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Laser-Induced Phosphor Thermometry - Feasibility and Precision in Combustion Applications

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Laser-Induced Phosphor Thermometry - Feasibility and Precision in Combustion Applications Laser-Induced Phosphor Thermometry - Feasibility and Precision in Combustion Applications Lindén, Johannes 2012 Link to publication Citation for published version (APA): Lindén, J. (2012). Laser-Induced Phosphor Thermometry - Feasibility and Precision in Combustion Applications. Total number of authors: 1 General rights Unless other specific re-use rights are stated the following general rights apply: Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal Read more about Creative commons licenses: https://creativecommons.org/licenses/ Take down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. LUND UNIVERSITY PO Box 117 221 00 Lund +46 46-222 00 00 Laser-Induced Phosphor Thermometry Feasibility and precision in combustion applications Johannes Lindén Copyright © Johannes Lindén Lund Report on Combustion Physics, LRCP-154 ISBN 978-91-7473-361-7 ISSN 1102-8718 ISRN LUTFD2/TFCP-154-SE Printed in Sweden by Media-Tryck, Lund University Lund 2012 Till Magda och Elvy Contents Abstract vii Populärvetenskaplig sammanfattning ix List of papers xii 1 Introduction 1 2 Temperature Measurements 5 2.1 History 5 2.2 Thermocouples 6 2.3 Infrared thermometry 7 2.4 Phosphor thermometry 9 3 Thermographic Phosphors 11 3.1 History and Terminology 11 3.2 Fundamentals of Phosphors 12 3.3 Temperature Properties 13 4 Laser-Induced Phosphor Thermometry 21 4.1 Laser diagnostics 21 4.2 Point measurements 22 4.3 Two-dimensional surface measurements 24 4.4 Detectors 28 4.4.1 Point detectors 28 4.4.2 2D detectors 29 iv 5 Results 33 5.1 Free-flow measurements 33 5.1.1 Potential heating by laser 34 5.1.2 Obstructing optical access 35 5.1.3 Relaxation time for achieving local thermal equilibrium 37 5.2 Thickness of coating 38 5.3 Non-linear features 40 5.3.1 Point detectors 40 5.3.2 2D detectors 44 5.4 Precision 50 6 Summary & Outlook 59 Acknowledgements 61 References 63 Summary of Papers 69 v Abstract Temperature is one of the most fundamental parameters to be measured in many research disciplines. In combustion science, a thorough knowledge of temperature is essential for improving and optimizing combustion processes. Increasingly strict environmental legislation, greater demands for energy, and efforts to reduce dependence upon fossil fuels are forcing the combustion industry to obtain more adequate knowledge of combustion processes generally. Laser Induced Phosphorescence (LIP) can serve as a tool for measuring temperature. It utilizes the temperature-dependent properties of the phosphorescence emitted by inorganic luminescent materials referred to as thermographic phosphors, after these have been illuminated by laser radiation. Either point- or two-dimensional measurements are usually performed by the use of either the temporal or the spectral temperature-dependent properties involved. The technique has many advantages. When used wisely it can be close to non-intrusive, offers remote sensing capabilities, and allows a high degree of temporal resolution, accuracy and precision to be obtained. The phosphor is usually applied to a surface either as a point or covering some area. The phosphorescence is detected using either a point light detector, such as a photomultiplier tube (PMT), or an image detector, such as a CCD camera. By seeding phosphor particles in free flow it is also possible to perform temperature measurements in gaseous media. In the present thesis, temperature measurements using laser-induced phosphorescence will be described and certain limitations of it will be addressed. Since the phosphor is applied to a surface as a very thin layer, the measurements performed are often considered non-intrusive. However, in situations in which the temperature changes very rapidly, such as in a combustion engine, and large temperature gradients are present for short periods of time, a relevant question to ask is whether the temperature of the phosphor agrees with the temperature it is intended to measure. One can also ask whether the phosphor layer acts as an insulator, making the measurements indeed intrusive. These are questions investigated in the thesis. At the same time, the technique is quite susceptible to small systematic errors if measurements of high precision and accuracy are to be performed. The technique also requires highly stable detectors in order for correct temperature values to be