Mariano Sirignano - Academia.edu (original) (raw)

Papers by Mariano Sirignano

A new developed model is used to gain insights on chemical composition and internal structure of ... more A new developed model is used to gain insights on chemical composition and internal structure of nano-sized particles formed in diffusion flames. Model allows simulating total amount of particulate matter produced in flames, its H/C ratio and morphological details. These enhancements result of great interest to better understand the pathways controlling the molecule-to-particle transition in flames. In particular, diffusion flames allow the evolution of gas phase and particle inception and growth in pyrolytic and oxidative condition to be followed. The two different combustion environments lead to quite different nascent particles in terms of chemical composition and internal structure. In pyrolytic region, aromatic molecule dimerization is the controlling mechanism leading to particles inception resulting in well ordered particle nuclei. Radical-molecule mechanism is the controlling growth mechanism in high temperature oxidation region. Dehydrogenation and surface addition reactions affect the final structure of the emitted soot particles as well.

Particle formation in fuel-rich combustion has been the subject of many studies in the last years... more Particle formation in fuel-rich combustion has been the subject of many studies in the last years in order to clarify the controlling pathways leading to their nucleation. Studies on premixed laminar flat flames have allowed to focus the attention on the physical-chemical processes occurring, almost neglecting the fluid-dynamic effects due to different and more complicated geometries. In rich premixed flames a bimodal particle size distribution function has been generally observed. The modal peak at around 20nm is universally accepted as soot mode, whereas the origin and evolution of the smaller particle mode, which has typical size of 2-3nm, is still controversial. The smaller particles can be formed through fast radical reactions occurring just downstream of the flame front and can survive in flame as consequence of their low coagulation rate. Moreover a competition between a persistent nucleation and a coagulation mechanism might be responsible for the evolution of the small particles along the flame. Particles formation process can also vary with flame temperature and fuel composition. The object of the present work is to perform a comparative study on the effect of fuel chemical structure on particle inception in rich premixed flames. Both experimental and modeling approach are used. In situ optical techniques based on the interaction of UV-light with particles have been adopted. Modeling of flame structure and particle formation has been performed by using a complete detailed scheme that provides both for gas phase and for particles reactions, the latter adopting a sectional method. This approach allows to model simultaneously major and minor species in gas phase and to predict the size distribution functions for particles. Benzene and n-hexane have been studied. Particle inception in these flames has showed a sensitivity to the chemical nature of the fuels. Results obtained from measurements have been confirmed by the modeling.

In the this work an experimental and numerical study on particles formation in opposedflow diffus... more In the this work an experimental and numerical study on particles formation in opposedflow diffusion flames, burning methane and ethylene as fuel is presented. Spectrally resolved laser induced emission spectroscopy techniques in the ultraviolet and visible, such as fluorescence and incandescence , are here used in order to investigate particles inception and growth with high spatial resolution and in a wide range of flame conditions. The fourth harmonic of a Nd:YAG laser, 266 nm, is used with the purpose to follow combustion generated compounds and their dynamics in flame. A complete kinetic scheme, which provides both to gas-phase and particle reactions, is adopted for numerical simulation. The comparison between experimental results and numerical predictions gives a qualitative view of the mechanism of particle formation. Both the experimental and numerical results carried out in this work demonstrate and explain the sensibility of inception and growth of soot to radicals concentrations and temperature conditions.

In order to go deep inside the nature and chemistry of combustion produced particles a kinetic mo... more In order to go deep inside the nature and chemistry of combustion produced particles a kinetic modeling approach is proposed. It is based on the modeling of gas-to-particle transition through sections in which 100 lumped species are used having a C number ranging from 24 to 4x10(8) and H/C ratio ranging from 0.2 to 1. This approach gives the possibility to numerically follow not only the mass evolution of particles but also their size distribution function and hydrogen content. The model is tested in a premixed flat flame of ethylene/oxygen with C/O=0.8. Comparison of modeled results with experimental data is reasonable both in terms of concentration of species and H/C ratio.

