Rotor Tip Leakage Research Articles

Impact of High-Pressure-Turbine Purge Flow on the Evolution of Turbulence in a Turbine Vane Frame

Abstract This paper describes the measurement and postprocessing of turbulence data. The experiments were carried out in a two-stage, two-spool transonic turbine test rig at the Institute for Thermal Turbomachinery and Machine Dynamics at the Graz University of Technology, which includes relevant purge and turbine rotor tip leakage flows. The test setup consists of a high-pressure turbine (HPT) stage, a turbine vane frame (TVF) with turning struts and splitters, and a counter-rotating low-pressure turbine (LPT) to allow engine realistic measurements. Time-resolved area traverse measurements have been performed for three different operating conditions in three measurement planes downstream of the HPT rotor, which enable the measurement of the turbulence quantities at the TVF inlet and outlet as well as the LPT outlet. The turbulence quantities are evaluated using triaxial- and single hot-film probes by means of Constant-Temperature-Anemometry, and their results were validated with five-hole probe (FHP) measurements. Ensemble- and time-averaging, as well as Fourier transforms, were applied for data reduction. It is shown how the turbulence intensity and integral length scale vary over different purge flowrates (PFR). The acquired measurement data illustrate that the interaction of the ejected purge flow with mean flow enhances the turbulent mixing in the secondary flow structures at the TVF inlet and TVF outlet, respectively. Furthermore, the flow downstream of the LPT rotor is affected by TVF and LPT rotor secondary flow structures, which are identified as high turbulence regions. These regions strongly depend on the purge flowrate. These results acquired under engine realistic rig flow conditions enable a deeper understanding of the relationship between loss and turbulence quantities and will help improve the application of computational fluid dynamics turbulence models to turbomachinery modeling.

Influence of the ending position of controllable speed casing on a transonic compressor rotor tip leakage flow

Abstract The controllable speed casing is a novel casing treatment approach that makes partial casing rotate at adjustable and proper speed to achieve stability expansion. Structural parameters of casing treatment are found to influence the effect of stability expansion by many studies. In this paper, the effect of the ending position of the rotatable ring in controllable speed casing on the tip leakage flow and the stability expansion was studied numerically. The results show that when the rotatable ring rotates at 30 % and 50 % rotor design speed, the controllable speed casing achieves the stability expansion of the compressor rotor no matter where the ending position is. The upstream movement of the ending position decreases the axial pressure gradient in the middle and rear of the tip passage. It pushes the shock wave downstream, which reduces blockage region at tip leading edge. The upstream movement of the ending position contributes to an approximately linear increase in the stable operating margin.

Spike type of stall inception in the tandem rotor

There are two known mechanisms of inception of rotating stall in the compressor, modal wave inception and spike inception. However, the stall inception mechanisms for a tandem-rotor have not been explored so far in detail. The paper explores the unsteady behaviour of the tandem rotor. Unsteady casing pressure measurements are carried out with the help of seven equispaced unsteady pressure sensors. The casing pressure traces are analysed using wavelet transforms, fast Fourier transform, discrete spatial Fourier series, travelling wave energy method and correlation techniques to identify the nature of the stall cells. The changes observed in the stall cells during the transition from the stable mode of operation to the fully stalled mode and back from the stall regime to the stable operating condition are investigated in detail. It is observed that the stall incepts in the form of a low-intensity spike, which later evolves as a long-length scale disturbance in the fully developed stall region. The initial stall disturbance is found to be rotating with 75% of the rotor speed. The speed of the stall cells is reduced to 61% of the rotor speed in a fully developed region. The number and size of stall cells increase as the mass flow is throttled. The number and size of the stall cells are strongly coupled with the leading-edge incidence pattern. During the transition from the stall regime to the stable operating condition, the mass flow of the compressor increases, which reduces the flow incidence angle of the tandem rotor. Thus, the separation region shrinks during this transition leading to a reduction in the size of the stall cells. The stall in the tandem-rotor is primarily associated with the tip leakage flow. The rotating disturbances in the tandem-rotor originate due to the leading-edge flow separation of the forward rotor blade. Leading-edge separation of the aft rotor, which arises due to interaction of the forward rotor tip leakage vortex, is effectively suppressed with the help of the gap-nozzle flow. For a comprehensive unsteady analysis, the different techniques need to be used in combination.

