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Title: Phase change of molten-salt flows in energy systems
Author: Le Brun, Niccolo
ISNI:       0000 0004 7228 9333
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
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The possibility of molten salt freezing in pipe flow systems is a key concern for the solar-energy industry and a safety issue in the Generation-IV molten-salt reactors, worthy of careful consideration. The overriding aim of this thesis is to address this issue by providing an approach to quantify the solidification of molten salts in piping systems (in terms of mass build-up, effect on the flow and heat transfer, etc.). In light of this aim, several aspects needed to be investigated which affect how molten salt solidification can be predicted. Specifically, the work described in this thesis is hereby described: 1) An experimental method was developed to measure the thermal conductivity of molten salts, whose uncertainties significantly affect further modelling efforts. The method can be applied to measure the thermal conductivity of molten salts up to temperatures around 760 K, with an overall error better than 4%. 2) The thermal conductivities of NaNO3 -NaNO2 -KNO3 eutectic (HTS) and LiCl -KCl eutectic were measured up to temperatures of 700 K and 760 K respectively. In addition, data in the literature were re-evaluated by taking into account the thermal losses present in a particular experimental apparatus; the revised results were found to be in good agreement with other studies. These re-evaluated data and the measurements conducted in the present study were used to critically review and suggest the values of the thermal conductivities of common salts, including FLiNaK. 3) A 1-dimensional thermo-hydraulic model was developed under the steady state assumption and validated to predict transient freezing in internal pipe flows. The model can be incorporated in standard thermo-hydraulic codes and can be used to predict the solidi cation process in complex piping system where CFD is computationally expensive. 4) An experimental apparatus equipped with laser-based diagnostic measurement techniques was built to measure the growing thickness of an ice layer in contact with a cold surface and liquid water flow. The developed freezing model was validated against these experimental data and the discrepancies were considered. 5) The freezing model was then applied to study the behaviour of the Direct Reactor Auxiliary Cooling System (DRACS) under Loss of Forced Circulation (LOFC) with blackout. DRACS was found to be prone to failure due to freezing in the molten salt/air heat exchanger; its transient response was characterised and discussed.
Supervisor: Markides, Christos Sponsor: Imperial College London
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral