Analysis of two-phase constitutive relation models implemented in thermal-hydraulic system analysis codes

Abstract

A system thermal hydraulic analysis code is generally used for the nuclear power plant safety analysis, and it solves the fluid field equations to model two phase flow. System codes typically solve mass, momentum and energy equations of each phase numerically by approximating to one dimensional flow. However, the constitutive relation based on empiricism must be included for mathematical closure of the approximated governing equations. This thesis work is conducted on the assumption that the constitutive relation models implemented in the thermal hydraulic analysis code has significant effect. A new methodology for evaluating the constitutive relations is proposed, and applied to US TRACE and Korean MARS-KS to analyze the constitutive relation models of each code. TRACE and MARS-KS are very different in the implemented constitutive relation models even though the field equations are almost identical. To compare different constitutive relations of each code, each model and correlations are studied and compared. Then constitutive relation modules are extracted in a separate computational environment. The developed platform is based on MATLAB and REFPROP, and it was proven to reproduce the values of TRACE 5.0 patch4 and MARS-KS 1.5 successfully. In the wall heat transfer models, different heat transfer regimes are occasionally chosen in each code and shows substantial deviation. In the interfacial heat transfer models, while MARS-KS includes void generation term, TRACE doesn’t consider that the term, which results in big difference in the calculation of interfacial heat transfer. In the wall friction models, almost all regimes show a good agreement between TRACE and MARS-KS except for slug and transition flow regimes. In the interfacial friction models, interfacial drag coefficient has large difference than wall drag coefficient between TRACE and MARS-KS.