• Energy Research

In this work, a set of four partially premixed ethylene-air laminar flames, characterized by different equivalent ratios, was studied. The aim was to investigate the effect of partial premixing on soot formation, which is a relevant parameter for real combustion devices. Quantitative and qualitative information on soot particles were obtained using a modified version of the thermophoretic particle densitometry (TPD) method. Particle thermal emissivity values were measured from the transient thermocouple temperature response. Results show that the emissivity of flame-formed carbon nanoparticles ranges from about 0.5 for soot precursor particles up to 0.95, i.e., the typical emissivity value attributed to mature soot particles. Particle volume fraction was then accurately evaluated by means of the TPD method, using the values of particle emissivity previously measured. The changing of particle features throughout the flame, evidenced by the variation in thermal emissivity, was further investigated using Raman spectroscopy. The analysis of Raman spectra showed that the increase in thermal emissivity follows a reduction in the hydrogen-to-carbon ratio of the investigated soot nanoparticles. Measurements performed with the TPD method also evidence that both particle emissivity and volume fraction are strongly affected by oxidation, which prevents to accurately measure soot concentration in oxidative regions of the flames. An attempt to include soot oxidation in the TPD method for a correct evaluation of volume fraction procedure is also illustrated. OH concentration profiles, obtained from a modeling analysis and validated by radical chemiluminescence measurements, were used in the TPD method to correct particle concentration due to oxidation.

In this work, a set of four partially premixed ethylene-air laminar flames, characterized by different equivalent ratios, was studied. The aim was to investigate the effect of partial premixing on soot formation, which is a relevant parameter for real combustion devices. Quantitative and qualitative information on soot particles were obtained using a modified version of the thermophoretic particle densitometry (TPD) method. Particle thermal emissivity values were measured from the transient thermocouple temperature response. Results show that the emissivity of flame-formed carbon nanoparticles ranges from about 0.5 for soot precursor particles up to 0.95, i.e., the typical emissivity value attributed to mature soot particles. Particle volume fraction was then accurately evaluated by means of the TPD method, using the values of particle emissivity previously measured. The changing of particle features throughout the flame, evidenced by the variation in thermal emissivity, was further investigated using Raman spectroscopy. The analysis of Raman spectra showed that the increase in thermal emissivity follows a reduction in the hydrogen-to-carbon ratio of the investigated soot nanoparticles. Measurements performed with the TPD method also evidence that both particle emissivity and volume fraction are strongly affected by oxidation, which prevents to accurately measure soot concentration in oxidative regions of the flames. An attempt to include soot oxidation in the TPD method for a correct evaluation of volume fraction procedure is also illustrated. OH concentration profiles, obtained from a modeling analysis and validated by radical chemiluminescence measurements, were used in the TPD method to correct particle concentration due to oxidation.

McKenna burners are widely used in the combustion community for producing "flat" premixed flames. These flames are considered as standards for the development and calibration of optical techniques. Rich premixed flames produced by McKenna burners are frequently investigated in order to understand soot formation processes both by optical and by sampling techniques. Measurements are normally performed along the axis of the flames, with a uniform distribution of temperature and species concentration assumed in the radial direction. In this work it is shown that the soot radial profiles of rich premixed ethylene-air flames produced by a McKenna burner with a stainless steel porous plug may be far from being "flat." Soot is mainly distributed in an annular region and nonsoot fluorescing species are present in the core of the flames. This surprising result was verified under several working conditions. Furthermore, flames cannot be considered axial-symmetric but present a skewed soot distribution. Another McKenna burner with a bronze porous disk was used to produce flames of the same equivalence ratio and flows. These flames show a completely different soot radial profile, closer to the claimed flat distribution. These results cast doubts about the conclusions drawn in several studies on soot formation performed with a stainless steel McKenna burner.

