OCR Text |
Show reduced temperature level of burned gases as well as its recycling flow rate. Combustion with normal ambient air usually becomes unstable when the exhaust gas recycling rate, defined as the mass ratio between exhaust gas to fresh reactants, exceeds 30 % [26]. As is shown in Fig.4, however, stable combustion domain appears for high rates of exhaust gas recycling, if combustion air is preheated exceeding the autoignition temperature of the fuel. Actually, very diluted air with burned gases, whose oxygen concentration as low as 3%, can sustain combustion when it is preheated up to 1200 K. ed (L> I* -*-» .a < 200 o • X Ignition Boundary Stable Boundary Auto-ignition point Non Auto-ignition point Unstable Point (co<100) 20 15 10 0 0 2 concentration Fig.4 Autoignition temperature of propane with air and air diluted with nitrogen. The influences of preheated temperature and oxygen content in diluted combustion air are shown in Fig.5, where temperature profile in laminar diffusion flames between fuel of ambient temperature and preheated air or its dilution with combustion products in counter flow configuration are drawn. The peak temperature in flames increases with the preheating of combustion air (Fig.5(a)), although the rise in peak temperature becomes smaller due to dissociation of combustion products in high temperature range. The variation of peak |