High-pressure Turbine Stage Research Articles

The Effects of Swirling Flows in Entropy Wave Convection Through High-Pressure Turbine Stage

Abstract First stages of aeronautical high-pressure turbines are subjected to significant inlet distortions generated by the combustor system. These disturbances are characterized by velocity and temperature fluctuations convected downstream by the flow. Such perturbations are commonly defined as vorticity and entropy waves and interact with the turbine stages affecting the aerodynamic performance, the heat exchange, and generating indirect noise. Moreover, the presence of a swirling flow highly influences the convection and migration of the entropy wave and thus its interaction with the stage. This article presents an in-depth study of the impact of the swirling flows on the entropy wave evolution by means of experimental campaigns and numerical simulations. Experimental campaigns have been carried out at Politecnico di Milano where a high-pressure turbine rig was equipped with a novel combustor simulator able to generate entropy waves and swirl profiles. Numerical simulations have been performed at the University of Florence by applying time accurate simulation schemes, including incoming disturbances, implemented in the CFD TRAF code. Two different entropy waves (featuring frequencies of 10 and 110 Hz) injected in a counterclockwise swirling region at mid-span have been analyzed at two clocking positions: passage aligned and vane aligned. An excellent agreement is found between experimental acquisitions and numerical results: both show an important reduction of the temperature fluctuations through the stage and highlight the effect of the swirling profile on secondary flows and blade wakes. The extensive comparison reported in this article validates the numerical approach (based on unsteady simulations postprocessed by a dedicated filtering technique), which has been further applied to study the impact of swirling flows with an opposite rotation (clockwise). The broad numerical investigation combined with the extensive experimental campaign leads to a deeper understanding of the aerodynamic, thermal, and acoustic implications related to entropy wave evolution in a swirling flow, highlighting the interaction phenomena and suggesting how to minimize the impact of entropy waves by comparing the results of the different injection positions and swirling flow directions.

Investigation of a High-Pressure Turbine Stage in a High-Speed Rotating Transient Test Facility for Rotor Tip Study and a Parametric Study for Improved Heat Transfer Calculation

Abstract The first part of the paper presents commissioning of a single-stage high-pressure (HP) turbine employed in a series of extensive experiments to study the aerodynamics and heat transfer on the rotor surface and casing liner. The Oxford Turbine Research Facility (OTRF), a high-speed rotating transient test facility has the capability to take unsteady aerodynamic and heat transfer measurements at engine representative conditions with a variety of inlet temperature profiles including radial distortion and swirl. A temperature profile survey was conducted at the inlet of the HP nozzle guide vane (NGV). Static and total pressure and temperature measurements have been taken at various locations on the rig including NGV surface, inlet and exit, and rotor exit to establish rig operating conditions. Detailed description of mass flow rate measurements along with calculation of heat loss factor in the rig is presented. The second part of the paper presents a parametric study performed to improve heat transfer measurement calculations from high-frequency response thin-film gauges. The effect of parameters like material properties and thickness of substrate on heat flux has been studied. A detailed uncertainty analysis for heat flux is also presented. A thermal model calibrated with analytical solutions has been developed to optimize thin-film gauge configurations and to study side-conduction effects.

Effects of Flow Coefficient on Turbine Aerodynamic Performance and Loss Characteristics

The effects of flow coefficient on the gas flow and loss characteristics inside the high-pressure turbine is investigated using a numerical simulation. In this paper, the midspan of the first stator of the “Lisa” 1.5 stage high-pressure turbine is used as a prototype to obtain different flow coefficients by changing the stagger angle and the exit angle. The boundary conditions of all cases are consistent with the experimental data of “Lisa”. The results show that the flow coefficient is decreased from 0.478 to 0.374 as the stagger angle is varied from 44.2° to 56.2° and from 0.630 to 0.341 as the exit angle is varied from 63° to 75°. Large stagger angle or large exit angle both cause an increase in turbine aerodynamic losses. The similarity between the two is that both cause enhanced effect of transverse secondary flow in the passage. The difference is that with large stagger angle, the adverse pressure gradient affects a large area, resulting in large boundary layer losses; with large exit angle, the passage vortex is weakened but with a large influence area.

