报告时间：12月7日（周三），15：00-17：00

报告地点：东三楼东汽报告厅

邀请人：王秋旺教授

报告题目：The role of dissolved gas in boiling heat transfer on biphilic surfaces。

报告人：Biao Shen博士

报告摘要：

Because of the complementary effects of surface hydrophilicity and hydrophobicity on boiling characteristics—to facilitate and restrain bubble spreading, respectively—boiling heat transfer can be greatly enhanced using heterogeneous surfaces coated with a spatially alternating wettability pattern. In this work, we show experimentally that a vast majority of the improved heat transfer is actually attributable to an unusually strong gas aggregation near the hydrophobic surface. Equivalent to extremely gassy boiling, the profound impact of the persistent gas enrichment on boiling includes significant reductions of the incipient degree of superheating to levels even below the nominal saturation temperature. Moreover, by differential degassing efforts, it is revealed that bubble dynamics on the same mixed-wettability surface differ fundamentally with and without dissolved gas, which are also captured by numerical simulations of bubble evolution in a two-component system using the diffuse-interface method. Specifically, the results show that dependent on the solute gas concentrations and the strength of the solute-solvent molecular interaction (gas solubility, bubble behavior diverges between the sustained bubble growth (leading to actual pinch-off) and the eventual collapse following brief periods of dynamic equilibrium between evaporation and condensation. Interestingly, the growing bubble, while being firmly attached to the hydrophobic subsurface, is found to be surrounded by strong Marangoni flow, which is driven by pronounced surface tension stress imbalance.

报告人简介：Biao Shen博士，日本九州大学无碳能源国际研究中心机械工程和热物理工程分部博士后研究员，主要研究兴趣为相变换热，沸腾现象等。

报告题目：Effect of micro- and nano-structural effects in condensation heat transfer

报告人：Daniel Orejon博士

报告摘要：

The effect of micro-texture heterogeneities, spatial distribution, and surface wettability on wetting and on liquid propagation have been extensively addressed, nonetheless the role of nano-scale roughness has received lesser attention. In this talk I will address first the effect of micro-pillar density and then that of nano-roughness orientation on condensation dynamics. On a completely hydrophilic micropillared configuration the spacing between pillars is found to have a strong impact on the dynamics of condensation and on the heat transfer performance. Micropillared surfaces with short spacing between pillars allow for the continuous liquid propagation between pillars and condensation takes place in a mixed dropwise and filmwise condensation fashion. On the other hand, in the case of large spacing, the condensate is able to grow above the pillars and the final condensation behavior is that of filmwise condensation. Next, for the same micropillar geometry, different condensation mechanisms are observed for different orientations of the nano-roughness imprinted on the pillars. Longitudinally oriented nano-ridges (perpendicular to liquid propagation) are found to suppress the liquid propagation between pillars whereas circumferentially oriented ones (parallel to liquid propagation) facilitate the propagation of the condensate. As a result of the liquid propagation suppression, the liquid interface is constrained in an upwards direction rising above the micropillars’ height eventually flooding the substrate, i.e., filmwise condensation. On the contrary, in the case of circumferentially aligned nano-ridges the condensate does not rise above the micropillars. In this latter case, condensation takes place as filmwise condensation with active dropwise condensation at the micropillars’ tops. Differences observed in liquid propagation and on the final condensation behavior are solely induced by the nano-roughenss orientation. In addition, a thermal resistance based model is proposed to account for the different theoretical heat transfer performance depending on the micropillar density and on the nano-roughness orientation. We point out to the importance to account for nano-scale roughness orientation for the effective design of surfaces for condensation and microfluidics applications.

报告人简介：Daniel Orejon博士，日本九州大学无碳能源国际研究中心机械工程和热物理工程分部博士后研究员，主要研究兴趣为纳米与微纳米尺度的换热机理。