Rotor Tip Leakage Flow 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.

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.

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.

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.

Evaluation of the effectiveness of typical casing treatments for a low-speed compressor by an integral method

A novel integral method is proposed to quickly assess the effectiveness on stability improvement with end-wall casing treatments. With this low-cost method, efficacies for various casing treatment configurations can be evaluated instead of calculating the entire performance curves, which can be used in place of expensive and costly experiments. The underlying mechanism for this approach is based on the hypothesis that the spike stall precursors can be triggered by the forward spillage of the rotor tip leakage flow, and the onset condition of such a spillage is determined by the axial momentum balance within the rotor tip region. Based on the simulation, a series of control volumes are set at the rotor tip region in order to catch the axial momentum balance between the incoming main flow and the reversed tip leakage flow. Cumulative axial momentum distributions for these control volumes named as “bell-shaped curves” are presented to evaluate the effectiveness of different configurations. The axial location of the bell curve peak indicates the time- and spatial-averaged interface position between the main flow and tip leakage flow, which moves upstream during throttling. Three types of typical casing treatments: multiple circumferential grooves, skewed axial slots and self-injection configurations for a low-speed in-house compressor are evaluated by their bell-shaped curves. Among these configurations, the skewed axial slots has the most downstream of the peak, followed by the multiple-grooves, while the self-injection configuration shows the least. Based on an existed experimental result for a double-groove configuration, the effectiveness of the three studied casing treatments are predicted by their locations of the bell-shape curve's peak without simulating the entire performance curves. The assessments are then validated by experiments.

Vane Clocking Effects on Stator Suction Side Boundary Layers in a Multistage Compressor

The stator inlet flow field in a multistage compressor varies in the pitchwise direction due to upstream vane wakes and how those wakes interact with the upstream rotor tip leakage flows. If successive vane rows have the same count, then vane clocking can be used to position the downstream vane in the optimum circumferential position for minimum vane loss. This paper explores vane clocking effects on the suction side vane boundary layer development by measuring the quasi-wall shear stress on the downstream vane at three spanwise locations. Comparisons between the boundary layer transition on Stator 1 and Stator 2 are made to emphasize the impact of rotor-rotor interactions which are not present for Stator 1 and yet contribute significantly to transition on Stator 2. Vane clocking can move the boundary layer transition in the path between the wakes by up to 24% of the suction side length at midspan by altering the influence of the Rotor 1 wakes in the 3/rev modulation from rotor-rotor interactions. The boundary layer near the vane hub and tip experiences earlier transition and separation due to interactions with the secondary flows along the shrouded endwalls. Flow visualization and Stator 2 wakes support the shear stress results.

Considerations for Measuring Compressor Aerodynamic Excitations Including Rotor Wakes and Tip Leakage Flows

The unsteady flow field generated by the rotor provides unsteady aerodynamic excitations to the downstream stator, which can result in vibrations such as forced response. In this paper, measurements of the rotor wake and rotor tip leakage flow from an embedded rotor in a multistage axial compressor are presented. A unique feature of this work is the pitchwise traverse of the flow field used to highlight the changes in the rotor exit flow field with respect to the position of the surrounding vane rows. Results acquired at midspan focus on characterizing an average rotor wake, including the effects on the frequency spectrum, from a forced response perspective. While many analyses use an average rotor wake to characterize the aerodynamic forcing function to the downstream stator, this study explores the factors that influence changes in the rotor wake shape and the resulting impact on the spectrum. Additionally, this paper investigates the flow near the endwall where the tip leakage vortex is an important contributor to the aerodynamic excitations for the downstream vane. For the first time, experimental data are presented at the rotor exit, which show the modulation in size and radial penetration of the tip leakage vortex as the rotor passes through the upstream vane wake. As computational models become more advanced, the ability to incorporate these aerodynamic excitation effects should be considered to provide better predictions for vane vibratory response.

Tip Clearance Investigation of a Ducted Fan Used in VTOL Unmanned Aerial Vehicles—Part II: Novel Treatments Via Computational Design and Their Experimental Verification

