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Experimental measurements of water mist systems and implications for modelling in CFD

Husted, Bjarne LU (2007)
Abstract
The use of water mist for fire extinguishment has increased rapidly in recent years. The main reason is the abandonment of halon-based extinguishing systems in favour of environmentally friendlier systems. Furthermore the use of water mist systems has spread from mainly marine applications to also include the protection of buildings. The main problem in this regard is to verify the effectiveness of the system. At present time this can only by done by full-scale tests. This is however expensive and in some cases also unrealistic and expensive when it comes to water mist systems for buildings. The aim of this thesis is to provide experimental data that can serve as a basis for simulations of the interaction of water mist and a fire and to... (More)
The use of water mist for fire extinguishment has increased rapidly in recent years. The main reason is the abandonment of halon-based extinguishing systems in favour of environmentally friendlier systems. Furthermore the use of water mist systems has spread from mainly marine applications to also include the protection of buildings. The main problem in this regard is to verify the effectiveness of the system. At present time this can only by done by full-scale tests. This is however expensive and in some cases also unrealistic and expensive when it comes to water mist systems for buildings. The aim of this thesis is to provide experimental data that can serve as a basis for simulations of the interaction of water mist and a fire and to demonstrate that CFD can predict the performance of a water mist system. The physics of water mist systems has been studied by theoretical considerations as well as experimental work. Measurements of droplet velocities, diameters and volumetric water distribution were carried out on the spray from a high-pressure system of 100 bars. Experiments have been conducted on a hollow cone nozzle without fire and with fire, as well as a full cone nozzle without fire. Relevant measurement results were obtained with Phase Doppler Anemometry and Particle Image velocimetry as well as Laser Tomography and High Speed camera. Suggestions were made for improvement of the water density apparatus. The measurements have been the basis for simulations of water mist with CFD. Initial simulations involving the complex zone around the nozzle resulted in droplets with radial velocities and insufficient transfer of momentum to the air. A new approach has been used for the simulations with the LES model in FDS 4.07. In this approach the simulations of the water mist spray is not done in the zone close to the nozzle. Instead the boundary conditions are set further downstream, based on the conducted measurements. This approach resulted in droplets and air moving downwards at relatively high velocities as expected. However, the momentum transfer is limited, and the simulations did not give sufficient mixing. Suggestion are made of how sufficient mixing can be obtained with the new approach, with regards to implementation of spray boundary conditions and treatment of the turbulence model interacting with the movement of the droplets. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Dr. Keski-Rahkonen, Olavi, VTT, Finland
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Teknik, Technological sciences, fire suppression, CFD, FDS, fire safety engineering, PDA, water mist, PIV
pages
249 pages
publisher
Department of Fire Safety Engineering and Systems Safety, Lund University
defense location
Room V:A, V-Building John Ericssons väg 1 Lund University Faculty of Engineering Sweden
defense date
2007-09-21 14:15
external identifiers
  • other:ISRN:LUTVDG-1036-SE
ISSN
1402-3504
ISBN
978-91-628-7240-3
language
English
LU publication?
yes
id
77ee3e13-3aee-40a2-8dc5-de446e26d571 (old id 548923)
date added to LUP
2007-09-10 09:23:03
date last changed
2016-09-19 08:44:58
@phdthesis{77ee3e13-3aee-40a2-8dc5-de446e26d571,
  abstract     = {The use of water mist for fire extinguishment has increased rapidly in recent years. The main reason is the abandonment of halon-based extinguishing systems in favour of environmentally friendlier systems. Furthermore the use of water mist systems has spread from mainly marine applications to also include the protection of buildings. The main problem in this regard is to verify the effectiveness of the system. At present time this can only by done by full-scale tests. This is however expensive and in some cases also unrealistic and expensive when it comes to water mist systems for buildings. The aim of this thesis is to provide experimental data that can serve as a basis for simulations of the interaction of water mist and a fire and to demonstrate that CFD can predict the performance of a water mist system. The physics of water mist systems has been studied by theoretical considerations as well as experimental work. Measurements of droplet velocities, diameters and volumetric water distribution were carried out on the spray from a high-pressure system of 100 bars. Experiments have been conducted on a hollow cone nozzle without fire and with fire, as well as a full cone nozzle without fire. Relevant measurement results were obtained with Phase Doppler Anemometry and Particle Image velocimetry as well as Laser Tomography and High Speed camera. Suggestions were made for improvement of the water density apparatus. The measurements have been the basis for simulations of water mist with CFD. Initial simulations involving the complex zone around the nozzle resulted in droplets with radial velocities and insufficient transfer of momentum to the air. A new approach has been used for the simulations with the LES model in FDS 4.07. In this approach the simulations of the water mist spray is not done in the zone close to the nozzle. Instead the boundary conditions are set further downstream, based on the conducted measurements. This approach resulted in droplets and air moving downwards at relatively high velocities as expected. However, the momentum transfer is limited, and the simulations did not give sufficient mixing. Suggestion are made of how sufficient mixing can be obtained with the new approach, with regards to implementation of spray boundary conditions and treatment of the turbulence model interacting with the movement of the droplets.},
  author       = {Husted, Bjarne},
  isbn         = {978-91-628-7240-3},
  issn         = {1402-3504},
  keyword      = {Teknik,Technological sciences,fire suppression,CFD,FDS,fire safety engineering,PDA,water mist,PIV},
  language     = {eng},
  pages        = {249},
  publisher    = {Department of Fire Safety Engineering and Systems Safety, Lund University},
  school       = {Lund University},
  title        = {Experimental measurements of water mist systems and implications for modelling in CFD},
  year         = {2007},
}