Sealing Flow Rate Research Articles

Swirl Enhancement Effect on Turbine Rim Seal Performance

Abstract In gas turbines, sealing flow is extracted from the high-pressure compressor and supplied into the turbine wheel-space to suppress hot gas ingestion. Such hot gas ingestion is thought to be driven by the difference in either the static pressure or swirl between the mainstream and wheel-space flows. For the first time, experiments have been conducted in a low-speed single-stage axial turbine with a single-radial clearance rim seal and a “swirler” on the rotor disk. The objective is to evaluate the impact of enhanced wheel-space flow swirl on the rim seal performance. The swirler does not affect the mainstream flow. In the wheel-space, however, the swirler reduces static pressure; increases swirl; and improves rim seal performance (i.e., reduces the minimum sealing flowrate needed for hot gas ingestion prevention). Thus, the seal performance improvement occurs with increased difference in pressure and reduced difference in swirl between the mainstream and wheel-space flows. Therefore, it can be inferred that the rim seal performance depends more strongly on swirl than pressure. Preliminary gas turbine cycle performance studies indicate that net cycle efficiency benefits can be obtained.

Aerostatically sealed chamber as a robust aerostatic bearing

Aerostatic bearings are typically used in ultra precision and high speed applications in controlled environments. The present study expands this operating domain. The present study experimentally investigates the performance and feasibility of a novel design for a robust air bearing consisting of an aerostatically sealed pressurized volume. A suitable operating domain for the bearing system was characterized based on measurements of the load capacity, friction moment, chamber flow, and seal flow rate at various opposing surface run-outs and supply pressures. The highest measured load capacity was 18.86 kN at 0.330 mm run-out, and decreased to 12.22 kN load at 3.804 mm run-out. The study provided corroborative evidence on the feasibility of the proposed chamber based bearing design.

Endwall contour design targeting on optimum aerodynamic efficiency of turbine stage and sealing effectiveness of rim seal

Endwall contour optimization design utilized to improve the aerodynamic performance has impact on the flow field near rim seal, and it consequently affects the sealing performance. Combined with Kriging agent model, NSGA-II genetic algorithm and CFD which solves the three-dimensional unsteady Reynolds-averaged Navier–Stokes equations coupled with a fully developed shear stress transport turbulent model, this paper constructs an optimization design system for turbine non-axisymmetric endwall contour design in order to improve aerodynamic efficiency and sealing effectiveness simultaneously. The numerical method for predicting the flow in wheel-space and sealing performance of turbine rim seal is validated on the basis of published experimental data. The base flat endwall design, and two optimal endwall contour designs are selected to investigate the aerodynamic and sealing performance. The results show that the aerodynamic and sealing performance can be improved at the same time. Compared with the base flat endwall design, the total-to-total efficiency is increased by 0.23% and the sealing effectiveness is increased by 11.49% for optimal design 1 at optimal design point. Optimal design 1 can significantly reduce the aerodynamic loss and for different sealing flow rates, the total-to-total efficiency is greater than that of base flat endwall design. Compared with the base flat endwall design, the total-to-total efficiency is increased by 0.15% and the sealing effectiveness is increased by 15.43% for optimal design 2 at optimal design point. The influence of endwall contour on sealing effectiveness is obvious when the cooling flow rate is small. The work of this paper provides a reference for the design of non-axisymmetric endwall with comprehensive optimization of turbine aerodynamic and rim sealing performance.

Numerical study of turbine rim seals performance with different sealing structures

The unsteady large-scale vortex near the turbine rim has an important influence on the sealing performance. Characteristics and performance of four sealing structures are researched in this paper. Three-dimensional unsteady numerical simulation was adopted to deeply reveal the characteristics of the rim sealing vortex and its influence mechanism on the rim sealing performance. The results show that the rim seal vortex structure induced by the interaction between ingested gas and sealing flow in the gap is the leading cause of unsteady flow in the rim. The vortex size is suppressed with the increasing seal flow rate or a Chute seal structure. However, the rim seal vortex exit in the cavity gap under a low seal flow rate can suppress the gas intrusion and improve the sealing efficiency of the turbine cavity even with a simple sealing structure. The Chute sealing structure achieves better performance among the four sealing structures studied in this paper. It can achieve complete sealing under a low sealing flow rate of 0.5% and has less impact on the aerodynamic performance of the mainstream even with high sealing flow rate. The research of this paper has guiding significance for further understanding the sealing mechanism and optimizing the design of the sealing structures.

