Deposition layers formed by a turbulent aerosol flow of micron and sub-micron particles (original) (raw)
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Advanced Powder Technology, 1998
The effects of air velocity and particle diameter on the thickness and the interval of striped deposition layers were studied as a simultaneous phenomenon of particle deposition and reentrainment ina rectangular channel flow. The powders used were alumina of three different sizes (Dp50 = 2.6, 4.9 and 5.6 µm) and the average air velocity was in a range of 10-23 m s-1 (turbulent aerosol flow). The experimental results show that the thickness increases with time elapsed and becomes constant. The time dependence can be represented by an exponential equation. Although the thickness tends to decrease with increasing average air velocity, the variation is small as long as the deposition layers take the striped pattern. The velocity range showing the striped pattern shifts to higher velocity for smaller particles. Furthermore, the interval between the striped deposition layers increases with average air velocity and/or particle diameter. It is suggested that the particle inertia takes an important role in the formation of the striped deposition layers. NOMENCLATURE De hydraulic diameter of a rectangular channel (m) Dp50 mass median diameter (m) dop distance between a fixed obstacle and a striped layer (m) dsso interval between striped deposition layers (m) FS cumulative distribution function of interval between layers (-) FL cumulative distribution function of layer thickness (-) Jd particle deposition flux (kg m-2 s-I) Jr particle re-entrainment flux (kg m-' ç 1) Re Reynolds number (-) t time elapsed (s)
Aerosol Science and Technology, 2000
Formation of particle deposition layers on rough wall surfaces was studied as a simultaneous phenomenon of particle deposition and reentrainment. The test powders used were alumina of micron sizes and the experiments were conducted under turbulent aerosol flows. After particle deposition and reentrainment reached an equilibrium state, deposition layer of a striped pattern was formed. The striped pattern was characterized by the interval and the thickness, which decreased with increasing the roughness of wall surface. Further, the striped deposition layers moved slowly downstream, and the velocity of the moving deposition layers decreased with increasing the surface roughness. A moment balance model was used to explain the effect of the surface roughness on the velocity of the moving deposition layers.
Reentrainment of deposited particles by drag and aerosol collision
Journal of Aerosol Science, 2003
The reentrainment of deposited particles in turbulent aerosol ow has been studied theoretically and experimentally. The moments of forces, i.e., particle adhesion, gravity, aerodynamic drag, and aerosol collision, acting on a small aggregate adhering to a wall are calculated as a function of particle diameter. The analytical solutions indicate that the collision of an aerosol particle larger than several micrometers plays an important role in the reentrainment, whereas, the e ect of the aerodynamic drag dominates for sub-micron particles. Furthermore, the critical velocity of aerosol ow for the reentrainment is calculated on the basis of a moment balance. The critical velocity decreases with increasing particle diameter. Experiments were conducted using alumina particles of size 3.3-10:3 m in mass median diameter. The particles were fully dispersed into air ow and fed into a glass tube. The variation in the state of the particle deposition layer was observed through a digital video camera, and the critical velocities for no particle layer formation were obtained under various conditions. Although the experimental data on the critical velocities deviate somewhat from the theoretical values, the trends are in reasonable agreement. In particular, it was found that the inertial collision by aerosol particles is e cient for the removal of the particle deposition layer.
Controlling factors of film-thickness in improved aerosol deposition method
Journal of the Ceramic Society of Japan, 2009
To understand the controlling factor of film thickness in aerosol deposition method (ADM), the deposition apparatus was improved at first and the effect of pretreatment of raw barium titanate powder was studied. A developed aerosol generator where the carrier gas was separated from the aerosol generating gas was effective to avoid the agglomeration of powders during the deposition. Two dimensional scanning of the substrate decreased the film-thickness distribution caused by the imhomogeneity of deposition rate in a line-type nozzle. Effect of pretreatments of raw powders, including sieving, drying, planetary ball milling and heating was examined, respectively. There was an optimum rotational velocity of planetary milling to increase the film thickness, indicating that adequate agglomeration of raw powders enhance the film deposition. The film thickness decreased as the heating temperature increased. The heating strengthened the agglomeration of raw powders which restricted the film deposition because the kinetic energy of particles in the aerosol was consumed to break the agglomerations rather than making film. Weakly agglomerated powders with optimum size enhanced the film thickness in ADM.
High-Resolution Analysis of Particle Deposition and Resuspension in Turbulent Channel Flow
Aerosol Science and Technology, 2015
Particle deposition and resuspension during turbulent flow were investigated using a rectangular channel with glass side walls. Micrometer-sized alumina particles were used in the experiments. Particle behavior in the rectangular channel was observed through a high-speed microscope camera with a resolution of 0.3 mm and a speed of 87,600 fps, and particle deposition and resuspension fluxes were quantified using digital image analysis. The experimental results showed that particle resuspension was caused by the collision of airborne particles with those deposited on the surface. The resuspension flux was found to be correlated with the deposition flux. Furthermore, the average residence time between particle deposition and resuspension was several tens of milliseconds, which was very short but much longer than the contact time at the collision. Additionally, the residence time decreased as the particle diameter increased.
