The Numerical Study of Heat Transfer and Multiphase Flows in Sub-Atmospheric Industrial Evaporators

The Sellafield site in Cumbria, UK maintains three steam heated evaporators which operate at sub-atmospheric pressure. They are used to evaporate and thus concentrate nitric acid based highly active liquor, which is a by-product arising from reprocessing spent nuclear fuels. Corrosion occurs on the internal surfaces of the evaporators in contact with the liquors. The rates of corrosion are a function of the local surface temperatures and heat transfer rates. Therefore accurate heat transfer predictions inside the evaporators is highly desirable as it enables good predictions to the rates of corrosion. The aim of this study was to use engineering calculations and computational fluid dynamics (CFD) to provide predictions of the continuity, momentum and energy transfer occurring inside evaporators operating at sub-atmospheric pressure. Three evaporator scales were chosen for this study: (i) two small unscaled cylindrical test rigs which were 0.1 m diameter, and had liquid fill depths of 0.1 m and 2.215 m respectively; (ii) a test rig representing a quarter scale slice of an industrial evaporator, and had a liquid fill depth of 0.8m; (iii) and lastly Sellafield Evaporator C, which represented a full scale industrial evaporator design, and had a fill depth of 2.35 m. Thermal resistance investigations were performed on the unscaled cylindrical test rigs which proved conduction heat transfer through the walls removed all sensitivity to the specified boundary conditions. Single phase CFD simulations were also performed on the test rigs which showed a symmetrical geometry assumption could not be used to simplify the modelling approach. Two phase Eulerian-Eulerian CFD simulations were performed on the one quarter scaled test rig. A custom length scale for use in the interfacial area density was developed and used. The length was a function of a user prescribed rate constant. In the simulations evaporation at the free surface was modelled. Published experimental data was used to validate the simulations, and showed that the length scale required a rate constant of 1 Hz to simulate evaporating flows at sub-atmospheric pressure. The CFD models which were developed were applied to the simulations of the full scale industrial evaporator design, Sellafield Evaporator C. The results shows the evaporator behaved similar to an unconstrained thermosyphon reboiler with distinct counter rotating convection cells. Indications of nucleate boiling was not present on all heat transfer surfaces as previously thought, which meant surface corrosion rates may be lower than anticipated.