Critical heat flux on heterogeneous fractal surfaces with micro-pin-fins in pool boiling – Part II: Model establishment and analysis

Abstract

The heterogeneous fractal surfaces can achieve higher critical heat flux (CHF) than that with the uniformly distributed surfaces, which is reported in Part Ⅰ. In order to explain the phenomenon and understand the trigger mechanism of CHF, a theoretical model of CHF for the heterogeneous structure surfaces is proposed based on the bubble coalescence caused by the hydrodynamic instability and the wall dryout induced by the liquid supply impediment. The continuous expansion and coalescence of the dry spots lead to the formation of the vapor blanket covering the whole surface, which triggers the occurrence of critical phenomenon. The CHF is obtained according to the arrangement of the vapor columns on the heating surface. Both the distribution of the heterogeneous structure and the subcooling of liquid affect the bubble behaviors and liquid supply, which result in an optimal structure distribution to reach the highest CHF under different subcoolings. Compared with the uniform distribution, the width of smooth region surrounding the bubble increases with the increase of heat flux and bubble diameter on the fractal structure, which can effectively provide liquid replenishment at high heat fluxes and achieve higher CHF. The CHFs calculated from this model agree well with the experimental results, and the vapor column arrangement is consistent with the bubble distribution observed from the experimental phenomenon. Moreover, the model can also be used to calculate the CHF of the uniformly distributed heterogeneous surfaces and the homogeneous microstructure surfaces with good precision, which verifies the correctness and extensive applicability of the model.