TURBULENT HEAT TRANSFER IN SUPERCRITICAL CARBON DIOXIDE IN A HEATED HORIZONTAL TUBE

摘要

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 (10.9 mm ID, 12.7 mm OD) was heated over 274 cm by hot water flowing countercurrent to the carbon dioxide in an annular gap between the inner tube 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; average wall heat flux was varied from 12.2 kW/m{sup}2 to 65.7 kW/m{sup}2; average CO{sub}2 temperature was varied from 16.9°C to 38.4°C; and the Reynolds number at the CO{sub}2 average temperature was 34 300 to 154 600. 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%, which was less than the calculated combined standard uncertainty. At the highest pressure tested when the heat flux was low, the measured Nusselt numbers also agreed to the constant property Petukhov-Gnielinski correlation for turbulent flow in a tube. As the pressure was lowered toward the critical pressure, the measured Nusselt numbers diverged from the predictions of the constant properly 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. This is a contribution by an agency of the US Government and is not subject to copyright.