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Performance evaluation of turbine trailing edge cooling channels under static and rotating conditions
Full text linkLa presente tesi riporta l’analisi sperimentale delle prestazioni aerotermiche di un innovativo canale di raffreddamento per il bordo d’uscita di pale di una moderna turbina a gas di alta pressione sia in condizioni statiche che rotanti. Inoltre sono descritte anche la la progettazione e lo sviluppo di un banco prova e di innovative metodologie di indagine per il campo di moto all’interno di canali di raffreddamento posti in rotazion
Analisi sperimentale delle prestazioni aero-termiche di canali di raffreddamento per il bordo d'uscita di pale di turbina a gas
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Flow field analysis inside a gas turbine trailing edge cooling channel under static and rotating conditions: Effect of ribs
No full textThe present work is part of a wider research program which concerns the aero-thermal characterization of cooling channels for the trailing edge of gas turbine blades. The selected passage model is characterized by a trapezoidal cross-section of high aspect-ratio and coolant discharge at the blade tip and along the wedge-shaped trailing edge, where seven elongated pedestals are also installed. In this contribution, a new channel configuration provided with inclined ribs installed inside the radial development region is analyzed, extending the previous results and completing the already available data base, thus providing an overall review of the aero-thermal performance of the considered passage. The velocity field inside the channel was measured by means of 20 and Stereo-Ply techniques in multiple flow planes under static and rotating conditions. The tests were performed under engine similar conditions with respect to both Reynolds (Re 20,000) and Rotation (Ro = 0, 0.23) numbers. Time averaged flow fields and velocity fluctuation data inside the stationary and rotating channels are analyzed and also critically compared with the data acquired without ribs. In this way the effects on the flow field induced by both rotation and ribs are clearly described. In particular, the ribs modify substantially both the flow field on the channel walls where they are installed and the 3D separation structures that surround the pedestals. If also rotation is taken into account, the relative flow field is characterized by a considerable guiding effect of the ribs coupled with a stronger flow separation on the obstacles that further enhances the heat transfer performances. This behavior was confirmed exploiting the wide thermal data base already available, obtaining a direct link between the observed flow features and the heat transfer performance
Effects of injection conditions and Mach number on unsteadiness arising within coolant jets over a pressure side vane surface
No full textThe thermal performance of a gas turbine airfoil with a cooled pressure side and a trailing edge cutback is investigated and discussed in relationship with the unsteady behavior of coolant injection. The focus is on the pressure side coolant injection through discrete holes. The cascade was tested at an exit Mach number of 0.2 and 0.6 for different coolant to mainstream mass flow ratios. Laser Doppler Velocimetry (LDV) and high speed flow visualizations were used to investigate the unsteady mixing process taking place between coolant and main flow downstream of the holes. This behavior was correlated with the film cooling effectiveness distributions over the vane surface. In the first row, hairpin clockwise rotating vortices and counterclockwise shear layer vortices were observed, depending on the coolant to mainstream velocity ratio. The second row turned out to be characterized by a lower velocity ratio and lower turbulent activity, consistently with the higher thermal protection observed also at high injection rates. The increase in mainstream Mach number kept the jets closer to the wall, resulting in lower turbulent characteristics and lower mixing, consistently with higher thermal protection
Coriolis Effects on the Flow Field Inside a Rotating Triangular Channel for Leading Edge Cooling
No full textThe flow field inside a rotating smooth radial channel with a triangular shaped cross section is investigated. Test conditions resemble those pertaining to the passages used for the internal cooling of the gas turbine blade's leading edge. Heat transfer data are also available from the literature on the same geometry and at comparable working conditions and have been profitably used for a combined aerothermal analysis. The model consists of a straight smooth channel with an equilateral triangle cross section. The rotation axis is aligned with one of the triangle bisectors. Two dimensional particle image velocimetry (PIV) and stereo-PIV were used in order to characterize the inlet flow (in static conditions) and the rotation-induced secondary flow in the channel cross section at Re = 20,000, Ro = 0.2 and Re = 10,000, Ro = 0.4. A wider range of working conditions (Re = 10,000-40,000, Ro = 0.2-0.6) was explored by means of Reynolds averaged Navier-Stokes (RANS) simulations carefully validated by the available PIV data. The turbulence was modeled by means of the shear stress transport (SST) model with a hybrid near-wall treatment. The results show that the rotation-induced flow structure is rather complicated and show relevant differences compared to the flow models that have been considered thus far. Indeed, the secondary flow turned out to be characterized by the presence of two or more vortex cells, depending on channel location and Ro number. No separation or reattachment of these structures is found on the channel walls but they have been observed at the channel apexes. The stream-wise velocity distribution shows a velocity peak close to the lower apex and the overall flow structure does not reach a steady configuration along the channel length. This evolution is fastened (in space) if the rotation number is increased while changes of the Re number have no effect. Finally, due to the understanding of the flow mechanisms associated with rotation, it was possible to provide a precise justification of the channel thermal behavior
Flow field analysis inside a gas turbine trailing edge cooling channel under static and rotating conditions
No full textThe flow field inside a modern internal cooling channel specifically designed for the trailing edge of gas
turbine blades has been experimentally investigated under static and rotating conditions. The passage is
characterized by a trapezoidal cross-section of high aspect-ratio and coolant discharge at the blade tip
and along the wedge-shaped trailing edge, where seven elongated pedestals are also installed. The tests
were performed under engine similar conditions with respect to both Reynolds (Re = 20,000) and Rotation
(Ro = 0, 0.23) numbers, while particular care was put in the implementation of proper pressure conditions
at the channel exits to allow the comparison between data under static and rotating conditions.
