气-液界面位置对微凹槽结构内两相流场和减阻特性的影响研究

EFFECTS OF GAS-LIQUID INTERFACE POSITION ON THE TWO-PHASE FLOW FIELDS AND DRAG REDUCTION IN MICRO-GROOVES

在超疏水表面增加微凹槽结构,有利于在表面张力作用下留存部分空气形成气-液界面,从而大大降低流动阻力,在流动减阻方面展示了巨大的应用潜力.然而,由于流体剪切、空气溶解等影响,微凹槽内的空气容易发生损耗,导致气-液界面位置向凹槽底部移动,进而降低减阻效果.为了揭示微凹槽内空气损耗后气-液界面位置对流动减阻效果的影响,开展了显微高速摄影实验和计算流体动力学数值模拟,研究了微凹槽内不同气-液界面位置对两相流场特性的影响,并分析了等效滑移长度和减阻率的变化规律.结果表明,随着气-液界面位置的下移,气-液界面上方的流体从附着流转变为分离涡流,空气内部的涡流方向也从顺时针转变为逆时针.形成分离涡的临界空气损耗率为h_(c)^(*)=0.3~0.4,减阻率从12%降低到5%.界面下降后雷诺数Re、微通道宽度W和凹槽长度占比δ对微凹槽减阻性能也有重要影响,研究结果有助于深化对超疏水表面微凹槽两相流动减阻问题的认识,为微凹槽结构设计及应用提供重要的理论指导.

The addition of micro-groove structures on superhydrophobic surfaces is conducive to the formation of gasliquid interface under the action of surface tension,which greatly reduces the flow resistance and shows great application potential in flow drag reduction.However,due to the effects of fluid shear and air dissolution,the air in the micro-groove is prone to loss,resulting in a shift in the position of the air-liquid interface toward the bottom of the groove,and thereby reducing the drag reduction effect.To reveal the influence of the position of air-liquid interface on the drag reduction after the air loss in the micro-groove,high-speed microscopic photography experiments and computational fluid dynamics numerical simulations were carried out to study the influence of air-liquid interface positions on the two-phase flow field characteristics,and the variations of the equivalent slip length and the drag reduction rate were also analyzed.The results show that the fluid above the gas-liquid interface changes from attached flow to separation vortex as the gasliquid interface moves downward,and the direction of the vortex inside the air also changes from clockwise to counterclockwise.The critical air loss rate for the formation of the separation vortex is h_(c)^(*)=0.3~0.4,and the drag reduction rate is reduced from 12%to 5%.The Reynolds number Re,microchannel width W and groove length proportionδalso have important effects on the drag reduction performance of microgrooves after the interface descent.The results of this study could help to deepen the understanding of the drag reduction of two-phase flow in micro-grooves on superhydrophobic surfaces,and provide important theoretical guidance for the design and application of micro-groove structures.