Mechanical properties of aluminium alloys highly depend on their phase composition and microstructure. High strength can be achieved among others by introduction of a high volume fraction of fine, homogeneously distributed second phase particles and by a refinement of the grain size. Powder metallurgy allows to prepare fine grained materials with increased solid solubility which are favourable precursors for further precipitation strengthening. Gas atomization was used for the preparation of powders of the commercial Al7075 alloy and its modification containing 1 wt% Zr. A part of gas atomized powders was mechanically milled at different conditions. Mechanical milling reduced the grain size down to the nano-size range and the corresponding microhardness exceeded the value of 300 HV. Powders were consolidated by the spark plasma sintering method to nearly fully dense compacts. Due to a short time and relatively low temperature of sintering the favourable microstructure can be preserved in the bulk material. The grain size of compacts prepared from milled powder was retained in the submicrocrystalline range and the microhardness close to 200 HV exceeded that of the specially heat treated ingot metallurgical counterparts. The prepared compacts retained their fine grained structure and high...

Für technische Zerstäubungsprozesse wird häufig eine Flüssigkeitsmenge durch die kinetische Energie eines Hochgeschwindigkeitsgases in einem Luftstromzerstäuber in Einzeltropfen dispergiert. In einem Prefilming-Luftstromzerstäuber befindet sich die zu zerstäubende Flüssigkeit zuerst auf einer Oberfläche (Prefilming-Oberfläche) um einen dünnen Flüssigkeitsfilm zu bilden, bevor sie einem Hochgeschwindigkeitsluftstrom ausgesetzt wird. Das erste Ziel dieser Untersuchungen ist, den Zerstäubungsmechanismus der Prefilming-Zerstäuber zu verstehen und den Effekt variierender Parameter des Sprühsystems beim Zerfallsmechanismus zu ermitteln. Zerfallsregime in der Nähe des Zerstäuberauslasses wurden mittels Schattenverfahren und begleitend durch Partikelverfolgung bestimmt. Im nächsten Schritt wird die Sprühleistung des Prefilming-Luftstromzerstäubers in einer Reihe von Testbedingungen charakterisiert. Die Sprühcharakterisierung wurde mittels Phasen-Doppler-Anemometrie (PDA) durchgeführt um den Einfluss verschiedener Parameter auf die lokale Tropfengröße und Geschwindigkeit im Spray zu untersuchen. Zuletzt werden Zukunftsansätze zu Entwicklung und Design eines Prefilming-Luftstromzerstäubers aufgezeigt. Um einen einzigartigen funktionellen Zusammenhang der experimentellen Daten zu entwickeln, wurde eine Dimensionsanalyse durchgeführt. Darauffolgend zeigt der Einfluss von zwei dimensionslosen Kennzahlen unterschiedliche Sensitivitäten in Abhängigkeit vom Druckbereich und es wurde durch Anpassen der Daten eine geeigneten Korrelationsfunktion hergeteiltet. / A bulk of liquid dispersed into single droplets using the kinetic energy of a high-velocity gas in an air-blast atomizer is frequently employed in technical atomization processes. In a prefilming air-blast atomizer, the atomizing liquid is primary situated on a surface (prefilming surface) to form a thin liquid film before exposing to a high-velocity air flow. The first purpose of this study is to understand atomization mechanisms close to prefilming atomizers and to determine the effect of spray system parameter variations on breakup mechanisms. Breakup regimes in the vicinity of the atomizer exit were determined using the shadowgraphy technique associated with particle tracking. In a next step, the spray performance of prefilming air-blast atomizers are characterized in a wide range of test conditions. For the spray characterization, a phase Doppler anemometry (PDA) was utilized to investigate the influence of variable parameters on the local droplet size and velocity in a spray. Finally, prediction approaches are determined for the development and design of a prefilming air-blast atomizer. In order to develop a unique functional relationship from experimental data, a dimensional analysis has been performed. Subsequently, the influence of two main nondimensional numbers shows different sensitivities depending on the pressure range and was quantified by fitting the data to appropriate correlation functions.

