Gas Turbine Rotor Research Articles

Film Cooling Effectiveness on the Leading Edge Region of a Rotating Turbine Blade With Two Rows of Film Cooling Holes Using Pressure Sensitive Paint

Detailed film cooling effectiveness distributions are measured on the leading edge of a rotating gas turbine blade with two rows (pressure-side row and suction-side row from the stagnation line) of holes aligned to the radial axis using the pressure sensitive paint (PSP) technique. Film cooling effectiveness distributions are obtained by comparing the difference of the measured oxygen concentration distributions with air and nitrogen as film cooling gas respectively and by applying the mass transfer analogy. Measurements are conducted on the first-stage rotor blade of a three-stage axial turbine at 2400rpm (positive off-design), 2550rpm (design), and 3000rpm (negative off-design), respectively. The effect of three blowing ratios is also studied. The blade Reynolds number based on the axial chord length and the exit velocity is 200,000 and the total to exit pressure ratio was 1.12 for the first-stage rotor blade. The corresponding rotor blade inlet and outlet Mach numbers are 0.1 and 0.3, respectively. The film cooling effectiveness distributions are presented along with discussions on the influence of rotational speed (off design incidence angle), blowing ratio, and upstream nozzle wakes around the leading edge region. Results show that rotation has a significant impact on the leading edge film cooling distributions with the average film cooling effectiveness in the leading edge region decreasing with an increase in the rotational speed (negative incidence angle).

Trailing-Edge Cooling for Gas Turbines

The trailing-edge section of modern high-pressure turbine airfoils is an area that requires a high degree of attention from turbine performance and durability standpoints. Aerodynamic loss near the trailing edge includes expansion waves, normal shocks, and wake shedding. Thermal issues associated with trailing edge are also very complex and challenging. To maintain effective cooling ensuring metal temperature below design limit is particularly difficult, as it needs to be implemented in a relatively small area of the airfoil. To date, little effort has been devoted to advancing the fundamental understanding of the thermal characteristics in airfoil trailing-edge regions. Described in this paper are the procedures leading to closed-form, analytical solutions for temperature profile for four most representative trailing-edge configurations. The configurations studied are 1) solid wedge shape without discharge, 2) wedge with slot discharge, 3) wedge with discrete-hole discharge, and 4) wedge with pressure-side cutback slot discharge. Comparison among these four cases is made primarily in the context of airfoil metal temperature and resulting cooling effectiveness. Further discussed in the paper are the overall and detail design parameters for preferred trailing-edge cooling configurations as they affect turbine airfoil performance and durability. Also described in this treatment is a current experimental investigation of heat transfer over a trailing-edge configuration. The trailing edge is preceded with an internal cooling channel of pedestal array. The pedestal array consists of both circular pin fins and oblong shaped features or exit teardrops. Downstream to the pedestal array, the trailing edge exits in a pressure side cutback partitioned by the oblong-shaped teardrops. The local heat-transfer coefficient over the entire wetted surface in the internal cooling chamber has been determined using a “hybrid” measurement technique based on transient liquid crystal imaging. The hybrid technique employs the transient conduction model in a semi-infinite solid for resolving the heat-transfer coefficient on the end-wall surface uncovered by the pedestals. The heat-transfer coefficient over a pedestal can be resolved by the lumped capacitance method with an assumption of low Biot number. The overall heat transfer for both the pedestals and end-walls combined shows a significant enhancement compared to the case with thermally developed smooth channel. Near the most downstream section of the suction side, the land, caused by pressure side cutback, is exposed to the stream mixed with hot gas and discharged coolant. Both the adiabatic effectiveness and heat-transfer coefficient on the land section are characterized by using this hybrid liquid-crystal technique. Dr. Cunha is a Principal Engineer at Pratt & Whitney responsible for advanced turbine designs of commercial and military engine programs. Dr. Cunha is also responsible for establishing technical direction for a consortium of Universities as well as in-house research programs in the area of state-of-the-art cooling technologies suitable for use with extremely aggressive gas turbine rotor inlet temperatures. Dr. Cunha’s professional career spans a period of 27 years with turbine original equipment manufacturers including General Electric and Siemens. Dr. Cunha has authored numerous technical papers and gas turbine design standards. Dr. Cunha holds 21 U.S. patents in turbomachinery and is presently a member of the IGTI-ASME heat transfer committee.

