Large Eddy Simulation of Turbulent Premixed and Partially Premixed Combustion
(2010) Abstract
 In this thesis, a computational fluid dynamics (CFD) approach is used to study turbulent premixed and partially premixed combustion. The CFD approach is based on large eddy simulation (LES) in which the largescale structures of the flow are resolved on a grid, leaving only the smallscale structures (subgrid scales) to be modeled. The combustion modelling is based on the flamelet concept in which the scale separation of the flow and the chemistry is assumed. This thesis work is made up of the following parts.
First, focusing on the turbulent premixed combustion, the levelset Gequation flamelet model is applied to study high density ratio flames without explicit filter in order to capture the effect of thin reaction zone... (More)  In this thesis, a computational fluid dynamics (CFD) approach is used to study turbulent premixed and partially premixed combustion. The CFD approach is based on large eddy simulation (LES) in which the largescale structures of the flow are resolved on a grid, leaving only the smallscale structures (subgrid scales) to be modeled. The combustion modelling is based on the flamelet concept in which the scale separation of the flow and the chemistry is assumed. This thesis work is made up of the following parts.
First, focusing on the turbulent premixed combustion, the levelset Gequation flamelet model is applied to study high density ratio flames without explicit filter in order to capture the effect of thin reaction zone embedded in the LES subgrid. Conventional fractional step methods are shown to be numerically instable for high density ratio flames. A highly robust numerical method, which is known as the ghost fluid method (GFM), is implemented. The Gequation based flamelet model and the ghost fluid method are evaluated on a lean propane/air premixed flame stabilized by a bluff body. The methods are shown to be able to capture the density ratio effect on the flame dynamics, including the near flame holder wrinkling due to Kelvin Helmholtz (KH) instabilities, the downstream large scale wrinkling due to the lower frequency Bénard/vonKarman (BVK) instability at low density ratio conditions, and the suppression of BVK instability at high density ratio conditions. In LES, spatial filtering of the reaction zone leads to thickening of the reaction zone. It is shown that thickening of the reaction zone can lead to significant underprediction of the turbulence intensity at high density ratio conditions. The effects of flame thickening are further studied for the cases of the flame/vortex interaction and hydrodynamic instability. Essentially, with thickening of the reaction zones, the development of flame wrinkling and hydrodynamic instability are suppressed.
Second, focusing on the partially premixed combustion, a twoscalar flamelet approach for LES is developed. In this approach, the trailing edge of the flame is assumed to be controlled by diffusion of mass and heat and thereby it is modelled using a steady diffusion flamelet model. The stabilization of the flame is due to the propagation of the leading premixed flame front (triple flame) in turbulent flows. The leading premixed flame is modelled using the levelset Gequation. This model is applied to simulate partially premixed flames of various fuels in a conical burner to understand the structure and the dynamics of the turbulent partially premixed flames operating in the flamelet regime and in certain cases with thicker reaction zones and local flame extinction. It is found that in general partially premixed flames are more stable when the level of partial premixing of air to the fuel stream decreases. However, at high Reynolds number conditions, an optimal level of partial premixing is found where the flame is most stable. There are two possible flammable surfaces in the partially premixed flames in the conical burner, where the mixture is in stoichiometric condition. At low Reynolds number flows, the inner flame is observed experimentally whereas it is not possible to stabilize at high Reynolds number flows. Numerical results based on the twoscalar flamelet model correctly predicted the blowoff of the inner flame at high Reynolds number conditions.
It is well known that LES results are sensitive to the inflow conditions. The sensitivity of LES results to inflow turbulence and the mean flow profiles is systematically investigated based on the conical burner. It is shown that in the proximity of the burner the onset of the flow instability is not only dependent on the shape of the mean profiles, but also on the anisotropy of the inflow turbulence and the integral length scale of the inflow turbulence. This calls for special care in validation of LES models and more detailed experimental data for the inflow conditions when preparing for the database for model development and validation. (Less)
Please use this url to cite or link to this publication:
http://lup.lub.lu.se/record/1717064
 author
 Baudoin, Eric ^{LU}
 supervisor

 XueSong Bai ^{LU}
 opponent

 Professor D’angelo, Yves, INSA CORIA CNRS, St. Etienne, France
 organization
 publishing date
 2010
 type
 Thesis
 publication status
 published
 subject
 defense location
 Lecture hall M:B, MBuilding, Ole Römersväg. 1, Lund University Faculty of Engineering
 defense date
 20101214 10:15
 ISSN
 02821990
 language
 English
 LU publication?
