Influencing mechanisms of CO2 pressure drop characteristics in variable cross-sectional zigzag microchannel

Microchannel heat exchangers, widely used as recuperators in CO2 power cycles, with flow performance directly impacting cycle efficiency. In this study, the effects of thermophysical and operating parameters on CO2 flow performance in variable cross-sectional zigzag microchannel (UAPCHE) were decoupled and quantified by numerical simulation. Experimental measurements were then used to analyze the variation of total pressure drop (ΔPtot) and its components (friction loss ΔPfr, flow acceleration loss ΔPacc, and local form loss ΔPm) across operating conditions. Additionally, the dynamic response mechanism of flow characteristics under continuous variable conditions was revealed. Finally, the flow correlation suitable for UAPCHE was proposed. The results indicate that inlet temperature, pressure, and heat flux influence friction factor primarily through changes in fluid density and viscosity. In contrast, the inlet mass flow rate affects it both indirectly via these properties and directly to a significant extent. Under all conditions, ΔPfr/ΔPtot is approximately within the range of 60%–80%, ΔPacc/ΔPtot is approximately within the range of 0%–10%, and ΔPm/ΔPtot is approximately within the range of 20%–30%. Additionally, ΔPfr/ΔPtot reaches its minimum value at pseudo-critical temperature (Tpc), whereas ΔPacc/ΔPtot and ΔPm/ΔPtot reach their maxima at Tpc. As heat flux increases, although ΔPtot remains almost constant, the maximum value of ΔPacc/ΔPtot increases by 5.38%, and the minimum value of ΔPfr/ΔPtot decreases by 6.28%. Continuous variation in pressure has little effect on the ΔPtot but significantly alters the composition of pressure loss mechanisms. The accuracy of the new flow correlation prediction is within ±18%.