| Drag reduction technology is very important to improve transport efficiency and reduce transport energy consumption.Bubble lubrication is a promising drag reduction method.Due to its advantages of safe operation,low cost,environmental protection and high efficiency,this technology has attracted much attention.Among them,the surface drag reduction of superhydrophobic bubble mattress is particularly prominent,and its drag reduction mechanism is derived from the slip effect of gasliquid interface.In this paper,the slip theory of fluid interface is systematically summarized.The slip effect and resistance characteristics of continuous fluid and single droplet moving over the surface of superhydrophobic bubble mattress are studied by simulation and experiment.Firstly,the slip theory is systematically reviewed and summarized from the micro to macro perspective.From the microscopic point of view,the phenomenon that the slip flow of liquid molecules does not exist is clarified.The internal slip phenomenon is a molecular level phenomenon suitable for discontinuous fluids,and its size is related to the average free path of molecules.At the macro level,the influence of complex surface on fluid slip is comprehensively considered,and the definition of apparent fluid slip and slip length is proposed.Three measurement methods of apparent slip length are summarized,which are fluid flow tracking method,liquid extrusion method and pressure drop calculation method.Among them,the pressure drop calculation method is suitable for the calculation of drag reduction efficiency of fluid in the third chapter.Secondly,the VOF model in Fluent is used to simulate the two-dimensional unsteady flow of continuous fluid through the bubble mattress microchannel.It is found that when the inclination angle of the gas-liquid interface is 70°,it will hinder the fluid movement,and the inclination angle that is most conducive to the drag reduction of the bubble mattress surface is between 10° and 20°.In addition,the larger the porosity of the bubble mattress surface,the higher the drag reduction efficiency,while the aspect ratio of the micro-nano structure has little effect on drag reduction.However,the larger the porosity and the smaller the aspect ratio of the micro-nano structure,the instability of the gas film will be caused.In different types of models,the inverted triangle model has the best drag reduction effect,followed by the rectangular model,and the trapezoidal model has the worst drag reduction effect.In the inverted triangle model,the airflow is fully extended in the transverse space of the groove,and the occurrence of vortex motion in the gas phase is limited by the transverse development of the groove.We point out that the difference in drag reduction efficiency is essentially limited by the gas phase eddy current deviation caused by different micro-nano structure spaces,forming different relative slip velocities of gas-liquid two phases.In addition,with the increase of shear stress,the gas-liquid interface will experience dissipation and deformation,and the bubble mattress surface will experience four stages:stable gas film,phase interface deformation,gas phase connection and gas phase loss.Finally,superhydrophobic black silicon nanoarrays were prepared by two-step reactive ion etching technology,and the rolling experiment of droplets on its inclined surface was carried out.The theoretical resistance model of bubble mattress was established for analysis.It is found that the droplet obtains a larger inertial force on the surface of the bubble mattress with a larger inclination angle,but the droplet has a larger wetting radius when rolling on a large inclination angle,which increases the rolling resistance.At the same time,larger droplets are subjected to greater rolling resistance on the same inclined surface.The droplet rolling acceleration increases first and then decreases with the increase of droplet size.There is a critical droplet size that makes the droplet roll down the fastest.This is because the large droplet provides greater inertia force but the rolling resistance also increases accordingly.Under the illumination condition,the droplet rolling speed decreases obviously.As the illumination intensity increases,the rolling resistance of the droplet increases.The 4.26 sun illumination intensity increases by 68.9%compared with the rolling resistance without illumination.It is observed by infrared thermal imager that there is an obvious temperature gradient in the droplets under illumination,and the droplets tend to be spherical.At this time,the Marangoni force formed by the obvious surface tension gradient at the gas-liquid interface caused by the uneven heating of the droplets increases the rolling resistance of the droplets.Superhydrophobic bubble mattress drag reduction surface and related technologies have been applied to aircraft,wind turbines,power lines,photovoltaic cells,heat exchangers,ice slurry generators and many other fields.The instability of the gas layer on the surface of the bubble mattress and how to repair the gas-liquid interface are worth studying.It is necessary to further improve the theoretical research of the slip effect and provide theoretical support and guidance for engineering applications in related fields. |
