Advanced

Modelling and Experimental Investigations on Thermal Radiation in Combustion Environments

Hofgren, Henrik LU (2015)
Abstract (Swedish)
Popular Abstract in Swedish

Värmestrålning är högst relevant i förbränningsmiljöer. I dessa miljöer är förbränningsgaser och partiklar den huvudsakliga källan till värmestrålning. Förbränningsmiljön är komplex att beskriva. Analys av turbulenta flöden, kemiska reaktioner och transport av energi måste göras för att fullständigt beskriva miljön. Att kunna beskriva dessa fenomen är en viktig del i förståelsen av miljön men även för att kunna designa nya förbränningsmiljöer. Värmestrålning är en del av den energitransport som sker i förbränningsmiljön. Denna avhandling behandlar specifikt ett urval av de modeller som beskriver hur gaser och partiklar avger och tar upp värmestrålning, så kallade egenskapsmodeller. Två specifika... (More)
Popular Abstract in Swedish

Värmestrålning är högst relevant i förbränningsmiljöer. I dessa miljöer är förbränningsgaser och partiklar den huvudsakliga källan till värmestrålning. Förbränningsmiljön är komplex att beskriva. Analys av turbulenta flöden, kemiska reaktioner och transport av energi måste göras för att fullständigt beskriva miljön. Att kunna beskriva dessa fenomen är en viktig del i förståelsen av miljön men även för att kunna designa nya förbränningsmiljöer. Värmestrålning är en del av den energitransport som sker i förbränningsmiljön. Denna avhandling behandlar specifikt ett urval av de modeller som beskriver hur gaser och partiklar avger och tar upp värmestrålning, så kallade egenskapsmodeller. Två specifika förbränningsmiljöer har även studerats med avseende på värmestrålning, förbränningsmiljöer med hög andel kolmonoxid och de som existerar i rosterpannor.

Vatten i gasform och koldioxid är de gaser som huvudsakligen inverkar på värmestrålningen men även andra gaser kan bidra, t ex metan och kolmonoxid. Sot är en partikel som förekommer i många förbränningsmiljöer. I de miljöer som eldar fasta bränslen, t ex biomassa, kol eller avfall, kan bränslepartiklar och aska även finnas. Resultaten från studier av egenskapsmodeller visar att då en viss andel partiklar finns i förbränningsmiljön så är det inte alltid motiverat att använda alltför avancerade egenskapsmodeller för gaserna. Enkla modeller kan prediktera likvärdiga resultat för värmestrålningen med avsevärt mindre behov av dataresurser. Detta beror på att partiklar börjar dominera upptagandet och avgivandet av värmestrålning jämfört med gaserna. Olika egenskapsmodeller för partiklar kan även påverka hur väl värmestrålningen predikteras. Väldigt ofta används enkla egenskapsmodeller för partiklar. För aska, kan dessa enkla egenskapsmodeller kan ge stora fel i uppskattningen av värmestrålningen. Resultaten från studier av egenskapsmodeller i denna avhandling guidar oss till några av de modeller som kan användas och vilka som skall undvikas.