High-Fidelity Computational Fluid Dynamics Analysis of In-Serviced Shrouded High-Pressure Turbine Rotor Blades

Abstract In-service deterioration can lead to undesired shape variations on high-pressure turbine rotor blades. This can have a significant impact on efficiency, power generation, and component life. Loss of power production from the high-pressure turbine rotor causes engine over-throttling to compensate for the lower performance. This will in turn worsen the operating conditions and ultimately reduce the life of the component. The aim of this study is to provide a high-fidelity flow simulation of in-serviced shrouded high-pressure turbine (HPT) blades of a modern Jet Engine. Shape variation effects on the aerodynamic performance of several shrouded HPT blades with a different number of in-service hours have been investigated. In order to establish a digital model of the shape variation, a novel reverse-engineering procedure is carried out to come up with a parametrized definition of each blade's variations from nominal including any observable damage. The investigation is conducted by means of an in-house, full 3D steady-state Reynolds-averaged Navier–Stokes (RANS) simulation of the flow around a series of damaged rotor blade geometries, which are obtained through high-resolution optical blue-light “Gesellschaft für Optische Messtechnik” (GOM) scans. The analysis shows that the aerodynamic performance of the HPT rotor blades under investigation is primarily sensitive to shroud damage, which is found to account for efficiency losses often greater than 3%, and for more than 80% of the total performance loss. A secondary role on efficiency is found to be played by the blade shape deviation. A highly linear correlation is found between HPT stage efficiency and a combination of shroud damage parameters.

Analysis on tenon tooth cracks of a second stage high-pressure turbine blade

Several cracks were observed on the fifth tenon tooth root of some second stage high pressure turbine blades of an aero-engine after ground test run. Surface and sectional morphologies of cracks, recrystallization and microstructure of the near-surface were utilized to explore surface integrity of the tenon tooth root. Besides, the finite element analysis was carried out to estimate the static strength and temperature field of the turbine blade. The results showed that most of the cracks were along the near-surface carbides, where undergone grinding burn with the formation of the white layer and fine secondary γ′ phases, and then cracks fatigue propagated during operating. Some cracks above the microporosities were ascribed to the local stress concentration when contacting with the grinding wheel during the creep feed grinding. The recrystallization occurred on the tenon teeth surface during annealing at 1000℃ for four hours after grinding, and related cracks were not observed in the recrystallized fine grains layer during operating.

Loss evaluation and aerodynamics investigation of an aggressive intermediate turbine duct under off-design conditions

To maximize the performance of the intermediate turbine duct (ITD) under off-design conditions, the loss generation in a one-half stage turbine was quantified using entropy generation and the global entropy generation rate. The numerical results solved by the unsteady Reynolds-averaged Navier–Strokes equations were first verified. Then, the aerodynamic losses within the high-pressure turbine stage were evaluated by efficiency loss under nine operating conditions composed of three rotor speeds and three rotor tip gaps. Finally, the disturbance modes caused by the upstream wake were captured by the dynamic mode decomposition method. Different from the influence of tip gaps, losses of the high-pressure turbine and the ITD are due to the swirl angle display an opposite trend. Under the influence of the interaction between the tip leakage flow and the shroud flow of the ITD, the viscous dissipation and turbulent dissipation increase with a larger tip gap owing to the dominant counter-rotating vortices and secondary flow occurring near the upstream of the ITD shroud. In addition, a large gap seems to enlarge the swirl component of the inflow angle, especially over 80% passage height, leading to greater dissipation losses in these areas. At the ITD inlet, two pairs of counter-rotating vortices at the shroud and the hub are, respectively, captured by the axial velocity mode. Large tip gaps enhance endwall vortices near the shroud and make the up vortex pairs merge into one pair.

Effect of clocking on entropy noise generation within an aeronautical high pressure turbine stage

