the casing treatment improves the stall margin by 14.9%, without any efficiency loss. In terms of noise, the SPL of the tonal noise at the third order of BPF is reduced by 3.2 dB, while the SPL of the broadband noise is reduced up to 2.4 dB. The comparison in evolutions of the tip structure in a smooth casing condition and with a casing treatment indicates that the casing treatment affects the origination and the development of the tip leakage vortex. The working mechanism is also discussed.

Reliability analysis for stall warning methods in an axial flow compressor

The reliability of widely studied four stall warning methods, namely, auto- and cross-correlation, Root-Mean-Square (RMS), and fast wavelet analysis, were experimentally studied in a low-speed axial flow compressor. Unsteady pressures used for stall warning were measured by time-resolved sensors mounted on the casing along axial and circumferential spatial resolution. Three influence factors of rotational speed fluctuation, circumferential and axial location of pressure sensors for the stall warning reliability were analyzed and discussed. The rotational speed fluctuation that inevitably exists in the actual aero-engine would lead to the phase shift of phase-locked pressure data and thus deteriorates the stall warning reliability using the auto-correlation and RMS analysis methods. The circumferential location of pressure sensors would affect the reliability of proposed stall warning methods due to the uneven tip clearance distribution. Auto-correlation, RMS, and fast wavelet methods are more effective at the large tip clearance location than at the small tip clearance location. For the cross-correlation method that needs two symmetrically-arranged pressure sensors, the negative effect caused by the circumferential unevenness of tip clearance size can be avoided. The ideal axial location of the sensor installation for stall warning is near the rotor leading edge where the strong tip leakage flow fluctuation exists. In addition, considering the actual complex inflow condition, the stall warning methods' reliability with inlet circumferential distortion was studied. In the distorted region, the adverse effect of distortion on stability can be detected by above four methods. In the undistorted region, only cross-correlation coefficient correspondingly decreased to reveal the stability margin deterioration caused by circumferential distortion. Considering the random circumferential position of distortion, cross-correlation method was proved to have a promising application compared to others.

Effects of the foam metal casing treatment on aerodynamic stability and aerocoustic noise in an axial flow compressor

A kind of foam metal casing treatment (FMCT) is proposed and experimentally tested on a low-speed axial flow compressor in this work. Foam metal is a foam-like substance with a lightweight structure but a certain strength. Both time-mean and high-response instrumentation are applied to capture the steady and unsteady performances of the compressor. Initially, parametric experimental measurements on axial location of foam metal over rotors are performed with respect to stability enhancement and noise reduction. It is found that the stall margin improvement (SMI) and the aerodynamic efficiency loss are functions of the axial location of the foam metal. Particularly, the SMI maxima appears at CT7 with a certain amount of aerodynamic efficiency loss. In analyzing the results measured over and downstream of the rotor, it is suggested that the release of rotor tip loading may be a stability improvement mechanism of FMCTs. Furthermore, comparison in the stall inception process of the axial flow between the compressor with the solid-wall and the implementation configuration shows that, in solid-wall casing condition, the compressor experiences a spike-type stall inception, however in CT7, the compressor goes through a modal-type process. Finally, conclusions and suggestions are given based on the findings.

Experimental and Numerical Investigation on Effects of the Steam Ingestion on the Aerodynamic Stability of an Axial Compressor.

In order to investigate the influence of steam ingestion on the aerodynamic stability of a two-stage low-speed axial-flow compressor, multiphase flow numerical simulation and experiment were carried out. The total pressure ratio and stall margin of the compressor was decreased under steam ingestion. When the compressor worked at 40% and 53% of the nominal speed, the stall margin decreased, respectively, by 1.5% and 6.3%. The ingested steam reduced the inlet Mach number and increased the thickness of the boundary layer on the suction surface of the blade. The low-speed region around the trailing edge of the blade was increased, and the flow separation region of the boundary layer on the suction surface of the blade was expanded; thus, the compressor was more likely to enter the stall state. The higher the rotational speed, the more significant the negative influence of steam ingestion on the compressor stall margin. The entropy and temperature of air were increased by steam. The heat transfer between steam and air was continuous in compressor passages. The entropy of the air in the later stage was higher than that in the first stage; consequently, the flow loss in the second stage was more serious. Under the combined action of steam ingestion and counter-rotating bulk swirl distortion, the compressor stability margin loss was more obvious. When the rotor speed was 40% and 53% of the nominal speed, the stall margin decreased by 6.3% and 12.64%, respectively.