Kolmonoxid är en gas som har en mindre växelverkan med strålning än de två vanligaste gaserna, vatten och koldioxid. Av denna anledning så försummas oftast värmestrålningen från kolmonoxid. I förgasningsmiljöer kan volymandelen av kolmonoxid vara upp mot och över 50%. Detta är en väldigt hög andel jämfört med vanliga förbränningsmiljöer. Av denna anledning är det intressant att undersöka om kolmonoxid fortfarande går att bortse ifrån i förgasningsmiljöer, då värmestrålningen till väggarna avses. Denna avhandling visar att bidraget från kolmonoxiden fortfarande kan försummas i förgasningsmiljöer. Detta beror på att det oftast också finns små mängder av koldioxid och vatten vilka, på grund av dess starka strålningsegenskaper, överskuggar bidraget från kolmonoxiden. Den andra förbränningsmiljön som studerades var rosterpannan. Rosterpannan är en förbränningsteknologi som förbränner fasta bränslen. Förbränningen av bränslet startar längst ner i pannan på rostern och försätter ovan i förbränningsrummet av de brännbara gaser och partiklar som lämnar bränslebädden. I denna avhandling har fokus varit på värmestrålningen i förbränningsrummet. Både modelleringsstudier och experimentella studier har genomförts på olika rosterpannor. En första utredning av värmestrålning i rosterpannan var en parameterstudie. Parametrarna som studerades var partikelmängder från avfall och biomassa, storlekar på pannan och strålningsegenskaper för pannans ytor. Mest partiklar skapas vid förbränning av avfall så detta bränsle hade störst inverkan på värmestrålningen. Generellt ökade värmestrålningen till väggarna i pannan när storleken ökade. Valet av strålningsegenskaper för väggar och bädd visade sig ha stor inverkan på värmestrålningen. En 400 kW rosterpanna i pilotskala studerades genom att utföra mätningar och modellering. Mätningarna som utfördes på rosterpannan har inte tidigare utförts på någon rosterpanna. Uppmätt data användes som en input till modellering av pannan vilket möjliggjorde en djupare studie av värmestrålningen i pannan. Studien av pilotpannan visade att partiklar kan ha en stor inverkan på värmestrålningen till väggarna. Då partiklar betraktades så fördubblades värmestrålningen till väggarna i pannans nedre delar. (Less)
Abstract
Thermal radiation is an important physical phenomenon in combustion environments. For the understanding of existing- and the design of new combustion environments computational modelling is a useful tool as it can describe the different transport phenomena. This thesis has focused on studying thermal radiative property models of the participating media, gases and particles. Two specific combustion environments have also been studied, from a thermal radiation perspective. The focus is on radiative property models that are useful specifically for engineering purposes. These property models are often developed from more advanced spectral models by means of correlative or other methods to simplify the treatment of the radiative transfer. One... (More)
Thermal radiation is an important physical phenomenon in combustion environments. For the understanding of existing- and the design of new combustion environments computational modelling is a useful tool as it can describe the different transport phenomena. This thesis has focused on studying thermal radiative property models of the participating media, gases and particles. Two specific combustion environments have also been studied, from a thermal radiation perspective. The focus is on radiative property models that are useful specifically for engineering purposes. These property models are often developed from more advanced spectral models by means of correlative or other methods to simplify the treatment of the radiative transfer. One such model is the non-correlated statistical narrow band (SNB) model, which drops spectral relation between intensity and transmissivity, which is used in the correlated SNB model. By doing so a significant decrease in computational demand can be achieved. The accuracy of this model has been questioned in the literature although it has been suggested as an appropriate option for sooty environments. The assessment of the model revealed that it only gives good predictions in large geometries and highly sooty environments. Because of the model’s limited applicability and a still rather high computational demand it is not recommended for use even in sooty environments. A simpler model, and computationally much faster, called the weighted sum of gray gas (WSGG) model accompanied the assessment of the non-correlated SNB model. The WSGG model was shown to be a better choice than the non-correlated SNB model in sooty environments. A gray gas property model was compared with a WSGG model in combustion environments resembling the ones found in grate fired furnaces. In these environments, which also contains particles, the gray model was shown to give predictions of wall heat flux in close agreement with the WSGG model. This shows how the simplest model, which the gray model is, can sometimes be a suitable choice, especially when particles are present. Particle property models were evaluated in various combustion environments. A common approach is to use Planck mean coefficients to represent the particle properties. The use of Planck mean coefficients for fly-ash particles, common in furnaces combusting solid fuels, is shown to give large errors in prediction of both radiative heat flux and the source term in particular.