This article reports a detailed experimental and numerical study of the clocking effect between entropy wave spots and stator leading edges on the evolution of the entropy wave and on indirect noise generation in a high pressure turbine stage. Experimental campaigns considering burner-representative temperature fluctuations injected upstream of an uncooled high-pressure gas turbine stage have been performed in the high-speed closed-loop test rig of the Fluid Machine Laboratory (LFM) of the Politecnico di Milano. Acoustic measurements focused on entropy noise generation have been carried out by the German Aerospace Center, DLR with two flush mounted in-duct microphone arrays. URANS CFD simulations with and without entropy fluctuations imposed at the stage inlet were performed with the TRAF code, developed by the Università degli Studi di Firenze. A numerical post-processing procedure, based on the time and space discrete Fourier transform (DFT) of the conservative variables, has been implemented to extract the low-frequency content connected to the entropy fluctuations and indirect noise emissions. Simulations show an overall good agreement with the experiments, especially at the stage outlet thus confirming the possibility to predict entropy noise by means of URANS simulations with time varying inlet conditions. By exploiting the combination of experiments and simulations, the evolution of the entropy wave, the identification of entropy noise generation areas, and the evaluation of indirect noise emissions, for the different clocking positions were properly assessed. This study also demonstrates that this type of simulations can be used for the geometric design optimization of combustor/turbine coupling performing detailed parametric studies concerning the clocking position of the perturbation source in order to reduce the turbine-combustor interactions and the indirect noise emissions.

Experimental and Numerical Analysis of Loss Characteristics of Cooled Transonic Nozzle Guide Vanes

Abstract This article presents high-fidelity experimental traverse measurements downstream of an annular cascade of transonic nozzle guide vanes (NGVs) from a high-pressure (HP) turbine stage. The components are heavily cooled real engine components from a modern civil gas turbine engine, operated at scaled engine conditions. Tests were conducted in the high technology readiness level (TRL) Engine Component Aerothermal (ECAT) facility at the University of Oxford. High-resolution full-area traverse measurements of local kinetic energy (KE) loss coefficient are presented in several axial planes. In particular, we present circumferential loss coefficient profiles at several radial heights, full-area traverses at three axial planes, and fully mixed-out loss calculations. The analysis of these data gives insight into particular loss structures, overall aerodynamic performance, and wake mixing rates. The effect of exit Mach number on performance is also considered. The data address a gap in the literature for the detailed analysis of traverse measurements downstream of HP NGV engine components. Experimental data are compared with steady and unsteady Reynolds-averaged Navier–Stokes (RANS) simulations, allowing benchmarking of typical computational fluid dynamics (CFD) methods for absolute loss prediction of cooled components. There is relatively limited aerodynamic performance data in the literature for heavily cooled NGVs, and this study represents one of the most comprehensive of its type.

A novel transcritical-recuperative two-stage Organic Rankine Cycle for dual/multi-source heat recovery applications

The Transcritical Regenerative Series Two-stage Organic Rankine Cycle (TR-STORC) has improved performance over STORC and single-stage ORC. However, the TR design requires vapor extraction between turbine stages, which prevents the use of compact two-stage ORC turboexpanders. An alternative Transcritical-Recuperative (TREC) design is proposed which eliminates vapor extraction allowing direct integration of compact two-stage turbines. The novel TREC-STORC operates with a recuperator acting as an additional evaporator at the low-pressure turbine exit (LP mode). Another TREC design requiring vapor extraction from the high-pressure turbine stage (HP mode), similar to the TR design, is also presented for comparison purposes. TREC-STORC in LP mode presents a marginally improved performance of 1–3% over HP mode for various IC engine heat source conditions. With cyclopentane as working fluid, at engine design point conditions, LP mode delivers 4% higher power outputs over HP mode and TR-STORC. Compared to STORC and single-stage ORC, the LP mode of TREC-STORC delivers 21% and 28% higher power outputs, respectively. For other working fluids, TREC-STORC (LP mode) delivers between 2–5% additional power output over TR-STORC, with 18–38% and 20–56% increased power outputs over STORC and single-stage ORC, respectively. TREC-STORC (LP mode) presents a superior and robust ORC architecture for dual/multi-source heat recovery applications.

Tightness problems at the flange connection in transient temperature and high pressure condition

The main goal of the article is to study, analyze and explain the most apparent cause of transient steam leakage from a high pressure turbine stage body. The analyses focus on transient heat transfer on the metal-metal flange connection of an external turbine casing and start-up tests. The coupled effect of thermal expansions and their impact on the residual bolt pretension force was analyzed in detail using FE models. Series of experiments were performed to confirm the results obtained by numerical calculation. The conclusion provided in this article can be used to improve the design of the flange connection working in variable temperature conditions.

