A Computational Study of Spherical Diffusion Flames in Microgravity with Gas Radiation.

The ultimate goal of this research is to extend the current understanding of the characteristics of spherical diffusion flames in microgravity. In particular, one of the key objectives is to assess the effects of gas radiation as a means to promote flame extinction. To investigate these phenomena, a one-dimensional computational model was developed to simulate the evolution of a spherical diffusion flame with consideration of detailed chemistry and transport properties. Various levels of radiation models were implemented and the results were compared with experimental measurements of flame radius and temperature profiles. It was shown that the statistical narrow band model (SNB) combined with the discrete ordinate method (DOM) reproduced the experimental results with highest accuracy, and this combination of the radiation models were adopted in the subsequent parametric studies. The parametric studies explored the relative effectiveness of fuel- and oxidizer-side dilution on the flame radius and temperature behavior, with nitrogen, CO2, and helium as diluents. In the spherical configuration considered in this study, the oxidizer-side dilution has a stronger effect on flame transient behavior than the fuel-side dilution, thereby suggesting a more effective means to induce flame extinction by dilution. Study on various oxidizer-side dilution cases shows that CO2 has a larger suppression effect than helium or nitrogen with the same dilution level. CO2 dilution has multiple effects on flame behavior including radiation, thermodynamic, diffusion, and chemical effects. Quantitative analysis shows that the radiation effect is the primary factor accounting for flame temperature drop by approximately 60%, as compared to the thermal/diffusion (30%), and chemical effect (10%). Considering the dominance of the radiative heat loss on flame extinction, a unified extinction criterion that applies to a wide range of parametric conditions was sought. The compiled computational results indicated that a critical flame temperature of 1130 K at extinction appears to be valid for most of the conditions under study. Therefore, it is concluded that extinction of spherical diffusion flame is primarily dictated by the local condition in the flame zone rather than by the volumetric radiative heat transfer in the surrounding gases.Ph.D.Mechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/60647/1/tangst_1.pd