Theoretical and experimental evaluation of a high temperature reactor (HTR) cavity cooling system (RCCS).

Swart, Rikus (2018-12)

Thesis (MEng)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: The main focus of this project was an investigation into a full-scale, 27 m high, 6 m wide thermosyphon loop, which can be used as a fully passive high temperature reactor (HTR) cavity cooling system (RCCS). Thermosyphon loops are closed thermodynamic systems, in which the working fluid inside the loop is driven by a temperature induced density gradient. This density gradient causes the working fluid to be circulated naturally. The literature study that was conducted showed that extensive theoretical and experimental research has been done on thermosyphons. The literature study focused on understanding the safety, instabilities, control and mathematical modelling of these systems. A 27 m high, 6 m wide water-filled thermosyphon loop was recommissioned. The heat input was simulated with 25 heating elements, which were evenly spaced and positioned on the left-hand side vertical pipe. The heat removal system relied on counter-current heat exchangers on the right-hand vertical and top horizontal pipe of the system. The thermosyphon loop was open to the atmosphere by means of an expansion tank connected at the bottom of the loop and positioned 30 m in the air. The expansion tank ensured that the working fluid did not experience any pressure buildup, and ensured that it remained at a constant pressure. Three transparent sections were inserted into the system to observe the working fluid flow regime inside the loop. These sections were positioned above the heat input section, after the horizontal condenser section and before the vertical condenser section. The recommissioned thermosyphon was operated under different operating conditions. The different operating conditions were repeated, and they were observed to deliver almost the same result; thus showing that the experiments were repeatable. The flow pattern behaviours were established for the flow patterns observed in the transparent sections of the loop. A time-dependent mathematical simulation thermal-hydraulic model of the thermosyphon loop was developed. The simulation model is based on a one-dimensional axially symmetrical control volume approach, where the loop is divided into a series of discreet control volumes. The three conservation equations, namely, mass, momentum and energy, were applied to these control volumes and solved with an explicit numerical method. The following main assumptions were made: The flow is quasi-static, implying that the mass flow rate changes over time, but at any instant in time, the mass flow rate is constant around the loop; and that the expansion tank does not have an effect on the system. It was found that the Lockhart-Martinelli void fraction and Friedel frictional multiplier, compared to a number of correlations, predicted the separated two-phase flow regime of the working fluid the most accurately. The temperatures and mass flow rate of the theoretical model corresponded reasonably well with the experimental results. The conclusion was reached that the exploratory study on thermosyphon loops is a viable option for an high temperature reactor (HTR) cavity cooling system (RCCS), and that a series of loops could be used. The theoretical simulation model is a viable simulation tool for predicting the working fluid temperatures and flow regimes of this system. Several recommendations are made regarding the theoretical model and the experimental setup. The most important recommendation is to reconstruct the thermosyphon loop in a more controlled environment (indoors) to increase the accuracy of the theoretical simulation.

AFRIKAANSE OPSOMMING: Die hooffokus van hierdie projek was die ondersoek van 'n volskaalse 27 m hoë, 6 m wye termoheuwellus, wat gebruik kan word as 'n volledig-passiewe reaktorholte-verkoelingsisteem (RCCS). Termoheuwellusse is 'n geslote termodinamiese stelsel waarin die werkvloeistof in die lus gedryf word deur 'n temperatuurverskil digtheidsgradiënt wat veroorsaak dat die werkvloeistof natuurlik gesirkuleer word. 'n Literatuurstudie is uitgevoer en het getoon dat uitgebreide navorsing oor termosifone teoreties en eksperimenteel gedoen is. Die literatuur het gefokus op die begrip van veiligheid, onstabiliteit, beheer en die wiskundige modellering van hierdie stelsels. 'n 27 m hoë, 6 m wye watergevulde termoheuwellus is weer in gebruik geneem. Die hitte-insette is gesimuleer met 25 verwarmingselemente, wat eweredig gespasieer en geplaas is aan die linkerkantse vertikale pyp. Die hitteverwyderingstelsel is deur teenstroom warmtewisselaars aan die regterkantse vertikale en boonste horisontale pyp van die stelsel gedoen. Die termoheuwellus was bloot gestel aan die atmosfeer deur middel van 'n uitsettingstenk wat onderaan die lus verbind is en 30 m in die lug geplaas was. Die uitsettingsstenk het verseker dat die werkende vloeistof nie druk opgebou het nie en dat dit teen konstante druk bly. Drie deursigtige afdelings is in die stelsel geplaas om die werkvloeistroomregime binne die lus waar te neem. Hulle is bo die hitte-insetgedeelte geplaas, na die horisontale waterkoeler en voor die vertikale waterkoeler afdeling. Die her inwerkstelling termoheuwellus is onder verskillende bedryfstoestande bedryf. Die verskillende bedryfsomstandighede is herhaal en daar is waargeneem dat byna dieselfde resultaat gelewer is, en sodoende bewys dat die eksperimente herhaalbaar was. Die vloeipatrone se gedrag is vasgestel soos waargeneem in die deursigtige dele van die lus. 'n Tydafhanklike wiskundige simulasie termiese hidrouliese model van die termohuewellus is ontwikkel. Die simulasiemodel is gebaseer op 'n beheerde eendimensionele simmetriese volumebenadering, waar die lus in 'n reeks diskrete beheervolumes verdeel word. Die drie bewaringsvergelykings, massa, momentum en energie is toegepas op hierdie beheervolumes en opgelos met 'n eksplisiete numeriese metode.Die volgende hoofaannames is gemaak: die vloei is kwasi-staties, wat impliseer dat die massa vloei-tempo oor tyd verander, maar op enige tydstip, die massastroming is konstant om die lus en dat die uitbreidingstenk nie 'n effek op die stelsel het nie. Daar is bevind dat die Lockhart-Martinelli leemte fraksie en Friedel wrywingsvermenigvuldiging koëffisiënt, in vergelyking met 'n aantal korrelasies, die gesproke tweefase regime van die werkvloeistof die akkuraatste voorspel het. Die temperatuur en massa vloei van die teoretiese model het redelik goed met die eksperimentele resultate ooreen gestem. Die gevolgtrekking is gemaak dat die verkennende studie oor termoheuwellusse 'n lewensvatbare opsie is vir 'n reaktorholte-verkoelingstelsel (RCCS) en dat 'n reeks lusse gebruik kan word. Die teoretiese simulasiemodel is 'n lewensvatbare simuleringsinstrument vir die voorspelling van die werkvloeistowwe en vloeistelsels van hierdie sisteem. Verskeie aanbevelings is gemaak aangaande die teoretiese model en die eksperimentele opstelling. Die belangrikste aanbeveling is om die termoheuwellus in 'n meer beheerde omgewing (binnenshuis) te rekonstrueer om die akkuraatheid van die teoretiese simulasie te verhoog.

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