Numerical modeling of supercritical CO2 natural circulation loop

Abstract

The objective of this research project is to estimate steady state characteristics of supercritical natural circulation loop (SCNCL) using computational methodology and to compliment on-going experimental investigation of the same at the authors’ organization. For computational investigation, a couple of in-house codes are developed. At first, formulation and a corresponding computer program for the SCNCL based on conservation equations written in 1D framework is developed. Comparison of 1D code results with experimental data showed that, under some operating conditions, there is deviation between computed results and experimental data. To improve predictive capability, it was thought to model the SCNCL using conservation equations in 2D axi-symmetric framework. An existing 2D axi-symmetric code (named NAFA), which was developed and validated for supercritical fluid flow in pipes, is modified for natural circulation loop (NCL) geometry. The modified code, named NAFA-Loop, is subsequently used to compute the steady state characteristics of the SCNCL. These results are compared with experimental data. The steady state characteristics such as the variation of mass flow rate with power, velocity and temperature profiles in heater and cooler are studied using NAFA-Loop. The computed velocity and temperature fields show that the flow is hydro-dynamically and thermally developing at various sections of the loop. Such a situation where in some parts of the loop is fully developed and some other part is developing, is more accurately computed by CFD codes than 1D codes. One-Dimensional codes employ fully developed flow correlations for friction factor and heat transfer throughout the loop, hence show deviation from actual behavior of loop. The advantages and shortcomings of 1D as well as 2D models are discussed in the paper.Steady state characteristics of SCNCL are found to be much different than those of sub-critical low pressure NCLs. The computed steady state characteristics are also compared with Swapnalee correlation for SCNCL. Analysis of temperature fields in SCNCL show the existence of normal enhanced and deteriorated heat transfer regimes as observed by previous investigators.