Downstream Vane Research Articles

Analysis of The Flow-Field Distorsions Induced by Pneumatic Pressure Probes In Test Bench Experiments On Centrifugal Compressors: Are We Testing Right?

Abstract Experimental testing at the test rig still plays a key role in the development of new centrifugal compressor designs for industrial applications. Although a wide set of different probes is used as per common practice, the overall impact of the probes? dimensions on the stage performance and rangeability is often assumed to be negligible. In this study, we aimed at quantifying the intrusiveness effects of pneumatic pressure probes on the flow-field inside a vaned diffuser using a model stage of an industrial centrifugal compressor, for which different experimental setups were tested at the University of Florence test rig. Performance and rangeability were initially assessed using a conventional set of pneumatic probes inserted in the flow path. Then, comparative analyses were realized through dedicated tests by either removing all probes at the diffuser inlet or introducing only a single pressure probe in the upstream section and varying its angular position in the measuring section. Experiments revealed an impact of upstream probes much higher than expected, with a decrease in efficiency and a substantial reduction of stall margin. Moving from this experimental evidence, accurate 3D CFD simulations of the entire stage were carried out including the actual geometry of the probes for two probes configurations and three operating points. Simulations highlighted the effect probes? wakes on downstream vanes and the induced blockage effect, which both appear in line with experimental evidence. As a handout, a recommendation on the optimal probe positioning is suggested.

Forced Response Analysis of an Embedded Compressor Rotor Induced by Stator Disturbances and Rotor–Stator Interactions

Accurate predictions of the blade response in a multi-row compressor is one of the most important tasks within the design process of compressor blades. Some recent studies have shown that the decoupled method considering only the stator disturbances cannot obtain accurate results for cases with strong rotor–stator interactions, especially for the interaction between the rotor and downstream stator, and the coupled method with multi-row configurations is necessary. Factors determine what computational domains to model need to be clarified to find a balance between accuracy requirements and computational costs. To this end, this study conducted full-annulus unsteady calculations with decoupled and coupled configurations to investigate the forced response of an embedded compressor rotor induced by upstream and downstream stator disturbances and rotor–stator interactions, respectively. The results show that the upstream IGV disturbances were dominated by the wake, and the IGV and S1 potential fields had little effect on the R1 response. Meanwhile, the IGV–R1 interactions and S1–R1 interactions were dominated by one cut-on mode, respectively. The comparisons of the blade vibration amplitude and the unsteady pressure field calculated by decoupled and coupled methods revealed the mechanism of the forced response, namely, for the R1 response induced by upstream aerodynamic disturbances, the dominant excitation source was the IGV wake, and the blade vibration amplitude can be predicted by the decoupled method. In terms of the response induced by downstream disturbances, the cut-on S1-R1-interaction mode was dominant and the use of the decoupled method without considering its influence will lead to an inaccurate prediction. This study concluded that the formation process of rotor–stator interactions was the key factor that determines whether the decoupled method or coupled method should be used, and analogized a process independent of the downstream stator disturbance. The results can provide a preliminary configuration for accurate and efficient blade response predictions and explain the reason why including downstream stator vanes is very important.

Influence of Different Flow Solvers and Off-Design Conditions On the Determination of Fan-Rotor Wakes for Broadband Noise Prediction

Abstract The acoustic interaction of fan-rotor wakes with the downstream stator vanes is considered as an important noise source of an aircraft engine. The turbulence induced by the rotor generates a stochastic acoustic source that appears as broadband noise in the acoustic spectrum. During the preliminary design phase of an engine, established meanline and throughflow solvers usually do not resolve turbulence and associated unsteady flow parameters. But such solvers provide rotor pressure losses that can be used to estimate the mean and turbulent rotor wakes. A crucial step is the deduction of turbulence parameters from the mean wakes. A semi-empirical model for rotor-wake turbulence estimation is presented in this paper. The meanline method and the throughflow solver are compared to three-dimensional computational flow simulations investigating the capabilities of the different solvers to provide flow data for broadband wake interaction noise prediction. The methods are applied to a representative modern fan stage at a comprehensive number of operating points, comprising several speed lines from surge to choking conditions. Microphone measurements are consulted to assess the noise predictions. The evaluation confirms the applicability of the meanline and throughflow method in combination with the turbulence model for broadband noise estimation during the preliminary design phase. The underestimated turbulence in the tip region of the fan is found to be negligible even during off-design conditions.

