OpenSMOKE++ was used for modeling the combustion of isolated fuel droplets in microgravity. The work was carried out in collaboration with University of San Diego (CA) and NASA Glenn Research Center (Cleveland, OH)
Summary
In this work we applied a numerical model describing the combustion of isolated fuel droplets in microgravity, based on a detailed kinetic mechanism. The main objective was to investigate and explain the role of the initial diameter of the droplet and the amount of oxygen on the gaseous atmosphere on radiative extinction and formation of stable cool flames around the droplet. The numerical predictions, in satisfactory agreement with the experimental measurements, showed that only if the initial diameter of the droplet is larger than a critical diameter, the radiative extinction occurs, followed by a low-temperature, soot-free, cool flame. It was demonstrated that the squared critical diameter linearly increases with the mole fraction of oxygen in the atmosphere. In particular, it was found that the inert species in the gaseous atmosphere plays a major role, promoting or inhibiting the tendency to radiative extinction.
The results may have direct application in fire safety. In space, cool flame can persist after hot flame extinction and generate combustible vapor that can reignite (similar to smoldering combustion in solid fuels). Thus, the knowledge of “windows” of operating conditions in which those phenomena can occur is potentially relevant.
Cuoci, A., Saufi, A.E., Frassoldati, A., Dietrich, D.L., Williams, F.A., Faravelli, T., Flame extinction and low-temperature combustion of isolated fuel droplets of n-alkanes (2017) Proceedings of the Combustion Institute, 36 (2), pp. 2531-2539, DOI: 10.1016/j.proci.2016.08.019
Critical diameter
Only if the initial diameter is sufficiently large, radiative extinction occurs. Thus, we can recognize the existence, for a given amount of oxygen in the atmosphere, of an initial critical diameter: if the initial diameter is smaller, no radiative extinction occurs. Based on our calculations, we found that the squared critical diameter depends (with a good approximation) linearly on the amount of oxygen. If we look at the squared initial diameter vs the amount of oxygen, we can split the space in two regions through a line. Here below we reported the lines corresponding to the critical diameters for n-heptane and n-decane droplets, as resulting from our calculations. We also reported on the plots the symbols corresponding to the experimental cases showing radiative extinction. As you can see no experimental points can be found below the calculated line, which is an additional confirmation of the quality of numerical simulations. Only here we have this point which is very border line. You can also observe that the critical diameters for n-heptane and n-decane are very similar, basically indistinguishable. On the contrary, the pressure plays an important role, as you can see from the slope of the green and red lines.
Role of inerts
We numerically investigated the role played by the inert species on the critical diameter. The results are summarized here. Helium leads to a larger window where radiative extinction occurs, because of the significant increase of the conductive heat transfer from the flame, mainly due to its larger thermal conductivity. In addition, when helium is adopted as inert, Soret effect plays a major role. This is evident from the plot, where two lines for helium are reported, with and without Soret effect. Soret diffusion tend to drive out n-heptane of the hot reaction zone, decreasing the overall reaction rate, thus enhancing the tendency to extinction. The opposite was observed when nitrogen is substituted with argon. The larger molecular weight and the lower thermal conductivity increases the critical diameter, which means that the hot flame is stronger to survive.