Proceedings of the Combustion Institute, 2015

ABSTRACT Particle oxidation is one of the steps still not completely understood in combustion. Mo... more ABSTRACT Particle oxidation is one of the steps still not completely understood in combustion. Most of the approaches are based on semi-empirical reaction rates. Correct evaluation of oxidation is needed to predict the final emission of particles in diffusion flames. Fragmentation has recently been proposed to be a controlling step in determining global soot burn out as well as the size of particles emitted.The oxidation and fragmentation of soot particles is studied in a counterflow diffusion flames with in situ optical diagnostics, laser-induced incandescence and elastic light scattering. A sectional modeling approach is used to predict particle formation and burnout.Two counterflow diffusion flames have been chosen, a soot formation (SF) and a soot formation/oxidation (SFO) flame.Experimental data supported by model predictions show the role of fragmentation in controlling the burn-out and the size distribution of particles in flames. SF flame, where no soot oxidation occurs, shows large particles. By contrast in the SFO flame, the mean diameter of particles shows that when fragmentation is active coagulation is less effective, aggregates are hardly formed and primary particles with small size are mostly formed.

Proceedings of the Combustion Institute, 2015

ABSTRACT Furans have recently raised as possible transportation fuels which can be produced from ... more ABSTRACT Furans have recently raised as possible transportation fuels which can be produced from biological sources and biotechnological methods. Their role on combustion-generated particle formation results quite unexplored. Few studies showed that dimethylfuran (DMF) among the other furanic hydrocarbons seems to have a great tendency to form soot precursors. This unexpected trend should be kept in mind before a further usage of furans, especially DMF, as transportation fuels. The effect of furans as substituent to traditional fuels has been investigated both experimentally and numerically in a counter-flow diffusion flame. Furan, 2-methylfuran and 2,5-dimethylfuran have been chosen as standards for furanic fuels. Optical techniques, previously validated, namely laser induced fluorescence and incandescence, have been adopted to detect small nanoparticles and soot aggregates, respectively. On the modeling part, a sectional approach has been used to confirm the sooting tendency experimentally found for the investigated fuels. A gas phase kinetic mechanism available in literature for oxidation and decomposition of furans has been integrated in a recently developed detailed kinetic mechanism for particulate formation. Experiments confirmed by model show that furanic fuels increase particle production. Furan has the lowest impact on the particle production whereas 2,5-dimethylfuran and 2-methylfuran show a higher propensity; in particular, 2-methylfuran has a greater tendency to produce particles with respect to 2,5-dimethylfuran. Modeling analysis showed that 2-methylfuran produces large amounts of C4-species and thus benzene and polycyclic aromatic hydrocarbons (PAHs). 2,5-dimethylfuran mainly forms phenol during its decomposition, which leads to cyclopentadiene and thus naphthalene formation. However, in the operating conditions analyzed in this study the overall PAHs and first particle production result less strong than in 2-methylfuran. However, different combustion conditions can change the effectiveness of these channels and might invert the particle production tendency of furanic fuels.

Chemical Engineering Transactions, 2010

New challenge in combustion is the extended use of" clean&am... more New challenge in combustion is the extended use of" clean" fuels, such as methane, in" particle free" combustion devices. Although large efforts have been made, actual combustion systems burning methane are still source of particulate that might be emitted in the atmosphere. In this paper measurements of ultrafine particle emission from premixed laboratory flames and several practical combustion systems including domestic heaters and IC engines, all burning methane or natural gas are presented. A new diagnostic tool ...

