AbstractGas turbine designers continue to strive for higher engine efficiencies. The introduction of cooled turbine blades has increased the turbine entry temperature and thus the gas turbine efficiency. The cooling techniques employed can introduce aerodynamic losses in the turbine stage. The removal of mass flow from the compressor to supply cooling flow to the turbine stage results in a lower overall efficiency of the gas turbine cycle. Therefore, a designer endeavours to obtain the minimum required cooling flow to prevent thermal stresses from damaging the components. Endwall Contouring (EWC) can reduce the required cooling flow but also improve the efficiency of turbines by reducing the losses generated through secondary flow features, such as horseshoe vortex formation around turbine blades. These gains can be negated by the presence of cooling flow exiting the wheel-space between the stator and rotor. Computational Fluid Dynamic (CFD) simulations have been used to interrogate EWC designs but require experimental validation. This thesis describes the development of InfraRed Planar Laser-Induced Fluorescence (IR-PLIF) as an experimental technique applicable to the study of EWC and flow interactions in turbomachinery. To enable the study of EWC in the presence of purge flow a single-stage turbine facility was built and commissioned. IR-PLIF was developed through a series of test scenarios that incrementally built-up complexity.
The IR-PLIF system was successfully implemented into the Film Cooling Wind Tunnel (FCWT), a static environment ideal for optimisation. Two vibrational transitions were investigated for the IR-PLIF system, the (0000) → (1001) transition (2700 nm) and the (0000) → (2001) transition (2000 nm), with the former producing a high Signal-to-Noise-Ratio (SNR) for the low energy laser system at the University of Bath. The IR camera settings, used for fluorescence imaging, were optimised with a longer integration time increasing SNR. A delay between the laser emission and camera capture period was utilised to reduce the influence of reflections. Above a coolant-mainstream density ratio of 1.2, fluorescence self-absorption (quenching) becomes dominant, restricting the calibration scope. The laser energy, jet Reynolds number and hole geometry had no effect on the normalised calibration curve. It was concluded that a calibration curve should be generated before each experimental campaign.
Measurements were made with an IR gas analyser to validate the IR-PLIF measurements. The comparison between the two gas concentration measurement techniques indicated that the IR-PLIF captured the same qualitative trends as the gas analyser measurements. Importantly the comparison showed how the lack of resolution in gas analyser results can create an incomplete understanding of the fluid physics. The IR-PLIF technique benefits from a substantially reduced data acquisition time with an associated reduction in experimental cost.
IR-PLIF clearly showed the formation of kidney vortices and jet lift off for film cooling jets at high momentum flux ratios along with the following findings:
• Beyond a jet trajectory (s) to hole diameter ratio (D) of 1 the centreline concentration will decay at a rate of s-1.3 in the near field regardless of momentum flux ratio or boundary layer thickness.
• IR-PLIF clearly captures the detachment of the jet from the film cooling surface at an IR = 1.60 with the detachment point moving upstream as momentum flux ratio increases.
• A fixed jet detachment location of around x/D = 2.5 above a unity velocity ratio was discovered.
• A novel comparison of the locus of film cooling effectiveness compared to the kidney vortex location showed a cross-over point exists where the locus of effectiveness exceeds kidney vortex location in z/D regardless of momentum flux ratio at about x/D = 4.
This PhD thesis has demonstrated the first use of IR-PLIF of CO2 for investigation of turbomachinery flows and enabled the investigation of EWC and turbomachinery flows in future research. The non-invasive, high-resolution data generated by IR-PLIF provides powerful insights not available with traditional low resolution, experimentally time-consuming techniques for measuring gas concentration. The best practice for IR-PLIF developed in this thesis mean that researchers who have access to an IR camera and a CO2 laser targeted at either 2000nm or 2700nm can use this work to efficiently implement and optimise their own system to study gas concentration in a large variety of contexts.
Experience garnered from applying IR-PLIF to film cooling jets has already been used for future application to a more complex rotating environment for investigating EWC. The generality of the IR-PLIF technique has been demonstrated through multiple applications documented in this thesis. It is hoped that IR-PLIF will be employed by other researchers, both to augment existing fields (such as film cooling) or to permit new research avenues.
|Date of Award||14 Feb 2022|
|Supervisor||Carl Sangan (Supervisor), James Scobie (Supervisor) & Gary Lock (Supervisor)|