Constrained Optimization of Gas Turbine Tilting Pad Bearing Designs

This paper presents the constrained optimization of the tilting pad bearing design on a gas turbine rotor system. A real coded genetic algorithm with a robust constraint handling technique is used as the optimization method. The objective is to develop a formulation of the optimization problem for the late bearing design of a complex rotor-bearing system. Furthermore, the usefulness of the search method is evaluated on a difficult problem. The effects considered are power loss and limiting temperatures in the bearings as well as the dynamics at the system level, i.e., stability and unbalance responses. The design variables are the bearing widths and radial clearances. A nominal design is the basis for comparison of the optimal solution found. An initial numerical experiment shows that finding a solution that fulfills all the constraints for the system design is likely impossible. Still, the optimization shows the possibility of finding a solution resulting in a reduced power loss while not violating any of the constraints more than the nominal design. Furthermore, the result also shows that the used search method and constraint handling technique works on this difficult problem.

Heat Transfer Measurements Using Liquid Crystals in a Preswirl Rotating-Disk System

Preswirl nozzles are often used in gas turbines to deliver the cooling air to the turbine blades through receiver holes in a rotating disk. The distribution of the local Nusselt number, Nu, on the rotating disk is governed by three nondimensional fluid-dynamic parameters: preswirl ratio, βp, rotational Reynolds number, Reϕ, and turbulent flow parameter, λT. A scaled model of a gas turbine rotor–stator cavity, based on the geometry of current engine designs, has been used to create appropriate flow conditions. This paper describes how a thermochromic liquid crystal, in conjunction with a stroboscopic light and digital camera, is used in a transient experiment to obtain contour maps of Nu on the rotating disk. The thermal boundary conditions for the transient technique are such that an exponential-series solution to Fourier’s one-dimensional conduction equation is necessary. A method to assess the uncertainty in the measurements is discussed and these uncertainties are quantified. The experiments reveal that Nu on the rotating disk is axisymmetric except in the region of the receiver holes, where significant two-dimensional variations have been measured. At the higher coolant flow rates studied, there is a peak in heat transfer at the radius of the preswirl nozzles. The heat transfer is governed by two flow regimes: one dominated by inertial effects associated with the impinging jets from the preswirl nozzles, and another dominated by viscous effects at lower flow rates. The Nusselt number is observed to increase as either Reϕ or λT increases.

Growth of naturally initiated fatigue cracks in ferritic gas turbine rotor steels

Smooth test specimens of 12% CrNiMoV and 1% CrMoV steel have been subjected to strain-controlled low-cycle fatigue loading. Several specimens were instrumented with potential drop probes. As fatigue cracks grew deeper than 0.2–0.3 mm, they caused a steadily increasing potential drop over the length of the specimen. These LCF tests were terminated after a certain fraction of the nominal life. A stepwise increasing high-cycle fatigue load was subsequently used to propagate the crack to fracture. The contour of the low-cycle fatigue crack could be identified on the fracture surface. In agreement with a model suggested by Tomkins, its growth rate was found to be proportional to the crack depth and to be a power function of the plastic strain amplitude. For an assumed initial crack depth of 50 μm, this relationship generally provided conservative life predictions deviating from the observed number of cycles by less than a factor of 5. A second model assumed the crack growth rate to be proportional to a power of the cyclic J integral. Using standard Paris type crack growth data, this model also tended to give conservative predictions, in particular for the longer lives. The deviation from the observed number of cycles was generally less than a factor of 3. The high-cycle fatigue tests permitted the fatigue limit of the cracked specimens to be determined. The threshold stress intensity range obtained from these tests consistently fell below that from tests with standard fatigue crack growth specimens.

2218 Ni基鍛造材,鋳造材からなる摩擦圧接材の強度へ及ぼす微視組織の影響(J10-3 溶接接合強度評価,J10 締結・接合部の力学とプロセス)

To develop the compact and high reliable micro gas turbine rotor structure, turbine wheel and rotor stacking shaft were integrated by friction welding. And the characteristic strength of friction welding joints made of Ni-based cast alloy, MM247LC, Ni-based forged alloy, IN718, were investigated. 0.2% proof strength and rupture elongation of MM247LC were lower than IN718 and tensile strength of MM247LC was lower than that of interface or IN718. And Creep rupture occurred at the boundary between Nb depleted band and fine grain band of IN718. Moreover, fatigue strength decreased remarkably by the effect of welding defects at the interface, so it is considered to be very important to optimize the welding process to keep reliability of friction welding joints.