Proceedings of the Combustion Institute, 2009

An experimental and numerical study on particles inception and growth is performed in opposed-flo... more An experimental and numerical study on particles inception and growth is performed in opposed-flow diffusion flames of ethylene and air characterized by different sooting tendencies. Spectrally resolved UV-visible laser induced fluorescence, laser induced incandescence and laser light scattering measurements are used to characterize different classes of combustion-generated compounds. A detailed kinetic model accounting for both gas-phase and particle formation is

Proceedings of the Combustion Institute, 2013

ABSTRACT Coagulation of combustion-generated particles has been investigated in low and intermedi... more ABSTRACT Coagulation of combustion-generated particles has been investigated in low and intermediate temperature regimes in a tubular reactor with a residence time of 1.65 s. Particles, generated by premixed ethylene/air flames with equivalence ratios above the soot threshold limit, are fed to a tubular reactor, which can be operated at temperatures up to 650 K. A wide range of equivalence ratios are used to generate particles with different characteristics. The evolution of the particle size distributions has been evaluated by a differential mobility analyzer with high sensitivity in the 2–100 nm size range. The effect of the reactor temperature on coagulation has been systematically studied. Particles exhibit different coagulation efficiencies at the different temperatures. At room temperature, 2–4 nm particles fed to the reactor coagulate forming particles as large as 10–20 nm, whereas at higher temperatures the size distribution of the particles does not change with respect to that measured at the inlet of the reactor. This behavior suggests a very ineffective coagulation efficiency at higher temperatures for small nanoparticles. Larger particles do not exhibit this high sensitivity to temperature, substantially maintaining very high coagulation efficiencies. These considerations have been confirmed by numerical simulations conducted both with constant and size-dependent coagulation efficiency. The numerical results confirm that also at low and intermediate temperature regimes, the use of a size-dependent coagulation efficiency is mandatory to match the evolution of the particles during coagulation. On the other hand, the simple model of coagulation based on the van der Waals interactions between particles in the framework of gas kinetic collision theory is in slightly disagreement with the experimental results for very small particles, suggesting that more advanced modeling based on quantum mechanism and molecular dynamics are necessary to correctly reproduce the experimental data.

Proceedings of the Combustion Institute, 2011

The evolution of the molecular weight (MW) distribution and structural properties of carbon parti... more The evolution of the molecular weight (MW) distribution and structural properties of carbon particulate formed in methane, ethylene and benzene fuel-rich premixed flames, burning in similar conditions of maximum flame temperature, was experimentally measured and modeled.Both solubility and chromatographic separation of the carbon particulate allowed to follow the variation of molecular weight distributions of the different fractions. Structural properties of

Fuel, 2014

ABSTRACT The role of dimethyl ether (DME) as substituent to ethylene on particulate formation has... more ABSTRACT The role of dimethyl ether (DME) as substituent to ethylene on particulate formation has been evaluated in premixed and counter-flow diffusion flames. In the premixed flame, the equivalence ratio has been changed from 1.95 to 2.61 and dimethyl ether has been added from 2% to 30% of the total carbon fed. In the counter-flow diffusion flame, the addition of DME has been from 0% to 60% of total carbon fed. Laser induced fluorescence and incandescence have been used to follow the soot formation process: UV and visible fluorescence signals have been attributed to aromatic macromolecules and incipient nanoparticles, respectively, whereas incandescence has been attributed to soot particles and aggregates. In premixed flames results evidence that the formation of soot precursors is not so sensitive to DME addition. In very rich combustion environments, DME addition cannot completely avoid the formation of small precursors, although it can slow down the formation process. This behavior has been observed for all the equivalence ratios investigated. In the pyrolysis region of counter-flow diffusion flames, the formation of aromatic small precursors and soot particles is increased for small DME percentages, up to 20%. Then, the precursors are suppressed for larger amounts, going below the detection limit when 60% of DME is used. This suggests that DME can enhance the production of radicals and small reactive molecules in the pyrolytic side when it is added in small concentrations. For larger amounts of DME, oxidative pathways prevail and the conversion of carbon to precursors and then to soot is inhibited. In the oxidative region DME starts to effectively decrease the particle reduction also for amounts as small as 10–15%.

