Enemark, Søren

Rotor-leje-systemer er nøglekomponenter i meget industriel udstyr, så som turbiner, kompressorer og pumper. De er ofte underlagt høje krav med hensyn til energiomkostninger og stabilitet. Løsninger, som efterkommer kravene, findes ofte i: lav friktion mellem rotor og leje, høj dæmpning af laterale vibrationer og høj lastkapacitet. Gaslejer og passive magnetiske lejer er eksempler med meget lav friktion. Tilgengæld har begge eksempler også en ulempe i forhold til lav dæmpning og derfor tilbøjelighed til instabilitet. For at øge dæmpningen kan passiv adaptiv regulering ved brug af smarte materialer være relevant. I den forbindelse er formhukommelsesmetaller (FHM’er) interessante kandidater, fordi deres stivhed er yderst temperatursensitiv, og fordi de udviser mekanisk hysterese, som kan bruges til dæmpning. Denne afhandling fokuserer på tre aspekter omkring muligheden for at integrere FHM’er i rotor-leje-systemer. Det første omhandler modellering af metallernes konstitutive forhold med vægt på stabiliseret cyklisk mekanisk deformation underlagt temperaturstyring. To veletablerede fænomenologiske termomekaniske modeller er taget i brug, og modifikationer er lavet på to delelementer omkring faseoverganges udvikling. Modifikationerne gør det muligt at genskabe eksperimentelle observationer med højere nøjagtighed. Usikkerhedsanalyse af materialeparametre er et gennemgående tema i afhandlingen med henblik på at sikre fysisk validitet og identificerbarhed og for at gøre opmærksom på modelforudsigelsers uundgåelige usikkerhed. Det andet aspekt omhandler design og modellering af maskinelementer lavet af FHM’er. Forskellige brugsprincipper af FHM’erne gennemgås, og det er vist at forspændte pseudoelastiske elementer har de mest lovende egenskaber. Diverse elementgeometrier granskes, mens fokus er på spiralfjedre. Flere fjedermodeller præsenteres, som bruger forskellige niveauer af tilnærmelser til den mekaniske spændingstilstand. Modellerne sammenlignes med eksperimentelle resultater, hvor forskellige niveauer af temperatur, deformation og lasthastighed tages i betragtning. Der er generelt god overensstemmelse mellem modellerne og eksperimenterne. Det sidste aspekt handler om to dynamiske systemer, hvor FHM-fjedre er integrerede for at sikre passiv adaptiv regulerbarhed og øget dæmpning. Det primære system består af en stiv rotor understøttet af passive magnetiske lejer. En holistisk og multidisciplinær tilgang i systemets model, der forbinder de ikke-lineære FHM-fjedre, de passive magnetiske lejer, som er svagt ikke-lineære, og den dynamiske interaktion mellem rotor og lejehuse. Teoretiske og eksperimentelle resultater viser, at det er muligt at mindske rotorvibrationer markant ved at ændre resonansfrekvenserne gennem temperaturregulering. Samtidig er egensvingningsformer også styrbare, og store vibrationer bliver klart mindsket ved dæmpning gennem FHM’ernes hysterese.Rotor-bearing systems are key components in a variety of industrial equipment, as turbines, compressors, pumps etc. They are often subjected to strict requirements with respect to cost of energy and stability. Solutions that meet the requirements often involve: low friction between rotor and bearing, high damping of lateral vibrations and high load-carrying capacity. Gas bearings and passive magnetic bearings are examples with very low friction. However, both examples also have a drawback with respect to very low damping and therefore prone to instability. To increase damping it is relevant to use passive adaptive control through smart materials. Shape Memory Alloys (SMAs) are interesting candidates in that relation, because of their highly temperature sensitive stiffness and mechanical hysteresis, which can be used for damping purposes. The thesis focuses on three main aspects related to the feasibility of integrating SMAs into rotor-bearing systems. The first one involves modelling of the constitutive relations of the metals with emphasis on stabilized cyclic behaviour under controlled temperature conditions. Two well-established phenomenological thermo-mechanical models are employed, and modifications are made to two subparts dealing with the evolution of phase transformations. By using the modifications, it is possible to reproduce experimental observations with higher accuracy. Uncertainty analysis of material parameters is a general theme of the thesis in order to ensure physical validity and identifiability, and to call attention to the inherent uncertainties of model predictions. The second aspect concerns design and modelling of machine elements made from SMAs. Different actuation principles of SMAs are covered, and pseudoelastic elements in pre-tension are found to have the most promising properties. Different element geometries are investigated with focus on helical springs. Several spring models are presented, which use different levels of approximations to the mechanical stress state. The models are compared to experimental results covering different levels of temperature, deformation and loading rate. Generally, there is a good agreement between model predictions and experimental results. The last aspect involves dynamical systems integrated with SMAs to ensure passive adaptive control and damping enhancement. The main system consists of a rigid rotor supported by passive magnetic bearings. A holistic and multidisciplinary approach is used for modelling the system, linking the nonlinear SMA spring, the weakly nonlinear passive magnetic bearings, and the dynamic interaction between the rotor and bearing housings. Theoretical and experimental results show that it is possible to reduce rotor vibrations significantly by changing resonance frequencies through temperature control. At the same time, mode shapes are also controllable, and large vibrations are limited and reduced by hysteretic damping of the SMAs

Online Research Database In Technology

Integration of Shape Memory Alloys into Low-Damped Rotor-Bearing Systems:Modelling, Uncertainties and Experimental Validation

http://orbit.dtu.dk/ws/files/123902073/Integration_of_Shape_Memory_Alloys_into_Low_Damped_Rotor_Bearing_Systems.pdf

Integration of Shape Memory Alloys into Low-Damped Rotor-Bearing Systems:Modelling, Uncertainties and Experimental Validation

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