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An innovative application of existing technology is proposed for attenuating the effects of transient phenomena, such as rotor-stator and rotor-strut interactions, linked to noise and fatigue failure in turbomachinery environments. A computational study was designed to assess the potential of Passive Porosity Technology as a mechanism for alleviating interaction effects and radiated noise by reducing the fluctuating forces acting on the vane surfaces. The study involved a typical high bypass fan stator airfoil immersed in a subsonic free field and exposed to the effects of a transversely moving wake. Time histories of the primitive aerodynamic variables obtained from Computational Fluid Dynamics (CFD) calculations were input into an acoustic prediction code to estimate noise levels at a radial distance of ten chords from the stator airfoil. This procedure was performed on the solid airfoil to obtain a baseline, and on approximately fifty porous configurations in order to isolate those that would yield maximum noise reductions without compromising the aerodynamic performance of the stator. It was found that, for a single stator immersed in a subsonic flow field, communication between regions of high pressure differential - made possible by the use of passive porosity - tends to induce a time-dependent oscillatory pattern of small inflow-outflow regions near the stator leading edge (LE), which is well established before wake effects come into play. The oscillatory pattern starts at the LE, and travels downstream on both suction and pressure sides of the airfoil. The amplitude of the oscillations seemed to be proportional to the extension of the porous patch on the pressure side. Regardless of this effect, which may not have occurred if the airfoil were placed within a stator cascade, communication between regions of high pressure differential is necessary to significantly alter the noise radiation pattern of the stator airfoil. Whether those changes result in noise abatement or enhancement depends primarily on the placement and extension of the porous patches. For most viable configurations, porosity reduced loading noise but increased thickness noise. Variations in nominal porosity were of secondary importance. In general, the best aerodynamic performers (i.e., those configurations that were able to reduce unsteady lift without severely altering the lift and/or drag characteristics of the solid airfoil) were also the best acoustic performers. As a result of using passive surface porosity, overall peak radiated noise was reduced by approximately 1.0 dB. This reduction increased to about 2.5 dB when the effects of loading noise alone were considered. / Ph. D.

Detta examensarbete ämnar att undersöka värmebehandlingsmetoder för att skapaporösa austenitiska rostfria stål samt utvärdera ifall dessa ytor medför förbättringargentemot referensmaterialen. De porösa ytorna skapades genom en två-stegs metod.Prover oxiderades initialt i en fuktig eller torr argon atmosfär för att sedan reducerasi ren vätgas. De material som använts i denna studie är austenitiska rostrifa stål ienhlighet med standardena 1.4301 (304), 1.4404 (316L), 1.4435 (316L) och F-138 (316LVM).Oxidationsprocessen genomfördes under temperaturer över 630 ◦ C under 150 - 1200 s.Efteråt reducerades proverna i ren vätgas under 150 - 2400 s.För att karakterisera processen undersöktes proverna med svepelektronmikroskopsamt elektron-dispersiv röntgenspektrometer. Vissa prover har även analyserats medhjälp av ljusoptiskt mikroskop, bildanalys samt värmeledningsförmåga. Resultaten visarklart att alla undersökta material kan uppvisa porösa strukturer, dock kan inte allaprocessparametrar möjliggöra uppkomsten av dessa. Generellt sett beror porositeten påden underliggande strukturen skapade under oxidationen. Det har ännu inte varit möjligtatt karaterisera några kristallstrukturer, kemisk analys tyder dock på närvaro av kromoxid,(Cr,Fe) spineller samt järnoxider. Resultaten konstaterar att en kort oxidationstid normaltbildar kromoxid medan längre oxidation medför uppkomsten av järnoxider. En ökadreduktionstid leder till större och färre porer jämfört med kortare reduktionstider. Kemiskaanalyser visar enhälligt att porösa strukturer innahåller till största del järn. En trolig orsaktill detta beteende kan kopplas till det fenomen där krom förångas under oxidation i fuktigaatmosfärer. Följaktikligen blir även ytan känslig för korrosion då majoriteten av krom harförsvunnit.Utvärdering av egenskaperna hos dessa ytor visar en tydlig förbättring i värmeledning vianaturlig konvektion. Värmeväxlingen mellan två uider uppvisar dock inte någon störreskillnad förutom under låga ödeshastigheter då en porös yta tenderar att transporterabort mer värme / This work aim to investigate a novel process by means of process parameters for creationof porous austenitic stainless steels surfaces as well as investigate if they permit anyimprovements compared to reference materials. Porous surfaces were created by a two-step method; samples were initially oxidised in a wet argon atmosphere and subsequentlyreduced in a pure hydrogen. The materials used in this investigation are all stainless steelswith specications according to steel grades 1.4301 (304), 1.4404 (316L), 1.4435 (316L)and F-138 (316LVM). The oxidation process is performed above 630 ◦ C for 150 - 1200 s in aow of wet argon or dry air and reduction is performed with a ow hydrogen for 150 - 2400 s.In order to understand the heat treatment processes, samples were characterised by usinga scanning electron microscope together with an electron dispersive x-ray spectrometer.However, light optical microscopy, image analysis and heat transfer measurement werealso used. Results show that all materials can obtain porous structures even though itis evident some process parameters cannot support successful creation of porosity. Ingeneral, the porosity depends on the structure created upon oxidation. It has not beenpossible to identify any crystal structures but compositional analysis as well as previousliterature suggest presence of chromia, (Cr,Fe) spinels and iron oxides. It is concluded thatshort time oxidation frequently creates chromia layers and prolonged oxidation inducesiron oxides due to breakaway oxidation. An increased reduction time results in larger andfewer pores. Compositional analyses show that all porous morphologies are created withina very iron rich layer. It is suggested that wet oxidation promote chromium evaporation,which consequently reduces the amount of chromium in the surface and enables iron oxideto rapidly form. Furthermore, results show that the porous layer is very susceptible tocorrosion as almost no chromium is present after the heat treatment. Evaluation of the properties of porous surfaces reveals a signicant improvement in heattransfer due to natural convection. Heat exchange between uids is though not enhancedgreatly by porous surfaces, a small improvement can be seen for low ow rates but for largerows no improvement is found.