Rotor Blisk Research Articles

Reusable Turbine Material GH4586: Stochastic Analysis and Reliability Assessment of Low‐Cycle Fatigue Life Parameters at Multiple Temperatures

ABSTRACTTurbines, crucial in reusable rocket engines, benefit significantly from precise fatigue life forecasting in their force‐thermal cyclic loading conditions. Further, low‐cycle fatigue life predictions can be random due to uncertainties in material properties. This study aims to analyze the probabilistic low‐cycle fatigue life of GH4586 under different temperature. Mechanical and thermal tests were carried out for the temperatures between 87 K and 1173 K. Random distributions of low‐cycle fatigue parameters were obtained using various methods. The cyclic life reliability of a reusable rocket engine turbine rotor blisk was analyzed using stochastic low‐cycle fatigue parameters. Mitchell's method had a higher prediction accuracy when the reliability was greater than 0.8. Sensitivity analysis shows the significant impact of GH4586 fatigue parameters on cyclic life reliability. Sensitivity coefficients of strength coefficient and ductility coefficient are 32.73% and 27.06%, respectively. These findings provide valuable insights that inform the design of turbines and enhance their reliability.

An Experimental Study on the Effects of Nonuniform Vane Spacing on Forced Response Reduction in a Mistuned Rotor Blisk in a Multistage Axial Compressor

Abstract Stators with nonuniform vane spacing (NUVS) are known to effectively reduce the forced response of adjacent rotors by spreading the primary engine order (EO) excitation at a certain speed to a series of weaker excitations at nearby EOs over a wider speed range. To be used in a real gas turbine engine, it is essential to know the forced response vibratory reduction that can be achieved for a specific NUVS configuration at different operating conditions. The classical estimation method to predict the blade response reduction is to quantify the reduction of the forcing function at a potential resonance crossing by conducting a circumferential Fourier analysis of the asymmetric flow field. However, besides excitation reduction, the blade forced response also depends on other factors, such as damping and blade-to-blade interactions due to mistuning. To study the blade forced response reduction in a realistic environment, a comprehensive experimental study was conducted in the Purdue three-stage axial research compressor at three different loading conditions. The vibratory response of the first torsion (1 T) mode forced response of rotor 2 was measured by strain gages (SGs) for two different upstream stator 1 configurations: the symmetric 38-vaned stator 1 configuration and the NUVS stator 1 configuration with 18–20 vane halves. As expected, the strong 38EO-1T response from the symmetric stator reduced to a series of weaker responses from 35EO to 41EO in the NUVS configuration. The overlap of adjacent EO responses caused considerable beating phenomena in the blade SG time–history data. There was substantial blade-to-blade variation in blade response for both symmetric and asymmetric stator 1 configurations due to the inherent nonintentional mistuning in the rotor 2 blisk. This, in turn, causes a large blade-to-blade variation in the reduction factors. While higher responding blades tend to have larger reduction factors, some blades show almost no forced response reduction when switching to the NUVS stator. In addition, the reduction factors for each blade and the maximum reduction factor for the whole blisk also change significantly with loading conditions. The measured reduction factors and the peak response EO are not well predicted by the classical estimation method. This indicates that both damping and mistuning effects need to be considered in the NUVS reduction factor prediction.

Improving Aeromechanical Performance of Compressor Rotor Blisk with Topology Optimization

When it comes to modern design of turbomachinery, one of the most critical objectives is to achieve higher efficiency and performance by reducing weight, fuel consumption, and noise emissions. This implies the need for reducing the mass and number of the components, by designing thinner, lighter, and more loaded blades. These choices may lead to mechanical issues caused by the fluid–structure interaction, such as flutter and forced response. Due to the periodic aerodynamic loading in rotating components, preventing or predicting resonances is essential to avoid or limit the dangerous vibration of the blades; thus, simulation methods are crucial to study such conditions during the machine design. The purpose of this paper is to assess a numerical approach based on a topology optimization method for the innovative design of a compressor rotor. A fluid-structural optimization process has been applied to a rotor blisk which belongs to a one-and-a-half-stage aeronautical compressor including static and dynamic loads coming from blade rotation and fluid flow interaction. The fluid forcing is computed by some CFD TRAF code, and it is processed via time and space discrete Fourier transform to extract the pressure fluctuation components in a cyclic-symmetry environment. Finally, a topological optimization of the disk is performed, and the encouraging results are presented and discussed. The remarkable mass reduction in the component (≈32%), the mode-shape frequency shift from a fluid forcing frequency, and an overall relevant reduction in the dynamic response around Campbell’s crossing confirm the efficacy of the presented methodology.

