Jason Randolph Allen

Evaluation of Turbine Airfoil Cooling Holes by Transient Infrared Methods

Design margins in film-cooled turbine airfoils continue to shrink as specific fuel consumption and performance requirements drive new cooling technology. As a consequence, today’s airfoil heat transfer designs are more sensitive to engine aerodynamic and thermal loads, hot gas path profiles, and manufacturing variability than previous designs. Compounding the challenges of airfoil heat transfer design has been the continued use of inspection practices developed for legacy designs. Advancement of thermal inspection techniques for today’s advanced thermally sensitive airfoils needs to be addressed and advanced in conjunction with advanced cooling designs. Infrared non-destructive evaluation (NDE) techniques offer a number of solutions to inspect turbine airfoils for proper thermal performance before entry into service and during inservice repair. A prototype infrared inspection system demonstrated detection of blocked film cooling holes in turbine airfoils. The system implements IR thermal transient techniques to identify flawed film holes based on the IR thermal signature obtained for each film hole. Furthermore, the approach utilizes robotics, system automation, image processing, and a simple algorithm to discern flawed holes reliably. Traditionally, identifying blocked and undersized holes is manually performed with undersized pin gauges and water flow visualization. These manual processes are qualitative and subject to operator interpretation. Conversely, the IR approach eliminates operator subjectivity by using quantitative metrics. This approach resulted in a 35% reduction in inspection cycle time permitting reallocation of operators to value added tasks; thereby reducing manufacturing and inspection costs. Beyond these initial benefits, the approach used provides a basis for continued developments in turbine airfoil NDE inspection and heat transfer technology development.Copyright

Experimental Testing Techniques for Kevlar® Fiber Brush Seals

Non-metallic brush seals, and more specifically, Kevlar® (aramid) fiber brush seals, are an emerging sealing technology in low-pressure, low temperature applications. Compared to metallic brush seals, aramid fibers are an order of magnitude smaller in diameter and consequently offer much tighter sealing capability. Further, their compliant nature requires minimal pressure drops across the seal to encourage blow-down of the bristle pack onto the rotor during operation. Similarly, their compliant nature also enables the bristle pack to correct for alignment issues and to recover from radial growth transients of the rotor. Proper design of the bristle pack stiffness is critical to the successful operation of the seal. If the seal is designed to be too soft, frictional forces prohibit the recovery of the bristle pack if pressed away from the rotor. Conversely, if designed too stiffly, then the heat generation at the sliding interface of the seal accelerates the degradation of the seal. The goal of the present paper is to present the experimental techniques developed to guide the design of aramid fiber brush seals. Two experimental test methodologies will be presented: a direct stiffness measurement and a heat generation measurement. Both testing procedures have been used to successfully design seals for various GE turbomachinery products.Copyright