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Laser-Induced Incandescence for Soot Diagnostics: Theoretical Investigation and Experimental Development

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Laser-Induced Incandescence for Soot Diagnostics: Theoretical Investigation and Experimental Development Laser-Induced Incandescence for Soot Diagnostics: Theoretical Investigation and Experimental Development Johnsson, Jonathan 2012 Link to publication Citation for published version (APA): Johnsson, J. (2012). Laser-Induced Incandescence for Soot Diagnostics: Theoretical Investigation and Experimental Development. 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LUND UNIVERSITY PO Box 117 221 00 Lund +46 46-222 00 00 Doctoral esis Laser-Induced Incandescence for Soot Diagnostics Theoretical Investigation and Experimental Development Jonathan Johnsson Division of Combustion Physics Department of Physics Front coer image: Transmission electron microscope image of soot aggregates from a ame-based soot generator (see Sections 2.3.3 and 3.2). e carbon layer that the aggregates stick to has ruptured so that some aggregates can be seen hanging out from the edge of the layer. e large soot aggregate in the centre is approximately 2µm across. © Jonathan Johnsson and the respective publishers Printed by Tryckeriet i E-huset, Lund University, Lund, Sweden October 2012 Lund Reports on Combustion Physics, LRCP-164 ISSN 1102-8718 ISRN LUTFD2/TFCP-164-SE ISBN 978-91-7473-397-6 Jonathan Johnsson Division of Combustion Physics Department of Physics Lund University P.O. Box 118 SE-221 00 Lund, Sweden Abstract Laser-induced incandescence, LII, is a laser-diagnostic technique that can be used to measure the volume fraction and the sizes of soot particles suspended in a gas, such as within a combustion process or in its exhausts. e technique is based on the facts that the time-decay of the radiation from laser-heated soot particles is directly related to the particle size in the probed volume, and that the time-integrated radiation is related to the soot volume fraction. In the present work, the properties of LII were investigated, such as its de- pendence on different parameters, how it can be used to calculate soot prop- erties and how it relates to and can be combined with other techniques. Both theoretical and experimental studies were carried out. An LII model was used for the theoretical work and its predictions under various conditions were in- vestigated. e studies addressed different questions with regard to the sens- itivity of LII to various parameters. Simulations were made to determine the best choices for the laser system settings when measuring the soot volume frac- tion under different conditions. is resulted in general suggestions for what practices are best under various conditions. It was also found that if the two modes of a bimodal particle size distribution are of about the same magnitude the smaller mode has no signicant effect on the LII particle size evaluated. Studies of soot aggregates consisting of ramied clusters of spherical primary particles were carried out using numerically constructed aggregates, together with a Monte Carlo algorithm for calculating heat conduction. e effect of different degrees of bridging between the primary particles on primary particle size as evaluated with use of LII was examined, and found to be noticeable but relatively small. A laser system was designed for use in the experimental studies and eval- uation routines that use our LII model were designed and implemented, to be able to evaluate particle sizes from experimental LII signals. e experi- mental setup was designed so as to possess properties suitable for obtaining well-controlled LII particle size measurements. at setup was employed, with slight variations, for investigating the properties of LII. It was also applied in different laboratory soot sources for measurement of particle sizes and of soot volume fraction. i e same setup was used for studying the effects of the beam prole, which was found to have only a marginal effect on the evaluated size, other uncer- tain parameters involved in the LII technique being of greater importance in this respect. Two collaborative projects were also carried out for investigating how LII could be used for determining particle sizes both of cold soot aggreg- ates obtained from a ame-based soot generator and of industrial carbon black samples. With use of the soot generator it was investigated, when the primary particle distributions were similar, if the differing decay rates of the LII signals could be explained by the differing degrees of aggregation. is could be an ef- fect of the increase in shielding between the primary particles with an increase in aggregate size, as predicted by theory. e LII sizes as