An approach to power station boiler and turbine life management

The de-regulated electricity market which has existed in the UK for around 15 years has led to energy companies operating their power plant flexibly to maintain profitability in a very competitive commercial environment. To do this, the company maintains a very strong engineering capability in the area of structural integrity of boiler components and high temperature rotating plant. The key issues for the older 500 MW boiler plant, which is daily two-shifted and operates at temperatures well into the creep range for low-alloy steels, are fatigue and creep damage, the latter being influenced greatly by the operating hours seen by the older coal-fired units which have exceeded their design lives. Major components of the boiler which are subject to rigorous and thorough examination are headers, drums and high-energy steam pipework. Methodologies include on-line plant damage analysis for controlling degradation and optimising start-up procedures, plant modifications, innovative repair and retro-fitting with improved materials. Steam and gas turbine rotors operate at high speeds at elevated temperatures. Maintaining rotors in a good commercial and safe operational condition in compliance with any regulatory requirements can also bring increased efficiency and reduced costs, for example by incorporating proactive measures into planned overhauls. Procedures have been developed for both prevention of creep cavitation cracking and assessment of bore high strain fatigue life (for hollow rotors). These use material data, temperature and stress information along with the results of NDE inspections to determine any operational constraints (starts, temperatures, and running hours) which may be necessary. Vibration, which can be an indication of cracking in an operating rotor is monitored remotely and on-line.

Scatter in Fatigue Crack Growth Rate in a Directionally Solidified Nickel-Base Superalloy

Abstract Directional solidification of nickel-base superalloy components for gas turbine rotor blades offers a significant improvement in creep strength while at the same time yielding an anisotropic, heterogeneous microstructure. In such a material, some of the continuum assumptions of linear elastic fracture mechanics begin to break down, and local (microscopic) conditions can lead to significant deviation in fatigue crack growth rate from the global (macroscopic) trend. Fatigue crack life prediction for a large population of fielded components requires a probabilistic treatment of the material fatigue crack growth behavior. A common approach to probabilistic fatigue crack life prediction involves sampling the Paris law coefficients from a large number of crack growth experiments, which can lead to effectively “smoothing” the local intraspecimen variability out of the model. The length scale of variability is discussed as it relates to material microstructure and crack life prediction. Results from fatigue crack growth experiments on a directionally solidified superalloy are presented and spatial variation in the fatigue crack growth rate is examined. Periodicity of the crack growth rate variation is compared with the scale of microstructural heterogeneity.

Heat Transfer Coefficients and Film Cooling Effectiveness on the Squealer Tip of a Gas Turbine Blade

Experimental investigations were performed to measure the detailed heat transfer coefficients and film cooling effectiveness on the squealer tip of a gas turbine blade in a five-bladed linear cascade. The blade was a two-dimensional model of a first stage gas turbine rotor blade with a profile of the GE-E3 aircraft gas turbine engine rotor blade. The test blade had a squealer (recessed) tip with a 4.22% recess. The blade model was equipped with a single row of film cooling holes on the pressure side near the tip region and the tip surface along the camber line. Hue detection based transient liquid crystals technique was used to measure heat transfer coefficients and film cooling effectiveness. All measurements were done for the three tip gap clearances of 1.0%, 1.5%, and 2.5% of blade span at the two blowing ratios of 1.0 and 2.0. The Reynolds number based on cascade exit velocity and axial chord length was 1.1×106 and the total turning angle of the blade was 97.9 deg. The overall pressure ratio was 1.2 and the inlet and exit Mach numbers were 0.25 and 0.59, respectively. The turbulence intensity level at the cascade inlet was 9.7%. Results showed that the overall heat transfer coefficients increased with increasing tip gap clearance, but decreased with increasing blowing ratio. However, the overall film cooling effectiveness increased with increasing blowing ratio. Results also showed that the overall film cooling effectiveness increased but heat transfer coefficients decreased for the squealer tip when compared to the plane tip at the same tip gap clearance and blowing ratio conditions.

Heat Transfer Coefficients on the Squealer Tip and Near Squealer Tip Regions of a Gas Turbine Blade

Detailed heat transfer coefficient distributions on a squealer tip of a gas turbine blade were measured using a hue detection based transient liquid crystals technique. The heat transfer coefficients on the shroud and near tip regions of the pressure and suction sides of a blade were also measured. Tests were performed on a five-bladed linear cascade with a blow-down facility. The blade was a two-dimensional model of a first stage gas turbine rotor blade with a profile of a GE-E3 aircraft gas turbine engine rotor blade. The Reynolds number based on the cascade exit velocity and axial chord length of a blade was 1.1×106 and the total turning angle of the blade was 97.7 deg. The overall pressure ratio was 1.2 and the inlet and exit Mach number were 0.25 and 0.59, respectively. The turbulence intensity level at the cascade inlet was 9.7 percent. The heat transfer measurements were taken at the three different tip gap clearances of 1.0 percent, 1.5 percent, and 2.5 percent of blade span. Results showed that the overall heat transfer coefficients on the squealer tip were higher than that on the shroud surface and the near tip regions of the pressure and suction sides. Results also showed that the heat transfer coefficients on the squealer tip and its shroud were lower than that on the plane tip and shroud. However, the reductions of heat transfer coefficients near the tip regions of the pressure and suction sides were not remarkable.

