Numerical investigation of intermittent dryout in diabatic vapor-water annular flow in an upward vertical tube

Dryout is a key limitation to heat transfer efficiency in high heat flux systems, and its triggering mechanism must be elucidated through the analysis of liquid film interfacial behavior in diabatic annular flow. This study examines vertical upward vapor–liquid diabatic annular flows of water at 7 MPa, with a mass flux of 750 kg/(m²·s), and an inlet vapor quality of 0.5, under three distinct heat flux conditions representative of operating conditions in once-through steam generators (OTSGs). The spatial–temporal distributions of liquid film thickness are examined, along with the axial variations in base film thickness and wave amplitude. The relationship between liquid film thickness distribution and wall temperature was explored, with particular attention to the correspondence between the timing of temperature rise and the liquid film thickness reaching a local minimum. The results indicate that the mass source is higher in the base film region, and the rupture of the base film leads to direct vapor-wall contact, causing localized vapor superheating. Higher heat flux results in an increased intermittent dryout fraction.