Thesis (MEng)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: In the present study, two-phase natural circulation flow in a multi-parallel channel system was
investigated using experimental and numerical modelling. The investigation was carried out under
different power excitations and various system operations (open system, closed system and heat pipe
mode). The multichannel system was equipped at the upper end with a condenser enclosed within a
steam drum, while the lower portion of each channel was heated to heat the system. For the numerical
modelling, transient one-dimensional conservation equations were derived from first principle for both
single- and two-phase fluids and used to computer program the system’s discretised simulation model.
Temperatures and mass flow rates of the fluid responses as a result of different power excitations and
operations were obtained for both the experimental and numerical modelling.
It was observed from the results that the fluid experiences a start-up transient before accomplishing
steady-state conditions. It was further noted that the transient duration varies with power excitations
and system operation modes and hence with the stability of the system. A rise in power proved not to
necessarily increase the fluid mass flow rate, but invited oscillations with higher amplitudes, depending
on the system’s mode of operation. Type I instability and low-quality steam oscillations were witnessed
at low power and open system operation mode (system open to the atmosphere). Type II instabilities
and flashing instability were observed to be associated with medium and high-power excitations for the
open system mode of operation.
The fluid flow became more stable and less oscillatory at all power excitations for the closed system
operation mode (system not open to the atmosphere). However, a sub-cooling effect was evident at
higher power, where the two-phase fluid temperatures oscillated in a sinusoidal manner. However, the
mass flow rates oscillated with high amplitudes in the forward direction in some channels and assumed
a unidirectional flow in other channels. In general, steady-state conditions were obtained earlier when
the system was operated as a closed system. For the heat pipe mode of operation, the system transient
response in all channels exhibited a geysering instability followed by flashing-induced boiling. In-phase
(flow in channels exhibiting the same behaviour) and out-of-phase (flow in channels exhibiting
contradictory conduct) behaviour between adjacent channels were observed at all power excitations
and system operation modes. Flow reversal in heated channels of a natural circulation system were
proven to exists even under equal power excitations. / AFRIKAANSE OPSOMMING: In hierdie studie is tweefasige natuurlike sirkulasievloei in ’n parallelle multikanaalstelsel ondersoek
deur middel van eksperimentele en numeriese modellering. Die ondersoek is onder verskillende
kragopwekkings en verskeie stelselwerkings (oop stelsel, toe stelsel en hittepypmodus). Die
multikanaalstelsel is aan die bopunt met ’n kondensor binne ’n stoomdrom toegerus, terwyl die laer
gedeelte van elke kanaal verhit is om die stelsel te verhit. Vir die numeriese modellering, is oorgangseendimensionele
behoundsvergelykings vanaf die eerste beginsel vir beide een- en tweefasige
vloeistowwe afgelei en dit is gebruik om die stelsel se gediskretiseerde simulasiemodel vir ’n rekenaar te
programmeer. Temperature en massavloeitempo’s van die vloeistofrespons as gevolg van verskillende
kragopwekkings en -werkings is vir beide die eksperimentele en die numeriese modellering verkry.
Dit is in die resultate waargeneem dat die vloeistof ’n aansitoorgang ervaar voor dit vloeiewewigstoestande
bereik. Daar is verder waargeneem dat die duur van die oorgang wissel volgens
kragopwekkings en stelselwerkingsmodusse en dus op grond van die stabiliteit van die stelsel. ’n
Toename in krag het nie noodwendig die vloeitempo van die vloeistofmassa verhoog nie, maar het
aanleiding gegee tot ossillasies met groter amplitudes, afhangende van die stelsel se metode van
werking. Tipe I-onstabiliteit en stoom-ossillasies van ’n lae intensiteit is teen lae krag en oop
stelselwerkingsmodus waargeneem (stelsel oop aan die atmosfeer). Tipe II-onstabiliteit en
flitsingsonstabiliteit (flashing instability) is met medium- en hoë kragopwekkings vir die oop stelsel
modus van werking waargeneem.
Die vloeistofvloei het meer stabiel en minder ossillerend geraak by alle kragopwekkings in die geslote
stelsel van werking (stelsel nie oop na die atmosfeer nie). ’n Subverkoelingseffek was egter teen hoër
krag duidelik, waar die tweefasige vloeistof se temperature sinusvormig geossilleer het. Die
massavloeitempo’s het egter met hoë amplitudes in die vorentoe rigting in sommige kanale gevloei en
eenrigtingvloei in ander kanale vertoon. Oor die algemeen is vloei-ewewigstoestande vroeër verkry toe
die stelsel as ’n geslote stelsel bedryf is. Vir die hittepypmodus van werking het die stelsel se
oorgangsweergawe in alle kanale ’n geysering onstabiliteit getoon, gevolg deur flitsinggeïnduseerde
(flashing induced) kook. Gelykfasige gedrag (vloei in kanale vertoon dieselfde gedrag) en ongelykfasige
gedrag (vloei in kanale vertoon teenstrydige gedrag) tussen langsliggende kanale is met al die
kragopwekkings en stelselwerkingsmodusse waargeneem. Vloei-omkering in die verhitte kanale van ’n
natuurlike sirkulasiestelsel is bewys om selfs onder gelyke kragopwekkings te bestaan.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/97140 |
| Date | 03 1900 |
| Creators | Sangweni, Lucy Sithombesethu |
| Contributors | Dobson, Robert Thomas, Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 125 pages : illustrations |
| Rights | Stellenbosch University |
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