Development of CHF mapping method for fin structured surface

Abstract

Several factors have been identified to affect CHF enhancement: wettability, surface roughness, porosity, surface structure and so on. In this study, we performed CHF experiments using structured surfaces to validate the parameter effects and to understand their physical meanings. Experimental results show that the CHF has a peak value as fin geometry change. A fin with 0.5mm height produces a largest CHF around 1.9MW/m2, and fins longer than 2mm reduced CHF values. To explain the results, we developed a CHF mapping method describing liquid supply-side and demand-side limits. The liquid demand-side limit is governed by heat removal capability, mainly nucleates boiling at the heating surface and calculated using the hot spot model. We consider 3 liquid supply-side limiting mechanisms limiting liquid supply to the heating surface: capillary limit in the fin structure and CCFLs in the fin structure and free volume. The capillary limit is determined by balancing capillary pressure and viscous dissipation in liquid film on the fin side. The CCFL in the structure is calculated using a Wallis-type correlation. The CCFL in the free volume limits liquid downward flow by the vapor jetting from the heating surface. The CHF map drawn for our experimental results describes the CHF trend in the water pool successfully. Also, we show that the CHF mapping method well explain the experimental trend in terms of fin length using FC-72 as a bulk liquid. As a result, it is proven that the CHF mapping is an effective way of explaining the CHF in a pool boiling.