Ducted fan based vertical lift systems are excellent candidates to be in the group of the next generation vertical lift vehicles, with many potential applications in general aviation and military missions. Although ducted fans provide high performance in many “vertical take-off and landing” (VTOL) applications, there are still unresolved problems associated with these systems. Fan rotor tip leakage flow adversely affects the general aerodynamic performance of ducted fan VTOL unmanned aerial vehicles (UAVs). The current study utilized a three-dimensional Reynolds-averaged Navier–Stokes (RANS) based computational fluid dynamics (CFD) model of ducted fan for the development and design analysis of novel tip treatments. Various tip leakage mitigation schemes were introduced by varying the chordwise location and the width of the extension in the circumferential direction. Reduced tip clearance related flow interactions were essential in improving the energy efficiency and range of ducted fan based vehicles. Full and inclined pressure side tip squealers were also designed. Squealer tips were effective in changing the overall trajectory of the tip vortex to a higher path in radial direction. The interaction of rotor blades and tip vortex was effectively reduced and the aerodynamic performance of the rotor blades was improved. The overall aerodynamic gain was a measurable reduction in leakage mass flow rate. This leakage reduction increased thrust significantly. Experimental measurements indicated that full and inclined pressure side tip squealers increased thrust obtained in hover condition by 9.1% and 9.6%, respectively. A reduction or elimination of the momentum deficit in tip vortices is essential to reduce the adverse performance effects originating from the unsteady and highly turbulent tip leakage flows rotating against a stationary casing. The novel tip treatments developed throughout this study are highly effective in reducing the adverse performance effects of ducted fan systems developed for VTOL UAVs.

Tip Clearance Investigation of a Ducted Fan Used in VTOL Unmanned Aerial Vehicles—Part I: Baseline Experiments and Computational Validation

Ducted fans that are popular choices in vertical take-off and landing (VTOL) unmanned aerial vehicles (UAV) offer a higher static thrust/power ratio for a given diameter than open propellers. Although ducted fans provide high performance in many VTOL applications, there are still unresolved problems associated with these systems. Fan rotor tip leakage flow is a significant source of aerodynamic loss for ducted fan VTOL UAVs and adversely affects the general aerodynamic performance of these vehicles. The present study utilized experimental and computational techniques in a 559 mm diameter ducted fan test system that has been custom designed and manufactured. The experimental investigation consisted of total pressure measurements using Kiel total pressure probes and real time six-component force and torque measurements. The computational technique used in this study included a 3D Reynolds-averaged Navier–Stokes (RANS) based computational fluid dynamics model of the ducted fan test system. Reynolds-averaged Navier–Stokes simulations of the flow around the rotor blades and duct geometry in the rotating frame of reference provided a comprehensive description of the tip leakage and passage flow. The experimental and computational analysis performed for various tip clearances were utilized in understanding the effect of the tip leakage flow on the aerodynamic performance of ducted fans used in VTOL UAVs. The aerodynamic measurements and results of the RANS simulations showed good agreement, especially near the tip region.

A Physical Interpretation of Stagnation Pressure and Enthalpy Changes in Unsteady Flow

This paper provides a physical interpretation of the mechanism of stagnation enthalpy and stagnation pressure changes in turbomachines due to unsteady flow, the agency for all work transfer between a turbomachine and an inviscid fluid. Examples are first given to illustrate the direct link between the time variation of static pressure seen by a given fluid particle and the rate of change of stagnation enthalpy for that particle. These include absolute stagnation temperature rises in turbine rotor tip leakage flow, wake transport through downstream blade rows, and effects of wake phasing on compressor work input. Fluid dynamic situations are then constructed to explain the effect of unsteadiness, including a physical interpretation of how stagnation pressure variations are created by temporal variations in static pressure; in this it is shown that the unsteady static pressure plays the role of a time-dependent body force potential. It is further shown that when the unsteadiness is due to a spatial nonuniformity translating at constant speed, as in a turbomachine, the unsteady pressure variation can be viewed as a local power input per unit mass from this body force to the fluid particle instantaneously at that point.

Comparative investigation of tip leakage flow and its effect on stage performance in shrouded and unshrouded turbines

In order to provide reference for the choice of the turbine architecture in specific application, comparative investigation was conducted to investigate the shrouded and unshrouded rotor tip leakage flows by means of the commercial CFX software, which modeled the flow in a 1.5-stage turbine. Tip leakage loss production mechanisms as well as the effects of tip clearance and pitch–chord ratio on tip leakage and stage performances were discussed and compared in this article. Numerical results show that, the leakage flow at the shroud cavity inlet effectively removes the boundary layer at rotor leading edge, and then reduces the casing secondary flows inside rotor passage. There is a break-even tip clearance at which both the shrouded and unshrouded turbines have the same efficiency, and if tip clearance is less than it, the unshrouded turbine performs better than the shrouded turbine. Compared to the unshrouded turbine, the shrouded turbine can reduce the sensitivity of turbine performances to changes in tip clearance. Moreover, at the given relative change in leakage flow, the relative change in work is approximately twice the relative change in efficiency. Besides, pitch–chord ratio has the different impact on the tip leakage mixing processes in shrouded and unshrouded turbines. It is effective to achieve lower tip leakage losses by turning the leakage flows to reduce mixing losses in shrouded turbines, and by blade tip design or tip clearance treatment to reduce the tip leakage flow in unshrouded turbines.