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CFD prediction of sodium flow boiling in a 2-D asymmetric heated pipe is carried out using RPI wall heat flux partitioning model available in the commercial CFD code FLUENT. The two phase models are validated with published pressurized water and R12 flow boiling experimental data. The influence of radial mesh density on validation parameter is investigated. Void fraction, wall temperature and liquid temperature at different axial section are taken as the validation parameter for water experiment. Among the various parameters predicted, the void fraction of water at the outlet gave the largest deviation of 35% with experimental data. But its value is in close comparison near the location of boiling initiation. Further the void fraction calculated based on heat balance compares well with the CFD results. Radial distributions of void fraction, vapour velocity, bubble diameter at the end of heated section are the parameters compared with R12 experimental data. Except bubble diameter, all other parameters gave reasonably good comparison. But when invoking breakup and coalescence model the predicted value of bubble diameter value approaches experimental data. Finally, sodium boiling simulations were carried out with the chosen models. It was found that, unlike water and R12, in the case of sodium, stable boiling was not achieved and boiling is highly localized to the end of heated section. Periodic growth and re-condensation of sodium vapour is found. Further, temporal oscillation in the heat transfer coefficient up to 8% was observed in the boiling zone. During sodium bubble expansion the evaporative heat flux dominates and once the bubble detaches, the quenching heat flux dominates. Unlike pressurised water evaporative and quenching heat flux have almost equal participation in boiling region.

CFD Simulation of Sodium Boiling in Heated Pipe using RPI Model