Heat Transfer of Supercritical Carbon Dioxide Flowing in a Cooled Horizontal Tube
Douglas A. Olson
We report measurements of heat transfer coefficients of flowing supercritical carbon dioxide (7.38 MPa critical pressure, 31.1 C critical temperature) in a cooled horizontal tube. The tube was 10.9 mm ID, was cooled over 274 cm, and had an adiabatic entrance section of 55.9 cm. Cooling was accomplished by flowing cold water countercurrent to the carbon dioxide in an annular gap between the inner tube (12.7 mm OD) and an outer tube (16.6 mm ID). This set a convective boundary condition similar to what would be encountered in a counterflow heat exchanger. The four experimental parameters found to affect the heat transfer coefficients, and their ranges tested, were: operating pressure (7.409 MPa to 13.0 MPa), CO2 mass flow rate (1.1 kg/min to 5.0 kg/min), rate of heat removal by cooling (1780 W to 6220 W), and CO2 average temperature (4.3 C below to 24.2 C above the pseudocritical temperature). The Reynolds number range at the CO2 average temperature was 63 250 to 291 700. Conditions which resulted in the highest heat transfer coefficients were pressure close to the critical, high mass flow rate, and temperature close to the pseudocritical. For CO2 average temperatures lower than 1 C below the pseudocritical, higher rates of cooling increased the heat transfer coefficient, whereas for temperatures higher than 1 C below the pseudocritical, higher rates of cooling reduced the heat transfer coefficient. The measured Nusselt numbers were predicted to within experimental uncertainty by the Krasnoshchekov-Protopopov correlation for supercritical flow, when the CO2 average temperature was above the pseudocritical. This correlation was developed for constant heat flux heating instead of cooling. No existing correlation was found to predict the measured Nusselt number when the CO2 average temperature was lower than the pseudocritical. The measured heat transfer coefficients were higher than those predicted by the constant property correlations throughout the range of parameters tested.
Heat Transfer of Supercritical Carbon Dioxide Flowing in a Cooled Horizontal Tube, NIST Interagency/Internal Report (NISTIR), National Institute of Standards and Technology, Gaithersburg, MD
(Accessed June 4, 2023)