Reliability analysis of reusable turbine rotor blisk: An application of parametric modelling method under multi-field coupling

As a critical component of reusable rocket engines (RRE), the cyclic life of the turbine rotor blisk often varies due to uncertainties such as machining errors and other factors. However, considering the difficulty of structural reconstruction and the large number of parameters related to the blade configuration, few studies have considered the blade configuration variation due to machining errors in the reliability analysis process. To investigate the influence of factors in reliability analysis, a reliability analysis framework is established by combining parametric modelling methods with finite element analysis methods under fluid-thermal-solid sequential coupling. The influence of blade lattice changes due to machining errors on the reliability of turbine rotor blisks in reusable rocket engines (RRE) is investigated. The effects of machining errors, material, and load variability on the cyclic life reliability of rotor blisks are effectively quantified. With the suggested method, a reliability analysis is conducted for an RRE turbine rotor blisk, and the effect of the machining error range on the analysis results is investigated. It is found that the tolerance level to meet the high reliability requirements is at least medium for the basic cyclic life requirements of the model. In terms of cyclic life reliability, the more influential random factors are blade axial chord, low-cycle fatigue plasticity coefficient, and low-cycle fatigue ductility coefficient, with sensitivity factors of 29.09%, 24.4% and 22.13%, respectively. This study provides an efficient and reliable modelling method for the reliability analysis of reusable turbine rotor blisks based on machining errors, investigates the influence of the range of machining errors, quantifies the ability of the random factors to influence, and provides valuable insights and recommendations for the reliability design and optimization of reusable turbine blisks.

Stator Blades Manufacturing Geometrical Variability in Axial Compressors and Impact on the Aeroelastic Excitation Forces

AbstractThe manufacturing geometrical variability is a source of uncertainty, which cannot be avoided in the realization of machinery components. Deviations of a part geometry from its nominal design are inevitably present due to the manufacturing process. In the case of the aeroelastic forced response problem within axial compressors, these uncertainties may affect the vibration characteristics. For this reason, the impact of geometrical uncertainties due to the manufacturing process onto the modal forcing of axial compressor blades is investigated in this study. The research focuses on the vibrational behavior of an axial compressor rotor blisk. In particular, the amplitude of the forces acting as a source of excitation on the vibrating blades is studied. The geometrical variability of the upstream stator is investigated as input uncertainty. The variability is modeled starting from a series of optical surface scans. A stochastic model is created to represent the measured manufacturing geometrical deviations from the nominal model. A data reduction methodology is proposed in order to represent the uncertainty with a minimal set of variables. The manufacturing geometrical variability model allows to represent the input uncertainty and probabilistically evaluate its impact on the aeroelastic problem. An uncertainty quantification is performed in order to evaluate the resulting variability on the modal forcing acting on the vibrating rotor blades. Of particular interest is the possible rise of low engine orders due to the mistuned flow field along the annulus. A reconstruction algorithm allows the representation of the variability during one rotor revolution. The uncertainty on low harmonics of the modal rotor forcing can be therefore identified and quantified.

Ceramic shell moulds for investment casting of low-pressure turbine rotor blisk

The investment casting process was adopted to cast a low-pressure turbine rotor (LPTR) blisk. The casting was carried out in vacuum at 1525 °C by using BZL12Y superalloy. Ceramic shell moulds for the casting were prepared by two different fillers (mullite and zircon), two types of colloidal silica binders (polymer-free binder-A and polymer-containing binder-B) and cobalt aluminate solution. The shell specimens having binder B showed improved green strength and self-load sag resistance in comparison to their counterparts having binder-A. Samples prepared from the slurry compositions having binder-A showed higher residual fired strength than the samples prepared from slurry compositions having binder-B. Defect-free castings of LPTR blisks with fine equiaxed grains were obtained by using ceramic shell moulds containing binder-B and cobalt aluminate solution. The LPTR blisks cast from the ceramic shell moulds having binder-A showed hot tear defect along-with coarse grains. Irrespective of the filler, the LPTR blisk cast by using ceramic shell moulds and cobalt aluminate showed fine grains. The average surface roughness values of LPTR blisks cast from ceramic shell moulds, having binder-B and binder-A, were observed to be in the acceptable range even though castings with binder-A showed marginally lower surface roughness.