determined for the carbon black samples were closely correlated with the equivalent particle sizes as determined by industrial surface-area analysis of the particles. e properties of a at ethylene/air ame on a McKenna burner were also investigated, as a function both of height and of radial position. e absorp- tion function for soot was found to increase with an increase in height from 6 to 14 mm above the burner, due probably to the changes that occurred in soot composition from nascent soot to more mature soot. In the central part of the ame, at a height of 10 mm above the burner, the temperatures and the soot particle sizes were found to be nearly constant. ii Populärvetenskaplig sammanfattning Mer än 75 % av energianvändningen i världen kommer idag från förbränning av fossila bränslen, biobränslen och avfall. Trots att alternativa energikällor är på framfart kommer förbränning stå för den största andelen av världens energi- användning även de kommande decennierna. Tyvärr medför förbränning era negativa konsekvenser, såsom ökad koldioxidhalt i atmosfären och utsläpp av olika slags hälsofarliga mikroskopiska partiklar. Därför är det viktigt utveckla metoder för att rena utsläppen och förbättra effektiviteten i de förbrännings- processer som används. Sotpartiklar bildas vid förbränning då det nns ett tillräckligt stort över- skott på bränsle i förhållande till mängden syre. I vissa fall förbränns dessa par- tiklar senare i samma förbränningsprocess, och i andra följer partiklarna med avgaserna ut i atmosfären. Ett problem med det senare är att cancerogena mo- lekyler sitter på sotpartiklarnas ytor. Om man andas in sotpartiklarna så kan de följa med luströmmen ner i lungorna och de minsta partiklarna kan därifrån ta sig vidare in i kroppen. När sotpartiklarna släpps ut i atmosfären påverkar de dessutom jordens strålningsbalans och därmed klimatet, genom att de mycket effektivt absorberar solljus. Det är även intressant att veta var och när sot bildas i själva förbränningsprocessen. Eersom motorer, gasturbiner, värmekraverk och andra apparater baserade på förbränning redan har utvecklats under mer än hundra år så är avancerade verktyg och kunskap om hur förbränningen sker till stor hjälp för att processerna ska kunna förbättras ytterligare. Sotpartiklar- na är en del i den komplicerade förbränningsprocessen och påverkar hur effek- tiv till exempel en motor är och vilka utsläpp som sker. I alla dessa fall är det viktigt att ha kunskap om sotpartiklarnas egenskaper för att kunna förstå och beräkna vilken inverkan de har i olika miljöer. I det dagliga livet kan vi se sotpartiklar till exempel när vi tänder ett stea- rinljus eller vid en lägereld. Det gulorangea ljuset och värmen kommer från de glödheta mikroskopiska sotpartiklarna. Vi kan inte se de enskilda partiklarna eersom de är mycket små, omkring 1000 gånger mindre än tjockleken av ett hårstrå. Anledningen till att sotpartiklarna lyser och utstrålar så mycket värme iii är att de har en så hög temperatur, ca 1000–1500 °C. Vid rumstemperatur är sotpartiklarna däremot svarta. Det kan vi se om vi fångar in sotpartiklar på ett metallföremål genom att sticka in det en kort tid i amman. En laser kan användas för att mäta på sotpartiklarna direkt i förbrännings- processen. Laserljus i mycket korta pulser sänds in i förbränningsgasen och het- tar upp sotpartiklarna så att de blir mycket varmare än omgivningen, oa över 3000 °C. Då lyser de mycket starkare än de omgivande partiklarna som inte träffats av laserpulsen. Med hjälp av noggranna detektorer eller kameror kan ljuset, eller signalen, från dessa sotpartiklar mätas och användas för att ta fram var sotpartiklarna fanns, men även koncentrationer och partikelstorlekar. För att räkna ut dessa värden från signalen så behövs en avancerad fysikalisk dator- modell av mätprocessen. I allmänhet är det dock en bra uppskattning att en högre koncentration av partiklar leder till en starkare signal och att mindre par- tiklar ger en signal som avtar snabbare. De upphettade sotpartiklarna svalnar av på mindre än 0,000001 sekund så avancerad utrustning krävs för att kunna mäta signalen under denna tid. Metoden som just beskrivits kallas laserindu- cerad inkandescens, LII, och är en av de mest avancerade metoderna för att beröringsfritt mäta egenskaper hos sotpartiklar, dvs mäta utan att föra in nå- got instrument i gasen och därmed störa förbränningsprocessen. LII kan även användas för att mäta sotkoncentrationer och partikelstorlekar i avgaser. I denna avhandling har mättekniken LII undersökts och utvecklats och även använts för att undersöka sotpartiklarna i olika laboratorieammor. Utveck- lingen har skett genom att använda avancerade lasrar med noggranna detek- tionssystem, tillsammans med stabila laboratorieammor.
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