Study On Inner Wall Wettability And Flow Property In Liquid Pipe

With the development of modern testing methods and analytical techniques,some recent studies have found that the slip of fluid may appear on a solid wall,which is directly related to the surface wettability.Therefore,the no-slip assumption of solid-liquid interface in classical fluid mechanics is hardly valid in some practical cases.Actually,the boundary slip can change the flow resistance of fluid,and the surface wettability is very important for the boundary slip.However,the effect of surface wettability on flow resistance and slip property is still controversial at present.For this purpose,around the pipe and liquid types,the inner connection between surface wettability and liquid flow property has been systematically investigated by experimental research,numerical simulation and theoretical analysis in this paper.It has an important theoretical and practical significance to reach the new process development of drag reduction and transportation-increasing for liquid pipe through the change of pipe surface wettability.The main works and results are as follows:(1)The pressure drops,frictional coefficients and poiseuille numbers of tap water flow in five pipes,ethylene glycol flow in four pipes,#26 white oil flow in four pipes and four liquid flows in a polytetrafluoroethylene pipe were tested and analyzed using the contact angle meter and the self-developed indoor circulation pipeline.The effects of surface wettability on flow resistance characteristic were systematically studied.The researches showed that wall wettability had a certain effect on the frictional coefficient of liquid in different flow regions.The frictional coefficient decreased with the increase of contact angle and the decrease of sliding angle.Meanwhile,the frictional coefficient in laminar flow decreased more than that in turbulent flow.The magnitude of the effect of surface wettability on frictional coefficient from the point of pipe material was greater than that from the point of liquid type.(2)The quantitative relationships between frictional coefficient and surface wettability in different flow regions were established by the dimension and regression analyses.The verification of the corresponding experimental data and the field operation data showed that the modified frictional coefficient formulas had high prediction accuracy.To avoid changing the original pipe material and the transported liquid,a prewetting method was put forward.The effects of water prewetting on the flow resistance of white oil in 304 steel pipe and plexiglass pipe were studied experimentally.Results showed that the pressure drop and frictional coefficient of white oil flowing in 304 steel pipe prewetted by water were less than that in the original steel pipe.However,the pressure drop and frictional coefficient in plexiglass pipe prewetted by water were greater than that in the original plexiglass pipe.(3)The slip parameters of water,ethylene glycol and#26 white oil flowing in different pipes were calculated by the slip boundary condition respectively,which referred to the laminar flow resistance of liquid in a fully developed section of a no-slip pipe under the constant pressure drop condition.The influence of surface wettability on the flow slip characteristics and the mechanism of drag reduction were discussed.The study found that at a constant average velocity of liquid,wall shear stress decreased with the increase of contact angle,which increases the sliding velocity and sliding length,resulting in the lower flow resistance of liquid.The negative slip phenomenon existed at low-speed flow for part of liquid.The slip length did not tend to be constant until the average velocity increased to a certain value.(4)The flow property of single-phase liquid in a circular pipe at different flow regions were simulated with FLUENT by directly setting the wall slip boundary condition.The effects of apparent contact angle and pipe diameter on liquid slip flow were studied.The researches showed that the flow resistance of the same liquid in the same pipe can be reduced by increasing the apparent contact angle of the solid-liquid interface.The greater the increase of contact angle and the smaller pipe diameter,the more obvious the effect of surface wettability on the pressure drop of liquid.(5)Using the contact angle of solid-liquid interface as evaluation index,the main factors of surface wettability were studied by single factor test and uniform design test.A new prediction model of contact angle was proposed and verified.The significance of each factor affecting the surface wettability was analyzed.Results showed that the smoother material surface could not always lead to the smaller frictional coefficient.The contact angle decreased with the increase of surface energy of a solid and increased with the increase of surface tension of liquid.The microscopic geometry and the element type of a material surface can improve the wettability.The surface tension of liquid was the most important index,the surface energy of a solid was the second and the surface roughness was the least.