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Unformatted text preview: 5.2. STOICHIOMETRY 161 5.2.2 Fuel-air mixtures In combustion with air, one often models air as a simple mixture of diatomic oxygen and inert diatomic nitrogen: ( O 2 + 3 . 76 N 2 ) . (5.100) The air-fuel ratio , A and its reciprocal, the fuel-air ratio , F , can be defined on a mass and mole basis. A mass = m air m fuel , A mole = n air n fuel . (5.101) Via the molecular masses, one has A mass = m air m fuel = n air M air n fuel M fuel = A mole M air M fuel . (5.102) If there is not enough air to burn all the fuel, the mixture is said to be rich . If there is excess air, the mixture is said to be lean . If there is just enough, the mixture is said to be stoichiometric . The equivalence ratio , , is defined as the actual fuel-air ratio scaled by the stoichiometric fuel-air ratio: F actual F stoichiometric = A stoichiometric A actual . (5.103) The ratio is the same whether F s are taken on a mass or mole basis, because the ratio of molecular masses cancel....
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