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Open Access Publications from the University of California

The Combustion Processes Laboratories (CPL) is the research facility in the Department of Mechanical Engineering at the University of California at Berkeley (USA), specializing in combustion, heat and mass transfer, and reactive systems. The Principal Investigators are Prof. Carlos Fernandez-Pello, Prof. Robert Dibble and Prof. Jyh-Yuan Chen.

Cover page of Flame Structure and Soot Formation in Inverse Diffusion Flames (Ph.D. Dissertation)

Flame Structure and Soot Formation in Inverse Diffusion Flames (Ph.D. Dissertation)


Flame structure and soot and carbon monoxide (CO) formation were studied in laminar co-flowing co-annular inverse diffusion flames (IDFs) in normal and microgravity. An IDF is a non-premixed flame that consists of an inner air flow surrounded by a fuel flow. Soot formation is important to understand because soot particles are a health concern and have a strong influence on flame radiation, while CO formation is important because of the role it plays in fire-related deaths. Soot formation in normal diffusion flames (NDFs) is difficult to study because soot forms in the center of the flame and is oxidized so that soot cannot be sampled easily. Soot formation can be studied more easily in IDFs because soot forms on the fuel side of the reaction zone, is convected away from the reaction zone without oxidizing, and cools quickly. Therefore, in IDFs, newly formed soot is easier to sample than in NDFs. Soot formation was studied in microgravity IDFs because buoyancy-induced vortices that affect soot formation are eliminated in the absence of gravity. Flame structure of IDFs must be understood because it determines where soot and CO are formed and how they evolve.

IDFs are also useful for studying soot and CO formation in underventilated fires because both IDFs and underventilated fires have insufficient oxygen for complete combustion. Therefore, they both emit unoxidized soot, CO, and other products of incomplete combustion. Soot emission from underventilated fires also contributes to rapid fire spread due to increased radiation heat transfer.

Laser diagnostics, including planar laser-induced fluorescence of hydroxyl radicals (OH PLIF) and polycyclic aromatic hydrocarbons (PAH PLIF) and planar laser-induced incandescence of soot particles (soot PLII), thermocouple probing, and video imaging, were used to study the flame structure and distributions of temperature, soot and PAH in ethylene and methane IDFs. Comparison of OH PLIF with visible images and thermocouple temperature measurements in an ethylene IDF showed that peak OH PLIF occurred on the air side very close to the reaction zone and coincided with peak temperature. Reaction zone height determined by OH PLIF was found to be less than visible flame height because the reaction zone was masked by luminous soot in an annular region around and above the reaction zone. Reaction zone heights increased with air flow rate and were predicted well using a modified version of Roper’s analysis for co-flowing co-annular NDFs.

Soot PLII was observed in ethylene IDFs on the fuel side very close to the reaction zone. Comparisons of radial profiles of soot PLII to radiation-corrected thermocouple temperature measurements in an ethylene IDF showed that, close to the reaction zone, peak soot PLII coincided with a temperature of about 1300 K. This temperature is known from the literature to be the minimum critical temperature for soot formation. Radially integrated soot PLII increased with axial position until a position downstream of the flame height, leveling off to a maximum constant value, in agreement with the trend reported in a previously published IDF soot extinction study. The fact that the soot PLII continued to increase downstream of the flame tip suggests that soot inception ceases above the reaction zone and that soot growth can occur at temperatures below the critical inception temperature, and consequently that IDFs may be used to isolate soot growth. The maximum radially integrated soot PLII increased linearly with increasing air flow rate, indicating that bulk soot production rate is proportional to near-reaction zone residence time.

In both methane and ethylene IDFs, PAH PLIF was observed on the fuel side of the reaction zone, outside the soot zone but close to it. The proximity of PAH to the soot layer suggests it may be a constituent in soot formation, in agreement with the literature. The intensity of PAH PLIF was an order of magnitude greater in ethylene IDFs than in methane IDFs. Radially integrated PAH PLIF increased monotonically with axial position for both fuels. PAH PLIF and predicted streamlines from computer modeling indicated that buoyancy-induced vortices occurred above the reaction zone where PAH and soot were present. Comparisons of PAH PLIF to computer modeling of an ethylene IDF showed that PAH PLIF followed predicted streamlines, temperature isotherms, and contours of constant mixture fraction.

Microgravity IDFs did not flicker, were more luminous, were slightly longer, were more rounded, and radiated more energy than IDFs in normal gravity. Soot, PAH, and other condensed materials collected from post combustion gases of IDFs had a greater fraction of elemental carbon in microgravity than in normal gravity. The elemental carbon fraction increased as air flow rate increased, and was also greater in ethylene IDFs than methane IDFs for both gravity conditions, which indicates that IDFs produce carbonized soot as well as soot precursor particles and that microgravity IDFs produce more carbonaceous soot than normal gravity NDFs. In both gravity conditions, IDFs emitted CO as a major component of the exhaust stream. Emission of CO was slightly greater in microgravity than in normal gravity and increased with air flow rate, which suggests that CO formation depends on residence time. Buoyancy-induced vortices that affected soot and PAH formation in normal gravity were eliminated in microgravity.

