Abstract
Industrial gas turbines form an integral part of electricity generating networks, providing base load at high efficiencies. This high efficiency has been developed over time through both aerodynamic improvements and increased turbine entry temperatures. While materials science has improved the high temperature performance of many components it is still beneficial, and in some cases necessary, to protect these parts from the high temperatures to prevent oxidation and creep. This is achieved through the use of component cooling technology and can take a myriad of forms.A key cooling technology is the use of cold gas bled from the compressor to seal the turbine wheelspace from hot gas ingestion, protecting the turbine disc from exposure to high temperatures. Minimising the amount of gas bled from the compressor is essential to maintaining high cycle efficiency, which must be balanced with the need for a fully sealed condition. A major drawback of this is that coolant gas escapes through the turbine rim seal into the mainstream ow, negatively affecting the mainstream ow through a blockage effect and a strengthening of secondary flows in the blade passage. These are both unsteady phenomena.
A number of authors have successfully utilised non-axisymmetric endwalls to
increase turbine stage efficiencies by weakening or controlling the secondary ow
features seen in a blade passage. This has primarily been achieved through the
use of steady state computational fluid dynamics and a thorough understanding of the desired flow field. However, it has also been shown that these non-axisymmetric endwalls are more sensitive to the introduction of purge flow egress than the cylindrical baseline and any benefit is quickly eroded. This is due to the interaction between purge and mainstream flows, which contributes additional losses in the stage. This has driven industry to pursue a combined approach to endwall design, encompassing rim-seal, seal clearance profile and contoured endwalls. Non-axisymmetric endwall contouring in the presence of purge flow forms the focus of this thesis, with emphasis on the application of numerical methods to the design of novel endwalls.
The new experimental facility at the University of Bath is described in detail. This facility is intended to provide researchers with the opportunity to investigate the fundamental fluid dynamics of purge-mainstream interactions. The design of the
facility inlet is presented, with emphasis on the delivery of well-conditioned flow to the turbine stage. The iterative design process introduced several features that enabled this to be achieved, as validated by test data.
A numerical model of the turbine stage was developed for a selection of geometric representations. This model was used to demonstrate grid and timestep sensitivity, assess iterative convergence and provide validation against test data. It was found that the inclusion of the full wheelspace extent prevented the solution from achieving periodic behaviour when purge ow was present. However, when purge flow was removed, periodic behaviour was achieved. Periodic behaviour for purged cases was obtained for reduced wheelspace extent models.
The interaction characteristics of purge and mainstream flows was numerically
investigated. Results are presented that enhance the understanding of both geometric effects and purge flow on the evolution of secondary ow structures. Flow in the blade passage matches the classical secondary ow structure for annulus only calculations. The flow becomes modified by the inclusion of rim-seal geometry, with both ingress and egress present for an unpurged case. Finally, for the purged case, a plume of egress is seen to exit the rim-seal near the blade suction-surface. This interacts with the secondary flow structures, causing the pressure-side leg of the horseshoe vortex to increase in strength. Radial displacement of the vortical structures occurs and the suction-side leg of the horseshoe is suppressed. The egress plume becomes entwined with the passage vortex as it lifts off the hub endwall.
A novel method for endwall design was developed and presented. Discrete features are implemented to obtain specific influences on the flow structures in the blade passage. These features were investigated in isolation and then combined, resulting in a notable improvement to stage efficiency. The suction-side trough was used to control the position of the egress plume and reduce unsteady fluctuations. The leading-edge feature was used to weaken the horseshoe vortex formation at the blade leading edge.
| Date of Award | 24 Jun 2020 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Michael Wilson (Supervisor) & Carl Sangan (Supervisor) |