Infoscience

Thesis

Experimental study on a heavy film cooled nozzle guide vane with contoured platforms

Nowadays, efficiency improvement of the modern gas turbines is usually achieved by increasing the pressure ratio which leads to an increase of the gas temperature in the combustion chamber. As a consequence, the temperature conditions imposed on the first stages of the turbine are well beyond the maximum allowable material of the engine. Therefore, it is necessary to effectively cool on these parts. Some of the highest thermal loading is applied to the nozzle guide vane located just behind the combustion chamber. Its function is to efficiently direct the hot and highpressure gas on the rotor blades of the turbine in order to extract maximum work. From the aerodynamic side, the loss level generated during the hot gas deviation defines the nozzle guide vane efficiency. From the thermodynamic side, an efficient nozzle guide vane is characterized by its cooling protection against the hot gas coming out of the combustion chamber. For this reason, the vane is internally cooled by coolant fluid circulating in channels; but it is also externally cooled by injection of coolant fluid on the surface. This process is usually called "film-cooling". If it is well designed, the film efficiently protects the vane airfoil and platforms of the engine; but if badly designed (unfavorable holes injections positions or inadequate blowing ratios), then the overall performance of the engine is reduced. Recently, studies have shown a decrease in the aerodynamic losses for nozzle guide vanes with contoured platforms. Moreover, it has also been noticed that temperature profiles at the exit of the combustion chamber are more and more flattened and consequently require a better cooling of the platforms. The problem is to efficiently film-cool a nozzle guide vane airfoil and its contoured platforms. A numerical approach of this problem is yet not possible as the numerical codes are not accurate enough to take into account all the physics governing such situations. Therefore, an experimental approach is required. So far, many film cooling studies have been performed but on relatively simple geometries (flat plates, cylinders etc.). The innovative part of this present work was to study film cooling on a complex nozzle guide vane geometry equipped with contoured platforms. For this, as well the aerodynamic side and the thermodynamic side related to film cooling were considered. The experiments were divided into two parts. The first part was dedicated to the study of the vane airfoil. The second part was dedicated to the film-cooling study on the contoured platforms. In both cases, heat transfer coefficients and film-cooling effectiveness were determined by transient measurement techniques using liquid crystals. For the vane airfoil, a preconditioning system followed by a rapid insertion was used. This installation was already used during previous projects, but in the frame of this research work, the liquid crystal technique was improved. For this, a new signal analysis and image processing system was developed. For the measurements on the platforms, a new measurement technique based on the use of a thin electrical heater-foil was developed. The innovation of this technique comes from the fact that it can be applied on complex geometries for which inhomogeneous heat fluxes are produced (curved surfaces, introduction of cooling holes). This novel measurement technique was first tested and validated on a simple geometry consisting of a film-cooled flat plate. The same technique was then applied to the film-cooled contoured platform and allowed obtaining interesting results of heat transfer coefficient and film-cooling effectiveness distributions. The latter result was also compared to values obtained from another innovative measurement technique based on Pressure Sensitive Paint with Nitrogen injection. The results obtained in the frame of this experimental work dedicated to film cooling applied on a complex geometry were obtained thanks to the development of novel measurement techniques. These measurement techniques provide the opportunity to perform systematic tests in order to gain further insight into the physics of the film cooling process on complex geometries such as contoured platforms.

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