Influence of wet compression on aerodynamic performance and stability boundary of transonic compressor

The gas–liquid two-phase flow in a transonic compressor stage, Stage 35, under wet compression was numerically simulated by Computational Fluid Dynamics (CFD) technique along with Euler–Lagrange approach. The evaporation and cooling effect of water droplets on compressor performance and stable flow range was investigated by changing the inlet spraying conditions (droplet sizes and injected water flow rates). Further, the influencing mechanism was explained by the changes of rotor velocity triangle, rotor specific work, blade loading, and tip leakage flow before and after spraying. The results indicate that wet compression can improve the pressure ratio and efficiency of the compressor, but, under different comparison conditions before and after spraying, the influence on compressor power consumption has different trends. Wet compression improves the compressor specific power consumption under constant flow rate conditions, while it reduces compressor specific power consumption under equal pressure ratio conditions. Additionally, the stalling flow rate of compressor increases by wet compression effect, and the increasing degree of stalling flow rate is positively correlated with the increasing level of pressure ratio. The evaporative and cooling effect of droplets reduces the airflow temperature at rotor outlet, leading to the increase of airflow density, the decrease of axial velocity, which results in the promotion of rotor specific work—this is the main reason for the improvement of pressure ratio under constant flow rate conditions. Under wet compression condition, the enhanced blade loading near tip strengthens the tip leakage flow, making the compressor achieve stall criteria at larger flow rate, which is the cause for wet compression influencing the compressor stall boundary.

Aerodynamic Influence of a Bleed on the Last Stage of a Low-Pressure Compressor and S-Duct

Abstract This paper uses computational fluid dynamics to investigate the effect of an engine handling bleed situated on the outer casing downstream of the last rotor stage of a low-pressure compressor and upstream of the outlet guide vane and S-shaped duct. The model, validated against existing experimental data, utilized an unsteady Reynolds-averaging Navier–Stokes (RANS) solver incorporating a Reynolds stress closure to examine the unsteady component interactions. The results showed that at bleed rates less than 25% of the mainstream flow, the bleed effects were negligible. However, at higher bleed rates, performance was significantly degraded. A uniform flow extraction hypothesis was employed to separate the positional bias effects of the bleed from the bulk flow diffusion. This revealed that the bleed-induced radial flow distortion can significantly affect the outlet guide vane (OGV) loading distribution, which thereby dictates the position and type of stall within the OGV passage. Extraction of the rotor tip leakage via the shroud bleed, combined with the radial flow distortion, contributed to a 28% reduction in duct loss at 10% bleed and up to 50% reduced loss at 25% bleed. The actual amount of flow required to be extracted for an OGV stall to develop was 30%. That was independent of the bleed location and the type of stall. For bleeds up to 20%, the S-duct displayed a remarkable resilience and consistency of flow variables at duct exit. However, a stalled OGV deteriorated the radial flow uniformity that was presented to the high-pressure compressor.

Effect of Rotor-stator Axial Gap on the Tip Seal Leakage Flow of a Steam Turbine

The tip leakage flow in different axial gaps is numerically investigated based on the Reynolds-Averaged Navier-Stokes equations and k-ω SST turbulent model using commercial CFD software ANSYS Fluent. The characteristics of vortexes in the flow field of tip seal and the distribution law of rotor tip leakage loss are analyzed under the change of axial gap. The results show that the flow field in the blade tip seal undergoes complex changes with the increase of axial gap. There are two opposite vortices in the seal cavity with small axial gap, namely, the large vortex in the cavity and the small vortex near the wall of the shroud. The small vortex gradually disappears with the increase of the axial gap. When the axial gap is increased to a certain value, the reverse large vortex is formed in the cavity behind the low teeth of seal, and the leakage rate increases significantly; the steam flow angle and the mixing loss also show different changes. Based on the traditional calculation formula of tip leakage rate, a more accurate calculation formula of tip leakage rate is constructed by introducing equivalent labyrinth seal teeth to consider the variation of axial gap.

Research on the Influence of Steam Turbine Seal Leakage

CFD was employed to simulate the steam flow in 1.5-stage cascades with three different seal clearances. When the seal clearance is 0, there is no steam seal leakage, no obvious secondary flow in the cascade, and the stage efficiency reaches 88.27%. When the clearance of diaphragm seal and rotor tip seal is 1mm, the leakage of diaphragm seal strongly interferes with the flow in the cascade, which promotes the formation and development of the end-wall secondary flow near rotor hub, and the rotor tip leakage flow has little effect on next stage, and the stage efficiency drops by about 2 percentage points. When the seal clearance is 4mm, the end-wall secondary flow near rotor hub and next stator casing is strengthened significantly, and the attack angle loss increases, so the stage efficiency decreases by about 13 percentage points.

