bluuebunny/crossref_metadata_2025 huggingface dataset:
The Influence of Variable Gas Properties on Turbomachinery CFD
Authors: Northall John D.
Published: November 2008

This paper describes the inclusion of variable gas properties within a Reynolds average Navier-Stokes solver for turbomachinery and its application to multi-stage turbines. Most current turbomachinery CFD models the gas as perfect with constant specific heats. However, the specific heat at constant pressure, CP, can vary significantly. This is most marked in the turbine where large variations of temperature are combined with variations in the fuel air ratio. In the current model CP is computed as a function of the local temperature and fuel air ratio using polynomial curve fits to represent the real gas behaviour. The importance of variable gas properties is assessed by analysing a multi-stage turbine typical of the core stages of a modern aero-engine. This calculation includes large temperature variations due to radial profiles at inlet, the addition of cooling air and work extraction through the machine. The calculation also includes local variations in fuel-air ratio resulting from the inlet profile and the dilution of the mixture by the addition of coolant air. A range of gas models is evaluated. The addition of variable gas properties is shown to have no significant effect on the convergence of the algorithm, and the extra computational costs are modest. The models are compared with emphasis on the parameters of importance in turbine design, such as capacity, work and efficiency.
Mechanism of the Interaction of a Ramped Bleed Slot With the Primary Flow
Authors: Leishman B. A. , Cumpsty N. A.
Published: November 2008

An experimental and computational study of the ramped bleed slot in a compressor cascade is presented. The geometry is a circumferential slot downstream of the stator blade trailing edge, with endwall ramps inside the blade passage, and the paper builds on work previously reported for different bleed off-take geometries [1, 2]. The strong interaction between any bleed slot and the primary flow through the cascade can be strong, thereby causing the levels of loss and blockage in the primary flow leaving the blade passage to be increased at some bleed flow rates. Radial flow into the bleed slot is highly non-uniform because the blade-to-blade pressure field causes flow to enter the bleed slot preferentially where the static pressure is high, and to spill out into the primary flow where the static pressure is low. The mechanism for the ramped bleed slot is different from that described in the earlier papers for other geometries. For the ramped bleed slot a static pressure field, with large variations of static pressure in the circumferential direction, is set up in the slot because the endwall flow entering the slot has higher stagnation pressure downstream of the pressure surface than downstream of the suction surface of the upstream blades. The flow entering the slot with high stagnation pressure is brought to rest in a stagnation point on the downstream surface of the slot, and the consequent variation in static pressure on the rear surface sets tangential and radial components of velocity which are a large fraction of the free-stream velocity. As well as demonstrating the mechanism for the flow behaviour, the paper presents results of experiments and calculations to demonstrate the behaviour and gives guidance for the design of bleed slots by stressing the fundamental features of the flow.
Buoyancy Effects on Heat Transfer in Five Different Aspect-Ratio Rectangular Channels With Smooth Walls and 45-Degree Ribbed Walls
Authors: Fu Wen-Lung , Wright Lesley M. , Han Je-Chin
Published: November 2008

This paper experimentally studies the effects of the buoyancy force and channel aspect ratio on heat transfer in two-pass rotating rectangular channels with smooth walls and 45° ribbed walls. The channel aspect ratios include 4:1, 2:1, 1:1, 1:2 and 1:4. Four Reynolds numbers are studied: 5000, 10000, 25000 and 40000. The rotation speed is fixed at 550 rpm for all tests, and for each channel, two channel orientations are studied: 90° and 45° or 135°, with respect to the plane of rotation. Rib turbulators are placed on the leading and trailing walls of the channels at an angle of 45° to the flow direction. The ribs have a 1.59 by 1.59 mm square cross section, and the rib pitch-to-height ratio (P/e) is 10 for all tests. The effects of the local buoyancy parameter and channel aspect ratio on the regional Nusselt number ratio are presented. The results show that increasing the local buoyancy parameter increases the Nusselt number ratio on the trailing surface and decreases the Nusselt number ratio on the leading surface in the first pass for all channels. However, the trend of the Nusselt number ratio in the second pass is more complicated due to the strong effect of the 180° turn. Results are also presented for this critical turn region of the two-pass channels. In addition to these regions, the channel averaged heat transfer, friction factor, and thermal performance are determined for each channel. With the channels having comparable Nusselt number ratios, the 1:4 channel has the superior thermal performance because it incurs the least pressure penalty.
CFD Investigation of the Performance of a Military HP Axial Turbine Subjected to Inlet Temperature Distortion
Authors: Zilli Andrea , Pachidis Vassilios , Jackson Anthony , Pilidis Pericles
Published: November 2008

