An investigation into transonic flows over high pressure gas turbine blade tip

Propulsion, in particular the gas turbine engines are a significantly important theme in the aerospace industry, any enhancement in the performance of these engines will have a great impact on the economics of the aviation sector [Stephens et al, 2007]. One way to increase the performance of the gas turbine engines is to increase the temperature at the inlet to the turbine stage. However this exposes the turbine blades to significant thermal load, degrades the blades and decreases their operational life. The blade tip in particular is exposed to a much higher thermal damage and hence is an important design area.  

 One way to overcome thermal damage is to seal the blades’ tips through interconnecting them using a shroud with drawbacks including high blades weight and centrifugal loading. Therefore using un-shrouded blades remains attractive. For the unshrouded blade tip, an unavoidable gap exists between the turbine blade tip and the casing where the pressure difference between the two surfaces of the blade gives rise to the development of the tip leakage flows from the pressure side to the suction side edge[Lee et al., 2008, Thorpe et al., 2005]. These flows not only expose the tip to high temperature gases and thermal damages but also contribute to a significant 30% of the aerodynamic losses in a turbine stage. Hence any incremental enhancement in reducing these flows will provide a significant improvement on the engine performance. [Chen et al., 1993, Saravanamuttoo et al, 2001, Denton et al., 1993]  

 Several investigations have been carried out to study the flow behaviour in the tip gap area and the important factors affecting its development. One of the very important factors affecting the tip flows is the tip geometry. Hence the tip flows have been studied for different tip models including flat tip, suction-side squealer tip, pressure-side squealer tip, cavity tip and winglet tip models with the aim of reducing the leakage flows and their associated losses [Azad et al., 2002, Key et al., 2006, Papa et al., 2003, Saha et al, 2006].  

 Studies on tip leakage flows show that as the flow approaches the leading edge of the flat blade tip (the simplest blade tip model), it accelerates and turns to adjust itself around the tip. Since the tip edge is sharp, flow separates on its arrival and develops a separation bubble. The separation bubble acts like a vena contracta and can accelerate the tip flow to transonic speed provided the pressure difference across the tip is sufficient. In addition this separation increases the blockage effect, reduces the effective tip gap and hence reduces the leakage flows and associated losses. Hence designing the blade tip geometry to increase the blockage is one solution. Another well-known approach to control the leakage flows is contouring the casing. Optimum contouring can reduce the leakage flows and hence their adverse effects and losses. Gao et.al [2012] studied the effect of the casing contour profile on the tip leakage flows. It was observed that optimising the contour profile resulted in a considerable reduction of the tip leakage flows and decreased the associated losses significantly.  

 This project aims to design and test an innovative integrated design of tip and casing contour making use of advantages from both tip design and casing contour profiling to reduce the tip leakage flows and associated losses. Literature shows that there have been very few studies on integrated design and its advantages [Kroger et al., 2009, Zuojun et al., 2014]. Hence the large gap of knowledge in this area leads to a great potential of accomplishing great improvement in the turbine efficiency through innovative integrated design of tip and casing. The project will first carry out two-dimensional simulations and then will be extended to fully three dimensional simulations.  

 This project will be suitable for the candidate with aerospace or mechanical engineering background, with very good design and computational fluid dynamics skills and a candidate who is confident in using ANSYS software package such as ICEM, FLUENT, TURBOGRID and etc.