Enhancement of submicron particle deposition on a semi-circular surface in turbulent flow (original) (raw)
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Deposition layers formed by a turbulent aerosol flow of micron and sub-micron particles
Powder Technology, 2001
The particle deposition layers formed by a turbulent aerosol flow have been studied as a simultaneous phenomenon of particle deposition and reentrainment. The experiments were conducted in a rectangular channel using alumina powders of D s 0.5-3.3 mm. p50 The state of the deposition layer was observed through a video microscope, and the surface profiles were measured with a scanning laser focus displacement meter. The experimental results showed that micrometer-sized particles formed a striped-pattern deposition layer as well as a filmy deposition layer, while sub-micron particles only formed a filmy deposition layer; the difference in the formation of these deposition layers was described in detail. Furthermore, a theoretical analysis taking account of the balance of the adhesive strength and the separation strength was carried out to explain the thickness of the deposition layer.
e-Journal of Surface Science and Nanotechnology, 2014
We demonstrate one-step deposition of submicrometer-sized particles suspended in the gas-phase onto a plate type substrate using an electrostatic-assisted spray system. The spray nozzle was set perpendicularly to the substrates (facing the front surface). The particles were deposited on plate-type metallic surfaces, on both front and rear sides of the substrate. This "both-side" deposition can be ascribed to deflection of charged particles in the front side, and then drifting of the particles around the rear side. A numerical simulation also showed that the deposition mechanism was found to be dependent on the center and the edge of the substrate. The electrostatic effect is more effective on both the center and the edge than the diffusion effect.
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.
Penetration efficiency of nanometer-sized aerosol particles in tubes under turbulent flow conditions
Journal of Aerosol Science, 2012
In order to quantify losses of nanometer-sized particles in turbulent flows through tubes, their penetration efficiencies were measured as a function of the particle size, Stokes number and Reynolds number. The penetration efficiency of tungsten oxide and ammonium nitrate particles with diameters between 3 and 17 nm was investigated in turbulent flow conditions with Reynolds numbers (Re) extending from 4500 to 10,500. The measured penetration efficiencies in straight tubes were found to deviate from the empirical correlation of Lee and Gieseke (1994). In contrast, the empirical equation of Fan and Ahmadi (1993) agrees better with our experimental results, also in comparison with the corresponding Wells and Chamberlain (1967) and Wood (1981) empirical correlation. Additional experiments were conducted to quantify the penetration efficiency of nanoparticles in tubes having 901 bends for Dean numbers between 1426 and 2885. Penetration efficiencies of particles through 901 bends were found to increase with increasing curvature ratios. The influence of Reynolds number (when varied between 4500 and 10,500) on the penetration efficiencies was found to be insignificant within the uncertainty of the measurements. We compared our experimental results of the penetration efficiencies through 901 bends as a function of the Stokes number with the Pui et al. (1987) parameterization which was validated for particles larger than 100 nm. For particles larger than 12 nm our data agree with the Pui et al. (1987) parameterization. However, for smaller particles the measured penetration efficiencies increased with the Stokes number while their parameterization predicts the opposite.
Particulate Fouling in Micro-Structured Devices
2005
Micro-structured equipment is used for many applications in analytical or chemical reaction devices. A literature review shows the major fouling problems in micro-channels like clogging by gas bubbles, chemical reactions, corrosion, and particulate fouling. Experimental investigations of particle flow in micro-mixers indicate two major precipitation mechanisms of nanoscale particles, which are described by dimensionless numbers. Small particles reach the wall by diffusion and attach there for Peclet numbers smaller than 5 • 10 6 . Larger particles touch the wall and accumulate in bent and curved flow, due to their inertia. The Stokes number describes the influence of the particle inertia in bent flow and has to be lower than 0.05 to prevent wall attachment. Additionally, surface properties have to be controlled and bends, curves, nozzles, and expansions have to be avoided to mitigate fouling.
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. ?
This work presents the results of experiments conducted to provide quantitative information on particle detachment from surfaces. The e ects of certain, controllable factors on the detachment of 70 m-diameter stainless-steel spheres from a glass surface exposed to accelerated air ow in a wind tunnel were studied. Changes in the free-stream velocity required to detach 50% of the particles, the threshold velocity for detachment, were measured for variations in the controlled factors. These factors were air relative humidity, residence time between particle deposition onto the substrate and ow application, mean ow acceleration, deposition density, ÿnal free-stream velocity, and ÿnal ow Reynolds number. Results reveal that deposition density was the most e ective factor that enhanced detachment at all relative humidities. Residence time was found to be the most e ective factor that suppressed detachment at high relative humidity. The threshold velocity increased with increasing relative humidity and was lower for turbulent ow versus laminar ow. Within the uncertainty limits, the mean ow acceleration in the transient period was found not to a ect the threshold velocity in the range from 0.014 to 0:34 m=s 2 . The ÿnal free-stream velocity also did not a ect the threshold velocity, provided it was greater than the threshold. A set of experimental conditions that lead to a relatively small uncertainty is presented. ?
Submicron particles removal by charged sprays. Fundamentals
Journal of Electrostatics, 2013
New regulations regarding the PM1 emission by power plants and transport vehicles require novel technical solutions for the abatement of particulate matter emission in submicron size range. Particles of this size are difficult to remove by conventional methods, and therefore various variants of wet electrostatic scrubbers have been developed. In such devices, the electrostatic forces between charged particles and collectors (water droplets) are used to permanently remove them from the gas. The paper focuses on the state-of-the-art of wet electrostatic scrubbing technique used for the removal of submicron particulate matter from exhaust gases with particular emphasis upon marine Diesel engines.
Journal of Aerosol Science, 1993
A~traet--The sublayer model for deposition of spherical particles from turbulent air streams in vertical ducts with smooth and rough walls is described. The formulation is based on the coherent vortical structure of near-waU turbulence and the detailed analysis of particle trajectories in this flow field. The Stokes drag, the Saffman lift force, and the gravitational force are included in the equations of particle motion. The wall roughness is taken into account by an appropriate modification of the boundary condition for the particle capture trajectories. The predicted deposition rates under various conditions are compared with the available experimental data and a reasonable agreement is observed. It is noticed that the particle-to-fluid density ratio, the shear-induced lift force, the wall roughness, the gravity direction and the flow Reynolds number have profound effects on the particle deposition rate. Based on a perturbation method, a simplified empirical equation for evaluating the turbulent deposition rate is also proposed.
Preparation of spray dried submicron particles: Part A – Particle generation by aerosol conditioning
International Journal of Pharmaceutics, 2018
The preparation of submicron-sized particles is relevant in chemical, food and pharmaceutical applications. In pharmaceutics, spray dried submicron-sized particles (0.1-1 µm) can increase the dissolution rate as well as the solubility of poorly water-soluble drugs. Since the particle size during spray drying is mainly influenced by the droplet size, the preparation of uniform droplets smaller than 3 µm is of particular interest. In this work, a two-fluid nozzle was combined with a cyclone droplet separator. Droplets larger than the cutoff size were separated with a cyclone droplet separator and returned to the liquid feed. The aerosol at the outlet of the droplet separator was subsequently dried. The drop size of the conditioned aerosol was small, , and independent of the liquid-togas mass flow ratio and the viscosity of the liquid feed. Thus it only depended on the characteristics of the separator. Finally, the dried particles were spherical in shape and in the submicron-sized range.