Effects of additives on the non-premixed ignition of ethylene in air

The ignition characteristics of heated C2H4 counterflowing against heated a ir were numerically investigated in the presence of additives such as NO, F -2, and H-2. C2H4 and air temperatures were chosen to resemble conditions r elevant to high-Mach number, air-breathing propulsion. The numerical simula tions were conducted along the stagnation streamline of the counterflow and included detailed descriptions of chemical kinetics and molecular transpor t. Addition of NO, at concentrations of about 10,000 ppm (1%), results in a substantial increase of ignition strain rates, from 300s(-1) to values up to 32,000s(-1). This ignition promotion is caused by enhanced radical produ ction, initiated through the interaction between NO and HO2. Further increa ses in NO amount lead to reduced improvements. Small additions of F-2 and H -2 were also found to promote ignition, but to a 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. The system becomes hypergolic for F -2 and NO concentrations larger than 25,000 ppm. The present investigations suggest that the use of C2H4, NO, and F-2 may permit ignition at condition s of relevance to SCRAMJET's.