A Comparison of Secondary Kinetic Energy Definitions Using Time-Resolved Flow Field Measurements in a Turbine

Improving turbine efficiency to reduce emissions is of high priority in the development of gas turbine engines. Secondary flows account for a significant proportion of turbine efficiency loss thereby requiring an understanding of their influence. A common parameter used to quantify the strength of these secondary flows is Secondary Kinetic Energy (SKE). SKE represents the kinetic energy of the velocity component of the flow normal to the bulk flow direction. Complex three-dimensional airfoils and hub-to-tip profile distributions in rotating test rigs lead to different requirements for definitions of primary flow in the SKE parameter. A study at the Steady Thermal Aero Research Turbine (START) lab was conducted to compare the applicability of multiple definitions of SKE using different definitions of primary flow direction, their respective magnitude, and what can be highlighted about the flow using each definition. To support this analysis, a radially traversed fast-response aerodynamic probe (FRAP) operated in a virtual four-hole mode was installed at the exit plane of a single stage cooled turbine. The resulting time-resolved measurements of the three components of velocity were used to visualize secondary flows and calculate resulting kinetic energy. The methods examined for calculating SKE include the trailing edge midspan camber line angle, the midspan flow direction per dataset, a time-averaged radial profile from a reference case, and a time-averaged radial profile of the specific dataset. Whereas the midspan velocity of a given dataset represents the most commonly applied primary flow direction, particularly for two-dimensional stationary cascade environments, this study identified that it is less applicable for more complex flow fields such as rotating facilities with three-dimensional airfoils. These comparisons can be a valuable reference for other facilities in the turbomachinery community to help select appropriate diagnostic methods and extract meaningful conclusions from measured data.