Brush seal design and secondary air system modification of a heavy-duty gas turbine to improve the output power

The overall performance of heavy-duty gas turbines is strongly affected by the mass flow distribution of the secondary air system. Reducing the mass flow of the secondary air is a remarkable method to improve the performance of a gas turbine. The aim of this article is to implement a brush seal as an alternative to labyrinth seal. Based on the experiences gained from the integrity of brush seals into a gas turbine, installing a brush seal would not necessarily improve turbine performance. This is because of SAS mass flow redistribution, which may not lead to overall SAS mass flow reduction. Therefore, some other components in the SAS arrangement should be modified as controlling variables to overcome this problem. The effect of these modifications on the airflow distribution is investigated using an in-house network analysis code. To establish an optimum solution (optimized brush seal geometry and controlling variables), the network analysis code is coupled with an in-house optimization code that benefits from the Genetic Algorithms and Artificial Neural Networks. Some constraints including upstream and downstream cavity purge flow and vane cooling air are considered in the optimization process. The final network results show a 33.27% reduction in overall SAS mass flow. To ensure an improvement in the performance of the new three-stage turbine, a CFD analysis is conducted, which indicates 1.0% more power with respect to the original turbine. In the end, the aerodynamic and mechanical behavior of the brush seal is analyzed using CFD and FEM.

Investigations on the interaction between the front and aft purge flow and the downstream vane of 1.5-stage turbine

Abstract The present work reports the influence of the 1.5-stage turbine flow field by the front and aft rim seal flow. The interaction between the front and aft purge flow and the mainstream of a 1.5-stage turbine was numerically simulated, and the influence of the front and aft purge flow on the downstream vane was analyzed separately. The results show that the front purge flow is distributed at the higher radius of second vane inlet, which changes the position of the blade hub secondary flows, and the aft purge flow is distributed at the low radius. The purge flow at different locations in the aft cavity exit forms shear induced vortex, pressure and suction side legs of the egress, which converges with the suction and pressure side legs of the horse vortex to form vane hub passage vortex. The increased purge flow rate in both the front and aft cavities significantly increases the sealing effectiveness of the rim seal, but also causes a reduction in turbine efficiency. The combined effect of the front and aft purge flow reduces the turbine efficiency of the end-wall structure by 0.3619, 0.9062, 1.5004, 2.0188 and 2.509% at IR = 0, IR = 0.5%, IR = 0.9%, IR = 1.3% and IR = 1.7%.

Comparative numerical study of the dynamic motion of OWC impulse turbine in the starting process

OWC impulse turbine can be effectively used in utilizing the wave energy. The starting process of rotor is crucial in influencing the operation stability and is investigated in this study. With adding the source terms on the momentum equation considering the angular acceleration, the dynamic motion of OWC turbine rotor is numerically studied. By comparing simulation and experiment, the starting process is found with two stages including speed increasing stage and stably-variating stage. The speed of rotor pulses and the flow direction alternates due to the reciprocating piston motion. The power is generated by the pressure difference between the two sides of rotor. Local high and low velocity can be found in rotor channels and downstream vane channels. The moment of inertia of rotor and wave motion law is comparatively studied to have a better understanding. Results found that a smaller moment of inertia of rotor, a slower wave motion, a stronger wave and a longer piston movement distance will be better to stably capture the energy. The compressibility is found influential mainly on pressure field and cause the rotor speed difference. This study provides guidance on enhancing the performance and improving the design technique of OWC impulse turbines.

