Embedded flame heat flux method for simulation of quasi-steady state vertical flame spread

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Embedded flame heat flux method for simulation of quasi-steady state vertical flame spread. / Li, Kaiyuan; Hostikka, Simo.

In: Fire Safety Journal, Vol. 104, 01.03.2019, p. 117-129.

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@article{0b1920179ae64f3489a82939dd5faf0a,
title = "Embedded flame heat flux method for simulation of quasi-steady state vertical flame spread",
abstract = "Flame spread has been an important but unsolved problem for decades as its numerical solution requires simultaneous accurate solutions in solid and gas phases. In the current study, a numerical model involving gas-phase Large Eddy Simulation (LES) and comprehensive solid pyrolysis model was built as the first step of the research with a sufficient resolution for reproducing the flame spread rates in a set of upward flame spread experiments on birch rods. In the current conditions, the flame spread is dominated by the convective heat transfer and the predicted spread rates are thus sensitive to the heat transfer model. The convective and radiative heat fluxes during steady state were then extracted from the model results and used to improve the accuracy of coarse mesh simulations by embedding them as thermal boundary conditions for the solid phase. Three alternative techniques were investigated: Normalizing the heat flux profile with the length of the pyrolysis zone (fully coupled technique) resulted in accurate spread rates only if the initial pyrolysis zone was specified precisely and the gas phase resolution remained sufficiently fine. In the second, uncoupled technique, the length of the pyrolysis zone was fixed. Accurate steady state flame spread rates were then achieved regardless of the mesh resolution and the initial length of the pyrolysis zone. Finally, the most promising technique involved a normalized heat flux profile within the pyrolysis zone and a generalized heat flux function within the preheating zone. Correct spread rates were achieved with up to ten times increase in cell size regardless of the initial pyrolysis zone properties.",
keywords = "Flame spread, Wood, Pyrolysis, Numerical modelling, Heat flux, LES",
author = "Kaiyuan Li and Simo Hostikka",
year = "2019",
month = "3",
day = "1",
doi = "10.1016/j.firesaf.2019.01.011",
language = "English",
volume = "104",
pages = "117--129",
journal = "Fire Safety Journal",
issn = "0379-7112",

}

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TY - JOUR

T1 - Embedded flame heat flux method for simulation of quasi-steady state vertical flame spread

AU - Li, Kaiyuan

AU - Hostikka, Simo

PY - 2019/3/1

Y1 - 2019/3/1

N2 - Flame spread has been an important but unsolved problem for decades as its numerical solution requires simultaneous accurate solutions in solid and gas phases. In the current study, a numerical model involving gas-phase Large Eddy Simulation (LES) and comprehensive solid pyrolysis model was built as the first step of the research with a sufficient resolution for reproducing the flame spread rates in a set of upward flame spread experiments on birch rods. In the current conditions, the flame spread is dominated by the convective heat transfer and the predicted spread rates are thus sensitive to the heat transfer model. The convective and radiative heat fluxes during steady state were then extracted from the model results and used to improve the accuracy of coarse mesh simulations by embedding them as thermal boundary conditions for the solid phase. Three alternative techniques were investigated: Normalizing the heat flux profile with the length of the pyrolysis zone (fully coupled technique) resulted in accurate spread rates only if the initial pyrolysis zone was specified precisely and the gas phase resolution remained sufficiently fine. In the second, uncoupled technique, the length of the pyrolysis zone was fixed. Accurate steady state flame spread rates were then achieved regardless of the mesh resolution and the initial length of the pyrolysis zone. Finally, the most promising technique involved a normalized heat flux profile within the pyrolysis zone and a generalized heat flux function within the preheating zone. Correct spread rates were achieved with up to ten times increase in cell size regardless of the initial pyrolysis zone properties.

AB - Flame spread has been an important but unsolved problem for decades as its numerical solution requires simultaneous accurate solutions in solid and gas phases. In the current study, a numerical model involving gas-phase Large Eddy Simulation (LES) and comprehensive solid pyrolysis model was built as the first step of the research with a sufficient resolution for reproducing the flame spread rates in a set of upward flame spread experiments on birch rods. In the current conditions, the flame spread is dominated by the convective heat transfer and the predicted spread rates are thus sensitive to the heat transfer model. The convective and radiative heat fluxes during steady state were then extracted from the model results and used to improve the accuracy of coarse mesh simulations by embedding them as thermal boundary conditions for the solid phase. Three alternative techniques were investigated: Normalizing the heat flux profile with the length of the pyrolysis zone (fully coupled technique) resulted in accurate spread rates only if the initial pyrolysis zone was specified precisely and the gas phase resolution remained sufficiently fine. In the second, uncoupled technique, the length of the pyrolysis zone was fixed. Accurate steady state flame spread rates were then achieved regardless of the mesh resolution and the initial length of the pyrolysis zone. Finally, the most promising technique involved a normalized heat flux profile within the pyrolysis zone and a generalized heat flux function within the preheating zone. Correct spread rates were achieved with up to ten times increase in cell size regardless of the initial pyrolysis zone properties.

KW - Flame spread

KW - Wood

KW - Pyrolysis

KW - Numerical modelling

KW - Heat flux

KW - LES

UR - http://www.scopus.com/inward/record.url?scp=85061330878&partnerID=8YFLogxK

U2 - 10.1016/j.firesaf.2019.01.011

DO - 10.1016/j.firesaf.2019.01.011

M3 - Article

VL - 104

SP - 117

EP - 129

JO - Fire Safety Journal

JF - Fire Safety Journal

SN - 0379-7112

ER -

ID: 31848133