SUMMARYIschemic stroke classification is critical in conducting basic research and clinical practice. A precise analysis of stroke subtypes requires the integration of clinical features, findings from diagnostic tests, and knowledge about potential etiologic factors by competent diagnostic investigators. We performed a literature review of the published stroke classification systems and examined each for its benefits and limitations in the evaluation of the stroke etiology. Two major approaches to etiologic classifications of ischemic stroke are currently being used: the causative and phenotypic subtyping. The most widely used causative system is the Trial of Org 10172 in acute stroke treatment (TOAST) classification. With the advances in modern diagnostic technology, new stroke subclassification systems, such as the causative classification system (CCS) and Chinese ischemic stroke subclassification (CISS) system, have been developed to enhance the accuracy of TOAST. The A-S-C-O (Atherosclerosis, Smallvessel disease, Cardiac source, Other cause) phenotypic classification system makes efforts to identify the most likely etiology but not neglecting the possibility of other potential multiple causes. We conclude that the ideal stroke classification system needs to be valid, easy to use, evidence-based, and incorporate new information as it emerges.
The commercial computational fluid dynamics code ANSYS CFX 12.1 has been employed to carry out Unsteady Reynolds Averaged Navier Stokes (URANS) computations to investigate the fluid mechanics of two different rim-seal geometries in a 3D model of a turbine stage. The mainstream annulus, seal and wheel-space geometries are based on an experimental test rig used at the University of Bath. The calculated peak-to-trough pressure difference in the annulus, which is the main driving mechanism for ingestion, is in good agreement with experimental measurements. There is also good agreement between the computed and measured swirl ratios in the wheel-space. Computed values of concentration-based sealing effectiveness are obtained over a range of sealing flow rates for both an axial-clearance and a radial clearance rim-seal. Good agreement with gas concentration measurements is found for the axial-clearance seal over a certain range of sealing flow rates. Some under-prediction of the amount of ingestion for the radial-clearance seal is obtained. The computed mainstream pressure coefficient increases progressively with mainstream Mach number in moving from quasi-incompressible experimental rig conditions to the compressible flow conditions encountered in engines. It is shown that the minimum sealing flow rate required to prevent ingestion increases as mainstream Mach number increases. A scaling method is proposed to allow sealing flow rates to prevent ingestion obtained from low Mach number experiments to be extrapolated to engine-representative conditions.
Rim seals are fitted in gas turbines at the periphery of the wheel-space formed between rotor discs 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 URANS computations of a turbine stage at engine conditions. The 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 disc 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 bench-mark 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.
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