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Critical heat flux (CHF) is one of the most important thermal criteria for nuclear reactor design. To obtain a more widely applicable CHF prediction method, many mechanistic models have been proposed based on single channels with a good precision. Since the annular flow may occur in rod bundle in gas-liquid two-phase region for boiling water reactors (BWRs), it is essential to apply the annular film dryout (AFD) mechanistic models to rod bundle geometry. Among many existing AFD mechanistic models that are available to review, two major categories can be distinguished with considerable attention: the three-fluid model and the film analysis model. Many studies show that it is feasible to couple AFD mechanistic model with sub-channel analysis using suitable coupling strategies, but some crucial issues have not been solved. In view of these facts, this paper initially highlights the basis of AFD mechanistic model and its application to rod bundle. Then some factors which have predominant influence on the accuracy of the mechanistic model are reviewed and analyzed, including cross flow, onset of annular flow, inception fraction for droplet entrainment, boiling and spacer grid effect, dryout criteria. Finally, conclusions and prospects in this research field are summarized and proposed. • This study carried out a comprehensive review on annular film dryout (AFD) mechanistic model. • A detailed discussion was applied on coupling the AFD mechanistic model with subchannel analysis for rod bundle geometry. • Some critical factors were presented in this study and subsequent research direction is evaluated. [ABSTRACT FROM AUTHOR]

• A three-field mechanistic model is developed for dryout prediction in annular flow. • The model is verified by experimental data of the circular and rectangular channel with relatively large parameter range. • Some key constitutive relations in the model are discussed for the better prediction. • The model is effective on prediction of dryout in the rod bundles by coupling with subchannel code. Critical heat flux (CHF) is one of the most important thermal criteria for nuclear power plants. It has traditionally been evaluated using look-up tables or empirical correlations. In this paper an annular film dryout (AFD) mechanistic model has been developed based on the interaction of three fields in annular flow region, i.e. the liquid film, entrained droplets and vapor core. The model describes the mass and momentum conservation equations of three fields together with a series of constitutive relations. The effect of some constitutive correlations (the entrainment and deposition of droplets and the onset of annular flow) on the prediction accuracy of the model is studied. The results are compared with CHF experimental data in flow boiling. Fairly good agreements are observed for the CHF in circular tubes with uniform and axially non-uniform heating, as well as rectangular channels with uniform heating. Through coupling with the subchannel analysis method, the model is used to predict the dryout-type CHF in the rod bundles with a good precision. [ABSTRACT FROM AUTHOR]

The helium-xenon(He–Xe) gas mixture with low Prandtl number and good heat transfer characteristics suits for the advanced high heat flux and compact gas-cooled reactors. A numerical simulation was conducted on the flow, heat transfer, and turbulent mixing characteristics of He–Xe gas mixture in a tightly arranged triangular rod bundle subchannel. The transition characteristics of He–Xe gas mixture in rod bundle structure were obtained. The friction resistance coefficient, Nusselt number(Nu), and turbulent mixing factor β was corrected within the Reynolds number(Re) range of 3 × 102 - 1.8 × 105. The research results indicate that the transition occurs at Re equal to 2000 and forms a transition zone within the Re range of 2000–5000; The Blasius correlation can describe the wholly turbulent flow precisely, but it is suitable to adopt a new formula in the transition zone; The local Nu prediction performance of the modified Peckett correlation is in good agreement with the numerical simulation results, and the Petukhov correlation with shape correction and temperature correction factors has good prediction results on global Nu ; The turbulent mixing factor using Re and shape parameters can better describe the mixing characteristics. This study has reference value for the design of rod bundle reactors using He–Xe gas mixtures as working fluids. [ABSTRACT FROM AUTHOR]