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Numerical simulation of progressive inlet orifices in boiling water reactor fuel
2004 (English)Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

This thesis was carried out at Forsmark Nuclear Power Plant. The power plant in Forsmark consists of three boiling water reactors (BWR) which produce about 17% of Swedish electricity. In a BWR the nuclear reactions are used to boil water inside the reactor vessel. The water works both as a coolant and as a moderator and the resulting steam is used directly to run the turbines. A problem when running a BWR at low flow conditions is the density wave oscillations that might occur to the water flow inside the fuel assemblies. These oscillations arise due to the connection between power and flow rate in a heated channel with two-phase flow. In order to improve the stability performance of the channel an orifice plate is placed at the inlet of each fuel assembly. Today these orifice plates have sharp edges and a constant resistance coefficient. Experimental work has been done with progressive orifices, the edge of which is half-oval in shape. The advantage of progressive orifices is the lower pressure losses with an increase of the Reynolds number, a similar phenomenon that appears in external flow around curved bodies. Since there are high costs associated with experimental generation of high- temperature and high-pressure data, it is of some interest to be able to reproduce and generate data using Computational Fluid Dynamics (CFD). This work deals with the possibility to use the CFD-code Fluent to do numerical simulations of the flow through progressive orifices. The following conclusions may be drawn from the numerical results: • All simulations using Reynolds-Averaged Navier-Stokes (RANS) turbulence models, two-dimensional and three-dimensional, capture an abrupt decrease of the resistance coefficient at higher Reynolds numbers. • Two-equation models seem to under-predict the critical Reynolds number. • The five-equation Reynolds Stress Model (RSM) gives a critical Reynolds number of the same order of magnitude of that measured in experiments. • No major differences have been observed between resistance coefficients obtained by two- and three-dimensional simulations. However, the grid and the wall treatment of the three-dimensional simulations had a lower accuracy than that of the two-dimensional simulations. Thus, CFD simulations may constitute a viable alternative to high-pressure, high-temperature experimental data if the RSM is used as turbulence model and care is taken to generate grids of god quality. However, the simulations should be validated by new experiments. Besides, detailed measurements of the boundary layers should be included in order to resolve the physical mechanism behind the drop in pressure losses.

Place, publisher, year, edition, pages
Keyword [en]
Technology, fluid mechanics, boiling water reactors, CFD, progressive orifices
Keyword [sv]
URN: urn:nbn:se:ltu:diva-52186ISRN: LTU-EX--04/267--SELocal ID: 952dbf29-516a-454b-8672-97490ac5576bOAI: diva2:1025555
Subject / course
Student thesis, at least 30 credits
Educational program
Engineering Physics, master's level
Validerat; 20101217 (root)Available from: 2016-10-04 Created: 2016-10-04Bibliographically approved

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