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An experimental system is built to study the thermal instability of supercritical CO 2 heated in vertical single-tube. The test section is heated uniformly by a DC power (0–6 kW) and the minimum Reynolds number in the test section inlet is about 8000 (it is much larger than 2300) for all cases. Under given working conditions (mass flow rate G = 240–400 kg/(m2·s), operating pressure P = 7.5–9 MPa, inlet temperature t in = 10–15 °C), the experimental results find that the working parameters generate oscillation instability once the heat flux surpasses a critical value. The maximum amplitude of the wall temperature arises in the zone of heat transfer deterioration. The mass flow rate and local wall temperature have large oscillation amplitude with more than ± 10 %. The results show that the maintenance of oscillation instability requires the existence of phase difference between inlet pressure and mass flow rate. The oscillation instability of supercritical CO 2 flowing is induced by the transition flow between turbulence and full re-laminarization, which is driven by the interaction among the expansion acceleration, buoyancy effect, inertia force, and friction force. An onset boundary of flow instability is proposed by connecting the buoyancy effect, friction force, and thermal acceleration based on hydrodynamic characteristics. The analysis results show that the buoyancy effect is the key factor for the occurrence of thermal oscillation for supercritical CO 2 heated in the vertical single-tube. • The maximum amplitude of wall temperature arises in heat transfer deterioration zone. • The thermal oscillation is caused by the change of flow state and heat transfer mode. • A criterion for judging the onset condition of the thermal oscillation is proposed. [ABSTRACT FROM AUTHOR]

• The emergence of deterioration and oscillation relates with the change of flow state. • Oscillation is driven by transition flow between partial and full re-laminarization. • The estimation criteria of HTD and oscillation are given with shearing-stress model. Trans-critical CO 2 Rankine cycle has great potential in converting low-grade heat into electricity, because of its good temperature glide matching between working medium and heat source. However, the drastic variation in thermo-physical properties of CO 2 around the pseudo-critical region causes the abnormal flow and heat transfer behaviors. In this paper, the heat transfer deterioration and thermal oscillation in the heat addition of trans-critical CO 2 cycle are investigated by experiments. The test section is heated uniformly by a DC power (0∼6 kW) and the minimum Reynolds number in the test section inlet is about 8000 (it is much larger than 2300) for the all cases. A shear stress reconstruction model is corrected by introducing the velocity profile of turbulence in the boundary layer to attempt to provide a quantitative criterion for deterioration and oscillation. The role of buoyancy and thermal acceleration in heat transfer deterioration and thermal oscillation is elucidated by connecting the shearing-stress reconstruction model and experimental results. It is found that, the thermal acceleration in the near-wall zone provides the key motivation to drive the heat transfer deterioration. The thermal oscillation is driven by the transition between partial and full re-laminarization flows in the local zone of heat transfer deterioration. When ( Δ τ a / τ w) < 1 & ( Δ τ / τ w) t o t > 1 or 0.1 < ( Δ τ / τ w) t o t < 1 , the turbulence is partially re-laminarized. When ( Δ τ a / τ w) > 1 & ( Δ τ a / τ w) > > ( Δ τ b o / τ w) , the intense thermal acceleration causes the turbulence to be fully re-laminarized in the near-wall zone and it maintains the new laminar boundary. The turbulence is partially re-laminarized in the lower boundary and it is almost full re-laminarization in the upper boundary of the oscillation wall temperatures. The results confirm that the emergence of heat transfer deterioration and oscillation instability in mixed turbulent convective relate with the change of flow states caused by acceleration and buoyancy effects. [ABSTRACT FROM AUTHOR]