Low Pressure Compression System Effects on Fan Assembly Forced Response

2007 ◽  
Author(s):  
Abdulnaser I. Sayma ◽  
Mehdi Vahdti ◽  
Mehmet Imregun ◽  
John Marshal
Keyword(s):  
Large Diameter ◽  
Pressure Fluctuations ◽  
Element Model ◽  
Low Pressure ◽  
Forced Response ◽  
Blade Vibration ◽  
Compression System ◽  
Engine Order ◽  
Modelling Methodology ◽  
Fan Blade

This paper describes a numerical modelling methodology for fan blade forced response calculations by considering the low-pressure compression system (LPCS) as a whole in order to include flow distortions caused by the asymmetric flight intake upstream, and the pylon downstream. Emphasis also is placed on blade mistuning or mis-placement which may be due to inherent manufacturing and assembly tolerances, or to small inservice displacements. Several levels of geometric complexity were used in the analysis, ranging from an isolated fan bladerow to a complete LPCS of a large-diameter aero-engine, consisting of the intake duct, the fan assembly, the outflow guide vanes, the pylon and a downstream nozzle. The aerodynamic model was coupled to a finite element model of the fan assembly for computing the blade vibration levels. The study revealed two major findings. The first is the unsteady forcing under one engine-order (1EO) excitation is found to be linked to the mean shock position on the fan blade, the highest forcing occurring when the shock is just swallowed since this position is particularly sensitive to pressure fluctuations. The second finding is that the 1EO fan assembly forcing resulting from an asymmetric intake and the pylon are of comparable magnitude but their relative phasing is the key parameter in determining the overall fan forced response levels.

Radial Decomposition of Blade Vibration to Identify a Stall Flutter Source in a Transonic Fan

2019 ◽  
Vol 141 (10) ◽  
Author(s):  
Q. Rendu ◽  
M. Vahdati ◽  
L. Salles
Keyword(s):  
Pressure Fluctuations ◽  
Navier Stokes ◽  
Local Vibration ◽  
Blade Vibration ◽  
Nonlinear Solver ◽  
Low Mass ◽  
Stall Flutter ◽  
Fan Blade ◽  
Unstable Oscillation ◽  
Transonic Fan

Abstract This paper investigates the three dimensionality of the unsteady flow responsible for stall flutter instability. Nonlinear unsteady Reynolds-averaged Navier–Stokes (RANS) computations are used to predict the aeroelastic behavior of a fan blade at part speed. Flutter is experienced by the blades at low mass flow for the first flap mode at nodal diameter 2. The maximal energy exchange is located near the tip of the blade, at 90% span. The modeshape is radially decomposed to investigate the main source of instability. This decomposition method is validated for the first time in 3D using a time-marching nonlinear solver. The source of stall flutter is finally found at 65% span where the local vibration induces an unstable oscillation of the shock-wave of large amplitude. This demonstrates that the radial migration of the pressure fluctuations must be taken into account to predict stall flutter.

The Mechanism on Prediction of Transient Maximum Amplitude for Tuned and Mistuned Blisks

2020 ◽  
Vol 142 (5) ◽  
Author(s):  
Luohui Ouyang ◽  
Hai Shang ◽  
Hua Chen ◽  
Qingzhen Bi ◽  
Li-Min Zhu
Keyword(s):  
Damping Ratio ◽  
Maximum Amplitude ◽  
Element Model ◽  
Forced Response ◽  
Lumped Parameter Model ◽  
Fundamental Parameter ◽  
Reduced Order ◽  
Lumped Parameter ◽  
Engine Order ◽  
Acceleration And Deceleration

Abstract Blisks are subjected to frequent acceleration and deceleration, which leads to a transient forced response; however, there is limited understanding of this response. In this work, the mechanism on prediction of transient maximum amplitude is found. An analytical link is proposed between the transient maximum amplitude and a fundamental dimensionless parameter which combines the damping ratio, natural frequency, acceleration, and engine order of the system to reveal the mechanism of the transient maximum amplitude. Therefore, the transient maximum amplitudes of tuned and mistuned blisks are predicted analytically. First, a lumped parameter model is used to study the mechanism of the transient maximum amplitude for a tuned blisk, and an approximated analytical expression is derived between the fundamental parameter and the transient amplification factor of a 1DOF (degree-of-freedom) model. The relationship is also applicable to a reduced order, tuned finite element model (FEM). Second, the mechanism of the transient response for a mistuned blisk is studied in the decoupled modal space of the blisk, based on the 1DOF transient relationship. The transient maximum amplitude in a reduced order, mistuned FEM is predicted. Two lumped parameter models and a FEM are employed to validate the prediction.

