Effects of Additives on the Non-Premixed Ignition of Ethylene in Air

The ignition characteristics of heated C_2H_4 counterflowing against heated air were numerically investigated in the presence of additives such as NO, F_2, and H_2. C_2H_4 and air temperatures were chosen to resemble conditions relevant to high-Mach number, air-breathing propulsion. The numerical simulations were conducted along the stagnation streamline of the counterflow and included detailed descriptions of chemical kinetics and molecular transport. It was found that addition of NO at concentrations of about 10,000 ppm (1%), results in a substantial increase of the ignition strain rate, from 300 /s to values up to 32,000/s. This ignition promotion is caused by enhanced radical production that is initiated through the interaction between NO and HO_2. A further increase in the NO amount leads to reduced improvements. Small additions of F_2 and H_2 were also found to promote ignition, but to lesser extent compared to NO. Results also show that with the addition of F_2 in the presence of NO, ignition promotion is further enhanced, and for F_2 and NO concentrations larger than 25,000 ppm, the system becomes hypergolic. The present investigations suggest that the use of C_2H_4, NO, and F_2 may permit ignition at conditions of relevance to SCRAMJET's.