Abstract
Due to the unique thermal and transport properties, supercritical natural circulation loop (NCL, or thermosyphon) has been proposed in many energy systems, such as solar heater, nuclear cooling, waste heat recovery, geothermal, etc. This chapter presents the principals of supercritical natural circulation loop 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 pressures from around 6.0 MPa to 15.0 MPa in the near-critical region. It is found that in a supercritical natural circulation 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 and 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 and 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 and 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).
Key Terms in this Chapter
Stability Map: The map that states the stability region, transition lines, and parameters for the system stability conditions.
Supercritical Fluid: The fluid state that above some critical temperature and pressure point, where the fluid will never be liquefied by pressurization process. The fluid status is special as the solubility, flow viscosity, thermal conductivity, compressibility will have large changes during the transitions.
Supercritical Heat Exchanger: The heat exchanger specially designed for supercritical fluids, which may be relatively compact as the supercritical fluid heat transfer rate is relatively high and it is operated under high pressure.
Natural Convection: The convection phenomena induced by buoyancy forces under gravity. Usually it indicates the ones different from forced convection under outer forces. In supercritical fluids, natural convection flow is strong as the fluid density changes are evident under temperature variations.
Critical Diverges: The thermal and transport property varies and show great diverges process when a fluid status is near the critical region.
Solar Thermal: The utilization of solar energy by converting it into thermal kinds, which is usually achieved by a collector system.
Supercritical Refrigeration: The concept of using supercritical fluids in refrigeration systems. As a working fluid, supercritical state connects high pressure region and the low pressure region (the gas phase) so as to achieve cooling process. Some designs also use the throttling process, or jet flow of sudden-expansion flow from supercritical status to achieve low temperature environment.
Heat Transfer Enhancement: The incensement of heat transport of a surface, body, apparatus. Usually it is a target to be achieved by improving the thermal and mechanical designs, or the choice of working fluids.
Thermosyphon: Another name for natural circulation loop (NCL), which is more focused on the thermal transportation aspect of a loop system utilizing natural circulation working fluid as the basic transport mechanism.
Stability Transition: The transitions among stable flow, quasi-stable flow, unstable flow, etc. It is critical for supercritical fluids for the definition of stability evolutions and changes.
System Design: The concept and construct of a targeted system, which may include the geometric design, fluid design, operation design, etc.
Supercritical Nuclear Cooling: The application of supercritical fluids in nuclear cooling and heat transportation process, which is possible for the secondary loop circulation system. The concept of supercritical fluids based nuclear system is now one of the trends for future nuclear systems.