Cooling Flow Effects on Rotor Heat Transfer Distribution in Highly Loaded Axial Turbines
Open access
Author
Date
2020Type
- Doctoral Thesis
ETH Bibliography
yes
Altmetrics
Abstract
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
Permanent link
https://doi.org/10.3929/ethz-b-000470381Publication status
publishedExternal links
Search print copy at ETH Library
Publisher
ETH ZurichSubject
HEAT TRANSFER; Turbomachinery; Cooling; Infrared Thermography; CFDOrganisational unit
03548 - Abhari, Reza S. / Abhari, Reza S.
More
Show all metadata
ETH Bibliography
yes
Altmetrics