1,721,008 research outputs found
Modelling of spray formation in a pressure swirl atomiser for aircraft engines
No full textThe internal flow and the liquid structure exiting the nozzle are investigated in a typical pressure swirl atomizer for aero-engine applications under isothermal non reacting environment, using a two-phase flow modelling according to the VOF numerical methodology. A parametrical analysis is performed to investigate the effect of model numerical parameters on the characteristics of the liquid jet exiting the nozzle and its interaction with the surrounding air. The outputs of the simulations have been used as input conditions for a primary atomization model, which is based on theoretical and empirical models available in the literature and here adapted for swirling flows. The characteristics of the newly formed liquid drops are predicted under the operating conditions suitable for an Ultra Low NOx injection system for aircraft engines
Processing of functional capsule powder particles based on multiple emulsions using a prilling process
No full textPresent studies show the ability of cold spray processing (prilling) to tailor the morphology of simple or double emulsion-based fluid systems as investigated for two types of air-assisted nozzle geometries under various processing conditions. The spray process parameters varied were: (i) gas to liquid flow rate ratio (GLR), (ii) spraying pressure and (iii) total mass flow rate. The results depicted that the emulsion flow inside the nozzle (liquid cap) as well as in the spray (outside nozzle) have a distinct impact on the resulting product structure due to the respective flow stresses acting. Increasing the flow stresses either lead to an additional dispersing impact or to separation and coalescence of the disperse fluid phase(s). Besides the process parameters, the material characteristics of the emulsion systems such as viscosity ratio λ of dispersed to continuous phase and the interfacial tension γ were varied in a wider range. The results demonstrated a systematic increase in structure stability for higher λ values within a range of 0.32–30. As representative dimensionless numbers, (a) a critical liquid Weber number Wel,Drop,cr/λ and (b) a critical gas Weber number Weg,Drop,cr/λ were defined to describe the effect of liquid cap-tip and air-assisted spraying, respectively, with respect to preserving the disperse microstructure of the treated emulsions. Above these critical We numbers, the dispersed emulsion phase drops were broken up and drop mean sizes were exponentially decreased due to the flow stresses acting either in the liquid flow inside the nozzle or in the spray filament outside the nozzle. Dynamic viscosity η and dynamic moduli (G′, G″) of treated emulsions increased with decreasing droplet size of the dispersed phase(s) thus altering the spraying performance as well as the properties of the liquid product systems reconstituted from resulting spray-chilled powders. A third critical Weber number Weg,Nozzle,cr was derived for the spray droplet (tertiary droplet) generation by the spray filament breakup providing information of the smallest spray droplet that could be attained, while keeping the dispersed emulsion (secondary) droplets unchanged in size. The impact of Weg,Nozzle on the resulting spray (tertiary) mean drop size was systematically explored for internal (INMIX) and external (EXMIX) liquid-gas mixing air-assisted nozzles. High-speed videography and laser shadowgraphy were applied to visualize liquid spray filament stretching and breakup, as well as the velocity distribution in the sprays. Sufficiently gentle spray conditions for complete preservation of the disperse emulsion structure were only achieved in the Rayleigh filament breakup regime.
Accordingly, a pressure controlled rotary “Rayleigh atomizer” was developed to study emulsion spraying by filament stretching and gently spray drop formation, preserving the emulsion (secondary) droplet structure. At the same time pressure adjustment enabled higher throughput rates compared to conventional rotary spraying nozzles for which only centrifugal forces determine filament stretch and throughput rate simultaneously. Filament length and drop size decreased with increasing rotational speed at a given total pressure (centrifugal pressure + static liquid pressure at the nozzle inlet) or flow rate, and the filament length and drop size increased with higher liquid pressures and related throughput rates at a given rotational speed. Chilling solidification of the spray drops was superimposed in selected cases. Prilling (spraying + chilling) was carried out for various emulsion systems in a prilling tower applying average air temperatures of ca. −10 °C for higher melting fat-continuous emulsions down to −50 °C for low melting oil- or water-continuous emulsions, in order to produce solid powder particles. The microstructure of the solid particles was analyzed in further detail by cryo-scanning electron microscopy (Cryo-SEM). Concerning emulsion structure preservation in the sprayed products, the results clearly demonstrated that the disperse structure can differ significantly from the initial emulsion structure if critical flow stress conditions are exceeded. Respective process-structure functions were also quantified.
