The characteristics and mechanisms of self-excited oscillation pulsating flow on heat transfer deterioration of supercritical CO2 heated in vertical upward tube

Heat transfer deterioration (HTD) of supercritical CO2 heated in a tube influences the efficiency and safe operation of the system due to the occurrence of local high temperature. To suppress and delay the HTD, the characteristics and mechanisms of self-excited oscillation pulsating flow on HTD of supercritical CO2 are studied by experiment and simulation at pressure 8 MPa, mass fluxes from 350 to 800 kg/m2.s, heat fluxes from 30 to 200 kW/m2. The Helmholtz oscillator is introduced into the inlet of the vertical tube for generating a pulsating flow of supercritical CO2. The heat transfer performance is compared with that of without Helmholtz oscillator at the inlet of the tube. The results show that the self-excited oscillation pulsating flow improve the heat transfer performance significantly. The heat transfer parameters present oscillations with small amplitude along flow direction before pseudo-critical point (Tpc). The average heat transfer coefficient can be up to 3.4 times and the enhancement takes place mainly at the HTD region which is located at the entrance section of the heated tube. The effect of suppressing HTD is more significant at higher heat flux, and the peak of wall temperature can be reduced by 100 K at a heat flux of 200 kW/m2. Compared to the steady flow, the mechanism analysis of the selfexcited oscillation pulsating flow based on radial distributions of velocity, turbulent kinetic energy (TKE), and density reveals that the velocity distribution of "M- shape" appears later and gentler. The production and diffusion of TKE are improved at log layer (30 < y+ < 0.2r). In addition, the period and the amplitude do not show monotonous trends on heat transfer performance. The effects of pulsating parameters on HTD are optimized that the heat transfer performance with the period of 0.016 s and the amplitude of 100 kg/m2.s is the best in calculated cases.