Axial Turbine Stage Research Articles

Effect of Ingestion on Temperature of Turbine Disks

This paper describes experimental results from a research facility which experimentally models hot-gas ingress into the wheel-space of an axial turbine stage with an axial-clearance rim seal. Thermochromic liquid crystal (TLC) was used to determine the effect of ingestion on heat transfer to the rotating disk; as far as the authors are aware, this is the first time that the measured effects of ingestion on adiabatic temperature have been published. An adiabatic effectiveness for the rotor was defined, and this definition was used to determine when the effect of ingress was first experienced by the rotor. Concentration measurements on the stator were used to determine the sealing effectiveness of the rim seal, and transient heat transfer tests with heated sealing air were used to determine the adiabatic effectiveness of the rotor. The thermal buffer ratio, which is defined as the ratio of the sealing flow rate when ingress first occurs to that when it is first experienced by the rotor, was shown to depend on the turbulent flow parameter. The local Nusselt numbers, Nu, which were measured on the rotor, were significantly smaller than those for a free disk; they decreased as the sealing flow rate decreased and as the ingress correspondingly increased. The values of Nu and adiabatic effectiveness obtained in these experiments provide data for the validation of CFD codes but caution is needed if they (particularly the values of Nu) are to be extrapolated to engine conditions.

Experimental Measurements of Ingestion Through Turbine Rim Seals—Part III: Single and Double Seals

This paper describes experimental results from a research facility, which experimentally models hot gas ingress into the wheel-space of an axial turbine stage. Measurements of CO2 gas concentration in the rim-seal region and inside the wheel-space are used to assess the performance of generic (though engine-representative) single and double seals in terms of the variation of concentration effectiveness with sealing flow rate. The variation of pressure in the turbine annulus, which governs externally induced ingress, was obtained from steady pressure measurements downstream of the vanes. The benefit of using double seals is demonstrated: the ingested gas is shown to be predominately confined to the outer wheel-space radially outward of the inner seal; and in the inner wheel-space, radially inward of the inner seal, the effectiveness is shown to be significantly higher. Criteria for ranking the performance of single and double seals are proposed, and the performance limit for any double seal is shown to be one in which the inner seal is exposed to rotationally induced ingress. Although the ingress is a consequence of an unsteady, three-dimensional flow field and the cause-effect relationship between pressure and the sealing effectiveness is complex, the experimental data is shown to be successfully calculated by simple effectiveness equations developed from a theoretical model. The data illustrate that, for similar turbine-stage velocity triangles, the effectiveness can be correlated using two empirical parameters. In principle, these correlations could be extrapolated to a geometrically similar turbine operating at engine-representative conditions.

Experimental Analysis of the Interaction Between Rim Seal and Main Annulus Flow in a Low Pressure Two Stage Axial Turbine

The spoiling effects of rim seal flow are studied at the Large Scale Turbine Rig (LSTR) at Technische Universität Darmstadt. Detailed flow field measurements and efficiency measurements were performed for various ingress and egress setups and will be presented in this paper. Efficiency measurements show an efficiency decrease as the rim seal mass flow is increased. Five hole probe measurements upstream and downstream of the second stator row show that an increasing rim seal mass flow leads to an increased pressure loss across the stator, to altered incidence angles and to an intensification of secondary flow structures within the lower 50% span. Static pressure taps at the stator profile primarily show altered aerodynamic loading with increased rim seal air. In addition, the end wall profile pressure was measured at the stator 2 hub. It can be seen that seal air injection causes increased pressure fluctuations on the platform. Temperature measurements with a temperature difference between rim seal and main annulus flow show that rim seal air primarily enters the passage vortex.

