Showing papers on "Tip clearance published in 2013"

Pre-Stall Behavior of a Transonic Axial Compressor Stage Via Time-Accurate Numerical Simulation

Abstract: CFD calculations using high-performance parallel computing were conducted to simulate the pre-stall flow of a transonic compressor stage, NASA compressor Stage 35. The simulations were run with a full-annulus grid that models the 3D, viscous, unsteady blade row interaction without the need for an artificial inlet distortion to induce stall. The simulation demonstrates the development of the rotating stall from the growth of instabilities. Pressure-rise performance and pressure traces are compared with published experimental data before the study of flow evolution prior to the rotating stall. Spatial FFT analysis of the flow indicates a rotating long-length disturbance of one rotor circumference, which is followed by a spike-type breakdown. The analysis also links the long-length wave disturbance with the initiation of the spike inception. The spike instabilities occur when the trajectory of the tip clearance flow becomes perpendicular to the axial direction. When approaching stall, the passage shock changes from a single oblique shock to a dual-shock, which distorts the perpendicular trajectory of the tip clearance vortex but shows no evidence of flow separation that may contribute to stall.

An optical fiber bundle sensor for tip clearance and tip timing measurements in a turbine rig.

Abstract: When it comes to measuring blade-tip clearance or blade-tip timing in turbines, reflective intensity-modulated optical fiber sensors overcome several traditional limitations of capacitive, inductive or discharging probe sensors. This paper presents the signals and results corresponding to the third stage of a multistage turbine rig, obtained from a transonic wind-tunnel test. The probe is based on a trifurcated bundle of optical fibers that is mounted on the turbine casing. To eliminate the influence of light source intensity variations and blade surface reflectivity, the sensing principle is based on the quotient of the voltages obtained from the two receiving bundle legs. A discrepancy lower than 3% with respect to a commercial sensor was observed in tip clearance measurements. Regarding tip timing measurements, the travel wave spectrum was obtained, which provides the average vibration amplitude for all blades at a particular nodal diameter. With this approach, both blade-tip timing and tip clearance measurements can be carried out simultaneously. The results obtained on the test turbine rig demonstrate the suitability and reliability of the type of sensor used, and suggest the possibility of performing these measurements in real turbines under real working conditions.

Experimental Investigation of Centrifugal Compressor Stabilization Techniques

Abstract: Results from a series of experiments to investigate techniques for extending the stable flow range of a centrifugal compressor are reported. The research was conducted in a high-speed centrifugal compressor at the NASA Glenn Research Center. The stabilizing effect of steadily flowing air-streams injected into the vaneless region of a vane-island diffuser through the shroud surface is described. Parametric variations of injection angle, injection flow rate, number of injectors, injector spacing, and injection versus bleed were investigated for a range of impeller speeds and tip clearances. Both the compressor discharge and an external source were used for the injection air supply. The stabilizing effect of flow obstructions created by tubes that were inserted into the diffuser vaneless space through the shroud was also investigated. Tube immersion into the vaneless space was varied in the flow obstruction experiments. Results from testing done at impeller design speed and tip clearance are presented. Surge margin improved by 1.7 points using injection air that was supplied from within the compressor. Externally supplied injection air was used to return the compressor to stable operation after being throttled into surge. The tubes, which were capped to prevent mass flux, provided 9.3 points of additional surge margin over the baseline surge margin of 11.7 points.

Blade track assembly with turbine tip clearance control

Abstract: A turbine shroud or blade track assembly adapted to extend around a turbine wheel assembly is disclosed. The turbine shroud includes a carrier and a blade track coupled to the carrier. The blade track is movable between a radially-inward position having a first inner diameter and a radially-outward position having a second inner diameter larger than the first inner diameter.

