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An experimental investigation on the aerodynamic behavior of cooling channels for the leading edge of gas turbine blades
Full text linkThe purpose of this dissertation is to provide a strong background knowledge about the flow field behavior inside leading edge internal cooling systems while addressing the importance of aerodynamic data for cooling channels design. This research is mainly focused on the characterization of rotation effects inside a triangular channel since, in the open literature, the only channel geometry exhaustively studied had been the rectangular one. In fact, in recent years, the interest of the scientific community had been shifted to the direct analysis of the thermal field, which is indeed the final figure of evaluation of the cooling system. However, the thermal field characterization alone (i.e. Nusselt number distribution), is not able to provide enough information in order to allow a complete and deep understanding of the heat transfer problem because it describes an effect giving too little clues about its cause, which is the aerodynamic field indeed. Even in simple geometries, the aerodynamic behavior usually is too much complex to be inferred just exploiting the thermal data. This is a major problem if an optimization process of the cooling system is considered: with the thermal data only a trial-and-error kind of iteration is possible. On the contrary, the knowledge of the flow field allow to operate in a more thoughtful way. For the reasons explained above, the first part of the research has been dedicated to the complete aerodynamic characterization of the simplest geometry suitable to represent a leading edge cooling system: a straight channel with triangular equilateral cross section, without bleeding holes, and with squared turbulence promoters perpendicular to the bulk velocity and placed on both leading and trailing sides. This campaign is the following of a previous work, where a smooth channel had been studied and the rotation effect was deeply characterized with both experimental and numerical analysis. In this work, the effect of centrifugal buoyancy forces is also taken into account and characterized. The data gathered with the PIV investigation had also been used to validate a numerical model and therefore extend the analysis, providing a complete characterization of the flow field evolution along the channel. The second part of the research project involves the investigation and complete characterization of the flow field inside an advanced leading edge cooling system, with impingement cooling and film cooling extraction holes. The aim was firstly to be able to correctly set the experiment boundary conditions and constraints, such as equal repartition of mass flow rate between each film cooling hole and maintain the same distribution in both static and rotating tests, in order to avoid a wrong experiment conditioning. Flow field measurements have been conducted mostly in order to evaluate if rotation effect in the jet's alimentation channel could affect its aerodynamics and the interaction between the jet and the coolant extraction holes
Rotational effects on the flow field inside a leading edge impingement cooling passage
Full text linkThe work reports for the first time detailed experimental data about the flow field inside an advanced leading edge cooling channel for gas turbine blades. The geometry key features are multiple internal impinging jests and coolant extraction for blade film cooling protection. Measurements have been performed by means of 2D and Stereo PIV, in both static and rotating conditions, with Reynolds number of 30k and 10k and a Rotation number of 0.05, both defined with reference to the jet characteristics. Different crossflow conditions in the feeding channel have been used to simulate the three main blade regions (i.e. HUB, MID, and TIP). The flow inside the feeding channel is significantly affected by rotation, conversely, when moving inside the main impingement duct, the jet core has been found to be only marginal modified due to rotation. Finally, a substantial Reynolds number independence has been found
Buoyancy effects at high rotation number on the flow field inside a triangular shaped rib roughened channel
No full textThe flow field inside a triangular cooling channel for the leading edge of a gas turbine blade has been investigated. The efforts were focused on the interaction between effects of rotation and those induced by turbulence promoters, i.e. perpendicular square ribs placed on both leading and trailing sides of the duct. PIV and Stereo-PIV measurements have been conducted for Re=10000, rotation number of 0, 0.2, and 0.6, and buoyancy parameter equal to 0, 0.08, and 0.7. Coriolis secondary flows are detected in the duct cross section, but contrary to the smooth case, they are characterized by a single main vortex and are less affected by an increase of the rotation parameter. Moreover, their main topology is only marginally affected by the buoyancy forces. Conversely, the features of the recirculation structure downstream the ribs turned out to be more sensible to a change of the sense of rotation and to buoyancy forces
Effects of rotation and buoyancy forces on the flow field behavior inside a triangular rib roughened channel