obtained. In the thesis, the characteristics of different detectors, both for point- and for 2D measurement, are investigated in terms of non-linear features and saturation effects. vii Also the precision of the technique, and how this is related to the spatial resolution achieved, are investigated. In addition, the suitability of the technique for measurements in gaseous media is investigated in terms of the laser heating of the phosphor particles and the relaxation time required for thermal equilibrium to be achieved. viii Populärvetenskaplig sammanfattning 80 % av den globala energi som används härrör från fossila bränslen. Ytterligare 10 % baseras på förbränning av någon form av förnyelsebart bränsle. Således är 90 % av vår energianvändning beroende av någon typ av förbränning, oavsett det rör sig om förbränning av olja, kol, gas eller biomassa. I en tid då priset på olja ständigt slår nya rekord, lagstiftningen kring miljöutsläpp blir alltmer strikt samtidigt som efterfrågan på energi bara växer, är det av största intresse att öka kunskapen om förbränning i alla dess former. Detta för att effektivisera och optimera de förbränningsprocesser som redan finns, men också för att hitta nya och bättre sätt att genomföra förbränning. För att öka kunskaperna om förbränning behöver man mäta. Förbrännings- processer är dock mycket komplicerade på så sätt att de involverar många olika kemiska ämnen och reaktionen. De är också vanligtvis mycket snabba, t.ex. sker det, i en vanlig bilmotorcylinder, 1000 explosionsliknande förbränningsprocesser per sekund, var och en inte längre än några millisekunder. Med hjälp av laserbaserad förbränningsdiagnostik kan man mäta många olika parametrar under mycket korta tidsförlopp. De lasrar som används är pulsade, vilket innebär att de ungefär 10 gånger per sekund skjuter extremt korta laserpulser (omkring 10 nanosekunder, dvs. 10 miljarddels sekund). Under den korta tidsperioden kan en förbränningsprocess i princip betraktas som ”stillastående”. Laserljuset belyser de molekyler man vill studera, antingen format som en stråle eller som ett ark, och det ljuset som molekylen sprider detekteras med någon form av detektor. Med hjälp av specialbyggda motorer, delvis byggda av glas, är det möjlig att nå in med laserljuset och att detektera det ljus som sänds ut (s.k. optiskt access). På så sätt kan man mäta många olika parametrar så som temperatur, tryck, flödeshastigeter samt ämneskoncentrationer, i punkter eller tvådimensionella tvärsnitt av förbränningsrummet. De ämnen vars koncentration man kan mäta är bland annat bränsle, olika förbränningsradikaler och sot. Man kan även visualisera flamfronter och studera fenomen så som blandning och bränsleförångning. En stor fördel med laserbaserade förbränningsdiagnostik är att man kan mäta i stort sett beröringsfritt (eng. non-intrusive); att molekyler belyses med laserljus påverkar inte kemin i förbränningsprocessen. På så vis kan man mäta realistiskt i annars besvärliga och svåråtkomliga miljöer som inuti motorer, gasturbiner och förbränningsugnar. Naturligtvis finns det även nackdelar men tekniken. Den utrustning som används är vanligtvis relativt dyr och kräver ofta specialiserad personal och den ix optiska accessen medför ofta att man inte kan köra motorn under realistiska förhållanden. En av de viktigaste parametrarna att mäta i förbränningssammanhang är temperatur. En mer exakt kunskap om temperaturen är väsentlig för att förstå, förbättra och optimera flera förbränningsprocesser. Bl.a. är de största förlusterna i en bilmotor är de i form av värmeförluster till cylinderväggar och avgaser, en gasturbins livslängd avgörs huvudsakligen av livslängden hos de komponenter som utsätts för högst temperatur, etc. I denna avhandling beskrivs en metod som kallas för laserinducerad fosforescens (eng. Laser-Induced Phosphorescence, LIP) som används för att mäta temperatur, vanligtvis på ytor, antingen i en punkt eller över en del av ytan. Tekniken är på intet sätt ny, utan har varit ämne för utvecklig ungefär sedan 80-talet. Dock finns det utrymme för förbättringar. I avhandlingen redogörs för sådant som är viktigt att tänka på för att mätningar med hög precision och noggrannhet ska vara möjliga. Temperaturmätning med hjälp av laserinducerad fosforescens går ut på att detektera det ljus som så kallade fosforer ger ifrån sig, efter att de har blivit belysta med en mycket kort och intensiv laserpuls. Detta ljus kallas för fosforescens. Denna fosforescens lyser under en mycket kort tid (bråkdelar av en sekund) efter det att den har blivit belyst
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