McKenna burners are widely used in the combustion community for producing "flat" premixed flames. These flames are considered as standards for the development and calibration of optical techniques. Rich premixed flames produced by McKenna burners are frequently investigated in order to understand soot formation processes both by optical and by sampling techniques. Measurements are normally performed along the axis of the flames, with a uniform distribution of temperature and species concentration assumed in the radial direction. In this work it is shown that the soot radial profiles of rich premixed ethylene-air flames produced by a McKenna burner with a stainless steel porous plug may be far from being "flat." Soot is mainly distributed in an annular region and nonsoot fluorescing species are present in the core of the flames. This surprising result was verified under several working conditions. Furthermore, flames cannot be considered axial-symmetric but present a skewed soot distribution. Another McKenna burner with a bronze porous disk was used to produce flames of the same equivalence ratio and flows. These flames show a completely different soot radial profile, closer to the claimed flat distribution. These results cast doubts about the conclusions drawn in several studies on soot formation performed with a stainless steel McKenna burner.

Spectral, grey-scale and colour chemiluminescence measurements of C 2* and CH* radicals' emission are carried out on the flame front of a methane-air premixed flame at different equivalence ratios. To this purpose, properly spatially resolved optical equipment has been implemented in order to reduce the background emission from other burned gas regions. The grey-scale (ICCD + interference filters) and RGB colour (commercial digital camera) approaches have been compared in order to find a correspondence between the C2* and the green component, as well as the CH* and the blue component of the emission intensities. The C2*/ CH* chemiluminescence ratio has been investigated at different equivalence ratios and a good correlation has been obtained, showing the possibility of sensing the equivalence ratio in practical systems. The grey-scale and colour chemiluminescence analysis has then been applied to a meso-scale not premixed swirl combustor fuelled with a methane-air mixture and operating at 0.3 MPa. 2D results are presented and discussed in this work.

Spectral, grey-scale and colour chemiluminescence measurements of C 2* and CH* radicals' emission are carried out on the flame front of a methane-air premixed flame at different equivalence ratios. To this purpose, properly spatially resolved optical equipment has been implemented in order to reduce the background emission from other burned gas regions. The grey-scale (ICCD + interference filters) and RGB colour (commercial digital camera) approaches have been compared in order to find a correspondence between the C2* and the green component, as well as the CH* and the blue component of the emission intensities. The C2*/ CH* chemiluminescence ratio has been investigated at different equivalence ratios and a good correlation has been obtained, showing the possibility of sensing the equivalence ratio in practical systems. The grey-scale and colour chemiluminescence analysis has then been applied to a meso-scale not premixed swirl combustor fuelled with a methane-air mixture and operating at 0.3 MPa. 2D results are presented and discussed in this work.

The large use of portable devices imposes a new interest in the development of power backup systems with constraints in terms of compactness and safety. Such systems have to match the use as battery backups as well as a self-standing operating. Thermoelectric generators (TEGs) allow access to new ways of power supply thanks to their long lifetimes, their competitive efficiencies at low powers and their capability of providing multiple outputs. In this work, a TEG based on catalytic combustor has been proposed aiming to approach electrical output and dimension of the commonly used AA batteries. Catalytic combustion provides the possibility to profit from the high power densities of hydrocarbon in limited space and low burning temperatures meeting the needs of the TEGs as a heat engine. The system has been characterized for different fuel flow rates. The measured TEG efficiency is 3.4% with the electrical power output of 5.3 W. The system thermal behavior has been experimentally investigated according to literature models, by evaluating the effectiveness of the design and of the chosen practical solutions. The system produced reached the electrical output target, matching the characteristics proper of most of the common commercial AA batteries in a similar device volume.

The large use of portable devices imposes a new interest in the development of power backup systems with constraints in terms of compactness and safety. Such systems have to match the use as battery backups as well as a self-standing operating. Thermoelectric generators (TEGs) allow access to new ways of power supply thanks to their long lifetimes, their competitive efficiencies at low powers and their capability of providing multiple outputs. In this work, a TEG based on catalytic combustor has been proposed aiming to approach electrical output and dimension of the commonly used AA batteries. Catalytic combustion provides the possibility to profit from the high power densities of hydrocarbon in limited space and low burning temperatures meeting the needs of the TEGs as a heat engine. The system has been characterized for different fuel flow rates. The measured TEG efficiency is 3.4% with the electrical power output of 5.3 W. The system thermal behavior has been experimentally investigated according to literature models, by evaluating the effectiveness of the design and of the chosen practical solutions. The system produced reached the electrical output target, matching the characteristics proper of most of the common commercial AA batteries in a similar device volume.