OCR Text |
Show where r - is a constant that may depend on the flame structure and the rate of reaction between fuel and oxidant; mf is the amount of fuel remaining; e is the rate of dissipation of the turbulent kinetic energy; and k is the turbulent kinetic energy. In fuel-rich regions, the fuel concentration is high, the oxidant concentration is low (oxidant is mostly intermittent), and the rate of combustion can be expressed as: m "2-^dfl (24) where in is the amount of oxidant remaining, and r is the stoichiometric oxidant to burn 1 kg of fuel. In premixed flames, oxidant and fuel can exist in the same eddy. These eddies will be separated by eddies containing hot products. The rate of combustion will be determined by local fire spread. The dissipation of the hot eddies must be modeled when the concentration of hot combustion products is low. This gives rise to the rate which follows as: Rl,3 »" --pprrc l,"-rrc „2 Jr-+&T- STc (25 where r 2 is a constant, and in is the local concentration of the products. The three reaction rates are applicable to premixed and nonpremixed flames. The minimum rate is controlling and determines the local rate of combustion. Kinetic Reduction of Carbon Monoxide A conservation equation for the average concentration of carbon monoxide is found by averaging the instantaneous conservation equation over a probability -12- O |