Browsing by Author "Wu, KK"

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A numerical analysis of ignition to steady downward flame spread over a thin solid fuel
(2003-05-01)
Wu, KK
;
Chen, CH
;
機械工程學系
;
Department of Mechanical Engineering
A numerical analysis using an unsteady combustion model is presented to study the ignition and subsequent downward flame spread over a thermally thin solid fuel in a gravitational field. The solid-fuel temperature rises gradually in the heat-up stage and the pyrolysis becomes more intense. Ignition, including the induction period and thermal runaway, occurs as soon as a flammable mixture is formed and the gas-phase temperature, heated by the solid fuel, becomes high enough. During the induction period, the reactivity and temperature in the gas phase are mutually supportive. The thermal runaway consists of a burning premixed flame as the flow moves with the flame front. This is followed by a transition from a premixed flame into a diffusion flame. The flame front extends along and toward the upstream virgin fuel as the diffusion flame is formed. Finally, steady flame spread takes place as burnout appears. The ignition delay time is found to be controlled mainly by the time required to form the flammable mixture and is almost independent of the gravity level and the ambient oxygen index. The ignition delay time increases nearly linearly with an increase in solid-fuel thickness within the range of 0.005 cm less than or equal to (τ) over bar less than or equal to 0.02 cm and is proportional to ((Q) over bar (max))(-1.11) within 2W/cm(2) less than or equal to (Q) over bar (max) less than or equal to 8 W/cm(2). The steady downward flame-spread rate decreases with increases in the gravity level or fuel thickness and with decreases in the ambient oxygen index but is independent of the incident peak heat flux. The blowoff limit is around 6.7 (g) over bar (e) and the extinction limit is found to be Y (Oinfinity) = 0.131.
Downward flame spread over a thick PMMA slab in an opposed flow environment: experiment and modeling
(2003-03-01)
Wu, KK
;
Fan, WF
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Chen, CH
;
Liou, TM
;
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.
Radiation effects for downward flame spread over a thermally thin fuel in a partial-gravity environment
(2004-11-01)
Wu, KK
;
Chen, CH
;
機械工程學系
;
Department of Mechanical Engineering
This study explores the effects of radiation on a downward flame spread over a thermally thin solid fuel in a partial-gravity environment. The radiation effect and gravitational field strength (g) are predicted not to influence the ignition delay time. The flame-spread rate reaches a maximum at g = 0.01. At g > 0.01, the flame stretch effect dominates the behaviors of the flame. Radiation heat transfer and oxygen transport control the flame behaviors for g < 0.01. The predicted quenching limit is g = 5 x 10(-6), close to the value obtained experimentally elsewhere. Radiation has two simultaneous effects. One is to reduce the flame strength by carrying heat to the ambient. The other one is combined with upstream conduction to enhance the total forward heat transfer rate and thus preheat virgin fuel upstream. The solid fuel temperature is low and some fuel is left over in low gravity due to radiation loss. Energy analyses indicate that the conduction heat flux from the flame (<(q(c))over bar>) dominates its behaviors. However, radiation gradually competes with (q(c)) over bar as the gravity is reduced.