Full text linkThe flow field inside a triangular cooling channel for the leading edge of a gas turbine blade has been investigated. The efforts were focused on the investigation of the interaction between effects of rotation, of buoyancy forces, and those induced by turbulence promoters, i.e., perpendicular square ribs placed on both leading and trailing sides of the duct. Particle image velocimetry (PIV) and stereo-PIV measurements have been performed for Re-Dh = 10(4), rotation number of 0, 0.2, and 0.6, and buoyancy parameter equal to 0, 0.08, and 0.7. Coriolis secondary flows are detected in the duct cross section, but contrary to the smooth case, they are characterized by a single main vortex and are less affected by an increase of the rotation parameter. Moreover, their main topology is only marginally sensitive to the buoyancy forces. Conversely, the features of the recirculation structure downstream the ribs turned out to be more sensitive to both the buoyancy forces and to the stabilizing/destabilizing effect on the separated shear layer induced by rotation
Aerodynamic behavior under rotation of an advanced leading edge impingement cooling channel
No full textA preliminary characterization of the flow field inside an advanced leading edge cooling channel characterized by multiple internal impinging jests has been carried out by means of 2D and Stereo PIV. The measurements have been conducted both in the jets impingement region and in the feeding channel. Test conditions are characterized by a Reynolds number of 30k and a Rotation number of 0.05, both values are defined with reference to the jet characteristics. Different crossflow conditions, namely feeding channel over jet flow rates ratio, have been used to simulate the three main blade regions (i.e. HUB, MID, and TIP). The flow inside the feeding channel turned out to be significantly affected by rotation, with stronger effects found at blade TIP, where the local rotation number is higher. Conversely, when moving inside the main impingement duct, the jet core has been found to be only marginal modified due to rotation. Finally, Reynolds number effect has been also investigated by repeating measurements for the MID crossflow condition at Re=10k. A substantial Reynolds number independence has been found
Gas turbine blades internal cooling: Design, development and validation of a new rig for heat transfer measurements under rotation
No full textThe contribution describes part of the work carried out on a wider research project aimed to set up a new tool to study rotational effects on the heat transfer distribution inside realistic cooling passages for gas turbine blades.
Transient thermochromic liquid crystals (TLC) measurement technique is chosen in order to obtain spatially resolved heat transfer data. This obliges to perform the transient measurements with a cold temperature step on the coolant flow, in order to replicate correctly the buoyancy effects induced by rotation. This target is achieved by a new facility which components and working principle have been the subject of previous contributions. In the present paper, the progresses made in the development of the data processing methodology are described at first. Successively, a first step into the demanding rig and methodology validation process is commented by exploiting the results of a wide test campaign on a simple cooling channel geometry
Rotating heat transfer measurements on realistic multi-pass geometry
No full textIn this contribution, a novel rig was used to assess the heat transfer coefficients on a full internal multi pass cooling scheme.
Transient liquid crystal technique was used for the measurement of the heat transfer coefficient (HTC) on channel’s internal surfaces.
A first set of experiments were performed at engine similar conditions of Re=21000 and Ro=0.074. In order to assess the reliability of the measurement methodology and to explore the thermal behavior at higher rotation numbers, tests were also carried out at Re=17000 and Ro=0.074-0.11. From the spatially resolved HTC maps made available, it is possible to characterize the thermal performances of the whole passage and to highlight the effect of rotation
Rotating heat transfer measurements on a multi-pass internal cooling channel - II experimental tests
Full text linkThe present contribution describes the design and realization of a rotating test rig for heat transfer measurements on internal cooling passages of gas turbine blades. The aim is to study the effects of Coriolis and buoyancy forces on the heat transfer distribution inside realistic cooling schemes.
Spatially resolved heat transfer data are obtained by means of transient thermo-chromic liquid crystals (TLC) technique. In order to replicate the same buoyancy effects induced by the Coriolis forces during rotation, the transient measurements are performed with a cold temperature step on the coolant flow. New solutions are adopted to generate the cold temperature step, acquire the experimental data on board of the rotating test model and to control the experimental parameters during tests execution. The main components of the rig will be described in the paper, together with an overview of the data processing methodology that has been developed
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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