The magnetic Fe–6.5 wt% Si powder was produced by gas atomization and its microstructure was also investigated. The secondary dendritic arm spacing (SDAS) is related to the droplet size, λ = 0.29 · D⁰·⁵, and the numerical solidification model was applied to the system, giving rise to the correlation of microstructure to the solidification process of the droplet. It is found that the solid fraction at the end of recalescence is strongly dependent on the undercooling achieved before nucleation; the chances for the smaller droplets to form the grain-refined microstructures are less than the larger ones. Furthermore, the SDAS is strongly influenced by the cooling rate of post-recalescence solidification, and the relationship can be expressed as follows, λ = 74.2 · (T)⁻⁰·³⁴⁷. Then, the growth of the SDAS is driven by the solute diffusion of the interdendritic liquids, leading to a coarsening phenomenon, shown in a cubic root law of local solidification time, λ = 10.73 · (tf)⁰·²⁹⁶.

Tato práce se zaměřuje na oblast effervescentních sprejů a jejich aplikace na kapalné spalování s důrazem na průmyslové spalovací komory. Oba aspekty – modelování a experiment – jsou řešeny. Práce obsahuje obecný úvod, ve kterém jsou vysvětleny základní jevy rozpadu kapaliny a vířivého spalování a dále je představena effervescentní atomizace. Poté jsou popsány použité experimentální postupy jak pro měření spreje, tak pro měření tepelných toků do stěn při spalování. V následující kapitole jsou popsány numerické modely a jejich podstata je vysvětlena. Jsou zde uvedeny modely pro rozpad spreje, turbulenci a spalování použité během výzkumu. Vlastní výsledky práce jsou uvedeny formou samostatných článků (vydaných nebo přijatých) s dodatečnou částí věnovanou nepublikovaným relevantním výsledkům. Bylo zjištěno, že standardní modely sprejů jsou do jisté míry schopny popsat effervescentní spreje. Nicméně aby bylo možné predikovat plamen kapalného spreje, jsou zapotřebí detailnější modely sprejů, které dokáží přesně zachytit změnu průměrů kapek v radiálním a axiálním směru. Experimentální měření effervescentních sprejů bylo provedeno pomocí navrhnuté metodiky. Výsledky měření byly analyzovány s důrazem na radiální a axiální vývoj průměrů kapek a některé nové jevy byly popsány. Nepřímá úměrnost mezi gas-liquid-ratio a středním průměrem kapek byla potvrzena. Dále by popsán jev, kdy pro různé axiální vzdálenosti které dojde k úplnému převrácení závislosti středního průměru na axiální vzdálenosti. V závěru je uvedeno shrnutí, které rekapituluje hlavní výsledků a závěry. V závěrečných poznámkách je nastíněn možný budoucí postup. Experimentální data pro ověřování budoucích effervescentních modelů jsou poskytnuta.

No description available.

Pressure swirl atomizers are commonly used in IC, aero-engines, and liquid propellant rocket combustion. Understanding the atomization process is important in order to enhance vaporization, mitigate soot formation, design of combustion chambers, and improve overall combustion efficiency. This work utilizes non-invasive techniques such as ultra -speed imaging, and Phase Doppler Particle Anemometry (PDPA) in order to investigate the cascade atomization process of pressure-swirl atomizers by examining swirling liquid film dynamics and the localized droplet characteristics of the resulting hollow cone spray. Specifically, experiments were conducted to examine these effects for three different nozzles with orifice diameters .3mm, .5mm, and .97mm. The ultra-speed imaging allowed for both visualization and interface tracking of the swirling conical film which emanated from each nozzle. Moreover, this allowed for the measurement of the radial fluctuations, film length, cone angle and maximum wavelength. Radial fluctuations are found to be maximum near the breakup or rupture of a swirling film. Film length decreases as Reynolds number increases. Cone angle increases until a critical Reynolds number is reached, beyond which it remains constant. A new approach to analyze the temporally unstable waves was developed and compared with the measured maximum wavelengths. The new approach incorporates the attenuation of a film thickness, as the radius of a conical film expands, with the classical dispersion relationship for an inviscid moving liquid film. This approach produces a new long wave solution which accurately matches the measured maximum wavelength swirling conical films generated from nozzles with the smallest orifice diameter. For the nozzle with the largest orifice diameter, the new long wave solution provides the upper bound limit, while the long wave solution for a constant film thickness provides the lower bound limit. These results indicate that temporal instability is the dominating mechanism which generates long Kelvin Helmholtz waves on the surface of a swirling liquid film. The PDPA was used to measure droplet size and velocity in both the near field and far field of the spray. For a constant Reynolds number, an increase in orifice diameter is shown to increase the overall diameter distribution of the spray. In addition, it was found that the probability of breakup, near the axis, decreases for the largest orifice diameter. This is in agreement with the cascading nature of atomization.