A sublayer model for deposition of nano- and micro-particles in turbulent flows
Chemical Engineering Science, 2000
Deposition of aerosol particles from 10 nm to 50 m in turbulent duct #ows is studied. The sublayer model for the turbulent deposition process is extended to cover the e!ects of gravity, Brownian, and lift forces. The model is based on the detailed analyses of particle trajectories in turbulence coherent structures near a wall. The Stokes drag, the Sa!man lift, and the Brownian excitation are included in the particle equation of motion. Limiting trajectories for various conditions are evaluated, and the deposition velocity for a range of particle relaxation times and #ow conditions are presented. In particular, a new method for evaluating the deposition velocity of Brownian particles based on the limiting trajectory concept is presented. E!ects of #ow shear velocity on the deposition rates of di!erent size particles are also studied.
The microstructure of alumina coatings prepared by aerosol assisted spray deposition
Surface and Coatings Technology, 2004
This paper presents the deposition of alumina coatings using a low cost aerosol assisted spray deposition (AASD) in an open atmosphere at low temperature. AASD is a variant of CVD, which involves spraying atomized precursor droplets into a heated environment where the droplets undergo decomposition and chemical reaction near the heated substrate and produce solid stable coatings. Using the same deposition method but different starting precursor (e.g. solution or suspension), dense or porous deposits could be produced using the AASD method. Dense and thin alumina coatings were deposited using a precursor solution containing aluminium alkoxide. Whereas thick porous alumina deposits were prepared using a suspension of alumina powder in alcohol solution. Such porous alumina deposits could be densified to form thick alumina coatings. Continuous CO laser was used to 2 densify the alumina deposits. The microstructure of the coatings and deposits was characterized using a combination of X-ray diffraction and scanning electron microscope methods. The dense and porous alumina coatings could be deposited by varying the processing conditions. The relationships between the growth behavior and microstructure of the coatings were also discussed.
Revealing the effects of aerosol deposition on the substrate‐film interface using NaCl coating
Journal of the American Ceramic Society, 2019
Aerosol deposition is a feasible method of fabricating dense ceramic films at room temperature by the impact consolidation of submicron-sized particles on ceramic, metal, glass, and polymer substrates at a rapid rate. Despite the potential usefulness of the aerosol deposition process, there are issues, such as deposition mechanisms and structure of the filmsubstrate interface, that are not well understood. We have used complementary structural and microstructural analysis to capture the state of the substrate surface after the aerosol deposition process. The results reveal that modification of the substrate surface by the ejected submicron-sized particles is essential for the formation of anchoring layer, thereby, a change in internal residual stress state and surface free energy of the substrate is required to deposit film using AD process. Our analysis also suggests that the adhesion between the metal substrate and ceramic particles is possibly contributed by both physical bonding and mechanical interlocking.
Enhancement of submicron particle deposition on a semi-circular surface in turbulent flow
Indoor and Built Environment, 2019
Air pollution due to suspended particles in the built environment threatens people’s health. The collection of submicron particles using air cleaners is especially difficult due to their tiny sizes. Surface patterned structures can enhance submicron particle deposition, thus improving the performance of an air cleaner. In this study, a semi-circular micro-structured surface was designed and optimized by considering particle deposition enhancement and reduction of pressure differential. The maximum efficiency ratio at the pitch-to-height ratio ([Formula: see text]) of 3 reached 1137 for the 1 µm particles but only 3.4 for 0.01 µm particles. The main mechanism of enhancing submicron particle deposition depends on the capture of the recirculation wake induced by semi-circular structures and entrainments of large turbulence kinetic energy to the near-wall regions. Submicron particles was observed to deposit between cavities on the semi-circular surface at a small [Formula: see text]. Th...
Chemical Engineering Science, 2003
An experimental study of micron-sized particle deposition on at surfaces is presented, aimed at delineating the e ects of hydrodynamic and physicochemical interactions on particle transport and attachment e ciency, and at obtaining a better understanding of the particle sticking probability, a concept employed in modelling particulate fouling of industrial heat exchangers. Dilute particle suspensions are employed in a parallel-plate-laminar-ow channel, and hydrodynamic and physicochemical conditions are systematically varied. Deposition rates are determined by optical microscopy and image analysis techniques. It is observed that if gravity forces are present (in a horizontal channel) they control deposition at low wall shear stresses. As the hydrodynamic wall shear stress increases particle deposition rates are signiÿcantly reduced due to the e ect of hydrodynamic lift or drag forces inhibiting transport or attachment. In general, for hydrodynamic conditions similar to those encountered in industrial heat exchangers, it appears that the particle sticking probability is signiÿcantly lower than unity. ?