Forced convective heat transfer of supercritical carbon dioxide in mini-channel under low mass fluxes

Abstract The comprehensive understanding of heat transfer characteristics to supercritical carbon dioxide in flowing channels is significant for the design and safe operation in the advanced supercritical carbon dioxide systems, especially for the heat transfer deterioration conditions that is generally occurs at relatively low mass fluxes but high heat fluxes. This paper experimentally studied the heat transfer behaviors of supercritical carbon dioxide flowing in a vertical heated mini-tube with an inner diameter of 2 mm. The experiment was conducted with pressure of 7.5–9 MPa, low (G   200 kg·m−2·s−1) and heat flux of 45–300 kW·m−2. The effects of operational parameters, buoyancy force and flow acceleration on convective heat transfer were discussed. The results showed that at low mass fluxes, no obvious heat transfer deterioration behaviors observed even though heat load (q/G) was high. However, under the similar q/G conditions with high mass fluxes, a remarkable heat transfer deterioration occurs. Under low mass fluxes, heat transfer enhanced by the strong buoyancy force and obvious acceleration effect, but at high mass fluxes, deteriorated heat transfer mainly effected by buoyancy rather than acceleration effect. Based on the experimental data and multiple effects of buoyancy and thermal acceleration, an improved correlation considering these effects at low mass fluxes was proposed, where more than 90% data in the within ± 20% error range.