Heat Transfer in Supercritical Carbon Dioxide With Convective Boundary Conditions
Douglas A. Olson
We report measurements of heat transfer coefficients of flowing supercritical carbon dioxide (7.38 MPa critical pressure, 31.1 degrees C critical temperature) in a heated horizontal tube. The tube was 10.9 mm ID, was heated over 274 cm, and had an unheated entrance section of 55.9 cm. Heating was accomplished by flowing hot water counter-current 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 shell-in-tube heat exchanger. Operating pressure was varied from 7.8 MPa to 13.1 MPa; CO2 mass flow rate was varied from 1.0 kg/min to 5.1 kg/min; heating was varied from 1150 W to 6180 W; and CO2 inlet temperature was varied from -1.7 degrees C to 32.7 degrees C. The Reynolds number range at the Co2 average temperature was 34 300 to 154 600. At the highest pressure tested, the measured Nusselt numbers agreed to the constant property Petukhov-Gnielinski correlation for turbulent flow in a tube to within 6.6 %, except when Reynolds number was less than 72 000 and the heat flux was greater than 47 kW/m2. As the pressure was reduced toward the critical pressure, the measured Nusselt numbers diverged from the constant property correlation. At these lower pressures, conditions of high mass flow and low heat flow enhanced the heat transfer, while conditions of low mass flow and high heat flow degraded the heat transfer. The Krashnoschekov-Protopopov correlation for supercritical flow, developed for conditions of a constant heat flux boundary condition when buoyancy is negligible, predicted the measured Nusselt numbers to within one standard deviation of 3.0 % (range of -6.5 % to +10.1 %).
Proceedings of the 20th International Congress of Refrigeration