Special Issue "Secondary Air Systems in Gas Turbine Engines"

A special issue of Aerospace (ISSN 2226-4310).

Deadline for manuscript submissions: closed (31 January 2019).

Special Issue Editor

Dr. Erinc Erdem
Website
Guest Editor
School of Engineering, University of Glasgow, Glasgow G12 8QQ, Scotland, UK; TUSAS Engine Industries (TEI), Esentepe Mah. Cevreyolu Bulvari no.356, Eskisehir, Turkey
Interests: aerothermal analysis; turbomachinery; modelling and simulation; ground testing; measurement techniques; propulsion

Special Issue Information

Dear Colleagues,

Modern gas turbine engines are presently being pushed to the limits of thermal efficiency, owing to recent advancements in materials and cooling technologies. The hot section of a gas turbine engine works above the limits of material capabilities. Consequently, there is a high demand for cooling and sealing to assure safe and sound operation throughout the operational envelope of an engine. Secondary Air Systems (SAS) play a significant role in gas turbine engines to accomplish reliable operation of the individual modules as well as the whole engine. Main functions of SAS are to provide cooling flow to engine components, to seal bearing chambers (sumps) and to control bearing axial loads. Being a functional discipline, SAS owns the airflow that is essentially not the primary flowpath.

Traditionally, the design of secondary air systems utilized industrial friendly “one-dimensional modeling” for both compressible internal rotating/non-rotating fluid flow and heat transfer. Many correlations were developed to model/compute the flows with reasonable accuracy, taking into account of heat pickups on the way in flow circuits. Testing is an integral part of the design process comprising of flow testing of components, module testing and whole engine testing; providing essential data to characterize specific flow elements and circuits.

This collection invites papers that address the areas of SAS in gas turbine engines encompassing aviation, power generation and industrial applications. Of interest are papers that address novel approaches in flow network modeling, contemporary modeling and experimental efforts in rotor-stator/ rotor-rotor cavities, windage measurements and predictions, advanced flow network modeling to include transient behaviors, advanced sealing technologies, axial load control strategies, rim sealing developments and sump pressurization aspects.

Dr. Erinc Erdem
Guest Editor

Manuscript Submission Information

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Keywords

  • Gas turbine engines
  • Secondary air systems
  • Gas turbine sealing technologies
  • One-dimensional flow network modeling
  • Rotor-stator/rotor-rotor cavities
  • Compressible internal flows
  • Heat transfer Gas turbine engine testing

Published Papers (8 papers)

