Fan Stator Research Articles

Design and aerodynamic stability improvement study of a non-axisymmetric stator fan in a distorted insertion plate configuration

The design of anti-distortion fans must ensure the reliable operation and flight safety of propulsion systems under inlet distortion conditions. This research uses an insertion plate to obtain the inlet distortion flow field for a specific type of turbofan engine. Based on the characteristics of distortion effects, a novel strategy is proposed to rapidly implement a non-axisymmetric stator (NAS) quasi-three-dimensional design to create anti-distortion fan designs and improve the performance and stability of the fan in distortion environments. Numerical studies show that under three rotational speed operating conditions, the NAS effectively improves the total pressure ratio and efficiency characteristics of the fan under inlet distortion conditions. Compared to the prototype, the NAS can significantly increase the stability margins by 10.31 %, 8.31 %, and 7.39 % under the three different speed operating conditions, which effectively expands the stability boundaries. The NAS design directly affects the distortion region, effectively improves the incidence angle of stator vanes in the distortion region, suppresses the development and migration of vortices inside the stator passage, and significantly reduces the stator vane diffusion factor. Because it effectively eliminates corner separations and makes the incidence angle distribution of stator vanes relatively circumferentially uniform, the internal flow field of the fan's stator vanes and overall aerodynamic performance of the fan improve. Furthermore, the NAS design can improve the internal flow field downstream of the fan, direct the nonuniform flow field at the inner bypass outlet toward circumferential uniformity, and effectively improve flow field uniformity.

Three-dimensional effects of cascade perforations on rotor–stator interaction noise

One novel trend in reducing aero-engine noise is to utilize the silent flight mechanism of owls by applying perforations on fan stator vanes. Consequently, the establishment of relevant theoretical models is of particular interest. The current efforts made in this regard are just targeting the features based on two-dimensional models without including the three-dimensionality. In this paper, we present a three-dimensional solution for acoustic scattering by annular perforated cascades, and the dipole source corresponding to the unsteady pressure loading on the vanes is identified as the dominant sound source. By the singularity method, the acoustic response is obtained with the soft boundary condition applied on the vane surfaces. It is found that considerable noise reduction can be achieved for rotor–stator interaction with a modest uniform porosity, and accordingly two mechanisms are proposed to understand the effect of porosity on propagating sound. The first is that the perforations allowing a normal velocity across the vane reduce the unsteady loading induced by the incident disturbances. The second is that the three-dimensional interactions among the dipole sources at different positions are also dampened by the soft boundaries, thus the distribution of the unsteady pressure loading on the vanes will also change significantly compared to hard-vane cases. Non-uniform distributions of porosity are investigated further, indicating that perforations in the vane upstream area are more effective in reducing propagating noise. Our method is fully three-dimensional and capable of investigating non-uniform porosity, and thus is able to provide useful guidance for future soft vane designs.

Ice-Crystal Icing Investigation on a Honeywell Uncertified Research Engine in an Altitude Simulation Icing Facility

AbstractA Honeywell Uncertified Research Engine was exposed to various ice-crystal conditions in the NASA Propulsion Systems Laboratory (PSL). Simulations using NASA's one-dimensional (1D) Icing Risk Analysis tool were used to determine potential inlet conditions that could lead to ice-crystal accretion along the inlet of the core flowpath and into the high-pressure compressor. Baseline conditions were established, and parameters were varied to observe accretion characteristics. Data were acquired at altitudes varying from 5 kft to 45 kft, at nominal ice particle median volumetric diameters from 20 µm to 100 µm, and total water contents of 1 g/m3 to 12 g/m3. Metal temperatures were acquired for the inlet guide vane and vane stators 1–2. In situ measurements of the particle size distribution were acquired upstream and downstream of the engine fan face in order to study particle breakup behavior. Cameras were installed in the engine to capture ice accretions at the leading edge of the fan stator, splitter lip, and inlet guide vane. The goal of this study was to understand the key parameters of accretion, acquire particle breakup data aft of the fan, and generate a unique icing dataset for model development. Significant particle breakup downstream of the fan in the bypass was observed. The metal temperatures on the inlet guide vanes (IGVs) and stators show a temperature increase with increasing particle size. Accretion behavior at the fan stator and splitter lip across was very similar. However, accretion decreased with increasing particle size at the IGVs.

