This study integrates superhydrophilic/superhydrophobic modification with micro-pillar structure design in an innovative way to address the limitations of conventional flat copper surfaces in boiling heat transfer, and systematically explores the enhanced effect of this combined strategy on boiling performance and its application in gravity heat pipes.
Two types of copper-based surfaces, flat structure and micro-pillar structure, were fabricated, each with dimensions of 30 mm×30 mm×3 mm. The micro-pillar surface had uniformly distributed pillars. Each individual pillar had a height of 80 μm and a width of 100 μm, and the gap between adjacent pillars was 60 μm. Oxidation and chemical treatments were applied to achieve diverse wettability: For superhydrophilic surfaces (both flat and micro-pillar), copper substrates were immersed in an etching solution containing 2.5 mol/L potassium hydroxide (KOH) and 0.065 mol/L potassium persulfate (K2S2O8) at 70 ℃ for 30 minutes, followed by heating at 180 ℃ for another 30 minutes to form a hydrophilic oxide layer. For superhydrophobic surfaces (both flat and micro-pillar), the pre-prepared superhydrophilic surfaces were soaked in a 0.002 5 mol/L C18H38S solution at 70 ℃ for 30 minutes, then cleaned with ethanol to remove residues and air-dried to form a hydrophobic film. A total of six surfaces with different structure-wettability combinations were obtained, and their static contact angles were measured with a Data Physics-OCA30 contact angle meter according to the sessile drop method to confirm successful wettability adjustment.
A visualized pool boiling test platform consisting of three core modules was constructed. The heat source module included a portable power supply and a 30 mm×30 mm heating block to control heat flux, with thermal conductive grease and insulation materials used to minimize heat loss. The temperature module employed six K-type thermocouples: two were used to measure the average surface temperature, two were embedded in the heating block 3 mm vertically from the surface thermocouples, and two were used to monitor the temperature of saturated deionized water. The image module included a quartz glass dish (forming a boiling chamber with the test surface), a high-speed camera (608×500 pixels resolution, 500 frames per second frame rate), an LED light source for uniform illumination, and Viewer software for capturing bubble dynamics. Heat flux (q) was calculated via Fourier's law, wall temperature (Tw) via surface temperature correction, wall superheat (ΔT) as the difference between Tw and saturated water temperature, and heat transfer coefficient (HTC, h) as the ratio of q to ΔT. At a fixed heat flux of 1 350 kW/m2, bubble behaviors (detachment diameter, frequency, nucleation site density) were analyzed with image processing software.
The superhydrophilic micro-pillar surface (with optimal boiling performance) was integrated into gravity heat pipes (evaporator: 30 mm, adiabatic section: 40 mm, condensation section: 30 mm in axial length). A heat pipe test system was built, with deionized water as the working fluid, testing under 4%/12% filling ratios and 15 W/30 W/45 W/60 W heating powers, and cooling the condensation section with a 4-5 ℃ cold air jet. Thermal resistance (R) was calculated as the ratio of the evaporation-condensation temperature difference to heating power.
Results showed that compared with the original flat surface, the original micro-pillar surface increased critical heat flux (CHF) by 12.5% and maximum heat transfer coefficient (HTC) by 83.5%; the hydrophobic flat surface improved CHF by 23.7% relative to the original flat surface; the superhydrophilic micro-pillar surface further enhanced CHF by 28.5% and maximum HTC by 193.6% compared with the original micro-pillar surface, attributed to moderate bubble size (average 2.1 mm) and high detachment frequency (≈25 Hz). In gravity heat pipes, the modified surface reduced evaporator thermal resistance by up to 40.7% (20.2% at 4% filling ratio, 24.9% at 12% filling ratio vs. conventional heat pipes).
This study reveals a new synergistic mechanism: micro-pillar structures increase nucleation sites and the heat transfer area, while superhydrophilicity accelerates liquid replenishment after bubble detachment, jointly optimizing bubble dynamics. This combined modification strategy provides a novel approach for enhancing boiling heat transfer and improving gravity heat pipe performance in high-heat-flux scenarios.
Key words
wetability /
surface modification /
pool boiling /
boiling heat transfer /
micro-pillar structure /
gravity heat pipe
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Funding
The National Natural Science Foundation of China (51905275); Youth Natural Science Foundation of Jiangsu Province (BK20190752)
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