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Author: National Aeronautics and Space Administration (NASA)
Publisher: Createspace Independent Publishing Platform
ISBN: 9781722903831
Category :
Languages : en
Pages : 158
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Book Description
A numerical model is developed to examine laminar flame spread and extinction over a thin solid fuel in lowspeed concurrent flows. The model provides a more precise fluid-mechanical description of the flame by incorporating an elliptic treatment of the upstream flame stabilization zone near the fuel burnout point. Parabolic equations are used to treat the downstream flame, which has a higher flow Reynolds number. The parabolic and elliptic regions are coupled smoothly by an appropriate matching of boundary conditions. The solid phase consists of an energy equation with surface radiative loss and a surface pyrolysis relation. Steady spread with constant flame and pyrolysis lengths is found possible for thin fuels and this facilitates the adoption of a moving coordinate system attached to the flame with the flame spread rate being an eigen value. Calculations are performed in purely forced flow in a range of velocities which are lower than those induced in a normal gravity buoyant environment. Both quenching and blowoff extinction are observed. The results show that as flow velocity or oxygen percentage is reduced, the flame spread rate, the pyrolysis length, and the flame length all decrease, as expected. The flame standoff distance from the solid and the reaction zone thickness, however, first increase with decreasing flow velocity, but eventually decrease very near the quenching extinction limit. The short, diffuse flames observed at low flow velocities and oxygen levels are consistent with available experimental data. The maximum flame temperature decreases slowly at first as flow velocity is reduced, then falls more steeply close to the quenching extinction limit. Low velocity quenching occurs as a result of heat loss. At low velocities, surface radiative loss becomes a significant fraction of the total combustion heat release. In addition, the shorter flame length causes an increase in the fraction of conduction downstream compared to conduction to the fuel. The...
Author: National Aeronautics and Space Administration (NASA)
Publisher: Createspace Independent Publishing Platform
ISBN: 9781722903831
Category :
Languages : en
Pages : 158
Get Book Here
Book Description
A numerical model is developed to examine laminar flame spread and extinction over a thin solid fuel in lowspeed concurrent flows. The model provides a more precise fluid-mechanical description of the flame by incorporating an elliptic treatment of the upstream flame stabilization zone near the fuel burnout point. Parabolic equations are used to treat the downstream flame, which has a higher flow Reynolds number. The parabolic and elliptic regions are coupled smoothly by an appropriate matching of boundary conditions. The solid phase consists of an energy equation with surface radiative loss and a surface pyrolysis relation. Steady spread with constant flame and pyrolysis lengths is found possible for thin fuels and this facilitates the adoption of a moving coordinate system attached to the flame with the flame spread rate being an eigen value. Calculations are performed in purely forced flow in a range of velocities which are lower than those induced in a normal gravity buoyant environment. Both quenching and blowoff extinction are observed. The results show that as flow velocity or oxygen percentage is reduced, the flame spread rate, the pyrolysis length, and the flame length all decrease, as expected. The flame standoff distance from the solid and the reaction zone thickness, however, first increase with decreasing flow velocity, but eventually decrease very near the quenching extinction limit. The short, diffuse flames observed at low flow velocities and oxygen levels are consistent with available experimental data. The maximum flame temperature decreases slowly at first as flow velocity is reduced, then falls more steeply close to the quenching extinction limit. Low velocity quenching occurs as a result of heat loss. At low velocities, surface radiative loss becomes a significant fraction of the total combustion heat release. In addition, the shorter flame length causes an increase in the fraction of conduction downstream compared to conduction to the fuel. The...
