Research On The Heat Transfer Of The Endwall Film-cooling In Axial Flow Turbine Cascade

An increase in the specific work and cycle efficiency of gas turbines can be achieved through higher turbine entry temperatures. Maintaining adequate life at these high temperatures requires the development of materials and efficient cooling methods. One cooling method that has gained increasing importance is endwall film-cooling, where coolant air is discharged though discrete holes in the inner and outer endwalls of a turbine blade passage. After leaving the holes, the coolant forms a protective layer between the hot mainstream gas and the surface that is to be protected. The ejected coolant interacts with the external flow near the endwall and generates aerodynamic and thermodynamic losses in the process. This reduces turbine stage efficiency and together with the consumption of cooling air is detrimental to the overall cycle efficiency.The keystone of this research project is an experimental investigation using a large-scale low-speed linear turbine cascade. The distribution of adiabatic film-cooling effectiveness on the endwall of this cascade has been measured using a new technique that has been developed as part of this project. Using this new technique, the achieved levels of cooling effectiveness are quantified and over, and under-cooled regions on the endwall are identified. These measurements are complemented by measurements of the flow field downstream of the cascade. The integrated losses and locations of secondary flow features with and without endwall film-cooling are determined for variations of both coolant supply pressure and injection location. The aerodynamic losses and the coolant consumption due to endwall film-cooling are quantified, thus providing data on both the aerodynamic costs and the cooling benefits of endwall film-cooling.Measured hole mass flows and a constant static pressure mixing analysis, together with the measured losses, allowed the decomposition of the losses into three distinct entropy generation mechanisms: loss generation within the hole,loss generation due to the mixing of the coolant with the mainstream, and change in secondary loss generation in the "blade passage. Results from this investigation show that the loss generation within the coolant holes is substantial and that ejection into regions of low static pressure increases the loss per unit coolant mass flow. The results also reveal strong interactions between endwall coolant ejection and secondary flow in the blade passage. The secondary flow has a strong influence on coolant trajectories and coolant ejection delays the three-dimensional separation of the inlet boundary layer on the endwall, chang the secondary flow and reduces its associated losses.Understanding the three-dimensional nature of these flows and understanding the interactions between the ejected coolant and the endwall flow are the key to a successful endwall film-cooling design. Results from this investigation enhance this understanding.