Normal Diffusion Flame Research Articles

Static Flame Stability of Circumferentially Arranged Fuel Port Inverse Jet Non-Premixed Flame Burner

ABSTRACT This paper presents experimental studies on flame appearance and static flame stability characteristics of inverse jet flame (IJF) using a circumferentially arranged fuel port (CAFP) burner by following two different methods, namely, Protocol I and Protocol II. In the case of Protocol I, experiments are conducted for a fixed air flow rate, by varying fuel flow rate whereas, in the case of Protocol II, by varying air flow rate for a fixed fuel flow rate. In Protocol I, the flame is lifted off from the burner rim unlike in Protocol II. Moreover, the flame base becomes unstable at higher fuel flow rate before the occurrence of flame liftoff for Protocol I. Momentum flux ratio (MFR) and overall equivalence ratio parameters are considered to obtain insights on the phenomenon with variation of flame liftoff height in CAFP-based IJF burner. An attempt has also been made to analyze the hysteresis effect on the flame liftoff. In contrast to the normal jet diffusion flame, the hysteresis effect is observed at lower MFR, and is absent particularly when the base flame gets strongly anchored to the burner rim for higher MFR. On the other hand, in Protocol II, with an increase in air velocity, the inverse jet diffusion flame is no longer diffusion controlled, rather mixing controlled. In addition, the flame blowoff for Protocol I is attributed to occurrence of leaner overall equivalence ratio obtained by reduction of fuel flow rate whereas global strain rate governs flame blowout for Protocol II. In addition to flame blowout and liftoff, the main flame for Protocol II experiences local extinction and re-ignition at certain conditions with increase in the global strain rate at certain threshold value. In addition, semi-empirical correlations based on Peclet number for main flame extinction, flame blowout and flame blowoff are proposed in the present study which can be employed for designing CAFP-based IJF-based combustor system.

Measurement of organic carbon content during the growth of soot particles in propane normal and inverse diffusion flames using a multi-wavelength light extinction method

This paper proposes a novel method for in situ determination of the organic carbon (OC) content in soot particles using a multi-wavelength light extinction method for propane normal diffusion flames (NDFs) and inverse diffusion flames (IDFs). Light extinction at wavelengths of 450–900 nm was measured using white light and a spectrometer. The light extinction coefficient plotted as a function of the reciprocal wavelength revealed the extinction characteristics of the soot particles. Additionally, the wavelength dependency of the OC content in soot was determined using the ratio of the slopes at short and long wavelengths. For the NDF, the slope ratio was higher near the flame center and upstream of the flame, whereas for the IDF, a constant of slope ratio was observed in all measurement positions. By comparing the slope ratio with the soot component analysis, a quantitative relationship was derived between the organic mass fraction (OMF) of the soot and the slope ratio. The OMF was 0.1–0.27 depending on the position in the NDF, and the IDF showed a similar OMF of 0.25–0.32 at all measurement positions. The results of transmission electron microscopy and electron energy loss spectroscopy also confirmed the validity of the OMF measurement.

Improvement of IDF Burner Effects on Lean Non-Premixed Synthetic Thai Natural Gas Flames

The combustion characteristics of lean non-premixed Thai synthetic natural gas on improved inverse diffusion flame (IDF) burner were investigated. Air and fuel nozzles were improved from normal diffusion flame burner (NDF) and set as inside and outside exits on burner, respectively. The combustion flames were studied with variations of fuel and air velocity. The luminous flame length was lower and premixed flame length was higher by designed IDF burner due to enhance of mixing between inside air and ambient air. When the equivalence ratio decreased, lower flame length and flame temperature were obtained. Moreover, the highest premixed combustion flames and the maximum temperature were observed equivalence ratio between Φ = 0.76-0.80 on lean non-premixed synthetic Thai natural gas flames with designed IDF burner.

