Experimental studies on CO2 based single and two-phase natural circulation loops

In this paper, experimental results obtained on a natural circulation loop operating with CO2 as loop fluid and water as the external fluid are presented. The instrumented test rig consists of two end heat exchangers that act as heat source and heat sink and two insulated vertical legs which act as downcomer and riser. Experiments are conducted with the loop fluid (CO2) in vapour phase, liquid phase and liquid–vapour phases by varying the amount of CO2 charged into the loop as well as the external fluid temperatures. In line with the theoretical predictions, it is observed from the experimental results that for a given loop, operating at given external fluid temperatures, there exist an optimum value of CO2 inventory in the loop at which the heat transfer rate is maximum. Results also show that the time required to reach steady state is more in case of two-phase loops compared to single phase loops. The experimental data are compared with the results obtained from a theoretical model. By normalising the experimental results comparison is also made between the results obtained from the present study and with those available in the literature, and a reasonably good agreement is found between them.