Transient modelling of a loop thermosyphon : transient effects in single and two phase natural circulation thermosyphon loops suitable for the reactor cavity cooling of a pebble bed modular reactor

[摘要] ENGLISH ABSTRACT: The focus of this project was the application of a passive device in the form of a loopthermosyphon as a reactor cavity cooling system (RCCS) for a Pebble Bed ModularReactor. An extensive literature review showed that loop thermosyphons have beenwidely researched, both theoretically and experimentally. In the review attention hasspecifically been given to matters such as safety, instability, control and mathematicalmodelling.One of the objectives of the project was to build one of the axially symmetric sectionsof Dobson's (2006) proposed full scale RCCS using a scaled down version consistingof a single loop heated by a section of the reactor pressure vessel and cooled by a tankof water. The second objective was to derive a theoretical model that could be used in acomputer code to simulate the experiment. The theory and experiment would then becompared in order to verify the code.The mathematical model created used the following three major assumptions: quasistaticflow, incompressible liquid and vapour and one dimensionality. The conservationequations in the form of a set of difference equations with the appropriate closureequations were then solved explicitly. It was found that the theoretical results wereheavily influenced by the surface optical properties as well as the heat transfercoefficients. The emissivity influenced the transition point from single to two-phaseflow as well as the condenser outlet temperature. The single phase heat transfercoefficients influenced the condenser outlet temperature significantly while it wasfound that for two phase flow the combination of the available boiling and condensationheat transfer coefficients had only minor effects on the end results.A stainless steel and aluminium thermosyphon loop was built using water as theworking fluid. A stainless steel heater plate provided the heat input while a 200 L watertank was the heat sink. Temperature and flow rate measurements were recorded as afunction of time with various heating/cooling transients from start-up to steady state forthree operating modes. The three operating modes were single phase, two-phase andheat pipe mode. It was found that the theoretical temperatures correspond reasonably well with theexperimental temperatures. The time predicted by the theoretical model to reach theoperating temperature was however somewhat longer than for the experimental. This isto be expected when considering that there is some uncertainty pertaining to the heattransfer coefficients as well as surface emissive properties. The correspondence of thetheoretical and experimental fin temperatures was poor due to significant thermalstratification of the air separating the heater plate and fins. Several shortcomings in thetheoretical model as well as the experimental setup were identified and discussed.The conclusion was reached that this exploratory study showed that the loopthermosyphon is a viable option for the RCCS and that the mathematical model is aviable theoretical simulation tool. Several recommendations were made for furtherstudy to address and overcome the shortcomings identified in the theoretical andexperimental models in order to prove this conclusion. Amongst these is thedetermination of better material surface properties and heat transfer coefficients andimproved mass flow rate measurement. Investigating scaling issues, natural convectionoutside the loop and updating of the computer program is also recommended.

[发布机构] Stellenbosch University