Turbulent supercritical CO2 flow and cooling heat transfer in vertical alternating flattened tubes

초록

Tubular heat exchangers are commonly utilized for cooling CO2 in both supercritical and transcritical cycles. However, due to their suboptimal performance, researchers are exploring advanced heat exchangers or novel tube geometries to enhance their effectiveness. This study evaluates the feasibility of Alternating Flattened Tubes (AFTs) as enhanced tubes for heat exchangers, including pre-coolers in Brayton supercritical CO2 cycles and gas coolers in transcritical CO2 heat pumps. It examines different vertical AFT configurations for both downward and upward CO2 flow under different operating conditions. The results indicate that buoyancy effects become more pronounced as CO2 cools from a supercritical to a subcooled state, particularly in upward flows. Flow acceleration has less impact under supercritical cooling but becomes significant in transcritical cooling, particularly in downward flows. The use of AFTs significantly boosts heat transfer efficiency near the pseudo-critical point, improving the coefficient by up to 64.4 % for downward flow and 44.1 % for upward flow. Likewise, transitioning to a supercritical liquid-like state increases dependence of hydraulic characteristics on flow orientation in vertical AFTs by up to 15.2 %. A lower axis angle enhances the synergy between velocity vectors and temperature gradients in AFTs, particularly when the entire flow path is flattened.

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