Heat transfer in a radially rotating square duct fitted with in-line transverse ribs

This paper describes an experimental study of heat transfer in a radially rotating square duct with two opposite walls fitted by transverse ribs. The manner in which rotation modifies the forced heat convection is considered for the case where the duct rotates about an axis perpendicular to the duct's axis of symmetry and the flow within is radially outward with particular reference to the design of a gas turbine rotor blade. A selection of experimental results illustrates the individual and interactive effects of Coriolis and centripetal buoyancy forces on heat transfer along the centerline of each rotating rib-roughened surface. A number of experimental-based observations are revealed those confirm the manner for which the Coriolis force and centripetal buoyancy interactively modify the heat transfer even if the rib associating flow phenomena persist when the through flow transverses the ribs. An empirical correlation based on theoretical consideration and experimental data, which is physically consistent, has been developed to permit the evaluation of interactive effects of rib-flows, convective inertial force, Coriolis force and centripetal buoyancy on heat transfer.

터빈 로터의 익단 간극이 성능에 미치는 영향에 대한 수치해석적 연구

The effect of tip clearance is important part for turbomachinery performance. Tip leakage flow due to tip clearance is mixed with passage vortex. Large amount of loss is generated at the mixing region and overall performance of turbomachinery is reduced. Numerical calculation of the lst stage rotor of GE7FA gas turbine is carried out to investigate tip clearance effect on performance, pitchwise variations of velocity profiles, pressure distributions and loss coefficients. A commercial code, CFX-TascFlow is validated in this study.

Probabilistic Methods for Design Assessment of Reliability with Inspection

Conventional gas turbine rotor life prediction methodologies are based on nominal conditions that do not adequately account for material and manufacturing anomalies that can degrade the structural integrity of high-energy rotors. To account for these anomalies, the Rotor Integrity Subcommittee of the Aerospace Industries Association recommended adoption of a probabilistic damage tolerance approach to supplement the current safe-life methodology. The recommendation led to the development of a computer program called DARWIN that computes the probability of fracture as a function of flight cycles, considering random defect occurrence and location, random inspection schedules, and several other random variables. The probabilistic fatigue analysis methodology developed for DARWIN to address hard alpha material anomalies is presented. The capability of this computer program is demonstrated using several realistic rotor models provided by aircraft engine manufacturers. It is shown that the life approximation function and importance sampling methods significantly reduce computation time (nearly two orders of magnitude) compared to the Monte Carlo method. In addition, an optimal zone sampling strategy is presented that can minimize the total number of samples required to achieve e a a desired sampling accuracy result for a given confidence interval. This probabilistic methodology can be used to focus design efforts on variables that have the most influence on risk reduction.

Toward Improved Film Cooling Prediction

Abstract Computations of flow and heat transfer for a film-cooled high pressure gas turbine rotor blade geometry are presented with an assessment of several turbulence models. Details of flow and temperature field predictions in the vicinity of cooling holes are examined. It is demonstrated that good predictions can be obtained when spurious turbulence energy production by the turbulence model is prevented.

Thermal characterization of a Ni-based superalloy

Ni-based superalloys are usually employed in industrial applications where good mechanical performance, corrosion and oxidation resistance at high temperatures is required. Samples collected from a gas turbine rotor blade were used in the present study. The X-ray photoelectron spectroscopy (XPS) and inductive coupled plasma (ICP) techniques were used to identify and quantify the elements in the alloy. simultaneous thermal analysis (STA) has been used to study the behavior of the alloy during the heat treatment from room temperature to 1450°C, under argon and air atmospheres. Heat capacity has been evaluated and two solid–solid transitions have been identified below 1400°C. A small oxidative process was identified at temperatures over 1150°C.

