Abstract: To enhance boiling heat transfer for promoting the efficiency of the energy system, the porous surfaces with structural gradients were developed on pure copper substrates by employing the electrochemical deposition method. In this study, honeycomb-like porous structures and hierarchical axial honeycomb gradient porous structures were fabricated using constant current single-step deposition and constant current constant voltage two-step deposition methods, respectively. Saturated pool boiling heat transfer experiments were conducted using HFE-7100 as the working fluid to investigate the influence of the gradient pore size changes on the boiling heat transfer performance of porous surfaces. The results demonstrated that the hierarchical gradient porous surface, which had a total deposition time of 60 seconds and an increased second-step deposition voltage of 3 V, showed the most significant heat transfer enhancement. The wall superheat at the boiling initiation point was 9.5 K, a 43.00% decrease compared to the smooth surface at 16.8 K. Moreover, the critical heat flux and heat transfer coefficient reached 522.02 kW/m2 and 22.76 kW/(m2?K), respectively, exhibiting with enhancements of 193.40% and 261.01% compared to the smooth surface. The hierarchical porous surface had two types of nucleation sites: internal pores and dendritic protrusions. The micropores and the internal micropores of the dendrites exhibited a wide range of pore sizes. This extensive distribution of pore sizes not only increased the density of nucleation sites and effective heat transfer area but also reduced the nucleation energy barrier. The axial pore size gradient accelerated bubble evolution, and the capillary suction force provided by the gradient porous structure and dendrites facilitated the return flow of the working fluid to the nucleation sites both horizontally and vertically, thereby enhancing the boiling heat transfer coefficient and critical heat flux of the hierarchical gradient porous surface.

Key words: hierarchical gradient porous structure, pool boiling, electrochemical deposition, heat transfer enhancement, HFE-7100

摘要： 为了提高能源系统的效率，采用电化学沉积方法在铜基底上构筑梯度结构多孔表面以强化沸腾传热，通过恒电流单步沉积法和恒电流恒电压两步沉积法制备了蜂窝状多孔和层级轴向蜂窝状梯度多孔两种结构，以HFE-7100为工质进行饱和池沸腾传热实验，研究多孔表面梯度孔径层级变化对沸腾传热性能的影响。结果表明，沉积总时长60 s，提升第二步沉积电压为3 V的层级梯度多孔表面强化传热效果最为显著，沸腾起始点的壁面过热度为9.5 K，相较光滑表面(16.8 K)下降了43.00%；临界热流密度和传热系数分别高达522.02 kW/m2和22.76 kW/(m2?K)，相较光滑表面，其临界热流密度和传热系数的强化比分别为193.40%和261.01%。层级多孔表面具有内部孔穴和枝晶凸起两类汽化核心，微孔及枝晶内部的微米孔穴具备丰富的孔径范围，在增加表面汽化核心密度和有效传热面积的同时，降低核化能垒；轴向孔径梯度变化加速气泡演化，梯度多孔结构和枝晶提供的毛细芯吸力使工质从水平和垂直方向回流至核化区域，提升了层级梯度多孔表面的沸腾传热系数和临界热流密度。

关键词: 层级梯度多孔结构, 池沸腾, 电化学沉积, 传热强化, HFE-7100