3. Combustion Dynamics _ Flame Chemistry (Notes).pdf

1Carleton University, 82.575 (CVG7300), Fire Dynamics I, Winter 2002 Lecture # 3 1 Fire Dynamics I Lecture # 3 Combustion Dynamics Flame Chemistry Jim Mehaffey 82.575 or CVG7300 Carleton University, 82.575 (CVG7300), Fire Dynamics I, Winter 2002 Lecture # 3 2 Combustion Dynamics Flame Chemistry Outline ? Incomplete combustion ? Mechanisms of gas phase combustion ? Free radicals (scavenging) ? Temperature of flames ? Life safety considerations 2Carleton University, 82.575 (CVG7300), Fire Dynamics I, Winter 2002 Lecture # 3 3 Heat Release Rate: Idealized ? In Lecture # 2, assumed complete combustion: Fuel burns and CO2 H2O are generated. True if fuel burns in excess of pure O2. ? Net heat of combustion = HC (kJ / g) ? Heat release rate / unit area is (kW m-2) Eqn (3-1) ? For complete combustion get the theoretical maximum heat release rate / unit area mH C ? = Q max ? Carleton University, 82.575 (CVG7300), Fire Dynamics I, Winter 2002 Lecture # 3 4 Heat Release Rates: Well-ventilated Fires ? Well-ventilated fires experience incomplete combustion so: CO2, H2O, CO, hydrocarbons soot are generated. ? Reduction in combustion efficiency means net heat of combustion is not released ? Actual (chemical) heat release rate / unit area is (kW m-2) Eqn (3-2) Hch = Actual (chemical) heat of combustion (kJ / g) mH ch ? = Q chem ? 3Carleton University, 82.575 (CVG7300), Fire Dynamics I, Winter 2002 Lecture # 3 5 Heat Release Rates: Well-ventilated Fires ? Combustion efficiency is defined as χ = Hch / HC Eqn (3-3) ? Rate heat is convected above flame: (kW m-2) Eqn (3-4) ? Rate heat is radiated away by flame: (kW m-2) Eqn (3-5) mH con ? = Q conv ? Q rad ? mH rad ? = Carleton University, 82.575 (CVG7300), Fire Dynamics I, Winter 2002 Lecture # 3 6 Heat Release: Well-ventilated Fires (2) M aterial H C (kJ/g) H ch (kJ/g) H con (kJ/g) H rad (kJ/g) M ethane 50.1 49.6 42.6 7.0 E thane 47.1 45.7 34.1 11.6 Propane 46.0 43.7 31.2 12.5 B utane 45.4 42.6 29.6 13.0