Optimal flow field in unilaterally heated mini-channels for compact supercritical CO2 solar receivers

The compact receiver with mini-channels is a promising candidate for direct supercritical carbon dioxide (S-CO2) solar receiver owing to its high efficiency and high-pressure resistance. However, large pressure losses and limited thermal performance of S-CO2 make it crucial to enhance convective heat transfer while minimizing flow resistance. The optimal flow patterns solved by the flow field optimization equations can significantly enhance convective heat transfer at a given power consumption, thus guiding the design of the enhanced structures. Nevertheless, the equations are mostly limited to constant-property fluids and uniformly heated boundaries, making them unsuitable for compact solar receivers with variable-property S-CO2 and non-uniform heating conditions. In this work, the flow field optimization equations for supercritical fluids under far-from-critical conditions are derived and the optimal flow fields in compact receivers are obtained. The results indicate that the optimal flow fields exhibit an evolving composite multi-longitudinal vortex structure. In the upstream region, the multiple small longitudinal vortices near the heated wall are formed to enhance the local convective heat transfer. Owing to buoyancy and decreasing temperature gradient downstream, the vortices merge into stable symmetric double-vortices, ensuring a uniform temperature distribution and reducing unnecessary power consumption. The optimal flow field achieves efficient heat transfer with limited pressure loss, enhancing the performance evaluation criterion (PEC) by up to 265% with at most a 40% increase in flow resistance.