In order to determine the mechanical characteristics of various fixed and moving components (compressors, turbines, rotors, casings) and consequently their lifetimes it is essential that we take the heat transfers into account.
Two particular areas of application of aerothermodynamics are specific to turbomachinery. Firstly, it is a question of the so called internal flow that concerns the inter-rotor cavities as well as the cooling channels inside the blades of the fixed and moving turbines. The real geometries are complex, with discontinuities in order to improve the exchanges by convection. Secondly, the calculation of the so called external flow must take into account the presence of the cooling flows, which come out of the blades either through slots, especially on the trailing edge, or through perforations. The same thing goes for the internal and/or external casings. The complexity of the flow is thus increased and these effects must be taken into account in the general aerodynamic calculation.
Migration of Hot Spots in a High Pressure Turbine
Figure 1 : entropy field in the test section (mid-height)
(influence of the hot spots location)
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A 3D unsteady numerical simulation was performed using Navier-Stokes equations of the case of a high pressure turbine stage to model the temperature distortions (hot spots) on the turbine inlet. The computing configuration led to a mesh with 3.5 million points. The objective was to simulate the migration of a hot spot and understand its effects on the heat exchanges with the blades. The analysis of the flow confirmed experimental observations. Calculation showed that when a hot spot hits the turbine nozzle there are large reductions in heat flow on the rotor, in relation to a configuration in which the hot spot circulates in the middle of a stator channel (figure 1).
Turbine Blade Cooling by Film
Figure 2 : temperature field ( wall thermical print)
In order to improve engine performances, the temperature at the turbine inletmust be increased. Temperatures on input to turbines higher than 2,000 K can be achieved by using materials that resist heat stresses coupled with blade cooling systems. The impact of these cooling systems can be predicted by performing thermal calculations. A simulation of the cooling of turbine blades by the method known as "film cooling" was performed (fig. 2) using the Chimera technique implemented in the elsA software. This calculation was also used to validate the Chimera method which consists in interpolating the solution between superimposed blocks of meshes with points that do not coincide.
