Numerical Simulations of Brand Transport in Large Outdoor Fires
Menzemer, Leo Willem LU
(2020)
- Abstract
- In wildland fires, burning particles of vegetative material can be lofted by the fire plume and are subsequently transported horizontally by cross wind. Depending on the state of the firebrands (flaming, smoldering, inert hot, inert cold) upon landing they bear the potential to cause secondary fires. The present work introduces the reader to existing theoretical analyses of the problem of combustion and transportation of firebrands. Based on the theory, approaches to numerical modelling of the important aspects of the problem are discussed. Using the Fire Dynamics Simulator (FDS) a pyrolysis model is developed based on the reaction rates from first order Arrhenius-equations. Firebrands exposed to high temperatures are found to fully... (More)
- In wildland fires, burning particles of vegetative material can be lofted by the fire plume and are subsequently transported horizontally by cross wind. Depending on the state of the firebrands (flaming, smoldering, inert hot, inert cold) upon landing they bear the potential to cause secondary fires. The present work introduces the reader to existing theoretical analyses of the problem of combustion and transportation of firebrands. Based on the theory, approaches to numerical modelling of the important aspects of the problem are discussed. Using the Fire Dynamics Simulator (FDS) a pyrolysis model is developed based on the reaction rates from first order Arrhenius-equations. Firebrands exposed to high temperatures are found to fully pyrolyze shortly after exposure to the fire and lower initial temperatures enable pyrolysis to sustain for a longer time while also reducing the cooling rates of firebrands as energy losses due to endothermic pyrolysis are minimized. Increasing thickness and density of firebrands lead to the same effect due to increased thermal inertia of the particles. The mass loss is established as most accurate criterion to define whether a particle is still flaming or not. A three-dimensional numerical model is presented with a flow field from a 100 MW tree crown fire with cross wind at 6.7 m/s. Firebrands equivalent to disc-like geometries with densities from 50-200 kg/mΒ³, diameters between 4 and 10 cm, and thicknesses between 0.2 to 10 cm are released from the canopy. In compliance with indications from other studies, the product π Γ π (initial density x thickness) is found to be a controlling factor of the firebrand dynamics and results show that particles with a product β₯ 1 are not lofted by the plume and land on the ground flaming, at a maximum travel distance of 10 m downwind. Particles with π Γ π β€ 0.6 are found to be lofted and entrained into the fire plume with travel distances >50 m from the fire. Future investigations need to address the validation of the pyrolysis model and invoking a model for char oxidation to allow analysis of far-travelling firebrands. (Less)
Please use this url to cite or link to this publication:
https://lup.lub.lu.se/record/6187e837-e95a-4d3e-a404-5e8c2d5f7287
- author
- Menzemer, Leo Willem
LU
- supervisor
- publishing date
- 2020-04
- type
- Thesis
- publication status
- published
- subject
- keywords
- Wildfire, spotting, Fire Brands, Numerical study, Fire Dynamics Simulator (FDS), Wildland-urban interface (WUI), Fire
- pages
- 98 pages
- publisher
- International Master of Science in Fire Safety Engineering
- language
- English
- LU publication?
- no
- id
- 6187e837-e95a-4d3e-a404-5e8c2d5f7287
- alternative location
- https://static1.squarespace.com/static/5cdbcb5a7a1fbd56ceb62430/t/620a336293dd371732225172/1644835694144/Leo+Menzemer+Master+thesis.pdf
- date added to LUP
- 2024-08-13 15:31:25
- date last changed
- 2024-08-19 12:58:09
@misc{6187e837-e95a-4d3e-a404-5e8c2d5f7287,
abstract = {{In wildland fires, burning particles of vegetative material can be lofted by the fire plume and are subsequently transported horizontally by cross wind. Depending on the state of the firebrands (flaming, smoldering, inert hot, inert cold) upon landing they bear the potential to cause secondary fires. The present work introduces the reader to existing theoretical analyses of the problem of combustion and transportation of firebrands. Based on the theory, approaches to numerical modelling of the important aspects of the problem are discussed. Using the Fire Dynamics Simulator (FDS) a pyrolysis model is developed based on the reaction rates from first order Arrhenius-equations. Firebrands exposed to high temperatures are found to fully pyrolyze shortly after exposure to the fire and lower initial temperatures enable pyrolysis to sustain for a longer time while also reducing the cooling rates of firebrands as energy losses due to endothermic pyrolysis are minimized. Increasing thickness and density of firebrands lead to the same effect due to increased thermal inertia of the particles. The mass loss is established as most accurate criterion to define whether a particle is still flaming or not. A three-dimensional numerical model is presented with a flow field from a 100 MW tree crown fire with cross wind at 6.7 m/s. Firebrands equivalent to disc-like geometries with densities from 50-200 kg/mΒ³, diameters between 4 and 10 cm, and thicknesses between 0.2 to 10 cm are released from the canopy. In compliance with indications from other studies, the product π Γ π (initial density x thickness) is found to be a controlling factor of the firebrand dynamics and results show that particles with a product β₯ 1 are not lofted by the plume and land on the ground flaming, at a maximum travel distance of 10 m downwind. Particles with π Γ π β€ 0.6 are found to be lofted and entrained into the fire plume with travel distances >50 m from the fire. Future investigations need to address the validation of the pyrolysis model and invoking a model for char oxidation to allow analysis of far-travelling firebrands.}},
author = {{Menzemer, Leo Willem}},
keywords = {{Wildfire; spotting; Fire Brands; Numerical study; Fire Dynamics Simulator (FDS); Wildland-urban interface (WUI); Fire}},
language = {{eng}},
publisher = {{International Master of Science in Fire Safety Engineering}},
title = {{Numerical Simulations of Brand Transport in Large Outdoor Fires}},
url = {{https://lup.lub.lu.se/search/files/193178262/Leo_Menzemer_Master_thesis.pdf}},
year = {{2020}},
}