For emulsion-based prilled powders, the applicability and adjustability as functional component carriers for controlled release applications is of big interest in industries such as food, pharmaceutical, and cosmetics. For related functional component release experiments we designed an in vitro gastric/duodenal setup. With this, the release kinetics of functional components encapsulated/embedded in dispersed secondary or primary emulsion drop phase(s) were quantified under simulated gastric or duodenal digestion conditions. Accordingly, an iron compound (micronutrient) was encapsulated into the primary and/or secondary dispersed emulsion droplets of simple or double emulsions, and related solid emulsion powder particles were produced through prilling applying our selected air-assist atomizers. In a first testing step, the iron release kinetics for selected products were systematically investigated in the in-vitro gastric system at pH ≈ 2.0, and quantified as a function of prill powder particle size, secondary emulsion drop size and prill powder storage time under ambient conditions
Einfluss des Carbonitrierprozesses auf die Phasenumwandlung beim Einsatzhärten, Restaustenit und Restspannungen
No full textThe carbonitriding process is a surface hardening technique with an ultimate goal of improving surface hardness, fatigue properties and resistance to wear of highly stressed parts. As opposed to carburizing process which enriches engineering components with carbon atoms only, carbonitriding introduces both carbon and nitrogen atoms in the surface layer. The presence of nitrogen stabilizes austenite and depending on the level of carbon and nitrogen content reached, as high as 70 mass-% of austenite can be retained. The thermal and mechanical stability of such high amount of retained austenite is vital as retained austenite should remain stable to avoid shape and dimensional changes especially in close fittings. Moreover, such high amount of retained austenite affects the nature, magnitudes and distribution of residual stresses which can influence the service properties. In the present work, the influence of carbonitriding process on the phase transformation during case hardening, retained austenite and residual stresses were investigated. In particular, the following points were taken into consideration: (1) characterization of the state after carbonitriding, (2) analysis of the state during and after tempering, (3) investigation of the state after tempering coupled with cryogenic treatment, (4) investigation of the state after thermal stabilization, and (5) investigation of the mechanical stability of carbonitrided samples. Five carbonitriding variants with different carbon and nitrogen contents were considered. The phase compositions and residual stress analysis was carried out using X-ray diffraction. For each variant, the amount of retained austenite was dependent on the level of carbon and nitrogen reached which in turn depends on the carbon and nitrogen potential in the carbonitriding atmosphere. Besides the misfit between the case and the core, the amount and distribution of retained austenite in the case affects the nature, magnitudes and distribution of residual stresses in both retained austenite and martensite phase. The thermal stability of retained austenite and residual stress relaxation during the process of tempering was captured in situ, using a diffractometer equipped with a position sensitive detector with high resolution and a heating system. This study establishes the range of thermal stability of retained austenite and its kinetics of decomposition during continuous heating and isothermal holding. Further, it helped to quantify the magnitudes and kinetics of residual stress relaxation. Analysis of state after cryogenic treatment revealed that indeed tempering prior cryogenic treatment does stabilize retained austenite which then becomes difficult to transform to martensite during cryogenic treatment. The new martensite formed during the cryogenic treatment enhances significantly the compressive residual stresses in the martensite phase. Via shot-peening treatment it could be revealed that retained austenite was mechanically unstable and readily transforms; consequently high compressive residual stresses in both retained austenite and martensite phase are resulting
On fluid dynamics of lamella separator modelling and process optimisation
No full textThe suspensions in the treatment of water, domestic wastewater and industrial wastewater can be separated into clarified fluid and solid matter by gravity settlers. Inclined plate settler (IPS) is a type of gravity settler. Its performance strongly depends on the inlet zone configuration. Three different inlet structures were used to develop a model that can describe the effect of the inlet configuration on the hydraulic characteristics of lab-scale IPS numerically and validate it experimentally. The results show that feeding the IPS by a nozzle distributor can enhance the hydraulic performance of the IPS. Based on the previous results, the response surface methodology (RSM) combined with numerical simulation was used to optimise this distributor. The optimisation of nozzle has improved the hydraulic performance, flow distribution between the settlers and the separation efficiency of the IPS. Finally, a sediment gutter on the plates with an inclination angle to collect the sediment and disposes it via a lateral outlets is suggested to improve the separation efficiency
Investigation of Elementary Processes of Non-Newtonian Droplets Inside Spray Processes by Means of Direct Numerical Simulation
No full textA Comparative Study of Wear Behavior of Spray Formed Al-3.5Cu-10Si-20Pb Alloy in Air and Vacuum