Experimental Thermal and Fluid Science, 2010

Measurements of ultrafine particles have been performed at the exhaust of a low emission microtur... more Measurements of ultrafine particles have been performed at the exhaust of a low emission microturbine for power generation. This device has been fuelled with liquid fuels, including a commercial diesel oil, a mixture of the diesel oil with a biodiesel and kerosene, and tested under different loads. Primarily attention has been focused on the measurements of the size distribution functions of the particles emitted from the system by using particle differential mobility analysis. A bimodal size distribution function of the particle emitted has been found in all the examined conditions. Burning diesel oil, the first mode of the size distribution function of the combustion-formed particles is centered at around 2-3 nm, whereas the second mode is centered at about 20-30 nm. The increase of the turbine load and the addition of 50% of biodiesel has not caused changes in the shape of size distribution of the particles. A slightly decrease of the amount of particle formed has been found. By using kerosene the amount of emitted particles increases of more than one order of magnitude. Also the shape of the size distribution function changes with the first mode shifted towards larger particles of the order of 8-10 nm but with a lower emission of larger 20-30 nm particles. Overall, in this conditions, the mass concentration of particles is increased respect to the diesel oil operation. Particle sizes measured with the diesel oil have been compared with the results on a diesel engine operated in the same power conditions and with the same fuel. Measurements have showed that the mean sizes of the formed particles do not change in the two combustion systems. However, diesel engine emits a number concentration of particles more than two orders of magnitude higher in the same conditions of power and with the same fuel. By running the engine in more premixed-like conditions, the size distribution function of the particles approaches that measured by burning kerosene in the microturbine indicating that the distribution function of the sizes of the emitted particles can be strongly affected by combustion conditions.

Experimental Thermal and Fluid Science, 2012

ABSTRACT In the present study a laminar premixed ethylene flame was doped with different amounts ... more ABSTRACT In the present study a laminar premixed ethylene flame was doped with different amounts of ethanol in order to understand the influence on biofuels in the total amount of particulate matter and the size distribution functions of the formed carbonaceous particles. Four different flames were investigated: one of pure ethylene as reference and three with an increased amount of ethanol, i.e. 10%, 20% and 30%. The equivalence ratio of the four flames was kept constant at 2.01. The particle size distribution functions were obtained using a nano-Differential Mobility Analyzer (nano-DMA). The results showed a general reduction of the total volume fraction of particulate when ethanol was added. The reduction of the formed particles increased as a function of the amount of ethanol added. The particle size distribution for pure ethylene flame and ethanol doped flames at different heights with the same amount of particles are quite similar. This indicates that in premixed flames, ethanol mostly affects gas phase chemistry and consequently the formation of incipient particles.

Energy & Fuels, 2013

A detailed kinetic mechanism of aromatic growth, particulate formation, and oxidation is presente... more A detailed kinetic mechanism of aromatic growth, particulate formation, and oxidation is presented and is tested in nonpremixed laminar flames of methane and ethylene at atmospheric pressure. Model development is refined in strict connection with new experimental data on the formation and oxidation of high molecular mass compounds and incipient particles. Reaction pathways leading to the formation of incipient particles, their transformation to soot, their oxidation, and the oxidation-induced fragmentation of particles and aggregates have been included by using a multisectional approach for the particle process. Predictions within a factor of 2−3 are obtained for major oxidation and pyrolysis products as well as trace aromatic species and particulate concentrations. The newly developed model predicts the concentration of the particles, their sizes, morphology, and chemical properties in nonpremixed flames of methane and ethylene with a wide range of particle formation without any condition-dependent adjustments to the kinetic scheme. A wide range of particle sizes is covered from nanoparticles formed on the fuel side of the flames to larger soot particles and particle aggregates formed in the flame wings. The trend of the H/C ratio of the particles along the flame axis is also predicted well. It decreases to very low values typical of mature soot particles when large aggregates are produced. The new mechanism for particle oxidation, which includes the oxidation-induced fragmentation of particles and aggregates, has shown the importance of accurate modeling of particle oxidation to correctly predict particle burnout and particle size in nonpremixed flames.

