| Summary: | 碩士 === 國立交通大學 === 機械工程系 === 89 === An unsteady combustion model is developed and solved numerically to investigate the ignition and subsequent flame development behaviors over a vertically-oriented cellulosic thick fuel, subjected to a specified incident heat flux in a forced convection environment. The whole process is divided into two distinct stages, which are heating up and flame development, respectively. In the heating up stage, the maximum temperature, occurred at interface, increases with time. The flame development stage consists of ignition and transition processes. Ignition includes an induction period and a thermal run away process. During the induction period, a flammable fuel/oxidizer mixture is establishing at the pyrolyzing fuel surface, but chemical reaction is not strong enough to generate significant heat. Upon to thermal run away process, a burning of premixed flame, the temperature raises sharply. In transition process, the flame initially is in a transition from a premixed flame to a diffusion flame. Subsequently, flame starts to spread downward and upward, simultaneously, and grows with time. A steady downward flame spread is reached eventually. The parametric study is based on the variation of the incoming flow temperature and velocity and the peak of imposed heat flux, respectively. Results show that the ignition delay time and the maximum interface temperature at the instant of ignition decrease with an increase of the incoming flow temperature, and steady downward flame spread rate shows the opposite trend. Considering on the effect of the incoming flow velocity, the ignition delay time is invariant under the different flow velocities and steady downward flame spread rate decreases as the opposed forced flow velocity increases. For the effect of the peak of imposed heat flux, the ignition delay time decreases and the interface maximum temperature at the instant of ignition increases as the peak heat flux increases. A comparison with Lin' results (1999) is given. It exists a recirculation flow just ahead of the flame front in the present study, whereas it does not appear at all in Lin (1999). Ignition delay time and the interface temperature at the instant of ignition are smaller than those in Lin (1999) under the same peak heat flux. Finally, a set of computations for steady and unsteady combustion models, simulating the experiments of Pan (1999) and Chen (1999), is carried out. In general, the qualitative trends are completely the same, that is, the flame spread rate increases as the opposed flow velocity decreases, the inlet flow temperature increases and the fuel thickness decreases, separately. The predicted flame spread rates have an excellent quantitative agreement with the measurements in high velocity regime.
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