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978-3-8439-3388-9, Reihe Thermodynamik
Robin G. Brakmann
Increasing Heat Transfer in Convective Cooling Systems with Optimized Surface Structures
175 Seiten, Dissertation Universität Stuttgart (2017), Softcover, A5
The efficiency of a gas turbine depends mainly on the inlet gas temperature and the compression ratio. For modern gas turbine engines, the inlet gas temperature already exceeds the melting point of the blade and vane material, which makes internal cooling of the gas turbine blades and vanes a key technology.
This work focuses on impingement jet cooling in combination with turbulators on flat and on smooth curved surfaces; it includes a comprehensive literature review, the design of a new experimental rig and numerical and experimental investigations.
Two generic (enlarged) impingement cooling systems are studied; one models an array of impingement jets on a flat surface and the other models a row of impingement jets on a concave surface. The two systems represent the impingement jets in the mid-chord and the leading edge region of a turbine vane, respectively. The heat transfer is further enhanced using turbulators on the target plate. The effects of turbulators on flat surfaces are numerically studied and compared to experimental results. The heat transfer is measured experimentally´by the transient liquid crystal (TLC) method. The computational fluid dynamics (CFD) model is carried out within the software package ANSYS CFX. The model uses a steady-state three-dimensional Reynolds Averaged Navier Stokes (RANS) approach and the Shear Stress Transport (SST) turbulence model. Boundary conditions are chosen to simulate the experiments as close as possible. The numerical results agree well with the experimental data but over-predict the Nusselt numbers by around 10-15%.
The impingement array on the flat target plate consists of an impingement plate with 9×9 jet holes and a target plate equipped with different turbulators. The effects of different separation distances, crossflow schemes and jet Reynolds numbers are investigated. In total, about 150 turbulators have been tested numerically. Cubic-Micro-Pin-Fins are effective due to their larger target surface areas. Detached Ribs prevent separation, which would occur downstream of attached ribs. V-Ribs enhance the´secondary flow motion of the impingement system. Depending on the feature, the heat transfer can be increased by a factor up to 1.42.
A new test rig has been designed for impingement jets on concave surfaces. Different jet Reynolds numbers and crossflow schemes are investigated.