2,587 research outputs found
Cost/efficiency analysis of a model wire-plate electrostatic precipitator via DNS based Eulerian particle transport approach
No full textThe object of this work is to outline a methodology that can improve the current procedures used to size cost-optimized, efficient electrostatic precipitators (ESP). Focusing on a model wire-plate ESP initially, we develop a two-dimensional Eulerian, advection-diffusion type model for particle transport with distributed parameters. The Eulerian model is assessed against the accurate Lagrangian particle tracking database obtained for a model ESP using the parameter-free, highly accurate direct numerical simulation database obtained in previous work (Soldati 2000; Soldati and Banerjee 1998). Results show that the simplified Eulerian model can have good performances, provided that the functional form of the required transport parameters (i.e., turbulent dispersion coefficient, electromigration velocity, and convection velocity) are properly defined. Next, the cost function for a model ESP is defined and the influence of several design parameters on cost and collection efficiency is examined to identify guidelines to increase the collection efficiency at the lowest cost. Considering the cost associated with variation of precipitator length and width, wire-to-plate distance, and voltage applied to the wires, results show that the most cost-effective way to increase the collection efficiency of a wire-plate ESP is to decrease the wire-to-wire distance. Furthermore, the reasons for cost effectiveness of wider-spacing ESPs are demonstrated from a theoretical viewpoint, thus confirming the experimental observations of Navarrete et al. (1997)
On the Effects of ElectroHydroDynamic Flows and Turbulence on Aerosol Transport and Collection in Wire-Plate Electrostatic Precipitators
Full text linkPredicting transport of aerosols or particles in wire-plate electrostatic precipitators is complicated by the influence of EHD flows and turbulent flow field. In this work we use the direct numerical simulation by Soldati and Banerjee (1998) to analyze the effects of the EHD flows and of turbulence on particle transport and collection efficiency. Particles of different size and charge were tracked in two different Row fields corresponding to different potential applied to the wires of the precipitator. Results were compared against simulations in which the electrostatic field acted only on particles. - i.e. no EHD flows. It is apparent that EHD Rows are large advective motions with scale of the wire-to-wall distance and have a strong effect on the local behavior of particles. sweeping them into different regions of the channel. However, it was found that the overall collection efficiency of the precipitator is not significantly affected by the presence of EHD Rows. Even in the vicinity of the wall, EHD flows appear to have negligible influence on particle deposition
Soldati e pratica scrittoria: i graffiti parietali
No full textI graffiti vergati dai soldati dell'esercito romano mostrano che essi erano in possesso di un grado di alfabetizzazione poco approfondit
Influence large-scale streamwise vortical EHD flows on wall turbulence
No full textThe influence of large-scale electrostatically induced streamwise vortical flows superimposed on turbulent plane channel flow driven by a pressure gradient was analyzed using direct numerical simulation. This study may be relevant for designing a new drag-reduction optimized configuration for electrostatic precipitators. The EHD flows had a spanwise periodicity of 340 wall units. Regardless of intensity, EHD flows induce an initial transient of similar to600 shear-based time units with moderate drag decrease (similar to6-7%), followed by a steady state with slight drag modification. The behavior of shear stress at the wall was examined in connection with the shape of the EHD flows to identify future directions for further drag reductio
Lagrangian Simulation of Turbulent Particle Dispersion in Electrostatic Precipitators
No full textIndustrial design of electrostatic precipitators Is based on the transport theory developed by Deutsch (1922), which assumes that transverse turbulent mixing is effective enough to maintain the concentration profile uniform throughout the cross section (i.e., turbulent diffusivity is assumed infinite). To improve understanding of turbulent particle dispersion under the influence of electrostatic forces, a database on particle trajectories was first generated, based on the flow field from a direct numerical simulation of a plate-plate precipitator (Soldati et al, 1993). The effect of various parameters, such as particle size, charge and particle migration velocity, on dispersion and collection efficiency was investigated. Results show that particle concentration profiles are not uniform due to finite values of ''turbulent diffusion'' coefficient. The simulations indicate that the early stages of particle collection are controlled by particle migration velocity, while final stages are controlled by turbulence diffusion mechanisms