LES and URANS study on turbulent flow through 3 × 3 rod bundle with spacer grid and mixing vanes using spectral element method

In this paper, the turbulent flow in rod bundles with spacer grid and mixing vanes in nuclear reactor core was studied using spectral element method with large eddy simulation (LES) and unsteady RANS (URANS) method. Combining with the radial periodical boundary conditions, the turbulent mixing phenomena in central subchannels were investigated, especially focusing on the flow field after the spacer grid and mixing vanes. Other than setting up long bare rod bundle to establish the fully developed turbulent flow, only a 2-cm-long bare rod bundle was extruded upstream the spacer grid and mixing vanes, while the fully developed laminar velocity profile was applied on the inlet surface for velocity boundary settings. The results indicate that both of the models could predict time-averaged velocity at 0.5Dh, 1Dh, 4Dh downstream mixing vanes properly, while the URANS model underestimate the root-mean-square (RMS) value of fluctuating velocity due to the inlet treatment effect. Furthermore, turbulent kinetic energy contour with transverse velocity vector was combined to investigate the behaviors of vortices formed by split-type mixing vanes, revealing the behavior of vortices for preliminary dynamic and mechanic analysis in design process of fuel assembly.

Blade vibration measurement and numerical analysis of a mistuned industrial impeller in a single-stage centrifugal compressor

Understanding and predicting the dynamical behavior of the impeller structure under realistic condition is vital for resonance-free and reliable design. This paper presents a novel method for high fidelity measurement and efficient modeling of impeller blade vibration considering realistic operating environment and mistuning effect. To clarify the blade forced vibration, strain gauge and blade tip timing (BTT) measurement have been conducted. Measurement introduced mistuning and flow disturbance from strain gauges are further excluded with tip-timing. The blade mistuning is then identified and quantified with the help of parameter estimation method and updates the numerical mistuned forced response model. Aerodynamic forcing function due to downstream diffuser vanes is also numerically evaluated based on a decoupled two row configuration and time transformation method. Results shown that BTT system is well applied to capture the blade resonance response and shows a promising way for monitoring individual blade component. The aerodynamic forcing is also found to be well captured and presents a wave decay trend along the flow path towards the upstream. Finally, based on the proposed BTT parameter driven vibration prediction method, the blade mistuned forced response shows a good agreement with measurement results. Improvement also implies variations of parameters and mistuning pattern should be considered as a critical part and uncertainty source in the numerical prediction. This work is of scientific significance to enrich the knowledge of blade vibration in centrifugal compressor using tip-timing and of industrial application value to consider mistuning problems with efficient reduce-order modeling method.

Velocity rate of screw streams in the site of submerged vanes

Introduction. The formation mechanism of longitudinal screw streams (LSS) along the upstream and downstream face of submerged vanes (SV) is considered. Acting along with the artificial transversal circulation (ATC), these streams protect the water intake from the channel sediments. The intensities and directions of LSS and ATC depend on the regime of flow, the planned-geometric characteristics of the vanes. Recommendations concerning the purpose of SV’s rational characteristics in the aspect of steady formation of all three protective streams in the flow for river damless intakes are contradictory and require clarification. The purpose of the study is to analyze the velocity rate of the LSS in SV site at various planned-geometric characteristics of the vane and hydraulic modes of its operation based on a physical model with an erosion-resistant channel (without water separation), as well as to determine an efficient range of setting angles of the SV’s to the tray board in terms of formation of steady and intensive LSS along the upstream and downstream face of the vane.
 Materials and methods. Model physical hydraulic studies and theoretical calculations were used. Five hydraulic modes of SV’s operation were studied, with various planned-geometric characteristics, using the physical model with an erosion-resistant channel (without water separation). The obtained experimental data were summarized and analyzed.
 Results. Results of laboratory hydraulic studies of LSS velocity rate in LSS site were presented. Experimental graphic dependence diagrams were plotted characterizing the intensity and direction of the LSS along upstream and downstream faces of the vane.
 Conclusions. A determining influence of the setting angle of the vane to the tray board (bank line) on the intensity and direction of the LSS in SV’s site was found out. Experimentally, an efficient SV setting angle was determined in terms of the formation of steady and intensive LSS along the upstream and the downstream vane face with practically usable direction.