Experimental Investigation of the Flow Field in a Transonic, Axial Flow Compressor with Respect to the Development of Blockage and Loss

Abstract: A detailed experimental investigation to understand and quantify the development of loss and blockage in the flow field of a transonic, axial flow compressor rotor has been undertaken Detailed laser anemometer measurements were acquired upstream, within, and downstream of a transonic, axial compressor rotor operating at design and off-design conditions The rotor was operated at 100%, 85%, 80%, and 60% of design speed which provided inlet relative Mach numbers at the blade tip of 148, 126, 118, and 089 respectively At design speed the blockage is evaluated ahead of the rotor passage shock, downstream of the rotor passage shock, and near the trailing edge of the blade row The blockage is evaluated in the core flow area as well as in the casing endwall region Similarly at pm speed conditions for the cases of (1) where the rotor passage shock is much weaker than that at design speed and (2) where there is no rotor passage shock, the blockage and loss are evaluated and compared to the results at design speed Specifically, the impact of the rotor passage shock on the blockage and loss development, pertaining to both the shock/boundary layer interactions and the shock/tip clearance flow interactions, is discussed In addition, the blockage evaluated from the experimental data is compared to (1) an existing correlation of blockage development which was based on computational results, and (2) computational results on a limited basis The results indicate that for this rotor the blockage in the endwall region is 2-3 times that of the core flow region and the blockage in the core flow region more than doubles when the shock strength is sufficient to separate the suction surface boundary layer The distribution of losses in the care flow region indicate that the total loss is primarily comprised of the shock loss when the shock strength is not sufficient to separate the suction surface boundary layer However, when the shock strength is sufficient to separate the suction surface boundary layer, the profile loss is comparable to the shock loss and can exceed the shock loss

Rate-Based Model Predictive Control of Turbofan Engine Clearance

Abstract: An innovative model predictive control strategy is developed for a rapid-response, closed-loop active clearance control application, in which the objectives are to tightly regulate turbine blade tip clearances and also anticipate and avoid detrimental blade-shroud rub occurrences by optimally maintaining a predefined minimum clearance. At the heart of the controller is a rate-based linear parameter-varying model of a turbofan engine that extends performance to transient regimes in which conventional controllers begin to degrade. Engine-in-the-loop simulations of this rate-critical tip clearance control system with a variety of different actuators and uncertainty modes are presented, demonstrating the efficacy and versatility of this approach. Comparisons are made with a conventional linear quadratic control approach, where it is shown that substantial clearance gap reductions are possible by incorporating the strategy explored in this paper, thereby maximizing the cycle benefits that the tip clearance actuation/sensing hardware is capable of producing. Based on the results, it is concluded that the new strategy has promise for this and other nonlinear aerospace applications that place high importance on attaining strict control objectives during transient regimes.

The Influence of Tip Clearance Momentum Flux on Stall Inception in a High-Speed Axial Compressor

Abstract: Experimental and numerical studies were conducted to investigate tip-leakage flow and its relationship to stall in a transonic axial compressor. The computational fluid dynamics (CFD) results were used to identify the existence of an interface between the approach flow and the tip-leakage flow. The experiments used a surface-streaking visualization method to identify the time-averaged location of this interface as a line of zero axial shear stress at the casing. The axial position of this line, denoted x(zs), moved upstream with decreasing flow coefficient in both the experiments and computations. The line was consistently located at the rotor leading edge plane at the stalling flow coefficient, regardless of inflow boundary condition. These results were successfully modeled using a control volume approach that balanced the reverse axial momentum flux of the tip-leakage flow with the momentum flux of the approach fluid. Nonuniform tip clearance measurements demonstrated that movement of the interface upstream of the rotor leading edge plane leads to the generation of short length scale rotating disturbances. Therefore, stall was interpreted as a critical point in the momentum flux balance of the approach flow and the reverse axial momentum flux of the tip-leakage flow.

