(A) CHF mapping method based on dry spot model for finned surfaces

Abstract

The concern about nuclear safety related to the severe accident has been sharply raised after the Fukushima accident. Among various safety issues, the passive safety system and corium cooling are the mostly emerged ones. As a critical heat flux(CHF) is a main restricting factor of the heat removal capacities of both kinds of safety systems, a number of researches have been performed to understand and improve the CHF.

In order to enhance the CHF, various methods of surface treatment have been being actively studied for decades. Generally, degree of CHF enhancement by surface treatment is determined by a number of factors, such as surface roughness, porosity, structure shape, structure spacing, etc. Most of them are known to affect the bubble dynamics as well as dry spot dynamics for improvement of the heat removal ability. To discover the CHF enhance-ment mechanism by surface structure, we investigated the individual parametric effect on structured-surface CHF and developed the CHF mapping method.

We performed CHF experiments using structured surfaces in the water pool to validate the parameter effects and to understand their physical meanings. Experimental results showed that the CHF has a peak value as the fin geometry changes. Fins with height of 0.5 mm produced the largest CHF, 1.7 $MW/m^2$, and fins longer than 2 mm reduced the CHF values.

To explain the results, the CHF mapping method was developed by describing the liquid supply-side and demand-side limits. The liquid demand-side limit, governed by the nucleate boiling heat removal capability, is calculated using the dry spot model considering the extended surface and contact angle. In case of the effect of surface extension by fin structures, it is analyzed by relating fin efficiency and a local heat transfer coefficient on the fin surface with the CHF. The fin efficiency through the fin structure is calculated with the thermal conductivity of heaters, structure size, spacing, fin length and local heat transfer coefficients. We found out that contact angle is changed with the variation of fin geometry through its measurement. The effect of its change on the nucleation site density is considered in CHF calculation by the dry spot model. We considered three liquid supply-side limits which restrict the liquid supply to the heating surface: capillary limit, and counter-current flow limitations (CCFLs) in the structure and in the free volume. The capillary limit is determined by balancing the capillary pressure and viscous dissipation in the liquid film on the fin side. The CCFL in the structure is calculated using the Wallis correlation and the CCFL in the free volume limits the liquid downward flow by the vapor jetting from the heating surface. The CHF map for our experimental results successfully describes the CHF trend of the structured surfaces in the water pool. Additionally, the experiment performed in the FC-72 pool is well matched with the CHF map. As a result, we concluded that the CHF mapping method is an effective means of explaining CHF for the heater of fin structure in pool boiling.

It has been known through experimental observation that a dry patch plays an important role in CHF initiation. We developed a dry patch model based on the observation as well as the dry spot model. There exist quenchable and unquenchable dry patches at high wall heat flux. Experimental observation showed that the formation of the unquenchable dry patchs is 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 detach-ment. 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 succesfully predicted experimental CHF data obtained in pool boiling and forced convective flow boiling of water.