Interacting droplet combustion under conditions of extinction

Abstract

This paper presents the results of an experimental investigation into the extinction behavior of single droplets and binary droplet arrays of decane fuel in normal gravity and microgravity. The test ambients were air at reduced pressure for the normal gravity experiments and nitrogen/oxygen and helium/oxygen at reduced pressure for the microgravity experiments. The data show that interaction effects have a stronger influence on the extinction behavior of a binary array than the quasi-steady burning behavior. In normal gravity, the burning rate constant is only a very weak function of the interdroplet spacing at initial nondimensional separation distances greater than 5. The extinction droplet diameter, however, is much smaller (if it exists) for the droplet array at an instantaneous non-dimensional spacing of approximately 20. The microgravity experiments in nitrogen/oxygen (0.15 oxygen mole fraction) ambients exhibited different extinction behavior than the normal gravity tests. In this ambient condition, the flame surrounding the binary array always extinguished at a larger droplet diameter compared with single droplets. The microgravity experiments in the helium/oxygen (0.25 oxygen mole fraction) ambients exhibited behavior opposite of the nitrogen diluted tests with the binary array extinguishing at smaller droplet diameters than a single droplet. For the microgravity experiments, the observed difference between the two ambients is attributed to the importance of radiative loss. The nitrogen/oxygen tests had larger, weaker flames and smaller burning rates and consequently were affected more significantly by radiative loss from the flame zone compared to the helium/oxygen tests. A qualitative explanation of the different extinction behavior is detailed in the paper.