Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2010. / AFRIKAANSE OPSOMMING:Die navorsing wat hier uiteengesit word ondersoek die hipotese dat kondensasie
en die gevolglike verdamping van water wat teenwoordig is in die uitlaatgas van
‘n binnebrandenjin, plaasvind in die gedeeltes van die uitlaatstelsel tussen die
uitlaatklep en die katalitiese-omsetter se uitlaat. Daar word verder veronderstel
dat hierdie tweefasevloeieffekte die tydafhanklike temperatuurprofiele in die
uitlaatstelsel beïnvloed, wat moontlik kan lei tot ‘n vertraging in die tydsduur vir
die katalitiese omsetter om temperature van 200-300 °C te bereik, wat nodig is om
noemenswaardige omsetting te bewerkstellig.
Om hierdie veronderstelling te evalueer is ‘n tydafhanklike, eendimensionele
wiskundige model van die termo-vloei gedrag in die uitlaatstelsel gedurende ‘n
koue inwerktreeding, insluitende vogtigheidseffekte, opgestel en opgelos deur van
‘n rekenaaralgoritme gebruik te maak. Warmte- en massaoordragsteorie was
gebruik om die ongestadigde massa-, energie- en
momentumbehoudsvergeleikings te formuleer. Die tweefasige vogeffekte was
gemodelleer deur gebruik te maak van die verhouding tussen warmte- en
massaoordrag, wat verdamping en heterogeniese kondensasie (die kondensasie
van damp teen die pypwand) voorspel as gevolg van die dampdrukgradient tussen
die grootmaat damp en die versadigde damp by die oppervlak van die
vloeistoffilm. Homogene kondensasie (die kondensasie van vloeistof in die vorm
van druppels in die dampstroom) was ook in aanmerking geneem indien die
grootmaatgas temperatuur onder die versadigingstemperatuur van die
grootmaatdamp gedaal het.
’n Eksperimentele ondersoek was gedoen deur van twee enjins gebruik te maak,
’n 1.6 L Volkswagen Bora en ’n 1.6 L Ford RoCam, in die toetsselle van Cape
Advanced Engineering Pty (Ltd). Om die gastemperature so akkuraat moontlik te
meet, was spesiale radiasiegeskermde sensore met vinnige reaksietyd ontwerp en
installer in die pypseksies van die uitlaatstelsels van beide enjins. Die geskermde
sensore het temperature van tot 50 °C hoër as konvensionele termokoppels in
dieselfde areas gemeet. Dit is in koers is met resultate wat deur die
foutbeperkingsteorie, geassosieer met die meet van temperature in vloeïende gas
in uitlaatstelsels, voorspel word.
Vergelyking van die numeriese simulasie met die eksperimenteel gemete
temperature het aangedui dat in dele van die uitlaatstelsel voor die katalitieseomsetter,
die vog min uitwerking het op die termiese gedrag van die stelsel. In
hierdie gedeeltes is die konveksie warmte-oordrag dominant. In die katalitieseomsetter
was die vogeffekte invloedryk. Die eksperimentele resultate toon ‘n
duidelike vroeë toename in die gastemperature, gevolg deur ‘n tydperk van
konstante temperature by nagenoeg die versadigingstemperatuur van die
grootmaatdamp (verwys na as die temperatuurplato) by die katalitiese-omsetter se
kern en uitlaat. Die numeries gesimuleerde gastemperature het ook hierdie gedrag
getoon, maar ‘n baie hoë en skerp piek by die begin van die plato het voorgekom. Hierdie piek was nie te sien by die eksperimentele resultate nie en is toegeskryf
aan nie-ewewigstoestande in die verdampingsproses, wat aandui dat die tempo
van verdamping wat deur die massa-oordragmodel voorspel word te hoog is vir
die model en dat dit verfyn moet word. Verdere ondersoek van die invloed van
die individuele massa-oordragprosesse het getoon dat die homogene kondensasie
die dominante proses is in die vorming van vloeistof in die katalitiese-omsetter.
Heterogeniese kondensasie het plaasgevind, maar ‘n kleiner massa vloeistof is
produseer. Die maksimum hoeveelheid vloeistof wat voorspel is om in die
katalitiese-omsetter te vorm was 12 g/cm wat gelykstaande is aan ‘n film van
0.05.mm dik indien eweredig versprei oor die binneoppervlak van die monoliet.
Daar was in die simulasie gevind dat beide verdamping en kondensasie benodig
word om die temperatuurplato te simuleer, vanwaar die gevolgtrekking gemaak
kan word dat beide prosesse wel plaasvind en dat die eerste stelling in die
oorspronklike hipotese wel geldig is. Daar was egter teen die einde van die
toetsperiode gevind dat beide temperature wat met en sonder vogeffekte simuleer
was, die eksperimentele temperature nagevolg het, wat aandui dat die invloed van
vog beperk is tot die vroeë stadiums van die katalitiese-omsetter se
opwarmingstydperk. Die tweede gedeelte van die hipotese wat veronderstel dat
die voggedrag ‘n vertraging in die tydsduur om omsetting te bewerkstellig
veroorsaak, is dus bevind om ongeldig te wees.
