Flow/Soot-Formation Interactions in Nonbuoyant Laminar Diffusion Flames
01 May 1999-
TL;DR: The final report of a research program considering interactions between flow and soot properties within laminar diffusion flames is presented in this paper. But this work is limited to three phases, considering the shapes of non-buoyant round LJ flames in still air, the shape of round Lj flames in co-flowing air, and the hydrodynamic suppression of soot formation in lj flames.
Abstract: This is the final report of a research program considering interactions between flow and soot properties within laminar diffusion flames. Laminar diffusion flames were considered because they provide model flame systems that are far more tractable for theoretical and experimental studies than more practical turbulent diffusion flames. In particular, understanding the transport and chemical reaction processes of laminar flames is a necessary precursor to understanding these processes in practical turbulent flames and many aspects of laminar diffusion flames have direct relevance to turbulent diffusion flames through application of the widely recognized laminar flamelet concept of turbulent diffusion flames. The investigation was divided into three phases, considering the shapes of nonbuoyant round laminar jet diffusion flames in still air, the shapes of nonbuoyant round laminar jet diffusion flames in coflowing air, and the hydrodynamic suppression of soot formation in laminar diffusion flames.
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15 Dec 2001
TL;DR: The structure and soot properties of round, soot-emitting, non-buoyant, laminar jet diffusion flames are described, based on long-duration (175-230s) experiments at microgravity carried out on orbit in the Space Shuttle Columbia as discussed by the authors.
Abstract: The structure and soot properties of round, soot-emitting, nonbuoyant, laminar jet diffusion flames are described, based on long-duration (175-230-s) experiments at microgravity carried out on orbit in the Space Shuttle Columbia. Experimental conditions included ethylene-fueled flames burning in still air at nominal pressures of 50 and 100 kPa and an ambient temperature of 300 K with luminous flame lengths of 49-64 mm. Measurements included luminous flame shapes using color video imaging, soot concentration (volume fraction) distributions using deconvoluted laser extinction imaging, soot temperature distributions using deconvoluted multiline emission imaging, gas temperature distributions at fuel-lean (plume) conditions using thermocouple probes, soot structure distributions using thermophoretic sampling and analysis by transmission electron microscopy, and flame radiation using a radiometer. The present flames were larger, and emitted soot more readily, than comparable flames observed during ground-based microgravity experiments due to closer approach to steady conditions resulting from the longer test times and the reduced gravitational disturbances of the space-based experiments.
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TL;DR: In this paper, the structure and soot properties of round, soot-emitting, non-buoyant, laminar jet diffusion flames are described, based on long-duration (175-230 s) experiments at microgravity carried out on orbit in the Space Shuttle Columbia.
Abstract: Observations of the structure and soot properties of round, soot-emitting, nonbuoyant, laminar jet diffusion flames are described, based on long-duration (175-230 s) experiments at microgravity carried out on orbit in the Space Shuttle Columbia. Experimental conditions included ethylene-fueled flames burning in still air at nominal pressures of 50 and 100 kPa and an ambient temperature of 300 K with luminous flame lengths of 49-64 mm. Measurements included luminous flame shapes using color video imaging, soot concentration (volume fraction) distributions using deconvoluted laser extinction imaging, soot temperature distributions using deconvoluted multiline emission imaging, gas temperature distributions at fuel-lean (plume) conditions using thermocouple probes, soot structure distributions using thermophoretic sampling and analysis by transmission electron microscopy (TEM) and flame radiation using a radiometer. After an initial 20s flame stabilization period (caused by effects of ignitor disturbances, fuel flow rate adjustments and transient development of flame structure), the flames reached steady-state conditions aside from slow (quasisteady) changes due to pressure increases and ambient oxygen consumption within the test chamber caused by combustion. The present flames were larger, and emitted soot more readily, than comparable flames observed during ground-based microgravity experiments due to closer approach to truly steady conditions resulting from the longer test times and the reduced gravitational disturbances of the space-based experiments. Increasing the pressure from 50 to 100 kPa for soot-emitting flames of similar length caused maximum soot volume fractions to increase from 2 to 32 ppm and average primary soot particle diameters to increase from 24 to 40 nm, showing that soot emissions are the result of the relative rates of soot formation and oxidation and do not correlate closely with peak soot concentrations and primary particle sizes within the flames. In addition, comparable sootemitting buoyant laminar diffusion flames at normal gravity and 100 kPa have significantly smaller maximum primary soot particles (32 nm diameter implying roughly 50 percent less mass) than the nonbuoyant flames. It was also found that the tipopening phenomena associated with nonbuoyant sootemitting flames is caused by extinction of the flame near its tip due to radiative heat losses, which means that emissions of unburned fuel are associated with emissions of soot in the present nonbuoyant flames. Finally, soot production properties (characterized by maximum soot concentrations) are similar for various paths through the 50 kPa flame where effects of radiative extinction and soot particle thermophoresis are small, suggesting potential for a simple state relationship between soot concentrations and mixing level (mixture fraction) at flame conditions representative of many practical applications. This behavior follows because flame residence times are relatively independent of path for nonbuoyant laminar jet diffusion flames, and may help to explain the universality of many properties of soot emitted from practical flames (which generally are relatively nonbuoyant).