Testing of a Microwave Blade Tip Clearance Sensor at the NASA Glenn Research Center

Abstract: The development of new active tip clearance control and structural health monitoring schemes in turbine engines and other types of rotating machinery requires sensors that are highly accurate and can operate in a high-temperature environment. The use of a microwave sensor to acquire blade tip clearance and tip timing measurements is being explored at the NASA Glenn Research Center. The microwave blade tip clearance sensor works on principles that are very similar to a short-range radar system. The sensor sends a continuous microwave signal towards a target and measures the reflected signal. The phase difference of the reflected signal is directly proportional to the distance between the sensor and the target being measured. This type of sensor is beneficial in that it has the ability to operate at extremely high temperatures and is unaffected by contaminants that may be present in turbine engines. The use of microwave sensors for this application is a new concept. Techniques on calibrating the sensors along with installation effects are not well quantified as they are for other sensor technologies. Developing calibration techniques and evaluating installation effects are essential in using these sensors to make tip clearance and tip timing measurements. As a means of better understanding these issues, the microwave sensors were used on a benchtop calibration rig, a large axial vane fan, and a turbofan. Background on the microwave tip clearance sensor, an overview of their calibration, and the results from their use on the axial vane fan and the turbofan will be presented in this paper.

Blade Tip Shape Optimization for Enhanced Turbine Aerothermal Performance

Abstract: In high-speed unshrouded turbines tip leakage flows generate large aerodynamic losses and intense unsteady thermal loads over the rotor blade tip and casing. The stage loading and rotational speeds are steadily increased to achieve higher turbine efficiency, and hence the overtip leakage flow may exceed the transonic regime. However, conventional blade tip geometries are not designed to cope with supersonic tip flow velocities. A great potential lays in the modification and optimization of the blade tip shape as a means to control the tip leakage flow aerodynamics, limit the entropy production in the overtip gap, manage the heat load distribution over the blade tip and improve the turbine efficiency at high stage loading coefficients.The present paper develops an optimization strategy to produce a set of blade tip profiles with enhanced aerothermal performance for a number of tip gap flow conditions. The tip clearance flow was numerically simulated through two-dimensional compressible Reynolds-Averaged Navier-Stokes (RANS) calculations that reproduce an idealized overtip flow along streamlines. A multi-objective optimization tool, based on differential evolution combined with surrogate models (artificial neural networks), was used to obtain optimized 2D tip profiles with reduced aerodynamic losses and minimum heat transfer variations and mean levels over the blade tip and casing. Optimized tip shapes were obtained for relevant tip gap flow conditions in terms of blade thickness to tip gap height ratios (between 5 and 25), and blade pressure loads (from subsonic to supersonic tip leakage flow regimes) imposing fixed inlet conditions. We demonstrated that tip geometries which perform superior in subsonic conditions are not optimal for supersonic tip gap flows. Prime tip profiles exist depending on the tip flow conditions. The numerical study yielded a deeper insight on the physics of tip leakage flows of unshrouded rotors with arbitrary tip shapes, providing the necessary knowledge to guide the design and optimization strategy of a full blade tip surface in a real 3D turbine environment.Copyright © 2013 by ASME

Effects of Tip Clearance on the Stall Inception Process in an Axial Compressor Rotor

Abstract: The paper presents experimental and numerical studies on the effects of tip clearance on the stall inception process in a low-speed axial compressor rotor with a large tip clearance. It has been revealed that in the small tip clearance case, shortly after the spike disturbance which results from the leading-edge separation near the rotor tip appears, the tornado-like vortex is generated by the separation, and soon the compressor falls into stall. In the large tip clearance case, the experiment showed that the performance characteristic differs from that in the small tip clearance case at near-stall conditions. This implies that the stall inception process differs with the tip clearance size. The flow phenomenon in the stall inception leading to such difference has been investigated in this study.Pressure and velocity fields which were ensemble-averaged and phase-locked by the periodic multi-sampling technique were measured on the casing wall and downstream of the rotor, respectively. In addition, to capture the unsteady flow phenomena inside the rotor, “Instantaneous Casing Pressure Field Measurement” was carried out: instantaneous casing pressure fields in one rotor passage region were measured by 30 high response pressure transducers mounted on the casing wall. In order to investigate further details of near-stall flow field for the large tip clearance, DES (Detached Eddy Simulation) has been conducted using a computational mesh with 120 million points. The results are compared with those from previous studies for the small tip clearance.As expected, the measurement results show notable differences in the near-stall flow field between the two tip clearance cases. The results from the casing pressure measurement show that high pressure fluctuation appears on the pressure side near the rotor leading-edge in the large tip clearance case. In the result of the velocity field measurement downstream of the rotor, high turbulence intensity is found near the casing in the large tip clearance case. The numerical results reveal that the vortex breakdown occurs in the tip leakage vortex and induces the oscillation of the tip leakage vortex with its unsteady nature. The flow phenomena confirmed in the experimental results are clearly explained by considering the breakdown of the tip leakage vortex. The vortex breakdown gives rise to not only large blockage but also the rotating disturbance through the interaction of the fluctuating tip leakage vortex with the pressure surface of the adjacent blade, and governs the stall inception process.Copyright © 2013 by ASME

