An investigation of the thermo-hydraulic performance of a novel double tube heat exchanger with variable cross-section inner tube for trans-critical CO2 cycle

Abstract

Double tube heat exchangers (DTHE) is one of the most significant components in the trans-critical CO2 cycle system, whose performance significantly affects the efficiency and compactness of the system. This work aims to improve the heat transfer performance of CO2 at supercritical pressure in DTHE. In this paper, the cylindrical inner tube of traditional DTHE is changed into the diverging/converging tube as an innovative design without increment of its heat transfer areas. The thermal–hydraulic performance of traditional DTHE with uniform cross-section inner tube (UIT), DTHE with diverging (DIT)/converging (CIT) inner tube were numerically investigated by using SST k-ω turbulence model. The results show that under the same operating conditions, the DIT and CIT can effectively enhance the heat transfer compared with the UIT. Utilization of DIT and CIT instead of UIT, the maximum increase of the total heat transfer coefficient are 27.3 % and 24.2% can be obtained, respectively. However, the pressure drop of DIT and CIT also increases accordingly. The performance evaluation criterion (PEC) of DIT is always greater than 1, and the maximum is 1.16. In most cases, the PEC of CIT is greater than 1, but with the increase of SCO2-side mass flow rate, it gradually decreases to less than 1. The comprehensive performance of DIT is the best. The heat transfer mechanism is analyzed from the aspects of flow field distribution, thermos-physical property and field synergy theory. The large turbulent kinetic energy near the wall, high thermal conductivity and large specific heat of SCO2 in the tube are the main reasons for the enhanced heat transfer of DIT and CIT. Finally, the influence of SCO2-side pressure and mass flow rate on local heat transfer coefficient hCO2 in DIT are also further studied. This study can provide some guidance for the design and optimization of DTHE.