Ignition and behavior of laminar gas-jet diffusion flames in microgravity

Abstract

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.

Effects of Gravity on Soot Formation in a Coflow Laminar Methane/Air Diffusion Flame

“…4 Comparison of predicted soot volume fraction distributions under different gravity level with the peak values indicated flame height in microgravity is obviously taller than that at normal gravity (Kong and Liu 2009). This is in qualitative agreement with the experimental results that the observed microgravity flames are taller than their normal gravity counterparts (Bahadori et al 1990) where the fuel jet is issued into a quiescence environment. The maximum values of soot volume fraction in each case are in the annular region.…”

Shapes of nonbuoyant round luminous hydrocarbon/air laminar jet diffusion flames

“…Typical of many past observations of sootcontaining nonbuoyant laminar jet diffusion flames [8][9][10][11][12][13][14], present flame shapes could be grouped into closed-tip and opened-tip configurations, which were observed for fuel flow rates smaller and larger than the laminar smoke point fuel flow rates, respectively. In fact, the tip-opening phenomenon provided a convenient indicator of laminar smoke points for present test conditions because the associated dramatic change of the shape of the flame tip invariably corresponded to the first observations of soot emissions.…”

“…In spite of potential effects of unsteadiness for the ground-based studies of nonbuoyant laminanyet diffusion flames, however, Cochran and co-workers [4][5][6], Bahadori and co-workers [7][8][9][10][11][12], and Sunderland et al [14] all observed a linear correlation between luminous flame lengths and fuel flow rates, independent of jet exit diameter, for each fuel burning in air. This behavior also is typical of the luminous flame lengths of buoyant round laminar jet diffusion flames [19].…”

“…Studies following the work of Cochran and coworkers [4][5][6] due to Bahadori and coworkers [7][8][9][10][11][12] sought to resolve potential effects of transient flame development and soot luminosity on measurements of the shape of nonbuoyant round laminar jet diffusion flames using both 2.2 s and 5.2 s drop towers. In order to minimize problems of transient behavior, they ignited the flames shortly after the experimental package was released and observed nearly steady flame shapes near the end of free fall.…”

“…The objectives of this paper are to document these measurements and to develop a summary of the results, convenient for use by others, based on simplified analysis of nonbuoyant laminar jet diffusion flames. Past measurements of the shapes of nonbuoyant laminar jet diffusion flames have been carried out using either drop towers to provide microgravity environments [4][5][6][7][8][9][10][11][12][13] or aircraft facilities [14] to provide low-gravity environments. The earliest work along these lines was a series of studies of hydrocarbon-fueled laminar jet diffusion flames using a 2.2 s free-fall (drop) tower due to Cochran and co-workers [4][5][6].…”