Hi'/> Sustained drag reduction and thermo-hydraulic performance enhancement in textured hydrophobic microchannels
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Sustained drag reduction and thermo-hydraulic performance enhancement in textured hydrophobic microchannels

机译：结构化疏水微通道中的持续减阻和热工液压性能增强

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Highlights•A method to sustain drag reduction in textured hydrophobic microchannels is proposed.•Drag reduction is sustained by maintaining trapped air bubbles on the textured surface.•Air bubbles are maintained by controlling the temperature of water near the surface.•Presence of air bubbles reduce drag, but lower the thermal transport performance.•Thermo-hydraulicperformanceis enhanced whenbubblesare flushwith the surface.AbstractDrag reduction obtained on flow over textured hydrophobic surfaces has been ascribed to the presence of entrapped air within the surface micro-texture. To sustain the drag reduction, it is important that the entrapped air be maintained on the surface. However, the entrapped air bubbles tend to shrink with time and finally disappear, causing the drag reduction also to reduce and eventually vanish. Recent research shows that by controlling the absolute pressure of water, it is possible to sustain the entrapped air bubbles on the surface and hence the drag reduction for extended periods of time. In this paper, we explore the possibility of sustaining the entrapped air by varying the absolute temperature of water in the vicinity of the textured surface. For this, the textured surface is externally heated and the evolution in the size of trapped air bubbles with time is observed. Simultaneous pressure and temperature measurements are made along with the visualization, to study the concomitant effects on drag and heat transfer. We find that, varying the absolute temperature influences the trapped air bubble dynamics appreciably, which in turn affects the measured pressure drop across the channel. By varying the external heat input, it was found that the trapped air bubbles can be maintained on the surface for prolonged periods of time, at an optimum size suitable for drag reduction, such that sustained and maximized drag reduction can be achieved. The presence of trapped air bubbles is found to inhibit the heat transfer across the surface. However, when the pressure drop reduction achieved due to the presence of air bubbles is significant enough, the combined thermo-hydraulic performance is found to be enhanced. The results, not only provides important inputs towards achieving sustained drag reduction from textured hydrophobic surfaces, but also ascertains the feasibility of using such surfaces in micro-scale heat transfer applications.
机译： 突出显示 • 提出了一种在带纹理的疏水性微通道中维持减阻的方法。 • 减少阻力的方法是将气泡保留在气泡上 • 通过控制表面附近的水温来保持气泡。 • 气泡的存在降低了博士，但降低了热传输性能。 • 当气泡与表面齐平时，热工液压性能会增强。 < / ce：abstract-sec> 摘要 在带纹理的疏水性表面流动获得的减阻作用归因于残留在表面微纹理中的空气。为了维持减阻作用，重要的是将夹带的空气保持在表面上。但是，夹带的气泡会随着时间的流逝而逐渐收缩并最终消失，从而导致阻力减小也减小并最终消失。最近的研究表明，通过控制水的绝对压力，可以在表面上保持截留的气泡，从而在更长的时间内减少阻力。在本文中，我们探讨了通过改变纹理表面附近水的绝对温度来维持残留空气的可能性。为此，从外部加热纹理表面，并观察到滞留气泡的尺寸随时间的变化。同时进行可视化的压力和温度测量，以研究对阻力和热传递的伴随影响。我们发现，改变绝对温度会明显影响滞留的气泡动力学，进而影响跨通道测得的压降。通过改变外部热量输入，发现可以以适合于减阻的最佳尺寸将滞留的气泡长时间保持在表面上，从而可以实现持续且最大化的减阻。发现夹带的气泡的存在抑制了表面上的热传递。然而，当由于气泡的存在而实现的压降降低足够显着时，发现组合的热工液压性能得到增强。该结果不仅为实现从带纹理的疏水表面上实现持续减阻提供了重要的投入，而且还确定了在微尺度传热应用中使用此类表面的可行性。

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