A new developed model is used to gain insights on chemical composition and internal structure of ... more A new developed model is used to gain insights on chemical composition and internal structure of nano-sized particles formed in diffusion flames. Model allows simulating total amount of particulate matter produced in flames, its H/C ratio and morphological details. These enhancements result of great interest to better understand the pathways controlling the molecule-to-particle transition in flames. In particular, diffusion flames allow the evolution of gas phase and particle inception and growth in pyrolytic and oxidative condition to be followed. The two different combustion environments lead to quite different nascent particles in terms of chemical composition and internal structure. In pyrolytic region, aromatic molecule dimerization is the controlling mechanism leading to particles inception resulting in well ordered particle nuclei. Radical-molecule mechanism is the controlling growth mechanism in high temperature oxidation region. Dehydrogenation and surface addition reactions affect the final structure of the emitted soot particles as well.

Particle formation in fuel-rich combustion has been the subject of many studies in the last years... more Particle formation in fuel-rich combustion has been the subject of many studies in the last years in order to clarify the controlling pathways leading to their nucleation. Studies on premixed laminar flat flames have allowed to focus the attention on the physical-chemical processes occurring, almost neglecting the fluid-dynamic effects due to different and more complicated geometries. In rich premixed flames a bimodal particle size distribution function has been generally observed. The modal peak at around 20nm is universally accepted as soot mode, whereas the origin and evolution of the smaller particle mode, which has typical size of 2-3nm, is still controversial. The smaller particles can be formed through fast radical reactions occurring just downstream of the flame front and can survive in flame as consequence of their low coagulation rate. Moreover a competition between a persistent nucleation and a coagulation mechanism might be responsible for the evolution of the small particles along the flame. Particles formation process can also vary with flame temperature and fuel composition. The object of the present work is to perform a comparative study on the effect of fuel chemical structure on particle inception in rich premixed flames. Both experimental and modeling approach are used. In situ optical techniques based on the interaction of UV-light with particles have been adopted. Modeling of flame structure and particle formation has been performed by using a complete detailed scheme that provides both for gas phase and for particles reactions, the latter adopting a sectional method. This approach allows to model simultaneously major and minor species in gas phase and to predict the size distribution functions for particles. Benzene and n-hexane have been studied. Particle inception in these flames has showed a sensitivity to the chemical nature of the fuels. Results obtained from measurements have been confirmed by the modeling.

In the this work an experimental and numerical study on particles formation in opposedflow diffus... more In the this work an experimental and numerical study on particles formation in opposedflow diffusion flames, burning methane and ethylene as fuel is presented. Spectrally resolved laser induced emission spectroscopy techniques in the ultraviolet and visible, such as fluorescence and incandescence , are here used in order to investigate particles inception and growth with high spatial resolution and in a wide range of flame conditions. The fourth harmonic of a Nd:YAG laser, 266 nm, is used with the purpose to follow combustion generated compounds and their dynamics in flame. A complete kinetic scheme, which provides both to gas-phase and particle reactions, is adopted for numerical simulation. The comparison between experimental results and numerical predictions gives a qualitative view of the mechanism of particle formation. Both the experimental and numerical results carried out in this work demonstrate and explain the sensibility of inception and growth of soot to radicals concentrations and temperature conditions.

In order to go deep inside the nature and chemistry of combustion produced particles a kinetic mo... more In order to go deep inside the nature and chemistry of combustion produced particles a kinetic modeling approach is proposed. It is based on the modeling of gas-to-particle transition through sections in which 100 lumped species are used having a C number ranging from 24 to 4x10(8) and H/C ratio ranging from 0.2 to 1. This approach gives the possibility to numerically follow not only the mass evolution of particles but also their size distribution function and hydrogen content. The model is tested in a premixed flat flame of ethylene/oxygen with C/O=0.8. Comparison of modeled results with experimental data is reasonable both in terms of concentration of species and H/C ratio.

