Soot Deposition and Gravitational Settling Modeling and the Impact of Particle Size and Agglomeration

Floyd, Jason; Overholt, Kristopher J.; Ezekoye, Ofodike A.

doi:10.3801/iafss.fss.11-376

Cited by 14 publications

(8 citation statements)

References 15 publications

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“…Turbulent eddy diffusion typically subsumes diffusive transport due to Brownian motion. Gravitational deposition, also known as gravitational settling, occurs due to gravitational force and is more significant for large particles, on the order of 10 µm or more, that tend to appear from agglomeration over long time periods, on the order of 30 minutes or more [1]. The Fire Dynamics Simulator (FDS) has implemented a computational scheme for predicting soot deposition in fires due to these mechanisms [1]- [3], but lacks sufficient validation data to assess the performance of the models.…”

Section: List Of Figuresmentioning

confidence: 99%

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A soot deposition gauge for fire measurements

Mensch¹,

Cleary²

2018

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The goal of this exploratory project is to demonstrate the feasibility of a conductometric measurement to determine the time-resolved soot deposition on surfaces in fire environments. Quantitative soot deposition data enabled by this measurement method is lacking in the literature and would be useful to advance fire analysis and fire model development. Laminar flow through a thin rectangular channel with a transverse temperature gradient is used to generate thermophoretic deposition exposures on a target surface. Computational fluid dynamics (CFD) modeling is used to design the channel geometry to have well characterized laminar velocity and temperature profiles. The results predict that fully developed laminar flow and temperature profiles are established by the midpoint of the channel length. The linear temperature gradient between channel walls causes thermophoretic deposition of soot particles on the cold wall of the channel. The channel flow is also modeled with the National Institute of Standards and Technology's Fire Dynamics Simulator (FDS) to generate predictions of soot deposition in the channel. In the experiments particles depositing on the target surface cause an increase in the conductance between the interdigitated electrodes. The change in conductance is measured intermittently before, during and after the exposure using a picoammeter and an applied voltage. At the end of the exposure the mass loading of deposited soot is determined by two separate measurement methods, including a gravimetric method and a light transmission method. The relationship between the amount of deposition and the conductometric response is evaluated for both types of mass loading measurements. The gravimetric method produced a more coherent correlation to gauge conductance with less scatter than the transmission method. The measured mass loadings are also combined with measurements of the incoming soot concentration to calculate the overall deposition velocity. The deposition velocity is compared to the theoretically predicted thermophoretic velocity, as determined from the measured temperatures and an estimate of particle size. The deposition velocities derived from the gravimetric method corresponded well with the thermophoretic velocities, but both the vertical and horizontal transmission based deposition velocities generally were lower than the thermophoretic velocities.

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Section: List Of Figuresmentioning

confidence: 99%

“…The thermophoretic velocity, vth, is proportional to ∇T and related to properties of the gas and particle, as given in Eq. (1).…”

Section: List Of Figuresmentioning

confidence: 99%

A soot deposition gauge for fire measurements

Mensch¹,

Cleary²

2018

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“…Although soot aerosols can have a net electric charge, there is negligible electric field to cause electrostatic deposition in most fire scenarios. Gravitational settling is significant for large particles on the order of 10 μm or larger aerodynamic diameter [1], as their gravitational force compared to air resistance is more significant than for smaller particles. Deposition from turbulent flow, typically more significant than Brownian diffusion, occurs when turbulent eddies bring particles near a surface, and the particles tend to deposit due to direct interception by the surface or their inertia causing an impact with the surface.…”

Section: Introductionmentioning

confidence: 99%

“…The Fire Dynamics Simulator (FDS), a computational fluid dynamics code for fire prediction, calculates thermophoretic deposition, turbulent deposition and gravitational settling of aerosols [1,9,10] according to literature models, but there are limited experimental data available to assess the performance of the predictions. A model validation case requires accurate measurements of the soot deposition and the surrounding conditions, such as spatial and temporal variations in temperature, velocity, and soot concentrations, which can be difficult to obtain in fire experiments.…”

Section: Introductionmentioning

confidence: 99%

Measurements and predictions of thermophoretic soot deposition

Mensch

Cleary

2019

International Journal of Heat and Mass Transfer

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A thin laminar flow channel with a transverse temperature gradient was used to examine thermophoretic deposition of soot aerosol particles in experiments and modeled in Fire Dynamics Simulator (FDS) simulations. Conditions investigated included three flowrates, with nominal Reynolds number based on the hydraulic diameter of 55, 115 and 230, and two applied temperature gradients, nominally 10 °C/mm and 20 °C/mm, with repeats. Soot was generated from a propene diffusion flame. The burner exhaust was mixed with dilution air, and most large agglomerates greater than 1 μm aerodynamic diameter were removed prior to the channel inlet. The expected thermophoretic velocity of the aerosol was calculated from the applied temperature gradient. A calculated deposition velocity was determined from the mass of deposition, the channel inlet soot concentration, and the exposure time. Uniform soot deposition allowed targets to be used to measure the mass of deposition on the cold side of the channel. The mass of deposition was also determined by subtracting the mass of soot exiting the channel from the mass of soot entering the channel during the exposure time. The deposition velocities from these two methods generally agreed with the thermophoretic velocity and with each other. The deposition mass predicted by the FDS model also compared well with the experiments in most cases. The disagreements for the lowest flow rate cases are attributed to buoyant flow effects adding uncertainty to the actual temperature gradients present in the channel. (The opinions, findings, and conclusions expressed in this paper are the authors' and do not represent the views or policies of NIST or the United States Government.

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“…To improve the prediction of soot movement in large-scale enclosures, a model called Multi-Particle-Size model (MPS model) was developed by Hu et al [18]. The model was adopted in the study [19] within the framework of the widely used CFD fire simulation tool FDS to investigate soot deposition on solid surface. The MPS model is an improvement of an Eulerian approach called the drift flux model, by considering the uneven mass size distributions of soot particles and the effect of particle size on gravitational settling.…”

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confidence: 99%

Grouping methods for MPS soot transport model and its application in large-scale enclosure fires

Jia

Wang

et al. 2017

Fire Safety Journal

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A soot transport model called Multi-Particle-Size model (MPS model) was developed to improve the prediction of soot movement by considering the uneven mass size distribution of soot particles and the influence of particle size on the gravitational settling. The model requires a sophisticated grouping strategy to divide the soot particles into several groups and determine the representative size for each group. In this paper, several soot particle grouping strategies and the method to calculate the representative sizes are developed with the aim of balancing the computational efficiency and the prediction accuracy of the model. The performance of the MPS model when different grouping strategies are applied is investigated through the comparison of the predicted movement of soot particles generated from several materials. Based on this analysis a grouping strategy that results in the identification of three groups is shown to be sufficient to represent the influence of particle size on the gravitational settling for a variety of combustible materials and the computational cost of the extra governing equations for the transport of soot particles in the groups is acceptable. Furthermore, the efficiency of the model is demonstrated by simulating soot movement in a large-scale industrial building with a high ceiling.

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Soot Deposition and Gravitational Settling Modeling and the Impact of Particle Size and Agglomeration

Cited by 14 publications

References 15 publications

A soot deposition gauge for fire measurements

A soot deposition gauge for fire measurements

Measurements and predictions of thermophoretic soot deposition

Grouping methods for MPS soot transport model and its application in large-scale enclosure fires

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