Helical tube once-through steam generator is widely utilized in the design of advanced small reactors, forced circulation boiler and quench boiler. Flow instability is a critical thermal-hydraulic issue, but lack of comprehensive research on this phenomenon in parallel helical tubes.

This study aims to conduct experiments on a helical tubes thermal-hydraulic apparatus.

The effects of thermal parameters were investigated at the pressure range of 7~13 MPa, mass flow rate ranger of 100~550 kg?m^-2?s^-1 and helical coil diameter range of 370~800 mm. The flow instability in parallel helical tube channels with three different helical diameters was experimentally studied, and the effects of mass flow rate, pressure, inlet subcooling and helical diameter on the flow instability were analyzed. Based on the test data, the prediction relationship of system stability boundary was fitted. Finally, the prediction effect of straight tube heat transfer and resistance model on the stable boundary of parallel spiral tube channel was analyzed.

Experimental results show that high system pressure and high inlet flow rate contribute significantly to more stable system whilst inlet subcooling has a minimal impact on the critical load at conditions with low pressure, low flow rate and low inlet subcooling. The system stability boundary exhibits an "S" shape under high system pressure but transitions to an "L" shape under low system pressure, which is associated with the multivalued influence of inlet subcooling on system stability. It is found that the critical heat load predicted by the straight tube model is 31.1%~69.9% higher than the experimental value.

This study fills the gap in experimental data on flow instability in parallel helical tubes, providing a solid data foundation for in-depth analysis of flow instability phenomena in the helical tube once-through steam generator.