Aerothermal Sentencing for Manufacturing Variations on Turbine Blade Shrouds

Abstract

The tolerances used to sentence HPT blades need to be well-matched to the corresponding influence on aerothermal performance caused by the geometric deviation. If this condition is not fulfilled, this can lead to well-performing parts costing around $8000 being scrapped and suboptimal parts being used in service, which can trigger an early shop visit with an associated cost of up to $2M. The current tolerances used on shroud platform radial displacements, which are required to limit shroud platform steps, are at risk of being unmatched to the corresponding influence on aerothermal performance and introduce uncertainty in the sentencing. Therefore, aerothermal tolerances for shroud platform variations are developed in this research project. Steady RANS simulations are run on a three passage HPT rotor model with engine-representative shroud platform manufacturing variations applied to the middle blade passage. Engine representative shroud platform manufacturing variations, consisting of platform steps and inter-platform gap width variations, are obtained based on a statistical analysis of step heights and gap widths occurring in a sample of 100 casting scans and 26 finished part scans. Both a 4σ backward-facing step and the equivalent height forward-facing step, as perceived by the flow in the aftchord region, are studied. The gap widths is varied from the nominal value by both an increase and reduction with 1σ. This study shows that the shroud endwall flow is aligned with the wedge face until midchord and crosses the step in the aftchord region, where the flow field resembles the corresponding canonical 2D step flow. Since both the heat transfer enhancement and the step-normal component of velocity is largest in this quasi 2D (Q2D) aftchord region, a step heat transfer and loss correlation is developed using a parametric study on a Q2D model of the shroud endwall. The heat transfer correlation calculates the Nusselt number at the reattachment point as a function of the included parameters. The change in total pressure loss caused by a step is derived using a control volume analysis. Both correlations are tested on 3D platform steps. The heat transfer correlation predicts the reattachment Nusselt number within 20% for all but one test case, where the prediction error is increased to 30% due to a 3D effect not included in the model. Both the predicted loss and the value obtained from CFD are below the numerical uncertainty for platform steps on a shroud endwall. More notably, the total blade passage loss scales linearly with inter-platform gap width, which is the main shroud platform manufacturing variations altering the aerodynamic loss. When the loss correlation is tested on steps in the Harrison cascade, prediction errors below 20% are obtained.