David F. Cloud

Rim Seal Ingestion Characteristics for Axial Gap Rim Seals in a Closely-Spaced Turbine Stage From a Numerical Simulation

Turbine rim seal ingestion in gas turbines is influenced by many geometric and flow parameters. For turbine stages where the vanes and blades are closely spaced, the time-dependent pressure and flow fields near the seal strongly influence the rim seal ingestion. Numerical simulations of a close-spaced configuration, similar to that used in previous experiments, were made to determine the complex 3-D, time-dependent flow and ingestion characteristics of an axial gap rim seal. The calculated pressure fields were in general agreement with previously published experimental data. The radial velocities inward and outward in the axial gap seal were appreciable fractions of the hub tangential velocity and varied with position across the airfoil pitch and the axial location in the seal. The tangential velocities in the gap varied with flow direction, generally greater than hub velocity for flow ingress and less than hub velocity for flow ingress and less than hub velocity for flow egress. Velocity jets upstream of the blade leading edge penetrated into the disk cavity approximately 10 times the seal width. The ingestion velocities for this configuration were dominated by the blade bow wave pressure field. One conclusion of the authors is that the blade pressure field can be as or more significant than the vane trailing pressure field in influencing rim seal ingestion.© 2006 ASME

Impact of Rotational Speed on the Discharge Characteristic of Stepped Labyrinth Seals

Current and future gas turbine engines are subject to increasing performance requirements and improved fuel efficiencies. The resultant engine cycles increase core flow temperatures requiring additional cooling flow while requiring a reduction in parasitic leakage by more than 25% to meet the performance goals. The achievement of the reduced leakage requires that seal design concepts be tested and improvements validated in engine like conditions before they are introduced into the actual engine. A previous paper [1] described a set of rotating seal designs that have been tested in various combinations of rotating and static design features for low leakage potential. This paper is a continuation of the previous effort and focuses in particular on the effect of rotational speeds on the discharge characteristic of various stepped labyrinth seal designs. Leakage reductions will be characterized in terms of CD /DD,0 (i.e. ratio of leakage rate with rotation over leakage rate without rotation) as a function of circumferential to axial through flow velocity, U/Cax . For large velocity ratios of U/Cax > 5, leakage reductions of more than 20% were observed. Experimental data are compared with literature [2].Copyright

Rotating Seal Rig Experiments: Test Results and Analysis Modeling

Current and future gas turbine engines are subject to increasing performance requirements and improved fuel efficiencies. The resultant engine cycles increase core flow temperatures requiring additional cooling flow while requiring a reduction in parasitic leakage by 25%–50% to meet the performance goals. The achievement of the reduced leakage requires that seal design concepts be tested and improvements validated in engine like conditions before they are introduced into the gas turbine product. This paper describes the process of how a potentially low leakage seal design was evaluated and tested in an advanced seal test rig facility. How existing engine seal leakage rig data was used to validate physics based models (CFD) of baseline labyrinth seal configurations, then used to run back to back sensitivity studies to identify seal design characteristics that could provide low leakage seal designs. The paper discusses the use of an Advanced Seal Rig (ASR) facility to test seal design concepts for gas turbine engine applications. Test seal flow results are presented and compared to the base line seal tests. The differences between the new seal design flow test results and the base line seal flow test results are investigated. Further, seal test flow results are compared with the validated physics based model predictions (CFD) run at the advanced seal rig test conditions. Differences between rig data and analysis are discussed. Future seal testing and analysis work is suggested.Copyright

Advanced Seal Test Rig Validation and Operation

Advanced seals have been identified as critical in meeting engine goals for specific fuel consumption, thrust-to-weight ratio, emissions, durability, and operating costs. In a direct effort to reduce the parasitic leakage, a high-temperature, high-speed seal test rig with Active Clearance Control (ACC) has been designed, built and validated by the National Aerospace Laboratory (NLR) in the Netherlands within a collaborative program with Sulzer Metco Turbine Components (SMTC) and Pratt & Whitney (P&W). NLR’s new seal test rig is capable to evaluate seals for the next generation gas turbine engines. It will test air seals (i.e., labyrinth, brush, and new seal concepts) in near gas turbine engine environment conditions of high temperature to 815 °C (1500 °F), high pressure to 2400 kPa (335 psid), high surface speeds to 365 m/s (1200 ft/s). Seal flows for typical engine seal clearances between 0.12 mm (0.005 inch) and 0.65 mm (0.025 inch) can be measured without changing test articles but by using the ACC system. A compressed air facility at the German-Dutch Windtunnel, located at the NLR site, delivers the required compressed clean and dry air. This paper describes the design, the instrumentation, the control system and the validation of the test rig. The rig certification was achieved by validating test measurements using a known three knife-edges stepped labyrinth seal. This paper also addresses the NLR’s CFD and engineering tool development to predict the seal performance.© 2006 ASME