Experimental Reduction of Transonic Fan Forced Response by IGV Flow Control

2004 ◽  
Author(s):  
S. Todd Bailie ◽  
Wing F. Ng ◽  
William W. Copenhaver
Keyword(s):  
Flow Control ◽  
Forced Response ◽  
Resonant Response ◽  
Compressor Blades ◽  
Resonant Amplitude ◽  
Engine Order ◽  
Blade Vibrations ◽  
Fan Blade ◽  
Transonic Fan ◽  
Bending Response

The main contributor to the high-cycle fatigue of compressor blades is the response to aerodynamic forcing functions generated by an upstream row of stators or inlet guide vanes. Resonant response to engine order excitation at certain rotor speeds can be especially damaging. Studies have shown that flow control by trailing edge blowing (TEB) can reduce stator wake strength and the amplitude of the downstream rotor blade vibrations generated by the unsteady stator-rotor interaction. In the present study, the effectiveness of TEB to reduce forced fan blade vibrations was evaluated in a modern single-stage transonic fan rig. Data was collected for multiple uniform full-span TEB conditions over a range of rotor speed including multiple modal resonance crossings. Resonant response sensitivity was generally characterized by a robust region of strong attenuation. The baseline resonant amplitude of the first torsion mode, which exceeded the endurance limit on the critical blade, was reduced by more than 80% with TEB at 1.0% of the total rig flow. The technique was also found to be modally robust; similar reductions were achieved for all tested modal crossings, including more than 90% reduction of the second LE bending response using 0.7% of the rig flow.

Forced Response Prediction for Steam Turbine Last Stage Blade Subject to Low Engine Order Excitation

2011 ◽  
Author(s):  
Bin Zhou ◽  
Amir Mujezinovic ◽  
Andrew Coleman ◽  
Wei Ning ◽  
Asif Ansari
Keyword(s):  
Steam Turbine ◽  
Low Pressure ◽  
Forced Response ◽  
Operating Conditions ◽  
Low Flow ◽  
Analytical Prediction ◽  
Unsteady Pressure ◽  
Engine Order ◽  
Last Stage ◽  
Cfd Analyses

Low Engine Order (LEO) excitations on a steam turbine Last Stage low-pressure (LP) Bucket (or Blade) (LSB) are largely the result of flow unsteadiness (e.g. flow circulation and reversal) due to low steam exit velocity (Vax) off the LSB at the off-design conditions. These excitations at low frequencies impose major constraints on LP bucket aeromechanical design. In this study, bucket forced response under typical LEO excitation was analytically predicted and correlated to experimental measurements. First, transient CFD analyses were performed at typical low flow, low Vax operating conditions that had been previously tested in a subscale low pressure turbine test rig. The unsteady pressure distribution on the bucket was derived from the transient CFD analyses at frequencies corresponding to the bucket’s modes of vibration. Subsequently, these computed unsteady pressure were mapped onto a LSB finite element model, and forced response analyses were performed to estimate the bucket dynamic response, i.e. the alternating stresses and strains. The analytically predicted bucket response was compared against measured data from airfoil mounted strain gages and good correlation was found between the analytical prediction and the test data. Despite uncertainty associated with various parameters such as damping and unsteady steam forcing etc., the developed methodology provides a viable approach for predicting bucket forced response and in turn High Cycle Fatigue (HCF) capability during early phases of steam turbine LSB design.

A Cyclic Symmetry Analysis for Turbomachine Blade Flutter

1998 ◽  
Author(s):  
Hsiao-Wei D. Chiang ◽  
Meng-Hsuan Chung
Keyword(s):  
Element Model ◽  
Forced Response ◽  
Symmetry Analysis ◽  
Mode Analysis ◽  
Mode Shapes ◽  
Cyclic Symmetry ◽  
Blade Flutter ◽  
Blade Failure ◽  
Fan Blade ◽  
System Mode

A frequent cause of turbomachinery blade failure is excessive vibration due to flutter or forced response. One method for dealing with this problem is to increase blade structural damping using either tip or mid-span shroud designs. Unfortunately, most existing aeroelastic analyses deal with a blade alone model which can not be used for system mode analysis. Therefore, judgments based on past experience are used to determine the acceptability of a shrouded blade design. A new cyclic symmetry analysis has been developed to predict shrouded blade flutter. The method provides a system approach, over and above the standard blade alone approach, for predicting potential aeroelastic problems. Using the blade natural frequencies and mode shapes from a cyclic symmetry finite element model, the unsteady aerodynamic forces of the system mode are calculated. A cyclic symmetry flutter analysis is then performed. This analysis has been applied to a typical shrouded fan blade to investigate blade flutter. The predicted system mode flutter demonstrated that the blade alone analysis can be non-conservative.

