Heat transfer in turbulent supercritical carbon dioxide flowing in a heated horizontal tube

We report measurements of heat transfer coefficients of flowing supercritical carbon dioxide (7.38 MPa critical pressure, 31.1 °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 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 shell-in-tube heat exchanger.Operating pressure was varied from 7.8 MPa to 13.1 MPa; C0 2 mass flow rate was varied from 1.0 kg/min to 5.1 kg/min; heating was varied from 1 150 W to 6180 W; and C0 2 inlet temperature was varied from -1.7 °C to 32.7 °C.The Reynolds number range at the C0 2 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/m 2 .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 %).