Premixed Ethylene-air Flames Research Articles

Non-periodic shear layer instabilities driving local extinction in premixed ramjet cavity flames

The local extinction characteristics of turbulent premixed ethylene-air flames are experimentally investigated in a high-speed cavity combustor to provide a full characterization of the temporally evolving non-periodic hydrodynamic instabilities that result in local extinction. The local flame extinction phenomenon is captured temporally using high-speed particle image velocimetry (PIV) and broadband chemiluminescence. The evolution of the flame topology, turbulent structures, and flame strain rate are analyzed through the extinction process to distinguish the turbulent shear layer mechanisms contributing to flame instabilities and extinction. A frequency analysis of the extinction event was performed and found the flame extinction to occur at non-periodic temporal intervals. A further wavelet decomposition revealed the presence of a non-periodic hydrodynamic instability in the shear layer causing extinction. The instability mechanism is related to the stability of vortex pairing in the shear layer, which were found to correspond to large-amplitude wave generation on the flame surface. In instances of flame extinction, the vortex pairs exhibit a core deformation from an elliptic instability mechanism, leading eventually to the shredding of the vortical cores, which is the primary mechanism for excessive turbulent flame strain and resulting extinction. Although the local flame extinction mechanism is driven from flame and shear layer interactions, the condition for local flame extinction to occur requires the flame to interact with an elliptical instability in the shear layer. The results can guide strategies to enhance combustion stability, performance, and efficiency.

A Filter-Based Approach for the Measurement of Mass Absorption Coefficient of OC and EC Components in Soot

ABSTRACT There is a general consensus that soot produced by hydrocarbon combustion consists of two components, elemental carbon associated with the graphitic nanostructure and organic carbon. Despite the relevance of evaluating their radiative properties, particularly for organic carbon, the knowledge of the spectral absorption behavior remains limited. In this work, a dual-beam five wavelengths absorption photometer is used to estimate the optical properties of particulate matter generated by a laminar premixed ethylene-air flame at different residence times along the flame. The wavelength dependence of the light absorption of aerosols collected on filters is analyzed in terms of both the optical band gap and the absorption Ångström exponent, AAE. The aerosols are then analyzed in terms of the organic and elemental carbon content by a thermal-optical OC-EC analyzer. Combining the results of light absorption and OC-EC analysis, the spectral mass absorption coefficient of both the organic carbon and elemental carbon components in soot are obtained.

A priori and a posteriori analysis of flamelet modeling for large-eddy simulations of a non-adiabatic backward-facing step

A lean premixed ethylene–air flame in a backstep configuration is simulated on multiple grids using both direct numerical simulations (DNS) with reduced order kinetic mechanism and large eddy simulations (LES) with flamelet-based thermochemistry. The configuration includes preheated reactants and a recirculation zone that provides radicals and high temperature gases to stabilize the flame. Heat losses are present due to the proximity of cooled walls. The reacting flow obtained from DNS at different resolutions is first analyzed to investigate the property of heat transfer within the recirculation region. LES based on adiabatic flamelets with a correction of the heat capacity is then tested, and its ability to account for heat losses is compared to results obtained using a three-dimensional non-adiabatic flamelet approach. Mean fields and subgrid properties are compared to those obtained from DNS to assess the capability of the LES models. The results show that the non-adiabatic flamelet approach can predict recirculation region and temperature fields with good accuracy. The model with heat capacity correction is able to effectively correct the heat capacity behavior as observed by a priori comparisons. However, in the a posteriori context, it is observed to overestimate the temperature field, although the correct size of the recirculation region is predicted. The combined a priori and a posteriori analyses on the same configuration and at different mesh resolutions allow for a precise separation of modeling effects due to heat transfer at the wall and combustion closure, thus providing indications on the LES performance in the context of flamelets.

