Blade Outer Air Seal Research Articles

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

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.

Transient Thermal Analysis and Viscoplastic Damage Model for Life Prediction of Turbine Components

This paper reports the process and computer methodology for a physics-based prediction of overall deformation and local failure modes in cooled turbine airfoils, blade outer air seals, and other turbomachinery parts operating in severe high temperature and high stress environments. The computational analysis work incorporated time-accurate, coupled aerothermal computational fluid dynamics (CFD) with nonlinear deformation thermal-structural finite element model (FEM) with a slip-based constitutive model, evaluated at real engine characteristic mission times, and flight points for part life prediction. The methodology utilizes a fully coupled elastic-viscoplastic model that was based on crystal morphology, and a semi-empirical life prediction model introduced the use of dissipated energy to estimate the remaining part life in terms of cycles to failure. The method was effective for use with three-dimensional FEMs of realistic turbine airfoils using commercial finite element applications. The computationally predicted part life was calibrated and verified against test data for deformation and crack growth.

Validation of Heat-Flux Predictions on the Outer Air Seal of a Transonic Turbine Blade

It is desirable to accurately predict the heat load on turbine hot section components within the design cycle of the engine. Thus, a set of predictions of the heat flux on the blade outer air seal of a transonic turbine is here validated with time-resolved measurements obtained in a single-stage high-pressure turbine rig. Surface pressure measurements were also obtained along the blade outer air seal, and these are also compared to three-dimensional, Reynolds-averaged Navier-Stokes predictions. A region of very high heat flux was predicted as the pressure side of the blade passed a fixed location on the blade outer air seal, but this was not measured in the experiment. The region of high heat flux was associated both with very high harmonics of the blade-passing event and a discrepancy between predicted and measured time-mean heat-flux levels. Further analysis of the predicted heat flux in light of the experimental technique employed in the test revealed that the elevated heat flux associated with passage of the pressure side might be physical. Improvements in the experimental technique are suggested for future efforts.

Unsteady Effects in Turbine Tip Clearance Flows

The present study is concerned with the unsteady flow field on the blade outer air seal segments of high-work turbines; these segments are installed between the blade tip and outer casing and are usually subjected to extreme heat loads. Time-resolved measurements of the unsteady pressure on the blade outer air seal are made in a low-speed turbine rig. The present measurements indicate the existence of a separation zone on the blade tip, which causes a vena contracta to form at the entrance of the tip gap. In addition, a careful comparison between the ensemble-averaged pressure measurement and the corresponding result from steady computation suggests that the pressure on the blade outer air seal can largely be described as being due to a steady flow (in the rotating frame) sweeping past a stationary probe. The ensemble deviation measurement indicates that unsteadiness (from one revolution to the next) is confined to the tip gap. [S0889-504X(00)02304-7]

Enhanced film cooling slot for turbine blade outer air seals

Enhanced film cooling slot for turbine blade outer air seals