Work Package 4
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| The demand to achieve higher thermal
efficiency and increased power output from gas turbine systems has led
to higher turbine inlet temperatures. Despite continuous research
efforts of developing new materials and more effective cooling methods
for a High Pressure Turbine (HPT) blade, the region around the blade
tip remains one of the most critical areas inside a gas turbine engine.
The pressure gradient between the pressure and the suction side of a
turbine rotor blade causes a leakage flow through the tip gap between
the moving blade and the opposing wall. Mixing of this leakage flow
with the rotor passage flow causes total pressure loss and reduces
turbine stage efficiency by as much as a quarter of the total stage
losses (Harvey [2004]). In addition, the leakage flow transports hot
mainstream fluid into the blade tip region which is inherently
difficult to cool. This high operating temperature causes not only
oxidation and erosion, but also induces high thermal stresses as a
consequence of non-uniform heat transfer coefficients. The turbulent interactions between the leakage, cooling and mainstream flows lead to a complex threedimensional unsteady flow and vortex field, which in turn affects the cooling of this region. In order to keep the aerodynamic losses to a minimum while maintaining optimum cooling it is important to gain a full understanding of the flow physics in this region which will ultimately lead to improved specific fuel consumption (SFC) through the more effective use of cooling air. Continuous attempts to reduce the effects of leakage flows on the stage efficiency have resulted in mainly two types of modern HPT blades, shrouded and un-shrouded. In the former case, the sensitivity of turbine efficiency to tip gap is less than half that of an un-shrouded blade, but the additional weight and additional cooling requirements result in a reduction in engine efficiency (Harvey [2004]). Advanced cooling concepts other than active shroud cooling are therefore required. One such design is passive shroud cooling where coolant is ejected through injection holes in the stationary parts directly onto the blade shroud, thereby reducing the driving gas temperature. For the case of an un-shrouded blade, the available literature covers mainly investigations of plane-tip blades, which do not perform as well as when the blade tip is shaped. A shaped blade tip surface can be considered to operate like a labyrinth seal but the combined effects on the aerodynamic and thermal performance at close-to-engine conditions have as yet not been fully understood and require therefore detailed experimental investigations. The aim of this work package is to develop and test advanced cooling concepts and to provide experimental data of aerodynamic and thermal performance in the tip area of cooled highly loaded shrouded and unshrouded HPT blades. The work package is divided into two sub-tasks. The purpose of subtask 4.1 is to quantify the thermal performance and the steady and unsteady flow field of a shrouded blade. The aim of subtask 4.2 is to identify the optimum tip geometry of an un-shrouded high-lift blade with respect to its aerodynamic and thermal performance. This will allow the creation of a database with a full set of aerothermal performance data and recommendations for the design of the tip region of HPT blades. |