Showing papers on "Tip clearance published in 2015"

Numerical and Experimental Investigation of Tip Leakage Vortex Cavitation Patterns and Mechanisms in an Axial Flow Pump

Abstract: The tip leakage vortex (TLV) cavitating flow in an axial flow pump was simulated based on an improved shear stress transport (SST) k-ω turbulence model and the homogeneous cavitation model. The generation and dynamics of the TLV cavitation throughout the blade cascades at different cavitation numbers were investigated by the numerical and experimental visualizations. The investigation results show that the corner vortex cavitation in the tip clearance is correlated with the reversed flow at the pressure side (PS) corner of blade, and TLV shear layer cavitation is caused by the interaction between the wall jet flow in the tip and the main flow in the impeller. The TLV cavitation patterns including TLV cavitation, tip corner vortex cavitation, shear layer cavitation, and blowing cavitation are merged into the unstable large-scale TLV cloud cavitation at critical cavitation conditions, which grows and collapses periodically near trailing edge (TE).

Subharmonic tonal noise from backflow vortices radiated by a low-speed ring fan in uniform inlet flow

Abstract: In order to highlight the mechanisms responsible for subharmonic tonal noise, a complete aeroacoustic study of a ring fan in presence of a uniform inlet flow is conducted. Unsteady RANS simulations with a compressible flow solver are used to compute the flow field and identify the acoustic sources on the rotor. The tip clearance recirculation shows upstream vortices that impinge the rotor blades and create the main source of unsteadiness on the fan. Since these vortices rotate at a lower speed than the rotor, the frequency of the impact is lower than the blade passing frequency. The acoustic signature is computed by propagating the noise sources located on the rotor surfaces using two methods: A Ffowcs-Williams and Hawkings analogy in the time-domain and an analytical model in the frequency-domain based on the compact rotating dipole formulation. A comparison with experimental results confirms that the aeroacoustic phenomena responsible for the subharmonic tonal noise are well captured and properly propagated by the acoustic codes.

Self-Excited Blade Vibration Experimentally Investigated in Transonic Compressors: Rotating Instabilities and Flutter

Abstract: This paper investigates the vibrations that occurred on the blisk rotor of a 1.5-stage transonic research compressor designed for aerodynamic performance validation and tested in various configurations at Technische Universitat Darmstadt.During the experimental test campaign self-excited blade vibrations were found near the aerodynamic stability limit of the compressor. The vibration was identified as flutter of the first torsion mode and occurred at design speed as well as in the part-speed region. Numerical investigations of the flutter event at design speed confirmed negative aerodynamic damping for the first torsion mode, but showed a strong dependency of aerodynamic damping on blade tip clearance.In order to experimentally validate the relation between blade tip clearance and aerodynamic damping, the compressor tests were repeated with enlarged blade tip clearance for which stability of the torsion mode was predicted.During this second experimental campaign, strong vibrations of a different mode limited compressor operation. An investigation of this second type of vibration found rotating instabilities to be the source of the vibration. The rotating instabilities first occur as an aerodynamic phenomenon and then develop into self-excited vibration of critical amplitude.In a third experimental campaign, the same compressor was tested with reference blade tip clearance and a non-axisymmetric casing treatment. Performance evaluation of this configuration repeatedly showed a significant gain in operating range and pressure ratio. The gain in operating range means that the casing treatment successfully suppresses the previously encountered flutter onset. The aeroelastic potential of the non-axisymmetric casing treatment is validated by means of the unsteady compressor data.By giving a description of all of above configurations and the corresponding vibratory behavior, this paper contains a comprehensive summary of the different types of blade vibration encountered with a single transonic compressor rotor. By investigating the mechanisms behind the vibrations, this paper contributes to the understanding of flow induced blade vibration. It also gives evidence to the dominant role of the tip clearance vortex in the fluid-structure-interaction of tip critical transonic compressors. The aeroelastic evaluation of the non-axisymmetric casing treatment is beneficial for the design of next generation casing treatments for vibration control.Copyright © 2015 by ASME

Zonal Large-Eddy Simulation of a Fan Tip-Clearance Flow, With Evidence of Vortex Wandering

Abstract: The flow in a fan test-rig is studied with combined experimental and numerical methods, with a focus on the tip-leakage flow. A zonal RANS/LES approach is introduced for the simulation: the region of interest at tip is computed with full large-eddy simulation (LES), while Reynolds-averaged Navier–Stokes (RANS) is used at inner radii. Detailed comparisons with the experiment show that the simulation gives a good description of the flow. In the region of interest at tip, a remarkable prediction of the velocity spectrum is achieved, over about six decades of energy. The simulation precisely captures both the tonal and broadband contents. Furthermore, a detailed analysis of the simulation allows identifying a tip-leakage vortex (TLV) wandering, whose influence onto the spectrum is also observed in the experiment. This phenomenon might be due to excitation by upstream turbulence from the casing boundary layer and/or the adjacent TLV. It may be a precursor of rotating instability. Finally, considering the outlet duct acoustic spectrum, the vortex wandering appears to be a major contribution to noise radiation.

