Experimental investigation on influence of inlet subcooling to “S-shaped” density wave oscillation in parallel helical channels

Abstract Helical coil once-through steam generators, owing to their advantages of compact structure and high thermal efficiency, are applied in fourth-generation nuclear reactors. During the process where the subcooled water on the secondary side of the OTSG is heated into superheated steam, density wave oscillation is prone to occur. An experimental study was conducted on full-scale double-parallel helical tubes. The influence law of inlet subcooling on the DWO boundary was obtained, which is nonlinear and non-monotonic. As the inlet subcooling increases, the critical heat flux first increases, then decreases, and increases again, with two inflection points. It generally presents an “S-shaped” law in the dimensionless boundary map. Increases in the inlet throttling coefficient, system pressure, and inlet flow rate enhance system stability and do not affect the occurrence of the “S-shaped” law. The “S-shaped” stability boundary is affected by the heat load applied to the gas-liquid two-phase section. The oscillation period variation law and pressure drop distribution were described. At relatively low inlet quality (less than −50 %), the ratio of oscillation period to mixed transport time (Ω/ τ ) is less than 0.5; as the inlet quality increases, Ω/ τ gradually increases; when the inlet quality is −3.61 %, the ratio of fluctuation period to mixed transport time is greater than 2. The ratio of the pressure drop in the gas-liquid two-phase section to that in the subcooled fluid section exhibits a fixed relationship similar to an inverse proportion. This study provides an important scientific basis for the safe operation of new-type nuclear energy systems. Graphical abstract Highlights • The experimental investigation focused on a full-scale dual parallel helical tube. • The influence of inlet subcooling on DWO generally shows an “S-shaped” law. • Inlet throttling coefficient, system pressure, and inlet flow rate enhance stability. • The “S-shaped” law is affected by the heat load applied to the two-phase section. • As the inlet quality increases, Ω/ τ gradually increases.