Proceedings of the Combustion Institute, 2015

ABSTRACT Particle oxidation is one of the steps still not completely understood in combustion. Mo... more ABSTRACT Particle oxidation is one of the steps still not completely understood in combustion. Most of the approaches are based on semi-empirical reaction rates. Correct evaluation of oxidation is needed to predict the final emission of particles in diffusion flames. Fragmentation has recently been proposed to be a controlling step in determining global soot burn out as well as the size of particles emitted.The oxidation and fragmentation of soot particles is studied in a counterflow diffusion flames with in situ optical diagnostics, laser-induced incandescence and elastic light scattering. A sectional modeling approach is used to predict particle formation and burnout.Two counterflow diffusion flames have been chosen, a soot formation (SF) and a soot formation/oxidation (SFO) flame.Experimental data supported by model predictions show the role of fragmentation in controlling the burn-out and the size distribution of particles in flames. SF flame, where no soot oxidation occurs, shows large particles. By contrast in the SFO flame, the mean diameter of particles shows that when fragmentation is active coagulation is less effective, aggregates are hardly formed and primary particles with small size are mostly formed.

Proceedings of the Combustion Institute, 2015

ABSTRACT Furans have recently raised as possible transportation fuels which can be produced from ... more ABSTRACT Furans have recently raised as possible transportation fuels which can be produced from biological sources and biotechnological methods. Their role on combustion-generated particle formation results quite unexplored. Few studies showed that dimethylfuran (DMF) among the other furanic hydrocarbons seems to have a great tendency to form soot precursors. This unexpected trend should be kept in mind before a further usage of furans, especially DMF, as transportation fuels. The effect of furans as substituent to traditional fuels has been investigated both experimentally and numerically in a counter-flow diffusion flame. Furan, 2-methylfuran and 2,5-dimethylfuran have been chosen as standards for furanic fuels. Optical techniques, previously validated, namely laser induced fluorescence and incandescence, have been adopted to detect small nanoparticles and soot aggregates, respectively. On the modeling part, a sectional approach has been used to confirm the sooting tendency experimentally found for the investigated fuels. A gas phase kinetic mechanism available in literature for oxidation and decomposition of furans has been integrated in a recently developed detailed kinetic mechanism for particulate formation. Experiments confirmed by model show that furanic fuels increase particle production. Furan has the lowest impact on the particle production whereas 2,5-dimethylfuran and 2-methylfuran show a higher propensity; in particular, 2-methylfuran has a greater tendency to produce particles with respect to 2,5-dimethylfuran. Modeling analysis showed that 2-methylfuran produces large amounts of C4-species and thus benzene and polycyclic aromatic hydrocarbons (PAHs). 2,5-dimethylfuran mainly forms phenol during its decomposition, which leads to cyclopentadiene and thus naphthalene formation. However, in the operating conditions analyzed in this study the overall PAHs and first particle production result less strong than in 2-methylfuran. However, different combustion conditions can change the effectiveness of these channels and might invert the particle production tendency of furanic fuels.

Chemical Engineering Transactions, 2010

New challenge in combustion is the extended use of" clean&am... more New challenge in combustion is the extended use of" clean" fuels, such as methane, in" particle free" combustion devices. Although large efforts have been made, actual combustion systems burning methane are still source of particulate that might be emitted in the atmosphere. In this paper measurements of ultrafine particle emission from premixed laboratory flames and several practical combustion systems including domestic heaters and IC engines, all burning methane or natural gas are presented. A new diagnostic tool ...