Unsteady Flow Interactions Between a High- and Low-Pressure Turbine

Abstract This paper presents the unsteady flow interactions between an engine-representative high-pressure turbine (HPT) and low-pressure turbine (LPT) stage, connected by a turbine center frame (TCF) duct with nonturning struts. The setup was tested at the high-speed two-spool test turbine facility at the Institute for Thermal Turbomachinery and Machine Dynamics at Graz University of Technology and includes relevant purge and turbine rotor tip leakage flows. Due to the complexity of such a test, the unsteady component interactions in an HPT–TCF–LPT module have not received much attention in the past and require additional analysis to determine new approaches for further performance improvements on the system level. The flow downstream of an HPT is highly unsteady and dominated by stator–rotor interactions, which affect the flow behavior through the downstream TCF and LPT. To capture the unsteady flow structures, time-resolved aerodynamic measurements were carried out with a fast-response aerodynamic pressure probe (FRAPP) at three different measurement planes. In this paper, the time-resolved and phase-averaged flow fields with respect to the HPT and LPT trigger are studied. Since the two rotors are uncorrelated, the applied method allows the identification of the flow structures induced by either of them. Upstream of the LPT stage, the HPT flow structures evolving through the TCF duct dominate the flow fields. Downstream of the LPT stage, the flow is affected by both the HPT and the LPT secondary flow structures. The interactions between the various stator rows and the two rotors are detected by means of time-space plots and modal decomposition. To describe the fluctuations induced by both rotors, particularly the rotor–rotor interaction, the rotor synchronic averaging (RSA) is used to analyze the flow field downstream of the LPT. This paper highlights the need to account for the HPT-induced unsteady mechanisms in addition to the LPT flow structures and the interaction of both to arrive at improved LPT designs.

Experimental investigation of the tip leakage flow in a low-speed multistage axial compressor.

Casing pressure measurements and Stereoscopic Particle-Image Velocimetry (SPIV) measurements are used together to characterize the behavior of the rotor tip leakage flow at both the design and near-stall conditions in a low-speed multistage axial compressor. A three-dimensional Navier-Stokes solver is also performed for the multistage compressor and the prediction of tip leakage flow is compared with SPIV data and casing dynamic static pressure data. During the experiment 10 high-frequency Kulite transducers are mounted in the outer casing of the rotor 3 to investigate the complex flow near the compressor casing and Fourier analyses of the dynamic static pressure on the casing of the rotor 3 are carried out to investigate the tip leakage flow characteristics. At the same time, the two CCD cameras are arranged at the same side of the laser light sheet, which is suitable for investigating unsteady tip leakage flow in the multistage axial compressor. The SPIV measurements identify that the tip leakage flow exists in the rotor passage. The influence of tip leakage flow leads to the existence of low axial velocity region in the rotor passage and the alternating regions of positive and negative radial velocity indicates the emergence of tip leakage vortex (TLV). The trajectory of the tip leakage vortex moves from the suction surface toward the pressure surface of adjacent blade, which is aligned close to the rotor at the design point (DP). However, the tip leakage vortex becomes unstable and breaks down at the near-stall point (NS), making the vortex trajectory move upstream in the rotor passage and causing a large blockage in the middle of the passage.

An Alternative Numerical Model for Investigating Three- Dimensional Steam Turbine Exhaust Hood

A proper design of exhaust hood geometry is very much essential in order to improve the overall efficiency of the steam turbine plant. The geometry of the non-axisymmetric exhaust hood makes the fluid flow at the exit of the steam turbine to be radially and circumferentially non-uniform. This work involves computational simulation of steam turbine asymmetric exhaust hood flows by incorporating the actuator-disc concept. The ANSYS FLUENT, finite volume based CFD solver is used for the present computational study. In the present simulation, the implementation of actuator disc boundary conditions with and without tip leakage is described in detail. The Actuator disc model approach exhibits a similar steam turbine exhaust hood flow asymmetry and static pressure recovery compared with the results reported in the literature, highlighting the applicability of the present model in coupling the rotor tip leakage jet with the steam turbine hood flow structure with less computational effort.