Sealing of gas turbine disk cavities operating in the presence of mainstream external flow

Abstract Improvements in internal air sealing systems lead to increases in the overall efficiency of gas turbines, by reducing the flow rate of high-pressure coolant required. This paper describes an experimental determination of mainstream external flow ingestion in gas turbine rotor–stator disk cavities. The performance of two geometries operating in the presence of a mainstream external flow field has been investigated: a simple rotor–stator geometry equipped with an axial rim seal, and a realistic rotor–stator geometry equipped with a double-toothed-rim (DTR) seal. Tests were conducted up to an external flow Reynolds number of 6×10 5 and a rotational Reynolds number of 1.5×10 6 . The sealing flow necessary to prevent ingress of external fluid into the disk cavity was determined using static pressure and gas concentration measurements. For the simple rotor–stator geometry, the values of minimum sealing flow rates necessary to prevent ingress are primarily governed by mainstream flow rate except at very low flow rates, where rotational speed effects are equally important. The sealing performance of the realistic rotor–stator geometry far exceeds that of the simple rotor–stator geometry in terms of both a minimum sealing flow rate needed to prevent ingress, and a sealing effectiveness parameter defined in this paper. This observation is consistent with the results formerly presented by the present authors for the same disk cavity and rim seal geometries but in the absence of mainstream external flow.

MODAL UNCOUPLING OF DAMPED GYROSCOPIC SYSTEMS

A new approach for uncoupling the equations of motion typical for rotordynamical systems is presented. The method does not neglect the speed-dependent effects, such as gyroscopic effects, and can be particularly valuable in the controller design of actively controlled rotors. In the presence of hysteretic type of damping, the resulting uncoupled gyroscopic systems come with an equivalent viscous damping, equivalent in a sense of yielding the same natural frequency and decay rate. The approach is illustrated through three examples of technical interest: a Jeffcott rotor with hysteretic damping, a Stodola–Green rotor, and a rotor of a small gas turbine. The generated results demonstrate that the developed approach is correct and straightforward.

Some current research in rotating-disc systems.

Rotating-disc systems are used to model the flow and heat transfer that occurs inside the cooling-air systems of gas-turbine engines. In this paper, recent computational and experimental research in three systems is discussed: rotor-stator systems, rotating cavities with superposed flow and buoyancy-induced flow in a rotating cavity. Discussion of the first two systems concentrates respectively on pre-swirl systems and rotating cavities with a peripheral inflow and outflow of cooling air. Buoyancy-induced flow in a rotating cavity is one of the most difficult problems facing computationalists and experimentalists, and there are similarities between the circulation in the Earth's atmosphere and the flow inside gas-turbine rotors. For this case, results are presented for heat transfer in sealed annuli and in rotating cavities with an axial throughflow of cooling air.

Vibration Analysis of Multi Degree of Freedom System by Simplified Rotor Dynamics Model.

This paper describes the evaluation of analytical Simplified Rotor dynamics Model (SRM) under the multi-freedom condition. Rotor-dynamic of gas-turbine rotor supported by the flexible casing with Legs is selected as the evaluation Model. SRM which consists of the lumped-mass and beam-element is applied for the casing and rotor dynamic system. Proving procedure of SRM is based on 3 Steps. The first step is the fundamental modeling based on test data of casing with support-leg and rotor. Second is the evaluation compared with analytical and test results in revolution under the multi-freedom condition. Third is final step for the applied case. Through these proving procedures, casing and rotor coupled vibration modes of complex vibration structure were made clear by SRM. Further the Influence of support-structure elements were investigated in simple manner by SRM. SRM is conformed to be useful analytical tool for the rotor dynamic characteristics of multi-freedom vibration system.

Heat Transfer and Pressure Distributions on a Gas Turbine Blade Tip

Heat transfer coefficient and static pressure distributions are experimentally investigated on a gas turbine blade tip in a five-bladed stationary linear cascade. The blade is a two-dimensional model of a first-stage gas turbine rotor blade with a blade tip profile of a GE-E3 aircraft gas turbine engine rotor blade. The flow condition in the test cascade corresponds to an overall pressure ratio of 1.32 and exit Reynolds number based on axial chord of 1.1×106. The middle 3-blade has a variable tip gap clearance. All measurements are made at three different tip gap clearances of about 1, 1.5, and 2.5 percent of the blade span. Heat transfer measurements are also made at two different turbulence intensity levels of 6.1 and 9.7 percent at the cascade inlet. Static pressure measurements are made in the midspan and the near-tip regions as well as on the shroud surface, opposite the blade tip surface. Detailed heat transfer coefficient distributions on the plane tip surface are measured using a transient liquid crystal technique. Results show various regions of high and low heat transfer coefficient on the tip surface. Tip clearance has a significant influence on local tip heat transfer coefficient distribution. Heat transfer coefficient also increases about 15–20 percent along the leakage flow path at higher turbulence intensity level of 9.7 over 6.1 percent. [S0889-504X(00)00404-9]