Combustion - 國立中央大學機械工程學系

Ch 1 Introduction to Combustion ME776Sec. 4 Classifications of Fundamental Combustion Phenomena1.4.1 Premixed and Nonpremixed Combustion• Most important classification of combustion phenomena.• Reactions generally involve two or more reactants.Frequently a fuel & an oxidizer( molecular mixedness )Essential mixed at mlc levelElementsReaction implies that at least one of the reactantsshould be in either the gaseous or theliquid phase so that its mlc. can spreadaround those of the other reactant.• Premixed system : A + B 2B( A : fuel + oxidizer ; B : heat and/or radicals )• Nonpremixed system: Diffusional combustion reactants initiallyseparated, then being brought together, via molecular process ofdiffusion and the bulk convective motion, to a common region wheremixing and subsequently reaction take place.• Diffusion is still essential in transporting the premixture to, and thethermal energy and combustion products away from, the flame regionwhere reactions occur.• Bunsen Flames• As the fuel gas issues from the orifice, air is entrained throughthe adjustable air-intake port and is then mixed with the fuel gasas they travel along the burner tube.• The subsequent reactions between fuel and oxygen in thispremixture forms a premixed flame.• If this premixture is fuel rich (has a high concentration of fuelthan can be consumed by all the oxygen in the entrained air),after passing through the premixed flame, the excess fuel (orrather the fuel-related intermediate species, can further react withthe oxygen in the ambient air.Reactants initially separated need to be transported to acommon region where mixing and reactions occurDiffusion flame.The outwardly-directed excess fuel reacts almost completelyw/ the inwardly-directed oxygen.• It is obvious that one would not find many examples of premixtures innature, because they would have already reacted even if they are onlyslightly reactive.13