Experimental and Numerical Investigation of the Tip Clearance Noise of an Axial Fan

Abstract: The aerodynamic and aeroacoustic performance of axial fans are strongly affected by the unavoidable tip clearance. Two identical fan impellers but with different tip clearance ratio were investigated. Unsteady wall pressure fluctuations in the tip region of the rotating blades and on the interior wall of the duct type shroud and the overall sound radiated were analysed by an unsteady numerical Scale-Adaptive Simulation (SAS) and unsteady surface pressure measurements in both, the stationary and rotating system. Based on SAS-predicted pressure fluctuations on the blade surfaces the acoustic analogy according to Ffowcs Williams and Hawkings (FWH) was employed to calculate the sound pressure in the far field.In general, experimentally and numerically determined unsteady flow were found to be a tendentially good agreement. The spatial and temporal structure of the tip vortex system and the resulting unsteady pressure distribution on the surfaces in the vicinity of the blade tips was revealed in good detail. The vortices’ strength and trajectories as well as the unsteadiness are controlled by the size of the tip clearance and the operating point: As tip clearance is increased blade/vortex interaction becomes more prevalent and with it the unsteady surface pressure and eventually the sound radiated into the far field. The broadband tip clearance noise was acceptably predicted from the simulation results, while the prediction at discrete frequency should still be improved in the further work.Copyright © 2013 by ASME

Tip clearance control method

Abstract: Disclosed is a method of controlling tip clearance of rotor blades from a surrounding casing in a gas turbine engine and an associated clearance control system. The method comprises the steps of performing a first run of the gas turbine engine, shutting down the gas turbine engine and starting the gas turbine engine for a second run. At least one performance parameter, such as the air temperature in a cavity in the turbine engine, is measured during or after the first engine run but before air within the gas turbine engine, which is soaked to the approximate temperature of one or more surrounding engine components, is substantially displaced as a consequence of the second run. The thermal condition of at least part of the engine in accordance with the measured performance parameters is then recorded. Finally the recorded thermal condition is retrieved and the temperature of the casing is controlled during at least part of the second run in a manner dependent on it. The method allows for the air temperature used to control the diameter of the engine casing to take in to account any residual heat in the engine from a previous engine run.

Tip Clearance Effects on Cavitation Evolution and Head Breakdown in Turbopump Inducer

Abstract: The objectives of the present study were to investigate the effects of tip clearance on cavitation performance and flow characteristics in a turbopump inducer by using computational fluid dynamics Three different tip clearances were analyzed under design (Qd) and off-design (08Qd and 12Qd) cavitating conditions The Rayleigh–Plesset model was implemented in ANSYS CFX 130 by using rate equation controlling vapor generation and condensation in the context of two-phase one-fluid analysis to calculate the cavitating flows Numerical results in this study were validated by comparison with experimental results for suction performance Cavitation inception occurs at the leading edge of the blade tip For high cavitation numbers, the static pressure under cavitating conditions is almost the same as that under noncavitating conditions because tip vortex cavitation and tip leakage vortex cavitation do not affect the flow significantly or deteriorate the overall performance Tip vortex cavitation and tip leakage

Effects of Tip Clearance Gap and Exit Mach Number on Turbine Blade Tip and Near-Tip Heat Transfer

