From a diagnostic point of view, the possibility to follow both qualitatively and quantitatively the entire process of carbonaceous nanoparticle formation in flames is particularly valuable. In this study, quantitative and qualitative measurements of flame-generated soot nanoparticles are presented. To this aim, an improved version of the thermophoretic particle densitometry (TPD) method, introduced by McEnnaly et al. [Combust. Flame 109 (1997) 701-720] has been developed and used. When the thermocouple is inserted in a particle-containing flame, thermophoretic transport and deposition of particles on the thermocouple junction produces a change of measured temperature in time, which is strongly dependent not only on particle concentration but also on the properties of collected material. Indeed, flame-formed carbonaceous particles are shown to exhibit different size, chemical composition and nanostructure, resulting in a large variety of thermal emissivity values ranging from ? ? 0.4 - 0.5 for early nucleated particles and increasing to a value of ? = 0.95, i.e. the typical soot particle emissivity. The improved TPD method is first tested to investigate a premixed flame, whose particle physical/chemical evolution was the object of a recent work [M. Commodo, G. De Falco, A. Bruno, C. Borriello, P. Minutolo, A. D'Anna, Combust. Flame 162 (2015) 3854-3863]. The results constitute the basis for implementing the TPD experiment in a diffusion flame. The obtained data demonstrate that a correct determination of emissivity allows a more accurate evaluation of the particle volume fraction at the early stage of formation process, in a flame region where particle concentration measurement is particularly challenging. Data also evidence that oxidation has a severe role in affecting both particle emissivity and particle concentration determination. So, an attempt to include carbon nanoparticle oxidation in the TPD volume fraction procedure is also illustrated resulting in a good agreement of the data obtained with other techniques. Finally, a Raman spectroscopy analysis has been conducted, in order to better understand and to support the variation in the thermal emissivity values of the carbon nanoparticles that were observed, and to correlate it to particle physicochemical properties. Particularly, the thermal emissivity is shown to be related to the particle hydrogen content, and also to the optical bandgap.
A thermophoretic-thermocouple method for soot measurement in combustion
G De Falco;M Commodo;P Minutolo
2016
Abstract
From a diagnostic point of view, the possibility to follow both qualitatively and quantitatively the entire process of carbonaceous nanoparticle formation in flames is particularly valuable. In this study, quantitative and qualitative measurements of flame-generated soot nanoparticles are presented. To this aim, an improved version of the thermophoretic particle densitometry (TPD) method, introduced by McEnnaly et al. [Combust. Flame 109 (1997) 701-720] has been developed and used. When the thermocouple is inserted in a particle-containing flame, thermophoretic transport and deposition of particles on the thermocouple junction produces a change of measured temperature in time, which is strongly dependent not only on particle concentration but also on the properties of collected material. Indeed, flame-formed carbonaceous particles are shown to exhibit different size, chemical composition and nanostructure, resulting in a large variety of thermal emissivity values ranging from ? ? 0.4 - 0.5 for early nucleated particles and increasing to a value of ? = 0.95, i.e. the typical soot particle emissivity. The improved TPD method is first tested to investigate a premixed flame, whose particle physical/chemical evolution was the object of a recent work [M. Commodo, G. De Falco, A. Bruno, C. Borriello, P. Minutolo, A. D'Anna, Combust. Flame 162 (2015) 3854-3863]. The results constitute the basis for implementing the TPD experiment in a diffusion flame. The obtained data demonstrate that a correct determination of emissivity allows a more accurate evaluation of the particle volume fraction at the early stage of formation process, in a flame region where particle concentration measurement is particularly challenging. Data also evidence that oxidation has a severe role in affecting both particle emissivity and particle concentration determination. So, an attempt to include carbon nanoparticle oxidation in the TPD volume fraction procedure is also illustrated resulting in a good agreement of the data obtained with other techniques. Finally, a Raman spectroscopy analysis has been conducted, in order to better understand and to support the variation in the thermal emissivity values of the carbon nanoparticles that were observed, and to correlate it to particle physicochemical properties. Particularly, the thermal emissivity is shown to be related to the particle hydrogen content, and also to the optical bandgap.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