 yes
 id
 4f54ce6886a449079f6fec345014d457 (old id 1717064)
 date added to LUP
 20101117 10:30:48
 date last changed
 20160919 08:45:00
@phdthesis{4f54ce6886a449079f6fec345014d457, abstract = {In this thesis, a computational fluid dynamics (CFD) approach is used to study turbulent premixed and partially premixed combustion. The CFD approach is based on large eddy simulation (LES) in which the largescale structures of the flow are resolved on a grid, leaving only the smallscale structures (subgrid scales) to be modeled. The combustion modelling is based on the flamelet concept in which the scale separation of the flow and the chemistry is assumed. This thesis work is made up of the following parts.<br/><br> First, focusing on the turbulent premixed combustion, the levelset Gequation flamelet model is applied to study high density ratio flames without explicit filter in order to capture the effect of thin reaction zone embedded in the LES subgrid. Conventional fractional step methods are shown to be numerically instable for high density ratio flames. A highly robust numerical method, which is known as the ghost fluid method (GFM), is implemented. The Gequation based flamelet model and the ghost fluid method are evaluated on a lean propane/air premixed flame stabilized by a bluff body. The methods are shown to be able to capture the density ratio effect on the flame dynamics, including the near flame holder wrinkling due to Kelvin Helmholtz (KH) instabilities, the downstream large scale wrinkling due to the lower frequency Bénard/vonKarman (BVK) instability at low density ratio conditions, and the suppression of BVK instability at high density ratio conditions. In LES, spatial filtering of the reaction zone leads to thickening of the reaction zone. It is shown that thickening of the reaction zone can lead to significant underprediction of the turbulence intensity at high density ratio conditions. The effects of flame thickening are further studied for the cases of the flame/vortex interaction and hydrodynamic instability. Essentially, with thickening of the reaction zones, the development of flame wrinkling and hydrodynamic instability are suppressed.<br/><br> Second, focusing on the partially premixed combustion, a twoscalar flamelet approach for LES is developed. In this approach, the trailing edge of the flame is assumed to be controlled by diffusion of mass and heat and thereby it is modelled using a steady diffusion flamelet model. The stabilization of the flame is due to the propagation of the leading premixed flame front (triple flame) in turbulent flows. The leading premixed flame is modelled using the levelset Gequation. This model is applied to simulate partially premixed flames of various fuels in a conical burner to understand the structure and the dynamics of the turbulent partially premixed flames operating in the flamelet regime and in certain cases with thicker reaction zones and local flame extinction. It is found that in general partially premixed flames are more stable when the level of partial premixing of air to the fuel stream decreases. However, at high Reynolds number conditions, an optimal level of partial premixing is found where the flame is most stable. There are two possible flammable surfaces in the partially premixed flames in the conical burner, where the mixture is in stoichiometric condition. At low Reynolds number flows, the inner flame is observed experimentally whereas it is not possible to stabilize at high Reynolds number flows. Numerical results based on the twoscalar flamelet model correctly predicted the blowoff of the inner flame at high Reynolds number conditions. <br/><br> It is well known that LES results are sensitive to the inflow conditions. The sensitivity of LES results to inflow turbulence and the mean flow profiles is systematically investigated based on the conical burner. It is shown that in the proximity of the burner the onset of the flow instability is not only dependent on the shape of the mean profiles, but also on the anisotropy of the inflow turbulence and the integral length scale of the inflow turbulence. This calls for special care in validation of LES models and more detailed experimental data for the inflow conditions when preparing for the database for model development and validation.}, author = {Baudoin, Eric}, issn = {02821990}, language = {eng}, school = {Lund University}, title = {Large Eddy Simulation of Turbulent Premixed and Partially Premixed Combustion}, year = {2010}, }