Abstract
The ignition delay of combustible gas in stagnation-point flows is analyzed numerically in this dissertation. This dissertation includes two parts. The first part discusses the phenomena of ignition delay of stagnation-point oxidizing flows over a wall with the injection of fuel. Then, the transient ignition process of premixed stagnation-point flows over a catalytic surface of a solid plate with a finite thickness is investigated in the second part.In the first part, the validity of various criteria of the ignition delay, such as the adiabaticity criterion and the runaway criteria, is investigated for the problems of cold flow/hot wall and hot flow/cold wall, respectively. For cold flow/hot wall systems, the ignition delay decreases with the mass flux of fuel and the ignition delay is chemically kinetically controlled if values of mass flux are below a critical value. For values of mass flux great than the critical value, the ignition delay increases with mass flux and is diffusionally controlled by the deficient oxidizer of the flow. From the viewpoints of practice and simplicity, the adiabaticity criterion suggested to be used in the cold flow/hot wall systems. For hot flow/cold wall systems, the ignition delay decreases with mass flux and is diffusionally controlled by the deficient fuel. The criterion of reaction runaway is suggested both qualitatively and quantitatively. In addition, the effects of flow strain rate, Lewis numbers and Prandtl number on ignition delay are investigated.The results reveal that the thermal runaway criterion instead of the zero-gradient criterion is preferred for the problem of interest in the second part. Depending on system parameters, both the ignition delay and the critical rate of catalytic reactions at ignition are either conductively or catalytically controlled. The effects of catalytic reactions on ignition are positive and negative for catalytically and conductively controlled mechanisms, respectively. According to these two ignition mechanisms, the qualitative and quantitative results of the ignition delay and the critical rate of catalytic reactions at ignition are systematically analyzed. As the thermal diffusivity is a great value,the ignition delay increases with the catalytic reaction rate and the critical rate of catalytic reactions at ignition is a maximum one above which the ignition can not be achieved. For the thermal diffusivity is a small value, the ignition delay decreases with the catalytic reaction rate and the critical rate of catalytic reactions at ignition here becomes a minimum one above which the ignition is achieved. Therefore, the minimum ignition temperature can be reduced by increasing the rate of catalytic reactions. For an intermediate value of thermal diffusivity,a minimum value of ignition delay is observed at a critical value of catalytic reaction rate and the C shaped ignition curves are observed.