Effects of tip clearance size on the performance and tip leakage vortex in dual-rows counter-rotating compressor

Abstract: Based on the preliminary validation of the numerical results with an experiment, the steady results of different tip clearance size effects on the performance and tip leakage vortex of counter-rotating compressor were discussed. The predicted results showed that the stable margin and isentropic efficiency of the counter-rotating compressor decreased with increasing of the tip clearance size. Furthermore, increasing the tip clearance size had a greater negative effect on the performance of the upstream rotor than the downstream rotor. Detailed flow field analyses further demonstrated that the unstable flow occurred in the two counter-rotating rotors with greater tip clearance size. The rotor downstream was the main unstable factor which caused the counter-rotating compressor to breakdown when a smaller tip clearance size was used.

The Dual Mechanisms and Implementations of Stability Enhancement With Discrete Tip Injection in Axial Flow Compressors

Abstract: The mechanisms and implementation scheme of discrete tip air injection are studied in this paper. A map that summarized the routes to stall is then proposed. It is argued that there exists a critical tip clearance ratio that separates two different routes to stall, which infers that the stability enhancement can also be based on two different mechanisms. A summation of tip injection test data in the literatures demonstrates that this is actually the case. For each compressor, there are two trends in the curve of stall margin improvement (SMI) versus injected momentum ratio, which is separated by a demarcation ratio of injected momentum. A series of tests are done in a low-speed compressor to show that the micro injection, wherein the injected momentum ratio is less than the demarcation ratio, can only act on the tip leakage flow (TLF) and thus provide small SMI by weakening the self-induced unsteadiness of the tip leakage flow (UTLF), while in contrast the macro injection can provide much larger SMI by acting on the main flow, decreasing the inlet angle-of-attack and thus unloading the blade tip. Based on these findings, a novel detecting-actuating scheme is designed and implemented onto a low-speed axial compressor. A cross-correlation coefficient is used to detect the UTLF in the prestall process way before stall inception and then to guide the opening of proportional electromagnetic valves. The injected flow rate can be smoothly varied to cover both micro-and macroinjection, which saves energy when the compressor is stable, and provides protection when it is needed. The same principle is applied to a high-speed compressor with a recirculation injection and the preliminary test results are very encouraging.

An Experimental and Numerical Study on the Aerothermal Characteristics of a Ribbed Transonic Squealer-Tip Turbine Blade With Purge Flow

Abstract: Detailed heat transfer coefficient (HTC) and film cooling effectiveness (Eta) distribution on a squealer tipped first stage rotor blade were measured using an infrared (IR) technique. The blade tip design, obtained from a Solar Turbines Inc. gas turbine, consisted of double purge hole exits and four ribs within the squealer cavity, with a bleeder exit port on the pressure side close to the trailing edge. The tests were carried out in a transient linear transonic wind tunnel facility under land-based engine representative Mach/Reynolds number. Measurements were taken at an inlet turbulent intensity of Tu =12%, with exit Mach numbers of 0.85 (Reexit=9.75x10 5 ) and 1.0 (Reexit = 1.15x10 6 ) with the Reynolds number based on the blade axial chord and the cascade exit velocity. The tip clearance was fixed at 1% (based on engine blade span) with a purge flow blowing ratio BR = 1.0. At each test condition, an accompanying numerical study was performed using Reynolds Averaged Navier Stokes (RANS) equations solver ANSYS Fluent to further understand the tip flow characteristics. The results showed that the tip purge flow has a blocking effect on the leakage flow path. Furthermore, the ribs significantly altered the flow (and consequently heat transfer) characteristics within the squealer tip cavity resulting in a significant reduction in film cooling effectiveness. This was attributed to increased coolant-leakage flow mixing due to increased recirculation within the squealer cavity. Overall, the peak heat transfer coefficient on the cavity floor increased with exit Mach/Reynolds number. NOMENCLATURE BR Averaged blowing Ratio (BR = ρcUc /ρ∞ U∞,avg )

