The gas turbine engine, as an adaptable source of power, has been used extensively for electric power generation, mechanical drive and jet propulsion. Driven by competition within the industry, gas turbine engine manufactures strive to produce ever more efficient products that also comply with emission regulations. The overall efficiency, as a crucial parameter governing engine performance and life cycle operating costs, depends on a high turbine inlet temperature, as well as an appropriately high pressure ratio across the compressor. The turbine components operating at elevated temperatures could experience serious problems, such as unwanted creep, oxidation or thermal fatigue, which compromise the integrity and reduce lifespan. In modern gas turbine engines, cooling air bled from the compressor is used to prevent overheating of the turbine, through the secondary air system. As much as 25% of the compressor air bypasses combustion to be used for cooling and sealing purposes.Hot gas ingress is one of the most important and intricate problems of the secondary air system faced by engine designers. Ingress occurs when the hot gas from the mainstream is ingested into the wheel space, formed by the turbine disc and its adjacent casing. A rim seal is fitted at the periphery of the wheel-space, and a sealing flow of coolant is used to purge the cavity reducing or preventing ingress. Sufficient sealing flow is required, but an excessive use of coolant decreases overall engine efficiency. Therefore, from engine designers’ perspective the use of sealing air must be minimised. Optimisation of the rim-seal design is a crucial approach to fulfil the purpose. The double-clearance rim seal is widely employed in gas turbines, which consists of an outer-seal at the periphery of the wheel-space and an inner-seal located radially inboard. The annular cavity formed between the double-seal arrangement is known to predominantly confine the ingested hot gases, thus significantly reducing ingestion inboard of the inner seal. In order to study how double-clearance seals operate in depth, this thesis describes a new single stage turbine research facility, designed for conducting extensive and comprehensive experimental studies on ingress for different rim-seal configurations. Experimental study conducted with the new facility is reported for a variety of generic but engine-representative double-clearance seal configurations, to gain insights into the sealing effectiveness and fluid dynamics associated with different rim-seal design features. Besides being adaptable for various rim-seal configurations, the research facility was also designed to be highly versatile in respect of wheel-space geometries and gas path blading, in order to investigate the impact on ingress from the aerodynamics both inboard and outboard of the rim seal. Extensive instrumentation was incorporated into the turbine stage for the measurements of pressure, swirl velocities, gas concentration and temperature. Additionally, the facility is capable of modelling leakage flow paths found in actual gas turbines to explore novel techniques that could reduce ingress by utilising the leakage air. A parametric study is presented for a range of double-clearance rim seal configurations, characterised by various design features both on the stator side and rotor side of the turbine disc system. Gas concentration measurements were made for each configuration to assess the relative sealing performance. The sealing effectiveness was determined both in the outer wheel-space between the double clearances, and in the inner wheel-space radially inboard of the inner-seal. These measurements constitute a research database that will support the design approach at Siemens, in conjunction with the theoretical model previously developed at Bath, which treats the seal clearance as an orifice ring and uses adapted Bernoulli’s equation to correlate sealing flow rate and pressure difference across the seal. The sealing effectiveness is correlated with pressure and swirl ratio in the wheel-space to study the fluid dynamics associated with different rim-seal features, in support of rim-seal design and optimisations for secondary air systems.
|Date of Award||8 Jun 2016|
|Supervisor||Carl Sangan (Supervisor) & Gary Lock (Supervisor)|