Comparative Study of a Powerplant Life Consumption Rate When Installed in Two Different Aircraft Variants

The Hellenic Air Force (HAF) operates both EMB-145 and EMB-135 LR versions of Embraer aircraft, used in surveillance and civil missions respectively. These aircraft are equipped with the same version of Rolls Royce, AE 3007 turbofan engine. This study aims to quantify and compare the life consumption rate of this engine when installed in each of the two aircraft variants. Two typical missions, one for each variant, were constructed based on mission profile data dictated by the aircraft commanders. For each mission profile segment, corresponding engine data were matched out of the engine recordings archives held by the Hellenic Air Force. The life consumption rate was based on the Low Cycle Fatigue (LCF) and creep cumulative detrimental effect on the rotor blades of the 1st High-Pressure Turbine stage. For the LCF, the rainflow method was used to determine the respective loading cycles, whereas the Larson - Miller parameter method was used to determine the consumed life fractions due to creep. The main conclusion of the study was that the engine when installed in the EMB-145 military variant, is much more loaded. Despite the fact absolute life consumption values could hide a great level of uncertainty, the comparative outcomes wherein errors are, to a certain extent, cancelled out, could be used as a rule of thumb when monitoring engine life consumption rates.

Unsteady simulation of flow and heat transfer in a transonic turbine stage under non-uniform inlet conditions

In modern aero-engines, the temperature non-uniformity (hot-streak) caused by the combustors and secondary flows caused by the rotor blades have a considerable impact on the aerothermal performances of the high pressure (HP) turbine. In this study, two idealized hot-streaks (radial and rounded) from literature and one engine representative hot-streak (distorted) were evaluated within a HP turbine stage using unsteady numerical simulation. The results were analyzed and discussed and they include, the stage performance, unsteady flow visualization, unsteady hot-streak transport, secondary flows/hot-streak interaction and blades thermal loading. It was found that the rotor passage had various complex secondary flows and the boundary layer on its surface undergone several transitions. The hot-streak transport and its contact with vane and rotor blades differed from one case to another. The secondary flows had a remarkable impact on the rotor exit temperature. They induced wavy shapes and corrugations of temperature gradients near the endwalls. All hot-streaks were completely deformed and distorted across the turbine stage. The rotor surfaces suffered from hot temperatures under radial and distorted hot-streaks compared with rounded hot-streak. Very high levels of the heat transfer were generated on the rotor suction side near the endwalls due to the vortices.

Proof-of-Concept of a Thermal Barrier Coated Titanium Cooling Layer for an Inside-Out Ceramic Turbine

Abstract Increasing turbine inlet temperature (TIT) of recuperated gas turbines would lead to simultaneously high efficiency and power density, making them prime candidates for low-emission aeronautics applications, such as hybrid-electric aircraft. The inside-out ceramic turbine (ICT) architecture achieves high TIT by using compression-loaded monolithic ceramics. To resist inertial forces due to blade tip speed exceeding 450 m/s, the shroud of the ICT is made of carbon-polymer composite, wound around a metallic cooling ring. This paper demonstrates that it is beneficial to use a titanium alloy cooling ring with a thermal barrier coating (TBC), rather than nickel superalloys, for the interstitial cooling ring protecting the carbon-polymer from the hot combustion gases. A numerical design of experiments (DOE) analysis shows the design tradeoffs between the minimum safety factor and the required cooling power for multiple geometries. An optimized high-pressure first turbine stage of a 500 kW microturbine concept using ceramic blades and a titanium cooling ring in an ICT configuration is presented. Its structural performance (minimum safety factor of 1.4), as well as its cooling losses, (2% of turbine stage power) are evaluated. Finally, a 20 kW-scale prototype is tested at 300 m/s and a TIT of 1375 K during 4 h to demonstrate the viability of the concept. Experiments show that the polymer composite was kept below its maximum safe operating temperature and components show no early signs of degradation.

