Experimental studies on an air-cooled natural circulation loop based on supercritical carbon dioxide - Part A: Steady state operation

Experimental studies are carried out on an air-cooled supercritical carbon dioxide based natural circulation loop (NCL) to investigate the effects of different controlling parameters under steady state and transient conditions. The heat source used is an electrical heater with controlled heat flux and the heat sink is an air-cooled fin and tube type heat exchanger with controlled air flow rate and air inlet temperature. To minimize required system charge, pressurizer (expansion tank) is not included in the system. Experimental results obtained under steady state operation of this loop are presented in this part (Part A). Based on the experimental results under steady state conditions, a regression equation that relates Reynolds number with Grashof number is obtained. Results show that Reynolds number as high as 104 could be obtained for source to sink temperature difference as small as 7 K. Results show that at higher heat inputs, the system charge should be chosen properly to avoid problems due to over pressure when the charge is excessive and over temperature when the charge is deficient. There is a threshold velocity for the external coolant (air) in the heat sink, below which the system pressure rises very rapidly leading to, possibly, catastrophic loop failure. This study shows that in the absence of a pressurizer, there is a need for determining the correct charge and to devise proper control and safety system to safeguard the loop under the accidental case of fan failure. It is expected that this study will be useful in the design and safe operation of air-cooled supercritical CO2 based natural circulation loops.