Abstract: This study focuses on local heat transfer characteristics on the tip and near-tip regions of a turbine blade with a flat tip, tested under transonic conditions in a stationary, 2-D linear cascade with high freestream turbulence. The experiments were conducted at the Virginia Tech transonic blow-down wind tunnel facility. The effects of tip clearance and exit Mach number on heat transfer distribution were investigated on the tip surface using a transient infrared thermography technique. In addition, thin film gages were used to study similar effects in heat transfer on the near-tip regions at 94% height based on engine blade span of the pressure and suction sides. Surface oil flow visualizations on the blade tip region were carried-out to shed some light on the leakage flow structure. Experiments were performed at three exit Mach numbers of 0.7, 0.85, and 1.05 for two different tip clearances of 0.9% and 1.8% based on turbine blade span. The exit Mach numbers tested correspond to exit Reynolds numbers of 7.6 × 105, 9.0 × 105, and 1.1 × 106 based on blade true chord. The tests were performed with a high freestream turbulence intensity of 12% at the cascade inlet.Results at 0.85 exit Mach showed that an increase in the tip gap clearance from 0.9% to 1.8% translates into a 3% increase in the average heat transfer coefficients on the blade tip surface. At 0.9% tip clearance, an increase in exit Mach number from 0.85 to 1.05 led to a 39% increase in average heat transfer on the tip. High heat transfer was observed on the blade tip surface near the leading edge, and an increase in the tip clearance gap and exit Mach number augmented this near-leading edge tip heat transfer. At 94% of engine blade height on the suction side near the tip, a peak in heat transfer was observed in all test cases at s/C = 0.66, due to the onset of a downstream leakage vortex, originating from the pressure side. An increase in both the tip gap and exit Mach number resulted in an increase, followed by a decrease in the near-tip suction side heat transfer. On the near-tip pressure side, a slight increase in heat transfer was observed with increased tip gap and exit Mach number. In general, the suction side heat transfer is greater than the pressure side heat transfer, as a result of the suction side leakage vortices.Copyright © 2013 by ASME

Experimental study of the effect of blade tip clearance and blade angle error on the performance of mixed-flow pump

Abstract: The hydraulic performance test of the mixed-flow pump has been carried out through selecting different blade tip clearances and various blade angle errors. The ratio of the mixed-flow pump efficiency reduction and the blade tip clearance variation (Δη/Δδ) varies with the flow rate coefficient ϕ revealing a parabolic trend. An empirical equation has been developed for the mixed-flow pump model by parabolic fitting. For the same blade tip clearance variation Δδ, the mixed-flow pump efficiency reduction Δη increases rapidly as the flow rate rises. For any given flow rate, the efficiency, the head and the shaft power of the mixed-flow pump all decrease with the increase of the blade tip clearance. Among them, the efficiency reduction Δη varies approximately linearly with the blade tip clearance variation Δδ. When the angle of an individual blade of the mixed-flow pump has a deviation, the performance curves will move and change. These curves have consistent change directions with the performance curves under the condition of all the blades rotated at the same time, but have smaller offset and lower range of variation. When an individual blade angle error changes to ±2°, the optimal efficiency of the mixed-flow pump will have no significant difference. When the individual blade angle error increases to ±4°, the optimal efficiency will decrease by 1%.

Dynamic Characteristics of Rotating Stall in Mixed Flow Pump

Abstract: Rotating stall, a phenomenon that causes flow instabilities and pressure hysteresis by propagating at some fraction of the impeller rotational speed, can occur in centrifugal impellers, mixed impellers, radial diffusers, or axial diffusers. Despite considerable efforts devoted to the study of rotating stall in pumps, the mechanics of this phenomenon are not sufficiently understood. The propagation mechanism and onset of rotating stall are not only affected by inlet flow but also by outlet flow as well as the pressure gradient in the flow passage. As such, the complexity of these concepts is not covered by the classical explanation. To bridge this research gap, the current study investigated prerotation generated at the upstream of the impeller, leakage flow at the tip clearance between the casing and the impeller, and strong reserve flow at the inlet of the diffuser. Understanding these areas will clarify the origin of the positive slope of the head-flow performance curve for a mixed flow pump. Nonuniform pressure distribution and adverse pressure gradient were also introduced to evaluate the onset and development of rotating stall within the diffuser.

