CFD analysis of a CO2 based natural circulation loop with end heat exchangers

3-D steady flow simulation of a CO2 based, single-phase (subcritical and supercritical) rectangular natural circulation loop (NCL) with end heat exchangers is presented. Viscous dissipation in fluid and axial conduction in fluid as well as in solid wall are considered. Results are obtained for various inlet temperatures of water in the hot heat exchanger, for a fixed inlet temperature of cooling water in the cold heat exchanger. Effect of loop operating pressure (60–100 bar) on system performance is also investigated. Results show that due to the presence of bends and strong buoyancy effects near pseudo-critical zone, local velocity and temperature vary in all three dimensions. It is noticed that at a given loop pressure, the variation of heat transfer rate with loop fluid temperature changes sign in the vicinity of pseudo-critical region. Steady-state Reynolds number and heat transfer coefficients obtained through the CFD analysis are compared with those calculated using available correlations. Although qualitative trends match, quantitative differences exist that may be attributed to difference in configuration. Additionally, new correlations are proposed for Re in terms of modified Gr, and Nu in terms of Re and Pr which are expected to be useful in the design and analysis of NCLs with end heat exchangers.

► CFD analysis of a CO2 based NCL with end heat exchangers is carried out. ► Due to strong buoyancy effects, velocity and temperature vary in all directions. ► Heat transfer rate in supercritical case is higher than that of subcritical case. ► Heat transfer rate reaches maximum near the pseudo-critical region. ► New correlations are proposed for Reynolds number and Nusselt number.