Optimization of a transonic axial-flow compressor under inlet total pressure distortion to enhance aerodynamic performance

In a boundary layer ingestion (BLI) propulsion system, the compressor rotor operates under the condition of a severe inflow distortion because the boundary layers on the surface of aircraft have been ingested. In this situation, the low-momentum fluid would inevitably raise the blade load and aggravate the local aerodynamic losses, thus severely deteriorating the compressor overall aerodynamic performance. The paper has applied a sweep optimization design based on surrogate model and genetic algorithm to a transonic axial-flow compressor at the condition of total pressure distorted inflow. The optimization objective is to improve the compressor efficiency. The sweep design has been implemented through controlling the shape of the blade stacking line and the time-saving steady numerical simulations are mainly utilized in the optimization process. Then, the full-annulus unsteady CFD calculations have been conducted for validating the optimized design and further analyzing the flow mechanisms of performance improvements. The computational results show that the inflow distortion has led to non-uniform distributions of the tip leakage loss and the hub flow separation loss along the circumferential direction. It is found that the rotor blade has suffered from the highest tip leakage losses and hub separation losses during the blade leaving the distorted region. After rotor sweep optimization, both tip leakage and hub separation losses have been notably reduced along the full circumferential direction. Moreover, for different pitchwise locations, the optimized rotor sweep design has achieved more aerodynamic performance improvements in the process of the rotor blade leaving the distorted region than those in the processes of the rotor blade moving out of, moving towards and entering the distorted region. Overall, the optimized design performs better than the baseline design in terms of adiabatic efficiency in the entire operating range. In the meanwhile, the total pressure ratio has been maintained. Specifically, an approximately 0.5% increment of efficiency has been achieved by the optimized design at the optimization point. For this paper, the major purpose is to offer useful guidelines for fan/compressor design strategy to achieve high aerodynamic performances under distorted inflow working condition.

Features of the Vortex Flow Structure around a One Fin Shroud

Modern gas turbines have high efficiency. A further increase in their economic efficiency can be achieved, on the one hand, by enhancing the accuracy of the parameter prediction at the design stage and, on the other hand, by the possibility of improving the design for different components of the flow path based on the results of calculating the complicated viscous spatial structure of the flow. One of the tools for enhancing the efficiency of gas turbines is the minimization of the rotor-tip leakage, the rate of which is reduced by shrouding the rotor blades. In particular, using numerical methods and software tools based on the former, one can perform thorough computational analysis of the vortex structure of the flow in the vicinity of the tip shroud and sufficiently accurately determine the rotor-tip leakage rates and other parameters of the stage. Such an approach allows a more accurate assessment of the leakage than semiempirical approaches used in practice. In particular, the assessment by the correlation dependence showed that the leakage rate for the tip shroud design in question with a tip clearance of 5 mm exceeded the leakage rate calculated using the method that considers the specific features of the tip shroud design and the vortex structure of the flow in the vicinity of it by 8.65%. As exemplified by the last-stage rotor blade of a stationary gas turbine, the possibility of controlling the leakage of the main flow through the tip clearance is demonstrated based on results of a numerical experiment.

Turbocharger turbine rotor tip leakage loss and mass flow model valid up to extreme off-design conditions with high blade to jet speed ratio

Due to the power consumption restriction of the turbocharger compressor, common turbine maps are rather narrow. To extrapolate them, reliable physical submodels are needed that are valid for broad ranges. Plenty of research has been done referring to tip leakage losses in axial and traditional radial turbomachinery. However, less effort has been put into the tip leakage analysis of radial turbocharger turbines, whose characteristics including high rotational speed and geometry are rather different. Commonly developed tip leakage loss models in radial turbines are mainly based on correlations with the rotational speed, while in axial turbomachinery they are mainly based on blade loading assumptions. Wide range computational fluid dynamics (CFD) data of a medium sized automotive turbine have been used to analyze tip leakage mass flow under extremely diverse running conditions. To be able to fit a model in a broad range of the map, blade loading and rotational speed have to be considered. A novel tip clearance model has been derived from the Navier Stokes Equations. The model owns a dependency on the rotational speed and the blade loading. With this approach CFD data have been fitted in a very good quality to model the tip leakage mass flow rate and tip leakage losses.

Demonstrating Multistage Compressor Blockage Calculations Using Pressure Measurements for Large Tip Clearances

With future gas turbine designs seeking increased overall pressure ratios, blade heights in the rear stages of high-pressure compressors are expected to decrease. As this will lead to an increase of relative tip-clearance height, detailed knowledge of the associated rotor tip-leakage flows becomes increasingly important. In particular, the development of blockage due to large tip-leakage flows can be a key component leading to an understanding of stage matching for multistage machines. To assess blockage related to increased tip clearance, a series of measurements are presented here from a three-stage axial compressor at two loading conditions on the 100% corrected speed line. These data correspond to three tip-clearance configurations, up to 4% span. Three-component velocity measurements are used to validate a new blockage calculation technique based on measured steady and time-resolved pressures, and instrumentation techniques that are more robust and require fewer resources than multicomponent velocity tests. Ultimately, these simplified pressure-based blockage calculation techniques yield results that compare well with the velocity-based results, within 2% flow area or better.

