| dc.description.abstract | I dette prosjektet har tre forskjellige modeller for flammer blitt undersøkt: forblandet velblandet
reaktor, forblandet fritt-spredene flamme, og ikke-forblandet motstrøms flamme. Målet
med prosjekter var å først bestemme en måte å beregne entropiproduskjoenen i flammer, samt
hvordan den fordeleres seg i rom, og deretter utføre en slik analyse på modellene nevnt over,
med metan og syntese gass (syngas) som brensel. For hvert brensel ble det brukt detaljert,
redusert, og globale mekanismer. GRI-mech 3.0 [41] ble brukt som den detaljerte mekanismen
for begge brenslene, mens DRM19 [24] og Davis et al. [15] var de reduserte mekanismene
brukt for metan og syngas, respektivt. For metan ble det bruke en global mekanisme med en
reaksjon, laget av Westbrook og Dryer [47]. I midlertid ble det brukt to globale mekanismer
for syngas. For den vel-blandete reaktoren, og motstrøms flammen ble en global mekanisme
presentert av Cuoci [11] brukt. En annen mekanisme presenter av Marzouk og Huckaby [32]
ble brukt for den fritt-spredene flammen. I tillegg til variasjon av forenkling i den kjemiske
mekanismen var trykket variert mellom 1 atm, 10 atm og 20 atm. Dette resulterte i totalt 54
forskjellige tilfeller som skulle simuleres.
Entropi produksjonen ble beregnet i en post-prosess analyse i en separat kode for den frittutvidende
flammen og motstrøms flammen, mens den var inkludert i koden for den vel-blanda
reaktoren. For å validere koden ble entropi produksjonen beregnet for metan ved bruk av den
detaljerte og redusert mekanismen, sammenlignet med Nishida et al. [35]. Dermed ble kodene
for, den fritt-spredene flammen, entropi produksjonen, samt bruken av GRI-mech 3.0 [41] og
DRM19 [24] for metan validert. Videre ble koden for motstrømsflammen, samt bruken av GRImech
3.0 [41] og Davis et al. [15] for syngas, validert mot Som et al. [42]. Den vel-blandete
reaktoren var antatt å være såpass enkel at en omgående validering ikke var nødvendig. Den
var derfor kun sammenlignet med manuelle beregninger.
To større hindre, blant flere små, ble møtt i prosjekt. Den ene var at Cantera [16] ikke gir ut
flerkomponent diffusjons koeffisientene, nødvendig for entropi produksjon fra diffusjon, i hvert
punkt i løsningen. Derfor ble i stedet stoffenes molare diffusjon hentet ut i hvert punkt og brukt
i utregningene. Den andre var entropi produksjon fra intern varmeoverføring fra reaksjonene
til luft-brensel blandingen måtte tas med for reaktoren, i tillegg til produksjonen fra reaksjonene
som først var antatt å være den eneste kilden. Når hindrene var overkomne ble flammene
simulert, og entropi produksjonen og dens distribusjon i rommet ble beregnet.
Fra resultatene ble det oppdaget at både for den vel-blandete reaktoren og den fritt-spredene
flammen, økte den integrerte entropi produksjonen med trykket for begge brenselene, ved bruk
av alle mekanismene. I motsetning sank produksjonen for begge brenselene, ved bruk av alle
mekanismer i motstrøms flammen. Grafene som viste den lokale produksjonen ble høyere og
smalere for begge stoffene, og alle mekanismene i den fritt-spredene flammen. Profilen for den
lokale produksjonen i motstrøms flammen var mindre avhengig av trykket.
Videre ble det oppdaget at den redusert mekanismen for metan fungerte bra for å beregne
den integrerte og den lokale produksjonen for den fritt-spredene flammen, men hadde noen
avvik for motstrøms flammen. Den reduserte mekanismen for syngas fungerte bra i begge modellene.
Den globale mekanismen for methan fungerte bra for å beregne den integrerte produksjonen
i alle modellene. Den hadde noe avvik ved 1 atm for den fritt-spredene flammen, og ved
10 atm og 20 atm for motstrøms flammen. Den globale mekanismen brukt for syngas i den
fritt-spredene flammen fungerte ikke bra. Den globale mekanismen brukt i de andre modellene
fungerte derimot overraskende bra. | |
| dc.description.abstract | In this project, three different flame models have been investigated, namely premixed wellstirred
reactor, premixed freely-propagating flame, and non-premixed counterflow flame. The
objective of the project was to figure out a way to analyse the entropy production in flames,
and how it is distributed in space, and thereafter perform such an analysis on the flame models
mentioned. In the analyses, methane and syngas were used as fuels. For each fuel it was
used one detailed, one reduced and one global mechanism. GRI-mech 3.0 [41] was used as
the detailed mechanism for both fuels, while DRM19 [24] and [15] was used as the reduced
mechanisms for methane and syngas, respectively. For methane, a global mechanism consisting
of one equation created by Westbrook and Dryer [47] was used. Meanwhile, for syngas two
global mechanisms were used. For the well-stirred reactor, and the non-premixed counterflow
flame, a global mechanism presented by Cuoci et al. [11] was used. Another global mechanism
presented by Marzouk and Huckaby [32] was used for the last model. In addition to varying
the chemical mechanism, in degree of simplification, the pressure was varied between 1 atm,
10 atm, and 20 atm. This resulted in a total of 54 individual cases to be simulated.
The entropy production was estimated in a post-process analysis in a separate code for
the freely-propagating and counterflow flames, while it was included in the code for the wellstirred
reactor. To validate the codes, the entropy production estimated for methane using both
the detailed, and reduced mechanisms were compared with results obtained by Nishida et al.
[35]. Thus, the code for the freely-propagating flame, the use of GRI-mech [41] and DRM19
[24] for methane, as well as the post-process analysis code were validated. The code written
for the counterflow flame, as well as the use of GRI-mech [41] and Davis et al. [15] for syngas,
were validated against Som et al. [42]. The well-stirred reactor was considered to be of such
simplicity that an extensive validation was unnecessary. The values obtained for entropy was
however crosschecked with manually. This was also done for the freely-propagating flame.
Two larger obstacles, amongst more less time-demanding obstacles, were met during the
project. Firstly, the multi-component diffusion coefficient necessary for the calculation of entropy
production due to mass diffusion was not given by Cantera [16]. Therefore, the species
mole fluxes were retrieved rather than the coefficients. Secondly, it was discovered that the entropy
change in the reactor was not caused by the chemical reactions alone, as initially thought.
The entropy production due to internal heat transfer from the reactions to the fuel-air mixture
also had to be accounted for. With all obstacles sorted, the flames were simulated, and the
entropy production, with its distribution in space was estimated. It was discovered that both in the well-stirred reactor model, and the freely-propagating
flame model the integrated entropy production increased with pressure for both fuels, with all
mechanisms. In contrast, the production decreased for both fuels, with all mechanisms in the
counterflow model. The profiles of the local entropy production got thinner and taller for both
fuels in the freely-propagating flame. The counterflow model was less pressure dependent.
Furthermore, the reduced mechanism for methane worked well for the integrated entropy
production and the local production in the freely-propagating flame, but had some discrepancies
in the counterflow flame. The reduced mechanism for syngas worked well in both models.
The global mechanism for methane worked well to calculate the integrated production in both
models, but had discrepancies for the local production at 1 atm for the freely-propagating
flame, and at 10 atm, and 20 atm for the counterflow model. The global mechanism used
for syngas in the freely-propagating model did not work, and was not appropriate to use. The
global mechanism used in the reactor and counterflow flame models worked well. | |
