Numerical investigation of thermohydraulic performance in supercritical CO2- based dual natural circulation systems

• Thermohydraulic performance of supercritical CO 2 -based dual natural circulation loops was investigated. • The effect of five supercritical pressures (8 MPa-12 MPa) and two constant heat fluxes (1000 W and 2000 W) on three cooler orientations was studied. • Fully horizontal coolers exhibited the best performance, whereas mixed orientation demonstrated intermediate behaviour. • Fully vertical coolers showed strong buoyancy at low pressure but deteriorated under higher pressure loads. • Velocity contours revealed suppressed middle leg circulation in the horizontal cooler configuration. The present study examines how cooler orientation influences the thermohydraulic behaviour of a hydraulically coupled sCO₂ dual natural-circulation loop, a configuration vital for passive heat removal in advanced reactors. Steady-state numerical simulations were performed at two heating powers (1000 W and 2000 W) and pressures ranging from 8–12 MPa. Three dual-loop configurations were analysed: CASE I (vertical coolers), CASE II (horizontal coolers), and CASE III (a mixed vertical–horizontal arrangement). The heater is located in the left vertical leg for all cases. The novelty of this work lies in analysing hydraulically coupled sCO₂ natural circulation system, which has not been previously explored for different cooler orientations . At 1000 W, CASE II shows a pronounced increase in heater-side heat transfer coefficient up to 10 MPa (≈158% growth from 9–10 MPa) before stabilizing, while CASE I and CASE III show gradual enhancement peaking near 11 MPa. At 2000 W, the thermal performance in all configurations becomes linear and stable with pressure, eliminating the sharp sensitivity observed at lower powers. Mass flow rate analysis reveals reductions for CASE I and CASE III with increased power, particularly beyond 10 MPa, whereas CASE II uniquely exhibits recovery and growth (7.7% at 11 MPa, 16.6% at 12 MPa). These results identify CASE II as the most resilient configuration under elevated thermal and pressure loads and provide new quantitative insight into configuration-driven thermohydraulic behaviour, demonstrating that hydraulically coupled sCO₂ systems can improve passive safety and heat-removal capability in advanced reactor designs.