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.

The ingress of hot annulus gas into stator–rotor cavities is an important topic to engine designers. Rim-seals reduce the pressurized purge required to protect highly stressed components. This paper describes an experimental and computational study of flow through a turbine chute seal. The computations—which include a 360 deg domain—were undertaken using dlrtrace's time-marching solver. The experiments used a low Reynolds number turbine rig operating with an engine-representative flow structure. The simulations provide an excellent prediction of cavity pressure and swirl, and good overall agreement of sealing effectiveness when compared to experiment. Computation of flow within the chute seal showed strong shear gradients which influence the pressure distribution and secondary-flow field near the blade leading edge. High levels of shear across the rim-seal promote the formation of large-scale structures at the wheel-space periphery; the number and speed of which were measured experimentally and captured, qualitatively and quantitatively, by computations. A comparison of computational domains ranging from 30 deg to 360 deg indicates that steady features of the flow are largely unaffected by sector size. However, differences in large-scale flow structures were pronounced with a 60 deg sector and suggest that modeling an even number of blades in small sector simulations should be avoided.

The flow inside cavities between corotating compressor disks of aero-engines is driven by buoyancy, with Grashof numbers exceeding 1013. This phenomenon creates a conjugate problem: the Nusselt numbers depend on the radial temperature distribution of the disks, and the disk temperatures depend on the Nusselt numbers. Furthermore, Coriolis forces in the rotating fluid generate cyclonic and anticyclonic circulations inside the cavity. Such flows are three-dimensional, unsteady, and unstable, and it is a challenge to compute and measure the heat transfer from the disks to the axial throughflow in the compressor. In this paper, Nusselt numbers are experimentally determined from measurements of steady-state temperatures on the surfaces of both disks in a rotating cavity of the Bath compressor-cavity rig. The data are collected over a range of engine-representative parameters and are the first results from a new experimental facility specifically designed to investigate buoyancy-induced flow. The radial distributions of disk temperature were collected under carefully controlled thermal boundary conditions appropriate for analysis using a Bayesian model combined with the equations for a circular fin. The Owen-Tang buoyancy model has been used to compare predicted radial distributions of disk temperatures and Nusselt numbers with some of the experimentally determined values, taking account of radiation between the interior surfaces of the cavity. The experiments show that the average Nusselt numbers on the disk increase as the buoyancy forces increase. At high rotational speeds, the temperature rise in the core, created by compressibility effects in the air, attenuates the heat transfer, and there is a critical rotational Reynolds number for which the Nusselt number is a maximum. In the cavity, there is an inner region dominated by forced convection and an outer region dominated by buoyancy-induced flow. The inner region is a mixing region, in which entrained cold throughflow encounters hot flow from the Ekman layers on the disks. Consequently, the Nusselt numbers on the downstream disk in the inner region tend to be higher than those on the upstream disk.

In high-pressure turbines, cool air is purged through rim seals at the periphery of wheel-spaces between the stator and rotor disks. The purge suppresses the ingress of hot gas from the annulus but superfluous use is inefficient. In this paper, the interaction between the ingress, purge, and mainstream flow is studied through comparisons of newly acquired experimental results alongside unsteady numerical simulations based on the DLR TRACE solver. New experimental measurements were taken from a one-and-a-half stage axial-turbine rig operating with engine-representative blade and vane geometries, and overlapping rim seals. Radial traverses using a miniature CO2 concentration probe quantified the penetration of ingress into the rim seal and the outer portion of the wheel-space. Unsteady pressure measurements from circumferentially positioned transducers on the stator disk identified distinct frequencies in the wheel-space, and the computations reveal these are associated with large-scale flow structures near the outer periphery rotating at just less than the disk speed. It is hypothesized that the physical origin of such phenomenon is driven by Kelvin–Helmholtz instabilities caused by the tangential shear between the annulus and egress flows, as also postulated by previous authors. The presence and intensity of these rotating structures are strongly dependent on the purge flow rate. While there is general qualitative agreement between experiment and computation, it is speculated that the underprediction by the computations of the measured levels of ingress is caused by deficiencies in the turbulence modeling.

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.