In order to model heat transfer from the slab to the cooling agent in the continuous casting process the thermophysical properties of the surface layer must be taken into account. For this purpose thermal conductivities and thermal diffusivities of wustite as well as of two carbon steels were measured. The oxide scale growth for six steels was measured as a function of time and at temperatures between 900 and 1200°C in air and in steam. Complex heterogeneous oxide scales were examined by optical and scanning electron microscopy, by SIMS, as well as by M6ssbauer spectroscopy. Segregation of alloying elements in the metal-oxide contact zone was found. Cracks due to thermal stresses result in spalling of the scale. Therefore, a straightforward use of the data for process modelling can principally not be recommended without restrictions. Bestimmung der Stoffeigenschaften von Oxidschichten auf Stahl unter den Bedingungen der Spritzwasserkiihlung. FOr die modellrnafslqe Beschreibung des WarmeObergangs zwischen Werkstoff und Umgebung beim StranggieBen mOssen die thermophysikalischen Eigenschaften der Obertlachenschicht bekannt sein. Zu diesem Zweck wurden Warmeleitfahigkeit und Temperaturleitzahl fOr WOstit und zwei Kohlenstottstahle gemessen. FOr insgesamt sechs Stahle wurde das Wachstum der Oxidschicht in Luft und in Dampf im Temperaturbereich von 900 bis 1 200°C untersucht. Die komplexen Schichten wurden mittels optischer Mikroskopie, Rasterelektronenmikroskopie, Sekundarionenmassenspektrometrie und M6ssbauer-Spektrometrie analysiert. Es zeigten sich Segregationen der Legierungselemente in der Kontaktzone Oxid-Stahl. Thermische Spannungen fOhren zu einem Abplatzen der Oxidschicht. Eine Verallgemeinerung der an den untersuchten Stahlen erhaltenen Ergebnisse ist daher nur bedingt zulassiq.
For the cooling of steels in the continuous casting process it is necessary to know the heat transfer from the solidifying strand to the cooling water to enable calculation of the secondary cooling zone. Previous investigations have only determined this variable for non-oxidizing metallic surfaces. For many steels cast in practice, however, the formation of oxide layers prevents a direct transfer of the previous results. In the present research the influence of the oxide layers on the heat transfer has been investigated for spay-water cooling. Results have shown that heat transfer in the range of stable film boiling is determined for a constant spray-water temperature in the same way as for non-oxidizing metals, i.e. using the water mass flux density fils only. The changed surface qualities resulting from the oxide formation cause the Leidenfrost temperature, however, to shift considerably to higher values.
The heat transfer for film quenching was investigated theoretically and experimentally. With the aid of a mathematical model analytical equations were deduced, with which the locally transferred heat‐flux density and the local thickness of the vapour film can be calculated during stable film boiling. The equations were confirmed by experimental results. Accordingly, the heat transfer can be technically influenced only by the velocity of the water leaving the nozzle. With increasing jet velocity of the water not only the transferred heat flux during stable film boiling becomes higher but also the thickness of the vapour film becomes smaller, so that the film collapses earlier, causing the Leidenfrost temperature to shift to higher values.
This study investigates the influence of the fuel injection strategy on safety against flashback in a gas turbine model combustor with premixing of H2-air-mixtures. The flashback propensity is quantified and the flashback mechanism is identified experimentally.
The A2EV swirler concept exhibits a hollow, thick walled conical structure with four tangential slots. Four fuel injector geometries were tested. One of them injects the fuel orthogonal to the air flow in the slots (jet-in-crossflow-injector, JICI). Three injector types introduce the fuel almost isokinetic to the air flow at the trailing edge of the swirler slots (trailing edge injector, TEI). Velocity and mixing fields in mixing zone and combustion chamber in isothermal water flow were measured with High-speed-Particle-Image-Velocimetry (PIV) and Highspeed-Laser-Induced-Fluorescence (LIF). The flashback limit was determined under atmospheric pressure for three air mass flows and 673 K preheat temperature for H2-air-mixtures. Flashback mechanism and trajectory of the flame tip during flashback were identified with two stereoscopically oriented intensified high-speed cameras observing the OH* radiation.
We notice flashback in the core flow due to Combustion Induced Vortex Breakdown (CIVB) and Turbulent upstream Flame Propagation (TFP) near the wall dependent on the injector type. The Flashback Resistance (FBR) defined as the ratio between a characteristic flow speed and a characteristic flame speed measures the direction of propagation of a turbulent flame in the flow field. Although CIVB cannot be predicted solely based on the FBR, its distribution gives evidence for CIVB-prone states.
The fuel should be injected preferably isokinetic to the air flow along the entire trailing edge in oder to reduce the RMS fluctuation of velocity and fuel concentration. The characteristic velocity in the entire cross section of the combustion chamber inlet should be at least twice the characteristic flame speed. The position of the stagnation point should be tuned to be located in the combustion chamber by adjusting the axial momentum. Those measures lead to safe operation with highly reactive fuels at high equivalence ratios.
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