Detailed numerical simulations of the multistage self-ignition process of n-heptane, isolated droplets and their verification by comparison with microgravity experiments

Abstract

Detailed understanding of the basic physical and chemical processes of self-ignition phenomena for technical fuel sprays is required for many technical combustion applications. Because single droplets as the basic elements of fuel sprays allow the study of fundamental ignition behavior, this work focuses on the multistage self-ignition behavior of Φ 0.7 mm single n-heptane droplets in air. The investigated ambient conditions are temperatures from 580 to 1000 K and pressures from 0.3 to 1 MPa. A substantial physical and chemical model is developed for the detailed numerical simulation. The chemical reaction mechanism consists of a 62-step kinetic model with special consideration of the low-temperature reaction branch. The program was validated by comparison with results from microgravity experiments at Drop Tower Bremen, which allow clarification of the ignition process free from natural convection. Cool flame and hot flame appearances were obtained from non-intrusive interferometric measurement in a well-tested experimental setup. The calculated ignition delays resulting from temperature and temperature gradient criteria, respectively, were compared with these experimental results. Furthermore, the cool flame temperature was measured with a K-type thermocouple of Φ 25 μm at the place of its appearance and was also compared with the numerical simulations. A quantitative good agreement for first and total ignition delays as well as for the cool flame temperature could be achieved. With this detailed numerical model, the multistage ignition behavior was analyzed, and the ignition criteria employed in interferometric measurement, which are temperature and temperature gradient, respectively, were confirmed.