Prediction of Low Engine Order Inlet Distortion Driven Resonance in a Low Aspect Ratio Fan

2000 ◽  
Author(s):  
J. G. Marshall ◽  
L. Xu ◽  
J. Denton ◽  
J. W. Chew
Keyword(s):  
Aspect Ratio ◽  
Three Dimensional ◽  
Coupled Model ◽  
Response Prediction ◽  
Euler Method ◽  
Forced Response ◽  
Blade Vibration ◽  
Inlet Distortion ◽  
Low Aspect Ratio ◽  
Engine Order

This paper presents a forced response prediction of 3 resonances in a low aspect ratio modern fan rotor and compares with other worker’s experimental data. The incoming disturbances are due to low engine-order inlet distortion from upstream screens. The resonances occur in the running range at 3 and 8 engine orders which cross low modes (flap, torsion and stripe) of the blade. The fan was tested with on-blade instrumentation at both on- and off-resonant conditions to establish the unsteady pressures due to known distortion patterns. The resulting steady and unsteady flow in the fan blade passages has been predicted by three methods, all three-dimensional. The first is a linearised unsteady Euler method; the second is a non-linear unsteady Navier-Stokes method; the third method uses a similar level of aerodynamic modelling as the second but also includes a coupled model of the structural dynamics. The predictions for the 3 methods are presented against the test data, and further insight into the problem is obtained through post-processing of the data. Predictions of the blade vibration response are also obtained. Overall the level of agreement between calculations and measurements is considered encouraging although further research is needed.

On the Effect of Axial Spacing Between Rotor and Stator Onto the Blade Vibrations of a Low Pressure Turbine Stage at Engine Relevant Operating Conditions

2016 ◽  
Author(s):  
Andreas Marn ◽  
Florian Schönleitner ◽  
Mathias Mayr ◽  
Thorsten Selic ◽  
Franz Heitmeir
Keyword(s):  
Rotor Blade ◽  
Low Pressure ◽  
Forced Response ◽  
Operating Conditions ◽  
Turbine Stage ◽  
Blade Vibration ◽  
Structural Dynamic ◽  
Flow Measurements ◽  
Vibration Measurements ◽  
Excitation Mechanisms

In order to achieve the ACARE targets regarding reduction of emissions, it is essential to reduce fuel consumption drastically. Reducing engine weight is supporting this target and one option to reduce weight is to reduce the overall engine length (shorter shafts, nacelle). However, to achieve a reduction in engine length, the spacing between stator and rotor can be minimised, thus changing the rotor blade excitation. Related to the axial spacing, a number of excitation mechanisms with respect to the rotor blading must already be considered during the design process. Based on these facts several setups have been investigated at different engine relevant operating points and axial spacing between the stator and rotor in the subsonic test turbine facility (STTF-AAAI) at the Institute for Thermal Turbomachinery and Machine Dynamics at Graz University of Technology. In order to avoid upstream effects of supporting struts, these struts are located far downstream of the stage which is under investigation. For rotor blade vibration measurements, a novel telemetry system in combination with strain gauges is applied. To the best of the author’s knowledge, the present paper is the first report of blade vibration measurements within a rotating system in the area of low pressure turbines under engine relevant operating conditions. In addition, aerodynamic measurements including unsteady flow measurements have been conducted, but will not be presented in this paper. By analysing the flow field, aerodynamic excitation mechanisms can be identified and assigned to the blade vibration. However, this is not presented in this paper. Within this paper, the flow fields are analysed in both upstream and downstream of the turbine stage, visualised for two axial gaps and then compared to the forced response of the blading. Detailed structural dynamic investigations show critical modes during the operation which are identified by the telemetry measurements as well. Finally the influence of the axial spacing regarding the rotor blade excitation and vibration can be elaborated and is prepared to get a better understanding of basic mechanisms. The paper shows that reducing axial spacing is a promising option for reducing engine weight, but aeroelasticity must be carefully taken into account.

Numerical Investigation on Wide-Chord Fan Blade Forced Response due to Vortex Ingestion

2018 ◽  
Author(s):  
Zhonglin Wang ◽  
Yong Chen ◽  
Hua Ouyang ◽  
Anjenq Wang
Keyword(s):  
Computational Study ◽  
Impact Damage ◽  
Ground Surface ◽  
Rotating Stall ◽  
Forced Response ◽  
Hard Material ◽  
Flow Distortion ◽  
Response Level ◽  
Blade Vibration ◽  
Fan Blade

