Film splitting model with entrainment

• Alternate approach is used to evaluate Chun's model against the data.
• Entrainment of droplets into and out of film layer is the missing in Chun's model.
• Film split model with entrainment was developed.
• New film splitting model is compared against both annular and pipe data.
• Entrainment of droplets plays a significant role in a mechanistic dryout model.

Critical heat flux (CHF) is one of the major parameters for calculating safety margin of nuclear power plant systems based on US-Nuclear Regulatory Commission's safety regulations (defined under 10CFR50-46). Exceeding the CHF point suddenly increases the fuel clad temperature so that it may result in damage to the nuclear fuel pins. Dryout is a special CHF physical phenomenon occurred in Boiling Water Reactors (BWRs). Firstly, the present work identifies several existing dryout correlations and notes the major assumptions for the derivation of them. It follows with the derivation of a film splitting model (originally proposed by Chun et al., in 2003) for annular geometry where the liquid film is broken into an inner and outer film. Film splitting can result in better prediction of the critical film thickness where dryout can occur due to local variations especially due to turbulence and other effects. Secondly, an alternate approach to solving the film splitting model of Chun et al. is employed to evaluate the model against the same experimental data. It was observed that liquid entrainment and deposition into and out of the film layer were the missing components in the model and the model predictions shown in Chun et al. (2003) so that the model predictions with alternative solution methods were compared against experimental data by excluding entrainment/de-entrainment phenomena. Thirdly, the film split model is updated by adding entrainment/de-entrainment models to make the film splitting model more realistic. Fourthly, new film splitting model is compared against both tubular and pipe data. It was observed that entrainment and deposition are significant contributors in the new mechanistic dryout model. The mass flux of the inner film is mostly overestimated while the mass flux of the outer film is overestimated for some of the experimental cases when compared to the original correlation.