Effects of inlet temperature and pressure in sCO$_2$ heat transfer in a 1 mm diameter horizontal tube

Supercritical CO$_2$ (sCO$_2$) is of high interest for compact cooling applications, yet local heat transfer data in millimetric scale horizontal tubes under electronics-relevant conditions remain scarce. This work reports local heat transfer coefficients, pressure drops and temperature-inlet-to-outlet values for CO$_2$ at inlet pressures from 7.5 to 8.5 MPa flowing in a 1 mm horizontal stainless-steel tube with $G$ = 2130 kg m$^{−2}$ s$^{−1}$ (m $\approx$ 1.8 g s$^{−1}$) and uniform Joule heating over $L_h$ = 0.903 m at $q$ = 35 kW m$^{−2}$. A calorimetric data reduction based on inlet and outlet conditions is combined with an axial enthalpy integration to express results as a function of bulk enthalpy, thus enabling consistent comparison across inlet temperatures.
The obtained heat transfer coefficients span from 10 to 60 kW m$^{−2}$ K$^{−1}$ and show a systematic dependence on inlet temperature and pressure through the pseudo-critical region. When the fluid near the wall crosses the pseudo-critical point, a first enhancement is observed, associated with the sharp increase in c$_p$, causing the collapse of ($T_w - T_b$). The overall pressure drop increases steeply as inlet conditions approach the pseudo-critical temperature, whereas the inlet-to-outlet temperature rise decreases and tends to plateau, highlighting the thermal–hydraulic trade-off in this regime. ... mehrAdditionally, comparison with Petukhov (P) and Krasnoshchekov–Kuraeva–Protopopov (KKP) correlations shows that KKP captures about 85% of data within $\pm$ 20%, while P overpredicts at high heat transfer coefficients due to the lack of consideration of strong radial property variations.
These results provide a benchmark dataset of high Reynolds numbers, low heat flux sCO$_2$ heat transfer in 1 mm horizontal channels and show an improved performance of sCO$_2$ as a local single-phase refrigerant, making it a promising alternative for detector and electronics cooling at warm operating temperatures above 31°C.