Cooling Flow Effects on Rotor Heat Transfer Distribution in Highly Loaded Axial Turbines
- Doctoral Thesis
Rights / licenseIn Copyright - Non-Commercial Use Permitted
In addition to their efficiency, the versatility of gas turbines is a major driver in their development for aero engines and power production. Advanced materials, cooling and thermal management are the main factors driving the reliable operation of gas turbines with main gas temperatures above their metal melting temperature. While adequate cooling is highly necessary, it comes at a cost. An improved understanding of heat transfer can reduce cooling requirements for components and shift the trade-off between increasing turbine inlet temperatures and associated cooling requirements for higher efficiencies. Improvements to design methods and new manufacturing techniques-such as neural network-based computational fluid dynamic optimisations and 3D printing-broaden the design space for engineers and facilitate increasingly integrated component designs. This results in a strong need for cost- and time-effective experimental testing and verification of numerical predictions. Due to the complexity of measuring high-resolution heat transfer on rotating components in gas turbine research facilities, relatively little experimental data are available in the open literature. Instead, most investigations are based on experiments in linear cascades with low geometrical complexity and simple inflow conditions. In particular, the unsteady flow field and interactions between main flow and cavity flows is not representative in such experiments. Established techniques used on rotating components such as thin-film gauges provide only single-point measurement data. This thesis contributes to the experimental setup and instrumentation for high-resolution heat transfer measurements on rotating turbomachinery components by improving a novel setup using a high-speed infrared-based technique. The rotor hub endwall heat transfer coefficient and purge flow cooling effectiveness of an endwall with contouring extended into the disc cavity is investigated for various purge flow injection rates. Using numerical simulations, the capabilities and challenges on heat transfer are assessed. The work in this thesis was performed within a joint industrial and academic research project to investigate the effect of advanced airfoil designs on the aerothermodynamics of gas turbines with cooling flows, with a special focus on heat transfer. Experimental measurements were performed in a state of the art and highly accurate axial turbine research facility. Aerodynamic measurements using pneumatic five-hole probes and fast response aerodynamic probes were performed to characterise the flow field, define boundary conditions and validate the numerical simulations. Moreover, a previously developed heat transfer measurement setup was improved for highly curved surfaces and rotor blade tip measurements. Insert-based custom-made thin-film resistive heaters based on chemical deposited nickel on a low thermal conductive substrate were manufactured and integrated on an aluminium bladed disk rotor to create controlled heat flux boundary conditions. The pressure and temperature on the rotor and the power for the heating platforms were acquired and controlled using a rotating data acquisition system mounted on the rotor. The test facility was modified to integrate the injection of dry cold air from a secondary loop to create a temperature difference between the injection cooling air and the main flow. A modular tip instrumentation setup was introduced to allow tip cooling integration for bladed disk rotors and simultaneous tip heat transfer measurements in a rainbow rotor approach with multiple geometries in a single test run. Complementary numerical investigations were performed using a Navier-Stokes solver for turbomachinery applications on a high-performance computing cluster. The developed technique for nickel thin-film resistance heaters was demonstrated to be reliable and versatile for complex geometries with high uniformity in the heat flux produced. The variable blade tip instrumentation was successfully implemented, used for measurement and proven to be robust with over 120 turbine start-ups and nearly 1000 turbine operating hours. For the investigated endwall geometry, local variations in heat transfer coefficient above ±20 % were observed between the blade suction side and the endwall hill for purge flow rate variations between 0:0 % and 1:2 %. The steep contouring hill deflects the main flow into the cavity and promotes a jet-like purge flow ejection from the cavity, thereby limiting the cooled platform area from the purge flow. The effect of purge flow on the endwall is limited to the front part of the platform upstream from the cross-passage migration of the secondary flow structures. Notably, both steady-state and unsteady simulations predicted the distribution of local heat transfer coefficients reasonably well in comparison to the experimental results. Regarding cooling effectiveness, an overestimation by the steady-state solution was observed due to suppression of the unsteady interaction between the main flow and cavity as well as the mixing of the two. To the best of the author’s knowledge, this study provides the first high-resolution heat transfer measurements at rotor blade tips performed in a rotating facility using a high-speed infrared camera and heat flux controlled boundary conditions. A unique data set of heat transfer coefficient data for various rotor blade tip geometries-with and without film cooling-has been acquired. Pronounced geometrical features on the rotor platform, such as upstream and cavity extended endwall contouring, can introduce regions of increased heat transfer that are not caused by secondary flows structures created in the passage and should be considered in the design phase. Due to the unsteady nature of the cavity and main flow interaction, unsteady simulations are required to correctly predict the cooling effect of purge flow injection on the hub endwall. Show more
External linksSearch print copy at ETH Library
SubjectHEAT TRANSFER; Turbomachinery; Cooling; Infrared Thermography; CFD
Organisational unit03548 - Abhari, Reza S. / Abhari, Reza S.
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