Aerodynamic performance of turbine rim sealing flows

An investigation of the hot gas ingestion phenomenon in the Oxford Rotor Facility ex- amined rotor disc rotation, mainstream pressure asymmetries and unsteady structures in the cavity as main drivers. The facility underwent major modifications to allow different annulus flow configurations, higher resolution in the low and high bandwidth pressure instrumentation and the installation of a new system to quantify the sealing effective- ness using the tracer gas technique (±0.0068 uncertainty). Rotationally-driven ingestion was simulated with a bladeless annulus and conditions of pressure-driven ingestion with nozzle guide vanes. By decoupling each contribution, the fundamentals of hot gas ingestion are addressed. A chute rim seal arrangement has been studied with two gap sizes. The influence of non-dimensional purge flow, Cw, rotational Reynolds number, Reφ, axial Reynolds number, Reax, and seal clearance, sc, has been examined through the mean cavity pressure coefficient, Cp, sealing effectiveness, ε, and frequency spectra of the unsteady pressure signals. The maximum Reφ was 3.3 × 106 with a flow coefficient of 0.45 with NGVs. The mean pressure field in the cavity revealed the existence of two vortices. Values of sealing effectiveness showed good agreement with the disc pumping orifice model at low flow coefficients, suggesting operation in the rotationally-induced regime. This finding challenges the extended assumption that pressure-driven ingestion dominates when asymmetries exist in the annulus. However, this hypothesis matched the experimental data at high flow coefficients. The frequency spectra of the high bandwidth pressure signals revealed two sources of unsteadiness in the rim seal region: the large-scale flow features due to rotation of the disc and the interaction between the annulus and purge flows.</p

Flow and ingestion in a turbine disc cavity under rotationally-dominated conditions

An investigation of hot gas ingestion driven by the disc pumping effect in a chute seal was conducted at the Oxford Rotor Facility. Measurements of mean pressure, unsteady pressure and gas concentration have been logged and analysed under different operating conditions. The sensitivity of mean cavity pressure coefficient, frequency spectra of the unsteady pressures and sealing effectiveness to changing conditions of purge flow, annulus flow, rotor disc speed and seal clearance have been studied. The steady pressures revealed the development of two vortices in the cavity, induced by the sharp change in geometry of the stator wall. The increased shear at the interface between these two vortices strengthened the unsteady activity at this location. The addition of mainstream flow improved the sealing capability of the chute seal under certain operating conditions. The excitation of further frequencies when an axisymmetric annulus flow was introduced suggests a complex interaction between annulus and purge flows

FLOW AND INGESTION IN A TURBINE DISC CAVITY UNDER ROTATIONALLY-DOMINATED CONDITIONS

An investigation of hot gas ingestion driven by the disc pumping effect in a chute seal was conducted at the Oxford Rotor Facility. Measurements of mean pressure, unsteady pressure and gas concentration have been logged and analysed under different operating conditions. The sensitivity of mean cavity pressure coefficient, frequency spectra of the unsteady pressures and sealing effectiveness to changing conditions of purge flow, annulus flow, rotor disc speed and seal clearance has been studied. The steady pressures revealed the development of two vortices in the cavity, induced by the sharp change in geometry of the stator wall. The increased shear at the interface between these two vortices strengthened the unsteady activity at this location. The addition of mainstream flow improved the sealing capability of the chute seal under certain operating conditions. The excitation of further frequencies when an axisymmetric annulus flow was introduced suggested a complex interaction between annulus and purge flows. INTRODUCTION Decades of research in the topic of hot gas ingestion pointed towards disc pumping, circumferential annulus pressure asymmetries and, more recently, unstable rim seal flows as the main drivers for hot gas ingestion. These three mechanisms are depicted in Figure 1. Ingress of high temperature flow from the main gas path is undesired. Rotor discs are not thermally insulated and ingestion of hot gas from the mainstream can significantly reduce the operating life and damage the mechanical integrity of these highly stressed components. To prevent this from occurring, cool air is injected between the stator and rotor discs to purge and pressurise the cavity, however, this comes at the detriment of engine efficiency. The intrinsically convoluted flow field in the turbine rim region creates a complex environment in which fundamental understanding of the basic flow physics becomes paramount to comprehend the synergetic effect of each of the mechanisms mentioned above. This investigation focuses on the performance of a chute rim seal with no blades and vanes in the annulus, such that ingestion is expected to be driven by the effects of rotation. Bru Revert et al. (2021) showed that this scenario is relevant for high pressure turbines at low flow coefficients

Performance of a turbine rim seal subject to rotationally-driven and pressure-driven ingestion

This experimental study considered the performance of a chute rim seal downstream of turbine nozzle guide vanes (but without rotor blades). The experimental setup reproduced rotationally-driven ingestion without vanes and conditions of pressure-driven ingestion with vanes. The maximum rotor speed was 9000 rpm corresponding to a rotational Reynolds number of 3.3 &#xD7; 106 with a flow coefficient of 0.45. Measurements of mean pressures in the annulus and the disk rim cavity as well as values of sealing effectiveness deduced from gas concentration data are presented. At high values of flow coefficient (low rotational speeds), the circumferential pressure variation generated by the vanes drove relatively high levels of ingestion into the disk rim cavity. For a given purge flow rate, increasing the disk rotational speed led to a reduction in ingestion, shown by higher values of sealing effectiveness, despite the presence of upstream vanes. At Uax=(&#x3A9;b) = 0:45, the sealing effectiveness approached that associated with purely rotationally-driven ingestion. A map of sealing effectiveness against non-dimensional purge flow summarizes the results and illustrates the combined rotational and pressure-driven effects on the ingestion mechanism. The results imply that flow coefficient is a relevant parameter in rim sealing and that rotational effects are important in many applications, especially turbines with low flow coefficient

Wall-modeled large eddy simulations of axial turbine rim sealing

This paper presents wall-modeled large-eddy simulations (WMLES) of a chute-type turbine rim seal. Configurations with an axisymmetric annulus flow and with nozzle guide vanes fitted (but without rotor blades) are considered. The passive scalar concentration solution and WMLES are validated against available data in the literature for uniform convection and a rotor-stator cavity flow. The WMLES approach is shown to be effective, giving significant improvements over an eddy viscosity turbulence model, in prediction of rim seal effectiveness compared to research rig measurements. WMLES requires considerably less computational time than wall-resolved LES, and has the potential for extension to engine conditions. All WMLES solutions show rotating inertial waves in the chute seal. Good agreement between WMLES and measurements for sealing effectiveness in the configuration without vanes is found. For cases with vanes fitted, the WMLES simulation shows less ingestion than the measurements, and possible reasons are discussed