Increased attention to environmental considerations has renewed interest in combustion-generated carbonaceous particulate. Carbon is suspected to play a major role both for human health effects and for the radiative balance in the atmosphere. Therefore a convenient way for the characterisation and qualification of the carbonaceous structures emitted in the atmosphere is to employ optical techniques like Laser Light Scattering/Extinction or Laser-Induced Incandescence, which may be used inside the combustion chambers and finally in the exhausts. The soot formation and burnout mechanisms have been mainly studied employing Elastic Scattering and Extinction measurements techniques which can provide information about distribution, effective particle size, and volume fraction of soot particles. However, these methods suffer under a great sensitivity for detection of larger particles like cenospheres. Additionally, the line-of-sight nature of the extinction methods may be appropriate for steady laboratory flames but is less suitable for practical and turbulent combustion devices burning heavy oils. These challenges led to use an alternative technique in heavy fuel oil spray flames, the Time Resolved Laser-Induced Incandescence (TIRE-LII) successfully used in previous work by the same authors. This technique, based on the original work of Eckbreth and Melton, is widely employed for the determination of soot and aggregates dimensions. Particles are heated up by an intense laser pulse and start to emit thermal radiation. TIRE exploits the thermal radiation decrease with time of the heated particles. Therefore, the characteristic time of the temperature decay can be used as a measure of the particle sizes. This possibility offers an attractive alternative to scattering and extinction methods. In the case of spray flames , where the particle distribution function is polydisperse, each of the heated particle class causes a specific, size and concentration dependent contribution to the total intensity of the thermal radiation. The particle size distribution can be recovered or estimated by mean of an appropriate numerical model based on the theoretical analysis of the particle heating and cooling mechanisms for the different particle size fractions. In the present work, experimental measurements of TIRE-LII were performed to assess the sensitivity of the TIRE-LII to particle sizes in oil spray flames burning two heavy oils differing by their asphaltene contents. In fact, it is well known that cenosphere production is correlated to asphaltene content of the oil. Contemporaneously, improvement of the theoretical model of TIRE-LII has been performed in this study including larger particles. The aim of this work was to try to discriminate the different classes of carbonaceous material produced during heavy oil combustion using the different cooling time decays. An estimation of the particle sizes and their relative contributions was done for soot and the larger particles using the previous theoretical model. Moreover, the assumed size distributions used for TIRE-LII measurements were compared with those obtained by sampling performed in flame as well as in the exhaust, and successive SEM and EDX analyses. Atomic Force Microscopy will has been attempted to analyse the structures of the particles .
Soot and micronic carbonaceous particle measurement based on time resolved laser induced incandescence
C Allouis;F Beretta
2001
Abstract
Increased attention to environmental considerations has renewed interest in combustion-generated carbonaceous particulate. Carbon is suspected to play a major role both for human health effects and for the radiative balance in the atmosphere. Therefore a convenient way for the characterisation and qualification of the carbonaceous structures emitted in the atmosphere is to employ optical techniques like Laser Light Scattering/Extinction or Laser-Induced Incandescence, which may be used inside the combustion chambers and finally in the exhausts. The soot formation and burnout mechanisms have been mainly studied employing Elastic Scattering and Extinction measurements techniques which can provide information about distribution, effective particle size, and volume fraction of soot particles. However, these methods suffer under a great sensitivity for detection of larger particles like cenospheres. Additionally, the line-of-sight nature of the extinction methods may be appropriate for steady laboratory flames but is less suitable for practical and turbulent combustion devices burning heavy oils. These challenges led to use an alternative technique in heavy fuel oil spray flames, the Time Resolved Laser-Induced Incandescence (TIRE-LII) successfully used in previous work by the same authors. This technique, based on the original work of Eckbreth and Melton, is widely employed for the determination of soot and aggregates dimensions. Particles are heated up by an intense laser pulse and start to emit thermal radiation. TIRE exploits the thermal radiation decrease with time of the heated particles. Therefore, the characteristic time of the temperature decay can be used as a measure of the particle sizes. This possibility offers an attractive alternative to scattering and extinction methods. In the case of spray flames , where the particle distribution function is polydisperse, each of the heated particle class causes a specific, size and concentration dependent contribution to the total intensity of the thermal radiation. The particle size distribution can be recovered or estimated by mean of an appropriate numerical model based on the theoretical analysis of the particle heating and cooling mechanisms for the different particle size fractions. In the present work, experimental measurements of TIRE-LII were performed to assess the sensitivity of the TIRE-LII to particle sizes in oil spray flames burning two heavy oils differing by their asphaltene contents. In fact, it is well known that cenosphere production is correlated to asphaltene content of the oil. Contemporaneously, improvement of the theoretical model of TIRE-LII has been performed in this study including larger particles. The aim of this work was to try to discriminate the different classes of carbonaceous material produced during heavy oil combustion using the different cooling time decays. An estimation of the particle sizes and their relative contributions was done for soot and the larger particles using the previous theoretical model. Moreover, the assumed size distributions used for TIRE-LII measurements were compared with those obtained by sampling performed in flame as well as in the exhaust, and successive SEM and EDX analyses. Atomic Force Microscopy will has been attempted to analyse the structures of the particles .I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
