Critical heat flux on heterogeneous fractal surfaces with micro-pin-fins in pool boiling Part I: The effects of distribution and subcooling

Heterogeneous surfaces can offer superior heat transfer performance by using characteristics of different structures compared with the homogeneous surfaces. The micro-pin-finned structure facilitating bubble nucleation and the smooth area as a good liquid supply pathway constitute a heterogeneous surface for further enhancing boiling heat transfer. Inspired by the fractal geometry's enhancement for the heat and mass transfer in nature, the created surface is designed into a fractal pattern. In this paper, five fractal structures F0–F4 with the same micro-pin-finned area ratio of 49% were designed. The smooth surface S and homogeneous micro-pin–fin surface PF30-60 were also used as comparisons. Pool boiling experiments using FC-72 were conducted under different subcoolings of 0 K, 15 K, 25 K and bubble behaviors were observed with a high-speed camera. The results indicated that under saturated conditions, the critical heat flux (CHF) of fractal surfaces increases first, and then decreases with the increase of fractal dimension. Besides, with the increase of subcooling, the fractal dimension with the highest CHF surface also increases. These results are explained from the liquid supply and the dry spots on the heating surface based on the bubble behaviors. There exists an optimal fractal dimension that can effectively impede the bubble coalescence and promote the liquid supply to achieve the highest CHF. The subcooling has also influence on CHF by affecting the bubble size.