Die wiskundige model wat opgestel is tydens die ondersoek is weens
noodsaaklikheid ‘n vereenvoudigde simulasie van komplekse termo-vloei
prosesse. Dit dien as nuttige grondwerk vir verdere in-diepte ondersoeke en
afronding van die teorie met betrekking tot voggedrag en die uitwerking daarvan
op die tydsafhanklike temperature in ‘n uitlaatstelsel. / ENGLISH ABSTRACT: The research presented here investigates the hypothesis that condensation and
subsequent evaporation of water vapour present in the exhaust gas of an internal
combustion engine occur in the sections of the exhaust system between the
exhaust port and the catalytic converter exit. It is further hypothesised that these
two-phase moisture effects influence the transient temperature profiles in the
exhaust system, and potentially cause a delay in the time it takes for the catalytic
converter to reach temperatures of 200-300 °C, which are required for light-off to
occur.
In order to evaluate this hypothesis a transient, one-dimensional mathematical
model of the thermo-fluid behaviour in the exhaust system during a cold start,
including moisture effects, was created and solved by means of a computer
algorithm. Heat and mass transfer theory was used to formulate the unsteady
conservation equations for mass, energy and momentum. The two phase moisture
effects were modelled using the analogy between heat and mass transfer, which
predicts evaporation and heterogeneous condensation (the condensation of vapour
against the pipe wall) due to a vapour pressure gradient between the bulk vapour
and a saturated vapour at the surface of the liquid film. Homogeneous
condensation (the condensation of liquid in the form of droplets in the gas stream)
was also accounted for if the bulk gas temperature dropped below the bulk vapour
saturation temperature.
An experimental investigation was performed using two engines, a 1.6.L
Volkswagen Bora and a 1.6.L Ford RoCam, in the test cells of Cape Advanced
Engineering Pty (Ltd). In order to measure the gas temperatures as accurately as
possible specialised radiation shielded sensors with fast time response were
designed and installed in the pipe sections of the exhaust systems of both engines.
The shielded sensors measured temperatures up 50 °C higher than the
conventional thermocouples installed at the same positions, which is in keeping
with the results predicted by the theory governing errors associated with
temperature measurement in the flowing gas in the exhaust system.
Comparison of the numerically simulated and experimentally measured
temperatures indicated that in the sections of the exhaust system leading up to the
catalytic converter the moisture has little influence on the temperature behaviour
of the exhaust system. In these sections the convective heat transfer is dominant.
In the catalytic converter the moisture effects were found to be influential. The
experimental results clearly show an early rise in the gas temperatures, followed
by a period of constant temperature at approximately the saturation temperature of
the bulk vapour (referred to as the temperature plateau) at the catalytic converter
mid-bed and exit. The numerically simulated gas temperatures also exhibited this
plateau, but an initial very high and sharp peak in the simulated gas temperatures
occurred at the start of the plateau. This was not seen in the experimental results
and is attributed to non-equilibrium in the evaporation process, indicating that the rate of evaporation predicted by the mass transfer model used is too high for this
application and that the model needs to be refined. Further investigation of the
influence of the individual mass transfer processes indicated that the
homogeneous condensation is the dominant process in the formation of liquid in
the catalytic converter. Heterogeneous condensation was found to occur, but
produced a smaller mass of liquid. The maximum amount of liquid predicted to
form in the catalytic converter was 12 g/cm, which translates to a film 0.05 mm
thick if evenly distributed over the inner surface of the monolith.
In the simulation it was found that both evaporation and condensation are needed
in order to simulate the temperature plateau, from which it was concluded that
both these processes do occur and the first statement in the original hypothesis is
valid. However, by the end of the test period temperatures simulated both with or
without the moisture effects closely approached the final temperatures of the
experimental investigation, indicating that the influence of the moisture is limited
to the early stages of the catalytic converter warm-up. The second part of the
hypothesis, postulating that the moisture behaviour caused a delay in the time
taken to reach light-off temperature, is therefore concluded to be invalid.
The mathematical model constructed in this research is by necessity a simplified
solution to complex thermo-fluid processes. It serves as useful groundwork for
further elaboration and refinement of the theory related the moisture behaviour
and its influence on the transient temperatures in the exhaust system.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/4285 |
| Date | 03 1900 |
| Creators | Haworth, Leanne |
| Contributors | Dobson, R. T., University of Stellenbosch. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 1 v. (various pagings) : ill. |
| Rights | University of Stellenbosch |
Page generated in 0.0033 seconds