No full textThe wear behavior of spray formed Al-3.5Cu-10Si-20Pb alloy has been studied in different environmental conditions. The wear tests were carried out in air atmosphere and vacuum, using a pin-on-disc type wear testing equipment with environmental chamber. The wear behavior in air atmosphere clearly illustrated the influence of surface oxidation
on the mating surfaces. This resulted in formation of irregular shaped oxide particles having bright contrast in SEM investigation of worn surfaces and debris particles. The nature of variation in wear with sliding distance, both in air atmosphere and vacuum, remained similar showing two distinct wear regimes viz., running in wear and steady state wear. A relatively lower running in period and a large reduction in the frictional force were observed in vacuum as compared to that obtained in air under same conditions of sliding speed and applied pressure. The wear rate varied from 0.25 to 0.61 x 10-12 m3m-1 for a range of applied pressure of 0.2 to 1.8 MPa and at a constant sliding speed of 1.0 ms-1 in vacuum, whereas the wear rates observed in air atmosphere for the same sliding conditions range between 0.75 to 3.87 x 10-12 m3 m-1. The coefficient of friction in air atmosphere varied for low pressures and remained almost constant at 0.25 for high pressure regime, whereas in vacuum it decreased linearly and finally attained a value of 0.1. The wear rates for different sliding speeds of 0.5 ms-1 to 2.0 ms-1 at constant applied pressure of 1.0 MPa were compared. The results depicted two different regimes of variation in wear rate with sliding speed in vacuum as well as in air atmosphere. The wear rate increased with sliding speed in vacuum, on the other hand, in air atmosphere the wear rate decreased with increasing sliding speed, reaching a minimum at a critical sliding speed and then increasing with further increase in sliding speed. The coefficient of friction remained almost constant for different sliding speeds for both air and vacuum. However, under vacuum, the value of coefficient of friction was lower than that in air, through the range of sliding speed. The wear behavior of the spray formed alloy under different conditions is discussed as inferred from the microstructural features of the alloy and the nature of the worn surfaces of the wear test specimen
Commissioning of the Optical Swirling Spray Injector: A new Test Section for Investigations of Atomization Inside an Aero Engine Burner at Realistic Operating Conditions
No full textA new test section for fuel spray investigations of a generic aero engine burner is described in this paper. This testsection is designed for air-preheat temperatures up to 900 K, air pressures up to 2.5 MPa and air mass flows of 1.5 kg/s. The cross section of the spray chamber has a dimension of 102 x 102 mm². The fuel is not burned in the spray chamber itself, but in a catalytic combustor downstream of the test section. Due to this, optical investigations of the swirling spray without disturbing effects of the combustion are possible. Another outstanding feature of this test section is the possibility of investigations of the fuel atomization inside a burner with tomographic shadowgraphy [3]. Four large optical accesses to the burner flow field in the chamber and additional three accesses at the end of the chamber offer the possibility for a wide range of optical measuring methods like LDA, PDA or Mie- Scattering
Mikroskopische Analyse des Zerfalls motorischer Sprühstrahlen
No full textThe shape of a liquid jet is strongly influenced by primary breakup close to the nozzle. Especially for microscale and highly turbulent engine sprays this atomization process is still hardly understood. One reason for this circumstance is the poor optical accessibility of the phenomenon. Visual examinations are always limited by diffraction effects, motion blur and multiple scattering of light, as atomization results in a very large number of tiny primary liquid structures, moving at high speed. For this reason, spray characterization is often made on the basis of macroscopic parameters.This work also starts with a macroscopic characterization using high-speed visualization. A parameter variation is done based on a wide range of fluids and material properties. Different empirical cone angle correlations are derived and evaluated in an existing analytical spray model. Based on these findings, the atomization close to the nozzle is analyzed in a second step. To meet the requirements for such a study, a novel microscopic method is developed and applied in this work, the so-called “Double Pulsed Transmitted Light Microscopy“. This method allows a visual determination of sizes, shapes and velocities of primary liquid structures with high spatial and temporal resolution for engine-related injection conditions and ambient pressures. The method is combined with the “Laser Correlation Velocimetry“, which allows the measurement of radial profiles of axial velocity. Based on both methods a detailed characterization of inner jet core, film structures, ligaments and droplets is done for atmospheric conditions as well as for engine-related ambient gas densities. These investigations form the basis for an interpretation of the influence of shear effects, damping effects and the outlet conditions on atomization of an engine-relevant jet
Heat Transport in Structured Catalytic Reactors for Gas-Phase Reactions
No full textEs ist seit langem bekannt, dass das Management des Wärmetransports ein entscheidender Faktor für die Leistungsfähigkeit katalytischer Reaktoren ist. Diese
werden beispielweise für stark exo- und endotherme Gasphasenreaktionen eingesetzt.
