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One of the most important advantages of twin-fluid atomizers over other atomizers is the opportunity of atomizing liquids in a wide range of liquid properties efficiently. Thus, the fundamentals of spray dynamics, such as spray pattern, liquid breakup length, and spatial droplet size evolution using different liquid properties need to be investigated in some detail. The purpose of this study is therefore to examine and describe the influence of the liquid viscosity and surface tension on the spray performance of prefilming air-blast atomizers. A high-speed background shadowgraphy technique associated with particle tracking and Phase Doppler Anemometry (PDA) as a non-intrusive high-resolution local measurement technique were utilized to study the atomization process regarding the influence of the liquid properties on the spray quality. Generally, the break up mechanism is considerably influenced by the liquid viscosity. The breakup length (Lb) is dependent on liquid viscosity with α μ0.30and Lb μ 0.46 for Pa = 0.25 and 0.6 bar- air pressure, respectively. Droplet size distribution profiles of different liquids in axial and radial directions are studied. No significant influences of the surface tension and viscosity were observed on the mean droplet velocity downstream of the spray. A minor increase of the droplet mean diameter was observed with liquid viscosity variations from 22 to 111 mPas, however, the liquid surface tension was verified to play an important role in the atomization process at prefilming airblast atomizers as well. According to the available experimental data, it has subsequently been possible to develop an original and descriptive model based on a dimensional approach at different air pressure ranges. [ABSTRACT FROM AUTHOR]

A bulk of a 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. The atomized liquid is primary situated on a surface (prefilming surface) to form a thin liquid film before being exposed to high-velocity air flow. Moreover, the performance of spray processes is affected by the variation in the atomizer geometry, liquid physical properties and operational conditions. The purpose of this study is to examine and describe the influence of the nozzle geometry and a wide range of test conditions on the spray performance of prefilming air-blast atomizers. In order to evade the commonly complicated internal flow, an important but simple geometry was selected. Liquid break up mechanisms close to the atomizer exit were investigated using shadowgraphy associated with particle tracking. Furthermore, high-resolution local velocity and droplet size measurements were performed using phase Doppler anemometry (PDA). On the whole, the break up mechanism is considerably influenced by either air pressure and liquid flowrates or atomization edge size. Droplet size distribution profile of the different spray parameters in axial and radial directions are studied. The location of the maximum droplet mean velocity and the minimum Sauter mean diameter (SMD) within the spray are determined. The prefilming surface area and atomization edge size were observed to influence the liquid sheet breakup, droplet velocity and droplet size. With an atomization edge length increase of 5.7 mm, the global SMD increased to a maximum of 70% within different operation conditions. [ABSTRACT FROM AUTHOR]

The aim of the present work is the investigation of the turbulent flow field downstream of an axisymmetric sudden expansion with a large aspect ratio of D/d = 12,3. For the fundamentally understanding of the flow some numerical results are presented. They were achieved by using the RANS approach and SST turbulence model. The flow field is characterized by a jet-like flow near the nozzle exit and a large toroidal recirculation zone. The x-component of the velocity u was measured using one-component laser Doppler velocimetry. Axial and radial velocity distributions as well as some velocity spectra were measured. The spectra were calculated from the velocity signal using the Sample-And-Hold method together with the refinement technique. At the axial half length of the recirculation zone at the edge of the jet flow a narrow band peak was observed in spectra, suggesting the existence of large-scale fluctuations or instability of the flow field. Further investigations reveal that this effect is locally limited and shows no sensitivity against changes of the inlet conditions, e.g. the Reynolds number and velocity profile. [ABSTRACT FROM AUTHOR]