Numerical investigation of API 31 cyclone separator for mechanical seal piping plan for rotating machineries

API plan 31 uses a cyclone separator that taps a small portion of discharge line process liquid for mechanical seal cooling process. Despite its simplicity, this piping plan requires comprehensive designing process to ensure its performances (flow rate to mechanical seal and pressure drop) are satisfactory. In this research, API 31 cyclone separator geometrical parameters (length cylinder, conical and vortex finder and radius overflow and underflow) effects onto pressure drop and seal flow rate are investigated using numerical method via ANSYS FLUENT software. It is found that increasing overflow and underflow radius and length of cylinder and conical decrease pressure drop by 15.94%, 24.82%, 20.30% and 11.42% respectively. Increasing vortex finder length meanwhile increases pressure drop by 7.77%. Meanwhile, increasing radius overflow and length of vortex finder increase flow rate to seal by 43.79% and 9.63% respectively. Increasing radius underflow and length of cylinder and conical reduces flow rate to seal by 71.92%, 6.41% and 8.93% respectively. It is also observed that a generalization of a particular cyclone separator geometry is not always applicable as it depends on the cyclone separator base design itself where in this case all geometrical factors are significant to pressure drop and flow to seal.

Investigation of the flowfield on a vaneless counter-rotating turbine with seal cavity flow ejection

The sealing of the rotor-rotor gap and rotor disk cooling are vital to the safe operation of the vaneless counter-rotating turbine(VCRT). In order to quantifies the influence of the wheel-space cavity flow on the VCRT aerodynamic performance, and to improve turbine efficiency of the VCRT at certain rim seal ejection rates, numerical studies which considered the effects of rotor-rotor rim seal flow ejection are carried out in this paper. The three dimensional unsteady computational fluid dynamic analysis of a VCRT at the engine conditions are performed, and the seal flow ejected downstream of the high pressure rotor row at six sealing flow rate are modeled. The interaction among the high pressure rotor trailing shock wave, the downstream secondary flow and the seal flow has been studied and quantitatively characterized as a function of the purge ejection rate. Numerical results show that seal flow- mainstream flow interaction is entirely dominated by the high pressure rotor trailing edge shock at the hub, low pressure hub passage vortex and the mixing of the sealing flow from wheelspace and mainstream. When the mass flow rate of the coolant is smaller than some threshold value, the shock loss of the high pressure rotor and hub secondary flow loss of the low pressure rotor are decreased with the increasing of the coolant mass flow rate. It causes that the VCRT efficiency is gradually increased. On condition that the amount of the seal flows is beyond the threshold value, the key roles in modification of the VCRT performance are changed. The increment of the hub secondary flow loss and the mixing loss are gradually larger than the decrement of the shock loss. As a result, the turbine efficiency gradually decreases.