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Research

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Open AccessArticle
Unsteady Flow Structures within a Turbine Rim Seal Cavity in the Presence of Purge Flow—An Experimental and Computational Unsteady Aerodynamics Investigation
Aerospace 2019, 6(5), 60; https://doi.org/10.3390/aerospace6050060 - 25 May 2019
Abstract
Flow within the space between the rotor and stator of a turbine disk, and an area referred to as the rim seal cavity, develops azimuthal velocity component from the rotor disk. The fluid within develops unsteady structures that move at a fraction of [...] Read more.
Flow within the space between the rotor and stator of a turbine disk, and an area referred to as the rim seal cavity, develops azimuthal velocity component from the rotor disk. The fluid within develops unsteady structures that move at a fraction of the rotor speed. A test is designed to measure the number of unsteady structures and the rotational speed at which they are moving in the rim seal cavity of an experimental research rig. Data manipulation was developed to extract the speed, and the numbers of structures present using two fast-response aerodynamic probes measuring static pressure on the surface of the nozzle guide vane (NGV)-side rim seal cavity. A computational study is done to compare measured results to a transient unsteady Reynolds-averaged Navier–Stokes (URANS). The computational simulation consists of eight vanes and ten blades, carefully picked to reduce the error caused by blade vane pitch mismatch and to allow for the structures to develop correctly, and the rim seal cavity to measure the speed and number of the structures. The experimental results found 15 structures moving at 77.5% of the rotor speed, and the computational study suggested 14.5 structures are moving at 81.7% rotor speed. The agreement represents the first known test of its kind in a large-scale turbine test rig and the first known “good” agreement between computational and experimental work. Full article
(This article belongs to the Special Issue Secondary Air Systems in Gas Turbine Engines)
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Open AccessArticle
Effect of Ingress on Flow and Heat Transfer Upstream and Downstream of a Rotating Turbine Disc
Aerospace 2019, 6(5), 49; https://doi.org/10.3390/aerospace6050049 - 26 Apr 2019
Abstract
Ingress is the penetration of a hot mainstream gas in a turbine annulus through the rim seal into the wheel-space between the rotating turbine disc (the rotor) and the adjacent stationary casing (the stator). Purge flow is used to prevent or reduce ingress, [...] Read more.
Ingress is the penetration of a hot mainstream gas in a turbine annulus through the rim seal into the wheel-space between the rotating turbine disc (the rotor) and the adjacent stationary casing (the stator). Purge flow is used to prevent or reduce ingress, and the sealing effectiveness relates the flow rates of the purge and ingress. In this paper, an adiabatic effectiveness is used to relate the temperatures of a thermally-insulated rotor, the purge flow and the ingress. A non-dimensional buffer parameter, Ψ, is used to relate the sealing effectiveness on the stator and the adiabatic effectiveness on the rotor, respectively. This paper reports the first experimental study of the effect of ingress and purge flow on the adiabatic temperatures of both upstream and downstream surfaces of the rotor. Measurements of concentration and swirl over a range of purge have been obtained in wheel-spaces upstream and downstream of the rotor in a turbine rig. In transient heating tests, fast-response thermocouples were used to measure the temperature of the air in the wheel-space core; simultaneously, the temperatures of the upstream and downstream rotor surfaces were determined from infra-red sensors. The extrapolated steady-state temperatures (obtained using a maximum-likelihood estimation analysis) were used to determine the adiabatic effectiveness as a function of purge flow rate. The buffer effect of the purge flow for both wheel-spaces was quantified via comparisons between the variation of Ψ with purge flow rate. It was shown that the sealing effectiveness for the downstream wheel-space was larger than for the upstream. Consequently, and consistent with the theoretical model, the buffering effect of the purge flow was shown to be smaller downstream. Full article
(This article belongs to the Special Issue Secondary Air Systems in Gas Turbine Engines)
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Open AccessArticle
Discharge Coefficients of Ports with Stepped Inlets
Aerospace 2018, 5(3), 97; https://doi.org/10.3390/aerospace5030097 - 19 Sep 2018
Abstract
Components of aeronautical gas turbines are increasingly being constructed from two layers, including a pressure containing skin, which is then protected by a thermal tile. Between them, pedestals and/or other heat transfer enhancing features are often employed. This results in air admission ports [...] Read more.
Components of aeronautical gas turbines are increasingly being constructed from two layers, including a pressure containing skin, which is then protected by a thermal tile. Between them, pedestals and/or other heat transfer enhancing features are often employed. This results in air admission ports through the dual skin having a step feature at the inlet. Experimental data have been captured for stepped ports with a cross flow approach, which show a marked increase of 20% to 25% in discharge coefficient due to inlet step sizes typical of combustion chamber configurations. In this respect, the step behaves in a fashion comparable to ports with inlet chamfering or radiusing; the discharge coefficient is increased as a result of a reduction in the size of the vena contracta brought about by changes to the flow at inlet to the port. Radiused and chamfered ports have been the subject of previous studies, and empirical correlations exist to predict their discharge coefficient as used in many one-dimensional flow network tools. A method to predict the discharge coefficient change due to a step is suggested: converting the effect of the step into an equivalent radius to diameter ratio available in existing correlation approaches. An additional factor of eccentricity between the hole in the two skins is also considered. Eccentricity is shown to reduce discharge coefficient by up to 10% for some configurations, which is more pronounced at higher port mass flow ingestion fraction. Full article
(This article belongs to the Special Issue Secondary Air Systems in Gas Turbine Engines)
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Open AccessArticle
Continued Experimental Study on the Friction Contact between a Labyrinth Seal Fin and a Honeycomb Stator: Slanted Position
Aerospace 2018, 5(3), 82; https://doi.org/10.3390/aerospace5030082 - 07 Aug 2018
Cited by 1
Abstract
Labyrinth seals are a state-of-the-art sealing technology to prevent and control leakage flows at rotor–stator interfaces in turbomachinery. Higher pressure ratios and the economical use of cooling air require small clearances, which lead to potential rubbing events. The use of honeycomb liners allows [...] Read more.
Labyrinth seals are a state-of-the-art sealing technology to prevent and control leakage flows at rotor–stator interfaces in turbomachinery. Higher pressure ratios and the economical use of cooling air require small clearances, which lead to potential rubbing events. The use of honeycomb liners allows for minimal leakage by tolerating rub events to a certain extent. A previous study within an EU project investigated the complex contact conditions of honeycomb liners, with the idealized contact of a seal fin and a single parallel metal foil representing the honeycomb double foil section. In the present work, the results for the slanted foil position are shown and compared to the previous results. The variation of rub velocity, incursion speed, incursion rate, and seal geometry in a test rig allows for the identification of the influence on contact forces, temperatures, and wear. For the slanted position, significantly lower friction temperatures are observed, leading to a higher ratio of abrasive wear. Overall, the rub test results demonstrate strong interactions between the contact forces, friction temperatures, and wear. Full article
