Available online at www.sciencedirect.com Nuclear Engineering and Design 238 (2008) 1917–1929 Dimensionless parameters in stability analysis of heated channels with fluids at supercritical pressures Walter Ambrosini ∗ , Medhat Sharabi Universit` a di Pisa, Dipartimento di Ingegneria Meccanica, Nucleare e della Produzione, Via Diotisalvi 2, 56126 Pisa, Italy Received 4 December 2006; received in revised form 7 September 2007; accepted 7 September 2007 Abstract The paper proposes dimensionless parameters for the analysis of stability in heated channels with supercritical fluids. The parameters are devised basing on the classical phase change and sub-cooling numbers adopted in the case of boiling channels, proposing a novel formulation making use of fluid properties at the pseudo-critical temperature as a function of pressure. The adopted formulation for dimensionless density of a given fluid provides a unique dependence with respect to dimensionless enthalpy, in a reasonably wide range of system pressures, thus giving generality to the predictions of unstable conditions obtained as a function of dimensionless parameters. It is shown that these parameters allow setting up quantitative stability maps for a single heated channel with imposed overall pressure drop, in analogy with the ones proposed in previous work concerning boiling channels. Similarities with the boiling channel stability phenomena are pointed out, also supporting the conclusions with system code predictions. © 2008 Elsevier B.V. All rights reserved. 1. Introduction The supercritical light water reactor (SCLWR) is presently receiving consideration as a promising development of conventional light water reactor technology, having the capability to achieve larger power conversion efficiency. In fact, the use of water at supercritical pressure conditions allows to achieve higher temperatures in the fluid cycle, raising the useful power fraction from classical figures of 33%, as in conventional light water reactors (LWRs), up to 44% (Squarer et al., 2003). Reactor concepts of this kind have been the subject of design proposals taking into account the characteristic of fluids at supercritical pressure to undergo remarkable changes in density and other thermodynamic and thermophysical properties without the formation of interfaces and separate phases (Dobashi et al., 1998; Heusener et al., 2000a,b; Oka, 2000). Therefore, during normal operation, the complications arising in boiling water reactors (BWRs) from two-phase flows are avoided, though this is obtained at the price of challenging the reactor materials with the exposure to higher temperatures and a more corrosive environment. The use of direct cycles is also possible, routing the lighter fluid directly from the core to the turbine, thus considerably simplifying the plant lay out with respect to pressurised water reactors (PWRs). Nevertheless, different two-phase flow problems are anyway encountered in this proposed technology which cannot be completely disregarded (Laurien and Schulenberg, 2004). On the other hand, supercritical fluids have some well known fascinating properties that make them an attracting and challenging subject to be coped with by scientific research. In particular, the strong variation of fluid properties in the vicinity of the pseudo-critical temperature makes them very peculiar single-phase fluids, posing problems in the simulation of their heat transfer capabilities and in detailed turbulence modelling (Cheng and Schulenberg, 2001; He et al., 2004, 2005; Cheng et al., 2005). Among the aspects to be investigated when a fluid undergoes strong density changes inside heated channels, fluid-dynamic stability is given a major concern when dealing with nuclear reactors, also due to the coupling with the neutronic processes. Studies have been performed in recent times concerning both simple natural circulation loops (Chatoorgoon, 2001) and proposed plant ∗ Corresponding author. Tel.: +39 050 836673; fax: +39 050 836665. E-mail address: walter.ambrosini@ing.unipi.it (W. Ambrosini). 0029-5493/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2007.09.008