High concentrations of sediments is a serious problem for hydropower stations in the Himalayas and the Andes Mountains. For run-of-river power plants sediment causes heavy erosion even with settling basins. This leads to reduced operating hours and high maintenance cost. In addition, the original design experienced problem with heavy cavitation.The objective of this master thesis is to carry out new hydraulic design of the runner and guide vanes of the existing Francis turbines in La Higuera Power Plant with reduced velocity components. To achieve this the cause of the heavy cavitation, which made the turbine fail, has to be established.Results from numerical simulations indicates a low pressure zone causing heavy leading edge cavitation is the reason for the turbine failure. The off-design operation has made the cavitation even worse.To carry out a new design, the in-house design software Khoj was used. Some new parameters, like blade leaning, were included in the program. Blade leaning is an important tool for pressure balancing the runner blade. Further, a parameter study was carried out to investigate the effect of blade leaning, blade angle distribution and blade length. The numerical simulation indicates proper pressure balancing could have avoided the cavitation problems and a new design should have an X-blade shape. Because the power plant is already built, the number of variables is limited. The rotational speed, inlet and outlet diameter remained constant. This made it impossible to significantly reduce the relative velocities. Therefore, coating of all wet surfaces is proposed to reduce the effect of erosion.The main objective for this thesis has been to identify the cause of the turbine failure and develop a new design to fit in the existing power plant. Complete 3D-drawings of the design, including runner and guide vanes, has not been made due to lack of time.

Målet for prosjektet var å finne virkningsgradkurve for en turbin ved hjelp av Winter-Kennedy målinger og CFD-analyser. Det ble derfor laget en CAD-modell av gjennomstrømningsvolumet i spiraltromma på NTNUs vannkraftlaboratorium basert på en eksakt modell av spiraltromma. På denne modellen ble det så gjort CFD-analyser med et visst antall volumstrømmer for å finne kalibreringskonstantene til Winter-Kennedy målingene.I laboratoriet ble så Francis-turbinen kjørt i et passende område rundt det optimale driftspunkt, mens det ble utført absolutte volumstrømsmålinger og Winter-Kennedy-målinger på turbinen. Trykkmålingene ble utført ved både automatisk logging med differensialtrykktransducer og optisk avlesning av vannsøylemanometerVed hjelp av kalibreringen fra CFD-resultatene ble det utarbeidet virkningsgradskurver og Hill-diagram for både de relative og de absolutte målingene.Differensialtrykk simulert med CFD stemte godt overens med, og var i gjennomsnitt 0,3 % høyere enn, målingene fra vannsøylemanometeret. Målingene med transduceren ser ut til å være heftet med systematiske feil, og ligger gjennomsnittlig 40 Pa over vannsøylemanometermålingene. Det lot seg gjøre å opprette virkningsgradsdiagram med relative målinger. Formen på Hill-diagrammet basert på Winter-Kennedy-målingene med vannsøylemanometer stemte godt overens med de absolutte, mens målingene med trykktransducer ble forstyrret av systematiske måleavvik. Beste virkningsgrad målt med absolutte målinger var omtrent 0,2 % lavere enn hva som tidligere er blitt målt på samme turbin. Virkningsgraden fra relative målinger kalibrert med CFD var ca. 0,3 % lavere enn de absolutte målingene. Kalibrering med CFD ga totalt sett kun en liten økning (0,16 %) i forventet avvik av volumstrømmåling med Winter-Kennedy-metoden.

Boiling two-phase flow is found in many industrial applications such as boiling water reactors, two-phase flow heat exchangers and refrigeration systems. The physics of two-phase gas-liquid flow may lead to undesirable system instabilities, and in the literature density wave oscillations (DWO) is reported to be the most commonly observed instability phenomenon. However, the literature alsoprovides two opposing views on what the fundamental mechanism of DWO is. The so-called classical description of DWO focuses on the variation in mixture density as the governing mechanism, and the oscillation period will consequently be about one to two times the channel residence time. The findings presented in citet{Rizwan-Uddin1994} show that it is the variation in mixture velocity that hasthe dominating effect, and the oscillation period was reported to be closer to four times the channel residence time. citet{Ambrosini2000} united the two opposing views by stating that the governing mechanismdepends on the level of system subcooling. The classical description of DWO is based on a lower level of subcooling, while citet{Rizwan-Uddin1994} considered higher subcooling. Here, the fundamental mechanisms of DWO and the effect of system subcooling is investigated further by performing a numerical analysis using a one dimensional homogenous equilibrium flow model. The modeled system consists of a horizontal uniformlyheated boiling channel with an inlet- and exit restriction. The system is exposed to constant externally imposed pressure drop. The effect of system subcooling is investigated by comparing the self-sustained periodicoscillations which make out the modeled stability threshold. The flow model is validated by observing the above mentioned effects of subcooling on DWO. Further, it is found that the change from a density dominated exit restriction towards a velocity dominated exit restriction is a smooth transition for increased subcooling. The amplitude of the variations in exit mixture velocity increases continuously with subcooling, anddue to the squared relationship between the exit restriction pressure drop and the exit mixture velocity, velocity becomes the governing mechanism at high subcooling.The modeled stability threshold approaches a straight line at high subcooling. This line represents operating conditions which have the same mean boiling boundary location. However, the amplitude of the variations about this mean limit grows exponentially at high subcooling. The oscillation period of the observed DWO grows continuously with higher subcooling, and the period increases exponentially at high subcooling. In contrast, the mean boiling channel residence time approaches an upper mean limit at high subcooling. It is postulated that it is the transition towards a more mixture velocity dominated system that causes the oscillation period to evolve as it does with respect to the level of subcooling.