Gas atomization is regarded as a reliable method for creating high-quality metal powders from molten metal. The liquid metal is fed into the chamber as a free-falling stream through a nozzle, where it is impinged by high-velocity gas jets, causing degeneration and production of metal droplets, which solidify to create metal powders. As the metal droplets fall lower towards the collection hoopers, the solidification process begins. As a result, having a strong handle on the process parameters helps to produce metal powders that are fine, spheroidized, and have good characteristics. A free fall atomizer with twelve discrete nozzles, having a cylindrical internal profile, arranged in two different levels has been employed to introduce high-velocity gas jets into the chamber, for the current study. A cross-sectional sketch created by Uddeholm AB provided the geometric dimensions, and CFD was used to generate a simulation experiment for the system. Fluent setup input values were derived from literature data. The primary objective of the study is to analyze the influence of varying inlet pressure and the number of discrete gas jet nozzles, on the flow behavior of the atomizing gas. Additionally, the Discrete Phase Model approach was adopted to study the interaction of particles with the gas flow. The simulation model was validated by carrying out the visualization experiment, Schlieren imaging. From the study, it was realized that the results of the numerical model showed a mismatch relative to the experimental value. This can be attributed to the discretization technique, input parameters and the numerical model employed in this study. However, the parametric study provided a qualitative analysis regarding the influence of input parameters on flow behavior. It was studied that with increasing the inlet pressure and number of discrete nozzles there is a subsequent increase in the maximum velocity attained by the atomizing gas, resulting in a decrease in velocity of melt introduced into the system. Additionally, a radial pressure gradient was observed to be present that increased in accordance with the parameters, resulting in reduction of the melt film thickness produced during pre-filming mechanism. However, the Discrete Phase Model provided evidence that with increment in the gas to melt ratio, the number of particles that get dispersed to make collision with the domain wall increased. Thus, a subsequent increase in downstream velocity was required to maintain the particles within the domain walls as the study parameters were increased. / Gasatomisering betraktas vara en tillförlitlig metod för att skapa högkvalitativt metallpulver från smält metall. Den flytande metallen matas in i en kammare som en fri fallande ström genom ett munstycke, där den trycks in av höghastighetsstrålar. Vilket skapar en degeneration och en bildandet av metalldroppar som stelnar till att metallpulver kan skapas. Stelningsprocessen börjar när metalldropparna faller emot uppsamlingsbågarna. Ett fint och sfärisk metall metallpulver med goda egenskaper kan produceras genom att ha en god kontroll på processparametrarna. I denna studie har en ”Free Fall Atomizer” med tolv diskreta munstycken med en cylindrisk profil arrangerade i två olika nivåer använts för att introducera höghastighetsstrålar i kammaren. De geometriska dimensionerna var försedda från en ritning i tvärsnitt skapad av Uddeholms AB och samt användes en CFD för att generera ett simulationsexperiment av systemet. Ingångsvärden för inställningarna av flödena härleddes från litteraturdata. Huvudsyftet med studien var att studera flödesbeteendet av den atomiserande gasen genom att analysera inflytandet av att variera ingångstrycket och antalet diskreta gasstrålmunstycken. Dessutom togs det till en diskret fasmodell för att studera partiklarnas interaktion med gasflödet. Simulationsmodellen validerades genom att utföra ett visualiseringsexperiment genom Schlierenfotografering. Det framgick i studien att den numeriska modellens resultat inte stämde överens med det experimentella värdet. Detta kan attribueras till diskretiseringstekniken, inmatningsparametrarna och den numeriska modellen som användes i studien. Hur som helst försedde den parametriska studien en kvalitativ analys angående inflytandet av inmatningsparametrarna på flödesbeteendet. Det framgick att en ökning av ingångstrycket och av antalet av diskreta munstycken gav en påföljande ökning i den maximala hastigheten som den atomiserande gasen kan erhålla. Vilket resulterar till en sänkning av hastigheten av smältan som introduceras till systemet. Dessutom observerades en radiell tryckgradient vara närvarande som ökade i enlighet med parametrarna. Det resulterade i en reduktion av smältfilmtjockleken som producerades under förfilmingsmekaniskmen. Trots det visade den diskreta fasmodellen att en ökning av gas till smältförhållandet också ökade antalet partiklar som sprids vidare för att kollidera med domänväggen. Således krävdes en påföljande ökning av nedströmningshastigheten för att bibehålla partiklarna inom domänväggar när studiens parametrar ökade.

