| Summary: | 博士 === 國立交通大學 === 機械工程系 === 87 === The research topics in this dissertation consist of three parts. In part one, a time-dependent, two-dimensional combustion model was developed to describe the radiant ignition and subsequent transition to downward flame spread over a vertical thermally-thin solid fuel in normal gravity. An external radiant flux with a strength of 5 W/cm2 and a half width of 1 cm is used as the heat source to ignite cellulosic material in a quiescent environment with 23.3% oxygen concentration. The process consists of a heating stage and a flame developing stage, separated by an ignition. The solid fuel temperature rises as it is subjected to an incident heat flux. Later, pyrolysis generates enough fuel vapor to form the flammable mixture for ignition. A hot gas layer is generated adjacent to the heated surface and a natural convection is established nearby. During the ignition, a premixed flame propagates into the unburned mixture. Then, the flame spreads downward to further support itself. The spreading flame is mainly a diffusion flame. Eventually, a steady flame spread is reached. The ignition delay time is decreased as the peak flux value or the oxygen index increases. There are two limiting peak flux values, and a lower bound for oxygen concentration beyond which no ignition can occur.
In part two, the transient combustion model developed in part 1 was adopted but modified with a moving boundary of burnout point. It was used to study the downward flame spread, subjected to an opposed induced flow resulting from the buoyancy, over a thermally-thin solid fuel in various gravitational acceleration. The emphasis was on the transient period since the ignition behaviors were not affected by the removal of ash. Seven parametric cases at various gravity levels ranging from 0.5 to 5.8 times normal gravity under 23.3% oxygen concentration environment were conducted. At g=5.0, it was identified as an unstable flame spread. At g=5.8, blowoff extinction, occurred at 9.64s after irradiation, was predicted. Ignition delay time was found to increase with an increase in gravity level. Comparing to the measurements obtained by Altenkirch et al. (1980) and the prediction by steady model of Duh and Chen (1991), the present computed flame spread rates show excellent agreement with measurements in the experimental domain ranged from 1 and 4.25 . The predicted blowoff limiting value at 5.8 appears closer to the experimental value at 4.25 than that predicted by steady model, whose value is at 11 .
In part three, investigates how radiation heat transfer influences downward flame spread by presenting a gas phase radiation model, described by a two dimensional P-1 approximation method, to incorporate with the combustion model of Duh and Chen (1991). The parametric study is based on the variation of gravity, which changes the Damkohler number (Da) and radiation to conduction parameter (1/N ) simultaneously. Comparing the results with the previous studies of Duh and Chen (1991) and Chen and Cheng (1994), which only considered the radiation effect in cross stream direction, the role of stream-wise radiation was identified. The stream-wise radiation contributes to reinforce the forward heat transfer rate subsequently increasing the flame spread rate. However, this model also provides more directional radiation loss than that of Chen and Cheng (1994) and, in doing so, draws more energy out from the flame to further reduce its strength. The results indicates that the effect of heat loss is greater than that of enhancing the upstream heat transfer since the flame spread rate in the present model is always lower than the one predicted by Duh and Chen (1991). Finally, a contour of the Planck mean absorption coefficient distribution is illustrated to demonstrate the effectiveness of gas radiation distribution. It reveals that the strongest radiation occurs near the pyrolyzing surface and the other significant one is in the plume region.
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