Different Configurations of a Reflective Intensity-Modulated Optical Sensor to Avoid Modal Noise in Tip-Clearance Measurements

Abstract: Tip clearance is critical to the performance of rotating turbomachinery. The objective of this paper is to develop a noncontact sensor with a precision of 30 μm to measure tip clearance in a turbine rig assembled in a wind tunnel. To carry out the measurements, an optical sensor whose main component is a bundle of optical fibers is employed. We use four different configurations of this sensor, which are tested in two distinct turbines with the aim of minimizing the effect of the noise on the repeatability of the measurements. Each configuration serves to increase the precision until the required performance is achieved for the measurement of the tip clearance. Our results may be helpful to develop applications related to structural health monitoring or active clearance-control systems.

Blade track assembly with turbine tip clearance control

Abstract: A turbine shroud assembly or blade track assembly adapted to extend around a turbine wheel assembly is disclosed. The turbine shroud assembly 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.

Study on Internal Flow and External Performance of a Semi-open Impeller Centrifugal Pump with Different Tip Clearances

Abstract: Abstract To study the influence of tip clearance on internal flow characteristics and external performance of a prototype centrifugal pump with a semi-open impeller, the unsteady numerical simulation and performance experiments are carried out in this paper. The evolution process of leakage vortex with time t, the flow characteristics and the magnitude of leakage rate in tip clearance are obtained in details. The results indicate that the H-Q curve hump of centrifugal pump shows a weakened trend with the increasing of tip clearance Δc. Meanwhile, the leakage rate ΔQ and the ratio of leakage rate to discharge flow rate (ψ) gradually increase. At the same tip clearance, the leakage rate ΔQ increases, while the ratio of leakage rate to discharge flow rate (ψ) decreases with the increasing of discharge flow rate Q. It is found that higher volumetric loss account for a higher percentage of the total loss at small flow rate condition. There easily exist strong leakage vortexes in the impeller inlet, impeller passage and impeller outlet. The pressure difference between suction side and pressure side makes the fluid pass through the tip clearance layer to form a lower pressure region and leakage vortex.

Desensitization of Axial Compressor Performance and Stability to Tip Clearance size

Abstract: This paper presents a computational and analytical study to identify and elucidate fundamental flow features associated with the desensitization of performance and aerodynamic stability of an axial compressor rotor to tip clearance change. Parametric studies of various design change on a baseline double circular arc axial rotor led to the identification of two flow features associated with reducing sensitivity to tip clearance, namely high incoming meridional momentum in the tip region and reduction/elimination of double tip leakage. Numerical experiments were subsequently performed on the baseline rotor geometry to validate these two flow features and explain the associated flow physics by variations in incoming meridional momentum and pitch size. Finally, two designs were proposed, namely full forward chordwise sweep and partially low stagger angle, to exploit these flow features. The results indicated that both designs produce the intended flow effects and exhibit lower performance and aerodynamic stability sensitivity to tip clearance.Copyright © 2015 by ASME

Tip clearance measurement

Abstract: An aircraft is provided and includes an airframe, first and second main rotors rotatably supported on the airframe to rotate about a rotational axis in opposite directions, first and second emitters disposed on an emitter blade of the second main rotor, each of the first and second emitters being configured to emit an emission toward a detector blade of the first main rotor, a detector disposed on the detector blade of the first main rotor, the detector being configured to detect the emissions of the first and second emitters and a flight computer which determines a clearance between the first and second main rotors in accordance with detections of the emissions by the detector.

Experimental investigation of factors influencing operating rotor tip clearance in multistage compressors

Abstract: An analysis of compressor rotor tip clearance measurements using capacitance probe instrumentation is discussed for a three-stage axial compressor. Thermal variations and centrifugal effects related to rotational speed changes affect clearance heights relative to the assembled configuration. These two primary contributions to measured changes are discussed both independently and in combination. Emphasis is given to tip clearance changes due to changing loading condition and at several compressor operating speeds. Measurements show a tip clearance change approaching 0.1 mm (0.2% rotor span) when comparing a near-choke operating condition to a near-stall operating condition for the third stage. Additional consideration is given to environmental contributions such as ambient temperature, for which changes in tip clearance height on the order of 0.05 mm (0.1% rotor span) were noted for temperature variations of 15°C. Experimental compressor operating clearances are presented for several temperatures, operating speeds, and loading conditions, and comparisons are drawn between these measured variations and predicted changes under the same conditions.

