An investigation of microgravity droplet combustion in quiescent atmospheres and in slow flow

Results of measurements on the burning of free n-heptane droplets (droplets without fiber supports) and tethered n-heptane and n-decane droplets performed in Spacelab during the flights of the first Microgravity Science Laboratory (MSL-1) are presented along with a description of the experimental apparatuses used. A theory describing droplet combustion in slow flows with Reynolds numbers near one is also presented. This theory is compared to experimental results obtained from preliminary testing of a future International Space Station experiment.; The droplet combustion onboard MSL-1 occurred in oxidizing atmospheres whose ambient temperatures were within a few degrees of 300K. The free droplets were heptane and were burned in helium-oxygen atmospheres with oxygen mole fractions from 20% to 50%, at pressures from 0.25 bar to 1.00 bar. The tethered droplets were burned in Spacelab cabin air at 1.00 bar; some involved slow convective flows with Reynolds number from about 1 to 10. Most of the fiber supported results involve heptane, but the few decane results are presented. Summaries are given of measured burning-rate constants, final droplet diameters and final flame diameters, where available. Both diffusive and radiative extinctions were exhibited. For the quiescent results the principal intent is to provide a complete, documented data set for future analysis, although some interpretations are reported and conclusions drawn concerning the combustion mechanisms. The slow flow results, principally burning rates, are compared to the prevailing empirical model and the theoretical model presented for slower flow, and general conclusions are drawn.; Also, an idealized model for droplet combustion in the Burke-Schumann reaction-sheet approximation is analyzed in terms of a Peclet number based on Stefan velocity of order unity, for Lewis numbers of unity and for small values of a parameter ϵ, defined as the ratio of the convective velocity at infinity to the Stefan velocity at the droplet surface. Asymptotic solutions for the velocity, pressure and mixture-fraction are obtained through second order in ϵ. The results are employed to calculate the effects of convection on burning rate and on flame shape. Qualitative and quantitative comparisons with experiment help to identify the strengths and limitations of this model.