Rotor–Stator Interactions in a 2.5-Stage Axial Compressor—Part II: Impact of Aerodynamic Modeling on Forced Response

Abstract The main objective of this study is the validation of numerical forced response predictions through experimental blade vibration measurements for higher order modes of a blade-integrated disk (blisk). To this end, a linearized and a nonlinear frequency domain CFD methods are used, as well as a tip timing measurement system. The focus is on the blade excitation by downstream vanes, in particular, because this study shows that the correct prediction of acoustic modes is of key importance in this case. The analysis of these modes is presented, both experimentally and numerically, in Part I of this publication. The grid independence study for the aerodynamic work on the blade surface conducted in this part shows a possible prediction uncertainty of more than 100% when a coarse grid is chosen. For the validation of the numerical setup, a study was performed using different turbulence and transition models. The results are compared to the measured performance map, to a 2D field traverse conducted with a pneumatic probe, and to data gained by unsteady pressure sensors mounted in the casing of the compressor. Flow features relevant for the prediction of blade stresses are best represented using the SST turbulence model in combination with the γ–ReΘ transition model. Nonlinear simulations with this setup are able to predict the blade stresses due to downstream excitation with an average difference of 23% compared to tip timing measurements. Single row linearized CFD methods have shown to be incapable of making a correct stress prediction when acoustic modes form a major part of the exciting mechanisms. In summary, this two-part publication proves the importance of acoustic rotor–stator interactions for blade vibrational stresses excited by downstream vanes in a state-of-the-art high-pressure compressor.

Rotor–Stator Interactions in a 2.5-Stage Axial Compressor—Part I: Experimental Analysis of Tyler–Sofrin Modes

Abstract This two-part paper investigates the influence of rotor–stator interactions on the blade vibrational stresses of the first rotor, excited by the downstream stator. To this end, aeroacoustic and aeroelastic measurements and numerical setup studies for the solver TRACE are conducted in order to improve the predictive accuracy of blade vibrational stresses. Part I compares tip timing data for resonance crossings of three blisk modes to numerical predictions. Due to the single-row analysis within the linearized version of the flow solver TRACE, unsteady rotor–stator interactions are excluded by default. The findings show that leaving out these interactions in the numerical setup can lead to 97% lower vibrational stress predictions with respect to the absolute value measured. To validate the prediction of rotor–stator interactions by the nonlinear frequency domain method of TRACE, unsteady pressure measurements were conducted at the casing in the inter-row section of the first stage. The results were analyzed using an optimized measuring grid and applying a compressed sensing-based azimuthal mode analysis. Predicted azimuthal mode numbers are in accordance with the experiment, whereas amplitudes deviate from the measurements in part. Part II focuses on the prediction of blade vibrational stresses. To this end, a detailed grid study is performed and comparisons to steady and unsteady measurement data are made. In summary, this two-part paper confirms the importance of rotor–stator interactions for blade vibrational stresses excited by downstream vanes at a state-of-the-art high-pressure compressor.

