| Summary: | 碩士 === 國立成功大學 === 機械工程學系碩博士班 === 98 === The objectives of the present research were to study the mechanisms of flame acceleration and deflagration-to-detonation transition (DDT) in microscale tubes and channels. Both 3D transient reacting flow simulations with detailed chemical mechanisms and experimental approaches were applied. High speed cinematography, schlieren visualization, and soot film method are utilized to reveal the flame evolution, flame structure, and detonation cell structure, respectively.
In the numerical study, the effects of wall thermal boundary condition, tube diameter, shape of the channel cross-section, equivalence ratio, and ignition configuration on the flame propagation were investigated. Adiabatic wall, smaller diameter, circular cross-section, stoichiometric mixture, and ring-shaped ignition configuration were found to be able to faciliate DDT. Since turbulence was not modeled in the simulation, the results implied that the existence of turbulence was not essential for DDT to occur. The mechanism of DDT in microscale tubes relied on the choking effect. The wall constraint and expansion of the burned gas resulted in a pre-compression to the upstream unburned gas, which enhanced mass burning rate. The compression was further enhanced by the increased amount of burned gas. A local explosion eventually occurred due to the excessive compression, and deflagration-to-detonation transition was triggered.
In the experiments, stoichiometric ethylene/oxygen mixtures were diluted with nitrogen, argon, helium, and carbon dioxide to study to the effect of dilution on flame propagation in microscale channels. The results showed that flame emission intensities decreased. Asymmetrical flame propagation was observed for high dilution ratio. Tulip flame was observed using schlieren method during the early stage of hydrogen/oxygen flame propagation. For stoichiometric ethylene/oxygen mixture diluted with 10 % nitrogen, soot film visualization revealed that detonation cells were about 0.75 mm in the early stage of detonation, and developed to approximately 1.75 mm in stable detonation wave propagation regime.
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