Turbine Blade Tip Film Cooling With Blade Rotation: Part I — Tip and Pressure Side Coolant Injection

Abstract: This is the first in a two-part series of an experimental film cooling study conducted on the tip of a turbine blade with a blade rotation speed of 1200 RPM. In this part of the study, the coolant is injected from the blade tip and pressure side (PS) holes, and the effect of the blowing ratio on the heat transfer coefficient and film cooling effectiveness of the blade tip is investigated. The blade has a tip clearance of 1.7% of the blade span and consists of a cut back squealer rim, two cylindrical tip holes and six shaped pressure side holes. The stator-rotor-stator test section is housed in a closed loop wind tunnel that allows for the performance of transient heat transfer tests. Measurements of the heat transfer coefficient and film cooling effectiveness are done on the blade tip using liquid crystal thermography. These measurements are reported for the no coolant case and for blowing ratios of 1.0, 1.5, 2.0, 3.0 and 4.0. The heat transfer results for the no coolant injection show a region of high heat transfer on the blade tip near the blade leading edge region as the incident flow impinges on that region. This region of high heat transfer extends and stretches on the tip as more coolant is introduced through the tip holes at higher blowing ratios. The cooling results show that increasing the blowing ratio increases the film cooling effectiveness. The tip film cooling profile is such that the tip coolant is pushed towards the blade suction side thereby providing better coverage in that region. The shift in coolant flow profile towards the blade suction side as opposed to the pressure side in stationary studies can primarily be attributed to the effects of the blade relative motion.Copyright © 2015 by ASME

Benefits of Nonaxisymmetric Endwall Contouring in a Compressor Cascade With a Tip Clearance

Abstract: This paper describes the design of a non-axisymmetric hub contouring in a shroudless axial flow compressor cascade operating at near stall condition. Although, an optimum tip clearance reduces the total pressure loss, further reduction in the loss was achieved using hub contouring. The design methodology presented here combines an evolutionary principle with a three-dimensional CFD flow solver to generate different geometric profiles of the hub systematically. The resulting configurations were preprocessed by GAMBIT © and subsequently analyzed computationally using ANSYS Fluent ©. The total pressure loss coefficient was used as a single objective function to guide the search process for the optimum hub geometry. The resulting three dimensionally complex hub promises considerable benefits discussed in detail in this paper. A reduction of 15.2% and 16.23% in the total pressure loss and secondary kinetic energy, respectively, is achieved in the wake region. An improvement of 4.53% in the blade loading is observed. Other complimentary benefits are also listed in the paper. The majority of the benefits are obtained away from the hub region. The contoured hub not only alters the pitch-wise static pressure gradient but also acts as a vortex generator in an effort to alleviate the total pressure loss. The results confirm that non-axisymmetric contouring is an effective method for reducing the losses and thereby improving the performance of the cascade.

Numerical investigation of influence of tip leakage flow on secondary flow in transonic centrifugal compressor at design condition

Abstract: In a centrifugal compressor, the leakage flow through the tip clearance generates the tip leakage vortex by the interaction with the main flow, and consequently makes the flow in the impeller passage more complex by the interaction with the passage vortex. In addition, the tip leakage vortex interacts with the shock wave on the suction surface near the blade tip in the transonic centrifugal compressor impeller. Therefore, the detailed examination for the influence of the tip leakage vortex becomes seriously important to improve the aerodynamic performance especially for the transonic centrifugal compressor. In this study, the flows in the transonic centrifugal compressor with and without the tip clearance at the design condition were analyzed numerically by using the commercial CFD code. The computed results revealed that the tip leakage vortex induced by the high loading at the blade tip around the leading edge affected the loss generation by the reduction or the suppression of the shock wave on the suction surface of the blade.

Tip clearance measurement method based on all-fiber tip timing

Abstract: The invention discloses a tip clearance measurement method based on all-fiber tip timing. The method comprises the following steps: converting measurement on length measurement in a triangle method into measurement on a tip timing signal; adopting two fiber bundle type timing sensors, and enabling emission fibers in the timing sensors to emit two beams of auto-collimation emergent light with different wavelengths; enabling receiving fibers in the timing sensors to receive moment signals when leaves approach and leave a measurement region; monitoring the rotating speed of a rotor in real time by a rotating speed synchronization sensor; performing high-speed signal acquisition and circuit module processing on all the sensor signals, and reversely calculating a distance between the end face of each sensor and each leaf tip by an upper computer through a math model. According to the tip clearance measurement method, a clearance value is ingeniously converted into a tip timing signal relevant to the clearance value, so that a measurement system is irrelative to most environmental factors such as the leaf and electromagnetic interference, and the interference resistance and the clearance measurement precision of the system are improved.

