Parallel channel thermal-hydraulics

Natural-circulation loop and parallel channel thermal-hydraulics... 

The discussions in the following sections will focus on the thermal-hydraulic properties and characteristics of flows in parallel channels and NCLs. The hterature on general aspects of the analytical, experimental, mathematical modeling, numerical solution methods, and computational aspects of these flows will be briefly reviewed. These aspects when associated with specific Gen IV systems will also be discussed. 

In the following paragraphs a brief summary of some of the literamre associated with thermal-hydraulic properties and characteristics and performance, including onset of instability, in NCLs and parallel channels is given. In general, the literature on NCLs and associated physical phenomena and processes is far too enormous to be reviewed in detail here. Instead, many of the reviews already, and especially recently, given will be mentioned along with some of the earlier literature associated with nuclear power systems. 

Considerations of distributions across the flow channel, transverse to the primary flow direction, were first included in basically one-dimensional models by approximating the temperature distribution in the fluid parallel to the flow direction. Recently there is an increasing application of CFD to various single- and two-phase thermal-hydraulic analyses, including NCLs and supercritical fluid states, in nuclear power systems. These approaches also allow for resolution of the thermal stratification in horizontal and vertical sections of the loop as well as resolution of gradients normal to the primary flow direction and the consequent effects on calculated stability. Fully three-dimensional analyses are becoming the norm, but only for simple idealized single-phase cases. 

The RELAP5 model was used to simulate the thermal-hydraulics of the reactor vessel, the piping in all three primary coolant loops, the pressurizer, all three steam generators, and selected parts of the secondary systems. Reactor vessel nodalization, as developed by Bayless, is shown in Figure 1. As indicated, three parallel flow channels extend from the lower plenum through the core to the upper reactor vessel head. If the appropriate conditions exist, this arrangement will allow development of in-vessel natural circulation. 

One particular characteristic of conduction heat transfer in micro-channel heat sinks is the strong three-dimensional character of the phenomenon. The smaller the hydraulic diameter, the more important the coupling between wall and bulk fluid temperatures, because the heat transfer coefficient becomes high. Even though the thermal wall boundary conditions at the inlet and outlet of the solid wall are adiabatic, for small Reynolds numbers the heat flux can become strongly non-uniform most of the flux is transferred to the fluid at the entrance of the micro-channel. Maranzana et al. (2004) analyzed this type of problem and proposed the model of channel flow heat transfer between parallel plates. The geometry shown in Fig. 4.15 corresponds to a flow between parallel plates, the uniform heat flux is imposed on the upper face of block 1 the lower face of block 0 and the side faces of both blocks... 

FLOWTRAN-FI (Reference 21), a WSRC-developed code, calculates the assembly thermal response and coolant hydraulics. The code divides the fuel assembly into concentric fuel cylinders spaced apart by parallel coolant channels. The presence of integral rib spacers (fins) on different fuel cylinders is... 

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