Abstract
The boiling process is utterly fundamental to the design and safety of water-cooled fission reactors. Both boiling water reactors and pressurised water reactors use boiling under high-pressure subcooled liquid flow conditions to achieve high surface heat fluxes required for their operation. Liquid water is an excellent coolant, which is why water-cooled reactors can have such small sizes and high-power densities, yet also have relatively low component temperatures. Steam is in contrast a very poor coolant. A good understanding of how liquid water coolant turns into steam is correspondingly vital. This need is particularly pressing because heat transfer by water when it is only partially steam (‘nucleate boiling’ regime) is particularly effective, providing a great incentive to operate a plant in this regime. Computational modelling of boiling, using computational fluid dynamics (CFD) simulation at the ‘component scale’ typical of nuclear subchannel analysis and at the scale of the single bubbles, is a core activity of current nuclear thermal hydraulics research. This paper gives an overview of recent literature on computational modelling of boiling. The knowledge and capabilities embodied in the surveyed literature entail theoretical, experimental and modelling work, and enabled the scientific community to improve its current understanding of the fundamental heat transfer phenomena in boiling fluids and to develop more accurate tools for the prediction of two-phase cooling in nuclear systems. Data and insights gathered on the fundamental heat transfer processes associated with the behaviour of single bubbles enabled us to develop and apply more capable modelling tools for engineering simulation and to obtain reliable estimates of the heat transfer rates associated with the growth and departure of steam bubbles from heated surfaces. While results so far are promising, much work is still needed in terms of development of fundamental understanding of the physical processes and application of improved modelling capabilities to industrially relevant flows.
Highlights
Understanding the boiling process is of great importance for the design and operation of light water reactors (LWRs)
Both boiling water reactors (BWRs) and pressurised water reactors (PWRs) use nucleate boiling under high-pressure subcooled liquid flow conditions to achieve high surface heat fluxes required for their operation
The breakdown in nucleate boiling heat transfer at high heat fluxes is referred to as critical heat flux (CHF), and it is imperative to be able to predict heat transfer in boiling flows to ensure normal reactor operation far from CHF conditions
Summary
Understanding the boiling process is of great importance for the design and operation of light water reactors (LWRs). A significant amount of work and effort are underway in the nuclear thermal hydraulics research community to develop simulation methods for flow boiling in LWR conditions and enable reliable analysis of boiling flows at the scale of nuclear reactor components. Recent literature on boiling phenomena and their modelling is reviewed and interpreted in the wider context of current nuclear thermal hydraulics research. The current paper is structured as follows: Section 2 provides an overview of the simulation methodology, embedded in the computational fluid dynamics (CFD) modelling framework, almost universally used for reactor analysis at the scale of the single plant components. A more capable tool for the computational analysis of boiling phenomena, the so-called interface-capturing simulation approach, is discussed, where results of the initial application of this recently developed methodology are reviewed.
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