Parallel channel natural-circulation flows

Natural-circulation flows around loops and flows in parallel channels are both susceptible to departures from steady operation and excursions into oscillatory and, potentially, unstable states. Thus Gen IV nuclear reactor power systems combine the type of fluid flow and geometries that are known to potentially lead to undesirable states. In particular, undesirable oscillatory states under steady-state operation should be avoided. The complete system and associated operational envelope are designed to avoid unstable states. 

Professors W. Ambrosini and J. Ferreri, in a collaborative effort over a period of approximately 10 years, and with others, have presented an exhaustive investigation of the effects of numerical approximations to the continuous model equations, and the numerical solution method for these, on calculations of the onset of instabiUty for natural-circulation and parallel-channel flows. A few examples are Ferreri and Ambrosini (1999, 2002), Ambrosini and Ferreri (1997a,b, 1998, 2000, 2003, 2006, 1999), Ferreri et al. (1995), Ambrosini et al. (2001), and Ambrosini (2001, 2008), among many others. The investigations have used special-purpose computer codes in the frequency and time domains, and the RELAP5 system-analysis code. Mangal et al. (2012) have also compared RELAP5 calculations with NCL properties. 

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. 

In designing heat removal systems, we often rely on both single and two-phase natural circulation. The fluid heats and expands, and may boil and the two-phase flow generates an increased pressure drop. The inlet flow is derived from a pump, or a downcomer with a hydrostatic head, so that when there are many channels in parallel, as in a reactor, heat exchanger or condenser, the boundary condition is a constant pressure drop. 

It is important to note that stability limits in natural circulation systems arise before (and as a prelude to) CHF or DNB. Indeed, conventional forced flow CHF and DNB correlations cannot be applied to natural circulation and parallel channel systems if either the loss coefficients are unknown or not reported, or the appropriate constant pressure drop was not maintained or achieved in the tests. Throttling the inlet flow to set a flow boundary condition artificially stabilizes the channel. In actual plants, it is well known that the plant maintains a constant pressure drop, by having multiple parallel channels and/or a controlled downcomer hydrostatic head. 

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