| Summary: | 博士 === 國立交通大學 === 機械工程系 === 91 === This study consists of four parts. The first part studied 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 run-away, occurs as soon as a flammable mixture is formed and the gas phase temperature becomes high enough. During the induction period, the reactivity and temperature in gas phase are mutually supportive. The thermal run-away consists of a premixed-flame burning. This is followed by a transition from a premixed flame into a diffusion flame. Finally, steady flame spread takes place as burnout appears. The ignition delay time is mainly controlled by the flammable mixture formation time, and is independent of the induced flow strength and ambient oxygen index. The ignition delay time increases with the solid fuel thickness or a decrease of the incident peak heat flux. The steady downward flame-spread rate decreases with an increase in the gravity level or fuel thickness and a decrement of the ambient oxygen index, but is independent of the incident peak heat flux. The blow-off limit is around 6.7ge, and the extinction limit is Yo=0.131.
In the second part, the downward flame spread over a thick PMMA slab in mixed convection environment was studied theoretically. Simulation results indicate that the ignition delay time increases with decreasing opposed flow temperature or increasing velocity. the ignition delay time is nearly constant with a low opposed flow velocity, i.e. the opposed flow velocity is less than 30 cm/s. The qualitative variation trends of the flame-spread rate and thermal boundary layer thickness are identical between Pan’s measurements (1999) and the numerical predictions. From the perspective of quantitative comparison, the predicted and experimental flame-spread rates correlate well with each other, except at a low velocity regime. The discrepancies in thermal boundary layer thickness decrease with increasing flow velocity. The quantitative agreement in high velocity regimes indicates that the opposed flame spread is mainly controlled by the stream-wise heat conduction in flame front, whereas the discrepancies in the low velocity regime demonstrate the importance of radiation, finite fuel length and 3-D effects, which are not considered in the combustion model. Recirculation ahead of the flame front is predicted by the simulation and confirmed by Pan’s experiment (1999).
In the third part, the flame spread phenomena over a thermally thin solid fuel, where the radiation from solid is under consideration, in a quiescent, zero gravity environment was studied numerically. A derivation of critical thickness for flame extinction including radiation effect was given. Under fixed the ambient oxygen index, a dividing point is identified to distinguish the flame-spread rate into two regimes against the solid fuel thickness. For fuel thickness lessthan 0.006cm , the flame-spread rate decreases with fuel thickness. The flame is extinction when fuel thickness is lower than a critical value. For fuel thickness greatthan 0.006cm , the flame-spread rate gradually decreases as fuel thickness decreases. The controlling mechanisms in these two regimes are fuel control and heat transfer control, respectively. The flame is extinguished due to radiation loss as fuel thickness is great than extinction thickness, derived by using a theoretical analysis. The responses of solid fuel, including temperature, density, vaporized mass flux, conduction heat flux from flame to the solid fuel, solid radiation heat flux and net heat flux are presented as well. The flame-spread rate increases with ambient oxygen index. The predicted extinction limit is at Yo=0.195. The standoff distance increases with a decrease of ambient oxygen index. This result consists with experimental results of Olson (1987).
In the last part, the radiation effects for downward flame spread over a thermally thin solid fuel in partial gravity environment were explored. The simulation results indicate that the ignition delay time is influenced slightly by the radiation effect and gravity level. The flame-spread rate reaches a maximum at g = 0.01 and then decreases in spite of increasing or decreasing gravity level. For g > 0.01, the flame behaviors are mainly dominated by flame stretch effect. Radiation heat transfer and oxygen transport control the flame behaviors for g < 0.01. The predicted radiation quench limit is g=5x10E-6 that is close to the experimental results (Sacksteder and T’ien, (1994)). The radiation has two kinds of contributions simultaneously. One is to reduce the flame strength by losing heat to the ambient. The other one is to join the upstream conduction to enhance the total forward heat transfer rate and the subsequently preheat upstream virgin fuel. The solid temperature is low and the fuel leftover phenomenon is apparent in low gravity level due to radiation. Based on energy analyses, the conduction heat flux from the flame dominates the flame behaviors. However, the radiation effect can compete gradually with conduction heat flux when the gravity level decreases continuously.
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