It is known that the exit flow from the combustor entering a turbine stage will have wide spatial variations in temperature both radially and circumferentially. This phenomenon is amplified in military engines, due to the higher temperatures involved, and can affect also the performance of a highly loaded, high-pressure axial turbine. Computational Fluid Dynamic (CFD) simulations are an innovative and powerful tool for studying inlet temperature distortion and can be integrated in the early phases of the design process. This paper discusses the 3D CFD steady state simulation of the performance of a single stage axial flow high pressure turbine at design point, off design and on several constant speed lines. The study also addresses the behaviour of the turbine when subjected to uneven inlet total temperature distribution. In addition to the ideal case (uniform inlet temperature), three types of distorted inlet temperature conditions have been investigated. These are: simple radial distortion, hot streak aligned to the mid passage of the stator and hot streak impinging on the leading edges of the stator. The analysis demonstrated that temperature distortion does not have a significant effect on the performance of the high pressure turbine apart from a small reduction in efficiency. It has been found that this drop in efficiency can be reduced by directing (clocking) the hot streaks towards the stator blades. The commercial CFD package, CFX-TASCflow, has been used in this study.
Large Eddy Simulation of Flow and Heat Transfer in the 180° Bend Region of a Stationary Ribbed Gas Turbine Internal Cooling Duct
Authors: Sewall Evan A. , Tafti Danesh K.
Published: November 2008

LES of the 180° bend in a stationary ribbed duct is presented. The domain studied includes three ribs upstream of the bend region and three ribs downstream of the bend with an outflow extension added to the end, using a total of 8.4 million cells. Two cases are compared to each other: one includes a rib in the bend and the other does not. The friction factor, mean flow, turbulence, and heat transfer are compared in the two cases to help explain the benefits and disadvantages of the wide number of flow effects seen in the bend, including flow separation at the tip of the dividing wall, counter-rotating Dean vortices, high heat transfer at areas of flow impingement, and flow separation at the upstream and downstream corners of the bend. Mean flow results show a region of separated flow at the tip of the dividing region in the case with no rib in the bend, but no separation region is observed in the case with a rib. A pair of counter-rotating Dean vortices in the middle of the bend is observed in both cases. Turbulent kinetic energy profiles show a 30% increase in the mid-plane of the bend when the rib is added. High gradients of heat transfer augmentation are observed on the back wall and downstream outside wall, where mean flow impingement occurs. This heat transfer is increased with the presence of a rib. Including a rib in the bend increases the friction factor in the bend by 80%, and it increases the heat transfer augmentation by approximately 20%, resulting in a tradeoff between pressure drop and heat transfer.
Towards Defining Objective Criteria for Assessing the Adequacy of Assumed Axisymmetry and Steadiness of Flows in Rotating Cavities
Authors: Snowsill G. D. , Young C.
Published: November 2008

The need to make a priori decisions about the level of approximation that can be accepted — and subsequently justified — in flows of industrial complexity is a perennial problem for CFD analysts. This problem is particularly acute in the simulation of rotating cavity flows, where the stiffness of the equation set results in protracted convergence times, making any simplification extremely attractive. For example, it is common practice, in applications where the geometry and boundary conditions are axisymmetric, to assume that the flow solution will also be axisymmetric. It is known, however, that inappropriate imposition of this assumption can lead to significant errors. Similarly, where the geometry or boundary conditions exhibit cyclic symmetry, it is quite common for analysts to constrain the solutions to satisfy this symmetry through boundary condition definition. Examples of inappropriate use of these approximating assumptions are frequently encountered in rotating machinery applications — such as the ventilation of rotating cavities within aero-engines. Objective criteria are required to provide guidance regarding the level of approximation that is appropriate in such applications. In the present work, a study has been carried out into: • The extent to which local 3-D features influence solutions in a generally 2-D problem. Criteria are proposed to aid in decisions about when a 2-D axisymmetric model is likely to deliver an acceptable solution. • The influence of flow features which may have a cyclic symmetry that differs from the bounding geometry or imposed boundary conditions (or indeed have no cyclic symmetry). • The influence of unsteady flow features and the extent to which their effects can be represented by mixing plane or multiple reference frame approximations.
Effect of Maximum Camber Location on Aerodynamics Performance of Transonic Compressor Blades
Authors: Chen N. X. , Zhang H. W. , Du H. , Xu Y. J. , Huang W. G.
Published: November 2008

It is well known that to increase rotational velocity is one of the effective measures to increase total pressure ratio. With increasing velocity, under the condition of transonic flow, the obvious effect of maximum camber location on aerodynamics performance of compressor blades especially in the supersonics zone can be found. In order to reduce the blade losses and to improve the blade design methodology it is necessary to study this complex flow mechanism. This paper describes only the influence of relative maximum camber location on aerodynamics performance, mainly adiabatic efficiency. As an example an axial fan was designed and calculated by the methodologies developed at the Institute of Engineering Thermophysics, Chinese Academy of Sciences.
Fan-Shaped Hole Effects on the Aero-Thermal Performance of a Film Cooled Endwall
Authors: Barigozzi Giovanna , Benzoni Giuseppe , Franchini Giuseppe , Perdichizzi Antonio
Published: November 2008