En ny generasjon vedovner tilpasset dagens nye boliger er under utvikling, og i den sammenheng skal vedovner med varmelagring vurderes. Varmelagring bidrar blant annet til en mer stabil varmeavgivelse fra vedovnen ved at varmelageret absorberer effekttoppene. Hensikten med denne oppgaven er å beskrive en første gjennomgang av et konsept for et latent varmelager for vedovner. Latent varmelagring vil si at varmen lagres i et materiale som gjennomgår faseovergang. Fordelen med latent varmelagring, i forhold til tradisjonelle lagringsmetoder med kleberstein og keramikk som lagringsmedium, er at vekt og volum reduseres for samme mengde lagret varme, og at varmen tas opp og avgis ved tilnærmet konstant temperatur. Latent varmelagring for vedovner, er så vidt forfatteren kjenner til, ikke tidligere rapportert om i litteraturen. I dette studiet er ulike faseovergangsmaterialer (Phase Change Material, PCM) for varmelagring undersøkt, og blant dem har salthydratet natriumacetat trihydrat og sukkeralkoholet erythritol vist seg å ha egenskaper som gjør dem egnet for formålet. To utfordringer knyttet til bruk av PCM for vedovner er lav termisk konduktivitet og risiko for overoppheting med påfølgende degradering av materialegenskaper. Metoder for å unngå overoppheting av faseovergangsmaterialene er vurdert i rapporten, og et konsept for et varmelager foreslått. Lageret har konsentrisk geometri med innvendige metallfinner og oppvarmingen skjer hovedsakelig ved stråling. Et luftsjikt mellom lageret og ovnsoverflater gir mulighet for konveksjonskjøling, og luftstrømmen kan stenges og åpnes med spjeld. Det er gjennomført numeriske beregninger av modellen med tre ulike finneløsninger, ved bruk av simuleringsprogrammet COMSOL Multiphysics®, for henholdsvis oppvarmings- og nedkjølingssykluser. Sukkeralkoholet erythitol ble brukt som PCM i simuleringene, og faseovergangen er beregnet ved bruk av ekvivalent varmekapasitetsmetode. Resultatet viser at løsningen med fri luftgjennomstrømning og seks sirkulære finner kan unngå overoppheting i to timer og førtito minutter ved konstant fyring. De numeriske resultatene må imidlertid benyttes med forbehold, på grunn av forenklingene som er forklart i rapporten. Når mer informasjon om samspillet mellom ovnstemperaturer og varmelager etterhvert blir kjent, kan modellen videreutvikles og optimaliseres, og konseptet kan testes eksperimentelt i laboratorium.

The objectives of this master thesis have been to model and simulate oxy-combustion CO2 capture in a cement plant. The model developed is a process simulation of the calcination process with varying degree of air in-leakage, where heat is supplied by combustion in an oxygen rich environment, followed by capture of the CO2. The further gas separation after H2O condensation to achieve the required CO2 quality was evaluated. In addition to the process simulations, a review of literature related to oxy-combustion CO2 capture and cement production was performed, and an engineering evaluation of the necessary modifications to the cement plant conducted.A simulation model was built in Aspen HYSYS, and student Jelmer de Winter’s project work was utilized as a starting point. The model was developed with the aim to achieve results comparable to a process model constructed by the European Cement Research Academy (ECRA) in 2009. The goal was to capture as much of the CO2 as possible, and to achieve a CO2 purity of minimum 95 mol-% after the CO2 Compression and Purification Unit (CPU).CO2 purity in the dry flue gas of ~85 mol % was achieved, with a CO2 capture rate up to 96.4 %. Five different air in-leakages (2, 4, 6, 8 and 10 % of total flue gas flow) were tested. The results showed that the CO2 concentration in the flue gas decreased with increasing degree of air in-leakage. The decrease in CO2 concentration causes an increase of the power consumption of the CO2 CPU of ~2.6 % per percentage point of air in-leakage, and the CO2 capture rate was also reduced when the air in-leakage increased. These results agree well with results from previous oxy-combustion studies, and show the importance of minimizing air in-leakages in the cement plant.If oxy-combustion capture is to be utilized at a cement plant, some process modifications and additional equipment is required. An Air Separation Unit (ASU) is needed to provide almost pure oxygen for the combustion process. A Compression and Purification Unit (CPU) is also required, in order achieve the necessary CO2 purity and transport conditions. When using oxy-combustion technology, both the material conversion in the cement kiln system and the operational specifications of the overall process are different from those in conventional kiln operation. However, research made by ECRA in 2012 showed that the negative impacts of oxy-combustion on the product quality seem to be negligible.Other necessary process modifications when retrofitting with oxy-combustion are news design of the kiln burner and the clinker cooler in the cement plant. In addition, prevention of excessive air in-leakage by improving sealing locations at the cement plant is necessary, as the simulation results show. This is possible e.g. by waste gas flushed systems, or by an improved maintenance of inspection doors and similar devices. The CPU is up to a certain point capable of handling changes in the flue gas composition at short-term inspections; however it limits its efficiency.