Experimental Investigation and Design of Microchannel Heat Exchanger With S-shape Channel in the SCO₂ Pseudo-critical Region

Supercritical carbon dioxide (SCO₂) has a drastic change of thermophysical properties in the pseudocritical region, which significantly affects the flow heat transfer performance. This study experimentally investigated the flow heat transfer characteristics of SCO₂ within a microchannel heat exchanger with serpentine channels at temperature range of 308.15-473.15 K, pressure range of 7.49.6 MPa, and Re range of 11,200-70,000. It is found that when T_b/T_pc of SCO₂ is between 1 ∼ 1.08, the drastic change of thermophysical properties leads to the rapid increase of the heat transfer coefficient and drag factor, and the heat transfer coefficient reaches the maximum value near the T_b/T_pc of 1. The flow and heat transfer prediction model established by regression fitting to the experimental results has a prediction error of ± 2 5%. A flow and heat transfer prediction model was developed using regression analysis. Localised flow and heat transfer patterns within the heat exchanger were investigated through a non-uniform segmentation approach. Under precooler conditions, a pinch point was found at x/L=0.52, and the critical heat transfer region was identified within the x/L interval of 0.2 to 0.8, with the temperature difference of nearly 2° C.