Large eddy simulation of transient upstream/downstream vortex interactions

Experimentally validated large eddy simulations were performed on two NACA0012 vanes at various lateral offsets to observe the transient effects of the near field interactions between two streamwise vortices. The vanes were separated in the streamwise direction, allowing the upstream vortex to impact on the downstream geometry. These vanes were evaluated at an angle of incidence of $8^{\circ }$ and a Reynolds number of 70 000, with rear vane angle reversed to create a co-rotating or counter-rotating vortex pair. The downstream vortex merged with the upstream in the co-rotating condition, driven by the suppression of one of the tip vortices of the downstream vane. At close proximity to the pressure side, the vane elongated the upstream vortex, resulting in it being the weakened and merging into the downstream vortex. This produced a transient production of bifurcated vortices in the wake region. The downstream vortex of the co-rotating pair experienced faster meandering growth, with position oscillations equalising between the vortices. The position oscillation was determined to be responsible for statistical variance in the merging location, with variation in vortex separation causing the vortices at a single plane to merge and separate in a time-dependent manner. In the counter-rotating condition position oscillations were found to be larger, with higher growth, but less uniform periodicity. It was found that the circulation transfer between the vortices was linked to the magnitude of their separation, with high separation fluctuations weakening the upstream vortex and strengthening the downstream vortex. In the case of upstream vortex impingement on the downstream vane, the upstream vortex was found to bifurcate, with a four vortex system being formed by interactions with the shear layer. This eventually resulted in a single dominant vortex, which did not magnify its oscillation amplitudes as it travelled downstream due to the destruction of the interacting vortices.

Large Eddy Simulation of a scale-model turbofan for fan noise source diagnostic

A wall-modeled statistically converged Large Eddy Simulation (LES) of the turbulent flow in the NASA Source Diagnostic Test turbofan has been successfully performed for the first time. A good agreement with aerodynamic measurements is observed for both Reynolds Averaged Navier-Stokes and LES results, although the LES provides better results in the tip regions where large coherent structures appear and no flow separation on the stator vanes is observed. In the LES the boundary layer naturally transition to turbulence on the blade suction side but remains quasi laminar over most of its pressure side. The rotor-wake turbulence yielding the stage broadband noise is then seen to be quasi isotropic. Transition on the downstream stator vanes is not triggered by the wake impingement but rather occurs at mid-chord. Finally, acoustics are investigated using both Ffowcs Williams & Hawkings' and Goldstein's analogies from the recorded LES noise source on the stator vanes. The latter analogy provides levels closer to the measurements especially at high frequencies, although the results are most likely still influenced by too coherent rotor tip secondary flow at low frequencies.

Effects of Downstream Vane Bowing and Asymmetry on Unsteadiness in a Transonic Turbine

Accurate predictions of unsteady forcing on turbine blades are essential for the avoidance of high-cycle-fatigue issues during turbine engine development. Further, if one can demonstrate that predictions of unsteady interaction in a turbine are accurate, then it becomes possible to anticipate resonant-stress problems and mitigate them through aerodynamic design changes during the development cycle. A successful reduction in unsteady forcing for a transonic turbine with significant shock interactions due to downstream components is presented here. A pair of methods to reduce the unsteadiness was considered and rigorously analyzed using a three-dimensional (3D), time-resolved Reynolds-Averaged Navier-Stokes (RANS) solver. The first method relied on the physics of shock reflections itself and involved altering the stacking of downstream components to achieve a bowed airfoil. The second method considered was circumferentially asymmetric vane spacing which is well known to spread the unsteadiness due to vane-blade interaction over a range of frequencies. Both methods of forcing reduction were analyzed separately and predicted to reduce unsteady pressures on the blade as intended. Then, both design changes were implemented together in a transonic turbine experiment and successfully shown to manipulate the blade unsteadiness in keeping with the design-level predictions. This demonstration was accomplished through comparisons of measured time-resolved pressures on the turbine blade to others obtained in a baseline experiment that included neither asymmetric spacing nor bowing of the downstream vane. The measured data were further compared to rigorous post-test simulations of the complete turbine annulus including a bowed downstream vane of nonuniform pitch.

