Combustion - 國立中央大學機械工程學系

Finite reaction rates >> Thermodynamic equilibrium• All combustion processes have some finite, characteristic times definingour interest in the phenomena.• Chemical Kinetics is needed to prescribe the paths and rates.• Thermodynamics can be considered to be a special area of chemicalkinetics in that with infinite time a reaction will eventually achieveequilibrium.• A complex subject : a myriad of chemical species exist, each of which hasthe potential of interacting w/ the rest.• Some confidence on fuel oxidation system only for hydrogen, CO, and thelight alkanes.1.3.3 Fluid MechanicsChemical reactions occurring in a flowing medium.Combustion: Knowledge of F.M. is essential for a successfulunderstanding of many combustion phenomena.Highly-localized and exothermic nature of chemical reactions →significant temperature, and therefore density variations in a flow,implying that fluid incompressibility can be a rather poor assumption.1.3.4 Transport PhenomenaS L ~ 40 cm/s ; D ~ 0.2 cm 2 /sδ ~ 0.2 / 40 = 5×10 -3 cm = 0.05 mm比 一 根 頭 髮 還 細• Transfer of energy & mass from high to low; through the molecular process ofdiffusion.• For heat transfer, radiation is also important.• Diffusive transport is crucial in the sustenance of many types of flames in that itis only through these processes fresh reactants can be continuously supplied tothe flame while the heat generated there is also being continuously used to heatup and thereby cause ignition of these fresh mixture.12