Experimental and numerical modelling investigations of the response of a two-phase natural circulation multi-parallel channel system

Sangweni, Lucy Sithombesethu (2015-03)

Thesis (MEng)--Stellenbosch University, 2015.

Thesis

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.

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