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

Supercritical carbon dioxide (SCO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>) 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 SCO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> within a microchannel heat exchanger with serpentine channels at temperature range of $308.15-473.15 \mathrm{~K}$, pressure range of 7.49.6 MPa, and Re range of $11,200-70,000$. It is found that when $T_{\mathrm{b}} / T_{\mathrm{pc}}$ of SCO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> is between $1 \sim 1.08$, 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 $T_{\mathrm{b}} / T_{\mathrm{pc}}$ of 1. The flow and heat transfer prediction model established by regression fitting to the experimental results has a prediction error of $\pm \mathbf{2 5 \%}$. 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 $2^{\circ} \mathrm{C}$.