Effect of Relative Movement between the Shroud and Blade on Tip Leakage Flow Characteristics

An experimental and numerical investigation into the tip leakage flow of a turbine rotor is carried out using a particle image velocimetry (PIV) system and the commercial software ANSYS CFX 14.0. The specimen used in this work is a typical GE-E3 model with a new squealer tip design. The experimental data are used to create a turbulence model and numerical strategy. Through the validated turbulence model and numerical strategy, simulations are carried out to compare the characteristics of the tip leakage flow in three cases: (1) the blade is rotating, but the shroud is stationary, which is the real status of turbine rotor operation; (2) the blade is stationary, but the shroud moves, to simulate their relative movement; (3) the blade is stationary, and the shroud is also stationary, this is a simplified case, but has been widely used in the experiments on rotor tip leakage flow. Detailed analysis of the flow phenomena shows that the second case is a reasonable alternative approach to simulate the real state. However, the flow patterns in the third case exhibit some evident differences from the real status. These differences are caused by the inaccurate viscous force arising from the stationary blade and shroud. In this work, a modification method for the experiments conducted in the third case is firstly proposed, which is realized through adding an imaginary roughness at the shroud wall to be close to the real viscous effect, and to thereby reduce the deviation of the experiment from the real case. According to the results calculated by ANSYS CFX, the flow structure in the modification case is very close to the real status. Besides, this modification case is an easy and cheap way to simulate the real tip leakage flow.

Effects of Rotor Tip Blade Loading Variation on Compressor Stage Performance

Changes in loss generation associated with altering rotor tip blade loading of an embedded rotor–stator compressor stage are assessed with unsteady three-dimensional computations, complemented by control volume analyses. Tip-fore-loaded and tip-aft-loaded rotor blades are designed to provide variation in rotor tip blade loading distributions for determining a compressor design hypothesis that aft-loading a rotor blade tip yields a reduction in loss generation in a stage environment. Aft-loading a rotor blade tip delays the formation of tip leakage flow, resulting in a relatively less mixed-out tip leakage flow at the rotor outlet and a reduction in overall tip leakage mass flow, hence a lower loss generation. However, the attendant changes in tip flow angle distribution are such that there is an overall increase in the flow angle mismatch between tip flow and main flow, leading to higher loss generation. The latter outweighs the former; therefore, rotor passage loss from aft-loading a rotor tip is higher unless a constraint is imposed on tip flow angle distribution so that the associated induced loss is negligible. Tip leakage flow, which is not mixed-out at the rotor outlet, is recovered in the downstream stator. The tip leakage flow recovery process yields a higher benefit for a relatively less mixed-out tip leakage flow in the tip-aft-loaded rotor blades on a time-averaged basis. These characterizing parameters together determine the attendant overall loss associated with rotor tip leakage flow in a compressor stage environment. The revised design hypothesis is thus as follows: A rotor should be tip-aft-loaded and hub-fore-loaded while a stator should be hub-aft-loaded and tip-fore-loaded with tip/hub leakage flow angle distribution such that it results in no additional loss. For the compressor stage being assessed here, an estimated 0.15 points enhancement in stage efficiency is possible from aft-loading rotor tip only.

The Effect of Rotor Casing on Low-Pressure Steam Turbine and Diffuser Interactions

The present study aims to investigate the interaction between a last-stage steam turbine blade row and diffuser. This work is carried out using computational fluid dynamics (CFD) simulations of a generic last-stage low-pressure (LP) turbine and axial–radial exhaust diffuser attached to it. In order to determine the validity of the computational method, the CFD predictions are first compared with data obtained from an experimental test facility. A computational study is then performed for different design configurations of the diffuser and rotor casing shapes. The study focuses on typical flow features such as effects of rotor tip leakage flows and subsequent changes in the rotor–diffuser interactions. The results suggest that the rotor casing shape influences the rotor work extraction capability and yields significant improvements in the diffuser static pressure recovery.

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).

A New Particle Image Velocimetry Technique for Turbomachinery Applications

Nonintrusive measurement techniques such as particle image velocimetry (PIV) are growing in both capability and utility for turbomachinery applications. However, the restrictive optical access afforded by multistage research compressors typically requires the use of a periscope probe to introduce the laser sheet for measurements in a rotor passage. This paper demonstrates the capability to perform three-dimensional PIV in a multistage compressor without the need for intrusive optical probes and requiring only line-of-sight optical access. The results collected from the embedded second stage of a three-stage axial compressor highlight the rotor tip leakage flow, and PIV measurements are qualitatively compared with high-frequency response piezoresistive pressure measurements to assess the tip leakage flow identification.

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.