Coupled Fan–Stator Aerodynamic and Acoustic Response to Inflow Distortion

Abstract Turbofan rotor–stator aeromechanic and aeroacoustics modeling has been traditionally developed by considering separately the aerodynamic and acoustic response of the fan and the stator to inflow nonuniformities. The present paper develops a model for the coupled fan–stator response for a realistic 3D geometry. The coupling mechanism is assumed to be mainly carried by scattered waves bouncing back and forth between the fan and the stator. The relationship between sources and state elements at three regions (inlet (1), in-between fan and stator (2), and exit (3)) is derived in terms of a scattering matrix S. The model is applied to two fan configurations: (1) a fully subsonic fan and (2) a transonic tip speed fan. The scattering matrix terms, up to the 5th blade passing frequency (BPF), are calculated by using CAAT, an Euler-based code. Results show coupling adds about 6 dB to the sound power level (PWL) of the transonic fan configuration but has a small effect for the subsonic fan configuration. The analysis of the propagating modes shows that, for the transonic configuration, the inlet mode at 1BPF propagates in regions 2 and 3 but is cut off in region 1. This reinforces the coupling process by trapping the acoustic mode 1BPF in region 2. Although this trapped energy is mainly due to the 1BPF of the fan wake, the fan scatters this energy into higher order acoustic modes and thus produces redistribution toward higher frequency of the acoustic spectra. Finally, adding a liner in region 2 reduces the energy of 1BPF mode propagating upstream and impinging on the fan. This mitigates the effect of the fan–stator coupling.

Blowing–Suction Control in S-Shaped Inlet and Its Impact on Fan-Stage Performance

This paper presents a numerical investigation of flow control with blowing and suction in the whole structure consisting of an aggressive S-shaped air intake accompanied by a downstream fan stage. The results show that the effect of blowing–suction flow control is neither the simple summation of the effects of a single blowing control and a suction control nor the simple differences between them. It is the best one among three flow control techniques studied in this paper. The approach to selecting the optimum flow control scheme via simulating the single air intake, and afterward performing the numerical simulation of whole passage, has a certain rationality. With the blowing–suction flow control, the maximum isentropic efficiency and maximum total pressure ratio of the fan stage increase by 2.70 and 3.13%, respectively. Moreover, the rotor of the fan stage has a significant effect in reducing circumferential distortions. Due to the influence of the rotating fan rotor, the low-energy region at the lower side of the cross sections decreases gradually when approaching the fan-stage inlet. The low-energy fluid always exists in several blade passages after it enters the fan stage. Using the blowing–suction flow control, the relative total pressure of fluid in these low-energy passages increases to an extent. Streamlines in the fan stator flow passages and on stator blades show that the blowing–suction flow control in the air intake can improve the fan stator flow pattern, reducing the flow separation in general.

Nonaxisymmetric Stator Design for Boundary Layer Ingesting Fans

In a boundary layer ingesting (BLI) fan system, the inlet flow field is highly nonuniform. In this environment, an axisymmetric stator design suffers from a nonuniform distribution of hub separations, increased wake thicknesses, and casing losses. These additional loss sources can be reduced using a nonaxisymmetric design that is tuned to the radial and circumferential flow variations at exit from the rotor. In this paper, a nonaxisymmetric design approach is described for the stator of a low-speed BLI fan. First, sectional design changes are applied at each radial and circumferential location. Next, this approach is combined with the application of nonaxisymmetric lean. The designs were tested computationally using full-annulus unsteady computational fluid dynamics (CFD) of the complete fan stage with a representative inlet distortion. The final design has also been manufactured and tested experimentally. The results show that a 2D sectional approach can be applied nonaxisymmetrically to reduce incidence and diffusion factor at each location. This leads to reduced loss, particularly at the casing and midspan, but it does not eliminate the hub separations that are present within highly distorted regions of the annulus. These are relieved by nonaxisymmetric lean where the pressure surface is inclined toward the hub. For the final design, the loss in the stator blades operating with BLI was measured to be 10% lower than that for the original stator design operating with undistorted inflow. Overall, the results demonstrate that the nonaxisymmetric design has the potential to eliminate any additional loss in a BLI fan stator caused by the nonuniform ingested flow field.