Early Assessment of Defects and Damage in Jet Engines

The jet engine maintenance process is complex, expensive and time-consuming. It often requires engine disassembly or boroscopic examinations. In order to accelerate the process and reduce the down time of an engine we intend to develop a method to locate and characterize defects and damage at an early state, without having to disassemble the engine. The assumption is that various defects in the hot gas path of an engine have a noticeable impact on the spatial density distribution of the exhaust jet of an engine. The resulting differences in the exhaust jet pattern will be measured with the Background Oriented Schlieren method (BOS). We perform numerical simulations (CFD) in order to analyze the effects of various general defect types on the density pattern of the exhaust jet. The defects under investigation include the malfunction of one burner, the increase the turbine blade tip clearance and burned trailing edges of the blades. The changes in the pattern resulting from the defects are compared to the density distribution of the undamaged initial state. It is shown that different exhaust jet patterns can be linked to the investigated hot gas path defects. Furthermore, a BOS set-up is installed in a test cell of a helicopter engine with a two- stage axial turbine to demonstrate the applicability of the BOS method for the measurement of small density gradients resulting from temperature non-uniformities. A cold streak was inserted into the exhaust diffuser to simulate an artificial defect. The completed measurements show that the BOS method is able to detect these small variations. The present paper summarizes the results of different investigations. It presents a combination of BOS measurements of the exhaust jet and CFD simulations of defects within the hot gas path as a promising approach for evaluating the condition of a jet engine.

Experimental Measurements of Ingestion Through Turbine Rim Seals—Part I: Externally Induced Ingress

This paper describes a new research facility which experimentally models hot gas ingestion into the wheel-space of an axial turbine stage. Measurements of the CO2 gas concentration in the rim-seal region and inside the cavity are used to assess the performance of two generic (though engine-representative) rim-seal geometries in terms of the variation of concentration effectiveness with sealing flow rate. The variation of pressure in the turbine annulus, which governs this externally-induced (EI) ingestion, was obtained from steady pressure measurements downstream of the vanes and near the rim seal upstream of the rotating blades. Although the ingestion through the rim seal is a consequence of an unsteady, three-dimensional flow field and the cause-effect relationship between the pressure and the sealing effectiveness is complex, the experimental data is shown to be successfully calculated by simple effectiveness equations developed from a previously published orifice model. The data illustrate that, for similar turbine-stage velocity triangles, the effectiveness can be correlated using a nondimensional sealing parameter, Φo. In principle, and within the limits of dimensional similitude, these correlations should apply to a geometrically-similar engine at the same operating conditions. Part II of this paper describes an experimental investigation of rotationally-induced (RI) ingress, where there is no mainstream flow and consequently no circumferential variation of external pressure.

Simulation of a 3D unsteady flow in an axial turbine stage

The contribution deals with a numerical simulation of an unsteady flow in an axial turbine stage. The solution is performed using an in-house numerical code developed in the Aeronautical and Test Institute, Plc. in Prague. The numerical code is based on a finite volume discretization of governing equations (Favre aver- aged Navier-Stokes equations) and a two-equations turbulence model. The temporal integration is based on the implicit second-order backward Euler formula, which is realized through the iteration process in dual time. The proposed numerical method is used for solution of the 3D, unsteady, viscous turbulent flow of a perfect gas in the axial turbine stage. The flow path consists of an input nozzle, stator blade-wheel, blade-wheel, a shroud-seal gap and a diffuser. Attention is paid to the influence of a secondary flow structures, such as generated vortices and flow in shroud-seal gap. 1I NTRODUCTION The aim of this study is to investigate some properties of flow field in the axial turbine stage. At first the flow in the turbine stage was modeled in the middle section of the turbine stage as the two-dimensional turbulent unsteady flow. Furthermore, a calculation of the three-dimensional unsteady inviscid flow was performed. Boundary phenomena - secondary flow - have been studied using model of the three- dimensional steady turbulent flow, using the frozen rotor technique. Effect of the secondary flow through the seal of the was studied in a simplified model of a prismatic blade cascade using model of the three-dimensional steady turbulent flow. The turbine stage consists of the stator and wheels. The stator wheel contains 70 and the wheel 90 blades. Therefor it was possible consider only periodic field of seven stator and nine blades for the calculation. The hub diameter of the stage is 0.506 m, the tip diameter is 0.597 m (blades span is 0.0455 m), the axial chord of the stator is 0.01955 m, the axial chord of the is 0.025 m (both at the hub), the gap between stator and is 0.005 m. The shape of the blades is shown in Figure 1. The angular speed of the is 455.11 rad s −1 . The inlet total pressure is 1.311 · 10 5 Pa, the inlet total temperature is 55.81 ◦ C, the angle of attac is 0 ◦ and the outlet static pressure is 0.9538 · 10 5 Pa. The in-house URANS numerical code was developed for calculation of flow in the turbine stage.

