Turbulence Drag Reduction Of Combined Effects Of Triangular Microgrooves And Additives In A Duct Flow

The friction of the fluid on the solid surface accounts for a large proportion of the total resistance of fluid transport.Reducing the turbulence resistance will have a significant impact on energy conservation and consumption reduction.In this paper,a mathematical model for the resistance calculation of combined drag reduction of triangular microgrooves and additives was established by using the method of combining theoretical analysis and rheological measurement.The drag test measurement and Particle Image Velocimetry(PIV)measurement was used to study the drag reduction performance and drag reduction mechanism of wall turbulence drag reduction of microstructures combined with.The main contents and conclusions are as follows.Drag reduction performance of two types of microgrooves in triangle and rectangle shapes,cetyltrimethylammonium chloride(CTAC)surfactant and polyacrylamide(PAM)polymer additives,and triangular microgrooves combined with two additives was studied at room temperature.Triangular and rectangular microgrooves have a certain drag reduction effect on wall turbulence,and the drag reduction effect of microgrooves is related to the size of the grooves as well as flow velocity.In addition,the drag reduction rate of the triangular microgrooves is significantly higher than that of the rectangular microgrooves,and the effective dimensionless microgrooves width s+ range of drag reduction is relatively wide.The greater the aspect ratio h/s of the grooves,the better the drag reduction effect,and the drag reduction rate will first increase and then decrease as s+increases.The two additives of CTAC surfactant and PAM polymer also have obvious drag reduction effect on turbulence.The drag reduction effect is related to the concentration of the additive in the drag reduction fluid.The higher the concentration is,the better the drag reduction rate.The drag reduction rate of the two additives is also related to the Reynolds number.The drag reduction rate first increases with the increase of the Reynolds number,and then decreases rapidly after reaching the maximum value.Microgrooves have a gain effect on the drag reduction performance of CTAC surfactants,which affected by the size of the microgrooves and the Reynolds number.The microgrooves have a gain effect on the drag reduction performance of PAM polymers,which is closely related to the groove apex angle and the Reynolds number.The triangular microgrooves with a larger groove apex angle have a gain effect on the drag reduction performance of the PAM drag reduction fluid at low and medium Reynolds numbers.This gain effect first strengthens and then decreases with the increase of Reynolds number,and appears "weakening" effect under high Reynolds number.The mathematical model for the calculation of the drag reduction flow resistance of the triangular microgrooves was obtained by the method of theoretical analysis.The rheology test was used to measure the rheology of the additive drag reduction fluid,and the least square method was used to fit the rheological test results to return to the rheological model of the additive drag reduction fluid.On this basis,a mathematical model for calculating the resistance of the microgrooves and additives combined drag reduction was established,and the mathematical model was solved.The calculated results are in good agreement with the experimental results.The average flow field of the triangular microgrooves,additives of CTAC and PAM and the triangular microgrooves combined with the additives was measured.The flow field characteristics including the dimensionless flow direction average velocity distribution,Reynolds shear stress,turbulence intensity and vorticity intensity are analyzed.The existence of the triangular microgrooves increases the thickness of the viscous layer,which in turn causes the buffer layer and the logarithmic law layer to move upward,resulting in the dimensionless average flow velocity of the microgrooved wall surface being greater than that of the smooth wall in both the buffer layer and the logarithmic layer.CTAC and PAM additives can make the average velocity of the logarithmic layer near the wall of wall turbulence move upward,and the gradient is steeper,while the triangular wall micro groove can make this change of additive drag reduction more prominent.The triangular microgrooves,additives and the combination of the two have a certain inhibitory effect on the normal turbulence intensity in the near-wall area,but the inhibitory effect on the turbulence intensity in the flow direction is not obvious.Triangular microgrooves,additives and the combination of the two can greatly reduce the Reynolds shear stress.Especially when a higher concentration of CTAC surfactant reduces drag alone or combined with the triangular wall micro-groove,its Reynolds shear stress close to 0.The triangular microgrooves,additives and the combined of the two can inhibit the motion of the vortex near the wall and reduce the average vorticity of the near-wall region of the drag reduction flow,but have little effect on the vorticity intensity of the entire flow area.The instantaneous flow field of the triangular microgrooves,CTAC and PAM additives,and the friction reducing flow combined with the triangular microgrooves and additives were measured,and parameters including instantaneous Reynolds shear stress,turbulence intensity,and vorticity intensity were analyzed.The triangular wall micro-grooves and additives can reduce the instantaneous normal phase turbulence intensity,Reynolds shear stress and spanwise vorticity of the flow field The existence of triangular wall micro-grooves can enhance the instantaneous normal direction of the additive convection field Inhibition of velocity pulsation,Reynolds shear stress and spanwise vortex motion.According to the drag reduction characteristics,flow field characteristics and rheology of drag reduction fluid,the drag reduction mechanism of triangular wall micro grooves,additives and the combined of the two were analyzed.Microgrooves have competing "drag reduction effects" and "drag-increasing effects" for wall turbulence.The relative strength of these two effects in the competition determines whether the wall micro-grooves ultimately exhibit drag reduction or drag-increasing effect.The result of this competitive relationship is determined by the size and flow rate of the triangular wall microgrooves.Shearing will induce the formation of an ordered structure of micelles in the fluid.The ordered structure of micelles hinders the motion of the vortex near the wall,which makes CTAC have a drag reduction effect.But under the action of strong shear,the ordered structure of micelles will be mechanically degraded,so that CTAC loses its drag reduction effect.The shearing effect of the turbulent structure on the coiled loops of polymer molecules causes the random coiled chains to be stretched,and the stretched molecular chains affect the turbulence.The inhibitory effect of vortex is the reason for its drag reduction,but the molecular chain of the polymer will also be mechanically degraded under strong shear and lose its drag reduction effect.The microgrooves have both "isolation effect"and "spike effect" on the drag reduction flow of CTAC surfactant.The "isolation effect" can inhibit the movement of turbulent vortices,which is beneficial to joint drag reduction,and the "spike effect" will accelerate the degradation of SIS.The combined drag reduction of microgrooves and PAM polymer is the result of the combined effects of "mechanical degradation effect","polymer drag reduction effect","micro groove drag reduction effect"and"deaggregation effect",and these four effects Influence and restrict each other.