Development of a Wide-Range Pre-CHF Convective Boiling Correlation

Abstract

There has been a tendency, in the context of “best-estimate” thermal-hydraulics code developments, to forgo the body of pre-CHF correlations dedicated to specific flow regimes and operating conditions, for a single correlation, developed for saturated boiling conditions by Chen. Furthermore, there has also been a trend to implement increasingly detailed (so-called “mechanistic”) two-phased flow models. In that context, this important heat transfer relationship was also split to calculate the crucial wall vapour generation rate in subcooled void models. An assessment of convective boiling and its suppression led to the questioning of these procedures and a new interpretation and representation of the standard (Collier's) flow boiling (JNST, 45[2], 2008). The present paper is an attempt to follow up on those findings by developing a new pre-CHF correlation susceptible to be applicable over a wide range of flow conditions, including pressures ranging from near-atmospheric to 15 MPa. The originality and essence of this attempt stems from an effective separate-effect approach, whereby the boiling heat transfer has been isolated, purely on an experimental basis, without reliance on correlations. This was possible because in the (CISE) experiments used for the basic formulation, the “boiling curves” were obtained under fixed hydrodynamic conditions. While the convective component, Fhconv , in the Chen correlation was essentially corroborated, and thus the F factor was marginally modified, the boiling component, Sh boil, was not. The latter has been replaced by a new formulation that implements a wall superheat “offset,” below which boiling heat transfer remains inoperative.