Enhanced laminar convective heat transfer using microstructured superhydrophobic surfaces.

摘要

For many centuries, researchers have investigated the complex interactions between a solid surface and a fluid in motion relative to the surface. For many cases, the classical no slip boundary condition holds true. However, there are a subset of situations where this assumption is not valid, and slip between the surface and fluid must be considered. One such example is a micropatterned, superhydrophobic surface, which has been shown to enable slip resulting in a decrease in drag and pressure loss for both laminar and turbulent flow. The hydrodynamic effects of these surfaces have been studied in depth, but the effects on heat transfer are largely unknown. The primary goal of this research effort was to explore the effects of slip flow on laminar convective heat transfer resulting from micropatterned, superhydrophobic surfaces.;The first step toward achieving the research goal was to develop a model to study first order effects, predict the effect of slip flow on heat transfer, and design the experimental setup. The general momentum equation for Poiseuille flow was solved using modified boundary conditions consistent with slip flow, and the resulting velocity profile was input into the thermal balance equation which was numerically solved. The model assumed hydrodynamic slip but not thermal slip nor a temperature jump at the boundary, and as a result, it predicted a net increase in heat transfer performance.;For the experimental portion of the study, laminar Poiseuille flow in a parallel plate configuration with a constant temperature boundary condition at 273 K using an ice bath was studied. Four sets of copper sample plates measuring 15 cm by 3.8 cm were fabricated with different surface condition: 1) uncoated smooth, 2) hydrophobic coated smooth, 3) uncoated micropatterned, and 4) hydrophobic coated micropatterned. The micropattern was a laser machined array of 25 microm x 25 microm microridges oriented in the streamwise direction. Contact angle measurements were made on all of the test samples to ensure the coated plates were hydrophobic and the uncoated plates were not.;From the experimental results, several observations and conclusions were made. First, only the micropatterned, superhydrophobic coated sample achieved a slip state with an average slip length of 0.3 mm. Second, hydrodynamic slip was observed without the accompaniment of thermal slip since the heat transfer performance for the superhydrophobic sample was as good as or better than the baseline sample for all flow rates tested. Finally, it was concluded that micropatterned superhydrophobic surfaces reduce pressure loss and improve heat transfer as seen by the improved efficiency factor, which is the ratio between the Nusselt number and the friction loss.