73 citations
TL;DR: In this article, the effects of flow (hydrodynamic) properties on soot formation and oxidation in non-premixed hydrocarbon/air flames were studied, emphasizing conditions where effects of buoyancy are small in co-flowing laminar jet diffusion flames.
Abstract: Effects of flow (hydrodynamic) properties on soot formation and oxidation in nonpremixed hydrocarbon/air flames were studied, emphasizing conditions where effects of buoyancy are small in coflowing laminar jet diffusion flames. Effects of air/fuel-stream velocity ratios were of particular interest; therefore, the experiments were carried out at reduced pressures (0.19-0.50 atm) in order to minimize effects of flow acceleration due to buoyancy. Test conditions included acetylene, propylene, and 1,3-butadiene flames burning in air with air/fuel-stream velocity ratios in the range 0.4-6.7. Measurements included laminar smoke-point properties, as well as the following flame structure properties: soot volume fractions, temperatures, soot structure, concentrations of major gas species, and velocities. The measurements showed that laminar smoke-point flame lengths can be increased, and soot emissions possibly suppressed entirely, by increasing air/fuel-stream velocity ratios. The flame structure measurements suggest that the mechanism of this effect involves the magnitude and direction of flow velocities relative to the flame sheet. In particular, large air/fuel-stream velocity ratios cause soot to form in cool and fuel-rich gases, inhibiting soot nucleation, and then to be drawn directly toward the flame sheet with a limited residence time, inhibiting soot growth, so that capabilities to complete the oxidation of soot are enhanced and the tendencies to emit soot are reduced.
65 citations
TL;DR: In this paper, the effects of flow properties on the presence of soot in laminar opposed-jet diffusion flames were studied experimentally at atmospheric pressure, emphasizing effects of velocities normal to the flame sheet.
Abstract: Effects of flow (hydrodynamic) properties on the presence of soot in hydrocarbon-fueled laminar opposed-jet diffusion flames were studied experimentally at atmospheric pressure, emphasizing effects of velocities normal to the flame sheet. These velocities were varied for conditions corresponding to combustion in air by transferring nitrogen from the oxidizer stream to the fuel stream, which increases the stoichiometric mixture fraction of the flame and causes the stagnation plane of the flow to shift toward the fuel-rich side of the flame sheet. Fuels considered included acetylene, ethylene, ethane, propylene, propane, and 1-3 butadiene. Present measurements consisted of the critical strain rates for the flames to contain soot (the soot extinction limit) and for the flames to extinguish (the flame extinction limit). It was found that increasing the stoichiometric mixture fraction causes a progressive reduction of the critical strain rates for both flame and soot extinction; however, their ratio increases, and even becomes unbounded in most instances to yield a permanently-blue-flame regime. The results suggest that soot formation in nonpremixed flames can be controlled by varying velocities normal to the flame sheet. Nevertheless, definitive conclusions along these lines require evaluation of effects of corresponding variations of fuel and oxygen concentrations on soot formation when velocities normal to the flame sheet are changed by varying stoichiometric mixture fractions for laminar opposed-jet diffusion flames.
48 citations
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06 Aug 2013TL;DR: In this paper, the laminar smoke point properties of non-buoyant round Laminar jet diffusion flames were studied emphasizing results from long-duration (100-230 s) experiments at microgravity carried out in orbit aboard the space shuttle Columbia.
Abstract: The laminar smoke-point properties of non-buoyant round laminar jet diffusion flames were studied emphasizing results from long-duration (100-230 s) experiments at microgravity carried out in orbit aboard the space shuttle Columbia. Experimental conditions included ethylene- and propane-fueled flames burning in still air at an ambient temperature of 300 K, pressures of 35-130 kPa, jet exit diameters of 1.6 and 2.7 mm, jet exit velocities of 170-690 mm/s, jet exit Reynolds numbers of 46-172, characteristic flame residence times of 40-302 ms, and luminous flame lengths of 15-63 mm. Contrary to the normal-gravity laminar smoke point, in microgravity, the onset of laminar smoke-point conditions involved two flame configurations: closed-tip flames with soot emissions along the flame axis and open-tip flames with soot emissions from an annular ring about the flame axis. Open-tip flames were observed at large characteristic flame residence times with the onset of soot emissions associated with radiative quenching near the flame tip: nevertheless, unified correlations of laminar smoke-point properties were obtained that included both flame configurations. Flame lengths at laminar smoke-point conditions were well correlated in terms of a corrected fuel flow rate suggested by a simplified analysis of flame shape. The present steady and non-buoyant flames emitted soot more readily than non-buoyant flames in earlier tests using ground-based microgravity facilities and than buoyant flames at normal gravity, as a result of reduced effects of unsteadiness, flame disturbances, and buoyant motion. For example, present measurements of laminar smoke-point flame lengths at comparable conditions were up to 2.3 times shorter than ground-based microgravity measurements and up to 6.4 times shorter than buoyant flame measurements. Finally, present laminar smoke-point flame lengths were roughly inversely proportional to pressure to a degree that is a somewhat smaller than observed during earlier tests both at microgravity (using ground-based facilities) and at normal gravity.
35 citations
01 Jan 2000
27 citations