Prediction model for the initial dryout vapor quality in helically coiled tubes

Dryout phenomena in helical-coileded steam generator tubes represent a critical limiting factor in thermal performance and operational safety. In this study, the initial dryout is examined through a combined experimental and theoretical approach, with particular focus on the influence of thermohydraulic parameters and vapor-liquid two-phase interactions on the critical vapor quality. Results indicate that dryout vapor quality is strongly correlated with the distribution of the annular liquid film, the intensity of secondary flows, and interfacial shear forces. Parametric sensitivity analyses reveal that increasing the inlet mass flux promotes liquid film redistribution under centrifugal effects and enhances secondary flow strength, thereby reducing the vapor quality at dryout. Conversely, higher pressure conditions suppress vapor expansion and void fraction, resulting in delayed dryout at elevated vapor qualities. The interfacial shear force, quantified using the slip ratio, exhibits a monotonic inverse relationship with dryout vapor quality, underscoring the role of secondary vapor flows in accelerating liquid film collapse. To further characterize the interaction between vapor-phase secondary motion and wall-adhering liquid films, a Centrifugal Punch Shear Interface (CPSI) analysis is proposed. Based on the analysis of interfacial interaction behavior, dimensionless parameters are introduced to quantify the effects of liquid film distribution, secondary flow patterns, and interfacial shear intensity on dryout. A predictive model for the critical vapor quality at initial dryout is developed based on these factors, achieving agreement with experimental data within a 15% deviation.