Rim Seal Configuration Research Articles

On The Relationship Between Swirl And Unsteadiness Within Turbine Rim Seals

Abstract Unravelling the flow physics pertaining to hot gas ingress in turbines is crucial in enabling designers to realise global decarbonisation targets in aerospace. A turbine rim seal is fitted at the periphery of the rotor-stator cavity to minimise the ingress of annulus gas, which detrimentally affects cycle efficiency. The inherent unsteadiness in rim seal flows, arising from shear gradients between contiguous flow paths, introduces a consequential, yet presently unestablished, influence on sealing characteristics. A single-stage axial turbine facility in conjunction with an aeroengine architecture is employed to assess the steady and unsteady sealing characteristics of a range of industrially-relevant rim seals. Time-averaged measurements of gas concentration and swirl, acquired over a range of flow coefficients (CF), exhibited an inverse relationship between sealing performance and the purge-mainstream swirl difference (Δβ). Spectral analysis of unsteady pressure signals revealed an associated unsteadiness, induced by the strength of the annulus-cavity interaction. Across all CF, a low-frequency harmonic range consistently displayed proportionality between spectral activity and Δβ. Thus, a relationship between steady and unsteady characteristics was established. Examining a series of rim seal configurations with varying radial clearances signified that sealing performance was predominantly influenced by the radially outermost clearance. The configurations exhibiting superior performance presented heightened spectral activity, ascribed to an increased radial purge mass flux and establishing a definite relationship with concurrent steady measurements.

Effect of Rim Seal Geometry on Rotationally-Driven Ingestion

Abstract This study investigates turbine rim seal geometry effects within the rotationally-driven ingestion regime. Computations were performed with a wall-resolved unsteady Reynolds-averaged Navier–Stokes (URANS) model and a large-eddy simulation (LES) model including near-wall boundary layer modeling, that is, wall-modeled LES (WMLES). Use of simplified rim sealing models is proposed as an efficient method of ranking seal designs and investigating sensitivity to seal geometry. Four rim seal configurations, two chute seals, an axial seal and a radial seal which are representative of those used in gas turbines and in previous research were investigated. Furthermore, hybrid seals combining geometric characteristics from both the chute and radial seal were considered. Significant sensitivities of sealing performance to turbulence modeling are identified, but URANS and WMLES show similar trends in ranking of seal performance, and these are consistent with previous experimental work. The addition of an outer radial clearance section to a chute seal is effective in reducing ingestion levels.

Numerical investigation of the purge flow mechanisms and heat-transfer characteristics of turbine rim seals

In gas turbine and aeroengine, the rim seal is set between the turbine stator and rotor to prevent ingestion of the high-temperature mainstream into the rotating cavity. This study examined the sealing efficiency, flow mechanisms, and heat-transfer characteristics of three types of radial rim seal structures, which seriously affect the turbine aerodynamic efficiency and thermal safety. The flows and thermal fields were simulated using steady and unsteady numerical simulations, and dynamic modal decomposition was carried out to extract the unsteady characteristics. The results show that the seal cavity shape affects the seal efficiency. It was found that dolphin-nose with a hook and large cavity results in a higher sealing efficiency. The nose configuration of the cavity upper part also affects the outflow form of the purge flow. The purge flow from a shark-nose seal will inhibit the strength of the mainstream passage vortex, while that from a dolphin-nose seal will result in Kelvin-Helmholtz instability under certain flow rates and induce unsteady vortices. The purge flow will increase the thermal load of the blade and the hub at low flow rates. In particular, the overall thermal load with a dolphin-nose seal is 50% higher than that with a shark-nose structure. It is necessary to balance the sealing efficiency, flow instability characteristics, and heat-transfer characteristics to select an appropriate rim seal configuration.

Flow mechanism between purge flow and mainstream in different turbine rim seal configurations

The rim seal is used to prevent mainstream ingestion to the gap between the vane of a turbine and its blade. In this article, the dolphin lip with a hook configuration and a large seal cavity with hook structures are designed based on the high-pressure turbine datum single shark lip rim seal configuration. The sealing effect and parameters of the flow field are measured by an experiment method and a numerical simulation is used to explain the mechanism. For three configurations, the effect of the leakage slot vortex on the efficiency of the seal and the influence of leakage vortex, generated by the interaction between purge flow and mainstream flow, are discussed in depth. The result shows that the reverse vortex formed by the dolphin lip rim seal with hook structure will increase the sealing efficiency. The seal configuration with a large cavity improves sealing efficiency to a greater extent than the datum structure. At different purge flow rates and with unequal seal structures, the purge flow produces three types of leakage vortices in the passage. Besides, the seal configuration with dolphin lip produces a Kelvin-Helmholtz instability at the interface of the purge and the mainstream flows at a low purge flow rate to induce new leakage vortex branches in the passage of the blade.

