| The minor loss caused by a sudden expansion,a sudden contraction or other pipe elements is approximately 30% to the total loss in water-transporting pipe network systems,which can not be ignored.This paper attempts to reveal the characteristics of minor losses in a sudden expansion and a sudden contraction flow with surfactants,for providing a scientific reference for designing a water-transporting pipe network drag reduction system with surfactant drag reducers,and the minor loss characteristic of a non-Newtonian fluid flowing through a sudden expansion and a contraction is also clarified.A flow resistance test experiment system has been constructed firstly,based on the theory of hydrodynamics.The reliability of the system was verified with water,and then,how is the minor loss flowing through the sudden expansions with three kinds of sudden expansion ratios and sudden contractions with three kinds of sudden contraction ratios influenced has been measured both in a water flow and in a surfactant drag-reducing aqueous flow,after confirming that the pipe friction coefficient at the upstream and downstream straight pipe is consistent with Bulasius empirical formula.And,the parameter which influences the minor losses was also explored.The experimental results for the water show that the minor loss coefficient is not nearly changed with Reynolds number.The works of previous some researchers have big differences in whether the minor loss in a sudden expansion flow is changed by Reynolds number or not.It is due to the differences in the flow pattern at the upstream flow and downstream flow from the sudden expansion,in the geometry size precision of the sudden expansion and in the position at which the pressure tap is set.The minor loss coefficient in a sudden expansion flow is independent of Reynolds number from the present work.The experimental results for the surfactant solution show that at a constant concentration,a constant temperature,and a constant Reynolds number,the drag reduction rate of a smaller diameter pipe is greater than that of a bigger diameter pipe,we call this phenomenon “diameter effect”.But if the wall shear stress in different pipe diameter flow is a constant,the drag-reducing rate is the same.While the drag reduction effect in the upstream flow is not disappeared,the minor loss coefficient of the sudden expansion is smaller than that for the water flow,and it decreases with Reynolds number.The maximum drag reduction rate is approximately 40%.The influence of the sudden expansion on the downstream flow pattern exists at much farther downstream distance than 15 times of pipe diameter for the water flow.The minor loss coefficient in a sudden contraction at low Reynolds numbers is higher than that for the water flow and when the drag reducing effect in the upstream straight pipe is significant,it is less than that of the water flow,the drag-reducing rate in the sudden contraction flow is approximately 65%.The influence of surfactants on the minor loss in a sudden expansion and a sudden contraction flow is explained as resulting from the reduction of the wall-normal turbulent diffusion induced by surfactants. |