Offenzellige feste Schäume (oder auch Schwämme) kombinieren einen geringen
Druckabfall mit bemerkenswerten Wärmetransporteigenschaften und bieten
damit eine Kombination, die sie zu einer äußerst attraktiven Option als monolithische
Katalysatorträger macht. Für kleinvolumige dynamisch betriebene Reaktoren
(zum Beispiel für die Methanisierung von CO2) haben offenzellige Schäume gegenüber
herkömmlichen Pelletkatalysatoren vorteilhafte Wärmetransporteigenschaften
gezeigt. Dies gilt solange die Wärme hauptsächlich radial über Konduktion abgeführt
wird. Um effiziente und sichere strukturierte Schwammreaktoren zu entwerfen
und außerdem zuverlässige technische Modelle zu entwickeln, ist ein gründliches
Verständnis der drei Wärmetransportmechanismen, Konduktion, Konvektion
und Wärmestrahlung, erforderlich. Ziel dieser Arbeit ist es daher, einen tieferen Einblick in die gekoppelte konjugierte Wärmeübertragung und Wärmeerzeugung in offenzelligen Schäumen zu erhalten, welche in Rohrreaktoren (d.h. strukturierten Schaumreaktoren) verwendet werden. Mithilfe von 3D CFD Simulationen können Wärmeflüsse und Temperaturen von Gasen und Feststoffen gründlich untersucht werden. Um die Reaktionswärme während einer exothermen chemischen Reaktion abzubilden und um den Rechenaufwand zu reduzieren, werden volumetrische Wärmequellen homogen in den festen Schaum implementiert. Die Vereinfachung der Wärmequelle ermöglicht es, die Auswirkung der Wärmeerzeugung im Reaktor auf die Wärmeübertragung zu untersuchen. Die Ergebnisse dieser Arbeit quantifizieren dominante Wärmetransportmechanismen in Schaumreaktoren, etablieren Wärmequellen als Instrument zur Untersuchung des Wärmetransports in strukturierten Reaktoren und geben Designrichtlinien für die Auslegung von Reaktoren und deren Katalysatorträgern bei hoch exo- oder endothermen Reaktionen
Mikroskopische Analyse des Zerfalls motorischer Sprühstrahlen
No full textThe shape of a liquid jet is strongly influenced by primary breakup close to the nozzle. Especially for microscale and highly turbulent engine sprays this atomization process is still hardly understood. One reason for this circumstance is the poor optical accessibility of the phenomenon. Visual examinations are always limited by diffraction effects, motion blur and multiple scattering of light, as atomization results in a very large number of tiny primary liquid structures, moving at high speed. For this reason, spray characterization is often made on the basis of macroscopic parameters.This work also starts with a macroscopic characterization using high-speed visualization. A parameter variation is done based on a wide range of fluids and material properties. Different empirical cone angle correlations are derived and evaluated in an existing analytical spray model. Based on these findings, the atomization close to the nozzle is analyzed in a second step. To meet the requirements for such a study, a novel microscopic method is developed and applied in this work, the so-called “Double Pulsed Transmitted Light Microscopy“. This method allows a visual determination of sizes, shapes and velocities of primary liquid structures with high spatial and temporal resolution for engine-related injection conditions and ambient pressures. The method is combined with the “Laser Correlation Velocimetry“, which allows the measurement of radial profiles of axial velocity. Based on both methods a detailed characterization of inner jet core, film structures, ligaments and droplets is done for atmospheric conditions as well as for engine-related ambient gas densities. These investigations form the basis for an interpretation of the influence of shear effects, damping effects and the outlet conditions on atomization of an engine-relevant jet
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