Flow characteristics in turbine wheel space cavity

Unsteady flow structures unrelated to rotating frequency in the turbine wheel space cavity has been observed and reported in a number of recent rim sealing investigations. These flow structures are relatively large in scale and have a significant influence on the sealing effectiveness prediction. As a result, it is important to capture these flow structures in numerical simulation. Small computation sectors, due to the circumferential symmetry assumption, have been proved to fail to capture these flow structures. This paper aims to find a minimum computation sector size that can capture these flow structures, at the same time save computation resources and shorten the convergence process for a simple axial rim seal. Four different sector model (10, 20, 30, 180-degree) are set into simulation using RANS and URANS method. The steady and unsteady simulation results are compared. By comparison, the 20-degree sector model is considered appropriate to conduct successive investigations. Then the 20-degree model is set into unsteady simulation under four different sealing flow rates cw=0 (non-sealing flow case), cw=2500, cw=5000, cw=10000). It was found that due to the large-scale flow structure, a staggering pressure distribution is found in the cavity. Increasing the sealing flow rate diminishes these structures and stabilizes the flow in the wheel space cavity. The staggering pressure distribution causes the sealing effectiveness to show an abnormal variation trend. Unsteady pressure oscillation waves at two different circumferential positions are subjected to cross-correlation analysis, by which the rotating speed and number of the flow structure could be calculated.

Investigation of unsteady flow in the unscalloped radial turbine cavity

This paper presents an unsteady numerical study of the unsteady flow in the unscalloped radial turbine cavity considering the effect of computational sector size and sealing flow rate. A simplified U-shaped cavity with vanes and blades was simulated and the sealing efficiency was obtained using additional variables method. Unsteady low-pressure structures exist under a certain sealing flow rate, which is similar with axial turbine cavity, while the impact area is much larger. A comparison of computational domains sizes for 60° and 360° shows that whether the sector model can accurately predict the flow field depends highly on the number of low-pressure structures. As the sealing flow rate increases, the number and the speed of low-pressure structures at high radius decrease, and the corresponding low-frequency amplitude increases initially but decreases subsequently. The speed of low-pressure structure is positively correlated with its number, which seems contrary to the phenomenon observed in the axial turbine cavity.

Influence of Secondary Sealing Flow on Performance of Turbine Axial Rim Seals

Purge flow is of great importance in cooling turbine disks and sealing rotor-stator disc cavity to reduce hot gas ingestion in gas turbines. The amount of cooling air extracted from the compressor is crucial to engine efficiency. Excessive sealing air will cause not only a reduction in work transfer but also an increase in aerodynamic losses caused by the mixing of main and sealing flow. In order to simplify rim seal structure while ensuring high sealing efficiency, the current paper optimizes the flow path of the secondary air system and presents a new rim seal structure with auxiliary sealing holes transporting a certain amount of secondary sealing flow. The new structure was compared with the conventional counterpart using validated CFD methods, showing that the additional secondary sealing flow is possible to improve sealing efficiency in disk cavity. The current paper investigates the secondary sealing flow with and without swirl (the angle of auxiliary sealing hole inclination is 0° and 45° respectively), while maintaining the total amount of the sealing flow, flowrate ratio of sealing air (main sealing flow rate versus secondary sealing flow rate=1:1, 2:1, 3:1, 4:1), found that both two parameters have essential impacts on sealing efficiency. The relationship between these two parameters and sealing efficiency was obtained, and it provides a new philosophy for the design of rim seal in gas turbines.

Numerical investigation of the blockage effect caused by rim seal flow

Blockage effect caused by the rim seal flow is one of the most important sources of loss in the interaction between the turbine rim seal flow and mainstream and has not been investigated in detail. To research the blockage effect, numerical simulations were presented for different rim seal flow rates. The pressure distribution was investigated and losses resulting from the blockage effect were analyzed. In addition, the influences of swirl ratio, inclination angle, and width were revealed. Results show that the blockage effect originates from the pushing and compression of egress flow to the mainstream and results in local pressure increase. The egress flow mainly blocks the mainstream downstream of the stator suction side and above the egress flow. Furthermore, the blockage effect strengthens with the increase of rim seal flow rate and weakens with the increase of swirl ratio. The effect of inclination angle and width on the blockage effect could be neglected. With the increase of rim seal flow rate, the blockage on the stator suction side causes the secondary losses to be reduced and the increasing expansion in the rotor passage leads to more losses at the tip and surface. A loss isolation method that defines the space region of loss in the rotor domain is established and proven to be effective. On an average, relative to the design condition, the overall loss introduced by the blockage effect increases by about 89.44% per 1% increase in the rim seal flow rate.

