AbstractModern gas turbines employ Endwall Contouring (EWC) to improve the efficiency of gas turbines by reducing the losses associated with secondary flow. The secondary flow features are influenced by the egress flow exiting the wheel-space between stator and rotor. Previous studies have demonstrated that endwall contouring designs developed without egress flow may show a poorer performance once this is taken into account. This reveals the need to conduct a combined design of contoured endwalls in the presence of egress flow. CFD computations can be employed to generate these designs, but require experimental validation. This thesis describes the development of a novel experimental technique, Volumetric Velocimetry, in steps of increasing complexity.
The development can be divided into three main stages: (i) implementation of VV in testing a stationary facility for studying film cooling, (ii) the development of borescope probes to introduce the laser beam in rigs with restricted access, and (iii) implementation of VV inside a 1-stage constant duration turbine facility, and testing with different purge flow cases. These measurements show the full three-dimensional flow field including secondary flow features.
The novelty of the experimental technique required the design of a new facility, named the Film Cooling Wind Tunnel, to develop and debug the technique in a stationary turbomachinery scenario. The design was targeted towards optical techniques and included a modular design to accommodate a large range of flow and geometry parameters for studies of film cooling. Volumetric Velocimetry successfully captured the flowfield including the counter-rotating kidney vortices.
The measurements were validated against prior studies available in the literature. The effect of the momentum flux ratio, up to IR = 6.50, was assessed by investigating the 3D vortex structures of film cooling. The circulation of the kidney vortices was observed to increase significantly with increasing momentum flux ratio. For each momentum flux ratio case, the circulation was shown to increase to a peak and then decay along the streamwise direction. Vortex identification and tracking showed that the lateral separation of the counter-rotating vortex pair correlated well with the self-induced velocity dipole for high momentum flux ratios. The results demonstrated that VV is a powerful technique for capturing 3D flows. The experimental processes developed during this phase were taken forward to the rotating turbine.
Prior to implementation of Volumetric Velocimetry in a turbine facility it was necessary to develop borescope probes to introduce the laser beam into areas of limited optical access. To achieve this objective a modular software program was developed, capable of predicting the path of the laser beam through an array of lenses. The program features include: modularity of optical elements, laser power distribution, window refraction calculation, and an evaluation of laser power density at inlet and outlet. The program was employed to develop several designs, and also helped identify the challenges associated with this application, namely regarding the laser damage threshold of optical materials and the applicability of different optical elements. Several probe designs were manufactured in-house and tested. One of the probes was successfully employed to conduct Volumetric Velocimetry measurements in the Film Cooling Wind Tunnel, demonstrating good agreement with the results obtained previously. The program has been made open source for other researchers to benefit.
Volumetric Velocimetry was implemented for the first time in a rotating turbomachinery facility in the Large Annulus Rig, a new facility designed for studying both egress flow and endwall contouring. This implementation built upon the testing and borescope development carried out beforehand in the Film Cooling Wind Tunnel. The campaign in this facility required: (i) the development of a mathematical model of the wind tunnel, used for laser beam path prediction and for computing the clock angle of the turbine during the measurements; (ii) bespoke seeding; (iii) laser delivery using a borescope inserted in a windowed vane; and (iv) a method to transform the results from the camera reference frame into the absolute and the relative reference frames of the turbine. Measurements were performed without purge flow and with a sealed wheel-space. The results demonstrated that the presence of egress accentuates the pressure side horseshoe vortex, increasing the strength and penetration depth of the secondary flow features in the rotor passage. The purge flow was shown to enhance the velocity in the boundary layer just downstream of the rim-seal. Schreiner et al. (2019) conducted CFD computations for the same geometry and flow conditions and also concluded that the secondary flow features were enhanced by purge flow. The VV results validate the conclusions obtained with CFD computations.
This PhD thesis has demonstrated the first implementation of Volumetric Velocimetry for measurements in both static and rotating turbomachinery facilities. This has opened the door for future researchers to investigate the highly complex three-dimensional flowfields associated with egress flow and mainstream gas path interactions, as well as the impact of EWC.
|Date of Award||13 May 2020|
|Supervisor||David Cleaver (Supervisor), Carl Sangan (Supervisor) & Gary Lock (Supervisor)|
- Volumetric Velocimetry
- film cooling
- momentum flux ratio
- egress flow
- data visualization