Numerical simulation of subcooled boiling in a vertical pipe with twisted tape inserts

Subcooled flow boiling occurs in many industrial applications and it is characterized by a large heat transfer coefficient. In this paper, numerical simulation of subcooled flow boiling in a vertical pipe is investigated by ANSYS CFX 14. Variation of void fraction as a function of increasing enthalpy along the pipe is validated against the RELAP5 and experimental results available in the literature. The RELAP5 has a broad acceptance as a tool for analyzing the thermal-hydraulic behavior of the nuclear reactors. The code is based on a non-homogeneous and non-equilibrium two-phase model of six equations solved by a fast and partially implicit numerical scheme. The model does not have the ability to simulate two phase flow with a twisted tape insert. The effect of the twisted tape inserts on improving the subcooled boiling heat transfer is investigated in a vertical pipe of 2m in length and 154mm in diameter using CFX14. The pressure and wall heat flux is 45 bar and 0.57 MW m^-2, respectively. The range of the twisted tape pitch investigated to be between 0.04 m and 0.16 m. A comparison of the predicted void fractions for a plain vertical tube without tube inserts revealed a good agreement with the experimental data. It was found that, heat transfer and pressure drop both increase with a reduction of the twisted tape pitch. Also, within the pitch range investigated, the 0.08 m pitch had the highest performance with respect to the pressure drop and heat transfer.