Steady-state and transient analysis of supercritical natural circulation systems

Abstract Steady-state and transient analysis of open and closed supercritical loops are presented in this chapter. Explicit equations for steady-state flow rate are presented in dimensional and dimensionless forms. The generalized dimensionless equation developed makes use of the near universal relationship between dimensionless density and enthalpy proposed by Ambrosini and Sharabi in 2008. The dimensionless flow equation has been validated with experimental data reported by several authors. However, it requires three different dimensionless equations to cover the entire supercritical region. Based on a polynomial fit, Archana et al. have shown that it is possible to derive a single dimensionless equation valid for the entire supercritical region. Steady-state natural circulation flow with supercritical fluids exhibits buoyancy-dominant and friction-dominant regimes as observed with two-phase fluids. Increasing the system pressure increases the flow rate in the friction-dominant regime, whereas it decreases marginally in the buoyancy-dominant regime. The effect of orientation of the source and sink was studied for a uniform diameter rectangular loop keeping the height and width the same. This analysis showed that the highest flow rate was obtained when both the heater and the cooler were horizontal. The influence of loop diameter was found to be similar to that observed in single-phase and two-phase loops. The acceleration effect is significant at high power ratings. The heater inlet temperature has a significant influence on the flow rate. A significant reduction in flow rate is observed if the inlet temperature is more than the pseudocritical temperature. The loss coefficient in the subcritical region has less influence on the flow rate than the loss coefficient in the supercritical region. Most supercritical fluids generate more flow than supercritical water for specified loop geometry and operating conditions. However, the heat transport ratio for supercritical water is found to be more than most other supercritical fluids. Transient predictions have been made with the temperature formulation for a uniform diameter rectangular loop with both the heater and cooler vertical. Transients following start-up from rest, power raising, and power step back from a steady state were predicted. The predicted results follow the expected trend.