In the past, steam turbines were mostly used for base load operation. Nowadays, with the increased development of variable renewable technologies, these same steam turbines are withstanding higher cyclic operational regimes with more frequent start-ups and fast changing loads. As such, improving the operational flexibility of installed and future designed steam turbines is a key aspect to be considered by equipment manufacturers. Steam turbine start-up is a phase of particular interest since is considered to be the most intricate of transient operations. During this phase, the machine can potentially be subjected to excessive thermal stresses and axial rubbing due to differential thermal expansion. These two thermal phenomena either consume component lifetime or can lead to machine failure if not carefully controlled. As such, there is a balance to be considered between increasing turbine start-up speed while ensuring the safe operation and life preservation of these machines. In order to improve the transient operation of steam turbines, it becomes important to examine their thermal behavior during start-up operation. To do that, it is important to have tools able to predict the thermal response of the machine. In this thesis work the impact of different aspects and boundary conditions on the results of ST3M, a KTH in-house tool, were investigated with the aim of understanding how large was their impact on the way to capture the thermal behavior of the turbine in terms of metal temperature and differential expansion. A small industrial high pressure turbine was validated against measured data and implemented on a sensitivity study; this analysis showed that the geometrical approximation introduce errors in the results, that the use of empirical Nusselt correlations give similar results to the validated model and that the cavity assumptions have a large impact on the trend of the differential expansion. Lastly, a strategy to validate any other similar turbine to the one of the study case was proposed in order to give a guide to future works in how to validate a model and what are the most influent parameters to take care of.

Förr i tiden, ångturbiner har främst använts för baskraft operation. Numera med den ökade utvecklingen av varierande förnyelsbara är samma ångturbiner motstå högre cykliska operativa system med mer frekvent uppstarter och snabbt föränderliga laster. Som sådan, förbättra den operativa flexibiliteten hos installerade och framtida utformad ångturbiner är en viktig aspekt för att övervägas av utrustning. Ångturbin uppstart är en intressant fas eftersom anses vara den mest intrikata av transienter. Under denna fas kan maskinen potentiellt utsättas för omåttlig termiska spänningar och axiella gnugga på grund av differentiell termisk expansion. Dessa två termiska fenomen antingen konsumera komponent livstid eller kan leda till maskinhaveri om inte kontrolleras noggrant. Som sådan, det finns en balans som skall beaktas mellan ökande turbin uppstart hastighet samtidigt som säker drift och livslängd bevarande av dessa maskiner. För att förbättra den transienta operationer av ångturbiner, blir det viktigt att undersöka deras termiska beteende under uppstarter. För att göra detta, är det viktigt att ha verktyg som kan förutäga den termiska responsen hos maskinen. I denna avhandling fungerar effekterna av olika aspekter och randvillkor om resultaten av ST3M, en KTH internt verktyg, undersöktes med syfte att förstå hur stor blev deras inverkan på sättet att fånga den termiska beteendet hos turbinen i termer av metalltemperatur och differentiell expansion. En industriell högtrycksturbinen validerades mot uppmätta data och genomförs på en känslighetsanalys; denna analys visade att den geometriska approximation införa fel i resultaten, att användningen av empiriska Nusselt korrelationer ge liknande resultat som den validerade modellen och att håligheten antaganden har en stor inverkan på utvecklingen av expansionsskillnaden. Slutligen har en strategi för att validera någon annan liknande turbin till en av studien fallet föreslås för att ge en vägledning för framtida arbeten i hur att validera en modell och vilka är de mest inflytelserika parametrar att ta hand om. / In the past, steam turbines were mostly used for base load operation. Nowadays, with the increased development of variable renewable technologies, these same steam turbines are withstanding higher cyclic operational regimes with more frequent start-ups and fast changing loads. As such, improving the operational flexibility of installed and future designed steam turbines is a key aspect to be considered by equipment manufacturers. Steam turbine start-up is a phase of particular interest since is considered to be the most intricate of transient operations. During this phase, the machine can potentially be subjected to excessive thermal stresses and axial rubbing due to differential thermal expansion. These two thermal phenomena either consume component lifetime or can lead to machine failure if not carefully controlled. As such, there is a balance to be considered between increasing turbine start-up speed while ensuring the safe operation and life preservation of these machines. In order to improve the transient operation of steam turbines, it becomes important to examine their thermal behavior during start-up operation. To do that, it is important to have tools able to predict the thermal response of the machine. In this thesis work the impact of different aspects and boundary conditions on the results of ST3M, a KTH in-house tool, were investigated with the aim of understanding how large was their impact on the way to capture the thermal behavior of the turbine in terms of metal temperature and differential expansion. A small industrial high pressure turbine was validated against measured data and implemented on a sensitivity study; this analysis showed that the geometrical approximation introduce errors in the results, that the use of empirical Nusselt correlations give similar results to the validated model and that the cavity assumptions have a large impact on the trend of the differential expansion. Lastly, a strategy to validate any other similar turbine to the one of the study case was proposed in order to give a guide to future works in how to validate a model and what are the most influent parameters to take care of.