Development and assessment of a new CHF mechanistic model for subcooled and low quality flow boiling

Under subcooled and low quality flow boiling conditions, a general critical heat flux (CHF) mechanistic model is essential to the thermal-hydraulic design and safety analysis, especially at the conceptual design stage of a new pressurized water reactor (PWR). In this study, an overview of the physical mechanisms behind the most common CHF models was carried out and the additional effects to be considered in a new CHF model were indicated. Then, based on the mechanism of superheated liquid layer depletion, the net mass transfer from the bulk liquid to the superheated liquid layer caused by turbulent velocity fluctuations of flowing bubbles was determined and a new calculation method of superheated liquid layer thickness was developed. After matching with other constitutive equations, a novel CHF mechanistic model for subcooled and low quality flow boiling conditions has been developed. Furthermore, the present CHF mechanistic model was validated with 5 x 5 rod bundle CHF experimental data of Nuclear Power Institute of China (NPIC) and compared with other CHF models and CF-DRW correlation. Finally, the distribution of M/P was evaluated and the advantage of the present CHF mechanistic model was discussed. The results show that the present CHF model performs as well as the CF-DRW correlation and is superior to other well established CHF models without any correction factors or empirical constants from experimental data. (C) 2020 Elsevier Ltd. All rights reserved.