Experimental investigations of the flow phenomena in the rotor blades of the axial flow low speed compressor stage at the unstable part of the overall performance characteristic

Experimental investigations of the flow phenomena in the rotor blades of the axial flow low speed compressor stage at the unstable part of the overall performance characteristic

Numerical Investigation for the Impact of Single Groove on the Stall Margin Improvement and the Unsteadiness of Tip Leakage Flow in a Counter-Rotating Axial Flow Compressor

A low-speed counter-rotating axial flow compressor (CRAC) with single circumferential grooved casing treatment (CT) was investigated numerically. Both steady and time-accurate numerical calculations were performed to study the effects of the single grooved CTs over the rear rotor on the stability enhancement and the unsteadiness of tip leakage flow (TLF) in the CRAC. Parametric studies indicate that the best position of the single groove should be located near about 20% axial tip chord in terms of the stall margin improvement (SMI). The coincidence of the effective CT locations and the high fluctuating region on blade pressure surface in the smooth wall case shows that the unsteadiness of TLF plays an important role in the stall inception process. Frequency analysis for the static pressure signals near the blade tip shows that both the disappearance of the low frequency components and the suppression of unsteady TLF are beneficial to the SMI. Detailed observation of the flow structures illustrates that the action of the grooves on the different parts of TLF is responsible for the difference of SMI in the CTs. It is more effective to improve the flow stability by controlling the critical TLF released from near the mid-chord.

Self-Adaptive Stability-Enhancing Technology With Tip Air Injection in an Axial Flow Compressor

Self-adaptive stability control with discrete tip air injection and online detection of prestall inception is experimentally studied in a low-speed axial flow compressor. The control strategy is to sense the cross-correlation coefficient of the wall static pressure patterns and to feed back the signal to an annular array of eight separately proportional injecting valves. The real-time detecting algorithm based on cross-correlation theory is proposed and experimentally conducted using the axisymmetric arrangement of time-resolved sensors. Subsequently, the sensitivity of the cross-correlation coefficient to the discrete tip air injection is investigated. Thus, the control law is formed on the basis of the cross-correlation as a function of the injected momentum ratios. The steady injection and the on–off pulsating injection are simultaneously selected for comparison. Results show that the proposed self-adaptive stability control using digital signal processing (DSP) controller can save energy when the compressor is stable. This control also provides protection when needed. With nearly the same stall margin improvement (SMI) as the steady injection (maximum SMI is 44.2%), the energy of the injected air is roughly a quarter of the steady injection. Unlike the on–off pulsating jet, the new actuating scheme can reduce the unsteady force impinging onto the compressor blades caused by the pulsating jets in addition to achieve the much larger stability range extension.

Numerical investigation of the influence of the camber distribution at the rotor tip on the efficiency at different tip clearances

The present paper reports on numerical investigations into the relationship of compressor efficiency drop due to increased rotor tip clearance and the rotor tip camber distribution in a 1.5 stage low-speed axial flow compressor. Starting from a baseline compressor, six alternative designs were derived. In these redesigns the tip section camber line of the rotor was replaced by an analytically given camber distribution. These camber lines used for the redesigns ranged from extreme front load to extreme rear load. The new camber line styles were blended into the original blade over the upper 30 % of blade height. For each of these variations a design speed characteristic was calculated for five different rotor tip clearances. From these characteristics the compressor efficiency at the design flow rate was extracted. Based on these values an exchange rate could be calculated relating compressor overall efficiency to rotor tip clearance height. It turned out that the rotor tip camber line style does not have an impact on this exchange rate. It could be shown that rotor losses are only affected slightly by the rotor tip camber line style and that the pressure rise that the tip vortex experiences as it travels through the passage is generally unchanged from one tip camber line style to the other.

Investigation of the Shear Flow Effect on Secondary Flow and Losses in a Low Speed Axial Flow Compressor Cascade

This paper explores the effect of a prescribed intense shear flow on the flow structures in an axial flow compressor cascade. An approximate shear flow is generated in the test section of an open circuit cascade wind tunnel by using a planar grid of parallel rods with varying solidity. To study only the effect of shear, a compressor cascade based on NACA65 series with a relatively low camber was chosen. The cascade was analysed experimentally as well as computationally (using ANSYS Fluent). The computational results of the cascade were compared with the available measured data. The results agreed well with the experimental data. Detailed analysis of the numerical results was then carried out to explore the complex flow features caused by the shear flow in a cascade. With uniform flow, the secondary flow was found to be negligible from experiments as well as from the computations. Therefore, in an attempt to amplify secondary flow, a shear flow generator was placed upstream of the cascade. Experiments showed quite contrasting results with shear flow, as compared to the uniform flow, in terms of the wake loss. Numerical analysis revealed the formation of vortices in the wake of the cascade due to secondary flows caused by the incoming shear flow and other interesting flow features.

Investigation of the Shear Flow Effect and Tip Clearance on a Low Speed Axial Flow Compressor Cascade

This paper explores the effect of inlet shear flow on the tip leakage flow in an axial flow compressor cascade. A flow with a high shear rate is generated in the test section of an open circuit cascade wind tunnel by using a combination of screens with a prescribed solidity. It is observed that a stable shear flow of shear rate 1.33 is possible and has a gradual decay rate until 15 times the height of the shear flow generator downstream. The computational results obtained agree well with the available experimental data on the baseline configuration. The detailed numerical analysis shows that the tip clearance improves the blade loading near the tip through the promotion of favorable incidence by the tip leakage flow. The tip clearance shifts the centre of pressure on the blade surface towards the tip. It, however, has no effect on the distribution of end wall loss and deviation angle along the span up to 60% from the hub. In the presence of a shear inflow, the end wall effects are considerable. On the other hand, with a shear inflow, the effects of tip leakage flow are observed to be partly suppressed. The shear flow reduces the tip leakage losses substantially in terms of kinetic energy associated with it.