When a turbofan engine is taxing or taking-off, a vortex can form between ground surface and the intake. As the diameters of engines increase, intakes are closer to the ground and as a result the possibility of vortex ingestion is increasing. The vortex starts from the ground surface and enters the inlet at high rotating speed. It is likely to draw in hard material or dust from the ground, which leads to blade erosion or impact damage. This is harmful to the engine durability and safety. Besides the vortex, inlet flow separation could induce high level of blade vibration, or aerodynamic instability, such as rotating stall. Cross wind may also lead to both vortex and flow distortion, which is more challenging for engine stability. Therefore, vibration characteristics and forced response under vortex ingestion should be evaluated to ensure the stability and safety of the engine in design phase. This paper presents a computational study of the forced response of a wide-chord fan blade under vortex ingestion. A finite element model was built, and modal analysis was conducted to characterize the vibrating characteristics of the fan blade with a corresponding Campbell diagram. Transient simulations of vortex passing over the fan blade were conducted with and without the blade pre-vibration at the natural frequency of the first bending mode. The forced response level was evaluated under various conditions, including different hitting time and increasing intensity of vortex. Results showed that the ingested vortex is able to amplify the displacement and vibratory response to a significant level of 18% at most. Linear relation between vortex intensity and blade response was found. The results give a comprehensive prediction of forced response for a better blade design against vortex ingestion.

Experimental Study on Impeller Blade Vibration During Resonance: Part 1—Blade Vibration Due to Inlet Flow Distortion

2008 ◽  
Author(s):  
Albert Kammerer ◽  
Reza S. Abhari
Keyword(s):  
Experimental Study ◽  
Speckle Pattern ◽  
Pressure Fluctuations ◽  
Forced Response ◽  
Dynamic Strain ◽  
Flow Distortion ◽  
Blade Vibration ◽  
Impeller Blade ◽  
Inlet Flow ◽  
Forcing Function

Forming the first part of a two-part paper, the experimental approach to acquire resonant vibration data is presented here. Part 2 deals with the estimation of damping. During the design process of turbomachinery components, mechanical integrity has to be guaranteed with respect to high cycle fatigue of blades subject to forced response or flutter. This requires the determination of stress levels within the blade which in turn depend on the forcing function and damping. The vast majority of experimental research in this field has been performed on axial configurations for both compressors and turbines. This experimental study aims to gain insight into forced response vibration at resonance for a radial compressor. For this purpose a research impeller was instrumented with dynamic strain gauges and operated under resonant conditions. Modal properties were analysed using FEM and verified using an optical method termed Electronic-Speckle-Pattern-Correlation-Interferometry (ESPI). During the experiment, unsteady forces acting on the blades were generated by grid installations upstream of the impeller which created a distorted inlet flow pattern. The associated flow properties were measured using an aerodynamic probe. The resultant pressure fluctuations on the blade surface and the corresponding frequency content were assessed using unsteady CFD. The response of the blades was measured for three resonant crossings which could be distinguished by the excitation order and the natural frequency of the blades. Measurements were undertaken for a number of inlet pressure settings starting at near vacuum and then increasing. The overall results showed that the installed distortion screens generated harmonics in addition to the fundamental frequency. The resonant response of the first and second blade mode showed that the underlying dynamics support a single-degree-of-freedom model.

Forced Response Analysis of a Steam Turbine Shrouded Blade: Numerical Analysis and Experiments

2013 ◽  
Author(s):  
Francesco Piraccini ◽  
Salvatore Lorusso ◽  
Nicola Maceli ◽  
John Ryman
Keyword(s):  
Dynamic Response ◽  
Steam Turbine ◽  
Test Data ◽  
Element Model ◽  
Low Pressure ◽  
Forced Response ◽  
Operating Conditions ◽  
Response Analysis ◽  
Cfd Analysis ◽  
Pressure Development

Faced with the present transformation of the world economy, steam turbine manufacturers are seeking ways to remain competitive in their respective markets. Having longer Low Pressure (LP) blades and seeking for higher rotating speeds have always been two effective methods to improve the Steam Turbine efficiency, therefore to reduce steam consumption and related plant costs. Both trends have increased the risk of failure for forced response due to the occurrence of resonance or to the decrease of alternating stress margins. Because of large uncertainties in the estimation of friction damping and aerodynamic excitation, the prediction of dynamic response of the long blades in the LP section is still a challenge for the analytical tools; therefore, expensive activities for experimental validation are usually required. In order to reduce design costs and time, a set of tools has been developed and validated using the test data collected during a full-scale test vehicle campaign in steam (Low Pressure Development Turbine - LPDT). In this study, the validation activity related to the blade response due to nozzle stimulus is reported. As a first step, a steady state CFD analysis was performed at the operating conditions where significant response was observed, caused by the resonance with the Nozzle Passing Frequency (NPF). Then, an unsteady CFD analysis of the bucket blade was conducted considering the perturbation due to the nozzles. Subsequently, the computed unsteady pressure distribution on the blade airfoil was mapped onto a finite element model and forced response analyses were performed to estimate the bucket dynamic response. The predicted response was compared against measured test data and good correlation was found.

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