Browsing by Author "Georgakis, C."

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    Detailed study on stiffness and load characteristics of film-riding groove types using design of experiments
    (American Society of Mechanical Engineers (ASME), 2017-04-11) Tibos, S. M.; Georgakis, C.; Harvey, K.; Amaral Teixeira, Joao
    In the application of film-riding sealing technology, there are various groove features that can be used to induce hydrodynamic lift. However, there is little guidance in selecting the relative parameter settings in order to maximize hydrodynamic load and fluid stiffness. In this study, two groove types are investigated—Rayleigh step and inclined groove. The study uses a design of experiments approach and a Reynolds equation solver to explore the design space. Key parameters have been identified that can be used to optimize a seal design. The results indicate that the relationship between parameters is not a simple linear relationship. It was also found that higher pressure drops hinder the hydrodynamic load and stiffness of the seal suggesting an advantage for using hydrostatic load support in such conditions.
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    Experimental studies on volute-impeller interactions pf centrifugal compressors having vaned diffusers
    (Cranfield University, 2003) Georgakis, C.; Ivey, Paul C.
    It is well recognised that the volute plays a important role on the stage performance of a turbo machine in terms of pressure recovery, losses, efficiency and flow range, in addition to influencing stability and radial loads. Although there is a demand for further improvements in efficiency and operating range of compressor components, less attention has been paid to the effect of volute design. The goal of the present study was to experimentally measure the flow through a high pressure ratio 5:1 centrifugal compressor incorporating a vaned diffuser and a volute as part of a fully representative, production version turbocharger test facility. By running the unit as in a gas turbine cycle' (that is with the compressor flow passed through the combustors and then through the hot axial turbine of the turbocharger) it was possible to generate the 1.2MW of power that the test compressor required at its design point. This approach allowed the compressor to be tested at a duty that was relevant to todays industrial needs. The present study shows that the source of the circumferential pressure distortion at the impeller tip was the non-axisymmetric volute. The vaned diffuser did not fully attenuate this distortion, and consequently some of this distortion was measured at the impeller tip. The impeller tip pressure distortion varied as the operating point moved on the compressor characteristic and it was seen to be greatest at the surge point. The performance duties of the compressor components were examined in detail. The radial vaned diffuser had a major influence on both the stage and volute performance, and imposed a very narrow operating range on the test volute. The volute had a nearly constant loss coefficient and the pressure recovery therefore mainly depended on the dynamic head at the volute inlet at the duty points encountered on the tests. The test volute was modified to have a cutback° tongue which gave a better compressor matching at high flows. The flow pattern at the impeller exit was seen to comprise two distinct regions, firstly a wake° or accumulation of low relative energy fluid which occupied most of the shroud surface and extended towards the suction side, and secondly a lower loss flow which was present at the hub pressure comer. The survey also investigated the structure of the flow in the test volute. The volute did not operate as a constant pressure collector° and there was a swirling flow within it. This Vortex distribution was seen to depend on the radial velocity distribution at the volute inlet. Larger radial velocities contributed to stronger swirling flows. The lowest total pressure showed itself in the core of the Vortex over the volute cross sections. There was a radial decrease of the tangential velocities from the inner to the outer radius of the volute cross sections. The flow into the test volute is not uniform. Additional insight into the nature of the compressor flows was derived from a complimentary CFD analysis which is also described.
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    Investigation of effective groove types for a film riding seal
    (ASME, 2017-02-14) Teixeira, Joao Amaral; Tibos, S. M.; Georgakis, C.
    Over the past two decades, significant efforts have been made to introduce film riding sealing technology on large industrial or aerospace gas turbines. The main challenge comes from the high surface speeds and high temperatures, which lead to large thermal distortions. One approach to tackle the effect of thermally induced distortion is to design a seal to operate at a large film to limit the viscous heat generation. To design a seal pad that maximizes force at relatively high film heights, it is important to select the seal groove type that looks the most promising to deliver this characteristic. Several groove types have been assessed as part of this study. The most promising groove type is the Rayleigh step, which gives the strongest level of combined hydrostatic and hydrodynamic load support while also being easier to tessellate on individual seal segments. The results generated using a uniform grid Reynolds equation method show reasonable agreement with computational fluid dynamics (CFD) calculations. This provides confidence in the validity of the method, approach, and results.
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    Investigation of hydrostatic fluid forces in varying clearance turbomachinery seals
    (ASME, 2017-11-21) Tibos, S. M.; Georgakis, C.; Teixeira, Joao Amaral; Hogg, S.
    Varying clearance, rotor-following seals are a key technology for meeting the demands of increased machine flexibility for conventional power units. These seals follow the rotor through hydrodynamic or hydrostatic mechanisms. Forward-facing step (FFS) and Rayleigh step designs are known to produce positive fluid stiffness. However, there is very limited modeling or experimental data available on the hydrostatic fluid forces generated from either design. A quasi-one-dimensional (1D) method has been developed to describe both designs and validated using test data. Tests have shown that the FFS and the Rayleigh step design are both capable of producing positive film stiffness and there is little difference in hydrostatic force generation between the two designs. This means any additional hydrodynamic features in the Rayleigh step design should have a limited effect on hydrostatic fluid stiffness. The analytical model is capable of modeling both the inertial fluid forces and the viscous fluid losses, and the predictions are in good agreement with the test data.