Blending effect between the natural gas and the liquefied petroleum gas using multiple co- and cross-flow jets on NOx emissions

Abstract A burner comprising multiple co- and cross-flow jets was developed to address the effect of blending the natural gas (NG) with the liquefied petroleum gas (LPG) on the NOx emissions from laminar/turbulent diffusion and triple flames in the normal and inverse configurations. Increasing the NG mole fraction in the blend from 0.0 to 1.0 decreased the effective Schmidt number from 1.4 to 0.7 thus displacing the flame envelope into higher velocity regions such that the corresponding decrease in residence time caused NOx reduction by 46%. While the prompt NOx increased by 11%, there was an additional NOx reduction by 238% due to the residence time decrease pertaining to the density/velocity difference. Conversely, increasing the LPG mole fraction from 0.0 up to 1.0 extended the flame stability limit from 8.2 to 12.7 m/s whereby the residence time decrease caused NOx reduction by 49%. Furthermore, the peak temperature reduction by the flame increased soot-radiation additionally caused exponential NOx reduction. The NG optimum mole fraction in the blend for reaching minimum NOx emissions of about 0.035 g/kW hr in the normal diffusion flame decreased from 0.80 to 0.25 as the firing rate increased 9.5 times, while a further NOx reduction to 0.028 g/kW hr was obtained in the inverse diffusion flame. Employing cross-flow opposing air jets reduced the NOx emissions further to 0.017 g/kW hr and shortened the flame by 36% as increasing the strain rate to 2900 s−1 enlarged the peak turbulent kinetic energy to 11.9 m2/s2 with 12 jets. In the triple flame mode, supplying the LPG across the fuel-rich wing and the NG across the lean wing combined high radiation efficiencies, flame shortening and extensive stability limits of 4.8 m/s. The NOx emission index thus reached 0.3 g/kg fuel for the inverse triple flames.

Dilution effects on laminar jet diffusion flame lengths

Many studies have examined the stoichiometric lengths of laminar gas jet diffusion flames. However, these have emphasized normal flames of undiluted fuel burning in air. Many questions remain about the effects of fuel dilution, oxygen-enhanced combustion, and inverse flames. Thus, the stoichiometric lengths of 287 normal and inverse gas jet flames are measured for a broad range of nitrogen dilution. The fuels are methane and propane and the ambient pressure is atmospheric. Nitrogen addition to the fuel and/or oxidizer is found to increase the stoichiometric lengths of both normal and inverse diffusion flames, but this effect is small at high reactant mole fraction. This counters previous assertions that inert addition to the fuel stream has a negligible effect on the lengths of normal diffusion flames. The analytical model of Roper is extended to these conditions by specifying the characteristic diffusivity to be the mean diffusivity of the fuel and oxidizer into stoichiometric products and a characteristic temperature that scales with the adiabatic flame temperature and the ambient temperature. The extended model correlates the measured lengths of normal and inverse flames with coefficients of determination of 0.87 for methane and 0.97 for propane.

Effect of magnetic field on temperature profile and flame flow characteristics of micro flame using Talbot interferometer

In this paper an application of circular grating Talbot interferometer is investigated for measurement of temperature and temperature profile of micro flame under the influence of gradient and uniform magnetic field. Hilbert transform is used for phase extraction from a single Talbot interferometric fringe pattern. In addition to this a numerical study is undertaken to established the effect of magnetic field on flame flow characteristics such as flame length, fuel mass flow rate, magnetic susceptibility and mass fractions of oxygen which is the main oxidizer and products of combustion in candle flame is demonstrated. Experimental investigation reveals that the temperature of the flame is increased under the influence of the upward-decreasing magnetic field and decreased in upward-increasing magnetic field. In a uniform magnetic field the flame temperature is also increased, which is in contrast to the normal diffusion (large size) flame. The system is cost effective, easy to align, less prone to environmental perturbation and capable of measuring temperature of a large size (centimeter) flame to micro size (millimeter) flame.

