Studies on the flow acceleration and buoyancy effects on the heat transfer to supercritical CO₂ flowing through mini channel

In this paper, numerical simulations are performed using several low-Reynolds number k-ε turbulence models due to AKN, MK and V2F for the convection heat transfer of supercritical CO₂ flowing through a heated vertical mini tube with inner diameter of 0.27 mm for various heat fluxes at relatively low Reynolds number of 2900. The predictions are compared with the corresponding experimentally measured values. The performance of various low Reynolds number k-ε turbulence models under normal and deteriorated heat transfer conditions are assessed. Results show that all the three models produce fairly good predictions of local wall temperature variations under normal heat transfer condition. The MK and V2F models give good predictions when the flow acceleration is not very strong. However, they respond insufficiently to the flow acceleration effect when it gets stronger. The V2F model is able to capture the general trends of deteriorated heat transfer due to the combined flow acceleration and buoyancy effects. The AKN model over-responds to the flow acceleration effect in most cases, and distorts the velocity profiles more severely than the V2F model where local heat transfer deterioration occurs. Further improvements of the turbulence models are needed for quantitative predictions. Detailed information obtained from the numerical results using V2F model is qualitatively analyzed to understand the mechanism of the special characteristics of the reduced heat transfer resulted from the buoyancy and flow acceleration. The redistribution of flow field induced by the buoyancy and flow acceleration effects is the main factor leading to the local heat transfer deterioration. (author)