Supercritical CO2 flow and heat transfer through tapered horizontal mini-tubes: Parametric analysis and optimization study

Abstract

One effective method for enhancing heat transfer in tubular heat exchangers involves gradually altering the cross-section of their tubes. While extensive research has been carried out in typical operating conditions, a significant shortage of studies exists that specifically investigate how changes in tube cross-section affect the heat transfer of fluids under critical and pseudocritical conditions. Additionally, the absence of a comprehensive parametric study considering the effect of all operating conditions and geometric parameters, and providing an optimal model in this field, is keenly felt. This paper offers a finite volume-based numerical simulation of steady-state turbulent flow and heat transfer for supercritical CO2 inside circular horizontal mini-tubes with tapered lateral profiles. It is found that the impact of buoyancy on thermal and hydraulic characteristics is more noticeable in the diverging mini-tube than in the converging mini-tube. The results of the parametric study reveal that under the same geometric parameters, the heat transfer coefficient in the diverging mini-tube is higher than that in the converging mini-tube by 5.2–28.6 %, with a simultaneous reduction in pressure drop ranging from 52.1 % to 68.5 %. However, in the case of a high diameter or diameter ratio, the heat transfer coefficient of the converging mini-tube exceeds that of the diverging mini-tube. Subsequently, an optimization study is conducted using BBD-RSM. To achieve the maximum heat transfer coefficient and the minimum pressure drop, an additional 54 cases are investigated, covering diverse ranges of operating conditions and geometric parameters. The results indicate that the mass flow rate and tube diameter are the most influential parameters on thermal and hydraulic characteristics. Additionally, the impact of the diameter ratio on the pressure drop is more pronounced than on the heat transfer coefficient. This research can serve as a foundation for enhancing the design of heat exchange devices within innovative supercritical CO2 power generation cycles.