The present paper investigates the effects of a fan-shaped hole endwall cooling geometry on the aero-thermal performance of a nozzle vane cascade. Two endwall cooling geometries with four rows of holes were tested, for different mass flow rate ratios: the first configuration is made of cylindrical holes, whereas the second one features conical expanded exits and a reduced number of holes. The experimental analysis is mainly focused on the variations of secondary flow phenomena related to different injection rates, as they have a strong relationship with the film cooling effectiveness. Secondary flow assessment was performed through downstream 3D aerodynamic measurements, by means of a miniaturized 5-hole probe. The results show that at high injection rates, the passage vortex and the 3D effects tend to become weaker, leading to a strong reduction of the endwall cross flow and to a more uniform flow in spanwise direction. This is of course obtained at the expense of a significant increase of losses. The thermal behavior was then investigated through the analysis of adiabatic effectiveness distributions on the two endwall configurations. The wide banded TLC’s technique was used to determine the adiabatic wall temperature. Using the measured distributions of film cooling adiabatic effectiveness, the interaction between the secondary flow vortices and the cooling jets can be followed in good detail all over the endwall surface. Fan-shaped holes have been shown to perform better than cylindrical ones: at low injection rates, the cooling performance is increased only in the front part of the vane passage. A larger improvement of cooling coverage all over the endwall is attained with a larger mass flow rate, about 1.5% of core flow, without a substantial increase of the aerodynamic losses.
Improving the Performance of a Turbine With Low Aspect Ratio Stators by Aft-Loading
Authors: Pullan Graham , Denton John , Curtis Eric
Published: November 2008

Experimental data and numerical simulations are presented from a research turbine with low aspect ratio nozzle guide vanes (NGVs). The combined effects of mechanical and aerodynamic constraints on the NGV create very strong secondary flows. This paper describes three designs of NGV that have been tested in the turbine, using the same rotor row in each case. NGV 2 used three-dimensional design techniques in an attempt to improve the performance of the datum NGV 1 blade but succeeded only in creating an intense vortex shed from the trailing edge (as previously reported) and lowering the measured stage efficiency by 1.1% points. NGV 3 was produced to avoid the “shed vortex” while adopting a highly aft-loaded surface pressure distribution to reduce the influence of the secondary flows. The stage with NGV 3 had an efficiency 0.5% points greater than that with NGV 1. Detailed comparisons between experiment and CFD, including predicted entropy generation rates, are used to highlight the areas where the loss reduction has occurred and hence to quantify the effects of employing highly aft-loaded NGVs.
Optimization of Cooling Passages Within a Turbine Vane
Authors: Nowak Grzegorz , Wro´blewski Włodzimierz , Chmielniak Tadeusz
Published: November 2008

The trends in gas turbine technology aim to build more and more efficient cycles, which is usually achieved by the temperature increase at the inlet of the turbine. To prevent the negative effects of elevated temperature some actions are taken concerning, among others cooling of the high temperature components. Since the structure of the cooling system affects the turbine performance, it is essential to carry out the optimization to make it as efficient as possible. In this paper we show some aspects of passage optimization for internally cooled gas turbine vanes. In the present study the vane profile is taken as aerodynamically optimal. The analysis involves the optimization of the location and size of circular cooling passages within the vane. The analysis is performed by means of the genetic algorithm for the optimization task and FEM for the heat transfer predictions within the blade.
Numerical Investigation of the Rotation on the Film Cooling Over a Flat Surface
Authors: Young Patrick , Ding Shui Ting , Tao Zhi
Published: November 2008

A computational analysis was carried out to comprehend the mechanism of rotation on the film cooling which is important in understanding the mixing process between the film coolant and the hot stream air over the high pressure turbine blades. In this paper, a simple flat surface with a vertical film cooling hole was positioned parallel to the hot main stream and different rotating orientations were selected to simulate the blade pressure or suction sides. A polar coordinate frame was used with the center of the film cooling hole center as the origin point and three parameters S/D, the polar angle and either the adiabatic effectiveness or heat transfer coefficient could be chosen as the control parameters to produce a 2D graph of the effective film cooling areas. The rotation effects were clearly shown with the effective film cooling area graphs. The effective film cooling area was reduced with the rotating speed increasing.
Unsteady Tip Clearance Flow in a Low-Speed Axial Compressor Rotor With Upstream and Downstream Stators
Authors: Deng Xiangyang , Zhang Hongwu , Chen Jingyi , Huang Weiguang
Published: November 2008

In the course of advancing the understanding of the unsteady flow nature of compressor tip clearance flows, the present paper investigates the unsteady tip clearance flow in the second rotor of a two-stage low-speed axial compressor and its interaction with upstream and downstream stators. Numerical methods were adopted in the present study and the research focused on clarifying the unsteadiness of tip clearance flow behavior and its link to the change of rotor performance, subjected to the variables of axial gap sizes between the rotor and upstream and downstream stators. The result shows how and why the tip leakage vortex trajectory changes its shape with the change of gap size, and its impact on the rotor pressure rise characteristic. Within all the computed operating range, the pressure rise increases monotonically with the decrease of upstream axial gaps, but no monotonic variation was observed with the change of downstream axial gaps. This trend of performance change could be explained by the unsteady effect of upstream stator wakes, and the overall result is that the rotor performance was found to be more influenced by the upstream interaction than the downstream interaction. The frequency characteristic of the tip clearance vortex, under the influence of gap size and compressor operating condition, was also analyzed to provide a quantified estimation of its periodic flow behavior and a comparison with the recent results of other researchers.
Effects of the Inlet Boundary Layer Thickness on the Flow and Loss Characteristics in an Axial Compressor
Authors: Choi Minsuk , Park Junyoung , Baek Jehyun
Published: November 2008

A three-dimensional computation was conducted to understand effects of the inlet boundary layer thickness on the internal flow and the loss characteristics in a low-speed axial compressor operating at the design condition (φ = 85%) and near stall condition (φ = 65%). At the design condition, independent of the inlet boundary layer thickness, flows in the axial compressor show similar characteristics such as the pressure distribution, size of hub corner-stall, tip leakage flow trajectory, limiting streamlines on the blade suction surface, etc. But, as the load is increased, for the thick inlet boundary layer at hub and casing, the hub corner stall grows to make a large separation region between the hub and suction surface, and the tip leakage flow is more vortical than that observed in the case with thin inlet boundary layer and has the critical point where the trajectory of the tip leakage flow is suddenly turned to the downstream. For the thin inlet boundary layer, the hub corner stall decays to form the thick boundary layer from hub to midspan on the suction surface owing to the blockage of the tip leakage flow and the tip leakage flow leans to the circumferential direction more than at the design condition. In addition to these, the severe reverse flow, induced by both boundary layers on the blade surface and the tip leakage flow, can be found to act as the blockage of flows near the casing, resulting in a heavy loss. As a result of these differences of the internal flow made by the different inlet boundary layer thickness, the spanwise distribution of the total loss is changed dramatically. At the design condition, total pressure losses for two different boundary layers are almost alike in the core flow region but the larger losses are generated at both hub and tip when the inlet boundary layer is thin. At the near stall condition, however, total loss for thick inlet boundary layer is found to be greater than that for thin inlet boundary layer on most of the span except the region near the hub and casing. In order to analyze effects of inlet boundary layer thickness on total loss in detail, total loss is scrutinized through three major loss categories available in a subsonic axial compressor such as profile loss, tip leakage loss and endwall loss.
Rain Ingestion in Axial Flow Compressors at Part Speed
Authors: Day Ivor , Williams John , Freeman Christopher
Published: November 2008

New experimental work is reported on the effects of water ingestion on the performance of an axial flow compressor. The background to the work is the effect which heavy rain has on an aero-engine compressor when operating in ‘descent idle’ mode, i.e. when the compressor is operating at part-speed and when the aero-mechanical effects of water ingestion are likely to be more important than the thermodynamic effects. Most of our existing knowledge in this field comes from whole engine tests. The current work provides the first known results from direct measurements on a stand-alone compressor. The influence of droplet size on path trajectory is considered both computationally and experimentally to show that most rain droplets will collide with the first row of rotor blades. The water on the blades is then centrifuged towards the casing where the normal air flow patterns in the vicinity of the rotor tips are disrupted. The result of this disruption is a reduction in the delivery pressure and an increase the torque required to drive the compressor. Both effects reduce the efficiency of the machine. The behaviour of the water in the blade rows is examined in detail, and simple models are proposed to explain the loss of pressure rise and the increase in torque. The measurement were obtained in a low speed compressor making it possible to study the mechanical (increase in torque) and aerodynamic (reduction in pressure rise) effects of water ingestion without the added complication of thermodynamic effects.
Film Cooling From a Row of Holes Embedded in Transverse Slots
Authors: Lu Yiping , Nasir Hasan , Ekkad Srinath V.
Published: November 2008

