Chapter 1: Introduction
1.1. Basic Physics of Natural Circulation Loop
The cause of fluid flow from one point to another point is the effect of various driving forces. When this driving potential is generated by the use of external energy sources like, pump, compressor, then that type of flow is classified as forced flow. On the other hand when so called change engendered due to the gradient of fluid temperature/density, then it falls to the family of natural convection flow. If the initial and final position of fluid in a flow field is same then it is recognized as a circulation flow field and corresponding forced and natural flow fields are referred to forced and natural circulation respectively. The water cycle and human blood circulation (Figure 1-1) are the most common examples of natural and forced circulation systems respectively. The first cycle is purely completed by generation of density gradient of water, whereas, later one have a pumping system in the form of heart and that create sufficient pressure difference to build the flow field.
Natural circulation refers to the ability of a fluid in a system to circulate continuously. The difference of density or precisely available buoyancy force is the prime driving force of any natural circulation system. A fluid system designed for natural circulation will have a heat source and a heat sink and it has to be ensured that the position of the heat sink is always at a higher elevation than the heat source. The heat source and heat sink are connected in such a way that it forms a continuous circulation path or loop of fluid motion and that type of configuration is categories as natural circulation loops (NCLs) (Figure 1-2).
The primary physics behind the NCL is like, most of the materials are expanded when they are heated and becomes less dense and opposite phenomenon occurs when they are cooled. Fluid get heated at the heat source section of NCL and becomes lighter than the surrounding fluid and then rises.
While, at heat sink section fluid extract heat and becomes denser than nearby fluid and then it drawn downward by gravity. Together, these effects produces a flow field of fluid motion from the heat source to the heat sink and back again. The generated flow filed is purely buoyancy driven.
(a)
(b)
Figure 1-1. (a) Water cycle (natural circulation) (https://goo.gl/4suUqJ) and (b) human blood cycle (https://goo.gl/XTa1za).
NCLs, despite the mathematical intricacy, proposes a convenient route of energy and species transport. The density differential can be accomplished either by introducing a lighter phase into the primary fluid or by modulating fluid temperature through complementary energy interactions with the surrounding in different segments of the flow path. The later contrives a
proficient option of energy transport from a high-temperature source to a low- temperature sink, without them in direct contact. Warmer fluid from the source can rise to the sink owing to buoyancy, to dispense the accrued energy there, and return as a cooler medium, prepared to accumulate energy from the source again. Therefore it is obligatory to place the sink at a higher elevation than the source to establish the favorable buoyancy field, as is shown in Figure 1-2, and that is the only constraint for NCL configuration.
Such simplicity in construction to suit any physical silhouette and enhanced reliability due to the absence of rotating machinery have stimulated innumerable engineering innovations, ranging from gigantic power cycles, nuclear plants and automobiles, through domestic refrigerators, chemical processes and solar heaters, to miniature chip cooling, with undisputed success.
Figure 1-2. Schematic representation of a generalized natural circulation loop, with sink mounted at a higher elevation than the source.
It is improbable to converge on any initiation period for commercial utilization of NCLs as heat transport systems. One of the pioneering application can be identified in early-1950s for turbine rotor cooling. Several arrangements have historically been proposed and developed with varying nature of the working medium, shape and imposed body force. Zvirin, 1981 and Greif, 1988 are presented general reviews on NCLs which are the pioneer article on categorization of loops and basic understanding of the phenomena of NCLs. A complete list of classification is shown in Figure 1-6. While the other factors are typical to a situation, the selection of operating fluid is
SINK
SOURCE
RISER DOWN COMER
g
generally governed by the operation convenience and range of parameters explored. Single-phase, mostly liquid-based, loops are favored in PWRs, solar heaters and electronic cooling applications; whereas two-phase systems, comprising distinct boiling and condensing sections, are prevalent in power cycles, refrigerators and heat pipes. Both are well-explored devices and under scrutiny since the very inception of the concept of NCL. The supercritical NCL, however, is a relatively new concept, with the pioneering research paper being publicized only in late 1990s. Significant count of theoretical and experimental studies have followed over next one and half-decades, only to propose several contradicting theories about the system performance, on both thermalhydraulic and stability aspects, leaving a reasonably thin knowledge base.