Flow visualization for investigating stator losses in a multistage axial compressor

Abstract: The methodology and implementation of a powder-paint-based flow visualization technique along with the illuminated flow physics are presented in detail for application in a three-stage axial compressor. While flow visualization often accompanies detailed studies, the turbomachinery literature lacks a comprehensive study which both utilizes flow visualization to interrupt the flow field and explains the intricacies of execution. Lessons learned for obtaining high-quality images of surface flow patterns are discussed in this study. Fluorescent paint is used to provide clear, high-contrast pictures of the recirculation regions on shrouded vane rows. An edge-finding image processing procedure is implemented to provide a quantitative measure of vane-to-vane variability in flow separation, which is approximately 7 % of the suction surface length for Stator 1. Results include images of vane suction side corner separations from all three stages at three loading conditions. Additionally, streakline patterns obtained experimentally are compared with those calculated from computational models. Flow physics associated with vane clocking and increased rotor tip clearance and their implications to stator loss are also investigated with this flow visualization technique. With increased rotor tip clearance, the vane surface flow patterns show a shift to larger separations and more radial flow at the tip. Finally, the effects of instrumentation on the flow field are highlighted.

Experimental Identification of Rotating Instability on an Axial Fan with Shrouded Rotor

Abstract: The rotating instability phenomenon is investigated experimentally on a laboratory low-speed axial fan. Unsteady pressure measurements are performed close to the rotor leading edge, in the far-field in the upstream-duct section, and within the rotating frame by using Kulite sensors near the blade tip. As reference, the occurrence of rotating instability is proven for a rotor configuration with large clearance size. Existing explanations assume that rotating instability is induced by the tip clearance vortex. Following these models, the characteristic spectral and modal signatures of rotating instability are generated by the circumferentially coupled unsteady vortex systems within the individual blade passages. However, some early findings on a shrouded rotor setup and a study on an annular cascade without clearance have raised questions about the validity of the existing theories. To answer these questions, a second configuration is investigated using the identical rotor in a shrouded version. Here, a shrouding band covers the blade tips over the whole circumference. By using a comprehensive sensor arrangement in a wide variation of operating conditions, the existence of spectral and modal rotating instability signatures is proven on the shrouded-rotor configuration, as well. In this case, the tip clearance vortex system and, respectively, the leakage flow are suppressed. The present study underlines that novel explanations are required to describe the physical mechanism that generates rotating instability.

Turbine Blade Tip Cooling With Blade Rotation: Part II — Shroud Coolant Injection

Abstract: In this paper, blade-tip cooling is investigated with coolant injection from the shroud alone and a combination of shroud coolant injection and tip cooling. With a nominal rotation speed of 1200 RPM, each blade consists of a cut back squealer tip with a tip clearance of 1.7 % of the blade span. The blades also consist of tip holes and pressure side shaped holes, while the shroud has an array of angled holes and a circumferential slot upstream of the rotor section. Different combinations of the three cooling configurations (tip and pressure side holes, shroud angled holes and shroud circumferential slot) are utilized to study the effectiveness of shroud cooling as a complementary method of cooling the blade tip. The measurements are done using liquid crystal thermography. Blowing ratios of 0.5, 1.0, 2.0, 3.0 and 4.0 are studied for shroud slot cooling and blowing ratios of 1.0, 2.0, 3.0, 4.0 and 5.0 are studied for shroud hole cooling. For cases with coolant injection from the blade tip, the blowing ratios used are 1.0, 2.0, 3.0 and 4.0. The results show an increase in film cooling effectiveness with increasing blowing ratio for shroud hole cooling. The increased effectiveness from shroud hole cooling is concentrated mainly in the tip-region below the shroud holes and towards the blade suction side and the suction side squealer rim. Slot cooling injection results in increased effectiveness on the blade tip near the blade leading edge up to a maximum blowing ratio, after which the cooling effectiveness decreases with increasing blowing ratio. The combination of the different cooling methods results in better overall cooling coverage of the blade tip with the shroud hole and blade tip cooling combination being the most effective. The level of coolant protection is strongly dependent on the blowing ratio and combination of blowing ratios.