Film cooling performance for a row of cylindrical holes can be enhanced by embedding the row in transverse slots. The geometry of the transverse slot greatly affects the cooling performance downstream of injection. The effect of the slot exit area and edge shape is investigated. Detailed heat transfer coefficient and film effectiveness measurements are obtained simultaneously using a single test transient IR thermography technique. The study is performed at a single mainstream Reynolds number based on free-stream velocity and film hole diameter of 7150 at three different coolant-to-mainstream blowing ratios of 0.5, 1.0, and 1.5. The results show that the film cooling holes provide higher film effectiveness when embedded in a slot. However, in some geometries when the slot begins at the upstream edge of the hole, the film effectiveness diminishes. The heat transfer coefficient enhancement due to the embedding is not significantly higher compared to the typical unembedded cylindrical hole. The overall heat flux ratio comparing film cooling with embedded holes to unembedded holes shows that the full slot and downstream slot spacing after the hole exit produce the highest heat flux reduction. The holes-in-slot geometry is certainly very promising.
Effect of Reynolds Number and Periodic Unsteady Wake Flow Condition on Boundary Layer Development, Separation, and Re-Attachment Along the Suction Surface of a Low Pressure Turbine Blade
Authors: O¨ztu¨rk B. , Schobeiri M. T. , Ashpis David E.
Published: November 2008

The paper experimentally studies the effects of periodic unsteady wake flow and different Reynolds numbers on boundary layer development, separation and re-attachment along the suction surface of a low pressure turbine blade. The experimental investigations were performed on a large scale, subsonic unsteady turbine cascade research facility at Turbomachinery Performance and Flow Research Laboratory (TPFL) of Texas A&M University. The experiments were carried out at Reynolds numbers of 110,000 and 150,000 (based on suction surface length and exit velocity). One steady and two different unsteady inlet flow conditions with the corresponding passing frequencies, wake velocities, and turbulence intensities were investigated. The reduced frequencies chosen cover the operating range of LP turbines. In addition to the unsteady boundary layer measurements, surface pressure measurements were performed. The inception, onset, and the extent of the separation bubble information collected from the pressure measurements were compared with the hot wire measurements. The results presented in ensemble-averaged, and the contour plot forms help to understand the physics of the separation phenomenon under periodic unsteady wake flow and different Reynolds number. It was found that the suction surface displayed a strong separation bubble for these three different reduced frequencies. For each condition, the locations defining the separation bubble were determined carefully analyzing and examining the pressure and mean velocity profile data. The location of the boundary layer separation was dependent of the Reynolds number. It is observed that starting point of the separation bubble and the re-attachment point move further downstream by increasing Reynolds number from 110,000 to 150,000. Also, the size of the separation bubble is smaller when compared to that for Re = 110,000.
An Entropy Loss Approach for a Meanline Bladerow Model With Coupling to Test Data and 3D CFD Results
Authors: Reed John A. , Turner Mark G.
Published: November 2008

A methodology which couples a computational fluid dynamics simulation and a ID meanline bladerow model employing entropy-based loss terms is presented. A 3D APNASA CFD flow solution of the GE90-94B was used to provide input to the 1D bladerow model, which computed entropy-generation terms from the flow state. These terms accounted for losses in mixing of leakage and cooling flow, and gross aerodynamics using bladerow entropy loss coefficients as defined by Denton. A description of the 1D model is presented. The 1D bladerow model was implemented in the NPSS system making it possible to easily construct N-stage component simulations. The 1D model was used to generate partial performance maps of the HPT and LPT for use in a coupled 0D-3D simulation of the full GE90 engine. To validate the approach, a data match of the GE EEE HPT at the design point has been made and presented. An extrapolation of the model to off-design points has compared favorably to the experimental data.
Axial Turbine Stage Design: 1D/2D/3D Simulation, Experiment, Optimization — Design of Single Stage Test Air Turbine Models and Validation of 1D/2D/3D Aerodynamic Computation Results Against Test Data
Authors: Moroz Leonid , Govoruschenko Yuri , Pagur Petr
Published: November 2008

In recent decade, industry had started to use intensively 3D simulation in turbine flow path and its components design. At the same time, this remains a very labor- and time-consumable process that sufficiently hampers its usage, whereas unidimensional and axisymmetric analyses are still widely used in the industry practice. A comparison of the data obtained from experiments conducted on a single stage air turbine test model with the results of 1D and 2D modeling and 3D simulation using a CFD solver was performed. The results were analyzed to validate a judgement of the authors that along with 3D CFD methods the low-fidelity models can be successfully used for turbine flow path optimization with the help of DoE methods. The forthcomings and advantages of different models are also discussed.
An Empirical Prediction Method for Secondary Losses in Turbines: Part II — A New Secondary Loss Correlation
Authors: Benner M. W. , Sjolander S. A. , Moustapha S. H.
Published: November 2008