MEASURING CHAIN FOR CHECKING THE VIBRATION OF MECHANICAL PARTS

Urgency of the research. Interest in this topic is aroused, because mechanical vibration may damage the machinery or parts of machine. Therefore, it is appropriate to design systems that detect problems. Also these systems can help in the timely replacement of the worn part. Target setting. The main goal is to design a system that can detect in a timely manner a problem that could destroy the device. It is therefore necessary to design systems that can record this. Actual scientific researches and issues analysis. In recent years, there has been an increase in demand for equipment that can detect a timely problem. Many such devices already exist and are still being upgraded. This industry is called vibrodiagnostics. Uninvestigated parts of general matters defining. This paper is focused on the analysis of mechanical systems and the creation of a measuring chain. The research objective. The aim of this research is to analyse the mechanical systems and the assembly of the measuring chain. The functionality of the device can be verified on this measuring chain. Whether or not it is suitable for operation. In the future these systems will be upgraded with software that better records the vibrations. The statement of basic materials. The analysis consists of basic information about mechanical systems and sensors. The definition of this problem is described below. Based on this knowledge of mechanical systems, a measurement chain was designed. Conclusions. Our vision is to implement knowledge of mechanical systems not only on a simple fan stator. Problems of vibrodiagnostics are still progressing and increasingly in technical practice. We would continue to do more testing on more complex devices.

Loss and Deviation in Windmilling Fans

For an unpowered turbofan in flight, the airflow through the engine causes the fan to freewheel. This paper considers the flow field through a fan operating in this mode, with emphasis on the effects of blade row losses and deviation. A control volume analysis is used to show that windmilling fans operate at a fixed flow coefficient which depends on the blade metal and deviation angles, while the blade row losses are shown to determine the fan mass flow rate. Experimental and numerical results are used to understand how the loss and deviation differ from the design condition due to the flow physics encountered at windmill. Results are presented from an experimental study of a windmilling low-speed rig fan, including detailed area traverses downstream of the rotor and stator. Three-dimensional computational fluid dynamics (CFD) calculations of the fan rig and a representative transonic fan windmilling at a cruise flight condition have also been completed. The rig test results confirm that in the windmilling condition, the flow through the fan stator separates from the pressure surface over most of the span. This generates high loss, and the resulting blockage changes the rotor work profile leading to modified rotational speed. In the engine fan rotor, a vortex forms at the pressure surface near the tip and further loss results from a hub separation caused by blockage from the downstream core and splitter.

Bifurcation analysis of fan casing under rotating air flow excitation

A fan casing model of cantilever circular thin shell is constructed based on the geometric characteristics of the thin-walled structure of aero-engine fan casing. According to Donnelly’s shell theory and Hamilton’s principle, the dynamic equations are established. The dynamic behaviors are investigated by a multiple-scale method. The effects of casing geometric parameters and motion parameters on the natural frequency of the system are studied. The transition sets and bifurcation diagrams of the system are obtained through a singularity analysis of the bifurcation equation, showing that various modes of the system such as the bifurcation and hysteresis will appear in different parameter regions. In accordance with the multiple relationship of the fan speed and stator vibration frequency, the fan speed interval with the casing vibration sudden jump is calculated. The dynamic reasons of casing cracks are investigated. The possibility of casing cracking hysteresis interval is analyzed. The results show that cracking is more likely to appear in the hysteresis interval. The research of this paper provides a theoretical basis for fan casing design and system parameter optimization.

Effectiveness of combined aircraft engine noise suppressors

We consider the design features of fan noise suppressors in application to air intakes and the bypass duct of a turbofan engine. A combined liner is developed that has increased acoustic efficiency in comparison to conventional honeycomb liner. We demonstrate the important role of the area of the sound-absorbing liner between fan Rotor and Stator ensuring significant noise reduction.

Nonlinear Flutter in Fan Stator Vanes With Time Dependent Fixity

A new mechanism for fan stator vane failure in turbofan engines at high speed and high loading has been identified and reported in this paper. Highly destructive vane failures have been encountered at Honeywell in a development fan with composite stator vanes. Measured data indicated nonlinear high amplitude vibratory response in fan stator vanes on the stall side of the fan map at high speeds. Analysis showed that under certain steady loading, vane fixity at the hub could change, significantly reducing the vane natural frequency. At this lower natural frequency, the vane was found to be aeroelastically unstable, and calculated response exhibited characteristics similar to those observed during failure. An engine test conducted to validate the role of hub fixity in vane failures showed the failure to be a self-excited phenomenon and not driven by an external source of excitation. It was also shown that failures occur in vanes that are not rigidly fixed, validating the role of hub fixity in vane failures. Test results along with analysis confirm the role of time dependent hub fixity leading to the highly destructive flutter responsible for vane failures.

