Observation-based CHF model development: Dry spot - Dry patch models

Abstract

Recently advanced visualization techniques such as total reflection and IR methods have been applied to observe thermally and hydraulically the CHF mechanism at the surface as well as macroscopic bubble dynamics simultaneously. Based on observation Ha and No (1998a, b, 2000), and Choi et al. (2016) developed a Dry spot model and a Dry patch model, which is an extension version of the dry spot model. Experimental observations clearly showed that the production of unquenchable dry patches mainly contributes to the initiation of CHF. Based on the above observatory conclusions and extensive literature survey, we discussed the physical basis of the Dry Spot/Dry Patch model. In the dry spot model, we assume that the dry spot can become unquenchable one when it is surrounded by 5 neighboring dry spots based on the geometrical consideration, which was confirmed by validation process. In the dry patch model, both criteria from the hydraulic and thermal considerations were proposed to estimate the critical size of unquenchable dry patch at CHF. The wall dry area fraction can be calculated by applying a probabilistic concept for the creation of the unquenchable dry patch. We showed that the Dry Spot/Dry Patch model can be extended into CHF predictions in both pool boiling and forced convective boiling. For transition boiling, we proposed models to represent two suppression mechanisms deactivating potential nucleation sites: nucleation site deactivation and non-availability mechanism. For the physical model of the nucleation site deactivation mechanism we introduced the spatial randomness concept. For the non-availability mechanism we proposed the multi-stage calculation method which considers the sequential bubble activation and their interaction. Then, we showed that the dry spot model modified with the current transition model well predicted the whole boiling curve including CHF, nucleate boiling, transition boiling, and film boiling.