Soot reduction by addition of dimethyl carbonate in normal and inverse ethylene diffusion flames: Nanostructural evidence

Oxygenated fuel represents an attractive alternative as an additive for reducing soot emissions. Dimethyl carbonate (DMC) is an oxygenated compound which is a good option to reduce soot, but the detailed characteristics of soot produced from combustion of hydrocarbon fuels blended with DMC are still lacking. The present research studied the nanostructure and reactivity of soot particles in ethylene/DMC normal and inverse diffusion flames. High resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA) were used to analyze the nanostructure and reactivity of soot. It was found that DMC addition was effective in decreasing the average weights of soot formed in flames. The results of HRTEM images showed that soot particles obtained with DMC addition showed liquid-like material and tight bonding, and exhibited more highly disorganized layers, which give it higher reactivity than soot obtained without DMC addition. Furthermore, HRTEM was used to analyze soot fringe characteristics consisting of fringe tortuosity, fringe length, and fringe separation. XRD was used to crosscheck the results for fringe separation, and was consistent with HRTEM results. In addition, the mass loss curve of TGA experiments showed that DMC addition could enhance the reactivity of soot particles.

Effects of Flame Configuration and Soot Aging on Soot Nanostructure and Reactivity in n-Butanol-Doped Ethylene Diffusion Flames

Soot has received considerable attention since it is a major pollutant in exhaust gas from fossil fuel combustion and it causes adverse climate and health effects. This work focused on soot morphology, nanostructure, and reactivity variations regarding the soot collected in different flame configurations of n-butanol-doped ethylene inverse diffusion flame (IDF) and normal diffusion flame (NDF) at different heights above burner surface (HAB = 20, 30, and 40 mm). The effects of flame configuration and soot aging on structural and reactivity characteristics were analyzed with emphasis on the differences of young and mature soot generated in IDF and NDF without and with n-butanol addition at specific positions, respectively. The effects of fuel-side n-butanol addition on IDF and NDF soot structures and reactivity were also evaluated. The soot obtained using thermophoretic sampling technique was analyzed by transmission electron microscopy (TEM) to investigate soot morphology evolutions along the centerline an...

Temperature measurement of wick stabilized micro diffusion flame under the influence of magnetic field using digital holographic interferometry

This paper presents the effect of magnetic field (upward decreasing, uniform and upward increasing) on wick stabilized micro diffusion flame by using digital holographic interferometry (DHI). The investigations reveal that under the influence of upward decreasing and uniform magnetic field temperature inside the micro flame increases in comparison to temperature inside micro flame without magnetic field. This is in contrary to normal diffusion flame, where uniform magnetic field has a little or no effect on the temperature. DHI is inherently more accurate more precise and is having better spatial resolution. DHI is ideally suited to study micro flame.

Effects of butanol isomers additions on soot nanostructure and reactivity in normal and inverse ethylene diffusion flames

This paper explored the effects of butanol isomers, n-butanol, iso-butanol, sec-butanol, and tert-butanol, as fuel-side additives on soot nanostructure and reactivity in ethylene inverse diffusion flames (IDF) and normal diffusion flames (NDF). The variations of structures and oxidation rates among the soot samples derived from ten different flames were studied and compared using high resolution transmission electron spectroscopy (HRTEM), Raman spectroscopy, thermogravimetric analyzer (TGA), surface area and porosimetry analyzer, and elemental analyzer. Results demonstrated that soot from IDF with the additions of butanol isomers had irregular shapes with film-like materials and highly heterogeneous nanostructures, which showed both amorphous carbon and fullerenic lamellae. The shells presented more prevalently in tert-butanol-doped IDF soot, which had the longest fringe length and smallest fringe tortuosity. Whereas in the NDF as butanol isomers were individually added, the soot samples were aggregated by several tens or hundreds nearly rounded particles. All the NDF soot exhibited a typical core-shell structure with extended and planar lamellae. Soot from sec-butanol added NDF presented the lowest degree of graphitization with the shortest fringe length and largest fringe tortuosity. The TGA results revealed that the oxidation rates of the IDF soot particles were much higher than that of the NDF soot. Moreover, when butanol isomers were individually added in IDF, soot produced with n-butanol (1.34E-03s−1) showed the highest reactivity with the least degree of crystallization in structure, followed by iso-butanol (1.31E-03s−1), sec-butanol (1.29E-03s−1) and tert-butanol (1.14E-03s−1) in the sequence of reactivity. However, the oxidation rates of the soot samples generated from butanol isomers-doped NDF were in the order of sec-butanol (4.26E-04s−1), tert-butanol (3.70E-04s−1), n-butanol (3.52E-04s−1) and iso-butanol (3.23E-04s−1) from the highest to lowest. The results confirmed a relationship on structure-property between soot nanostructure and reactivity.