The two investigated combustion environments are those found in grate fired furnaces and environments with high CO mole fraction. The grate fired furnace was studied in both modelling- and measurement work. Specifically the importance of particle radiation was investigated in the grate fired combustion environment. A preliminary study of the grate fired furnace was a parametric study. The parameters investigated were different particles originating from combustion of biomass and municipal solid waste, different furnace size, and boundary emissivities. The most significant effect on the overall radiative heat transfer is that of particles from municipal solid waste; moderate effects are seen when particles come from biomass. Increased furnace size most affected the heat flux to the hot bed and source term compared with a case without any particles. The choice of emissivity can be as important as considering particles or not. The measurement and modelling work were carried out on a 400 kW grate fired furnace combusting biomass. The boundary temperatures, flue gas temperature, gas mole fraction, particle mass-size distribution, and wall irradiation were measured. A so-called indirect wall irradiation was retrieved when the furnace was modelled with the implemented measured parameters. This indirect wall irradiation was compared with the directly measured wall irradiation. The study revealed that particle radiation is very important in the evaluated furnace, it doubles the wall irradiation in the hot flame zone compared with irradiation only from gases. The CO contribution to the total directional radiation was studied in environments with high mole fractions of CO, often found in gasifiers. CO is normally disregarded in environments where the fuel is fully oxidised, as it is a weak radiating species and small mole fractions of CO exist in these types of environments. The evaluation reveal that disregarding CO is still a good approach even in gasifier environments. These environments still contain small volume fractions of CO2 and H2O, which compared with CO are much stronger radiators. The rotational-vibrational bands of these two species overlaps the important fundamental band of CO reducing the importance of CO to the total directional radiative heat flux. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Professor Mengüc, Pinar, University of Özyegins, Turkey
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Thermal radiation, Property model, SNB, WSGG, LBL, Planck, particle, fly-ash, soot, grate, furnace, combustion
pages
69 pages
publisher
Lund University
defense location
Lecture hall B, M-building, Ole Römers väg 1, Lund University, Faculty of Engineering, LTH.
defense date
2015-06-09 10:15
ISBN
978-91-7623-353-5
language
English
LU publication?
yes
id
29dd0d10-5c33-4fbb-ad66-60967d4bf64a (old id 5384229)
date added to LUP
2015-05-12 11:08:10
date last changed
2016-09-19 08:45:05
@phdthesis{29dd0d10-5c33-4fbb-ad66-60967d4bf64a,
  abstract     = {Thermal radiation is an important physical phenomenon in combustion environments. For the understanding of existing- and the design of new combustion environments computational modelling is a useful tool as it can describe the different transport phenomena. This thesis has focused on studying thermal radiative property models of the participating media, gases and particles. Two specific combustion environments have also been studied, from a thermal radiation perspective. The focus is on radiative property models that are useful specifically for engineering purposes. These property models are often developed from more advanced spectral models by means of correlative or other methods to simplify the treatment of the radiative transfer. One such model is the non-correlated statistical narrow band (SNB) model, which drops spectral relation between intensity and transmissivity, which is used in the correlated SNB model. By doing so a significant decrease in computational demand can be achieved. The accuracy of this model has been questioned in the literature although it has been suggested as an appropriate option for sooty environments. The assessment of the model revealed that it only gives good predictions in large geometries and highly sooty environments. Because of the model’s limited applicability and a still rather high computational demand it is not recommended for use even in sooty environments. A simpler model, and computationally much faster, called the weighted sum of gray gas (WSGG) model accompanied the assessment of the non-correlated SNB model. The WSGG model was shown to be a better choice than the non-correlated SNB model in sooty environments. A gray gas property model was compared with a WSGG model in combustion environments resembling the ones found in grate fired furnaces. In these environments, which also contains particles, the gray model was shown to give predictions of wall heat flux in close agreement with the WSGG model. This shows how the simplest model, which the gray model is, can sometimes be a suitable choice, especially when particles are present. Particle property models were evaluated in various combustion environments. A common approach is to use Planck mean coefficients to represent the particle properties. The use of Planck mean coefficients for fly-ash particles, common in furnaces combusting solid fuels, is shown to give large errors in prediction of both radiative heat flux and the source term in particular.<br/><br>
The two investigated combustion environments are those found in grate fired furnaces and environments with high CO mole fraction. The grate fired furnace was studied in both modelling- and measurement work. Specifically the importance of particle radiation was investigated in the grate fired combustion environment. A preliminary study of the grate fired furnace was a parametric study. The parameters investigated were different particles originating from combustion of biomass and municipal solid waste, different furnace size, and boundary emissivities. The most significant effect on the overall radiative heat transfer is that of particles from municipal solid waste; moderate effects are seen when particles come from biomass. Increased furnace size most affected the heat flux to the hot bed and source term compared with a case without any particles. The choice of emissivity can be as important as considering particles or not. The measurement and modelling work were carried out on a 400 kW grate fired furnace combusting biomass. The boundary temperatures, flue gas temperature, gas mole fraction, particle mass-size distribution, and wall irradiation were measured. A so-called indirect wall irradiation was retrieved when the furnace was modelled with the implemented measured parameters. This indirect wall irradiation was compared with the directly measured wall irradiation. The study revealed that particle radiation is very important in the evaluated furnace, it doubles the wall irradiation in the hot flame zone compared with irradiation only from gases. The CO contribution to the total directional radiation was studied in environments with high mole fractions of CO, often found in gasifiers. CO is normally disregarded in environments where the fuel is fully oxidised, as it is a weak radiating species and small mole fractions of CO exist in these types of environments. The evaluation reveal that disregarding CO is still a good approach even in gasifier environments. These environments still contain small volume fractions of CO2 and H2O, which compared with CO are much stronger radiators. The rotational-vibrational bands of these two species overlaps the important fundamental band of CO reducing the importance of CO to the total directional radiative heat flux.},
  author       = {Hofgren, Henrik},
  isbn         = {978-91-7623-353-5},
  keyword      = {Thermal radiation,Property model,SNB,WSGG,LBL,Planck,particle,fly-ash,soot,grate,furnace,combustion},
  language     = {eng},
  pages        = {69},
  publisher    = {Lund University},
  school       = {Lund University},
  title        = {Modelling and Experimental Investigations on Thermal Radiation in Combustion Environments},
  year         = {2015},
}