No description available.

Design of atomizers for gas-turbine purposes are an important ordeal. The per-formance of the atomizer directly impacts the efficiency of the gas-turbine, andconsequently, the energy extracted from the turbine. Furthermore, the design ofthe atomizer can have an impact on reducing toxic emissions. On a global scale,gas-turbines can be considered crucial for the transition to renewable energy. Forengineers, designing of atomizers are however challenging. Turbulent flow, multi-phase interaction and chemical reactions are some of the complex physics involvedwhich has to be taken into consideration when designing the atomizer. Engineerstraditionally uses experimental testing for investigation of designs, and it is still verymuch a useful methodology. However, numerical simulations and CFD have recentlygained popularity due to being a more cost-effective methodology. In this work, theprocedure for designing a prefilm atomizer involving CFD (single phase model andmulti phase model) and experimental testing is documented. The details of the twonumerical models (a single phase model and a multi phase model) has been doc-umented as well as the experimental setup. The single phase model is used for aparametric study and experimental testing is used for evaluation of designs. Themulti phase model is aimed to replicate the experimental results. The validity ofthe numerical models and the experimental setup are discussed, and possible mod-ifications of the methodology for future studies are suggested. Finally, suggestionsfor how the prefilmer should be designed is suggested.

<p dir="ltr">This thesis presents a comprehensive analysis of liquid injection through plain-orifice injectors into high-speed gaseous crossflows. Experimental data is collected for more than 1,000 injection events into a blowdown wind tunnel with Mach numbers ranging from 0.3 to 2.5, and sophisticated methodologies are developed and employed to quantify spray penetration and jet breakup behaviors. Despite the simplicity of a plain-orifice injector design, the flow field induced by the transverse streams is complex and three-dimensional, and the rapid jet breakup and high advection speeds of the resulting droplet cloud make for a difficult diagnostic environment. This results in a present need for accurate tools to predict the performance of plain-orifice injectors in high-speed crossflows and for specific details of jet breakup behaviors and of the resulting droplet distributions. The experiments conducted for this work constitute a substantial database of high-speed images and flow diagnostics, and the analyses conducted thereof provide critical new understandings of this class of flows. Transmittance images have been used extensively to characterize spray penetration profiles, but new analyses presented here use transmittance to quantify time-averaged droplet distributions and their variations with various flow properties. A novel combination of these with cross-sectional Mie-scatter images also enables the generation of three-dimensional spray profiles. A previously unidentified jet-in-crossflow breakup mode is found and distinguished from the catastrophic breakup mode by its instantaneous spray structures; additionally, both regimes are mapped with respect to momentum flux ratio and Weber number by analyzing peak frequencies in modal decompositions. Finally, a spray penetration correlation is developed that spans both subsonic and supersonic crossflows by applying a novel shock correction. Each of these contributions represents a significant advancement in the scientific understanding of liquid jets in high-speed crossflows and a valuable resource for engine design and model validation.</p>