Ignition delay of non-premixed stagnation-point flows

Ignition delay of stagnation-point oxidizing flows over a wall with the injection of fuel is analyzed numerically. The validity of various criteria of ignition delay, i.e., the adiabaticity criterion and the thermal runaway criteria (∂2Tmax/∂t2=0 and ∂2ωmax/∂t2=0), is investigated for the problems of cold flow/hot wall and hot flow/cold wall. For cold flow/hot wall systems, the ignition delay decreases with the mass flux of fuel (mw) if mw is below a critical value (mw,c). The ignition delay is kinetically controlled for mwmw,c, the ignition delay increases with mw and is diffusionally controlled by the deficient oxidizer. The adiabaticity criterion is suggested from the viewpoints of practice and simplicity. For hot flow/cold wall systems, the ignition delay decreases with mw and is diffusionally controlled by the deficient fuel. The criterion of ∂2ωmax/∂t2=0 is suggested both qualitatively and quantitatively. In addition, the effects of flow strain rate, Lewis numbers and Prandtl number on ignition delay are investigated.