The mechanism and onset boundary of flow instability for supercritical CO2 heated in vertical single-tube: Experimental study

An experimental system is built to study the thermal instability of supercritical CO2 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/(m(2)<middle dot>s), operating pressure P = 7.5-9 MPa, inlet temperature t(in) = 10-15 degrees 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 CO2 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 CO2 heated in the vertical single-tube.