Investigation Of Pool Boiling Heat Transfer On Pillar-Structured Surfaces With Heterogeneous Wettability

Boiling heat transfer is an efficient heat transfer method,which can utilize the latent heat of vaporization in the liquid phase transition process to rapidly absorb the redundant heat on the heating surface.Therefore,boiling heat transfer has great application potential in the field of heat dissipation of high-heat-flux devices.In this paper,the heat transfer performance in deionized（DI）water of pillar-structured surfaces with different wettability patterns and structural parameters was experimentally investigated.Besides,the synergistic boiling enhancement mechanism of wettability and pillar-structure were analyzed based on the bubble dynamics.The main research contents and results are as follows:（1）Three groups of pillar-structured surfaces were fabricated by a wire-cutting method.Compared with the plain surface,the heat transfer performance of the pillar-structured surfaces has been greatly enhanced.To be specific,the critical heat flux（CHF）and heat transfer coefficient（HTC）of the pillar-structured surfaces were significantly improved as compared with that of the plain surface.Besides,the wall superheats on the onset nucleate boiling（ONB）of the pillar-structured surfaces were reduced as compared with that of the plain surface.（2）Two homogeneous wettability patterns were prepared on the pillar-structured surfaces by chemical modification,i.e.,nearly superhydrophilic and hydrophobic patterns.The experimental results show that the boiling performance of the nearly superhydrophilic surface is much worse at low and moderate heat fluxes,but it is considerably enhanced in the boiling regime at high heat fluxes as compared with that of the corresponding base surface.Conversely,boiling performance of the hydrophobic surface is much better than that of the corresponding base surface in the boiling regime with low heat fluxes,but it is significantly deteriorated at moderate and high heat fluxes.（3）A heterogeneous pattern with hydrophobic bottom and hydrophilic pillar top has been prepared.The experimental results show that the modified surface yields a leftward shift of the boiling curve and an upward shift of the HTC curve,but causes a decrease of the CHF as compared with the corresponding base surface.（4）A heterogeneous pattern with hydrophilic bottom and hydrophobic pillar top has been prepared.The experimental results show that when the hydrophobic area on the pillar top is small,the modified surface not only achieves significant augmentation in the number of active nucleate sites but also owns faster bubble departure frequency at high heat fluxes.Consequently,the modified surface can significantly enhance the HTC and provides a higher CHF in comparison with the corresponding base surface.However,when the hydrophobic area at the top of the column is too large,the excessive bubble coalescence on the modified surface would result in the deterioration of boiling performance at high heat fluxes.（5）A series of heterogeneous fractal surfaces are prepared based on the pillar-top modified surfaces,namely fractal modified surfaces.The experimental results show that the fractal modified surface can effectively regulate the position of bubble nucleation,inhibiting the excessive bubble merge and facilitating the bubble departure.As a result,the heat transfer performance of fractal modified surface can be further improved.Compared with the plain surface,the CHF and HTC_max of the fractal modified surfaces can be increased by 89.5%and 141.9%,respectively.