Soot Size Determination In A Swirl Stratified Premixed Ethylene-Air Flame By A Sampling Probe And Elastic Light Scattering

An experimental study was performed to measure the size and number density distributions of soot particles produced in turbulent, stratified (high spatial equivalence ratio gradient), swirled (rotating flow) premixed ethylene-air flames at atmospheric pressure. Soot particle size and number density measurements are initially performed using a Scanning Mobility Particle Sizer (SMPS). For this purpose, two-stage sampling and dilution device designed for hot gas analysis was used. The dilution rate of the sampling apparatus was evaluated to ensure a most representative probing. In a second step, the ex-situ measurements were replaced by single-shot measurements recorded with a high cadency planar multi-angle light scattering (2D-MALS) laser diagnostic technique, based on the relationship between aggregate size and light scattering angle. In a third step, results obtained by the SMPS as well as by the optical diagnostic were compared. Because a direct confrontation of the obtained results obtained is not feasible due to difference in nature of both techniques, a two-step post-processing methodology as developed. For this purpose, the median electric mobility diameters measured by SMPS were converted into diameters of gyration by means of a semi-empirical conversion tool. Moreover, a time average data processing was applied to the soot size distributions recorded by 2D-MALS, considering that SMPS measurements performed over a relatively large acquisition time converge to a single value. Results obtained in stratified swirled premixed flames by SMPS exhibit an overall good agreement with the optical measurements. This good similarity between the results provides a remarkably high degree of confidence in the quality of the measurements for both measurement systems investigated. Furthermore, the results suggest that a comparison of soot aggregate size and number density measurements between a sampling probe and a light scattering technique is still possible in turbulent flames at atmospheric pressure. While the current work has focused on the performances of SMPS and the 2D-MALS approach on a one-to-one basis, future work will also address the application of these diagnostics to high-pressure two-phase flames for which measurements are made in confined environments.

Influence of potassium chloride on PAH concentration during soot formation studied using laser-induced fluorescence

Previous studies have shown that addition of potassium chloride (KCl) reduces soot concentration and primary particle size. To improve the understanding of this phenomenon, this work focuses on investigating the influence of KCl addition on formation of polycyclic aromatic hydrocarbons (PAHs), assumed to be precursors of soot. The method used is laser-induced fluorescence (LIF), for which different size classes of PAHs can be probed using selected combinations of excitation and detection wavelengths. KCl solutions of different concentrations were aspirated into premixed ethylene-air flames, and spectral and 2D imaging PAH-LIF measurements were performed using laser excitation at 266 nm and 532 nm. PAH-LIF spectra using 266 nm excitation showed a peak in the UV region at around 350 nm and a peak at around 550 nm in visible region, where the latter peak intensity increased strongly with flame height. The normalised PAH-LIF spectra obtained using 532 nm excitation showed no spectral change with flame height. The main results from this study showed that KCl addition, firstly, had negligible influence on the formation of smaller PAHs (measured using excitation at 266 nm and detection at ∼ 320 nm), and secondly, decreased the formation of larger PAHs at increasing flame heights (measured using excitation at 532 nm and detection in the visible region).

In situ characterisation of absorbing species in stationary premixed flat flames using UV\u2013Vis absorption spectroscopy

A one-dimensional premixed ethylene–air flame is investigated regarding the presence of various combustion intermediates and products relevant for the formation of carbonaceous particles for various equivalence ratios and spatial positions using in situ UV–Vis absorption spectroscopy. A laser driven light source in combination with a fast spectrometer allow to record absorption spectra at a high rate required for practical combustion devices. The approach is coupled with a least squares regression procedure using a database of several absorbing species in the flame. To account for the high temperature flame conditions, the absorption spectra are convoluted by a simplified Maxwell–Boltzmann distribution model. While the approach is based on several assumptions and a verification requires future detailed intercomparison with other techniques, a first semi-quantitative evaluation can be obtained. This novel approach opens a potential route to the in situ measurement of the evolution of polycyclic aromatic hydrocarbons (PAHs) in flames.

Molecular structures in flames: A comparison between SNapS2 and recent AFM results

Models capturing the growth of polycyclic aromatic compounds (PACs) and nanoparticles in combustion have always faced a large degree of uncertainty due to the paucity of detailed direct experimental validation. In particular, data on molecular structures, chemical composition, size, cross-linking, and aliphatic chains is still very limited. In the past few years, we have developed an atomistic code, SNapS2, that models the formation of PACs in combustion conditions, providing information on the chemical and structural evolution of these compounds. In this paper, we present a detailed analysis of the compounds formed in a premixed ethylene-air flame that was previously characterized experimentally by AFM (Commodo et al. 2019). The comparison is based on molecular structures and extends to other properties of PACs. The results demonstrate that SNapS2 is able to capture the vast array of PACs in terms of a large variety of functional groups. The simulations, confirmed to a large extent by the experimental data, show the presence of different types of oxygenated (e.g., phenol, ketone) and cyclic (acenaphthylene-type, acenaphthene-type, and fluorene-type five-membered rings) functional groups. Moreover, the structures that were not observed in the simulations (indane-type five-membered rings or six-membered rings containing oxygen) are missing due to the lack of formation pathways, highlighting the fact that important kinetic mechanisms are still missing in the literature. Finally, we observed that a preponderant number of high molecular mass PACs are curved, which may have an impact on their aggregation and sampling. Overall, while some discrepancies remain due to the inherent limitation of the model and the AFM techniques, this work demonstrates the unique capabilities of the SNapS2 code to provide insights on the structures and chemical pathways of PACs.