Turbine blade-mounted sensor fixture for tip gap measurement

Abstract: Turbine blade tip clearance is measured in a fully assembled turbine casing by mounting a non-contact displacement probe or sensor on a turbine blade that generates data indicative of sensor distance from the turbine casing that circumferentially surrounds the blade. The sensor is mounted on the blade with a sensor fixture, which includes a clamping mechanism and a sensor retention mechanism that retains and calibrates the sensor by selective movement of the sensor relative to the retention mechanism. Variations in sensor distance data are recorded when the turbine is operated in turning gear mode. Blade rotational position data are collected by a rotational position sensor. A data processing system correlates the distance and rotational position data with localized blade tip gap at angular positions about the turbine casing circumference. This method and apparatus facilitate assessment of turbine casing deformation impact on blade tip clearance and rotor/casing alignment.

Validation of Large-Eddy Simulation for the Prediction of Compressible Flow in an Axial Compressor Stage

Abstract: A better understanding of unsteady flows (including turbulence) is a necessary step towards a breakthrough in the design of modern gas turbine components. LES emerges as a very promising method to improve both knowledge of complex physics and reliability of flow solver predictions. However, there is still a lack of evidences in the literature that LES is applicable for rotor/stator configurations at industrial Reynolds numbers. In that context, the objective of the present work is to investigate the capability of LES to predict the turbulent flow in a stage of an axial compressor. The studied configuration operates at conditions relevant to industrial applications (M = 0.5, Re = 5 × 105). The whole 3D flow is considered, including the rotor tip clearance, as well as the natural periodicity of the compressor (i.e. the exact ratio between the number of rotor blades and stator vanes). First a validation of LES is obtained by comparing results on different mesh densities, in order to evaluate the grid requirements necessary to perform a wall-resolved LES (the finest grid is made of o(109) points to represent 3 rotor and 4 stator passages). Typical mesh scales are compared to the Kolmogorov length scale to estimate the regions where the subgrid scale model acts. Results obtained with both URANS and LES are then compared at nominal operating conditions. The analysis focuses on two dataset: time-averaged quantities (boundary layer profiles, turbulent kinetic energy, etc). and unsteady flow data. An evaluation of the flow physics shows that URANS predicts much more losses than LES in this configuration (discrepancy on efficiency is about 6%). The analysis indicates that LES predicts a laminar to turbulent transition at 50% of the rotor chord, leading to thinner boundary layers in both rotor and stator than with URANS. Actually LES also predicts the development of high-energy frequencies in the tip region that are uncorrelated to the blade passing frequency, related to the pulsation of the tip leakage flow.Copyright © 2013 by ASME

Numerical and experimental study of cavitating flow through an axial inducer considering tip clearance

Abstract: This paper presents three-dimensional numerical simulations and experimental investigations of cavitating flow through an axial inducer. Particularly, this work focuses on the influence of radial t...

Measurement of Blade Tip Heat Transfer and Leakage Flow in a Turbine Cascade With a Multi-Cavity Squealer Tip

Abstract: Tip leakage flow induces high heat transfer to the blade tip and causes significant aerodynamic losses. In this paper, we propose a multi-cavity squealer tip with an additional rib in the squealer cavity. Our study investigated the effects of the rib location and shape on the blade tip heat transfer and the total pressure loss. Experiments were performed in a five-bladed linear cascade using a low speed wind tunnel. The blade chord, pitch, and span length were 126mm, 102.7mm, and 160mm, respectively. The Reynolds number, based on the blade chord and cascade exit velocity, was 2.44×105, and a tip clearance of 1.25% of the blade span was considered. The additional rib was installed in the squealer tip cavity near the leading edge, the mid-chord, and the training edge, respectively. The shape of the rib was also varied from rectangular to triangular in order to minimize the rib surface area exposed to the hot gas. The secondary flow and total pressure loss were measured using a seven-hole probe at one-chord downstream of the blade trailing edge, and the heat transfer coefficient distributions were measured by utilizing the hue-detection based transient liquid crystal technique. Flow measurement results indicated that the proposed multi-cavity tip reduced the total pressure loss. The blade tip heat transfer measurement results showed that the proposed multi-cavity tip was able to reduce the maximum heat transfer region near the cavity floor near the leading edge, but the heat transfer on the second cavity floor increased due to the leakage flow reattachment.Copyright © 2013 by ASME

