Fig. 1. Schematic diagram of experimental facility

Fig. 2. Thermocouples distribution at the same temperature measuring cross-section

Fig. 3. Influence of system pressure on flow instability in parallel helical tube (color online)(a) G=300 kg∙m^-2∙s^-1, (b) G=500 kg∙m^-2∙s^-1

Fig. 4. The influence of system pressure on critical heat load of parallel helical tube (color online)(a) G=300 kg∙m^-2∙s^-1, (b) G=500 kg∙m^-2∙s^-1

Fig. 5. The influence of inlet mass flow rate on critical heat load of parallel helical tube (color online) (a) P=7 MPa, (b) P=11 MPa, 13 MPa

Fig. 6. The influences of coiled diameter on the stable boundary of the system (color online) (a) P=11 MPa, G=300 kg∙m^-2∙s^-1, (b) P=11 MPa, G=500 kg∙m^-2∙s^-1, (c) P=7 MPa, G=300 kg∙m^-2∙s^-1, (d) P=7 MPa, G=500 kg∙m^-2∙s^-1

Fig. 7. The influences of subcooling degree on gas-liquid length ratio and stable boundary (color online) (a) P=7 MPa, G=500 kg∙m^-2∙s^-1, (b) P=11 MPa, G=500 kg∙m^-2∙s^-1, (c) P=11 MPa, G=300 kg∙m^-2∙s^-1

Fig. 8. The oscillation periods under different system pressure conditions (color online)

Fig. 9. The variation of T/τ with air voids x_in under different system pressure conditions (color online)

Fig. 10. The variation of oscillation periods of HT-1 and HT-3 with air voids at different inlet quality (color online) (a) P=7 MPa, (b) P=11 MPa

Fig. 11. The variation of T/τ of HT-1 and HT-3 with air voids at different inlet quality (color online)(a) P=7 MPa, (b) P=11 MPa

Fig. 12. Schematic diagram of node visualization

Fig. 13. Comparation of measured values in system boundary experiment with predicted value of straight pipe model (color online)(a) P=11 MPa, G=300 kg∙m^-2∙s^-1, (b) P=11 MPa, G=500 kg∙m^-2∙s^-1, (c) P=7 MPa, G=300 kg∙m^-2∙s^-1, (d) P=7 MPa, G=500 kg∙m^-2∙s^-1