Author: Paul Vincent Ferkul
Publisher:
ISBN:
Category :
Languages : en
Pages : 160
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Book Description
Author: Ching-Biau Jiang
Publisher:
ISBN:
Category : Flame spread
Languages : en
Pages : 312
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Book Description
Author: Hsin-Yi Shih
Publisher:
ISBN:
Category :
Languages : en
Pages : 194
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Book Description
Author: Hsin-Yi Shih
Publisher:
ISBN:
Category :
Languages : en
Pages : 318
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Book Description
Author: Ama R. Carney
Publisher:
ISBN:
Category : Aerospace engineering
Languages : en
Pages : 125
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Book Description
Microgravity experiments are performed to study wind-assisted flame spread over discrete fuel elements. Ultra-thin cellulose-based fuel segments are distributed uniformly in a low-speed flow and flame spread is initiated by igniting the most upstream fuel segment. Similar to continuous fuels, flame spread over discrete fuels is a continual process of ignition. Flame propagation across a gap only occurs when a burning fuel segment, before it burns out, ignites the subsequent segment. During this process, gaps between samples reduce the fuel load, increasing the apparent flame spread rate and decreasing the heat transfer between adjacent segments. The reduction in heat transfer decreases the solid burning rate. In this study, sample segment length, gap size, and imposed flow velocity are varied to study the impacts on burning characteristics, including propensity of flame spread, flame spread rate, and solid burning rate. Detailed profiles of the transient flame spread process are also presented.
Author: Gary David Grayson
Publisher:
ISBN:
Category :
Languages : en
Pages : 166
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Book Description
Author: Hai-Tien Loh
Publisher:
ISBN:
Category :
Languages : en
Pages : 318
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Book Description
Author: Jeanhyuk Park
Publisher:
ISBN:
Category : Flame spread
Languages : en
Pages : 131
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Book Description
Building fire, Forrest fire, and warehouse compartment fire are some of the most frequently occurring practical fire hazards in modern world. Although these types of hazards seem irrelevant from one another, they have some things in common from the perspective of fire protection engineering, in that they all have a very similar fundamental fuel-gap configuration, or discrete fuel configuration. There has been some studies in the past regarding the subject, yet it is not the most popular in the field. Furthermore, there is even fewer, if not any, numerical analysis done to fires in discrete fuel configuration. Discrete fuel arrangements represent some practical fire hazard situations, such as compartment fires in enclosed vehicles. In this study, an unsteady two-dimensional numerical model (Fire Dynamics Simulator) was used to simulate concurrent flame spreadover paper-like thin solid fuels in discrete configurations in microgravity (0g, where a20cm/s flow is imposed) and in normal gravity (1g). An array of ten 1cm-long fuel segments is uniformly distributed in the flow direction (0g) or in the vertical direction (1g).A hot spot ignition source is applied at the upstream leading edge of the first fuel seg-ment. The separation distance between the fuel segments is a parameter in this study, ranging from 0 (corresponding to a continuous fuel) to 3cm. Using this setup, the spread rate of the flame base and the fuel burning rate were studied. The spread rate in 1g and 0g increases with increasing separation distance. This is due to the gaps in the discrete fuel that force the flame base to jump to the subsequent fuel segment when the upstream segment burns out. On the other hand, the fuel burning rate behaves differently in 1g versus 0g. At a flow velocity of 20 cm/s in 0g, the flame reaches a limiting length and the flame length is approximately the same ( 4cm) for all fuel configurations. Therefore, as the separation distance increases, the preheating length (the fuel area exposed to the flame) decreases, resulting in a smaller burning rate. In 1g, the buoyancy driven flow accelerates as it rises, resulting in a longer flame as the separation distance increases. In all simulated configurations, the flame extends to the last fuel segment before the first fuel segment burns out and the flame spans the entire set of fuel segments. However, flame standoff distance reduces at the gaps between fuel segments, and in some con-figurations, the flame breaks into multiple flamelets. The shorter standoff distance and intense burning at each flamelet base result in a larger total burning rate as the separation distance increases.
Author: Liming Zhou
Publisher:
ISBN:
Category : Ceilings
Languages : en
Pages : 480
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Book Description
The flow turbulence also has a significant effect on the flame extinction conditions, resulting in a smaller extinction velocity for larger flow turbulence intensity. For concurrent flow flame spread, it is found that the flow turbulence decreases the flame spread rate for both floor and ceiling geometries, mainly as a result of the flame length shortening at high turbulence intensity. It is also found that flow velocity intensifies the spread of the flame. The experimental data of flame spread rate, flame length and surface heat flux agree well with the formula obtained from a simplified thermal model, indicating that the heat transfer from flame to solid surface is the dominant controlling mechanism in the turbulent concurrent flame spread and, that the gas phase chemical reaction is of secondary importance.
