The heat transfer performance of supercritical CO2 (sCO2) in straight and three enhanced tubes were presented firstly, and then a distributed coordination principle was proposed for the design and optimization of heat exchangers. The field synergy principle could explain the thermal-hydraulic performance of sCO2 in different channels very well. The ratio of secondary number to Reynolds number Se/Re could give great predictions for the buoyancy effect. The local heat transfer coefficient also has a lot to do with the near-wall effective thermal conductivity. Zigzag channels with bend angles between 110° to 130° exhibit the best comprehensive performance. With smaller curvature diameter or larger camber, the serpentine channel has better overall performance. Two novel fins were proposed to further improve the performance of PCHE. The heat exchanger performance depends not only on the values of parameters but also on their distributed coordination, which provides a novel approach to heat exchanger optimization of sCO2 through the coordination improvement of distributed parameters.
TopIntroduction
CO2 is non-toxic, non-flammable and easy-available, which makes it one of the most promising candidates in the innovative air-conditioning and refrigeration systems, next generation nuclear reactors, the solar power cycles, etc. (Dostál et al., 2004; Ma et al., 2013; Pizzarelli, 2018; Turchi et al., 2013). Particularly, the supercritical carbon dioxide (sCO2) Brayton cycle has enormous potentials in many power generation systems in recent years for its simple structure and high efficiency (Gkountas et al., 2017; Hanak et al., 2016; Iverson et al., 2013; Le Moullec, 2013; Lee et al., 2014; Padilla et al., 2015; Wang et al., 2010). Heat exchanger is one of the most important components in sCO2 Brayton cycle, whose performance has crucial influences on the efficiency and stable running of the cycle (Dostál et al., 2004). The heat exchanger takes up a large proportion of total investment of sCO2 Brayton cycle (Iverson et al., 2013), whose performance optimization and efficiency improvement are very important for the development and application of the cycle. However, the drastic variation of thermophysical properties near the critical/pseudo-critical points makes the heat transfer and fluid flow of CO2 very complex, which challenges the conventional heat exchanger design and optimization theory seriously and have attracted much attentions since 1960s (Adebiyi et al., 1976; Guo, 2016; Guo et al., 2017a; Guo et al., 2018; Shiralkar et al., 1970; Wood et al., 1964).
The convective heat transfer characteristics of sCO2 in smooth circular tubes under cooling and heating conditions have been investigated substantially in the existing literature. Most of them focused on the effects of heat flux q, mass flux G, pressure P, diameter D and inlet temperature Tin on the thermal-hydraulic performance of sCO2. The general conclusions were that increasing heat flux tends to flatten the temperature and velocity profile in the radial direction (Wood et al., 1964) and goes against the increase of the heat transfer coefficient h (Niu et al., 2011; Oh et al., 2010; Son et al., 2006; Zhang et al., 2011). The higher mass flux would lead to greater heat transfer behavior due to the improvement of turbulent diffusion (Cheng et al., 2008; Dang et al., 2004; Liu et al., 2014; Yoon et al., 2003). When the operating pressure decreases, the peak heat transfer coefficient hpeak increases drastically and locates at lower temperature (Dang et al., 2004; Liu et al., 2014; Oh et al., 2010; Yoon et al., 2003), and the pressure drop would get larger because of more drastic property variation (Huai et al., 2005; Son et al., 2006). Yildiz and Groeneveld (Yildiz et al., 2014) investigated the diameter effect on supercritical fluids. They reported that the smaller inner diameter contributes to better heat transfer performance (Kim et al., 2008b; Oh et al., 2010) and heat transfer is more prone to deterioration in larger tube. Rao et al. (Rao et al., 2016) summarized that inlet temperature variations have little impacts on the heat transfer under cooling conditions but affect the heat transfer behavior a lot under heating conditions. Apart from that, some inconsistent and contradictory results were also published in previous research findings (Bruch et al., 2009; Jiang et al., 2008a; Jiang et al., 2008b; Jiang et al., 2004; Li et al., 2010). The thermal-hydraulic characteristics in tubes has such sophisticated changes even under uniform heating conditions, let alone the flow and heat transfer performance in tubes with non-uniform heat flux (Fan et al. 2019; Zhang et al. 2020).