The flow velocity was measured by means of 2D and Stereo-PIV techniques applied in the absolute frame
of reference. The relative velocity fields were obtained through a pre-processing procedure of the PIV
images developed on purpose.
Time averaged flow fields inside the stationary and rotating channels are analyzed and compared.
A substantial modification of the whole flow behavior due to rotational effects is commented, nevertheless
no trace of rotation induced secondary Coriolis vortices has been found because of the progressive
flow discharge along the trailing edge. For Ro = 0.23, at the channel inlet the high aspect-ratio of the cross
section enhances inviscid flow effects which determine a mass flow redistribution towards the leading
edge side. At the trailing edge exits, the distortion of the flow path observed in the channel central portion
causes a strong reduction in the dimensions of the 3D separation structures that surround the
pedestals
Comparison of rankine cycles for micro-CHP generation based on inward flow radial turbine or scroll expander
Full text linkThis contribution aims to analyze micro-CHP units based on Rankine cycles. Two types of expander are considered: a small scale inward flow radial turbine and a volumetric scroll type expander. This latter, should allow to overcome the limitation imposed by a standard steam-turbine that arise when the required shaft-power is very low. Moreover, the scroll expander will also allow to easily treat wet steams, which must be avoided when considering a turbo-expander. The final aim is to deduce which one of the two types of expander is more suitable, with a specified target performance and the availability of a certain hot source. In order to define the thermodynamic expansion process, the analysis uses a one-dimensional model of the radial turbine, previously developed by the authors, and of an estimation of the scroll expander efficiency. Also, the analysis is carried out for different working fluids, such as water, and two organic fluids, cyclohexane and toluene. Through the discussion of the results, for a specified set of constraints (e.g. expander inlet temperature, temperature of condensation, expander geometrical parameters) it is possible to deduce important indications on the most suitable expander for a given cycle layout
Experimental assessment of the aero-thermal performance of rib roughened trailing edge cooling channels for gas turbine blades
No full textBased on the combined analysis of detailed flow field and heat transfer experimental data, the aero-thermal behaviour of different trailing edge cooling channels is reported.
The reference geometry (GO) is characterized by a trapezoidal cross section of high aspect-ratio, inlet radial flow, and coolant discharge at both model tip and trailing edge, where seven elongated pedestals are also installed. Two variations of the reference geometry have squared ribs installed inside the channel radial central portion (G1) or inside the trailing edge exit region (G2). The forced convection heat transfer coefficient has been measured by means of a steady state Liquid Crystal Thermography (LCT) technique, while reliable and detailed flow measurements have been performed by means of Particle Image Velocimetry (PIV) or Stereo-Ply techniques. The experimental Reynolds number has been fixed at 20,000.
The heat transfer data for the three configurations have been analyzed and compared considering both local and channel-averaged features of the heat transfer fields. In particular, the flow mechanisms responsible for the existence of high or low heat transfer regions have been identified and explained. The effects of the different turbulence promoters on both the flow and heat transfer fields have been put in evidence as well. With the aim to determine the most effective configuration, area averaged heat transfer data have been compared, together with information about the channels pressure losses. Configuration G1 turned out to be the most promising, giving rise to a significant heat transfer enhancement associated to a moderate increase in pressure losses
Experimental Investigation of unsteadiness associated with film cooling flow ejection from the vane pressure side
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