Due to the unique thermal and transport properties, Supercritical natural circulation loop (NCL, or thermosyphon) has been proposed in many energy conversion systems. This chapter presents the principals of supercritical natural circulation loop system and its application challenges. A specially designed experimental prototype system is introduced and compared with numerical findings. The system is operated in wide range of high pressures in the critical region. It is found that in a supercritical NCL system, very high Reynolds number natural convection flow can be achieved only by simple heating and cooling. Thermal performance analysis and parameter effects are carried out along with the experimental development. The heat transfer dependency on operation and its mechanisms are also explained and summarized in this chapter. The comparison of experimental and numerical results contributes to better understanding of NCL stability phenomena and applications in energy systems.
TopBasic Design And Principals Of Supercritical Natural Circulation Loops (Ncls)
Thermosyphon (or called NCL: natural circulation loop) is one basic loop heat and mass transfer mechanism by fluid buoyancy forces under terrestrial conditions. Due to the absence of mechanical components, natural circulation provides a reliable way of heating/cooling and energy conversion from heat source to heat sink. Representative applications of NCL can be found in solar heating and cooling systems (Huang, 1980; Kalogirou, 2004; Yamaguchi et al., 2010), geothermal process (Kreitlow & Reistad, 1978), nuclear power plant (Zvirin, 1981; Dimmick et al., 2007) and others. Zvirin (1981) and Grief (1998) have made general reviews on NCL systems.
However, the efficiency of NCL systems is still a problem. Water based NCL is now mostly utilized, for example in solar collectors, which has very low circulation flow rate and it takes a long time for heat accumulation (Kalogirou, 2004). Therefore, it is not preferable for high intensity and fast energy conversion processes. Also, for applications in higher temperature and pressure, possible two phase flow and system instabilities are found in NCL (Huang, 1980; Kalogirou, 2004; Yamaguchi et al., 2010). Due to the of density and loop friction variations when the thermosyphon loop heat input is varied, fluid ‘density waves’ or flow reversals are also found, both for single-phase and two-phase flows (Vijayan et al., 1995; Chatoorgoon, 2001; Kumar and Gopal, 2012). Since 1960s, Welander (1967) and others (Holman & Boggs, 1960) have begun to study the fluid dynamics and stability laws for such NCL systems. Later, a lot of experimental and analytical/numerical studies have been carried out to investigate the system behaviors (for normal fluids) under various input and geometric effects as briefly reviewed by Chen et al. (2010). For some studies, NCL power-flow rate curves are proposed to describe the stability threshold (Dimmick et al., 2002; Jain & Rizwan-uddin, 2008). However, it is later reported that system stability cannot be judged by single parameters due to the complex effects from specific system design and fluid properties (Zhang et al., 2010). Therefore, the NCL controlling parameter correlations are analytically developed by one-dimensional modeling of Vijayan et al. (2004). Also, stability maps have been proposed analytically by Cammarata et al. (2003) and others (Misale et al., 2007).