Performance evaluation of non-uniform twisted designs in precooler of supercritical CO₂ power cycle

Twisted structures have been commonly used to improve heat transfer in tubular heat exchangers operating at regular pressures. However, their behavior under supercritical operating conditions remains unknown, and the thermal and hydraulic characteristics of a precooler equipped with twisted structures in a supercritical carbon dioxide (SCO₂) cycle have not been studied before. For this reason, 3D simulations are conducted to explore the flow and heat transfer of SCO₂ in a precooler featuring both twisted tubes and twisted tapes. Furthermore, four non-uniform configurations are designed for each twisted structure. The obtained results indicate that for SCO₂ in a gas-like state, the use of twisted tubes is more suitable, whereas, for SCO₂ in a liquid-like state, the use of twisted tapes is more appropriate. Twisted configurations exhibit a heat transfer coefficient ranging from 1.11 to 1.48 times higher than that of the straight mini-tube. Non-uniform structures can help control and tailor turbulence levels according to SCO₂'s thermophysical properties variations. The results reveal that shorter twist pitches at the beginning of the flow path in non-uniform twisted tubes and in the middle section for twisted tapes provide the best overall performance, with significant heat transfer enhancement and a relatively low pressure drop penalty.