Effect of system pressure on the steady state performance of a CO2 based natural circulation loop

In the present work the effect of system pressure on the steady state performance of a CO2 based natural circulation loop with end heat exchangers is studied. For a given geometry, the loop pressure (or CO2 inventory) is considered as an input and its influence on the system performance is obtained. The system operates as a single-phase loop or a two-phase loop depending upon the loop pressure, when other parameters such as external fluid temperatures and external fluid flow rates are kept fixed. Hence the model developed considers both single and two-phase flow regimes. For the two-phase flow, a homogeneous model is used and the influence of different two-phase viscosity models on system performance is observed. Results show that when other things are kept constant, the heat carrying capacity of the loop increases initially as the loop pressure is decreased, reaches a peak and then starts decreasing with further decrease in loop pressure. From the value of optimum pressure at which heat transfer rate becomes maximum, one can obtain the required optimum amount of CO2 to be charged into a particular system as the property variation of CO2 along the loop can be obtained by using the model presented. Thus it is expected that the present study will be useful in the design and optimization of natural circulation loops based on carbon dioxide.