Particles turbulence interactions in boundary layers - Plenary lecture presented at the 75th Annual GAMM Conference, Dresden/Germany, 22-26 March 2004
No full texturbulent dispersed flows in boundary layers are crucial in a number of industrial and environmental applications. In most applications, the key information is space distribution of particles which is known to be strongly non-homogeneous. Specifically, inertial particles distribute preferentially avoiding strong vortical regions and segregating into straining regions. The vortical boundary layer structures control momentum, mass, heat, and particle transfer. Coherent structures bring particles toward the wall and away from the wall and favour particle segregation in the viscous region giving rise to nonuniform particle distribution profiles which peak close to the wall. The reason for this behavior is particle inertia, which filters the high frequency turbulence fluctuations. The object of this work is to review the current understanding of turbulent boundary layer dynamics and to examine the mechanisms for particle transfer, segregation, and preferential distribution. The physical mechanisms discussed and proposed are based on Direct Numerical Simulations of turbulence and Lagrangian tracking of inertial particles
The Influence of Coalescence on Droplet Transfer in Vertical Annular Flow
No full textIn vertical annular flow, the motion of droplets in the gas core is dominated either by a diffusion mechanism, when the droplet size is small, or by inertial effects when droplets are large. These two mechanisms have to be considered when predicting deposition rates. Furthermore, since droplet-droplet interactions influence droplet motion, a deposition model should also account for collisions and coalescence among droplets. After reviewing the available deposition models, the effect of coalescence on droplet motion is theoretically analyzed. The results demonstrate that coalescence extends droplets residence time in the gas core thus decreasing the deposition coefficient. On the basis of these results, a new deposition model which accounts for the two deposition mechanisms and includes the effect of coalescence is proposed and compared against existing experimental data
Sediment transport in steady turbulent boundary layers: Potentials, limitations, and perspectives for Lagrangian tracking in DNS and LES
No full textIn this paper, we discuss the applicability of the Eulerian-Lagrangian point-particle approach to predict sedimentation and resuspension phenomena in solid-liquid flows. We start from results obtained for dilute systems and we examine to which degree such results can be applied to sedimentation phenomena, in which higher concentration values will be at some point relevant. We examine critical issues in state-of-the-art direct and large-eddy simulations of sediment dynamics in turbulence, starting from concepts and ideas that were derived from a systematic study of particle transport in turbulent boundary layer and applied to several canonical flow configurations (closed/open channel with flat/wavy bottom). We focus on issues related to high sediment concentration near the boundary and we examine critically results from original modelling strategies specifically aimed at dealing with boundary conditions where concentration peaks, with inter-particle collisions and with subgrid scale models in higher Reynolds number simulations employing LES
Numerical evaluation of mixing time in a tank reactor stirred by a magnetically driven impeller
No full textIn this work, we investigate numerically the mixing characteristics of a tank reactor stirred by a low off-bottom clearance magnetically coupled impeller. We calculate the fully three-dimensional, time-dependent flow field using the Reynolds average form of the Navier-Stokes equations and adopting the sliding mesh approach. Mixing of a scalar species is computed in the Eulerian framework. We compute power consumption, pumping capability, fluid dynamic efficiency, and mixing time to homogeneous distribution from pointwise release of a scalar. We identify relationships useful to optimize the choice of operational parameters. Numerical results are in qualitative agreement with empirical correlations available from the literature and may be used to improve understanding of the mixing processes. The numerical procedure used mimics possible experimental approaches with the advantage of a lower cost. The methodology outlined can be a reference method to derive guidelines for optimization of impeller/tank design and identification of operational parameters
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