A new empirical prediction method for design and off-design secondary losses in turbines has been developed. The empirical prediction method is based on a new loss breakdown scheme, and as discussed in Part I, the secondary loss definition in this new scheme differs from that in the conventional one. Therefore, a new secondary loss correlation for design and off-design incidence values has been developed. It is based on a database of linear cascade measurements from the present authors’ experiments (Benner [1]) as well as cases available in the open literature. The new correlation is based on correlating parameters that are similar to those used in existing correlations. This paper also focusses on providing physical insights into the relationship between these parameters and the loss generation mechanisms in the endwall region. To demonstrate the improvements achieved with the new prediction method, the measured cascade data are compared to predictions from the most recent design and off-design secondary loss correlations (Kacker and Okapuu [2], Moustapha et al. [3] using the conventional loss breakdown. The Kacker & Okapuu correlation is based on rotating-rig and engine data, and a scaling factor is needed to make their correlation predictions apply to the linear cascade environment. This suggests that there are additional and significant losses in the engine that are not present in the linear cascade environment.
A Numerical Approach for the Design of the Compressor Impeller Exit Diameter Variations in a Marine Engine Turbocharger
Authors: Kim Hong-Won , Ryu Seung-Hyup , Ghal Sang-Hak , Ha Ji-Soo , Lee Sang-Ki
Published: November 2008

A turbocharger has been designed to fulfill the requirements of medium speed marine engines, 900 kW output power at nominal operation point. The main objective of the design was to meet the requirements of engine power and increased engine operating range. This must be achieved by improving the degree of aerodynamic efficiency and the pressure ratio. The design was performed by the two stages. First, quasi-two dimensional program code was used to determine the main geometry of the compressor. Second, the detailed geometries of compressor blade were completed by using a three dimensional fluid flow analysis. The analyzed performance results were compared with the experimental data for the verification of their validity. Also, the designed three different impeller exits allow for a substantial performance variation. Increased compression ratio and mass flow rate of new optimized impeller were 10.08 and 27.3 percent higher than those of origin impeller, respectively. This simple change of design parameter offers considerable advantages to customers when upgrading their engines.
Long-to-Short Length Scale Transition: A Stall Inception Phenomenon in an Axial Compressor With Inlet Distortion
Authors: Lin Feng , Li Meilin , Chen Jingyi
Published: November 2008

A theoretical and experimental study of stall inception processes in a three-stage low-speed axial flow compressor with inlet distortion is presented in this paper. Since inlet distortion provides asymmetric flows imposing onto the compressor, the main goal of this research is to unveil the mechanism of how such flows initiate long and/or short length-scale disturbances and how the compression system reacts to those disturbances. It is found that the initial disturbances are always triggered by the distorted flows, yet the growth of such disturbances depends on system dynamics. While in many cases the stall precursors were the short length scale spikes, there were some cases where the compressor instability was triggered after the disturbances going through a long-to-short length scale transition. A Moore-Greitzer based (system scale) model was proposed to qualitatively explain this phenomenon. It was found that when the compressor operated in a region where the nonlinearity of the characteristics dominated, long length-scale disturbances induced by the inlet distortion would evolve into short length-scale disturbances before they disappeared or triggered stall. However, the model was not able to predict the fact that many disturbances that were triggered by the distorted sector(s) were completely damped out in the undistorted sector(s). It is thus suggested that in future research of compressor instability, one should consider the flows in blade passage scale, the dynamics in system scale and their interaction simultaneously.
The Experimental and Computational Study of a New Cooling Strategy for Turbomachinery Rotational Components
Authors: Elhabeshi N. I. , Guo S. M.
Published: November 2008

Many cooling methods have been developed for gas turbine rotational components. Film cooling is one of the most commonly used technique, which introduces cold air through film cooling holes onto the outer surfaces of gas turbine components to protect them from hot mainstreams. Although film cooling is very effective, it is well known that film cooling could reduce the engine power rating and introduce losses, mainly due to the direct coolant/mainstream interactions. In this paper, a preliminary study of a new closed-loop rotor cooling technique is reported. Filled with suitable heat transfer fluid, this closed loop design is applicable to turbomachinery rotors. Due to the combined buoyancy-centrifugal forces under turbine working conditions, the heat transfer fluid could transport the heat effectively from the outer surface of the blade to the internal cooling air. Instead of relying on the temperature gradient inside the blade walls for heat absorption from the high temperature mainstream side to the low temperature cooling airside, these closed loops act as heat transport superhighways. With proper insulation, the center part of the blade walls could be kept at a lower temperature, comparing to the conventional cooling designs. This would potentially provide vital important extra mechanical strength to the highly loaded turbine blades. Preliminary experimental work and CFD predictions have been conducted to prove this novel design. The tests were conducted using a rotational disk at a speed up to 1500 rpm. A liquid crystal based imaging system was employed to measure the surface temperature under transient conditions. Steady/unsteady 2D/3D CFD predictions were carried out to provide detailed information about the flow and temperature fields inside the proposed cooling configuration. Both experimental and computational work proved that this new design would work, providing the mechanical and manufacturing requirements could be met.
Effects of Surface-Roughness Geometry on Separation-Bubble Transition
Authors: Roberts Stephen K. , Yaras Metin I.
Published: November 2008

