Browsing by Author "Fan, WF"

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Downward flame spread over a thick PMMA slab in an opposed flow environment: experiment and modeling
(2003-03-01)
Wu, KK
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Fan, WF
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Chen, CH
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Liou, TM
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Pan, IJ
This work investigates experimentally and theoretically the downward spread of a flame over a thick polymethylmethacrylate (PMMA) slab with an opposed flow of air. Simulation results, using an unsteady combustion model with mixed convection, indicate that the ignition delay time increases with a decreasing opposed-flow temperature or increasing velocity. The ignition delay time is nearly constant at a low opposed flow velocity, i.e., (u(&INFIN;)) over bar less than or equal to 30 cm/s. Experiments were conducted at three different opposed flow temperatures and velocities, namely, (T-i) over bar = 313, 333, and 353 K and (u(&INFIN;)) over bar= 40, 70, and 100 cm/s, respectively. Measurements included the flame-spread rate and temperature distributions, using thermocouples and laser-holographic interferometry. The qualitative trends of the flame-spread rate and thermal boundary layer thickness, as obtained experimentally and from numerical predictions, were identical. For a quantitative comparison, the predicted and experimental flame-spread rates correlated well with each other, except at the lowest velocity ((u(&INFIN;)) over bar = 40 cm/s). The discrepancies between the measured and predicted thermal boundary layer thicknesses decreased with an increasing flow velocity. The quantitative agreement with a high velocity indicates that the spread of an opposed flame is mainly controlled by the flame front, whereas the discrepancies at low flow rates demonstrate the importance of radiation, the finite length of the fuel, and also three-dimensional effects, which were not considered in the model. The temperature profiles around the flame front measured by interferometric photographs indicate a recirculation flow there, as predicted by the simulation. (C) 2003 The Combustion Institute. All rights reserved.
Numerical analyses for radiative ignition and transition to flame spread over a horizontally oriented solid fuel in a gravitation field
(2001)
Lin, PH
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Fan, WF
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Chen, CH
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機械工程學系
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Department of Mechanical Engineering
This study numerically investigates the ignition behaviors of horizontal-oriented cellulosic materials subjected to radiant heat flux under a natural convective environment. This process can be divided into two stages: (1) the heating-up stage, during which the maximum temperature increases with time and (2) the flame development stage, consisting of the ignition and transition processes, The ignition process is marked with a sharp increase in maximum temperature. Meanwhile, the flame is in a transition from a premixed flame into a diffused one. In the transition process, the flame propagates upstream, forming a so-called opposed flame spread. There is a time lag between solid fuel pyrolysis and the gas-phase chemical reaction. The pyrolysis front overtakes the flame front in the initial heating-up process. As soon as the flammable mixture ahead of the flame front is ignited, the flame moves forward quickly, moving ahead of the pyrolysis front. The ignition delay time is shorter for a horizontal solid fuel than for a vertical solid fuel under the same environment, because the resultant high-temperature region is not cooled and the flammable mixture is not diluted by the induced flow in a horizontal solid fuel. For the effect of changing gravity, the ignition delay time decreases with a decrease in gravity level, owing to the smaller induced flow velocity.