Advanced Seal Rig Experiments and Analysis

Compliant Seals eg. Brush Seals, Finger Seals: a) 3-5X flow reduction; b) developing higher surface speed, temperature, and pressure levels; c) interference/debris/durability issues. Non Contact Seals eg. Aspirating, Film Riding: a) 5-10X flow reduction but still improving surface speed, temperature, and delta pressure levels; b) limited applications; c) interference issues. Labyrinth Seals still the workhorse seal in gas turbine engines: a) long history of use in compressors, turbines, around bearing compartments; b) cheaper to make than many other seals; c) small improvement x many seals (up to 50*) = big gain in performance/operability; d) well and still investigated by academia & industry; e) with a proper abradable seal land can handle interference.

Probabilistic Analysis of a Turbofan Secondary Flow System

This paper documents a probabilistic analysis of the secondary flow system in a modern commercial turbofan engine. The purpose of this analysis is to investigate the variability in the high and low rotor bearing loads and total secondary flow due to the inherent uncertainty in manufacturing processes and engine performance. In addition to quantifying the variability in bearing load and secondary flow, the sensitivity of the parameters to individual input variables is determined. The system was found to behave linearly, resulting in negligible mean shifts due to input variation. The importance of correlation among the performance parameters will be addressed, as well as the effects of different correlations. Methods used to reduce the time required for the analysis will also be discussed. This type of analysis has many applications in cost reduction, engine design, optimization, and root cause analysis that will be covered in this paper.Copyright

Rim Seal Ingestion in a Turbine Stage From 360-Degree Time Dependent Numerical Simulations

Numerical simulations of turbine rim seal experiments are conducted with a time-dependent, 360-degree CFD model of the complete turbine stage with a rim seal and cavity. The turbine stage has 22 vanes and 28 blades and is modeled with a uniform flow upstream of the vane inlet, a pressure condition downstream of the blades and three coolant flow conditions previously employed during experiments at Arizona State University. The simulations show the pressure fields downstream of the vanes and upstream of the blades interacting to form a complex pressure pattern above the rim seal. Circumferential distributions of 15 and 17 sets of ingress and egress velocities flow through the rim seal at the two modest coolant flow rate conditions. These flow distributions rotate at wheel speed and are not associated with the numbers of blades or vanes. The seal velocity distribution for a high coolant flow rate with little or no ingestion into the stator wall boundary layer is associated with the blade pressure field. These pressure field characteristics and the rim seal ingress/egress pattern provide new insight to the physics of rim seal ingestion. Flow patterns within the rim cavity have large cells that rotate in the wheel direction at a slightly slower speed. These secondary flows are similar to structures noted in previous a 360-degree model and large sector models but not obtained in a single blade or vane sector model with periodic boundary condition at sector boundaries. The predictions of pressure profiles, sealing effectiveness and cavity velocity components are compared with experimental data.© 2012 ASME

Probabilistic Thermal Analysis of Gas Turbine Internal Hardware

This paper documents the initial development of a method to perform probabilistic thermal analyses of gas turbine internal hardware and uses the turbine interstage seal of a turbofan engine as an example. The purpose of this analysis is to investigate the variability in steady state metal temperature due to variability in the secondary flow system. In addition to quantifying the variability in metal temperature, the sensitivity of the temperature to individual input variables is determined. As a prerequisite for a probabilistic thermal analysis, a probabilistic flow analysis was executed, with variability in engine performance and hardware geometry yielding variability in mass flow rates, heat generation and local swirl velocity. These outputs were used as stochastic inputs for the probabilistic thermal analysis. The analysis was run with correlated input as well as independently varying inputs. The results of this analysis showed that the metal temperature at the tip of the seal was sensitive and highly correlated to air source temperature, as expected. The mass flow rate of air across the seal and heat transfer coefficient also affected the metal temperature. By using correlated input variability, it is shown that variability in metal temperature is ultimately caused by variability in labyrinth seal clearance.© 2006 ASME