Mistuned Higher-Order Mode Forced Response of an Embedded Compressor Rotor—Part II: Mistuned Forced Response Prediction

This paper is the second part of a two-part paper that presents a comprehensive study of the higher-order mode (HOM) mistuned forced response of an embedded rotor blisk in a multistage axial research compressor. The resonant response of the second-stage rotor (R2) in its first chordwise bending (1CWB) mode due to the second harmonic of the periodic passing of its neighboring stators (S1 and S2) is investigated computationally and experimentally at three steady loading conditions in the Purdue three-stage compressor research facility. A nonintrusive stress measurement system (NSMS, or blade tip-timing) is used to measure the blade vibration. Two reduced-order mistuning models of different levels of fidelity are used, namely, the fundamental mistuning model (FMM) and the component mode mistuning (CMM), to predict the response. Although several modes in the 1CWB modal family appear in frequency veering and high modal density regions, they do not heavily participate in the response such that very similar results are produced by the FMM and the CMM models of different sizes. A significant response amplification factor of 1.5–2.0 is both measured and predicted, which is on the same order of magnitude of what was commonly reported for low-frequency modes. In this study, a good agreement between predictions and measurements is achieved for the deterministic analysis. This is complemented by a sensitivity analysis which shows that the mistuned system is highly sensitive to the discrepancies in the experimentally determined blade frequency mistuning.

Design and Experimental Verification of Mistuning of a Supersonic Turbine Blisk

A rotor blisk of a supersonic space turbine has previously been designed to allow for free flutter to occur in an air test rig (Groth Mårtensson, and Edin, 2010, “Experimental and Computational Fluid Dynamics Based Determination of Flutter Limits in Supersonic Space Turbines,” 132(1), p. 011010). Flutter occurred at several operating conditions, and the flutter boundary for the test turbine was established. In this paper the rotor blisk is redesigned in order to inhibit flutter. The design strategy chosen is to introduce a mistuning concept. Based on aeroelastic analyses using a reduced order model a criterion for the required level of mistuning is established in order to stabilize the lower system modes. Proposals in literature suggest and analyze mistuning by varying blade mode frequencies in random patterns or by modifying blades in an odd-even pattern. Here a modification of sectors of the blisk is introduced in order to bring a sufficient split of the system mode frequencies. To verify that the redesigned blisk efficiently could inhibit flutter, an experiment similar to that in the work of Groth et al. is performed with the mistuned rotor blisk. By running the redesigned blisk at operating conditions deep into the unstable region of the tuned blisk, it is demonstrated that a relative low level of mistuning is sufficient to eliminate rotor flutter.

Failure of turbine rotor blisk of an aircraft engine

Cracks were observed at the trailing edge of the blades of a turbine rotor blisk of a gas turbine engine during inspection following an endurance test. The integrally bladed disk was made of Mar-M-247 alloy, cast and hipped. Visual inspection, micro and macro fractography, and metallography of the failed blades indicated that the failure was caused by stress rupture at the trailing edge followed by fatigue crack propagation. A systematic analysis was carried out to ascertain the cause for stress rupture. Microstructural studies revealed that the columnar grains at the blade root airfoil region were oriented unfavorably leading to poor stress rupture property in the transverse direction. Coupled with this, the high operating temperatures were responsible for the failure of the blade by stress rupture. It was recommended that the casting parameters be controlled so that the columnar grains are oriented parallel to the principal stress axis of the blade. Coarse columnar grains with high aspect ratios in the longitudinal direction will improve the stress rupture property. It was also suggested to monitor the operating temperature to see that it does not exceed the specified temperature limit of the alloy.