Single droplet combustion of decane in microgravity: experiments and numerical modelling

This paper presents experimental data on single droplet combustion of decane in microgravity and compares the results to a numerical model. The primary independent experiment variables are the ambient pressure and oxygen mole fraction, pressure, droplet size (over a relatively small range) and ignition energy. The droplet history (D² history) is non-linear with the burning rate constant increasing throughout the test. The average burning rate constant, consistent with classical theory, increased with increasing ambient oxygen mole fraction and was nearly independent of pressure, initial droplet size and ignition energy. The flame typically increased in size initially, and then decreased in size, in response to the shrinking droplet. The flame standoff increased linearly for the majority of the droplet lifetime. The flame surrounding the droplet extinguished at a finite droplet size at lower ambient pressures and an oxygen mole fraction of 0.15. The extinction droplet size increased with decreasing pressure. The model is transient and assumes spherical symmetry, constant thermo-physical properties (specific heat, thermal conductivity and species Lewis number) and single step chemistry. The model includes gas-phase radiative loss and a spherically symmetric, transient liquid phase. The model accurately predicts the droplet and flame histories of the experiments. Good agreement requires that the ignition in the experiment be reasonably approximated in the model and that the model accurately predict the pre-ignition vaporization of the droplet. The model does not accurately predict the dependence of extinction droplet diameter on pressure, a result of the simplified chemistry in the model. The transient flame behaviour suggests the potential importance of fuel vapour accumulation. The model results, however, show that the fractional mass consumption rate of fuel in the flame relative to the fuel vaporized is close to 1.0 for all but the lowest ambient oxygen mole fractions.

Keywords:

• Droplet combustion
• Microgravity

Acknowledgments

The authors would like to thank J.S. T'ien, Y. Shu and P. Chang for the source code of the candle flame (the basis of the current model) and their input on the current model. PMS and DLD would like to thank I. Goodman and D. Lenhert for their assistance with the NASA GRC experiments and analysis of the experimental data. The authors would like to thank S. Honma, K. Ikeda, K. Kitano, and H. Nagaiishi for their assistance with the JAMIC experiments. DLD thanks K.A. Kutler for her critical review of the manuscript.

Notes

^‡The air was a precision gas blend of oxygen (0.21 mole fraction) and nitrogen mixed gravimetrically by the manufacturer.