Application of the Wavelet Transform of Pressure Signals for Detecting and Analyzing Rotating Stall Inception in Axial Flow Low Speed Compressor Stage

The paper presents a research program carried out to improve understanding of the fluid dynamics mechanisms that lead to rotating stall in the axial flow low speed compressor stage. The stalling behavior of this compressor stage was studied by measuring unsteady casing pressure by means of a circumferentially and axially spaced array of high frequency pressure transducers. Another probe used was a disc static pressure probe, with the pressure transducer, for in-flow and out-flow measurements along the blade span. It was expected that understanding of the fluid dynamics will facilitate at least two important tasks. The first was to accurately predict of when and how a particular compressor would stall. The second was to control, delay, or eventually suppress the rotating stall and surge. In consequence, one could extend the useful operating range of the axial compressor. Another motivation for the research was to compare the results from the three applied analysis techniques by using a single stall inception event. The first one was a simple visual inspection of the traces, which brought about a very satisfactory effect. The second one was application of spatial Fourier decomposition to the analysis of stall inception data, and the third method of analysis consisted in application of wavelet filtering in order to better understand the physical mechanisms which lead to rotating stall. It was shown that each of these techniques would provide different information about compressor stall behavior, and each method had unique advantages and limitations.

Aerodynamic Design and Optimization of Zero-stage for Three-stage Axial-flow Low-speed Compressor

The aerodynamic design of a new front stage for a three-stage axial-flow low-speed compressor based on the similarity theory is described,in order to increase the flow by 12.5% and the total pressure ratio.The design and optimization are carried out by using a 3D-viscocity controlled diffusion airfoil(CFD) technology on stator.The new computational fluid dynamics(CDA) profile is adopted to control the profile losses and adverse pressure gradient at trailing areas,so the three dimensional separation is reduced greatly.End-bending technology is also used to further mitigate the accumulation of low-energy fluid at stator end wall.It improves not only the flow at end zone but also the matching between the front stage and the base one,and enhances the zero-stage performance to arrive the design target.It is found that the surge margin is ensured by analyzing the characteristic curves and the base one keeps the similitude operating conditions by comparing the flow angles.

Stall characteristics and tip clearance effects in forward swept axial compressor rotors

Tilting the blade sections to the flow direction (blade sweep) would increase the operating range of an axial compressor due to modifications in the pressure and velocity fields on the suction surface. On the other hand, blade tip gap, though finite, has great influence on the performance of a turbomachine. The present paper investigates the combined effect of these two factors on various flow characteristics in a low speed axial flow compressor. For this present study, nine computational domains were modeled; three rotor sweep configurations (0°, 20° and 30°) and for three different clearance levels for each rotor. Commercial CFD solver ANSYS CFX 11.0 is used for the simulations. Results indicated that tip chordline sweep is found to improve the stall margin of the compressor by modifying the suction surface boundary layer migration phenomenon. Diffusion Factor (DF) contours showed the severity of stalling with unswept rotor. For the swept rotors, the zones of high probable stall are less severe and they become less in size with increasing sweep. Increment in the tip gap is found to gradually affect the performance of unswept rotor, while the effect is very high for the two swept rotors for the earlier increments. As a minimum clearance is unavoidable, swept rotors suffer relatively higher deviation from the idealistic behavior than the unswept rotor due to tip clearance.

Aerodynamic Performance of Low Speed Axial Flow Compressor Rotors with Sweep and Tip Clearance

Blade tip gap, though finite, has great influence on the performance of a turbomachine. Tilting the blade sections to the flow direction (blade sweep) would increase the operating range of an axial compressor by modifying the pressure and velocity fields on the suction surface. The present paper investigates the combined effect of these two factors on various flow characteristics in a low speed axial flow compressor. For the present study, six computational domains were modeled: two rotor sweep configurations (0° and 20°) and three different clearance levels for each rotor. Commercial CFD package ANSYS® CFX 11.0 is used for the simulations. Results indicated that the effect of tip clearance is more predominant in swept rotor than unswept rotor in terms of change in total pressure rise and efficiency. Traced streamlines indicated that blade sweep guides the suction surface boundary layer fluid towards the trailing edge, preventing it from getting accumulated near the tip. Studies near the tip region showed the pattern of velocity streamlines through the tip gap. Tip clearance vortices are found and their trajectory and effect on the flow are studied. Total pressure rise contours at the rotor exit showed that radial migration of low energy boundary layer fluid towards the tip sections is well prevented with sweep.