Cover page of Laser Extinction in Laminar Inverse Diffusion Flames

Laser Extinction in Laminar Inverse Diffusion Flames


Measurements of line-of-sight laser extinction in a co-annular ethylene-air laminar inverse diffusion flame (IDF) were made to determine soot concentration. Extinction has frequently been used in the literature to measure soot concentration in normal diffusion flames (NDFs), but it has rarely been applied to IDFs. A coflow IDF contains a primary air flow surrounded by a fuel annulus. Soot particles form on the outside of IDFs, advect upward, and eventually quench without being oxidized. It has been proposed in the literature that IDFs will produce less near-flame soot than NDFs because, for flames of comparable fuel, size and flow rates, movement of soot outward into cool regions of an IDF limits its simultaneous exposure to the high temperatures and fuel pyrolysis products needed for soot growth. A two-dimensional soot concentration map of an IDF using experimental data confirms this hypothesis by showing integrated soot volume fractions to be an order of magnitude lower than those reported for NDFs in the literature. Computer simulations of particle temperature histories in an NDF and IDF of similar height lend support to these results.

Cover page of Effect of Varied Air Flow on Flame Structure of Laminar Inverse Diffusion Flames

Effect of Varied Air Flow on Flame Structure of Laminar Inverse Diffusion Flames


The structure of laminar inverse diffusion flames (IDFs) of methane and ethylene was studied using a cylindrical co-flowing burner. Several flames of the same fuel flow-rate yet various air flow-rates were examined. Heights of visible flames were obtained using measurements of hydroxyl (OH) laser-induced fluorescence (LIF) and visible images. Polycyclic aromatic hydrocarbon (PAH) LIF and soot laser-induced incandescence (LII) were also measured. In visible images, radiating soot masks the blue region typically associated with the flame height in normal diffusion flames (NDFs). Increased air flow-rates resulted in longer flames. PAH LIF and soot LII indicated that PAH and soot are present on the fuel side of the flame and that soot is located closer to the reaction zone than PAH. Ethylene flames produced significantly higher PAH LIF and soot LII signals than methane flames, which is consistent with the sooting propensity of

Cover page of COSMIC: Carbon Monoxide and Soot in Microgravity Inverse Combustion

COSMIC: Carbon Monoxide and Soot in Microgravity Inverse Combustion


Almost seventy percent of fire related deaths are caused by the inhalation of toxins such as CO and soot that are produced when fires become underventilated.(1) Although studies have established the importance of CO formation during underventilated burning,(2) the formation processes of CO (and soot) in underventilated fires are not well understood. The goal of the COSMIC project is to study the formation processes of CO and soot in underventilated flames. A potential way to study CO and soot production in underventilated flames is the use of inverse diffusion flames (IDFs). An IDF forms between a central air jet and a surrounding fuel jet. IDFs are related to underventilated flames because they may allow CO and soot to escape unoxidized. Experiments and numerical simulations of laminar IDFs of CH4 and C2H4 were conducted in 1-g and µ-g to study CO and soot formation. Laminar flames were studied because turbulent models of underventilated fires are uncertain. Microgravity was used to alter CO and soot pathways. A IDF literature survey, providing background and establishing motivation for this research, was presented at the 5th IWMC.(3) Experimental results from 1-g C2H4 IDFs and comparisons with simulations, demonstrating similarities between IDFs and underventilated fires, were presented at the 6th IWMC.(4) This paper will present experimental results from µ-g and 1-g IDFs of CH4 and C2H4 as well as comparisons with simulations, further supporting the relation between IDFs and underventilated flames.

Cover page of Flow-Assisted Flame Propagation Through a Porous Combustible in Microgravity

Flow-Assisted Flame Propagation Through a Porous Combustible in Microgravity


Experiments were conducted to measure the flame propagation rate of a plug-flow flame through a combustible matrix of randomly oriented cubes of polyurethane foam in microgravity and normal gravity as a function of the forced air flow. The experiments in microgravity were conducted at the Japan Microgravity Center (JAMIC) drop tower, which provides 10s of microgravity. The normal gravity experiments were simulations of the microgravity experiments, and by comparison, were used to determine the effect of gravity on the flame propagation process. The experiment was conducted in a cylindrical geometry. Ignition was accomplished by means of a hot-surface igniter brought into direct contact with the foam at one end of the sample holder. The other end of the sample was sealed to a fan drawing air through the sample, which was adjustable using a variable DC power supply. In this configuration the flame propagation is flow-assisted. The flame propagation rate was determined by means of the temperature histories provided by thermocouples placed along the centerline of the sample. It is found that, both in normal and microgravity, as the air flow rate is increased the flame propagation velocity increases. Comparison between the normal and microgravity experiments shows that the microgravity combustion is greatly influenced by the ignition period. In microgravity the time to initiation of flame propagation is significantly longer than the corresponding time in normal gravity. This is due to the contribution of the buoyant flow that assists the forced flow during the initiation period in normal gravity. A simplified analytical model is presented for correlation of the velocity data.

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