Measurement of temperature and temperature profile of wick stabilized micro diffusion flame under the effect of magnetic field using digital speckle pattern interferometry

The effect of upward decreasing, uniform, and upward increasing magnetic fields on the temperature and temperature profile of a wick stabilized micro diffusion flame is investigated experimentally by using digital speckle pattern interferometry (DSPI). DSPI fringe patterns have inherent speckle noise which leads to inaccuracies in the measurements. To extract data more accurately, the high frequency speckle noise in a DSPI fringe pattern is reduced by using the combination of median filter and Symlet wavelet filter. The optical phase is extracted from the filtered DSPI fringe pattern by using Hilbert transform. The obtained phase is used to calculate the refractive index and temperature distribution in a microflame created by a candle. Temperature in the micro diffusion flame was determined experimentally both in the absence and in the presence of upward decreasing, uniform, and upward increasing magnetic fields. The experimental results reveal that temperature is increased under the effect of uniform and upward decreasing magnetic fields in comparison to the temperature of the microflame without a magnetic field. This is in contrast to the normal diffusion flame, where under a uniform magnetic field, there was no effect on temperature. In the case of an upward increasing magnetic field, the temperature of the microflame decreased.

Effect of Oxygen Enrichment in Propane Laminar Diffusion Flames under Microgravity and Earth Gravity Conditions

Diffusion flames are the most common type of flame which we see in our daily life such as candle flame and match-stick flame. Also, they are the most used flames in practical combustion system such as industrial burner (coal fired, gas fired or oil fired), diesel engines, gas turbines, and solid fuel rockets. In the present study, steady-state global chemistry calculations for 24 different flames were performed using an axisymmetric computational fluid dynamics code (UNICORN). Computation involved simulations of inverse and normal diffusion flames of propane in earth and microgravity condition with varying oxidizer compositions (21, 30, 50, 100 % O2, by mole, in N2). 2 cases were compared with the experimental result for validating the computational model. These flames were stabilized on a 5.5 mm diameter burner with 10 mm of burner length. The effect of oxygen enrichment and variation in gravity (earth gravity and microgravity) on shape and size of diffusion flames, flame temperature, flame velocity have been studied from the computational result obtained. Oxygen enrichment resulted in significant increase in flame temperature for both types of diffusion flames. Also, oxygen enrichment and gravity variation have significant effect on the flame configuration of normal diffusion flames in comparison with inverse diffusion flames. Microgravity normal diffusion flames are spherical in shape and much wider in comparison to earth gravity normal diffusion flames. In inverse diffusion flames, microgravity flames were wider than earth gravity flames. However, microgravity inverse flames were not spherical in shape.