Re-Ingestion of Upstream Egress in a 1.5-Stage Gas Turbine Rig

In gas turbines, rim seals are fitted at the periphery of stator and rotor discs to minimize the purge flow required to seal the wheel-space between the discs. Ingestion (or ingress) of hot mainstream gases through rim seals is a threat to the operating life and integrity of highly stressed components, particularly in the first-stage turbine. Egress of sealing flow from the first-stage can be re-ingested in downstream stages. This paper presents experimental results using a 1.5-stage test facility designed to investigate ingress into the wheel-spaces upstream and downstream of a rotor disk. Re-ingestion was quantified using measurements of CO2 concentration, with seeding injected into the upstream and downstream sealing flows. Here, a theoretical mixing model has been developed from first principles and validated by the experimental measurements. For the first time, a method to quantify the mass fraction of the fluid carried over from upstream egress into downstream ingress has been presented and measured; it was shown that this fraction increased as the downstream sealing flow rate increased. The upstream purge was shown to not significantly disturb the fluid dynamics but only partially mixes with the annulus flow near the downstream seal, with the ingested fluid emanating from the boundary layer on the blade platform. From the analogy between heat and mass transfer, the measured mass-concentration flux is equivalent to an enthalpy flux, and this re-ingestion could significantly reduce the adverse effect of ingress in the downstream wheel-space. Radial traverses using a concentration probe in and around the rim seal clearances provide insight into the complex interaction between the egress, ingress and mainstream flows.

Performance of Rim-Seals in Upstream and Downstream Cavities over a Range of Flow Coefficients

This paper presents measurements of CO2 concentration and pressure in a new, highly instrumented and versatile, 1.5-stage gas turbine facility. The rig, which has been specifically designed for investigations related to hot gas ingestion, features interchangeable rim-seals, blading configurations, and the capability to operate at a wide range of flow coefficients. The turbine section includes an upstream and a downstream wheel-space on either side of a rotor disc featuring turned blades. Measurements of CO2 concentration and steady static pressure were used to assess the pressure field in the turbine annulus and to investigate the performance of a radial clearance rim seal in both wheel-spaces. The wealth of data presented will be of great significance for computational fluid dynamics (CFD) validation studies considering downstream cavities. Pressure measurements were made at various locations in the turbine annulus for a range of flow coefficients. In the annulus upstream of the rotor blades the square root of the peak-to-trough pressure difference was shown to increase linearly with the flow coefficient. The radial variation in concentration effectiveness in the upstream and downstream wheel-spaces is provided for a range of sealing flow rates at an operating point near the design condition for the stage. In both cases, the concentration on the stator walls was virtually invariant with the radius and equal to that in the rotating core. The results also showed that for the same effectiveness, a smaller value of non-dimensional sealing flow is required in the downstream wheel-space, indicating a weaker driver for ingress. Off-design measurements of the value of the sealing flow parameter when the concentration effectiveness is 95% in both wheel-spaces are also provided for a range of flow coefficients. In the upstream wheel-space, the sealing flow parameter, and hence ingress, is shown to increase linearly with flow coefficient and be proportional to the square root of the peak-to-trough difference of the circumferential pressure variation in the annulus. Downstream of the blades, both the sealing flow parameter and the square root of the peak-to-trough pressure difference reach a minimum near the operating point, indicating that ingress is affected by the vane pressure field immediately downstream of the seal clearance.

