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Coupled flow field and heat transfer in an advanced internal cooling scheme

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VKI PHDT 2011-02, Filippo Coletti, Coupled flow field and heat transfer in an advanced internal cooling scheme, ISBN 978-2-87516-019-5, 133 pgs

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Coupled flow field and heat transfer in an advanced internal cooling scheme

By Filippo Coletti

PhD Thesis from the von Karman Institute/Universität Stuttgart, October 2010, ISBN 978-2-87516-019-5, 133 pgs

Abstract

State-of-the-art gas turbines are designed to operate at turbine inlet temperatures in excess of 1850 K. Such temperature levels are sustainable only by means of an aggressive and efficient cooling of the components exposed to the hot gas path. Not only the maximum metal temperature needs to be kept below the safety limits, but the thermal field must be reasonably uniform, in order to limit the thermal stresses. This requires from the designer the most accurate knowledge of the local heat transfer rate. The need for such detailed information is in conflict with some common practices in the cooling system design: numerical simulations are often compared against area-averaged experimental data; the link between coolant flow field and heat transfer rates is scarcely analyzed; moreover, the coupling between convection and conduction is hardly taken into account.

The present thesis aims at the aero-thermal characterization of a trailing edge cooling channel geometry. Cooling the trailing edge, one of the life-limiting parts of the airfoil, represents an especially challenging task since the aerodynamic requirement of high slenderness is conflicting with the need of integrating internal passages. The focus of the study is on three main aspects of the internal cooling technology: (i) the details of the coolant flow field; (ii) the contribution of the obstacles’ surface to the heat transfer; and (iii) the effect of the conduction through the cooling channel walls.

The investigated cooling channel geometry is characterized by a trapezoidal cross-section. It has one rib-roughened wall and slots along two opposite walls. The coolant passes through a smooth inlet channel upstream of the investigated cavity; it crosses the divider wall through a first row of inclined slots, producing crossing-jets; the latter impinge on the rib-roughened wall, and the jet-rib interaction results in a complex flow pattern, rebounding the coolant towards the opposite smooth wall; finally the air exits through a second row of slots along the trailing edge. Such a scheme represents a combination of internal forced convection cooling and impingement cooling. An engine-representative Reynolds number equal to 67500 is defined at the entrance of the inlet channel.

A comprehensive experimental investigation is carried out on a magnified model of the channel, at a scale of 25 : 1 with respect to engine size (applied to both the cavity volume and the walls thickness). The main flow structures are identified and characterized by means of particle image velocimetry, allowing to deduce a model of the highly three-dimensional mean flow. Each jet is shown to impinge on the three ribs in front of the slot, and the jet-rib interaction produces two upward deflections in each inter-rib domain.

Distributions of heat transfer coefficient are obtained by means of liquid crystals thermography on the rib-roughened surface as well as on the opposite smooth wall in a purely convective regime, with a uniform heat flux imposed at the solid-fluid interface. The thermal patterns on the channel walls show the footprints of the flow features detected by the velocity measurements. Globally, the top side of the rib shows the highest Nusselt number among the investigated surfaces. The presence of the ribs enhances the global heat transfer level (averaged on all surfaces) by 14%. 

The aero-thermal results suggest a definite margin for improvement of the heat transfer performance by varying one or more geometrical parameters. In this perspective, the ribs have been tapered and shifted with respect to the slots position. Both expedients have proven to be useful in reducing the extent and intensity of the aforementioned hot spots in the vicinity of the ribs: an enhancement of about 20% in area-averaged heat transfer rate is achieved with respect to the standard configuration.  The thermal behavior of the ribbed wall has also been investigated in conjugate heat transfer regime, in order to study the effect of the wall conduction on the thermal levels. By matching the solid-to-fluid thermal conductivity ratio found in an engine, the correct thermal boundary conditions for the convective problem are attained, which guarantees full similarity between laboratory model and engine reality. Infra-red thermography coupled to a finite element analysis is used to retrieve the whole thermal pattern through the considered rib-roughened wall. Nusselt number levels in conjugate regime differ by up to 30% locally and 25% globally with respect to the purely convective results. Neglecting wall conduction when evaluating the heat transfer coefficient leads to underestimating the maximum surface temperature by 26 to 33 K at engine conditions. Decreasing the wall thermal conductivity increases the overall heat transfer coefficient on the ribbed surface. However lower conductivities amplify local temperature gradients and hot spots.

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