Investigation of unsteady flow in axial turbine stage

The never ending process to increase the efficiency of turbine stages introduced an idea to create an axial turbine stage test rig as part of closed loop transonic wind tunnel at Vyzkumny a zkusebni letecky ustav (VZLU). Studying of unsteady flow field behaviours in turbine stages is nowadays essential in the development process. Therefore, the test rig was designed with focusing on possibility of detailed studying of unsteady flow fields in turbine stages. New methodologies and new instrumentations were developed at VZLU and successfully integrated to gain valuable information from experiments with turbine stages. Two different turbine stages were tested during the facility introduction process. The measurement of these two models demonstrated how flexible the test rig is. One of them was an enlarged model of small gas turbine stage. The other was scaled-down model of stage of intermediate-pressure steam turbine. The external characteristics of both models were acquired during experiments as well as the detailed unsteady flow field measurement. Turbine stages were tested in various regimes in order to check the capabilities of the facility, methodology and instrumentation together. The paper presents results of unsteady flow field investigation in the enlarged turbine stage with unshrouded rotor. The interaction of structures developed in a stator with the rotor flow field is depicted and the influence of turbine load on evolution of secondary flows in rotor is analysed.

Effects of Multiblocking and Axial Gap Distance on Performance of Partial Admission Turbines: A Numerical Analysis

In this paper, the effects of axial gap distance between the first stage stator and rotor blades and multiblocking on aerodynamics and performance of partial admission turbines are analyzed numerically. The selected test case is a two stage axial steam turbine with low reaction blades operating with compressed air. The multiblocking effect is studied by blocking the inlet annulus of the turbine in a single arc and in two opposing blocked arcs, each having the same admission degree. The effect of axial gap distance between the first stage stator and rotor blades is studied while varying the axial gap by 20% compared with the design gap distance. Finally, full admission turbine is modeled numerically for comparison. Performance of various computational cases showed that the first stage efficiency of the two stage partial admission turbine with double blockage was better than that of the single blockage turbine; however, the extra mixing losses of the double blockage turbine caused the efficiency to deteriorate in the downstream stage. It was shown that the two stage partial admission turbine with smaller axial gap than the design value had better efficiency of the first stage due to lower main flow and leakage flow interactions; however, the efficiency at the second stage decreased faster compared with the other cases. Numerical computations showed that the parameters, which increased the axial force of the first stage rotor wheel for the partial admission turbine, were longer blocked arc, single blocked arc, and reduced axial gap distance between the first stage stator and rotor blades.

J051015 航空エンジン用低圧タービン段の高効率化に関する研究 : 回転試験機による実験及びCFD([J05101]流体機械の研究開発におけるEFD/CFD(1))

This paper presents an experimental study and numerical simulation of the unsteady flow mechanisms in a single stage axial flow turbine which is simulated the LP turbine airfoils. The target of this study is to verify the influence of the purge flow from the Rim-seal cavity on the flow field in the rotor passage, which leads to changes of the secondary flow behavior especially in the rotor hub region. To observe the unsteady flow pattern at rotor downstream, detailed 3-D unsteady measurements using a slant-type hot wire probe is performed. The result of unsteady measurement observed the secondary flow and the leakage flow in the tip region without the purge flow as well as the steady flow measurement with 5-hole probe. A three-dimensional, time-dependent numerical simulation shows the unsteady flow interactions between the main flow and purge flow.