Computations on the flow characteristics and sealing performance of a stator well cavity for the second stage in gas turbine

In this paper, a numerical study is conducted to determine the effect of sealing flow rate, annulus pressure ratio as well as rotation speed on the flow dynamics and sealing performance for a rim cavity. The computational model is performed for a two-stage mainstream cascade passage with a stator-well cavity which consists of a pre-swirl cavity and two rim cavities beneath second vanes. The Reynolds-averaged Navier–Stokes equations, coupled with Shear Stress Transport turbulence model, and a scalar equation are solved. The results obtained in this paper indicate that the downstream rim cavity shows better sealing performance than that of upstream rim cavity. The flow field in the upstream rim cavity is sensitive to the sealing flow rate and rotation speed but is insensitive to the annulus pressure ratio. However, the sealing flow rate, rotation speed and annulus pressure ratio has negligible effect on the flow field within the downstream rim cavity. With an increase in sealing flow rate or rotation speed, the sealing effectiveness increases. Nevertheless, increasing annulus pressure ratio contributes to the hot gas ingestion. Moreover, the pressure distribution in the mainstream annulus and the variation of sealing effectiveness on the stator and rotor wall in upstream and downstream rim cavity with radius is discussed in detail.

Numerical Characterization of Hot-Gas Ingestion Through Turbine Rim Seals

This paper deals with a numerical study aimed at the characterization of hot-gas ingestion through turbine rim seals. The numerical campaign focused on an experimental facility which models ingress through the rim seal into the upstream wheel-space of an axial-turbine stage. Single-clearance arrangements were considered in the form of axial- and radial-seal gap configurations. With the radial-seal clearance configuration, computational fluid dynamics (CFD) steady-state solutions were able to predict the system sealing effectiveness over a wide range of coolant mass flow rates reasonably well. The greater insight of flow field provided by the computations illustrates the thermal buffering effect when ingress occurs: For a given sealing flow rate, the effectiveness on the rotor was significantly higher than that on the stator due to the axial flow of hot gases from stator to rotor caused by pumping effects. The predicted effectiveness on the rotor was compared with a theoretical model for the thermal buffering effect showing good agreement. When the axial-seal clearance arrangement is considered, the agreement between CFD and experiments worsens; the variation of sealing effectiveness with coolant flow rate calculated by means of the simulations displays a distinct kink. It was found that the “kink phenomenon” can be ascribed to an overestimation of the egress spoiling effects due to turbulence modeling limitations. Despite some weaknesses in the numerical predictions, the paper shows that CFD can be used to characterize the sealing performance of axial- and radial-clearance turbine rim seals.

Measurements and Modeling of Ingress in a New 1.5-Stage Turbine Research Facility

In gas turbines, hot mainstream flow can be ingested into the wheel-space formed between stator and rotor disks as a result of the circumferential pressure asymmetry in the annulus; this ingress can significantly affect the operating life, performance, and integrity of highly stressed, vulnerable engine components. Rim seals, fitted at the periphery of the disks, are used to minimize ingress and therefore reduce the amount of purge flow required to seal the wheel-space and cool the disks. This paper presents experimental results from a new 1.5-stage test facility designed to investigate ingress into the wheel-spaces upstream and downstream of a rotor disk. The fluid-dynamically scaled rig operates at incompressible flow conditions, far removed from the harsh environment of the engine which is not conducive to experimental measurements. The test facility features interchangeable rim-seal components, offering significant flexibility and expediency in terms of data collection over a wide range of sealing flow rates. The rig was specifically designed to enable an efficient method of ranking and quantifying the performance of generic and engine-specific seal geometries. The radial variation of CO2 gas concentration, pressure, and swirl is measured to explore, for the first time, the flow structure in both the upstream and downstream wheel-spaces. The measurements show that the concentration in the core is equal to that on the stator walls and that both distributions are virtually invariant with radius. These measurements confirm that mixing between ingress and egress is essentially complete immediately after the ingested fluid enters the wheel-space and that the fluid from the boundary layer on the stator is the source of that in the core. The swirl in the core is shown to determine the radial distribution of pressure in the wheel-space. The performance of a double radial-clearance seal is evaluated in terms of the variation of effectiveness with sealing flow rate for both the upstream and the downstream wheel-spaces and is found to be independent of rotational Reynolds number. A simple theoretical orifice model was fitted to the experimental data showing good agreement between theory and experiment for all cases. This observation is of great significance as it demonstrates that the theoretical model can accurately predict ingress even when it is driven by the complex unsteady pressure field in the annulus upstream and downstream of the rotor. The combination of the theoretical model and the new test rig with its flexibility and capability for detailed measurements provides a powerful tool for the engine rim-seal designer.

