Ignition and Combustion Characteristics of Flammable Gas Mixtures over Hot Surfaces
Author
Pedersen, Lea Duedahl
Term
4. term
Education
Publication year
2016
Submitted on
2016-06-01
Pages
76
Abstract
Dette speciale undersøger antændelses- og forbrændingsegenskaber for brændbare gasblandinger, der strømmer over varme overflader, med fokus på risici i gasturbiner på offshore olie- og gasanlæg. Arbejdet kombinerer et omfattende litteraturstudie af mekanismerne bag varm-overflade-antændelse og de fysiske/kemiske forhold, der påvirker den, med en state-of-the-art gennemgang af modelleringsmetoder. Med udgangspunkt heri udvikles og implementeres en funktionsdygtig CFD-model i ANSYS CFX til at simulere antændelse og forbrænding over varme overflader. Simulationerne reproducerer overordnede eksperimentelle tendenser og viser, at hot surface ignition temperature (HSIT) stiger med øget hastighed og turbulens, falder med højere initial temperatur og først falder med stigende tryk for derefter at stige igen over et vist niveau. På baggrund af risikovurderingen diskuteres praktiske krav til afværgeforanstaltninger, herunder cranking og tilsætning af fortyndingsmiddel; sidstnævnte kan kobles til antændbarhedsgrænser og anvendes uafhængigt af den specifikke turbinekonfiguration. Der foreslås desuden en procedure til at inkludere effekten af fortyndingsmidler i CFD-modellen, med henblik på anvendelse i industrirelateret risikostyring.
This thesis investigates the ignition and combustion behavior of flammable gas mixtures flowing over hot surfaces, with a focus on risks in gas turbines at offshore oil and gas facilities. The work combines an extensive literature review of hot-surface ignition mechanisms and the physical/chemical factors that influence them with a state-of-the-art review of modeling approaches. Building on these insights, a functional CFD model is developed and implemented in ANSYS CFX to simulate ignition and combustion over hot surfaces. The simulations reproduce overall experimental trends and indicate that the hot surface ignition temperature (HSIT) increases with higher flow velocity and turbulence, decreases with higher initial temperature, and decreases with increasing pressure up to a point before rising again. Based on the assessed risk, practical requirements for mitigation are discussed, including cranking and adding diluent; the latter can be tied to flammability limits and applied independently of a specific turbine configuration. A procedure is also proposed for incorporating diluent effects into the CFD model to support industry-relevant risk management.
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