Investigation of the Flow Field on a Transonic Turbine Nozzle Guide Vane With Rim Seal Cavity Flow Ejection

Ensuring an adequate life of high pressure turbines requires efficient cooling methods such as rim seal flow ejection from the stator-rotor wheel space cavity interface, which prevents hot gas ingress into the rotor disk. The present paper addresses the potential to improve the efficiency in transonic turbines at certain rim seal ejection rates. To understand this process, a numerical study was carried out, combining computational fluid dynamic (CFD) simulations and experiments on a single stage axial test turbine. The three dimensional steady CFD analysis was performed, modeling the purge cavity flow ejected downstream of the stator blade row at three flow regimes: subsonic M2=0.73, transonic M2=1.12, and supersonic M2=1.33. Experimental static pressure measurements were used to calibrate the computational model. The main flow field-purge flow interaction is found to be governed by the vane shock structures at the stator hub. The interaction between the vane shocks at the hub and the purge flow has been studied and quantitatively characterized as a function of the purge ejection rate. The ejection of 1% of the core flow from the rim seal cavity leads to an increase in the hub static pressure of approximately 7% at the vane trailing edge. This local reduction of the stator exit Mach number decreases the trailing edge losses in the transonic regime. Finally, a numerically predicted loss breakdown is presented, focusing on the relative importance of the trailing edge losses, boundary layer losses, shock losses, and mixing losses, as a function of the purge rate ejected. Contrary to the experience in subsonic turbines, results in a transonic model demonstrate that ejecting purge flow improves the vane efficiency due to the shock structure modification downstream of the stator.

Studies on Effects of Stator-Rotor Axial Gap on Aerodynamic Performance of a Single Stage Axial Turbine (Experiment and CFD)

Studies on Effects of Stator-Rotor Axial Gap on Aerodynamic Performance of a Single Stage Axial Turbine (Experiment and CFD)

Three-Dimensional, Unsteady, Inviscid Flow in an Axial Turbine Stage

Three-Dimensional, Unsteady, Inviscid Flow in an Axial Turbine Stage

Computational Investigation of Effect of Axial Spacing on Blade Row Interaction in a 1½ Stage Axial Flow Turbine

This paper presents computational investigation of the flow in a 1½ stage axial turbine using finite volume based commercial CFD package namely CFX 5.7.1. The blade row interaction effect is studied in unsteady computations. The effect of axial spacing on the flow field of the turbine is studied for four different axial spacings between the nozzle-rotor and rotor-stator, viz. Xs=15%, 25%, 35% and 50%. The computational results are utilized to suggest the optimum axial spacing between nozzle-rotor and rotor-stator blade rows. The results of static pressure coefficient distribution, spanwise variation of lift coefficient in nozzle, rotor and stator, variation of lift coefficient at midspan with axial spacing, contours of total loss coefficient at 5% down stream of nozzle, rotor and stator, distribution of mass averaged total pressure loss coefficient with axial distance, and variation of mass averaged total pressure loss coefficient with axial spacing are presented. The total pressure loss coefficient contours show the clear structure of the vortices and loss cores.

Relation of Clocking Effect and Secondary Flow in a 1.5 Stage Axial Turbine

Relation of Clocking Effect and Secondary Flow in a 1.5 Stage Axial Turbine

J0501-2-2 軸流タービン空力性能に及ぼす動静翼間距離の影響(流体機械の研究開発におけるEFD/CFD(2))

The purpose of this study is to investigate the effects of the axial gap between stator and rotor upon the performance and secondary flow field of a single axial flow turbine stage. We performed experiments for three axial gaps. In our test rig, it is possible to change the axial gap by moving the stator vane in axial direction. The tests were carried out at two different conditions, that is design and off-design conditions. The unsteady three-dimensional flow field was analyzed by time-accurate RANS (Reynolds-Averaged Navier-Stokes) simulations. The simulation results were compared with the experiments, in which total pressure loss and time-averaged flow field upstream and downstream of the rotor were obtained by five-hole probe measurements. The effect of the axial gap was confirmed in the endwall regions, and obtained relatively at the off-design condition. The turbine stage efficiency was improved almost linearly to reduce the axial gap at the off-design condition.