A Comprehensive Investigation of Blade Row Interaction Effects on Stator Loss Utilizing Vane Clocking

Blade row interactions drive the unsteady performance of high-pressure compressors. Vane clocking is the relative circumferential positioning of consecutive stationary vane rows with the same vane count. By altering the upstream vane wake's path with respect to the downstream vane, vane clocking changes the blade row interactions and results in a change in steady total pressure loss on the downstream vane. The open literature lacks a conclusive discussion of the flow physics governing these interactions in compressors. This paper presents the details of a comprehensive vane clocking study on the embedded stage of the Purdue three-stage axial compressor. The steady loss results, including radial total pressure profiles and surface flow visualization, suggest a shift in the stator 2 corner separations occurs between clocking configurations associated with the maximum and minimum total pressure loss. To better understand the flow mechanisms driving the vane clocking effects on the steady stator 2 performance, time-resolved interrogations of the stator 2 inlet flow field, surface pressure unsteadiness, and boundary layer response were conducted. The stator 2 surface flows, both pressure unsteadiness and boundary layer transition, are influenced by vane clocking and interactions between rotor 1 and rotor 2, but neither of these results indicate a cause for the change in steady total pressure loss. Moreover, they are a result of upstream changes in the flow field: the interaction between the stator 1 wake and rotor 2 results in a circumferentially varying pattern which alters the inlet flow field for the downstream row, including the unsteadiness and frequency content in the tip and hub regions. Therefore, under different clocking configurations, stator 2 experiences significantly different inlet blockage and unsteadiness from the rotor 2 tip leakage flow and hub corner separation, which, in turn, shifts the radial blade loading distribution and subsequent loss development of stator 2.

Aerodynamic performance of an un-located high-pressure turbine rotor

ABSTRACTThe rotor sub-assembly of the high-pressure turbine of a modern turbofan engine is typically free to move downstream because of the force imbalance acting on the disc and blades following an un-located shaft failure. This downstream movement results in a change in the geometry of the rotor blade, tip seals and rim/platform seals because of the interaction of the rotor sub-assembly with the downstream vane sub-assembly. Additionally, there is a change in the leakage flow properties, which mix with the main flow because of the change in engine behaviour and secondary air system dynamics. In the present work, the changes in geometry following the downstream movement of the turbine, are obtained from a validated friction model and structural LS-DYNA simulations. Changes in leakage flow properties are obtained from a transient network source-sink secondary air system model. Three-dimensional Reynolds-averaged Navier-Stokes simulations are used to evaluate the aerodynamic effect from the inclusion of the leakage flows, tipseal domains, and downstream movement of the rotor for three displacement configurations (i.e. 0, 10 and 15 mm) with appropriate changes in geometry and leakage flow conditions. It is observed from the results that there is a significant reduction in the expansion ratio, torque and power produced by the turbine with the downstream movement of the rotor because of changes in the flow behaviour for the different configurations. These changes in turbine performance parameters are necessary to accurately predict the terminal speed of the rotor using an engine thermodynamic model. Further, it is to be noted that such reductions in turbine rotor torque will result in a reduction of the terminal speed attained by the rotor during an un-located shaft failure. Therefore the terminal speed of the rotor can be controlled by introducing design features that will result in the rapid rearward displacement of the turbine rotor.

Interactions of a counter-rotating vortex pair at multiple offsets

The interactions between two streamwise vortices were investigated by wind tunnel testing of two NACA0012 vanes at various lateral offsets. One vane was spaced 10 chord lengths (C) downstream of the other, with both at an angle of incidence of 8 degrees and a Reynolds number of 7×104. The evolution of the vortex pair was observed until 6.5C behind the downstream vane using Particle Image Velocimetry (PIV). It was found that proximity of the upstream vortex to the downstream vane had a significant effect on the rotational rate of the subsequent vortex pair, with far offset cases having little rotation, and near field cases having angle changes of 19.6 degrees per chord length travelled downstream. At the point of vortex impingement on the downstream vane, the rotational rate dropped to near zero due to a significant strength reduction of both vortices. The point of strongest interaction was found to be laterally offset from the point of closest vortex proximity to the downstream vane by −0.15C, with the vortex on the suction side of the vane. In the offset range investigated, a significant instability was observed in only the upstream vortex. These instabilities increased as the proximity between the vortices decreased, peaking where the vortex interaction was strongest.