(This article belongs to the Special Issue Secondary Air Systems in Gas Turbine Engines)
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Open AccessArticle
Effects of the Back Plate Inner Diameter on the Frictional Heat Input and General Performance of Brush Seals
Aerospace 2018, 5(2), 58; https://doi.org/10.3390/aerospace5020058 - 25 May 2018
Cited by 2
Abstract
Reducing losses in the secondary air system of gas and steam turbines can significantly increase the efficiency of such machines. Meanwhile, brush seals are a widely used alternative to labyrinth seals. Their most valuable advantage over other sealing concepts is the very small [...] Read more.
Reducing losses in the secondary air system of gas and steam turbines can significantly increase the efficiency of such machines. Meanwhile, brush seals are a widely used alternative to labyrinth seals. Their most valuable advantage over other sealing concepts is the very small gap between the sealing package and the rotor and thus reduced leakage mass flow. This small gap can be achieved due to the great radial flexibility without running the risk of severe detrimental deterioration in case of rubbing. Rubbing between rotor and seal during operation might occur as a result of e.g., an unequal thermal expansion of the rotor and stator or a rotor elongation due to centrifugal forces or manoeuvre forces. Thanks to the flexible structure of the brush seal, the contact forces during a rubbing event are reduced; however, the frictional heat input can still be considerable. Particularly in aircraft engines with their thin and lightweight rotor structures, the permissible material stresses can easily be exceeded by an increased heat input and thus harm the engine’s integrity. The geometry of the seal has a decisive influence on the resulting contact forces and consequently the heat input. This paper is a contribution to further understand the influence of the geometrical parameters of the brush seal on the heat input and the leakage during the rubbing of the seal on the rotor. In this paper, a total of three seals with varied back plate inner diameter are examined in more detail. The experimental tests were carried out on the brush seal test rig of the Institute of Thermal Turbomachinery (ITS) under machine-relevant conditions. These are represented by pressure differences of 1 to 7 bar, surface speeds of 30 to 180 m/s and radial interferences of 0.1 to 0.4 mm. For a better interpretation, the results were compared with those obtained at the static test rig of the Institute of Jet Propulsion and Turbomachinery (IFAS) at the Technical University of Braunschweig. The stiffness, the blow-down and the axial behaviour of the seals as a function of the differential pressure can be examined at this test rig. It could be shown that the back plate inner diameter has a decisive influence on the overall operating behaviour of a brush seal. Full article
(This article belongs to the Special Issue Secondary Air Systems in Gas Turbine Engines)
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Open AccessArticle
Prediction of Heat Transfer in a Jet Cooled Aircraft Engine Compressor Cone Based on Statistical Methods
Aerospace 2018, 5(2), 51; https://doi.org/10.3390/aerospace5020051 - 01 May 2018
Cited by 1
Abstract
The paper presents the setup and analysis of an experimental study on heat transfer of a jet cooled compressor rear cone with adjacent conical housing. The main goal of the paper is to describe the systematic derivation of empirical correlations for global Nusselt [...] Read more.
The paper presents the setup and analysis of an experimental study on heat transfer of a jet cooled compressor rear cone with adjacent conical housing. The main goal of the paper is to describe the systematic derivation of empirical correlations for global Nusselt numbers to be used in the design process of a jet engine secondary air system. Based on the relevant similarity parameters obtained from literature, operating points are deduced leading to a full factorial design experiment to identify all effects and interactions. The varied similarity parameters are the circumferential Reynolds number, the non-dimensional mass flow, the non-dimensional spacing between rotor and stator, and the jet incidence angle. The range of the varied similarity parameters covers engine oriented operating conditions and is therefore suitable to predict Nusselt numbers in the actual engine component. In order to estimate measurement uncertainties, a simplified model of the test specimen, consisting of a convectively cooled flat plate, has been derived. Uncertainties of the measured quantities and derived properties are discussed by means of a linear propagation of uncertainties. A sensitivity study shows the effects of the input parameters and their interactions on the global Nusselt number. Subsequently, an empirical correlation for the global Nusselt numbers is derived using a multivariate non-linear regression. The quality of the empirical correlation is assessed by means of statistical hypotheses and by a comparison between measured and predicted data. The predicted values show excellent agreement with experimental data. In a wide range, accuracies of 15% can be reached when predicting global Nusselt numbers. Furthermore, the results of the sensitivity study show that pre-swirled cooling air does not have a positive effect on heat transfer. Full article
(This article belongs to the Special Issue Secondary Air Systems in Gas Turbine Engines)
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Open AccessArticle
Numerical Investigation on Windback Seals Used in Aero Engines
Aerospace 2018, 5(1), 12; https://doi.org/10.3390/aerospace5010012 - 20 Jan 2018
Cited by 2
Abstract
Seals are considered one of the most important flow elements in turbomachinery applications. The most traditional and widely known seal is the labyrinth seal but in recent years other types like the brush or carbon seals were introduced since they considerably reduce the [...] Read more.
Seals are considered one of the most important flow elements in turbomachinery applications. The most traditional and widely known seal is the labyrinth seal but in recent years other types like the brush or carbon seals were introduced since they considerably reduce the sealing air consumption. When seals are used for sealing of aero engine bearing chambers they are subjected to high “bombardment” through oil particles which are present in the bearing chamber. These particles mainly result from the bearings as a consequence of the high rotational speeds. Particularly when carbon or brush seals are used, problems with carbon formation (coking) may arise when oil gets trapped in the very tight gap of these seals. In order to prevent oil migration into the turbomachinery, particularly when the pressure difference over a seal is small or even negligible, significant improvement can be achieved through the introduction of so called windback seals. This seal has a row of static helical teeth (thread) and below this thread a scalloped or smooth shaft section is rotating. Depending on the application, a windback seal can be used alone or as a combination with another seal (carbon, brush or labyrinth seal). A CFD analysis carried out with ANSYS CFX version 11 is presented in this paper with the aim to investigate this seal type. The simulations were performed by assuming a two-phase flow of air and oil in the bearing compartment. Design parameters like seal clearance, thread size, scallop width, were investigated at different operating conditions. Full article
(This article belongs to the Special Issue Secondary Air Systems in Gas Turbine Engines)
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Review

