Abstract:
Maximizing the turbine entry temperature (TET) is fundamental to increase engine
efficiency and reducing fuel consumption. Nonetheless, safety and reliability
requirements have to be fulfilled. The life of gas turbine blades is strictly connected to
their temperature through the creep deformation process. For this reason temperature
monitoring is an essential requirement. Commonly this is achieved by means of
devices such as thermocouples which are placed in the bulk flow.
The usefulness of these devices as the means of supplying turbine blade temperature
information is limited given their slow response time and the fact that the blade
temperature is inferred from that of the surrounding gas rather than measured directly.
This in turn means that critical blades parts (e.g., trailing edge) or the presence of hot
spots are not identified in a discrete manner. These drawbacks can be addressed by
using instead a radiation pyrometer, which is characterized by a fast response time,
high accuracy, and by being contactless.
The pyrometer optical front-end is a lens which collects the radiation emitted by a
spot on the turbine blades. However, since the lens is exposed to the harsh engine
environment, contaminants entrapped by the turbine flow can therefore be easily
deposited on the lens thus filtering the radiation and resulting in an under-estimation
of the actual blade temperature.
The fouling of the lens is generally tackled by using a purge air system that employs
air bled from the compressor to divert those particles whose trajectory is directed
towards the lens.
Currently the employment of optical pyrometry is often confined to military
applications due to the fact that their turbine entry temperatures are higher than in civil applications. Besides, the maintenance schedule established for military engines
is far more frequent than what is practiced in airline engines. Therefore, the design of
current purge air systems reflects these facts. Before optical pyrometers can be
commonly used for civil applications more research is required since some of the
fundamentals of the fouling mechanisms remain to be clarified. This is then the
knowledge gap the present research sought to fill. Its aim was to conduct a comprehensive
investigation of the phenomena that underpins the lens fouling process in order to provide
a set of guidelines for optimising the design of purge air systems.
The initial part of the research was dedicated to the study of the purge flow inside a given
pyrometer configuration. The scope was to identify the main flow structure that
determines the fouling process and at the same time to validate the results obtained via
computational fluid dynamics (CFD) analyses conducted in a second phase of the
research. Given the reduced dimensions of the pyrometer purge system, it was not
possible to gain the appropriate optical access to take flow measurements.
Consequently, a large scaled experiment was performed, employing the Particle
Image Velocimetry (PIV) technique for the acquisition of experimental data of the
flow field. The distortion of the image and light reflection introduced by the presence
of curved glass surfaces was investigated by means of a feasibility experiment. The
experimental study highlighted the presence of a large recirculation zone that can trap
contaminants and direct them towards the pyrometer lens. The experimental data were
in agreement with computational fluid dynamics results obtained by using two
different turbulence models.
In a second instance, attention was focused on the particle deposition as seen from a fluid
dynamics perspective. A computational fluid dynamics analysis aimed at reproducing the
flow field of an existing pyrometer purge system enabled the identification of those
features that can significantly impact on the lens fouling process. It was found that the
geometry of the air curtain configuration plays a fundamental role. However, given the
high speeds involved, the main force governing the contaminants deposition is the
drag. Additionally, particles with high inertia hit the purge tube wall and then bounce
towards the pyrometer lens, while contaminants with low inertia can be trapped by a
large recirculation zone and subsequently directed towards the lens. In a third phase of the research, the impacts between the contaminant particles and the
lens were investigated through a finite element analysis (FEA) aimed at identifying the
most important factors that contribute to the lens fouling process. Particles moving at
low speed can be deposited on the lens by means of electrostatic and Van der Waals
forces. Conversely, particles with very high velocity can be deposited on the lens
through the same mechanisms involved in the cold spraying process, which is a
technique commonly used for coating deposition.
A local melting can occur at the interface between the lens and the contaminants due
to the high stresses created by the asperities and high sliding velocity of the particles.
As a result, while large particles bounce back, debris remains bonded to the lens
surface.
Last but not least, the findings of the several steps of the present research have been
brought together in order to produce guidelines to be followed by engineers engaged in
the redesign of more efficient pyrometer purge systems.