Review of Ingress in Gas Turbines

This review summarizes research concerned with the ingress of hot mainstream gas through the rim seals of gas turbines. It includes experimental, theoretical, and computational studies conducted by many institutions, and the ingress is classified as externally induced (EI), rotationally induced (RI), and combined ingress (CI). Although EI ingress (which is caused by the circumferential distribution of pressure created by the vanes and blades in the turbine annulus) occurs in all turbines, RI and CI ingress can be important at off-design conditions and for the inner seal of a double-seal geometry. For all three types of ingress, the equations from a simple orifice model are shown to be useful for relating the sealing effectiveness (and therefore the amount of hot gas ingested into the wheel-space of a turbine) to the sealing flow rate. In this paper, experimental data obtained from different research groups have been transformed into a consistent format and reviewed using the orifice model equations. Most of the published results for sealing effectiveness have been made using concentration measurements of a tracer gas (usually CO2) on the surface of the stator, and—for a large number of tests with single and double seals—the measured distributions of effectiveness with sealing flow rate are shown to be consistent with those predicted by the model. Although the flow through the rim seal can be treated as inviscid, the flow inside the wheel-space is controlled by the boundary layers on the rotor and stator. Using boundary-layer theory and the similarity between the transfer of mass and energy, a theoretical model has been developed to relate the adiabatic effectiveness on the rotor to the sealing effectiveness of the rim seal. Concentration measurements on the stator and infrared (IR) measurements on the rotor have confirmed that, even when ingress occurs, the sealing flow will help to protect the rotor from the effect of hot-gas ingestion. Despite the improved understanding of the “ingress problem,” there are still many unanswered questions to be addressed.

Numerical Investigations on the Sealing Effectiveness of Turbine Honeycomb Radial Rim Seal

This paper presented a numerical comparison of the sealing performance between conventional radial rim seal and new-designed honeycomb radial rim seal with three sealing flow rates. Three-dimensional unsteady Reynolds-averaged Navier–Stokes (URANS) equations, coupled with a fully developed shear stress transport (SST) turbulent model from ansys-cfx, were utilized to investigate the sealing effectiveness of rim seal and flow characteristics in the wheel-space of gas turbines. First, the numerical method for analysis the sealing performance of the rim seal was validated on the basis of published experimental data. Pressure distributions on the vane hub, sealing effectiveness distributions on the stator disk surface and swirl ratio distributions in the wheel-space of the experimental models were numerically computed and compared to the experimental data. The additional scalar variable was adopted in calculation to simulate the distribution of tracer gas concentration in experiment. The numerical results were in excellent agreement with experimental data. Then the sealing effectiveness of conventional and new-designed honeycomb radial rim seal are compared. The flow field in the wheel-space of the new-designed honeycomb and conventional turbine radial rim seal was illustrated and analyzed. Furthermore, three cases with different honeycomb cell depths were selected to investigate the influence of honeycomb cell depth on sealing performance of honeycomb radial rim seal. Compared with conventional radial rim seal, the honeycomb radial rim seal could improve the sealing effectiveness by 9–14% at the same sealing flow rate. The honeycomb cell depth has a pronounced effect on sealing performance of honeycomb radial rim seal. It shows that sealing effectiveness of the honeycomb radial rim seal increases with the increase of the honeycomb cell depth, as honeycomb cell depth increases from 1.6 mm to 4.8 mm, the sealing effectiveness is increased by about 8% at most. In addition, the flow pattern of the rim seal and wheel-space is provided to describe sealing flow characteristics.