Proceedings of the Combustion Institute, 2009

An experimental and numerical study on particles inception and growth is performed in opposed-flo... more An experimental and numerical study on particles inception and growth is performed in opposed-flow diffusion flames of ethylene and air characterized by different sooting tendencies. Spectrally resolved UV-visible laser induced fluorescence, laser induced incandescence and laser light scattering measurements are used to characterize different classes of combustion-generated compounds. A detailed kinetic model accounting for both gas-phase and particle formation is

Proceedings of the Combustion Institute, 2013

ABSTRACT Coagulation of combustion-generated particles has been investigated in low and intermedi... more ABSTRACT Coagulation of combustion-generated particles has been investigated in low and intermediate temperature regimes in a tubular reactor with a residence time of 1.65 s. Particles, generated by premixed ethylene/air flames with equivalence ratios above the soot threshold limit, are fed to a tubular reactor, which can be operated at temperatures up to 650 K. A wide range of equivalence ratios are used to generate particles with different characteristics. The evolution of the particle size distributions has been evaluated by a differential mobility analyzer with high sensitivity in the 2–100 nm size range. The effect of the reactor temperature on coagulation has been systematically studied. Particles exhibit different coagulation efficiencies at the different temperatures. At room temperature, 2–4 nm particles fed to the reactor coagulate forming particles as large as 10–20 nm, whereas at higher temperatures the size distribution of the particles does not change with respect to that measured at the inlet of the reactor. This behavior suggests a very ineffective coagulation efficiency at higher temperatures for small nanoparticles. Larger particles do not exhibit this high sensitivity to temperature, substantially maintaining very high coagulation efficiencies. These considerations have been confirmed by numerical simulations conducted both with constant and size-dependent coagulation efficiency. The numerical results confirm that also at low and intermediate temperature regimes, the use of a size-dependent coagulation efficiency is mandatory to match the evolution of the particles during coagulation. On the other hand, the simple model of coagulation based on the van der Waals interactions between particles in the framework of gas kinetic collision theory is in slightly disagreement with the experimental results for very small particles, suggesting that more advanced modeling based on quantum mechanism and molecular dynamics are necessary to correctly reproduce the experimental data.

Proceedings of the Combustion Institute, 2011

The evolution of the molecular weight (MW) distribution and structural properties of carbon parti... more The evolution of the molecular weight (MW) distribution and structural properties of carbon particulate formed in methane, ethylene and benzene fuel-rich premixed flames, burning in similar conditions of maximum flame temperature, was experimentally measured and modeled.Both solubility and chromatographic separation of the carbon particulate allowed to follow the variation of molecular weight distributions of the different fractions. Structural properties of

Fuel, 2014

ABSTRACT The role of dimethyl ether (DME) as substituent to ethylene on particulate formation has... more ABSTRACT The role of dimethyl ether (DME) as substituent to ethylene on particulate formation has been evaluated in premixed and counter-flow diffusion flames. In the premixed flame, the equivalence ratio has been changed from 1.95 to 2.61 and dimethyl ether has been added from 2% to 30% of the total carbon fed. In the counter-flow diffusion flame, the addition of DME has been from 0% to 60% of total carbon fed. Laser induced fluorescence and incandescence have been used to follow the soot formation process: UV and visible fluorescence signals have been attributed to aromatic macromolecules and incipient nanoparticles, respectively, whereas incandescence has been attributed to soot particles and aggregates. In premixed flames results evidence that the formation of soot precursors is not so sensitive to DME addition. In very rich combustion environments, DME addition cannot completely avoid the formation of small precursors, although it can slow down the formation process. This behavior has been observed for all the equivalence ratios investigated. In the pyrolysis region of counter-flow diffusion flames, the formation of aromatic small precursors and soot particles is increased for small DME percentages, up to 20%. Then, the precursors are suppressed for larger amounts, going below the detection limit when 60% of DME is used. This suggests that DME can enhance the production of radicals and small reactive molecules in the pyrolytic side when it is added in small concentrations. For larger amounts of DME, oxidative pathways prevail and the conversion of carbon to precursors and then to soot is inhibited. In the oxidative region DME starts to effectively decrease the particle reduction also for amounts as small as 10–15%.

