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978-3-8439-2573-0, Reihe Luftfahrt
Unsteady Work Processes in a Low Pressure Turbine
133 Seiten, Dissertation Universität Stuttgart (2016), Softcover, A5
In this thesis, the unsteady turbine work and the underlying physical processes are analyzed. Methods are developed to validate and quantify the so-called wake differential work effect, an inviscid stator wake momentum recovery in subsequent turbine rotors due to unsteady work. If low-momentum wake fluid receives work through an unsteady, inviscid work mechanism, the momentum difference compared to the undisturbed free stream fluid is reduced, which offers a potential reduction in the overall mixing loss.
In a combined experimental and numerical approach, the static pressure field of a two-stage low-pressure turbine is analyzed in order to isolate and investigate local unsteady work regions, which are responsible for the differential work in upstream originated wakes.
Through a Lagrangian analysis, in which a large number of stator wake- and free stream particles are tracked through the subsequent turbine rotor, the existence of the wake differential work effect is verified on a statistically representative basis. With the Lagrangian method, on average a 14% elevated stagnation enthalpy of the stator wake fluid behind the first rotor is shown, which serves as a confirmation for the existence of the differential work effect.
A simplified, inviscid, one-dimensional model is created, which expresses unsteady work as an interaction of fluid particles with propagating static pressure waves. Through this model, the thermodynamic response of the fluid particles on a static pressure increase or decrease is described, in terms of estimating the kinetic energy and static enthalpy change due to the pressure wave.
In a final step, the developed pressure wave model is incorporated into a steady mixing sum, through which the influence of a differential work transfer between the two streams on the downstream mixing loss is approximated. It is concluded that a mixing loss reduction due to unsteady work effects is possible, but requires a work transfer region which is well-aligned in order to only transfer energy from the high-momentum free stream into the low-momentum wake. If this is not given, the unsteady work mechanism may act as significant loss source, instead.
The research presented here is based on the ATRD turbine, a full-scale engine- representative turbine rig preserving both Mach and Reynolds number similarity. The turbine is installed and tested at the Altitude Test Facility at Stuttgart University.