This paper presents measurements of separation-bubble transition over a range of surfaces with randomly distributed roughness elements. The tested roughness patterns represent the typical range of roughness conditions encountered on in-service turbine blades. Through these measurements, the effects of size and spacing of the roughness elements, and the tendency of the roughness pattern toward protrusions or depressions (skewness), on the inception location and rate of transition are evaluated. Increased roughness height, increased spacing of the roughness elements, and a tendency of the roughness pattern toward depressions (negative skewness) are observed to promote earlier transition inception. The observed effects of roughness spacing and skewness are found to be small in comparison to that of the roughness height. Variation in the dominant mode of instability in the separated shear layer is achieved through adjustment of the streamwise pressure distribution. The results provide examples for the extent of interaction between viscous and inviscid stability mechanisms.
Large-Eddy Simulation of Transition in a Separation Bubble
Authors: Roberts Stephen K. , Yaras Metin I.
Published: November 2008

In this paper, large-eddy simulation of the transition process in a separation bubble is compared to experimental results. The measurements and simulations are conducted under low free-stream turbulence conditions over a flat plate with a streamwise pressure distribution typical of those encountered on the suction side of turbine airfoils. The computational grid is sufficiently refined that the effects of sub-grid scale turbulence are adequately represented by the numerical dissipation of the computational algorithm. The large-eddy simulations are shown to accurately capture the transition process in the separated shear layer. The results of these simulations are used to gain further insight into the breakdown mechanisms in transitioning separation bubbles.
Optimization of Inlet Ring Groove Arrangement for Suppression of Unstable Flow in a Centrifugal Impeller
Authors: Ishida Masahiro , Sakaguchi Daisaku , Ueki Hironobu
Published: November 2008

An optimization of the inlet ring groove arrangement has been pursued in the present study for obtaining better impeller characteristics and a wider operation range at both small and large flow rates in a high specific speed type centrifugal impeller with inducer. The effects of the shape parameters with respect to the inlet ring groove on the impeller characteristic and the flow incidence were analyzed mainly based on numerical simulations, but also compared to the experimental results. At small flow rates, a significant improvement in the impeller characteristic is achieved due to reduction in the excessive-positive flow incidence by optimizing both location and width of the rear groove near the inducer tip throat. On the other hand, the impeller characteristic is improved at large flow rates by implementing the corner radius at the rear groove edge and by placing another front ring groove in the suction pipe. As a result, by the optimized configuration of the front and rear ring grooves, the unstable flow range of the test impeller can be reduced by about 50% without deterioration of the impeller characteristic even at the 125% flow rate.
Dimensional Analysis and Scaling of Rotating Seals
Authors: Denecke J. , Fa¨rber J. , Dullenkopf K. , Bauer H.-J.
Published: November 2008

To characterize contactless seals in turbo machinery, their discharge behavior, the development of the circumferential velocity (swirl) and the loss induced total temperature increase (windage heating) are of special interest for the designer. For the discharge behavior of non-rotating labyrinth seals, a well established set of non-dimensional numbers already exists: the discharge coefficient of two seals with different sizes but similar geometry is identical, if pressure ratio, axial Reynolds number, fluid properties and turbulence level are also identical. In this paper, the set of non-dimensional numbers is extended to cover swirl and windage heating using the well established Buckingham-π theorem to derive possible candidates. First, as a proof of concept, the known set of numbers for the non-rotating case was redeveloped and subsequently the influence of rotation was included. To validate the candidates, a comprehensive numerical parametric study was conducted. A variety of convergent and divergent stepped labyrinth seals was scaled from laboratory to typical engine conditions such that the dimensionless numbers stayed constant. Then, simulations at different rotational speeds, radii, and inlet circumferential velocities were performed to investigate the effects of rotation while maintaining nearly constant discharge behavior. The numerical data were used to validate the new non-dimensional numbers and to derive laws for the scaling of labyrinth seals. The non-dimensional numbers can also be applied to other seal types, such as brush or finger seals, because their theoretical deduction does not imply a specific geometry.
Experimental Investigation of the Total Temperature Increase and Swirl Development in Rotating Labyrinth Seals
Authors: Denecke J. , Dullenkopf K. , Wittig S. , Bauer H.-J.
Published: November 2008

