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Numerical Studies of Wall Effects of Laminar Flames
KTH, Superseded Departments, Chemical Engineering and Technology.
2001 (English)Licentiate thesis, comprehensive summary (Other scientific)
Abstract [en]

Numerical simulations have been done with the CHEMKINsoftware to study different aspects of wall effects in thecombustion of lean, laminar and premixed flames in anaxisymmetric boundary-layer flow.

The importance of the chemical wall effects compared to thethermal wall effects caused by the development of the thermaland velocity boundary layer has been investigated in thereaction zone by using different wall boundary conditions, walltemperatures and fuel/air ratios. Surface mechanisms include acatalytic surface (Platinum), a surface that promotesrecombination of active intermediates and a completely inertwall with no species and reactions as the simplest possibleboundary condition.

When hydrogen is the model fuel, the analysis of the resultsshow that for atmospheric pressure and a wall temperature of600 K, the surface chemistry gives significant wall effects atthe richer combustion case (f=0.5), while the thermal andvelocity boundary layer gives rather small effects. For theleaner combustion case (f=0.1) the thermal and velocityboundary layer gives more significant wall effects, whilesurface chemistry gives less significant wall effects comparedto the other case.

For methane as model fuel, the thermal and velocity boundarylayer gives significant wall effects at the lower walltemperature (600 K), while surface chemistry gives rather smalleffects. The wall can then be modelled as chemically inert forthe lean mixtures used (f=0.2 and 0.4). For the higher walltemperature (1200 K) the surface chemistry gives significantwall effects.

For both model fuels, the catalytic wall unexpectedlyretards homogeneous combustion of the fuel more than the wallthat acts like a sink for active intermediates. This is due toproduct inhibition by catalytic combustion. For hydrogen thisoccurs at atmospheric pressure, but for methane only at thehigher wall temperature (1200 K) and the higher pressure (10atm).

As expected, the overall wall effects (i.e. a lowerconversion) were more pronounced for the leaner fuel-air ratiosand at the lower wall temperatures.

To estimate a possible discrepancy in flame position as aresult of neglecting the axial diffusion in the boundary layerassumption, calculations have been performed with PREMIX, alsoa part of the CHEMKIN software. With PREMIX, where axialdiffusion is considered, steady, laminar, one-dimensionalpremixed flames can be modelled. Results obtained with the sameinitial conditions as in the boundary layer calculations showthat for the richer mixtures at atmospheric pressure the axialdiffusion generally has a strong impact on the flame position,but in the other cases the axial diffusion may beneglected.

Keywords:wall effects, laminar premixed flames,platinum surfaces, boundary layer flow

Place, publisher, year, edition, pages
Stockholm: Kemiteknik , 2001. , 45 p.
Series
Trita-KET, ISSN 1104-3466 ; 145
Keyword [en]
wall effects, laminar premixed flames, Chemkin, boundary layer flow
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-1258OAI: oai:DiVA.org:kth-1258DiVA: diva2:7066
Presentation
(English)
Note
QC 20100504Available from: 2001-08-20 Created: 2001-08-20 Last updated: 2010-05-04Bibliographically approved
List of papers
1. Wall Effects of Laminar Hydrogen Flames over Platinum and Inert Surfaces
Open this publication in new window or tab >>Wall Effects of Laminar Hydrogen Flames over Platinum and Inert Surfaces
2000 (English)In: AIChE Journal, ISSN 0001-1541, E-ISSN 1547-5905, Vol. 46, no 7, 1454-1460 p.Article in journal (Refereed) Published
Abstract [en]

Different aspects of wall effects in the combustion of lean, laminar and stationary hydrogen flames in an axisymmetric boundary-layer flow were studied using numerical simulations with the program CRESLAF. The importance of the chemical wall effects compared to thermal wall effects caused by heat transfer to a cold wall was investigated in the reaction zone by using different combustion systems at atmospheric pressure. Surface mechanisms include a catalytic surface, an inert surface that promotes radical recombinations, and a completely inert wall used as reference was the simplest possible boundary condition. The analysis of the results show that for the richer combustion case ( = 0.5) the surface chemistry gives significant wall effects, while the thermal and velocity boundary layer gives rather small effects. But for the leaner combustion case ( = 0.1) the thermal and velocity boundary layer gives more significant wall effects, while surface chemistry gives less significant wall effects compared to the other case. As expected, the overall wall effects were more pronounced for the leaner combustion case.

Keyword
MODEL; CHEMISTRY; OXIDATION
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-12245 (URN)10.1002/aic.690460718 (DOI)000088372000016 ()
Note
QC 20100504Available from: 2010-03-30 Created: 2010-03-30 Last updated: 2017-12-12Bibliographically approved
2. Numerical studies of wall effects with laminar methane flames
Open this publication in new window or tab >>Numerical studies of wall effects with laminar methane flames
2002 (English)In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 128, no 1-2, 165-180 p.Article in journal (Refereed) Published
Abstract [en]

Wall effects in the combustion of lean methane mixtures have been studied numerically using the CHEMKIN software. To gain a deeper understanding of the flame-wall interaction in lean burn combustion, and in particular the kinetic and thermal effects, we have simulated lean and steady methane/air flames in a boundary layer flow. The gas-phase chemistry is modeled with the GRI mechanism version 1.2. Boundary conditions include an inert wall, a recombination wall and catalytic combustion of methane. Different pressures, wall temperatures and fuel-air ratios are used to address questions such as which part of the wall effects is most important at a given set of conditions. As the results are analyzed it can be seen that the thermal wall effects are more significant at the lower wall temperature (600 K) and the wall can essentially be modeled as chemical inert for the lean mixtures used. At the higher wall temperature (1,200 K), the chemical wall effects become more significant and at the higher pressure (10 atm) the catalytic surface retards homogeneous combustion of methane more than the recombination wall because of product inhibition. This may explain the increased emissions of unburned fuel observed in engine studies, when using catalytic coatings on the cylinder walls. The overall wall effects were more pronounced for the leaner combustion case (phi = 0.2). When the position of the reaction zone obtained from the boundary layer calculations is compared with the results from a one-dimensional premixed flame model, there is a small but significant difference except at the richer combustion case (phi = 0.4) at atmospheric pressure, where the boundary layer model may not predict the flame position for the given initial conditions.

Keyword
INERT SURFACES, COMBUSTION, HYDROGEN, PLATINUM, MODEL, AIR
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-12249 (URN)10.1016/S0010-2180(01)00342-X (DOI)000173661900011 ()
Note

QC 20100504

Available from: 2010-03-30 Created: 2010-03-30 Last updated: 2017-12-12Bibliographically approved

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