Comparative flame structure investigation of normal and inverse turbulent non-premixed oxy-fuel flames using experimentally recorded and numerically predicted Rayleigh and OH-PLIF signals

The structure and characteristics of a turbulent inverse and normal oxy-fuel diffusion flame are investigated. Previous investigations reported in the literature looked at flame characteristics of laminar inverse diffusion flames and their differences to normal diffusion flames. Only few investigations are reported for turbulent inverse diffusion flames and they did not compare the results to the corresponding normal configuration.The present study uses a combined experimental and numerical approach to compare and analyze a turbulent non-premixed inverse oxy-fuel and a corresponding normal flame, both are non-piloted. Measurements were conducted using simultaneously recorded planar Rayleigh scattering and OH-LIF signals. Due to the significant variation of the effective Rayleigh cross section in mixture fraction space and the unknown OH quenching contributions, a comparison of derived quantities such as temperature and OH mole fraction is not possible. Therefore, the Rayleigh and OH-LIF signals were incorporated in the LES flamelet/progress variable approach used here. This allows for a direct comparison of experimentally recorded and numerically predicted Rayleigh and OH-PLIF signals for the flame structure analysis, which includes the joint PDF of both quantities. The two flames are compared in terms of the local flame structure. In addition, differences in the mixing field and especially in the location of turbulent/non-turbulent interface are investigated.

Formation of a cool diffusion flame and its characteristics

Low-temperature (∼700 K) “cool” flames formed with n-heptane fuel were observed in the experiments conducted under microgravity. Recently, Won et al. demonstrated experimentally that such “cool” flames could be established under normal gravity. However, as ozone was added to air for supporting the flames in that experiment, the fundamental question on the formation of a self-supported n-heptane/air “cool” flame remained unanswered. A numerical investigation is conducted for (1) finding an answer to this question and (2) determining a procedure for establishing a “cool” flame. A standard opposing-jet burner is considered. Investigations are performed using a well-tested CFD code. A reduced mechanism that successfully captured the “cool” flames in microgravity is used. Calculations have demonstrated that a “cool” diffusion flame can be established with n-heptane under normal gravity without resorting to flame-speed promoters such as ozone, pressure or temperature. Using velocities of 4 and 7 cm/s and temperatures of 400 and 300 K for the fuel and air jets, respectively, a stable “cool” flame was obtained. Flame was ignited through increasing the temperature of the air jet to a value >724 K and then a self-sustained “cool” flame was obtained by decreasing the temperature back to 300 K. Parametric studies are performed on the applied gravitational force such that the heavy n-heptane flowed upward from the bottom nozzle or downward from the top nozzle. In general, flame remained flat with or without the gravity. However, when the fuel jet was at the bottom, it expanded faster and moved the flame closer to the fuel nozzle. “Cool” flame could not be stabilized for the same velocity and temperature conditions when fuel flowed from the top. “Cool” and normal diffusion flames obtained with identical flow conditions are compared. Limiting stretch rate for extinguishing the “cool” flame is determined.

Effect of CO2 and O2 enhancement and gravity variations on normal and inverse laminar diffusion flames

Steady-state global chemistry calculations for 15 different flames in both normal earth gravity and micro-gravity conditions are implemented using an axi-symmetric CFD code. 9 different normal laminar diffusion flames of methane-air in normal earth gravity condition with varying CO2 mole fraction have been implemented using 5.5 mm diameter burner. Out of these 9 different flames, 4 flames are different mole fraction of CO2 dilution in fuel (20%, 30%, 40%, 50% CO2 and rest are CH4), 4 flames are different mole fraction of CO2 in air inlet (50%, 60%, 70%, 79% CO2 and rest are O2) and 1 flame is methane-air (21% O2 and 79% N2). The effect of CO2 enhancement in fuel and air inlet on flame shape, size and maximum temperature has been described in this paper. Additionally, 6 different inverse laminar diffusion flames of ethane-air with varying O2 mole fraction (21%, 30%, 50% O2 and rest are N2) with both normal earth gravity and micro-gravity have been implemented using 5.5 mm diameter burner and the maximum temperature results are compared with the available literature. For methane-air normal laminar diffusion flames, effect of CO2 enhancement in both air and fuel inlet cause a decrease in flame temperature. For ethane-air inverse laminar diffusion flames, effect of O2 enrichment in air inlet cause an increase in flame temperature.