Situ blade mounted tip gap measurement for turbines

Abstract: Turbine blade tip clearance is measured in a fully assembled turbine casing by mounting a contact or non-contact displacement probe on a turbine blade that generates data indicative of probe distance from the turbine casing that circumferentially surrounds the blade. Variations in probe distance data are recorded as the blade circumferentially sweeps the turbine casing when the turbine is operated in turning gear mode. Blade rotational position data are collected by a rotational position sensor. A data processing system correlates the distance and rotational position data with localized blade tip gap at angular positions about the turbine casing circumference. An optical camera inspection system may be coupled to a turbine blade to obtain visual inspection information within the turbine casing. This method and apparatus provide an accurate and cost effective solution for accessing turbine casing deformation impact on blade tip clearance and rotor/casing alignment.

Influence of the Clearance Size on Rotating Instability in an Axial Compressor Stator

Abstract: In order to improve the understanding of unsteady secondary flow structures in axial turbomachines, the flow in a scaled axial compressor stator was studied. It was operated in the vicinity of the stability line and could be equipped with variable hub clearances. Results from unsteady aerodynamic and acoustic pressure measurements, high-speed Particle Image Velocimetry measurements, and turbulence resolving numerical simulations are presented. Typical characteristics of the rotating instability (RI) phenomenon were found. A parameter study varying the clearance size and the blade loading, respectively, revealed the surprising occurrence of the RI even for the stator configuration without hub clearance. This contradicts the hitherto presumption of a hub or tip clearance needed to be in place for a rotating instability to emerge

Experimental study of centrifugal compressor tip clearance and vaneless diffuser flow fields

Abstract: Three vaneless diffuser designs, varying in diffuser width, for centrifugal compressor were studied experimentally. Along with the diffuser width, the tip clearance was altered. The compressor over...

High-Speed, capacitance-based tip clearance sensing

Abstract: This paper discusses recent advances in tip clearance measurement systems for turbine engines using capacitive probes. Real time measurements of individual blade pulses are generated using wideband signal processing providing 3 dB bandwidths of typically 5 MHz. Subsequent mixed-signal processing circuitry provide real-time measurements of maximum, minimum, and average clearance with latencies of one blade-to-blade time interval. Both guarded and unguarded probe configurations are possible with the system. Calibration techniques provide high accuracy measurements.

Effects of tip clearance on performance and characteristics of backflow in a turbopump inducer

Abstract: The objectives of the present study were to investigate the effects of tip clearance on the performance and flow characteristics of a turbopump inducer by using computational fluid dynamics. Three ...

Investigation of the tip leakage flow at turbine rotor blades with squealer cavity

Abstract: Understanding of the tip leakage flow (TLF) in turbine rotors is one key aspect in the design for improving the efficiency of turbines. This requires measurements and simulations of the TLF, especially when investigating new rotor blade designs with blade tip treatments. However, flow measurements in the tip gap of a rotating machine are highly challenging because of the small gap size of about 1 mm and the high unsteadiness of the flow requiring a high temporal resolution of about 10 μs. For this purpose, an optimized non-intrusive measurement concept based on frequency modulated Doppler global velocimetry is presented, which fulfills the requirements. Three component velocity fields of the TLF were obtained in a turbine test rig at a blade passing frequency of 930 Hz. The rotor blades were equipped with a squealer tip, and the TLF in the squealer cavity region was successfully measured. The measurement agrees well with calculated results showing gradients in the tip gap above the squealer cavity. Furthermore, the development of the tip clearance vortex was resolved at the suction side of the blades.