Experimental Thermal and Fluid Science, 2010

Measurements of ultrafine particles have been performed at the exhaust of a low emission microtur... more Measurements of ultrafine particles have been performed at the exhaust of a low emission microturbine for power generation. This device has been fuelled with liquid fuels, including a commercial diesel oil, a mixture of the diesel oil with a biodiesel and kerosene, and tested under different loads. Primarily attention has been focused on the measurements of the size distribution functions of the particles emitted from the system by using particle differential mobility analysis. A bimodal size distribution function of the particle emitted has been found in all the examined conditions. Burning diesel oil, the first mode of the size distribution function of the combustion-formed particles is centered at around 2-3 nm, whereas the second mode is centered at about 20-30 nm. The increase of the turbine load and the addition of 50% of biodiesel has not caused changes in the shape of size distribution of the particles. A slightly decrease of the amount of particle formed has been found. By using kerosene the amount of emitted particles increases of more than one order of magnitude. Also the shape of the size distribution function changes with the first mode shifted towards larger particles of the order of 8-10 nm but with a lower emission of larger 20-30 nm particles. Overall, in this conditions, the mass concentration of particles is increased respect to the diesel oil operation. Particle sizes measured with the diesel oil have been compared with the results on a diesel engine operated in the same power conditions and with the same fuel. Measurements have showed that the mean sizes of the formed particles do not change in the two combustion systems. However, diesel engine emits a number concentration of particles more than two orders of magnitude higher in the same conditions of power and with the same fuel. By running the engine in more premixed-like conditions, the size distribution function of the particles approaches that measured by burning kerosene in the microturbine indicating that the distribution function of the sizes of the emitted particles can be strongly affected by combustion conditions.

Experimental Thermal and Fluid Science, 2012

ABSTRACT In the present study a laminar premixed ethylene flame was doped with different amounts ... more ABSTRACT In the present study a laminar premixed ethylene flame was doped with different amounts of ethanol in order to understand the influence on biofuels in the total amount of particulate matter and the size distribution functions of the formed carbonaceous particles. Four different flames were investigated: one of pure ethylene as reference and three with an increased amount of ethanol, i.e. 10%, 20% and 30%. The equivalence ratio of the four flames was kept constant at 2.01. The particle size distribution functions were obtained using a nano-Differential Mobility Analyzer (nano-DMA). The results showed a general reduction of the total volume fraction of particulate when ethanol was added. The reduction of the formed particles increased as a function of the amount of ethanol added. The particle size distribution for pure ethylene flame and ethanol doped flames at different heights with the same amount of particles are quite similar. This indicates that in premixed flames, ethanol mostly affects gas phase chemistry and consequently the formation of incipient particles.

Energy & Fuels, 2013

A detailed kinetic mechanism of aromatic growth, particulate formation, and oxidation is presente... more A detailed kinetic mechanism of aromatic growth, particulate formation, and oxidation is presented and is tested in nonpremixed laminar flames of methane and ethylene at atmospheric pressure. Model development is refined in strict connection with new experimental data on the formation and oxidation of high molecular mass compounds and incipient particles. Reaction pathways leading to the formation of incipient particles, their transformation to soot, their oxidation, and the oxidation-induced fragmentation of particles and aggregates have been included by using a multisectional approach for the particle process. Predictions within a factor of 2−3 are obtained for major oxidation and pyrolysis products as well as trace aromatic species and particulate concentrations. The newly developed model predicts the concentration of the particles, their sizes, morphology, and chemical properties in nonpremixed flames of methane and ethylene with a wide range of particle formation without any condition-dependent adjustments to the kinetic scheme. A wide range of particle sizes is covered from nanoparticles formed on the fuel side of the flames to larger soot particles and particle aggregates formed in the flame wings. The trend of the H/C ratio of the particles along the flame axis is also predicted well. It decreases to very low values typical of mature soot particles when large aggregates are produced. The new mechanism for particle oxidation, which includes the oxidation-induced fragmentation of particles and aggregates, has shown the importance of accurate modeling of particle oxidation to correctly predict particle burnout and particle size in nonpremixed flames.