Experimental Study On Pipe Flow Transition Of Xanthan Gum Solution And Drag Reduction Characteristics At Different NaCl Concentrations

Toms published the paper about the drag-reducing effect in the first international conference in 1948.Subsequently it was proved to be of great significance to improve flow efficiency and energy saving.Among them,polymer drag reduction especially natural polymer has captured more attention from humans due to the reason of economic value and environmental friendliness.Nowadays,the application of drag reduction technology has been widely and deeply used in various engineering applications such as petroleum transportation and ship navigation.Xanthan gum(XG)is a biological natural polymer.It owns a relatively large molecular weight,ranging from 2×10~6 to 5×10~7.The structure of the XG molecule is stable which makes it less susceptible to external factors.it is harmless to human body,instead it even has a certain medical health care function.Overall,XG is a kind of excellent drag reduce additive.The domestic and foreign research results in recent several decades were analyzed in this paper;two different mechanism hypothesis were summarized.Based on the self-built experimental equipment of the research group,the experimental error was analyzed and the actual pipe diameter was calculated according to the clear water calibration.In order to research the pipe flow transition of XG solution and drag reduction characteristics at different NaCl concentrations,some works were carried out which are as follows:the laminar-turbulent and turbulent-laminar transition phenomena of XG solution was observed and measured according to the Reynolds experiment;some phenomenon including the onset of drag reduction,concentration effect and salt concentration effect of XG solution at different NaCl concentrations were investigated by using the self-built drag reduction experimental device;the shearing resistance of XG solution at different NaCl concentrations were compared through the process of long-term cyclic shearing,and the effect of the shearing process on the drag reduction characteristics of the XG solution was investigated also.The main achievements are:(1)Characterized by generalized Reynolds number;the flow transition lower critical Reynolds number is delayed with the increase of XG concentration in an approximately linear mode,while the value of the upper critical Reynolds number rapidly drops to about 3000.(2)Combined with the effects of salt on molecular entanglement,hydrodynamic volume and rheological properties,salt can advance the onset of drag reduction of XG solution;the saturated drag reduction efficiency of semidilute XG solution(100?400ppm)decreases significantly with the dissolution of NaCl under the high Re number nevertheless a better drag reduction effect under the low Re number;the addition of appropriate amount of salt can increase the drag reduction efficiency of the concentrated XG solution(500?600 ppm);the turbulence of XG solution behaves a transform from type A drag reduction at low Re number to type B at high Re number,the low concentration solution will convert to type B drag reduction completely with the addition of salt;the drop in temperature will delay the onset of drag reduction significantly.(3)The shearing resistance of the XG aqueous solution goes up as the increase of concentration,which shows a concentration effect;the addition of an appropriate amount of salt can improve its shearing resistance slightly;the impact of the shearing process on the drag reduction characteristics of the XG solution is mainly concentrated in the high Re number section and has almost no effect on the onset of drag reduction.