WIDECARS multi-parameter measurements in premixed ethylene-air flames using a wavelength stable ultrabroadband dye laser.

Width-increased dual-pump enhanced coherent anti-Stokes Raman spectroscopy (WIDECARS) measurements were used to determine the temperature and major species mole fractions in laminar, premixed, ethylene-air flames operating at atmospheric pressure. Conventional ultrabroadband dye lasers for WIDECARS, which use Pyrromethene dyes, have historically suffered from day-to-day wavelength shifting. To overcome this problem, a new ultrabroadband dye laser was developed in this study to provide a stable wavelength and power generation. A new dye laser pumping scheme and a mixture of Sulforhodamine 640, Kiton Red 620, and Rhodamine 640, was used to generate the desired FWHM ${\sim}{15}\;{\rm nm}$∼15nm (${410}\;{{\rm cm}^{ - 1}}$410cm-1) bandwidth. The WIDECARS measured mole fraction ratios of ${{\rm CO}_2}$CO2, CO, and ${{\rm H}_2}$H2 with ${{\rm N}_2}$N2 agreed well with chemical equilibrium calculations.

Hybrid Eulerian-Lagrangian method for soot modelling applied to ethylene-air premixed flames

Soot formation has become an important issue in the design of gas turbine combustors due to its environmental impact and its contribution to radiative heat transfer in the combustion chamber. However, efficient and accurate prediction of soot particles formation, growth, oxidation and interaction in gas turbine combustors is still an open field in computational fluid dynamics. The present approach proposes to combine a reduced gas-phase chemistry, a sectional model for polycyclic aromatic hydrocarbons, and a Lagrangian description of soot particles dynamics. The Lagrangian description has been chosen for its ability to simulate the evolution of the particle size distribution. A numerical procedure is proposed to minimise its CPU cost. This approach was successfully applied to the simulation of steady laminar premixed ethylene-air flames at three fuel equivalence ratios, which constitutes a prerequisite towards its use in an aeronautical combustion chamber.

Lagrangian tracking of soot particles in LES of gas turbines

Expected stringent legislation on particulate matter (PM) emission by gas turbine combustors is currently motivating considerable efforts to better understand, model and predict soot formation. This complex phenomenon is very difficult to study in detail with experiment, and numerical simulation is an essential complementary tool. Considering that the chemistry of soot particles strongly depends on their size, the numerical prediction of soot formation requires the description of their size distribution. To do so, either Eulerian methods (sectional or moments), or stochastic Lagrangian approaches are reported in the literature. In the present work a far more simple semi-deterministic Lagrangian approach is proposed. Combined to the semi-empirical model of Leung et al. (1991) for soot chemistry, the Lagrangian approach is first validated on a one-dimensional premixed ethylene-air flame. The model is then applied to a gaseous non-premixed ethylene-air burner measured at DLR and computed with Large Eddy Simulation (LES). The gaseous chemistry is described with an Analytically Reduced Chemistry (ARC) to guarantee a good prediction of combustion and gaseous soot precursors. Results are validated against experiment and compared, in terms of accuracy and CPU cost, to an Eulerian semi-empirical model. To the authors knowledge, it is the first time that such Lagrangian particle tracking approach is used for soot. Results obtained in terms of accuracy and computing time are very encouraging.

