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 Choi and No [1] developed a dry patch model based on the observation, which is an extension version of the dry spot model. Experimental observation showed that the formation of the unquenchable dry patchs are a main source for CHF initiation. In the dry patch model we proposed thermal and hydraulic criteria for the onset of the unquenchable dry patch at the high heat flux: an unquenchable dry patch with a critical size can be generated when the following two criteria are satisfied. As a hydraulic criterion, we assume that the coalescence of the whole dry spots takes place generating a dry patch if all of them just contacts each other. As a thermal criterion, we consider the temperature of the dry patch. An unquenchable dry patch will be formed if its peripheral temperature reaches Leidenfrost temperature so that it may not be rewetted even with bubble detachment. The critical size of the dry patch is obtained by CFD simulation so that its peripheral temperature may be the same as Leidenfrost temperature. Wall dry area fraction can be obtained by calculating the probability of the formation of the unquencable dry patchs satisfying both criteria for its critical size. We demonstrated that the dry patch model successfully predicted experimental CHF data obtained in pool boiling and forced convective flow boiling of water.