Scaling Concept for Axial Turbine Stages With Loosely Assembled Friction Bolts: The Linear Dynamic Assessment

In the early 1980s, Siemens developed a last stage fast rotating condensation blading (SK) blade with strongly twisted and tapered profiles for industrial condensing steam turbines, which operate with variable speed under high steam mass flow and excessive condensing pressures. To suppress alternating stresses of the lowest blade resonances, conical friction bolts are loosely mounted at the upper parts of adjacent airfoils. Also, these bolts couple the rotating blades, since steam excitation is lower than the friction threshold force on the bolt contacts. These coupling and damping capabilities were proven experimentally for the smallest SK blade at the test rig of the real turbine. By considering the similar mechanical and aerodynamic characteristics based on the tested smallest airfoil, the entire SK-blade family has been scaled up for reliable utilization in more than 500 industrial turbines operating for diverse ranges of power and speed. A recent trend to very large compression units, like gas to liquids, acid terephtalic, or methanol plants, imposes a need for further enlargement of the SK-blade family and its friction bolt, whose mechanical properties have been proven experimentally for the smallest airfoil. In this paper, the mechanical capabilities of the smallest and large SK blades coupled by the bolts are verified by using the finite element (FE) method. The static analyses with friction sliding on airfoil interfaces and the linear dynamic behavior of tuned disk assemblies are considered. The FE mesh quality and the proper boundary conditions at the radial fork root are accomplished by getting good agreements between the computed and measured resonance frequencies of the large freestanding blade at standstill. The validated mesh refinement and root boundary conditions are used further in all numerical FE analyses. For the large SK-disk assembly under spin-pit conditions, the obtained FE results are in very good agreement with the experimental Campbell diagrams, which are measured with the three gauges that also identify the stick-slip and stuck bolt’s contact conditions. Concerning the gauge outputs and the FE steady-state blade resonances computed for the analytically determined air jet excitation, the experimental spin-pit results demonstrate that the bolts are mainly in stuck contact conditions. Only in very narrow frequency ranges around resonance peaks, microslips on the bolts occur due to the resonance amplification of blade vibrations. This is proven indirectly by comparison of the overall damping values evaluated from the blade resonances at standstill and in the spin pit. The described linear dynamic concept assesses properly static stresses and free vibrations of the scaled disk assembly with friction bolts. For the steam excitation, which generates dynamic contact reactions bigger than the friction threshold forces, the realistic blade responses need to be obtained from the blade simulation with friction (Szwedowicz, J., Secall-Wimmel, T., and Duenck-Kerst, P., 2007, “Damping Performance of Axial Turbine Stages With Loosely Assembled Friction Bolts; the Non-Linear Dynamic Assessment; Part II,” Proceedings of ASME Turbo Expo 2007, Montreal, Canada, May 14–17, ASME Paper No. GT2007-27506).

Damping Performance of Axial Turbine Stages With Loosely Assembled Friction Bolts: The Nonlinear Dynamic Assessment