Coherent anti-Stokes Raman spectroscopy of a premixed ethylene–air flame in a dual-mode scramjet

As part of a broader effort to provide detailed measurements of turbulent flames in dual-mode scramjets, promote a deeper understanding of the relevant combustion physics, and aid appropriate computational model development, a high-subsonic cavity-stabilized premixed ethylene–air flame (typical of ramjet operation) is studied using coherent anti-Stokes Raman spectroscopy. This technique provides simultaneous measurements of temperature, separate O2 temperature, and mole fraction of N2, O2, CO, CO2, and C2H4. The experiments reveal a highly unsteady turbulent flame, approximately two-dimensional in the mean, which propagates downstream from the cavity and towards the observation wall. Measurements in the flame region reveal a flow that is largely divided into reactants (freestream fluid) and (nearly) equilibrium products, separated by flames that cannot be resolved by the CARS measurement volume, which is about 1 mm long. Mean and standard deviation of the resolved fluctuations of the temperature and mole fractions of species are quantified. The peak standard deviation in each profile across the flame occurs where the mean gradient is steepest, and is about 37% the difference between reactants (freestream) and products conditions. Several cases were investigated including limiting combustion cases near the lean fuel and low air temperature blowouts; in all cases the flame propagation angles are the same and distributions of suitably normalized mean and fluctuation parameters are similar at all locations and for all cases.

Electrical characterization of flame-soot nanoparticle thin films

Carbon nanoparticles, with diameter d ∼10nm, were produced in a premixed ethylene-air flame and used to grow self-assembled thin films by thermophoretic deposition on Si++/SiO2/Gold multilayer substrates inserted in flame.The particles’ size was measured by a Scanning Mobility Particle Sizer. UV–vis light absorption, Raman spectroscopy and Atomic Force Microscopy were used for the chemico-physical and morphological investigation of the carbon nanoparticle thin films. The electrical characterization was then performed in different environmental conditions with the basic aim to assess the potentiality of these films to be employed as electrically-active components in electronic and sensing applications.The films are composed by the aggregation of nanoscale grains whose size increases with the deposition time and are consequently highly porous with the electrical conduction properties showing a dependence on thickness suggesting a percolation-like behavior. Raman spectrum indicates that the chemical/structural composition of the film does not change with deposition time and consists in small graphitic crystallites with constant size.The electrical measurements show also the ohmic behavior of the IV curves. Moreover, an ambipolar response of the CNPs channels, when investigated in the field-effect configuration, was observed and, in parallel with conductivity, charge carrier mobility values were found to increase with thickness.

Soot with 1013 cm−3 high concentration and 25 Å radius of gyration as detected by small-angle X-ray scattering in a premixed ethylene-air flame at sooting threshold

SAXS experiments on a premixed ethylene-air flame burning at the so-called sooting threshold, i.e., a flame with Carbon-to-Oxygen molecular ratio between fuel C2H4 and air equal to [C]/[O] = 0.65 (ΦAir=25cm3/min, ΦC2H4=11.7cm3/min) is the object of this work. Measurements are performed at synchrotron SOLEIL / beamline SWING under the approved proposal #20130463. The height above burner considered for this case study is z=21mm (τres about 10ms). The background is made by considering an identical lean flame with [C]/[O]=0.19. The data processing approach consisting in subtracting to the signal S, the X-ray scattering from the air molecules surrounding the flame as background BCKG, is here revealed to be incorrect, owing to the fact that sometimes the S-BCKG evaluated as above is a negative quantity. This is caused by the much higher cool gas molecule number density at ambient conditions (300K) with respect to the lower gas molecule density inside the flame at hot temperatures (about 1800–2100K). Vice versa, by using a lean flame one obtains as BCKG the scattering contribute from the gas molecules alone in the absence of soot at about the identical premixed flame under test, which results in appreciating even the low signal from ultrafine incipient soot nanoparticles. The main results from the fit of SAXS data at larger q's are two: first, the demonstration of ultrafine nanoparticles, with gyration radius Rg=25±10Å, as inferred by the Guinier regime fit, and overall size D=45±10Å, as evaluated through Kratky plots; second, the fact that these entities are present in the premixed flame with a huge concentration, about N=1013cm−3. Such nanoparticles with D=45Å (Rg=25Å), do coexist contemporarily to isolated 20nm primaries and 160nm nanoparticle aggregates, which are observed with a number concentrations 6 and 8 order of magnitudes smaller with respect to the one of ultrafine nanoparticles.Our SAXS results do accord very well previous findings on size of premixed flame soot nanoparticles reported both by Wersborg et al. (1973) from molecular beam sampling and TEM analysis and, with concern to the number concentration, the results by Minutolo et al. (1998) and Sgro et al. (2009).

