Aero-Thermal Numerical Predictions of Trailing Edge and Leading Edge Cooling Channels

State-of-art gas turbine are designed to operate at turbine inlet temperatures higher than 2000[K]. Such temperature levels are sustainable only by means of aggressive and efficient cooling of the components exposed to the hot gas path. It should be pointed out that not only must the maximum metal temperature be kept below safety limits, but the thermal field must be reasonably uniform too, in order to limit thermal stresses. Moreover, mod- ern blade cooling systems consist of a combination of internal cavities with cross-sections specifically developed for each different blade portion; therefore, specific studies are essen- tial to describe their performances in detail in order to provide designers with the most accurate knowledge. The need for such detailed information is in conflict with some common practices in cool- ing system design: most of the studies deal with square or rectangular channels cross- section (hence resembling ducts in the central body of the blade), the link between coolant flow field and heat transfer rates is seldom analyzed; finally, the coupling of rotation and different channel orientations is rarely taken into account. Over the last few years CFD (Computational Fluid Dynamic) has been exploited to pro- vide valuable information on complex flow fields and heat transfer in internal cooling passages; indeed, it is already used as an engineering tool in design and optimization processes of gas turbine cooling. On the other hand, the reliability of the numerical tools available at present is not sufficiently high and, hence, detailed experimental analyses are still required for numerical validation purposes. The present thesis focuses on the aspects pertaining to the suitability of CFD for the prediction of the aero-thermal performances inside cooling channels designed for two es- sential portions of the blade, namely trailing edge and leading edge, whose sizes and shapes are quite different from those resembling cooling channel in the central portion of the blade. The trailing edge cooling 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 side, where seven lengthened pedestals are also installed. Three different configurations are taken into account, namely the smooth channel and two others characterized by the use of ribs in different portions of the duct. Firstly, an extensive comparison with detailed experimental data including local flow velocities, turbulence proprieties and local heat transfer coefficient in static (Ro = 0) and orthogonal rotating conditions (Ro = 0.23) is carried out using the Shear Stress Transport (SST) turbulence model. Moreover, for one rib-roughened configuration in static condition (Ro = 0) different turbulence models are tested in order to enhance all computational results. Finally, the CFD code is exploited to analyzed more engine-like conditions, namely Ro = 0.46 and γ = 22.5 − 45[◦]. The results show that rotation and channel orientation produce contrasting effects which are more significant in the rib-roughened configuration. In fact, on the radial central portion rotation/orientation generates an increase/decrease in the heat transfer; conversely, on the trailing side region, rotation/orientation has a negative/positive effect on the thermal field. The leading edge cooling model consists of a straight, smooth channel with an equi- lateral triangle cross-section. Geometry and test conditions resemble those pertaining to the passages used for the internal cooling of gas turbine blades leading edge. On the same geometry and at comparable working conditions, heat transfer data are also available from literature. Experimental data are used for CFD validation purposes at Re = 20000 Ro = 0.2 and Re = 10000 Ro = 0.4. Consequently, a wide range of work- ing conditions, namely Re = 10000 − 40000 Ro = 0.2 − 0.6 are numerically explored by the SST turbulence model. The results show that the rotation-induced flow structure is rather complicated showing relevant differences compared to the flow models that have been supposed by the research community so 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 are 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 has- ten (in space) if the rotation number is increased while changes of the Re number have no effects. Moreover, thanks to the understanding of the flow mechanisms associated to rotation, it was possible to provide a precise justification for the channel thermal behav- ior. Finally, different channel orientations (namely γ = 22.5 − 45[◦]) are numerically investigated. The results further demonstrate that the variation of the channel orien- tation to more engine-like conditions significantly affects the flow field and, hence, the aero-thermal behavior.