Transient flow, flow pattern characterisation and performance evaluation of a two-phase natural circulation heat transfer loop.

Senda, Paul Tudieji (2018-03)

Thesis (MEng)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: This thesis project focusses on the experimental and theoretical analysis of the transient behaviour of a single and two-phase flow natural circulation heat transfer loop using water as the working fluid. The background theory provided by a literature survey shows that natural thermosyphon loops offer attractive solutions for passive cooling and heat transfer, and safety systems; especially in the nuclear and process industries. In particular the thesis considers a passive reactor cavity cooling system (RCCS) for next generation high temperature nuclear reactors such as “the pebble bed modular reactor (PBMR)”. A time-dependent mathematical computer simulation program of a 7 m high by 8 m wide, 32 mm inside diameter vertically orientated rectangular natural circulation loop was developed. The major assumption was that the flow is quasi-static; implying that although the mass flow rate might be changing with time, that at any instant in time that the mass flow rate is constant at axial position along the loop. To theoretically simulate the loop the working fluid was discretised into a series of one dimensional control volumes. By applying the conservation of mass, momentum and energy and suitable property functions to each control volume as series of time dependent partial differential equations were generated and then solved using an explicit finite difference method. The one vertical side of the loop was heated by a series of electrical heating elements and the other vertical side cooled using a series of water-cooling jackets. Transparent pipe lengths were inserted in the loop to observe the two-phase flow patterns. The loop was supplied with an expansion tank and operated in single and two-phase modes with water as the working fluid at a mass flow rate determined using an orifice-plate flow meter. A reasonable correlation between the experimental and theoretical simulations was found using a separated two-phase flow with frictional multipliers and vapour-liquid void fraction correlations, similar to those originally suggested by Martinelli. It was concluded that the as developed theoretical model adequately captured the actual transient and dynamic flow and heat transfer behaviour of the loop. It is thus recommended that a series of such loops could be used with confidence for a RCCS.

AFRIKAANSE OPSOMMING: Hierdie studie het gefokus op die eksperimentele en teoretiese ontleding van die oorgangsgedrag van ʼn natuurlikesirkulasie-hitteoordragslus in enkel en tweefasevloei deur water as die werkvloeistof te gebruik. Die agtergrondse teorie wat deur die literatuuroorsig verskaf word, toon dat natuurlike termosifonlusse aanloklike oplossings vir passiewe verkoeling en hitteoordrag asook veiligheidstelsels bied, veral in die kern- en prosesbedrywe. Die studie het spesifiek ʼn passiewe reaktorholte-verkoelingstelsel (RHVS) ondersoek vir volgendegenerasie-hoëtemperatuur-kernreaktors soos die spoelkliplaag- modulêre reaktor. ʼn Tydafhanklike wiskundige rekenaarsimulasieprogram van ʼn vertikaal georiënteerde reghoekige natuurlike sirkulasielus, 7 m hoog, 8 m breed, met ʼn binnedeursnee van 32 mm, is ontwikkel. Die vernaamste aanname was dat die vloei kwasi-staties is, wat impliseer dat alhoewel die massavloeikoers wel moontlik met verloop van tyd verander, dit by ’n aksiale posisie in die lus in enige tydsoomblik konstant sal wees. Ten einde die lus teoreties te simuleer, is die werkvloeistof in ʼn reeks eendimensionele beheervolumes verdeel. Deur die behoud van massa, momentum en energie en geskikte eienskapsfunksies op elke beheervolume toe te pas, is ʼn reeks tyd-afhanklike gedeeltelike differensiaalvergelykings gegenereer en met behulp van ʼn eksplisiete eindigeverskil-metode opgelos. Die een vertikale kant van die lus is deur ʼn reeks elektriese verhittingselemente verhit en die ander kant met ʼn reeks waterverkoelende mantels verkoel. Deursigtige pypstukke is in die lus geplaas om die tweefase-vloeipatrone waar te neem. Die lus is met ʼn uitsittenk toegerus. Die massavloeikoers is met behulp van ʼn gaatjiesplaatvloeimeter bepaal. Die lus is in enkelfase- en tweefasebedryfsmodusse in werking gestel. ʼn Redelike korrelasie tussen die eksperimentele en teoretiese simulasies is gevind met behulp van ʼn geskeide tweefasevloei met wrywingvermenigvuldigers en dampvloeistof-leegtebreukkorrelasies, soortgelyk aan dié wat oorspronklik deur Martinelli voorgestel is. Die gevolgtrekking was dat die ontwikkelde teoretiese model die werklike oorgangs- en dinamiese vloei en hitteoordrag-gedrag van die lus genoegsaam vasgelê het. Die aanbeveling word dus gemaak dat ʼn reeks van hierdie lusse met gerustheid vir ʼn RHVS gebruik kan word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103807
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