Computational and Experimental Study of the Unsteady Convection of Entropy Waves Within a High-Pressure Turbine Stage

Abstract This article describes the transport and the interaction of pulsating entropy waves generated by combustor burners within a high-pressure turbine stage for aeronautical application. Experiments and computational fluid dynamics (CFD) simulations were carried out in the context of the European Research Project RECORD. Experimental campaigns considering burner-representative temperature fluctuations (in terms of spot shape, fluctuation frequency, and total temperature variation percentage) injected upstream of an un-cooled high-pressure gas turbine stage have been performed in the high-speed closed-loop test rig of the Fluid Machine Laboratory (LFM) of the Politecnico di Milano (Italy). The pulsating entropy waves are injected at the stage inlet in streamwise direction at four different azimuthal positions featuring a 7% overtemperature with respect to the main flow with a frequency of 90 Hz. Detailed time-resolved temperature measurements (in the range of 0–200 Hz) upstream and downstream of the stage, as well as in the stator–rotor axial gap, were performed. Time-accurate CFD simulations with and without entropy fluctuations imposed at the stage inlet were performed with the TRAF code, developed by the Università degli Studi di Firenze (Italy). A numerical postprocessing procedure, based on the discrete Fourier transform (DFT) of the conservative variables, has been implemented to extract the low-frequency content connected to the entropy fluctuations. Measurements highlighted a significant attenuation of the entropy wave spot throughout their transport within the stator channel and their interaction with the rotor blade rows, highly depending on their injection azimuthal position. Simulations show an overall good agreement with the experiments on the measurement traverses, especially at the stage outlet. By exploiting the combination of experiments and simulations, the aerodynamic and thermal implications of the temperature fluctuation injected upstream of the stage were properly assessed, thus allowing suggest useful information to the designer. The comparison with the experiments confirms the accuracy of the CFD method to solve the periodic, but characterized by a low-frequency content event, associated with the entropy wave fluctuation.

Numerical study on the biomimetic trailing edge in the environment of a high-pressure turbine stage

In this study, the whisker structure of a harbor seal is applied to the trailing edge of a high-pressure turbine stage to reduce energy loss. The method to produce biomimetic geometry is provided. Two cases are studied in this paper including a prototype of a high-pressure turbine stage and this stage with the biomimetic trailing edge. The delayed detached-eddy simulation (DDES) method is used to simulate the flow of the stages. The energy loss coefficient of the turbine stage with the biomimetic trailing edge is significantly reduced. The analysis of the local entropy generation rate shows that the wake loss of the biomimetic stage and the loss in the blade passage are greatly reduced. Two reasons for the widening of the wake of the prototype vane during the downstream movement are analyzed. For the biomimetic stage, the Karman vortices are completely suppressed and transformed into small vortices with strong three-dimensional motion. The wake width of the biomimetic trailing edge is narrowed and the speed recovery becomes faster. This leads to the reduction of the wake loss and the interaction between the wake of the vane and the downstream blade.

Impact of Turbine-Strut Clocking on the Performance of a Turbine Center Frame

Abstract Previous studies have indicated a potential for improving the performance of a turbine center frame (TCF) duct by optimizing the clocking position between the high-pressure turbine (HPT) vanes and TCF struts. To assess the impact of clocking on the performance, a new test vehicle with a clockable ratio of HPT vanes to TCF struts, consisting of an HPT stage (aerodynamically representative of the second-stage HPT engine), a TCF duct with nonturning struts, and a first-stage low-pressure turbine vane, was designed and tested in the transonic test turbine facility (TTTF) at Graz University of Technology. This article quantifies the performance impact of clocking and describes the mechanisms causing TCF flow field changes, leveraging both experimental and numerical data. Other areas in the TCF duct impacted by the choice of the HPT vane circumferential position including the strength of unsteady HPT-TCF interaction modes, TCF strut incidence changes, and carryover effects to the first low-pressure turbine (LPT) vane are additionally highlighted. Five-hole-probe (5HP) area traverses and kielhead-rake traverses were used to assess the flow field at the TCF exit and to calculate the pressure loss. The flow field at the TCF exit shows significant differences depending on the circumferential position of the HPT vane. A relative performance benefit of 5% was achieved. A series of unsteady RANS simulations were performed to support the measured results, understand, and characterize the relevant loss mechanisms. The observed performance improvement was related to interaction between the HPT secondary-flow structures and the TCF struts. The impact of the HPT vane clocking on the unsteady flow field downstream of the TCF was investigated using fast-response aerodynamic pressure probe (FRAPP) area traverses and analyzed by means of modal decomposition. In this way, the individual azimuthal modes were ranked by their amplitude, and a dependency of the clocking position was observed and quantified.