Effect of Ingress on Turbine Disks

The ingress of hot gas through the rim seal of a gas turbine depends on the pressure difference between the mainstream flow in the turbine annulus and that in the wheel-space radially inward of the seal. This paper describes experimental measurements which quantify the effect of ingress on both the stator and rotor disks in a wheel-space pressurized by sealing flow. Infrared (IR) sensors were developed and calibrated to accurately measure the temperature history of the rotating disk surface during a transient experiment, leading to an adiabatic effectiveness. The performance of four generic (though engine-representative) single- and double-clearance seals was assessed in terms of the variation of adiabatic effectiveness with sealing flow rate. The measurements identify a so-called thermal buffering effect, where the boundary layer on the rotor protects the disk from the effects of ingress. It was shown that the effectiveness on the rotor was significantly higher than the equivalent stator effectiveness for all rim seals tested. Although the ingress through the rim seal is a consequence of an unsteady, three-dimensional flow field, and the cause–effect relationship between pressure and the sealing effectiveness is complex, the time-averaged experimental data are shown to be successfully predicted by relatively simple semi-empirical models, which are described in a separate paper. Of particular interest to the designer, significant ingress can enter the wheel-space before its effect is sensed by the rotor.

Design of an Improved Turbine Rim-Seal

Rim seals are fitted in gas turbines at the periphery of the wheel-space formed between rotor disks and their adjacent casings. These seals, also called platform overlap seals, reduce the ingress of hot gases which can limit the life of highly stressed components in the engine. This paper describes the development of a new, patented rim-seal concept showing improved performance relative to a reference engine design, using unsteady Reynolds-averaged Navier–Stokes (URANS) computations of a turbine stage at engine conditions. The computational fluid dynamics (CFD) study was limited to a small number of purge-flow rates due to computational time and cost, and the computations were validated experimentally at a lower rotational Reynolds number and in conditions under incompressible flow. The new rim seal features a stator-side angel wing and two buffer cavities between outer and inner seals: the angel-wing promotes a counter-rotating vortex to reduce the effect of the ingress on the stator; the two buffer cavities are shown to attenuate the circumferential pressure asymmetries of the fluid ingested from the mainstream annulus. Rotor disk pumping is exploited to reduce the sealing flow rate required to prevent ingress, with the rotor boundary layer also providing protective cooling. Measurements of gas concentration and swirl ratio, determined from static and total pressure, were used to assess the performance of the new seal concept relative to a benchmark generic seal. The radial variation of concentration through the seal was measured in the experiments and these data captured the improvements due to the intermediate buffer cavities predicted by the CFD. This successful design approach is a potent combination of insight provided by computation, and the flexibility and expedience provided by experiment.

Steady computations of ingress through gas turbine rim seals

In gas turbines, rim seals are fitted at the periphery of the wheel-space between the turbine disc and its adjacent casing; their purpose is to reduce the ingress of hot mainstream gases. This paper describes the use of a three-dimensional, steady-state model to investigate ingress through engine-representative single and double radial-clearance seals. The three-dimensional Reynolds-averaged Navier–Stokes computations of a simplified turbine stage are carried out using the commercial computational fluid dynamics code ANSYS CFX v13, and the model is based on the geometry of an experimental test rig at the University of Bath. The measured variation of the peak-to-trough pressure difference in the annulus, which is the main driving mechanism for ingress, is reproduced well qualitatively by the computations; quantitatively, the maximum local differences between computation and experiment are less than 20% of the measured peak-to-trough circumferential variation. The radial variation of swirl ratio in the rotor–stator wheel-space is well predicted over the range of flow rates and rim seal geometries studied. The radial distribution of sealing effectiveness determined from experiments is reproduced inward of the mixing region near the seal clearance over a range of sealing flow rates; some over-prediction of the effectiveness was found for both seals at high radius, probably due to limitations in the turbulent mixing modelling. The three-dimensional steady-state approach may be a practical tool for the engine designer where there is a lack of experimental data, providing quantitative predictions of the flow structure within the rotor–stator wheel-space and qualitative predictions of the sealing effectiveness for a given rim seal geometry.