Diffusion flame

About: Diffusion flame is a research topic. Over the lifetime, 9266 publications have been published within this topic receiving 233522 citations.

Structure and Soot Properties of Nonbuoyant Ethylene/Air Laminar Jet Diffusion Flames

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).

The flickering candle: transition to a global oscillation in a thermal plume

Abstract: A number of experiments have been performed on the properties of propane diffusion flames at relatively low fuel flow rates and using a variety of burner types. Optical methods were used to observe the flame and plume above it. We have studied the transition from a steady flame, with a plume exhibiting a helical instability at low flow rates, to an axisymmetric instability of the whole flame/plume, that grows from the base of the flame and oscillates at a well-defined and robust frequency, at higher flow rates. These results and the observation of the effects of various external modifications, e.g. the generation of an annular counter flow, a pressure perturbation, etc. are consistent with the view that the transition to the axisymmetric state, at which the flame flickers, is one to a globally excited oscillation forced by a region of absolutely unstable flow at or near the base of the burners used in this study.

Diffusive-thermal instability and flame extinction in nonpremixed combustion

Abstract: A systematic study of cellular instabilities in nonpremixed flames is performed by examining shapes, heights, and extinction conditions of over- and under-ventilated slot-jet flames. A variety of fuels and diluents are used to vary Lewis numbers of fuel and oxidizer. Only nonpremixed flames near extinction having sufficiently low effective Le were found to exhibit cellular instability. It is proposed that near-extinction conditions are required so that reactant intermixing occurs before combustion takes place and that the effective Le is that of the more completely consumed reactant according to Linan's criterion for fuel or oxidizer leakage in diffusion flames, which msut be 0.8 for cellular instability. It is shown that all known observations of cellular nonpremixed flames can be described this way. Mechanisms proposed in previous work, such as preferential diffusion of light reactants and stoichiometric considerations, are disputed. Flame-height and extinction limit scalings suggest that flame height may be controlled by either buoyancy or jet momentum and extinction by blowoff or heatloss mechanisms. The characteristics of the observed cellular structures are consistent with the proposed mechanisms. Temperatures at cellular fronts are found to be higher than the calculated equilibrium values, but extinction limits appear not to be extended by diffusivethermal effects as much as in premixed flames. It is concluded that diffusive-thermal instability of nonpremixed flames occurs in a manner simililar to that of premixed flames, regardless of the mechanism leading to reactant intermixing.

Structure and stabilization of hydrogen jet flames in cross-flows

Abstract: The structure and stabilization of heated hydrogen jet flames in heated cross-flows was experimentally investigated in a configuration that is analogous to terrestrial gas turbine components. Three flames, with jet velocities ranging from 100 to 200 m/s, were investigated using particle image velocimetry and OH planar laser induced fluorescence in a total of 11 x–y and y–z planes. Additionally, laser Raman scattering was performed in the 200 m/s jet to characterize the thermo-chemical state. In all cases, the flame along the jet centerline plane consisted of two branches, one stabilized in the jet lee and one lifted above the jet trajectory. The positional stability of the lee-stabilized branch was greater in the higher jet velocity cases due to the larger and stronger recirculation zones created downstream of the injection point. The lifted flame branch was much more dynamic, with measured flame base axial positions ranging from the jet near field to the flame tip. This flame branch instantaneously resided downstream of regions with high extensive principal strain-rate, and the strain-rate significantly affected the thermo-chemical state. The Raman measurements indicated that the base of the lifted flame branch existed in locations where both tribrachial and/or stratified premixed flame behaviors are expected, depending on the instantaneous flame location. Accurately modeling these complex flame structures and flow-flame interactions therefore is necessary to properly simulate jet flames in cross-flows.