Measurement of Ingress through Gas Turbine Rim Seals

One of the most important problems facing gas turbine designers today is the ingestion of hot mainstream gases into the wheel-space between the turbine disc and its adjacent casing. A rim seal is fitted at the periphery and a superposed sealant flow is used to prevent ingress. The aim of this PhD research was to design a new rotor-stator testing facility, from which both flow physics and future heat transfer characteristics in relation to ingress could be measured and analysed, along with a detailed investigation into the sealing characteristics of turbine rim-seals there from. The rig was constructed as an engine representative model of a gas turbine wheel-space, from which data correlations could tentatively be scaled and applied to actual engine design. The novel testing facility was designed in great detail for both sealing effectiveness research and to investigate the thermal effects of hot gas ingress; insight never previously achieved. An extensive commissioning process was undertaken to ensure that the correct, albeit benign, fluid-dynamic conditions were created inside the single stage turbine rig. Effectiveness data are presented for single-clearance rim-seals in a variety of ingress conditions from which a fundamental understanding is developed for both rotationally-induced and externally-induced ingress. A newly developed orifice model is validated against the experimental data, resulting in theoretical predictions of the sealing effectiveness characteristics of various rim-seals. It is suggested that these predictions could be scaled to engine representative conditions where they could act as a future design tool for secondary air system engineers. The theory is then extended to the application of double clearance-seal configurations, whereby the beneficial aspects are shown both theoretically and experimentally, leading on to the suggestion of a possible optimisation process resulting in an ultimate double seal. It is postulated that this would be the highest performance that can ever be achieved with a double clearance configuration.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Measurement of ingress through gas turbine rim seals

One of the most important problems facing gas turbine designers today is the ingestion of hot mainstream gases into the wheel-space between the turbine disc and its adjacent casing. A rim seal is fitted at the periphery and a superposed sealant flow is used to prevent ingress. The aim of this PhD research was to design a new rotor-stator testing facility, from which both flow physics and future heat transfer characteristics in relation to ingress could be measured and analysed, along with a detailed investigation into the sealing characteristics of turbine rim-seals there from. The rig was constructed as an engine representative model of a gas turbine wheel-space, from which data correlations could tentatively be scaled and applied to actual engine design. The novel testing facility was designed in great detail for both sealing effectiveness research and to investigate the thermal effects of hot gas ingress; insight never previously achieved. An extensive commissioning process was undertaken to ensure that the correct, albeit benign, fluid-dynamic conditions were created inside the single stage turbine rig. Effectiveness data are presented for single-clearance rim-seals in a variety of ingress conditions from which a fundamental understanding is developed for both rotationally-induced and externally-induced ingress. A newly developed orifice model is validated against the experimental data, resulting in theoretical predictions of the sealing effectiveness characteristics of various rim-seals. It is suggested that these predictions could be scaled to engine representative conditions where they could act as a future design tool for secondary air system engineers. The theory is then extended to the application of double clearance-seal configurations, whereby the beneficial aspects are shown both theoretically and experimentally, leading on to the suggestion of a possible optimisation process resulting in an ultimate double seal. It is postulated that this would be the highest performance that can ever be achieved with a double clearance configuration.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

A New Interpretation of Hot Gas Ingress Through Turbine Rim Seals Influenced by Mainstream Annulus Swirl

Rim seals are fitted at the periphery of the stator and rotor disks to reduce the adverse effects of hot gas ingress on highly stressed turbine components limited by temperature. Ingress is induced by rotational effects such as disk pumping, as well as by asymmetric pressure-driven unsteady phenomena. These influences superpose to form a complex flow-physics problem that is a challenge for computational fluid dynamics. Engine designers typically use practical low-order models that require empirical validation and correlating parameters. This paper identifies the swirl ratio in the mainstream annulus as a dominant characterizing parameter to predict ingress. This is a new interpretation that is supported by extending a low-order model based on turbulent transport using an effective eddy mixing length based on the difference in swirl between the annulus and seal clearance. Experimental measurements were made using a 1.5-stage turbine rig at low Reynolds number. The influence of annulus swirl ratio was investigated over a range of flow conditions and two rim-seal geometries, with the ingress quantified using CO 2tracer concentration in the sealing flow. The concentration data were complemented by measurements in the annulus using a five-hole aerodynamic probe.</p

Flow Instability Effects Related to Purge through a Gas Turbine Chute Seal

This paper investigates flow instabilities inside the cavity formed between the stator and rotor disks of a high-speed turbine rig. The cavity rim seal is of chute seal design. The influence of flow coefficient on the sealing effectiveness at constant purge flow rate through the wheel-space is determined. The effectiveness at different radial positions over a range of purge flow conditions and flow coefficients is also studied. Unsteady pressure measurements have identified the frequency of instabilities that form within the rim seal, phenomena which have been observed in other studies. Frequencies of these disturbances, and their correlation in the circumferential direction have determined the strength and speed of rotation of the instabilities within the cavity. Large scale unsteady rotational structures have been identified, which show similarity to previous studies. These disturbances have been found to be weakly dependent on the purge flow and flow coefficients, although an increased purge reduced both the intensity and speed of rotation of the instabilities. Additionally, certain uncorrelated disturbances have been found to be inconsistent (discontinuous) with pitchwise variation.QC 20220503</p