Effect of furanic biofuels on particles formation in premixed ethylene–air flames: An experimental study

Furanic fuels have been recently investigated because they are considered possible fossil fuel substituents due to their high energy density, their renewable origin and consequently low green-house gas emissions. Few data are available about the behaviors of these fuels in laboratory flame reactors. We have recently studied their sooting tendency in a counter-flow diffusion flame showing a different propensity to form particulate matter depending on the richness of the combustion conditions. In this paper, we explore their tendency in forming particulate matter in atmospheric pressure premixed flames burning mixtures of ethylene and furanic fuels at four different equivalence ratios, namely 2.01, 2.16, 2.31 and 2.46, thus covering the range from slightly sooting to fully sooting conditions. Liquid furan, 2-methylfuran and 2,5-dimethylfuran have been added to ethylene as 10% and 20% of the total carbon fed to the flame.In-situ spectroscopy, namely laser UV-induced emission, has been used as diagnostic tool to detected different classes of precursor nanoparticles by changing the detection wavelength from the UV to the visible. Laser induced incandescence has instead been used to detect soot particles. Equivalence ratios, the total carbon flow rate and cold gas velocity have been kept constant, allowing the combustion temperature to be almost the same also when furans were added.LII signal decreases significantly when furans are added showing a strong effect of furanic fuels in decreasing the amount of soot formed in atmospheric pressure premixed flames. Particularly the reduction increases as the equivalence ratio increases. On the other side, LIF UV and LIF VIS slightly decrease when furans are added. The absence of a significant reduction of the LIF UV signal when furans are added suggests that small particle formation is not effectively inhibited by these additives. This behavior is similar to what found for ethanol and dimethyl ether addition to ethylene premixed flames, showing a similarity in the behavior of these oxygenated fuels.

WIDECARS spectra fitting in a premixed ethylene–air flame

WIDECARS measures temperature and mole fractions of most of the major species in ethylene–air flames. One of the issues in implementing this technique is fitting the experimental spectra to theory to obtain flame conditions (temperature, species mole fractions). Individual spectra contain many species resonances, and theory is slow to compute. Libraries of precalculated spectra can be used, but a library of sufficient density for accurate interpolation is large given the many variables. A new fitting algorithm is presented which utilizes a less‐dense library, and additional spectra are calculated during fitting to maintain accuracy. The iterative convergence method converts the problem of minimizing fit error, which converges slowly, to a zero finding problem, which converges reliably, rapidly, and accurately to best fit. Various practical fitting issues, such as the effects of dye laser mode noise and variability, phase‐matching efficiency, and shifts of the spectrum on the spectrometer are addressed. The technique is demonstrated in the analysis of experimental measurements in an equivalence ratio 2.1 ethylene–air flame above the surface of a McKenna burner. Precision errors because of experimental and fitting effects are discussed. Copyright © 2015 John Wiley & Sons, Ltd.

Simultaneous CO concentration and temperature measurements using tunable diode laser absorption spectroscopy near 2.3 μm

Simultaneous measurements of carbon monoxide (CO) mole fraction and temperature using tunable diode laser absorption spectroscopy (TDLAS) near 2.3 μm are reported. The measurement method uses ro-vibrational transitions [R(27): v″ = 1 → v′ = 3] and [R(6): v″ = 0 → v′ = 2] in the first overtone band of CO near 2.3 μm (~4,278 cm−1). The measurements were performed in the post flame environment of fuel rich premixed ethylene–air flames with a N2 co-flow, stabilized over a water cooled McKenna burner. Non-uniformity in the temperature and CO mole fraction, along the absorption line of sight, in the mixing layer of the co-flow, was considered during data analysis. The TDLAS based temperature measurements (±80 K) were in good agreement with those obtained using N2 vibrational coherent anti-Stokes Raman scattering (±20 K), and the CO mole fraction measurements were in good agreement with the equilibrium values, for equivalence ratios lower than 1.8. A signal to noise ratio of 45 was achieved at an equivalence ratio of 1 for a CO concentration of 0.8 % at 1,854 K.

