Heat Transfer Correlation for Supercritical Carbon Dioxide Flowing Upward in a Vertical Bare Tube

by Sarah Mokry & Igor Pioro
University of Ontario Institute of Technology

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It is expected that the next generation of water-cooled nuclear reactors will operate at supercritical pressures (~25 MPa) and high coolant temperatures (350–625°C). In support of the development of SuperCritical Water-cooled Reactors (SCWRs), research is currently being conducted for heat-transfer at supercritical conditions. Currently, there are no experimental datasets for heat transfer from power reactor fuel bundles to the fuel coolant (water) available in open literature. Therefore, for preliminary calculations, heat-transfer correlations obtained with bare-tube data can be used as a conservative approach.  

In support of developing SCWRs, studies are being conducted into heat transfer at supercritical conditions using carbon dioxide as a modelling fluid as a less expensive alternative to using supercritical water. Experiments with SuperCritical Water (SCW) are very expensive. Therefore, a number of experiments are performed in modeling fluids, such as carbon dioxide and refrigerants. However, there is no common opinion if supercritical modeling fluids’ correlations can be applied to SCW and vice versa.

The MR-1 loop at Chalk River Laboratories (CRL) is a former steam/water loop that has been adapted for use with supercritical CO2. The objective of the experimental research was to obtain a detailed reference dataset on heat transfer in supercritical CO2 flowing upward in a vertical tube at SCWR-equivalent operating conditions. The supercritical carbon dioxide dataset was obtained at reactor-equivalent conditions at three pressures above the critical point (7.6, 8.4 and 8.8 MPa), mass fluxes from 840 to 3000 kg/m2s, heat fluxes up to 600 kW/m2 and with inlet temperatures from 20°C to 40°C.  The conducted analysis also meets the objective of improving our fundamental knowledge of the transport processes and handling of supercritical fluids.

Two modes of heat transfer at supercritical pressures were observed: normal and deteriorated heat transfer. The latter was observed within the entrance region and near the middle of the test section. The deteriorated heat transfer regime is characterized by higher wall temperatures (lower HTC values) than the normal heat transfer regime. This phenomenon is affected by pressure, bulk-fluid temperature, mass flux and heat flux.

The obtained correlation for forced convective heat transfer to supercritical carbon dioxide, in a bare vertical tube with upward flow, showed a reasonable fit for the analyzed dataset. This correlation can be used for future comparisons with other independent datasets and for the verification of scaling parameters between water and CO2.