Large-Eddy Simulation of the Unsteady Full 3D Rim Seal Flow in a One-Stage Axial-Flow Turbine

The flow field in a complete one-stage axial-flow turbine with 30 stator and 62 rotor blades is investigated by large-eddy simulation (LES). To solve the compressible Navier-Stokes equations, a massively parallelized finite-volume flow solver based on an efficient Cartesian cut-cell/level-set approach, which ensures a strict conservation of mass, momentum and energy, is used. This numerical method contains two adaptive Cartesian meshes, one mesh to track the embedded surface boundaries and a second mesh to resolve the fluid domain and to solve the conservation equations. The overall approach allows large scale simulations of turbomachinery applications with multiple relatively moving boundaries in a single frame of reference. The relative motion of the geometries is described by a kinematic motion level-set interface method. The focus of the numerical analysis is on the flow inside the rim seal between the stator and the rotor disks. Full $360^{\circ }$ computations of the turbine stage are performed for two rim seal configurations. First, the impact of the mesh resolution on the LES results is analyzed for the single lip rim seal configuration. Second, the LES results are compared to experimental data, followed by a detailed analysis of the unsteady flow field. For the single lip rim seal configuration, two modes unrelated to the rotor frequency and its harmonics are identified inside the rotor-stator wheel space, where the first more dominant mode shows a major impact on the ingress of the hot gas into the rotor-stator wheel space. The second mode is a counter-rotating mode which results from the interaction of the first mode with the flow field downstream of the stator blades. Third, at the same operating condition a modified configuration with a double lip rim seal is investigated and compared to the reference configuration to demonstrate the impact of the rim seal geometry on the overall flow field. The additional lip on the rotor disk damps the aforementioned modes and reduces the ingress of the hot gas resulting in an increase of the cooling effectiveness inside the rotor-stator wheel space, which is in a good agreement with the experimental results.

Performance Evaluation of Various Rim-Seal Geometries

A numerical investigation has been performed to evaluate the performance of various rim seals in a gas turbine. Two performance functions of the rim seal related to sealing effectiveness and the loss coefficient of the main flow were analyzed for two rim-seal inlet velocities and three leakage fractions using three-dimensional Reynolds-averaged Navier–Stokes equations. The Spalart–Allmaras model was used as the turbulence closure model. The Reynolds number, based on the axial chord at the mainstream outlet, was fixed at 500,000. Computational results for the sealing effectiveness were validated by comparison to experimental data. To examine the effects of the rim-seal configurations on the sealing effectiveness and loss coefficient of the main flow, five different rim-seal geometries were tested. The results indicate that a newly designed rim seal, which includes the inner cavity attached to the rotor side and the narrow interaction plane between the mainstream and rim seal, shows the best performance in sealing effectiveness and loss coefficient among the five tested cases for most of the tested leakage fractions and rim-seal inlet velocities.

Numerical investigation of the effect of rim seal on turbine aerodynamic design parameters and end wall flows in low-aspect ratio turbine

Turbomachinery design is associated with a comprehensive understanding of the end wall loss mechanism which is more complicated when the rim seal purge flow is taken into consideration. Ensuring an adequate life of high pressure turbine requires efficient cooling method to keep the disk temperature at an acceptable level. Hence, purge flow is injected through the rim seal in axial turbine in order to prevent hot gas ingestion into the rotor disk. This paper focused on the effect of upstream rim seal purge flow on turbine aerodynamic design parameters in a low aspect ratio axial turbine. Besides, end wall secondary flows and interaction between the rim seal purge flow and mainstream were also highlighted. Two different rim seal configurations were taken into account, i.e., inclined axial rim seal configuration and radial rim seal configuration. The purge flow rate approximately equaled to 1% of main flow which is the typical value in modern aero-engine secondary air system. Results show that inclined axial rim seal configuration is obviously superior to the radial one in terms of aerodynamic and sealing performance. The turbine aerodynamic design parameters and end wall flows are significantly modified by the purge flow. The change of pressure field and vortex structure near the hub region directly leads to the variation of turbine aerodynamic design parameters. In addition, the pressure leg of horse shoe vortex nearly disappears and cavity induced vortex appear when the flow is injected from rim seal. Moreover, the two legs of cavity induced vortex have the same rotating direction which form the passage vortex and propagate downstream in the main passage. Current research demonstrates that it is important to consider the effect of rim seal purge flow in turbine aerodynamic design and end wall profile optimization.

Rim Seal Experiments and Analysis for Turbine Applications

An experimental investigation was conducted to determine the sealing effectiveness and the aerodynamic characteristics of four rim seal models for a number of flow conditions. The experiments were conducted to obtain an extended data base for advanced turbine rim seal design. The class of rim seals investigated are those found on the downstream side of the rotor where the boundary layer on the disk is pumped directly into the seal gap. The experiments were conducted at disk tangential Reynolds numbers up to 5.1 × 106 with a simulated gas path flow across the top of the seal. The simulated gas path flow was injected with various amounts of swirl to determine the effect of swirl on the seal effectiveness. The radial gap and the axial overlap of the seal were varied and results compared with a baseline configuration. A rim seal configuration intended to prevent disk pumping directly into the seal gap was also investigated. A mass transfer analogy was used to characterize the rim seal ingestion characteristics and the trace gas chosen for this technique was CO2. The results of this investigation indicate that decreasing the radial gap of the seal produces a better improvement in seal effectiveness than increasing the axial overlap of the seal, that seal effectiveness increases only modestly as the swirl across the top of the seal decreases, and that the trace gas technique employed to determine seal effectiveness is an accurate alternative to pressure measurement or flow visualization techniques used by other investigators. The results of this investigation were compared with results from a boundary layer model for rim seals with axial gap geometries.