Experimental Heat Transfer to Supercritical CO2 Flowing Upward in a Bare Vertical Tube

by Sarah J. Mokry, Igor L. Pioro and Romney B. Duffey*
University of Ontario Institute of Technology
*Chalk River Laboratories (CRL), AECL

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A new reactor concept under development at AECL has the main design objective of achieving a 50% reduction in unit energy cost relative to existing reactor designs. A major contribution to this energy cost reduction would come from raising the average temperature of the fuel-channel coolant, thereby increasing the thermal efficiency of the nuclear power plant. The target coolant conditions are a pressure of 25 MPa, inlet temperature of 350ºC, and outlet temperature of 625ºC. The coolant would pass through its pseudocritical temperature before it reaches the channel outlet, so the reactor is termed the SuperCritical Water-cooled Reactor (SCWR). This approach builds on existing SuperCritical Water (SCW) experience and supercritical turbines at coal-fired power plants.

In support of developing a SCW CANDU® reactor, studies are being conducted into heat transfer at supercritical conditions using carbon dioxide as a modelling fluid as a less expensive alternative to using SCW. Another objective is to improve our fundamental knowledge of the transport processes and handling of supercritical fluids.

The MR-1 loop at CRL is a former steam/water loop adapted for use with supercritical CO2. This paper describes new heat-transfer test results obtained with this facility. The objective of the present experimental research is to obtain detailed reference dataset on heat transfer in supercritical CO2 flowing upward in a vertical tube at SCWR-equivalent operating conditions.

Supercritical CO2 heat-transfer data were 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 inlet temperatures from 20 to 40ºC.

Results are given for supercritical heat transfer for several combinations of wall and bulk fluid temperatures that were below, at, or above the pseudocritical temperature. The experimental data on heat transfer are presented in the form of graphs of internal wall and bulk-fluid temperatures and Heat Transfer Coefficient (HTC) vs. heated length and bulk-fluid enthalpy.

Two regimes of heat transfer at supercritical pressures have been recorded: (1) so-called normal heat-transfer regime and (2) deteriorated heat-transfer regime characterized with lower-than-expected HTC values than those in the normal heat-transfer regime.

A generalized heat-transfer correlation for normal regime in supercritical carbon dioxide flowing in a vertical bare circular tube is proposed.