Labyrinth seals are widely used as reliable components in many areas of turbo machines, e.g. the cooling air system in gas turbines. While the discharge behavior is generally well predictable, the uncertainty predicting the exit circumferential velocity (exit-swirl) and the total temperature increase due to internal losses (windage heating) is comparably large. In order to evaluate analytical correlations and for the validation of numerical simulations convergent and divergent stepped labyrinth seals were investigated experimentally. The change in total temperature across the labyrinth seal was measured in a test rig capable to establish different rotational speeds, pressure ratios and various inlet swirls. In an engine, honeycomb abrasive liners on the stator protect the seal fins. To simulate real engine conditions honeycombs were applied in the test setup, too and the influence of these liners on the windage heating was compared to smooth stator configurations. Detailed velocity profiles within the seal chambers were determined using a 2D Laser-Doppler-Velocimeter. Additionally, the ability of axisymmetric numerical k-ε simulations to predict the data was evaluated. The present study provides important data for the design of future turbo machines, because the exact knowledge of the labyrinth seal exit swirl and temperature is expected to further improve the design of downstream components such as the pre-swirl system. Additionally, more accurate boundary conditions for the thermal analysis will be available and the rotor dynamic stability of the seal can be estimated better.
Effects of Inlet Distortion on the Stability of an Advanced Military Swept Fan Stage With Casing Treatment
Authors: Madden Donald S. , West Matt A.
Published: November 2008

It is possible to improve the stability of a fan in both clean and distorted inlet flow through the use of forward sweep and/or rotor casing treatment. However, for a practical design, there is the need to know whether sweep, casing treatment or a combination of both is most effective for enhancing stability. A further complication is that both these measures can have implications on downstream blade rows. There is therefore a requirement to be able to analyse 3D blade designs with casing treatments within a multi-stage environment. This paper presents both experimental and numerical studies of an advanced low hub-tip-ratio military fan stage of pressure ratio 2.2 subjected to an undistorted and tip-low radial distorted inlet total pressure profile. The case discussed is the second build of this research stage and features a rotor which has been successfully re-designed with forward sweep to improve stability. The stage was tested with and without casing treatment. The intention of this paper is therefore to outline an efficient method whereby a rotor casing treatment can be modelled unsteadily as part of the full compressor or fan stage. To the authors knowledge, all previously published unsteady numerical simulations of slotted casing treatments have been confined to rotor only calculations. Numerical studies of the stage use a mixed approach to speed convergence, employing a fully unsteady simulation of a single rotor passage with casing treatment coupled via a mixing plane to a steady simulation of the downstream stator vane. These studies, supported by test results, indicate that the swept blade in undistorted flow does not stall at the rotor tip and interacts significantly towards stall with the downstream stator vane. However, with the introduction of tip-low inlet distortion the swept rotor can be made to stall at the tip. Both experimental and numerical studies show that in the case of tip-low radial distortion the stability line can be successfully extended by employing rotor casing treatment.
Part Load Operation of a Four-Stage Turbine
Authors: Herzog N. , Gu¨ndogdu Y. , Kang G. , Seume J. R. , Rothe K.
Published: November 2008

The design of bigger and more efficient steam turbines in the last decades led to a development of bigger blades even in high pressure steam turbines and therefore to increased problems with windage effects at certain operation points and during the shut down and particularly during trips of entire steam turbines. When the steam mass flow is low or even interrupted, it cannot cool down the rotating blades of the turbine. The kinetic energy of the rotor blades is thus transformed into thermal energy of the enclosed steam. The temperature increase in the steam leads to an increase in blade temperature which can lead to serious blade damage and therefore must be prevented. To better understand the aerodynamic characteristics and the flow mechanisms at part-load, investigations of the flow field at low Mach numbers have been undertaken in a four-stage research air turbine. Temperature, pressure, velocity and flow angles were measured in 6 different planes along the turbine annulus for different rotational speeds and different relative mass flows. CFD-simulations with the flow solver TRACE have been carried out and compared to the experimental data at partial load. The results show that the highest temperatures are developed in the last third of the turbine, which corresponds to experiences with observed damage in real turbines. The calculation results help to predict the critical operating conditions which lead to the highest thermal stresses and to quantify the rise in temperature. Such computational results for real multi-stage high-pressure turbines will help to develop rules for steam turbine operation and can help to prevent serious damage.
An Unsteady RANS Transition Model With Dynamic Description of Intermittency
Authors: Lodefier Koen , Dick Erik
Published: November 2008

A transition model for describing wake-induced transition is presented. It is based on the SST turbulence model by Menter, with the k–ω part in low-Reynolds form according to Wilcox, and two dynamic equations for intermittency: one for near-wall-intermittency and one for free-stream-intermittency. The total intermittency factor, which is the sum of the two, multiplies the turbulent viscosity computed by the turbulence model. The quality of the transition model is illustrated on the T106a test cascade using experimental results for flow with low free-stream turbulence intensity and transition in separated state and for flow with high free-stream turbulence intensity and transition in attached state. The unsteady results are presented in S–T diagrams of the shape factor and wall shear stress on the suction side. Results show the capability of the model to capture the basics of unsteady transition.