This paper presents the results of studies on the ignition and behavior of cylindrically symmetric, laminar diffusion flames of methane and propane in quiescent air under microgravity conditions. The experiments were conducted in the 2.2 s NASA-Lewis Research Center Drop Tower. The characteristics of gas-jet diffusion flames ignited in microgravity environments have not been reported in the past. In prior research, similar flames were ignited in normal gravity and then subjected to the microgravity condition once the flame was established. The different ignition methods result in different flame behavior and conclusions in relation to extinction, transient adjustment, and approach toward steady state in microgravity. Specifically, some of the flames of the previous studies, which were in a transient state or reportedly extinguished, reach a near-steady state when ignited in microgravity. In addition, low-flow-rate methane flames in the present study (filmed at 15 frames/s) were entirely blue, whereas blue was not discernible in the microgravity flames of the previous works (filmed at 400 frames/s).Based on these findings, it is concluded that some of the previously reported "extinguished" flames may have been blue (and not visible on the high-speed film). In this paper, comparisons between the two methods of ignition and observations of the flame behavior and distinct nature of flame color and luminosity are presented. Application of a steady-state, parabolic model has shown satisfactory agreement between the predicted and observed flame heights for those flames that reached a near-steady state in the 2.2 s period of microgravity.
Upon the completion of the Structure and Liftoff in Combustion Experiment (SLICE) in March 2012, a comprehensive and unique set of microgravity coflow diffusion flame data was obtained. This data covers a range of conditions from weak flames near extinction to strong, highly sooting flames, and enabled the study of gravitational effects on phenomena such as liftoff, blowout and soot formation. The microgravity experiment was carried out in the Microgravity Science Glovebox (MSG) on board the International Space Station (ISS), while the normal gravity experiment was performed at Yale utilizing a copy of the flight hardware. Computational simulations of microgravity and normal gravity flames were also carried out to facilitate understanding of the experimental observations. This paper focuses on the different sooting behaviors of CH 4 coflow jet flames in microgravity and normal gravity. The unique set of data serves as an excellent test case for developing more accurate computational models. Experimentally, the flame shape and size, lift-off height, and soot temperature were determined from line-of-sight flame emission images taken with a color digital camera. Soot volume fraction was determined by performing an absolute light calibration using the incandescence from a flame-heated thermocouple. Computationally, the MC-Smooth vorticity-velocity formulation was employed to describe the chemically reacting flow, and the soot evolution was modeled by the sectional aerosol equations. The governing equations and boundary conditions were discretized on an axisymmetric computational domain by finite differences, and the resulting system of fully coupled, highly nonlinear equations was solved by a damped, modified Newton's method. The microgravity sooting flames were found to have lower soot temperatures and higher volume fraction than their normal gravity counterparts. The soot distribution tends to shift from the centerline of the flame to the wings from normal gravity to microgravity.
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