Roger Paolillo

Experiment on Gas Ingestion Through Axial-Flow Turbine Rim Seals

It has been suggested by researchers that ingestion, through rim seals, of mainstream gas into axial-flow turbine disk cavities is a consequence of the prevailing unsteady three-dimensional flow field. The cause-effect relationship is complex-to help understand it, experiments were performed in a model single-stage turbine rig using two different vaneblade configurations. Selected measurements from one of the configurations were reported earlier (1999-2001). The second configuration is new, featuring smaller numbers of vanes and blades and a larger vane turning angle. Selected measurements are presented and compared to those from the first configuration. The measurements include unsteady and rotor revolution time-average static pressure spatial distributions, and spatial distribution, in the rotor-stator cavity, of time-average ingestion. The parameters in the experiments were the main airflow rate, the purge/seal airflow rate, and the rotor speed. Unsteady three-dimensional CFD simulation may be helpful in identifying the roles of the many intertwined phenomena in the ingestion process.

The Flow Field and Main Gas Ingestion in a Rotor-Stator Cavity

The ingestion of mainstream gas into turbine rotor-stator disk cavities and simultaneously, the egress of cavity gas into the main gas path are consequences of the prevailing unsteady, three-dimensional flow field. To understand these processes, we are carrying out a study that combines experiments in a model single-stage axial turbine with computational fluid dynamic (CFD) simulations. The turbine stage features vanes, blades, and axially overlapping radial clearance rim seal. In this paper, we present time-resolved velocity maps, obtained by particle image velocimetry, of the flow in the disk cavity at four experimental conditions as defined by the main air flow rate, rotor speed, and purge air flow rate. Time-averaged but spatially local measurement of main air ingestion is also presented. Significant ingestion occurred at two of the four experimental conditions where the purge air flow rate was low — it is found that high tangential (swirl) velocity fluid intersperses with lower tangential velocity fluid in the rim region of the cavity. It is argued that the high tangential velocity fluid is comprised of the ingested main air, while the lower tangential velocity fluid is the indigenous cavity air. This interpretation is corroborated by the results of the unsteady, three-dimensional CFD simulation. When the purge flow rate was high, no ingestion occurred as expected; also, large-scale structures that were unsteady appeared in the cavity flow giving rise to large velocity fluctuations. It is necessary to obtain time-resolved information from experiments and computation in such a flow because even when the vane-blade relative position is matched during a particular experiment, the instantaneous flow field does not necessarily remain the same. As such, some of the flow patterns will be smeared out if the interrogation time scale is large.Copyright

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

Flow Characteristics and Stability Analysis of Variable-Density Rotating Flows in Compressor-Disk Cavities

Previous heat transfer experiments showed that significant differences in the flow and heat transfer characteristics can occur in models of aircraft gas-turbine, high-compressor drums. Experiments with heated disks and colder flow show large-scale instabilities that cause mixing between the cooling flow and the flow in the trapped cavities. The general result of this mixing is relatively high heat flux on the disks. Other heat transfer experiments, simulating the aircraft take-off condition with cold disks and hotter coolant, show decreased heat transfer due to the stabilizing effects of positive radial density gradients. A stability analysis for inviscid, variable-density flow was developed to quantify the effects of axial velocity, tangential velocity, and density profiles in the bore region of the disk cavities on the stabilizing or destabilizing characteristics of the flow. The criteria from the stability analysis were used to evaluate the axial velocity and density profile conditions required to stabilize three tangential-velocity profiles, obtained from previous experiments and analyses. The results from the parametric study showed that for Rossby numbers, the ratio of axial velocity to disk bore velocity, less than 0.1, the flow can be stabilized with ratios of cavity density to coolant density of less than 1.1. However for Rossby numbers greater than 1, the flow in the bore region is unlikely to be stabilized with a positive radial density gradient. For Rossby numbers between 0.1 and 1.0, the flow stability is a more complex relationship between the velocity and density profiles. Results from the analysis can be used to guide the correlation of experimental heat transfer data for design systems.

CFD Analysis Prediction of Heat Transfer Coefficient in Rotating Cavities With Radial Outflow

This paper documents two related investigations. The first investigation was to benchmark commercial CFD code Fluent in rotating cavities for velocity profiles and beat transfer coefficients. The second investigation was to evaluate the methods of extracting heat transfer coefficients from CFD solution with direct method and Reynolds analogy approach. The rotating cavities examined include rotor-stator, contra-rotating and co-rotating disks. The velocity profiles benchmark was conducted prior to heat transfer coefficient benchmark. Several turbulence models were compared for closed rotating cavity flows. The comparisons between test data and CFD results of tangential and radial velocity profiles showed that the SST k-ω turbulence model performed the best among turbulence models tested. Hence, the SST k-ω model was chosen for heat transfer coefficient benchmarking. The comparisons of heat transfer coefficients between test data and CFD results were presented in the form of local Nusselt number. The thermal wall boundary conditions applied to all the computations were curved-fitted wall temperature distributions from available test data. The wall temperature distributions include approximately constant, positive and negative profiles. It was found that the accurate information of the thermal wall temperature distribution was critical to the benchmark and that only the CFD results with well defined information of wall temperature distributions matched well with test data. The Nusselt number extracted from the CFD solution with the Reynolds analogy approach tends to over predict the heat transfer coefficient on the higher radii and only matched test data at low Reynolds number with positive wall temperature profile. The error increases with higher Reynolds number and decreases with larger flow rate.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.

Experiments on Gas Ingestion Through Axial-Flow Turbine Rim Seals

It has been suggested by researchers that ingestion, through rim seals, of mainstream gas into axial-flow turbine disk cavities is a consequence of the prevailing unsteady three-dimensional flow field. The cause-effect relationship is complex — to help understand it, experiments were performed in a model single-stage turbine rig using two different vane-blade configurations. Selected measurements from one of the configurations were reported earlier (1999–2001). The second configuration is new, featuring smaller numbers of vanes and blades and a larger vane turning angle. Selected measurements are presented and compared to those from the first configuration. The measurements include: unsteady and time-average static pressure spatial distributions, and spatial distribution, in the rotor-stator cavity, of time-average ingestion. The parameters in the experiments were: main air flow rate, purge/seal air flow rate, and rotor speed. Unsteady three-dimensional CFD simulation may be helpful in identifying the roles of the many intertwined phenomena in the ingestion process.Copyright