Experimental investigation on the flow instability of supercritical CO2 in vertical upward circular tube in trans-critical CO2 Rankine system

The trans-critical CO2 Rankine cycle is a promising low-grade waste heat recovery technology. The research about the heat transfer deterioration of supercritical CO2 in tube has been attracted more and more attentions recently, but there is not much attention that it produces the flow instability of supercritical CO2 under a certain range of working conditions as well in the trans-critical CO2 Rankine system. The present experimental work is aimed at the investigation of flow instability of supercritical CO2 in vertically upward flow tube. The experiments are conducted at the conditions of the inlet temperature from 10 to 20 °C, the inlet pressure from 7.5 to 8.5 MPa, the mass flux from 315 to 477 kg/m2·s and the heat flux from 59.95 to 90.05 kW/m2. The influences of the operating parameters on the heat transfer deterioration and the flow instability are discussed. For the upward flow, it is observed that the flow instability occurs with the continued increasing of the heat flux after the heat transfer deterioration. Moreover, the flow instability occurs in the transition from the heat transfer deterioration to the heat transfer recovery, and the flow remains stability after heat transfer recovery with the increasing of the heat flux, pressure, inlet temperature or mass flux. Compared with the upward flow, the downward flow is stable without any oscillation. It follows from analysis that the oscillation in the upward flow tube results from the buoyancy effect with the variation of heat transfer mode. The experimental results also show that the periodical oscillation of local temperature occurs within the region where the density variation is greatest. The growth and collapse of the fluid particle between the near-wall flow and the core flow makes the volumes of the fluid vary greatly, resulting in the occurrence of oscillation phenomenon.