Influence of Flow Coefficient on Ingress Through Turbine Rim Seals

Rim seals are critical in terms of limiting the temperature of highly-stressed engine components but function with a penalty to the power output and contribute to entropy gain stemming from mixing losses in the turbine. Ingress through rim seals is influenced by the presence of rotor blades and stator vanes, and the mainstream flow coefficient in the annulus that determines the corresponding swirl. This paper presents an experimental study of ingress upstream and downstream of the rotor disc in a 1.5-stage rig with double radial clearance rim seals. Two rotor discs were used, one with blades and one without, and two platforms were used downstream of the rotor, one with vanes and one without. Tests were conducted at two rotational speeds and a range of flow conditions was achieved by varying the annulus and sealing mass flow rates. Concentration effectiveness, swirl and steady pressure measurements separated, for the first time, the influence of the blades and vanes on ingressover a wide range of flow conditions. Measurements on the downstream stator platform provide added insight into the complex interaction between the egress and the mainstream.Measurements of unsteady pressure revealed the presence of large-scale structures, even in the absence of blades. The number and speed of the structures was shown to depend on the flow coefficient and the purge flow rate

Fluid-dynamics of Turbine Rim Seal Structures:A physical Interpretation Using URANS

Unsteady Reynolds-averaged Navier-Stokes modeling (URANS) is a valuable and costeffective tool for computational fluid dynamics (CFD), including the investigation of mainstream-cavity interaction in turbines. Despite the gap in accuracy with higher order CFD methodologies, URANS is among the few simulation strategies of industrial interest suitable for predicting ingress/egress over a wide range of conditions. This paper presents a numerical study of the flow-field in the upstream double-radial seal of a 1.5 stage turbine. Various configurations are tested, including nonpurged and purged conditions. Rigor of the approach is ensured by a set of sensitivity analyses, allowing the delineation of a best practice on the use of URANS in rim seal simulations: this includes an assessment of the effects of sector size, cavity domain size, and blade count. Timeaveraged and time-resolved flow predictions capture coherent structures in the rim gap. An association between the three-dimensional (3D) morphology of these structures and different ingress/egress mechanisms is proposed. Regions of enhanced radial activity are identified to correspond with the blade leading edges. A frequency analysis of unsteady pressure signals probed in the rim gap leads to a calculation of the structure number and speed. The structures are synchronous with the disk rotation for nonpurged cases but rotate at slower speed when purge is introduced. The relative number of blades and vanes directly influences the structure count and velocity. The configuration with no blades is characterized by the slowest structures. The calculations have been conducted at three different flow coefficients for the annulus flow. There is a reduction in radial activity and structure speed at lower flow coefficient, fundamentally related to the reduced pressure asymmetry and gradient of swirl across the rim seal.</p

The Effect of Vanes and Blades on Ingress in Gas Turbines

This paper presents experimental and computational results using a 1.5-stage test rig designed to investigate the effects of ingress through a double radial overlap rim-seal. The effect of the vanes and blades on ingress was investigated by a series of carefully-controlled experiments: firstly, the position of the vane relative to the rim seal was varied; secondly, the effect of the rotor blades was isolated using a disc with and without blades. Measurements of steady pressure in the annulus show a strong influence of the vane position. The relationship between sealing effectiveness and purge flow-rate exhibited a pronounced inflexion for intermediate levels of purge; the inflexion did not occur for experiments with a bladeless rotor. Shifting the vane closer to the rim-seal, and therefore the blade, caused a local increase in ingress in the inflexion region; again this effect was not observed for the bladeless experiments.Unsteady pressure measurements at the periphery of the wheel-space revealed the existence of large-scale pressure structures (or instabilities) which depended weakly on the vane position and sealing flow rate. These were measured with and without the blades on the rotor disc. In all cases these structures rotated close to the disc speed.<br/

The Inter-Bristle Pressure Filed in a Large-Scale Brush Seal

Brush seals promise improvements to the widely used labyrinth seal in regulating turbomachinery leakages. Enhanced resistance to the flow is provided by a static ring of densely packed fine wire bristles that are angled in the direction of rotation and flex to accommodate rotor excursions. A large-scale brush seal was constructed to study the leakage characteristics in direct relation to the pressure field within and surrounding the bristle pack for multiple clearance conditions, therefore developing the understanding of brush seal fluid dynamic behaviour. The governing parameter controlling leakage behaviour transitioned from pressure ratio for a large clearance, to pressure load for a line-on-line configuration. In all cases, leakage flow converged to an asymptotic value once maximum levels of bristle blow-down and pack compaction were attained. For both clearance configurations, this occurred at a pressure ratio corresponding to that at which axial distributions of pressure converged; equivalent behaviour was noted for the line-on-line configuration with pressure drop. Comparatively small changes were experienced in leakage behaviour and to the inter-bristle pressure field with increasing pressure drop for the line-on-line brush seal. This indicated that brush seal performance is more influenced by changes in bristle blow-down than bristle pack compaction.<br/

Dynamic Characterization of an Adaptive Film-Riding Seal

Shaft seals control the leakage of fluid between areas of high pressure and low pressure around rotating components inside turbomachinery. Static seals are often subject to damaging rubs with the shaft, caused by assembly misalignments and rotordynamic vibrations during operation. Adaptive seals aim to reduce leakage flows whilst minimizing wear. The Film Riding Pressure Actuated Leaf Seal (FRPALS) is one such design which utilizes a large installation clearance and is blown down towards the shaft under pressure.This paper presents a numerical model which can be used in the design and development of adaptive shaft seals, validated by experimental data from the literature. The model uses a modified version of the Reynolds equation to predict the dynamic, frequency-dependent stiffness and damping coefficients of the fluid film. The dynamic coefficients have been solved for different operational clearances and pressure differences to generate coefficient maps. These maps have been incorporated into a blow down model with compliant mechanical leaves to predict the transient translational and angular displacement paths of the FRPALS when subject to an increasing pressure drop.The blow down model has been compared against experimental measurements collected from a specially designed test facility for the characterization of shaft seal performance. Eddy current probes were used to measure the displacement paths of the FRPALS with the experimental values showing that the model can accurately predict the dynamic movement of the seal when subject to a pressure difference