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Direct numerical simulation of turbulent premixed flames at high Karlovitz numbers: structure and modelling

Nilsson, Thommie LU (2019)
Abstract
There has been recent increased interest in premixed flames and in particular for use in gas turbines for power generation. This is a development that follows from increasing demands for high efficiency and low emissions of NOx, which can be achieved by using premixed flames and lean pre-heated mixtures, often close to the lean flammability limit. In combination with high levels of turbulence, which is needed to burn significant amounts of fuel in a limited space, this trend leads to higher Karlovitz numbers. The Karlovitz number is a dimensionless quantity that compares the time scale of flame propagation with the time scale of turbulent micro-mixing. It has been hypothesized, for example in classical regime diagrams, that premixed flames... (More)
There has been recent increased interest in premixed flames and in particular for use in gas turbines for power generation. This is a development that follows from increasing demands for high efficiency and low emissions of NOx, which can be achieved by using premixed flames and lean pre-heated mixtures, often close to the lean flammability limit. In combination with high levels of turbulence, which is needed to burn significant amounts of fuel in a limited space, this trend leads to higher Karlovitz numbers. The Karlovitz number is a dimensionless quantity that compares the time scale of flame propagation with the time scale of turbulent micro-mixing. It has been hypothesized, for example in classical regime diagrams, that premixed flames enter a special combustion regime, the distributed reaction zone regime, at high enough Karlovitz numbers and in this regime turbulent mixing would distort all layers of the flame. This has important implications for computational fluid dynamics (CFD) simulation of premixed flames since such simulations rely on models that are often based on the flamelet assumption which states that the internal flame structure should not be distorted. In the recent literature there have been a limited number of reports of flames, simulated and experimental, that are claimed to probe the high Karlovitz regime and observations include changes in the chemical reaction path, low temperature heat release and average broadening of the flame, but also the existence of locally thin or compressed regions and the persistence of a layered structure well above the speculated regime boundary of distributed reactions. In addition to remaining questions about structure changes in high Ka flames, there is the important question about how to model them. In particular, under what conditions can flamelet based models be used, when can they not be used, and is there a need for different models? This thesis is devoted to two main questions. First, the thesis concerns the structure of premixed turbulent flames, in particular at high Karlovitz numbers, and the focus is on the flame thickness, how it evolves in a transient developing flame, and the balance between contributing mechanisms. Direct numerical simulations (DNS) are used to investigate these topics. Second, the thesis concerns modelling in premixed flames in the large eddy simulation (LES) framework which is assessed using filtered DNS data. The focus is on a flamelet-based presumed probability density function (PDF) model for the filtered reaction rate, investigated for a range of premixed flames of varying complexity and Karlovitz number, as well as on the related sub-filter variance equation which is an essential ingredient of the studied flamelet model. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Professor Chakraborty, Nilanjan, Newcastle University, United Kingdom.
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Direct numerical simulation, Computational fluid dynamics, Combustion model, Premixed flame, Turbulence, Karlovitz number
publisher
Department of Energy Sciences, Lund University
defense location
Lecture Hall M:B, M-Building, Ole Römers väg 1, Lund University, Faculty of Engineering LTH
defense date
2019-05-31 10:00:00
ISSN
0282-1990
0282-1990
ISBN
978-91-7895-110-9
978-91-7895-109-3
language
English
LU publication?
yes
id
198df51a-0b68-4ffe-a057-308877cb2b6e
date added to LUP
2019-05-06 10:16:21
date last changed
2019-05-07 11:02:35
@phdthesis{198df51a-0b68-4ffe-a057-308877cb2b6e,
  abstract     = {There has been recent increased interest in premixed flames and in particular for use in gas turbines for power generation. This is a development that follows from increasing demands for high efficiency and low emissions of NOx, which can be achieved by using premixed flames and lean pre-heated mixtures, often close to the lean flammability limit. In combination with high levels of turbulence, which is needed to burn significant amounts of fuel in a limited space, this trend leads to higher Karlovitz numbers. The Karlovitz number is a dimensionless quantity that compares the time scale of flame propagation with the time scale of turbulent micro-mixing. It has been hypothesized, for example in classical regime diagrams, that premixed flames enter a special combustion regime, the distributed reaction zone regime, at high enough Karlovitz numbers and in this regime turbulent mixing would distort all layers of the flame. This has important implications for computational fluid dynamics (CFD) simulation of premixed flames since such simulations rely on models that are often based on the flamelet assumption which states that the internal flame structure should not be distorted. In the recent literature there have been a limited number of reports of flames, simulated and experimental, that are claimed to probe the high Karlovitz regime and observations include changes in the chemical reaction path, low temperature heat release and average broadening of the flame, but also the existence of locally thin or compressed regions and the persistence of a layered structure well above the speculated regime boundary of distributed reactions. In addition to remaining questions about structure changes in high Ka flames, there is the important question about how to model them. In particular, under what conditions can flamelet based models be used, when can they not be used, and is there a need for different models? This thesis is devoted to two main questions. First, the thesis concerns the structure of premixed turbulent flames, in particular at high Karlovitz numbers, and the focus is on the flame thickness, how it evolves in a transient developing flame, and the balance between contributing mechanisms. Direct numerical simulations (DNS) are used to investigate these topics. Second, the thesis concerns modelling in premixed flames in the large eddy simulation (LES) framework which is assessed using filtered DNS data. The focus is on a flamelet-based presumed probability density function (PDF) model for the filtered reaction rate, investigated for a range of premixed flames of varying complexity and Karlovitz number, as well as on the related sub-filter variance equation which is an essential ingredient of the studied flamelet model.},
  author       = {Nilsson, Thommie},
  isbn         = {978-91-7895-110-9},
  issn         = {0282-1990},
  language     = {eng},
  publisher    = {Department of Energy Sciences, Lund University},
  school       = {Lund University},
  title        = {Direct numerical simulation of turbulent premixed flames at high Karlovitz numbers: structure and modelling},
  url          = {https://lup.lub.lu.se/search/ws/files/63820200/Thommie_Nilsson_komplett.pdf},
  year         = {2019},
}