Experimental study of the inverse diffusion flame using high repetition rate OH/acetone PLIF and PIV

Most previous work on inverse diffusion flames (IDFs) has focused on laminar IDF emissions and the soot formation characteristics. Here, we investigate the characteristics and structure of methane IDFs using high speed planar laser-induced fluorescence (PLIF) images of OH, particle image velocimetry (PIV), and acetone PLIF imaging for non-reacting cases. First, the flame appearance was investigated with fixed methane loading (mass flux) but with varying airflow rates, yielding a central air jet Reynolds number (Re) of 1,000 to 6,000 (when blow-off occurs). Next, it was investigated a fixed central air jet Re of 4500, but with varied methane mass flux such that the global equivalence ratio spanned 0.5 to 4. It was observed that at Re smaller than 2000, the inner air jet promotes the establishment of an inverse diffusion flame surrounded by a normal diffusion flame. However, when the Re was increased to 2500, two distinct zones became apparent in the flame, a lower entrainment zone and an upper mixing and combustion zone. 10kHz OH-PLIF images, and 2D PIV allow the identification of the fate and spatial flame structure. Many flame features were identified and further analyzed using simple but effective image processing methods, where three types of structure in all the flames investigated here: flame holes or breaks; closures; and growing kernels. Insights about the rate of evolution of these features, the dynamics of local extinction, and the sequence of events that lead to re-ignition are reported here. In the lower entrainment zone, the occurrence of the flame break events is counterbalanced by closure events, and the edge propagation appears to control the rate at which the flame holes and closures propagate. The rate of propagation of holes was found to be statistically faster than the rate of closure. As the flames approach blow-off, flame kernels become the main mechanism for flame re-ignition further downstream. The simultaneous OH-PLIF/Stereo PIV measurements indicate that the individual breaks events could be correlated to local vortical flow structure and strain rates fields. The detailed measurements provide a more complete understanding of IDF flame characteristics and structure than was previously possible.

Immaturity of soot particles in exhaust gas for low temperature diesel combustion in a direct injection compression ignition engine

The particulate matter sampled in exhaust gas from low temperature diesel combustion (LTC) with an exhaust gas recirculation rate of 60% in a direct injection compression ignition engine was comprehensively assessed and compared to conventional diesel combustion using thermogravimetric analysis and elemental analysis. With LTC, relatively significant mass reduction occurred due to the oxidation and desorption of hydrocarbons. 95% of mass was reduced at around 520°C in LTC, which is lower than for conventional combustion (595°C). The weight fractions of carbon and hydrogen species found by the elemental analysis after thermogravimetry with inert gas up to 400°C revealed that the change of weight fractions into higher carbon and lower hydrogen contents were larger for soot particles from LTC. The atomic ratios of carbon to hydrogen (C/H) as a measure of soot maturity were 6.74 and 3.84 for conventional combustion and LTC, respectively. The C/H ratio for LTC is close to that of the precursor particles (1.80) from a normal diffusion flame at low height or young soot particles (2.2) from an inverse diffusion flame, which demonstrates the immaturity of the soot particles from LTC, while the range of the C/H ratio in the conventional combustion was suggestive of highly matured soot particles. Transmission electron microscopy revealed the primary particle and agglomerates from LTC appeared small without distinguishable boundaries. The LTC mode also exhibited clumps of amorphous materials and translucent precursor particles, which were attributed to intermediate substances from the low combustion temperature and abundant hydrocarbon species.