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Open AccessReview
Buoyancy-Induced Heat Transfer inside Compressor Rotors: Overview of Theoretical Models
Aerospace 2018, 5(1), 32; https://doi.org/10.3390/aerospace5010032 - 17 Mar 2018
Cited by 1
Abstract
Increasing pressures in gas-turbine compressors, particularly in aeroengines where the pressure ratios can be above 50:1, require smaller compressor blades and an increasing focus on blade-clearance control. The blade clearance depends on the radial growth of the compressor discs, which in turn depends [...] Read more.
Increasing pressures in gas-turbine compressors, particularly in aeroengines where the pressure ratios can be above 50:1, require smaller compressor blades and an increasing focus on blade-clearance control. The blade clearance depends on the radial growth of the compressor discs, which in turn depends on the temperature and stress in the discs. As the flow inside the disc cavities is buoyancy-driven, calculation of the disc temperature is a conjugate problem: the heat transfer from the disc is coupled with the air temperature inside the cavity. The flow inside the cavity is three-dimensional, unsteady and unstable, so computational fluid dynamics is not only expensive and time-consuming, it is also unable to achieve accurate solutions at the high Grashof numbers found in modern compressors. Many designers rely on empirical equations based on inappropriate physical models, and recently the authors have produced a series of papers on physically-based theoretical modelling of buoyancy-induced heat transfer in the rotating cavities found inside compressor rotors. Predictions from these models, all of which are for laminar flow, have been validated using measurements made in open and closed compressor rigs for a range of flow parameters representative of those found inside compressor rotors. (The fact that laminar buoyancy models can be used for large Grashof numbers (up to 10 12 ), where most engineers expect the flow to be turbulent, is attributed to the large Coriolis accelerations in the fluid core and to the fact that there is only a small difference between the rotational speed of the core and that of the discs.) As many as 223 separate tests were analysed in the validation of the models, and good agreement between the predictions and measurements was achieved for most of these cases. This overview paper has collected together the equations from these papers, which should be helpful to designers and research workers. The paper also points out the limitations of the models, all of which are for steady flow, and shows where further experimental evidence is needed. Full article
(This article belongs to the Special Issue Secondary Air Systems in Gas Turbine Engines)
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