Cheng-Zhang Wang

Main Gas Ingestion in a Turbine Stage for Three Rim Cavity Configurations

Experiments were carried out in a model air turbine stage to study the influence of rotor-stator rim cavity configuration on the ingestion of mainstream gas into the cavity. The three rim cavity configurations differed in their aspect ratio (height/width); the rim seal geometry remained the same. The aspect ratio was changed from the baseline ratio by installing an inner shell on the stator at an appropriate radius; this effectively introduced an axial-gap seal between the rim cavity and the cavity radially inboard. The initial step in each experiment was the measurement of time-average static pressure distribution in the turbine stage to ascertain that proper flow condition had been established. Subsequently, tracer gas concentration and particle image velocimetry techniques were employed to measure the time-average but spatially local main gas ingestion and the instantaneous velocity field in the rim cavity. At low purge air flow, regions of ingestion and egress could be identified by inspecting the instantaneous radial velocity distribution near the rim seal obtained from cavity gas velocity maps close to the stator. While the tangential velocity tended to be slightly larger for the so determined ingested gas, a more clear-cut indicator of ingestion was the strong inward gas radial velocity. Information provided by ensemble-average velocity maps was not sufficient for identifying ingestion because the averaging smeared out flow details, which varied from instant to instant. Velocity fields obtained from three-dimensional, time-dependent numerical simulation of a rim seal-cavity sector with similar dimensions qualitatively showed similar characteristics in the outer part of the cavity and provided insight into the complex flow in the seal region.

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

Comparison of Flow Characteristics in Axial-Gap Seals for Close- and Wide-Spaced Turbine Stages

3D unsteady computational fluid dynamics analyses were performed for both close-spaced and wide-spaced turbine stages with axial gap seals and a cavity. Turbine stages, with airfoil configurations similar to those previously studied at United Technologies Research Center (UTRC) and Arizona State University (ASU), were simulated for vane-blade spacing at 34 percent and 70 percent of the vane axial chord length, L. Three configurations were investigated, with the first one placing an axial gap rim seal at 17 percent L upstream of the blade for close-spaced stage, and the other two placing the axial gap seal at either 17 or 34 percent L upstream of the blade for the wide-spaced stage. The seal velocity ingestion characteristics were strongly dependent on axial location for the wide-spaced stage. The seals placed at equal distances upstream of the blade leading edge for the wide- and close-spaced stages had approximately the same average ingestion velocity characteristics. However, the ingestion velocity profiles for the wide-spaced stage were less influenced by vane wakes than for the close-spaced stage. The calculated variation of radial velocity in all gaps was consistent with previous tangential and radial velocity measurements in the seal gap measurements at the University of Aachen.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

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

A Rim Seal Orifice Model With 2 Cds and Effects of Swirl in Seals

Rim seal ingestion models for gas turbines are formulated to estimate the amount of hot fluid ingested through “clearance” seals into the disk cavity. Previous numerical and experimental studies showed the complex time-dependent, three-dimensional characteristics of the flow through the seals and in the outer region of the disk cavity. The present model is developed for estimating ingress and egress flow through the seal that is driven by the azimuthal variation in gas path pressure near the vane and blade platforms. Most published rim seal orifice models have used one “lumped parameter” Cd for both ingress and egress across the seal. However, the flow path from the gas path through the seal is often more convoluted than the flow returning to the gas path. The present Rim Seal Orifice Model includes (i) a Cd value for ingress from the gas path into the disk cavity, (ii) a Cd value for egress from the disk cavity to the gas path and (iii) an estimate for effects of swirl from the seal outer radius to the inner radius of the seal mixing region. The use of two Cd values provides two parameters for characterizing the flow through the seal. The ingress and egress Cd values for a turbine rim seal configuration and flow condition are estimated by comparing the modeled seal effectiveness for a parametric range of ingress and egress Cd values with experimental stator wall measurements. The combination of Cd values, which best matches experimental data over a range of coolant flow ratios, characterizes the seal and flow condition. Arizona State University experimental data were used to estimate the Cd values for an overlap seal configuration.© 2008 ASME