The Influence of Thermal Transient Rates on Coated Turbine Parts' Life Expectancy

Abstract

During rapid engine throttling operations, turbine airfoils can experience very rapid heating and cooling, particularly at take-off conditions. These rapid transient events lead to the generation of high thermal gradients and nonuniform stress distributions through the thermal barrier coating (TBC), environmental barrier/bond coating, and substrate. This, in turn, can lead to coating delamination, overheat of the substrate materials, creep, and thermo-mechanical fatigue of the part. We present the process and computer modeling methodology for a physics-based prediction of deformation, damage, crack propagation and local failure modes in coated turbine airfoils and other parts operating at hot section turbine environment conditions as a function of engine operational regimes, with a particular emphasis on rapid transient events. The overall goal is to predict the effects and severity of the cooling and heating thermal rates on transient thermal mechanical fatigue life of coated hot parts (turbine airfoils, blade outer air seals, and combustor liners). The computational analysis incorporates time-accurate, coupled aerothermodynamics with nonlinear deformation thermal-structural finite element modeling, and fracture mechanics modeling for high-rate thermal transient events. TBC thermal failure and spallation are introduced by the use of interface fracture toughness and interface property evolution as well as dissipated energy rate. The spallation model allows estimations of the part remaining life as a function of the heating/cooling rates. Applicability of the developed model is verified using experimental coupons and calibrated against burner rig test data for high-flux thermal cycles. Our results show a decrease in TBC spall life due to high-rate transient events.