Particle Image Velocimetry in a High-Pressure Turbine Stage at Aerodynamically Engine Representative Conditions

Abstract Particle image velocimetry (PIV) is a well-established technique for determining the flow direction and velocity magnitude of complex flows. This paper presents a methodology for executing this nonintrusive measurement technique to study a scaled-up turbine vane geometry within an annular cascade at engine-relevant conditions. Custom optical tools such as laser delivery probes and imaging inserts were manufactured to mitigate the difficult optical access of the test section and perform planar PIV. With the use of a burst-mode Nd: YAG laser and Photron FASTCAM camera, the frame straddling technique is implemented to enable short time intervals for the collection of image pairs and velocity fields at 10 kHz. Furthermore, custom image processing tools were developed to optimize the contrast and intensity balance of each image pair to maximize particle number and uniformity while removing scattering and background noise. The preprocessing strategies significantly improve the vector yield under challenging alignment, seeding, and illumination conditions. With the optical and software tools developed, planar PIV was conducted in the passage of a high-pressure stator row, at midspan, in an annular cascade. Different Mach and Reynolds number operating conditions were achieved by modifying the temperature and mass flow. With careful spatial calibration, the resultant velocity vector fields are compared with Reynolds-averaged-Navier–Stokes (RANS) simulations of the vane passage with the same geometry and flow conditions. Uncertainty analysis of the experimental results is also presented and discussed, along with prospects for further improvements.

가스터빈 연소기 출구 열·유동장 분포가 터빈에 미치는 효과의 수치해석 연구

Effects of temperature and flow fields at a combustor outlet on a high pressure gas turbine stage are investigated by using numerical methods. Numerical simulations of the annular type combustor are carried out by using large eddy simulation (LES) and unsteady Reynolds-averaged Navier-Stokes equations (URANS) techniques. The temperature distribution is found to be significantly affected by vortical structures at the primary zone and dilution jets. Numerical simulations of high pressure turbine stage are also conducted by using the Reynolds-averaged Navier-Stokes equations (RANS). Temperature distributions and aerodynamic loads at blade and end wall surfaces are drastically altered by changes in temperature, momentum, turbulent kinetic energy and turbulence dissipation rate at the exit of the combustor which are imposed as turbine inlet boundary conditions.

Aerothermal and aerodynamic performance of turbine blade squealer tip under the influence of guide vane passing wake

In this paper, the performance of the turbine blade squealer tip has been studied detailed aimed to highlight the impact of the upstream guide vane passing wake. The first stage of GE-E3 high-pressure turbine has been employed to perform the three-dimensional simulation and the computational domain has been scaled based on the domain scaling method. Boundary conditions are consistent with operating conditions of the annular cascade testing. Circumferential averaged and realistic non-uniform interface conditions have been used to obtain steady and unsteady characteristics respectively. The validation of the turbulent model and mesh independent test has been conducted detailed in previous work. Three squealer tips, including two widths and heights, have been designed and investigated to understand its influence. Results show that the aerothermal performance of the squealer tip is remarkably influenced by the upstream passing wake. Although steady and time-averaged results have a good agreement, the variation of instantaneous heat transfer coefficient (HTC) would be over 30%, especially on the Cavity Floor region. Changing the geometry of the squealer also has different impacts on both steady and unsteady performance. The unsteady aerodynamic has relatively small fluctuation within 10%, and the distribution of steady and time-averaged leakage flow as well as total pressure loss coefficient still have a satisfactory agreement.

Heated thin film gauge arrangements to reduce uncertainty in transient heat transfer measurements

This paper describes the development of heated double-sided thin film gauge configurations for transient heat transfer measurements. By heating the substrate it is possible to measure the heat flux over a range of surface temperatures and deduce the adiabatic wall temperature and the external heat transfer coefficient. The accuracy of the measurement depends on the stability of the regression of heat flux against wall temperature and can be improved by extending the range of wall temperature over which the regression is performed. In this paper we compare two methods of local heating: double-sided gauges with an underside thin film heater and self-heating double-sided gauges. Both arrangements have been used in the Oxford Turbine Research Facility to measure the heat transfer on the uncooled turbine shroud of the MT1 high-pressure turbine stage at engine-representative conditions. These measurements yield improved regressions compared to conventional techniques to determine the adiabatic wall temperature and the heat transfer coefficient.