This paper reports on the advancement in the study of thermal-hydraulic dynamic instabilities with reference to the helical-coiled tube geometry. A full-scale open-loop experimental facility simulating a helically coiled steam generator was built and operated at SIET labs in Piacenza (Italy). The facility comprises two helical tubes (1 m coil diameter, 32 m length, 8 m height), connected via lower and upper headers. Nearly 100 flow instability threshold conditions were identified, in a test matrix of pressures (80 bar, 40 bar, 20 bar), mass fluxes (600 kg/m2 s, 400 kg/m2 s, 200 kg/m2 s), inlet subcooling (from −30% up to ∼0), and inlet throttling (four different entrance resistance conditions). The long test section feature and the helical-...