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Wall Effects of Laminar Hydrogen Flames over Platinum and Inert Surfaces
KTH, Superseded Departments, Chemical Engineering and Technology.
KTH, Superseded Departments, Chemical Engineering and Technology.
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.

Place, publisher, year, edition, pages
2000. Vol. 46, no 7, 1454-1460 p.
Keyword [en]
MODEL; CHEMISTRY; OXIDATION
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-12245DOI: 10.1002/aic.690460718ISI: 000088372000016OAI: oai:DiVA.org:kth-12245DiVA: diva2:306697
Note
QC 20100504Available from: 2010-03-30 Created: 2010-03-30 Last updated: 2017-12-12Bibliographically approved
In thesis
1. Wall Related Lean Premixed Combustion Modeled with Complex Chemistry
Open this publication in new window or tab >>Wall Related Lean Premixed Combustion Modeled with Complex Chemistry
2002 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Increased knowledge into the physics and chemistrycontrolling emissions from flame-surface interactions shouldhelp in the design of combustion engines featuring improvedfuel economy and reduced emissions.

The overall aim of this work has been to obtain afundamental understanding of wall-related, premixed combustionusing numerical modeling with detailed chemical kinetics. Thiswork has utilized CHEMKIN®, one of the leading softwarepackages for modeling combustion kinetics.

The simple fuels hydrogen and methane as well as the morecomplex fuels propane and gasified biomass have been used inthe model. The main emphasis has been on lean combustion, andthe principal flow field studied is a laminar boundary layerflow in two-dimensional channels. The assumption has been madethat the wall effects may at least in principle be the same forlaminar and turbulent flames.

Different flame geometries have been investigated, includingfor example autoignition flames (Papers I and II) and premixedflame fronts propagating toward a wall (Papers III and IV).Analysis of the results has shown that the wall effects arisingdue to the surface chemistry are strongly affected by changesin flame geometry. When a wall material promoting catalyticcombustion (Pt) is used, the homogeneous reactions in theboundary layer are inhibited (Papers I, II and IV). This isexplained by a process whereby water produced by catalyticcombustion increases the rate of the third-body recombinationreaction: H+O2+M ⇔ HO2+M. In addition, the water produced at higherpressures increases the rate of the 2CH3(+M) ⇔ C2H6(+M) reaction, giving rise to increased unburnedhydrocarbon emissions (Paper IV).

The thermal coupling between the flame and the wall (theheat transfer and development of the boundary layers) issignificant in lean combustion. This leads to a sloweroxidation rate of the fuel than of the intermediatehydrocarbons (Paper III).

Finally in Paper V, a well-known problem in the combustionof gasified biomass has been addressed, being the formation offuel-NOx due to the presence of NH3 in the biogas. A hybridcatalytic gas-turbine combustor has been designed, which cansignificantly reduce fuel-NOx formation.

Keywords:wall effects, premixed flames, flamequenching, numerical modeling, CHEMKIN, boundarylayerapproximation, gasified biomass, fuel-NOx, hybrid catalytic combustor.

Place, publisher, year, edition, pages
Stockholm: Kemiteknik, 2002. 73 p.
Series
Trita-KET, ISSN 1104-3466 ; 164
Keyword
Wall effects, premixed flames, flame quenching, numerical modeling, chemkin, boundary layer approximation, gasified biomass, fuel-NOx, hybrid catalytic combustor.
National Category
Natural Sciences
Identifiers
urn:nbn:se:kth:diva-3455 (URN)91-7283-391-2 (ISBN)
Public defence
2002-12-17, 00:00 (English)
Note
QC 20100504Available from: 2002-12-11 Created: 2002-12-11 Last updated: 2010-05-04Bibliographically approved
2. Numerical Studies of Wall Effects of Laminar Flames
Open this publication in new window or tab >>Numerical Studies of Wall Effects of Laminar Flames
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
wall effects, laminar premixed flames, Chemkin, boundary layer flow
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-1258 (URN)
Presentation
(English)
Note
QC 20100504Available from: 2001-08-20 Created: 2001-08-20 Last updated: 2010-05-04Bibliographically approved

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