LII–lidar: range-resolved backward picosecond laser-induced incandescence

A novel concept for remote in situ detection of soot emissions by a combination of laser-induced incandescence (LII) and light detection and ranging (lidar) is presented. A lidar setup based on a picosecond Nd:YAG laser and time-resolved signal detection in the backward direction was used for LII measurements in sooty premixed ethylene–air flames. Measurements of LII–lidar signal versus laser fluence and flame equivalence ratio showed good qualitative agreement with data reported in literature. The LII–lidar signal showed a decay consisting of two components, with lifetimes of typically 20 and 70 ns, attributed to soot sublimation and conductive cooling, respectively. Theoretical considerations and analysis of the LII–lidar signal showed that the derivative was proportional to the maximum value, which is an established measure of soot volume fraction. Utilizing this, differentiation of LII–lidar data gave profiles representing soot volume fraction with a range resolution of ~16 cm along the laser beam propagation axis. The accuracy of the evaluated LII-profiles was confirmed by comparison with LII-data measured simultaneously employing conventional right-angle detection. Thus, LII–lidar provides range-resolved single-ended detection, resourceful when optical access is restricted, extending the LII technique and opening up new possibilities for laser-based diagnostics of soot and other carbonaceous particles.

Development of laser absorption techniques for real-time, in-situ dual-species monitoring (NO/NH3, CO/O2) in combustion exhaust

Simultaneous dual-species monitoring offers potential for control of large-scale practical combustion systems. The development and demonstration of two dual-species sensors for characterizing NOx abatement (NO/NH3) and combustor performance (CO/O2) are described for potential application in boiler exhaust at coal-fired electric utilities. Tunable laser absorption sensors for simultaneous in-situ detection of these paired species were developed using fundamental-band vibrational transitions in the mid-infrared near 5.2μm for NO, combination-band transitions near 2.25μm for NH3, overtone-band transitions near 2.3μm for CO, and electronic transitions in the b-X system near 760nm for O2. Scanned-wavelength, 1f-normalized wavelength modulation spectroscopy with second harmonic detection (WMS-2f) was employed for real-time data processing. Spatial- and time-demultiplexing strategies were used to combine and separate the laser signals. The sensors were tested for simultaneous, continuous monitoring in laboratory combustion exhaust from a premixed ethylene-air flame at atmospheric pressure and varied equivalence ratios with exhaust temperature of ∼620K. A retro-reflected 3.58m beam-path was used to mimic a single-ended installation in a boiler exhaust duct. NH3 mixtures were metered into the flame at different rates to test the response of the NO/NH3 sensor, and the CO/CO2 ratio was adjusted by fuel/air equivalence ratio. Trends in the measured concentration ratio of NO to NH3 were found to agree qualitatively with theoretical expectation, and the CO and O2 measurements were confirmed by analysis of sampled gases. The laser absorption exhibited the fast time response needed for control sensors. These fast-response, simultaneous dual-species sensors for NO/NH3 and CO/O2 show excellent promise for control and optimization of NOx abatement and furnace efficiency in practical combustion systems.

Soot characterization with laser-induced incandescence applied to a laminar premixed ethylene–air flame

Laser-induced incandescence (LII) has emerged as a promising non-invasive technique for measuring spatially and temporally resolved soot volume fraction and size. In this investigation we try to assess its performance in more detail by characterizing primary particle sizes using time-resolved LII and soot volume fractions by 2D LII. The experiments were performed at a fixed location in premixed ethylene/air flames burning on a sintered stainless-steel plug (McKenna) burner with varying values of the equivalence ratio for primary particle sizing and on the burner axis for concentration measurements. Maximum soot concentrations follow a power law behavior with equivalence ratio. The primary particle sizes obtained from LII decay curves are in good agreement with the values measured by other techniques and show a clear rise of particle size with equivalence ratio. The data analyzed with the help of a validated LII model will be useful for the further development of soot formation models.

Synchrotron radiation studies of additives in combustion I: Water

Small Angle X-ray Scattering (SAXS) measurements have been performed on a partially pre-mixed ethylene–air flame with and without the introduction of a water aerosol for the same flame height. Our measurements are sensitive to particles with radii in the range of 6–78 nm. It is found that water addition leads to soot suppression below the detection limit at low heights along the flame centerline (<11 mm) and to an increased sooting tendency higher up in the flame. There, larger primary particles (Rg∼20–45 nm) shaped as agglomerates with fractal dimension about 2 are observed together with a large number of smaller (Rg ∼ 10 nm) smooth particles, which we identify with sub-primary units observed in previous work on pure diffusion flames. In the case of the un-doped partially pre-mixed flame, however, only relatively (Rg ∼ 20–35 nm) large spherical particles (Porod’s exponent about 4) are observed. An intriguing result is that water addition induces the enhanced formation of sub-primaries, with these having more than 2 orders of magnitude higher number concentration with respect to the spherical particles in the water free flame.