Blade-Row Interaction Effects on Unsteady Stator Loading in an Embedded Compressor Stage

Blade-row interactions drive unsteady blade forces in compressors. This paper presents a perspective on understanding how the pitchwise variations in the flow at the exit of the rotor affect the surface pressures on the downstream vane. The rotor wakes and tip leakage flows are the primary unsteady flow features that drive unsteady lift on the vane. However, those rotor flow features are affected by their interaction with the wakes from upstream vanes. Thus, the interaction with stator 1 and rotor 2 must be understood to adequately characterize the interaction between rotor 2 and stator 2. This paper uses vane clocking, or the circumferential shift in successive vane rows of similar counts, to illuminate these blade-row interaction effects on the downstream vane surface pressure distribution. To accomplish this, experiments are performed in a three-stage axial compressor where high-frequency response pressure transducers are flush-mounted in the stator 2 pressure and suction surfaces are flush-mounted at 50 and 80% spans. Results show that rotor 1–rotor 2 interactions contribute significantly to the changes in unsteady stator surface pressure over the course of a rotor revolution. Also, the rotor 1–rotor 2 interaction levels change in the pitchwise direction downstream of the rotor; thus, the changes in surface pressure are affected by vane clocking. Furthermore, the rotor tip leakage flow is an additional contributor to the unsteady stator surface pressures measured at 80% span, providing an additional pressure peak per blade passing, and thus more high-frequency content (twice the blade-passing frequency).

Experimental Characterization of Tip Leakage Flow Trajectories in a Multistage Compressor

Fast-response pressure measurements collected in a three-stage axial compressor highlight the development of the rotor tip leakage flow. Data collected from an array of high-frequency-response pressure transducers measure time-resolved static pressure over the rotors at several loading conditions for three tip clearance heights. The influence of surrounding vane rows on the rotor tip leakage flow is investigated by adjusting the position of the vanes with respect to fixed sensor positions. One key result is that the wake from the upstream vane creates a modulation of the leakage flow. However, the upstream propagating potential field from the downstream vane row has no measureable impact on the leakage flow trajectory. In some cases, variations of the leakage flow trajectory angle due to these blade row interactions are greater than the differences due to a doubling of the rotor tip clearance height, an important finding not previously reported in the literature. Differences of the leakage flow trajectory trends with tip clearance are identified for a high loading condition compared to other operating points. Based on these observations, alternate methods are introduced for achieving collapsed leakage flow trajectories with changing tip clearance in two different flow ranges along the speed line.

Unsteady Aerodynamic Interaction in a Closely Coupled Turbine Consistent With Contrarotation

The focus of the study presented here was to investigate the interaction between the blade and downstream vane of a stage-and-one-half transonic turbine via computation fluid dynamic (CFD) analysis and experimental data. A Reynolds-averaged Navier–Stokes (RANS) flow solver with the two-equation Wilcox 1998 k–ω turbulence model was used as the numerical analysis tool for comparison with all of the experiments conducted. The rigor and fidelity of both the experimental tests and numerical analysis methods were built through two- and three-dimensional steady-state comparisons, leading to three-dimensional time-accurate comparisons. This was accomplished by first testing the midspan and quarter-tip two-dimensional geometries of the blade in a linear transonic cascade. The effects of varying the incidence angle and pressure ratio on the pressure distribution were captured both numerically and experimentally. This was used during the stage-and-one-half post-test analysis to confirm that the target corrected speed and pressure ratio were achieved. Then, in a full annulus facility, the first vane itself was tested in order to characterize the flowfield exiting the vane that would be provided to the blade row during the rotating experiments. Finally, the full stage-and-one-half transonic turbine was tested in the full annulus cascade with a data resolution not seen in any studies to date. A rigorous convergence study was conducted in order to sufficiently model the flow physics of the transonic turbine. The surface pressure traces and the discrete Fourier transforms (DFT) thereof were compared to the numerical analysis. Shock trajectories were tracked through the use of two-point space–time correlation coefficients. Very good agreement was seen when comparing the numerical analysis to the experimental data. The unsteady interaction between the blade and downstream vane was well captured in the numerical analysis.