Chemiluminescence Studies of Coke Oven Gas/O2 Coflow Normal/Inverse Diffusion Flames

In order to analyze the difference between the inverse diffusion flame (IDF) and normal diffusion flame (NDF) under various conditions, the emission spectra of OH* and CH* chemiluminescence in two dimensions measured by hyperspectral and ultraviolet (UV) cameras are described in this article. The results show that CH* mainly appears in the fuel side near the flame front, while OH* distribution can reflect the reaction region of flame. According to the OH* radial distributions in IDF and NDF, the flame can be divided into three parts: the core area of the flame, the transition region of the flame, and the developed region of flame. The peak intensity of CH* in IDF is higher than that in NDF. Moreover, the length of reaction region in NDF increases with O/C equivalence ratio ([O/C]e) until it reaches a steady value, while in IDF the length decreased with the increase of [O/C]e.

Assessment of semi-empirical soot production models in C1–C3 axisymmetric laminar diffusion flames

The objective of this work is to assess the accuracy and limitations of three different semi-empirical soot models: an acetylene/benzene flamelet based two-equation soot model developed by Lindstedt (1994) [4] (in: H. Bockhorn, pp. 417–441) and two laminar smoke point (LSP) based soot models developed by Lautenberger et al. (2005) [15] (Fire Saf. J., 40, pp. 141–176) and by Yao et al. (2011) [14] (Fire Saf. J., 46, pp. 371–387). This task is performed by simulating twenty-seven C1–C3 hydrocarbon-air flames, consisting of normal (NDF) and inverse (IDF) diffusion flames and covering a wide range of residence times and fuel sooting propensities. The acetylene/benzene based soot model predicts the maximum soot volume fractions and the peaks of the integrated soot volume fraction within a factor of 2 of measurements. However, two pre-exponential factors for the surface growth process, one for the C1–C2 hydrocarbons and the other for the C3 hydrocarbons, were necessary in order to obtain this agreement. LSP soot model results are also within a factor of 2 of the measurements for most of the propane, ethylene, propylene and acetylene NDFs. For weakly sooting fuels, such as methane and ethane, the LSP concept overestimates soot production rates. In addition, the model of Lautenberger et al. was found to provide a better description of the soot production in the ethylene IDFs than that of Yao et al. Eventually, computed radiant fractions were compared with experimental data for propane, ethylene and propylene NDFs, showing that predictions using the models of Lindstedt and Lautenberger are on the whole within 10% of the experimental data.

Role of Fluid-Dynamics in Soot Formation and Microstructure in Acetylene-Air Laminar Diffusion Flames

Residence time and thermo-chemical environment are important factors in the soot-formation process in flames. Studies have revealed that flow-dynamics plays a dominant role in soot formation process. For understanding the effect of flow dynamics on soot formation and physical structure of the soot formed in different combustion environments two types of laminar diffusion flames of Acetylene and air, a normal diffusion flame (NDF) and an inverse diffusion flame (IDF) have been investigated. The fuel and air supply in the reaction zone in two flame types were kept constant but the interchange of relative position of fuel and air altered the burner exit Reynolds and Froude numbers of gases, fuel/air velocity ratio and flame shape. Soot samples were collected using thermophoretic sampling on transmission electron microscope (TEM) grids at different flame heights and were analyzed off-line in a Transmission Electron Microscope. Soot primary particle size, soot aggregate size and soot volume fraction were measured using an image analysis software. In NDF the maximum flame temperature was about 1525 K and 1230 K for IDF. The soot primary particles are distinctly smaller in size in IDF (between 19 – 26 nm) compared to NDF (between 29–34 nm). Both NDF and IDF show chainlike branched structure of soot agglomerate with soot particles of a nearly spherical shape. The average number of soot primary particles per aggregate in NDF was in the range of 24 to 40 and in IDF it varied between 16 to 24. Soot volume fraction was between 0.6 to 1.5 ppm in NDF where as it was less than 0.2 ppm in IDF. The change in sooting characteristics of the two flame types is attributed to changed fuel/air velocity ratio, entrainment of gas molecules and thermophoresis on soot particles.