Application of Tandem Cascade to Design of Fan Stator with Supersonic Inflow

This article proposes a tandem cascade constructed to tackle the thorny problem of designing the high-loaded stator with a supersonic inflow and a large turning angle. The front cascade adopts a supersonic profile to reduce the shock wave intensity turning the flow into subsonic, while the rear cascade adopts a subsonic profile with a large camber offering the flow a large turning angle. It is disclosed that the losses would be minimized if the leading edge of the rear cascade lies close to the pressure side of the front cascade at a distance of 20% pitch in pitch-wise direction without either axial spacing or overlapping in axial direction. The 2D numerical test results show that, with the inflow Mach number of 1. 25 and the turning angle of 52°, the total pressure loss coefficient of the tandem cascade reaches 0. 106, and the diffusion factor 0. 745. Finally, this article has designed and simulated a high-loaded fan stage with the proposed tandem stator, which has the pressure ratio of 3. 15 and the efficiency of 86. 32% at the rotor tip speed of 495. 32 m/s.

On the interaction of a fan stator and acoustic treatments using the transfer element method

In the present investigation, a theoretical model is suggested to study the interaction of a fan stator and acoustic treatments using the transfer element method. Firstly, the solution of an acoustic field caused by a fan stator in an infinite duct is extended to that in a finite domain with all knowns and unknowns on the interface plane. Secondly, the related numerical results for an annular cascade are compared with the data obtained by directly solving an integral equation based on the blade boundary condition, which have good agreement with each other. Finally, more emphasis is placed on studying how a fan stator interacts with both upstream and downstream acoustic treatments. It is found that the interaction has an important influence on sound attenuation. In addition, optimal sound attenuation will depend on the combined design of both acoustic treatment and the fan stator.

A mass addition approach to the bypass turbomachine through flow inverse design problem

In the design of a low bypass ratio fan stage, this paper links the through flow inverse design problems of the fan rotor, the core channel fan stator and the bypass channel fan stator together, as an organic whole, with the fan splitter aerodynamic design problem. It forms thus a unified through flow inverse problem for two-channel turbomachines. While an overall quasi-radial-equilibrium of the streamlines is iteratively achieved among five parts — the fan rotor, the core channel fan stator, the bypass channel fan stator, the fan splitter and the position of the fan splitter, the blading processes for one fan blade, two fan vanes and one fan splitter ring are performed. This method designs in one pass a fan stage and a splitter with a precise bypass ratio. Its primary and key approach scheme is the introduction of a mass addition flow half way through the through flow streamline development. This scheme can serve as a new feature which can be inserted into those existing through flow programs. When subtracting a mass flow after adding a mass flow, alternatively, this scheme can also be used to treat the inverse design problem of a fan/compressor rotor blade with part-span shrouds.

Loss Mechanisms of High-Turning Supercritical Compressor Cascades

This paper presents an experimental and numerical study on loss mechanisms of two high-turning compressor cascades at supercritical flow conditions. It is part of a comprehensive advanced compressor blade development program, in which three cascades were designed at a higher supercritical speed (M 1 = 0.87) for a transonic fan stator hub section. The cascades had the same solidity and blade chord but differed significantly in blade profiles. The baseline profile was a conventional controlled diffusion airfoil. The other two were state-of-the-art optimized designs and were experimentally confirmed to outperform the baseline. The first optimized cascade (optimized A) and its performance was published in Song and Ng (Song, B., and Ng, W., Performance and Flow Characteristics of an Optimized Supercritical Compressor Stator Cascade, Journal of Turbomachinery, Vol. 128, July 2006, pp. 435-443). The current paper focuses on comparing the baseline and the second optimized cascade (optimized B) to complement the first publication. Cascade test results are presented to show significant loss reduction (about 30%) from the baseline to the optimized profiles. Experimental and numerical analyses to understand the flow physics underlying the loss reduction are detailed using the following: wake profile, blade surface Mach number, shadowgraph, and blade surface flow visualization. It was found that severe boundary-layer separation occurred in the baseline cascade, whereas the boundary layer of the optimized cascades was well controlled from separation. An understanding of the low-loss flow pattern and design philosophy for high-turning compressor blades at higher supercritical flow conditions, exemplified by the two optimized cascades and relative to the controlled diffusion airfoil, is summarized.