An entire family of twisted and tapered low pressure steam turbine fast rotating condensation blading (SK) blades with pinned radial root and loosely assembled conical bolts is designed by scaling the aerodynamic and mechanical properties of the smallest airfoil. For SK blades operating with variable speed, the friction bolts, mounted in the upper airfoil part, provide either damping or coupling capabilities for the blades with respect to resonance conditions. The damping and coupling performance have been proven experimentally in the test rig of the real turbine. The measurements of the smallest SK-disk assembly under different operating conditions have allowed us to understand the dynamic and damping behavior of the bolts that are either friction dampers or coupling devices for the vibrating blades depending on the excitation level. In this paper, nonlinear dynamic analyses of the smallest and large SK-turbine stage are performed and compared with the experimental data. The modal blade dynamics is defined by 30 complex finite element (FE) mode shapes of the freestanding blades coupled by the disk whereby the bolt’s motion is described by six rigid body modes. The sticking contact condition between the blades and bolts is represented by the normal and tangential contact stiffnesses. These values are firstly estimated analytically with Hertz’s formulas for the FE reaction forces and contact areas. More realistic contact stiffness values are obtained from the iterative process, in which the resonance frequencies are calculated with the steady-state simulations and compared to the FE nodal diameter curves for sticking contact conditions that meet the experimental frequencies very well (Szwedowicz, J. et al., 2007, “Scaling Concept for Axial Turbine Stages With Loosely Assembled Friction Bolts: The Linear Dynamic Assessment Part 1,” Proceedings of ASME Turbo Expo 2007, Montreal, Canada, May 14–17, ASME Paper No. GT2007-27502). In nonlinear simulations, in case of exceeding the sticking contact condition, the induced friction forces are linearized by the harmonic balance method. In this manner, the microslipping and sticking contact behavior at all contact points are calculated iteratively for the specified excitation amplitudes, friction coefficient, contact roughness, and aerodamping values that are known from the experiment. The computed results of the tuned smallest SK blades agree with the experimental resonance stresses of 12 measured blades. Differences between the computed and measured stresses are caused by mistuning, which was not quantified in the experiment. The nonlinear dynamic analyses provide evidence of good damping performance for the smallest and large SK blades with respect to a wide range of excitation forces, different friction coefficients, and various aerodynamic damping values. For the analyzed resonances of the eighth engine order, the scalability of damping performance is found for the SK blades of different sizes.

1712 単段軸流タービンの動静翼列間隔による非定常効果(S19-2 流体機械に関連した流動現象とその解析・計測(2)不安定流動・非定常流動の計測と解析,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

This paper provides experimental investigations of unsteady flow behaviors in a single axial flow turbine stage with two axial gaps. The aim of this study is to clarify the effect of the axial gap on the unsteady flow field, where the 3-D unsteady flow field downstream of rotor blades is measured using a slant-type hot wire probe. Since the flow field is likely to be affected by the disk and tip cavity flow when the stator-rotor axial gap increased, the cavities on both hub side and casing side were kept constant (about 1 mm) by attaching several rings to openings of the cavities. In addition, we have compared time-averaged flow fields measured by a five-hole probe with unsteady experimental results.

Evaluation of mass flowrate through an axial gas turbine stage using different computational methods

Mass flowrate is very important, as it determines the frontal area, volume, size, and weight of the engine, and consequently, the momentum drag at certain flight conditions, specific fuel consumption, and the aircraft total loading. The mass flowrate is also needed to determine the shaft power and the thrust power developed by the engine when the specific work output is obtained. Different computational methods have been used in this research to find out their relative accuracy in determining the mass flow passing through an already designed, but not built, stage. The methods employed are free vortex variation of density using approximate scheme including mean radius, arithmetic average density, and numerical integration using the trapezoidal and Simpson's rule. Moreover, closed-form integration (theoretical) with free vortex, linear variation, and parabolic variation of density along the height of the blade was considered. The arithmetic average method was found to give the highest accuracy. The mean-radius method is the least accurate but the simplest. Hence, it is advised to be used especially in quick estimates. The difference in accuracy is not so great to justify using tedious and time-consuming methods.

Turbulence Measurements and Analysis in a Multistage Axial Turbine

This paper presents turbulence measurements and detailed flow analysis in an axial turbine stage. Fast response aerodynamic probes were used to resolve aperiodic fluctuations along the three directions. Assuming incompressible flow, the effective turbulence level and Reynolds stress are retrieved by evaluating the stochastic velocity component out of the measured time-resolved pressure and flow angle fluctuations along the streamwise, radial, and circumferential direction. A comparison between turbulence intensity and measured total pressure shows that flow structures with higher turbulence level are identified in the region of loss cores at the exit of the second stator passage. Turbulence intensity is evaluated under isotropic and nonisotropic assumption in order to quantify the departure from isotropic conditions. The measurements show that locally the streamwise fluctuating component can be twice bigger than the radial and tangential component. The current analysis shows that multisensor fast response aerodynamic probes can be used to provide information about the mean turbulence levels in the flow and the Reynolds stress tensor, in addition to the measurements of unsteady total pressure loss.