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 ̇ ≈ 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. Additionally, comparison with Petukhov (P) and Krasnoshchekov–Kuraeva–Protopopov (KKP) correlations shows that KKP captures about 85% of data within ± 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 o C. • Local CO 2 heat transfer coefficients experimentally obtained in a 1 mm horizontal tube. • Heat transfer coefficients of 10–60 kW m −2 K −1 with sometimes two local peaks near T p c . • Temperature rise drops and pressure loss peaks near pseudo-critical conditions. • CO 2 can be a viable alternative to water for electronics cooling in small channels.