Nonsynchronous Vibration (NSV) due to a Flow-Induced Aerodynamic Instability in a Composite Fan Stator

Abstract This paper describes the identification and prediction of a new class of nonsynchronous vibration (NSV) problem encountered during the development of an advanced design composite fan stator for an aircraft engine application. Variable exhaust nozzle testing on an instrumented engine is used to map out the NSV boundary, with both choke- and stall-side instability zones present that converge toward the nominal fan operating line and place a limit on the high-speed operating range. Time-accurate three-dimensional viscous CFD analyses are used to demonstrate that the NSV instability is being driven by dynamic stalling of the fan stator due to unsteady shock-boundary layer interaction. The effects of downstream struts in the front frame of the engine are found to exasperate the problem, with the two fat service struts in the bypass duct generating significant spatial variations in the stator flow field. Strain gage measurements indicate that the stator vanes experiencing the highest vibratory strains correspond to the low static pressure regions of the fan stator assembly located approximately 90 degrees away from the two fat struts. The CFD analyses confirm the low static pressure sectors of the stator assembly are the passages in which the flow-induced NSV instability is initiated due to localized choking phenomena. The CFD predictions of the instability frequency are in reasonable agreement with the strain gage data, with the strain gage data indicating that the NSV response occurs at a frequency approximately 25% below the frequency of the fundamental bending mode. The flow patterns predicted by the CFD analyses are also correlated with the results of an engine flow visualization test to demonstrate the complex nature of the flow field.

A Note on Blade Wake Interaction Influence on Compressor Stator Row Aerodynamic Performance

Attention is given to the effect of blade wake interactions on the performance of an axial flow compressor's stator row, for the case of compressor and fan stator flows without shocks. The measured midspan loss of total pressure can be related to stator surface boundary layers, the chopped rotor wakes passing through the stator row, and the interaction between these flows. The interaction between rotor blade wake segments and stator blade surface boundary layers generates much higher losses than expected in both the stator wake and in the region of flow between stator boundary layers/wakes, if interaction effects are ignored.

Fan noise reduction achieved by removing tip flow irregularities behind the rotor

One of the major noise producing mechanisms in a fan stage for a turbofan engine is the interaction of the rotor wakes with the downstream stator vanes. Previous work has indicated that the rotor tip flow irregularities (vortices and velocity defects) may be as large or larger a source of blade passage tone noise as the usually considered mean rotor wake. Flow was removed between the rotor and stator of an existing fan stage by an outer wall slot and noise measurements were made to investigate the rotor tip flow irregularity‐stator interaction as a noise source. Farfield noise data taken in an inlet arc showed little tone noise reduction with flow removal, probably the result of rotor‐stator interaction noise not propagating upstream through the rotor. In‐duct measurements behind the fan stator did show noise reduction with flow removal and proportionally more reduction occurred with the first increment of flow removal than with subsequent amounts. Since the tip flow irregularities were assumedly removed with the first increment of flow, the duct measurements indicate that the tip flow irregularity‐stator interaction is probably as large a noise producing mechanism as the normally considered rotor wake‐stator interaction.

Spectrum of rotor noise caused by inlet guide vane wakes

A blade loading model is formulated for the interaction of an axial-flow fan rotor with turbulent wakes from inlet guide vanes. From this, the spectra of both the blade lift and the associated sound field are computed. The analysis and results arc similar to those in the analogous case treated previously by the author for a stator interacting with the wakes from an upstream rotor [D. B. Hanson, “Unified Analysis of Fan Stator Noise,” J. Acoust. Soc. Am. 54, 1571–1591 (1973)]. The turbulence in the wakes causes a smooth broad-band spectrum component, while the spatially periodic portion of the wakes causes periodic components in the blade lift and sound field. The spectrum peaks do not exhibit bandspreading or “haystacking,” except for fans with inlet guide vane wakes which are wider than the gap between adjacent rotor blades.

Unified analysis of fan stator noise

A theory is developed which predicts the acoustic radiation of an axial-flow fan stator due to interaction with rotor viscous wakes. Calculations are compared in detail with experimental data. Both the harmonic and broadband noise-spectrum components are calculated from a unified model using methodology from the theories of random pulse amplitude modulation, PAM, and pulse position modulation, PPM. The stator is modeled as a circular array of pulsed dipoles. The amplitude and arrival time of each pulse are random variables whose means correspond to the values calculated from harmonic rotor-stator interaction theory. The standard deviations of these random variables are measures of the turbulence levels in the blade wakes. For the model proposed the solution is exact: when PAM is imposed on a periodic stator source, new broadband energy is generated whose spectrum shape is similar to the envelope of harmonics at high frequencies and the harmonic radiation is unchanged; however, when PPM occurs, new broadband energy is radiated but at the expense of harmonic energy. At frequencies significantly above duct cutoff it is shown for a fixed stator solidity that the spectrum is essentially independent of the number of stator vanes.