Analysis of the enhanced heat transfer characteristics of supercritical CO ₂ and CO ₂ / Xe mixture working fluids in PCHE and prediction of heat transfer correlation

To investigate the enhanced heat transfer behaviour of supercritical carbon dioxide and CO 2 /Xe mixed working fluids in printed circuit heat exchangers (PCHE), and to improve the cycle efficiency of the Brayton cycle, the flow and heat transfer performance of the mixed working fluid were analyzed by varying the Xe mass fraction in CO 2 , the inlet mass flow rate, and the inlet temperature. Results show that under supercritical conditions, the change in the mass fraction of Xe is combined with the heat transfer characteristics of mixed working fluids and the thermal efficiency of the Brayton cycle, filling a gap in the combination of these two research areas. As the mass fraction of Xe increases from 0% to 30%, the peak heat transfer coefficient decreases by 36.8%, but the thermal efficiency is significantly improved. When the Reynolds number reaches 77,000, the comprehensive heat transfer evaluation index (PEC) increases with increasing Xe mass fraction, improving the heat transfer performance. As the mass flow rate increases from 400 to 1000 kg/(m 2 s), the peak heat transfer coefficient increases by 378.29%, and the average value of the comprehensive heat transfer evaluation index increases by 100.21%. Before the inlet temperature reaches the critical temperature, as the temperature increases, the peak heat transfer coefficient increases by 40.73%, the average Nusselt number increases by 38.09%. A heat transfer correlation for CO 2 /Xe mixtures was derived with an error range within ±20%. The research results will provide a theoretical foundation for the design of CO 2 /Xe binary mixed working fluid PCHEs.