Study On The Drag Reduction Of Elbow And T-junction Close-coupled Pipes By PIV And CFD

Close-coupled pipe fittings are widely used in HVAC and other ductwork system, Local components coupled with each other makes the fluid flow in the ductwork system complexity,the resistance calculating of the ductwork system becomes more difficult and the stability of the ductwork system operation is poor,this directly affects the selection of power equipment,energy consumption,operating costs and safe operation of pipeline systems.Therefore,the design of pipeline drag reduction is significance in engineering.PIV and CFD code Fluent is used to study the fluid flow in the elbow and T-junction close-coupled pipe with and without local components in this paper,the inner diameter of the pipe is 32mm and the separation distance between elbow and T-junction is 1.128 pipe diameters.The comparison of turbulent flow field with and without local components of seven different combine flows between PIV and CFD shows that it is capable to study fluid flow in the elbow and T-junction close-coupled pipes using CFD.The drag reductions are optimized based on this case,and combination of physical drag reduction technology in different fields,there are eight different schemes of the drag reduction components form is designed to used in dividing and combining pipe flow for calculations,and the results are compared and analyzed.The results of this paper are as follows:(1) PIV can be successfully used in the measurement of the pipe flow,revealing the structure of the flow field,providing guidance to reveal the mechanism of drag reduction and drag reduction scheme design.SST k-ωturbulence model can accurately simulate the fluid flow in the straight tube sections and the elbow and T-junction close-coupled pipe,CFD and PIV flow field are similar and the maximum error is less than 15 percent. (2) The mechanism of drag reduction is to delay and inhibit the boundary layer transition and separation,inhibit the effect of proliferation and contraction at the time of fluid flow through the bends and tees,reduce the convergence and separation loss of fluid,inhibit the emergence and development of eddy,reduce the energy dissipation, so that to flow more smooth and stable.Due to different flow status with different work conditions,therefore the drag reduction effect of drag reduction components changes with the flow status.If the drag reduction components appropriately match with the work condition,pressure loss can be reduced effectively,but it is likely to receive the opposite effect if conditions do not match.(3) Based on the shape of experimental drag reduction components,by changing the largest radial distance of pipe to carry out simulation for combining flow condiction,the results show that,the largest radial distance of drag reduction components should not exceed 1/4 pipe diameters in the common scope of the flow conditions for project.(4) Without drag reduction components,the simulation data of local pressure loss coefficient has the same trend with the Eurovent experimental local pressure loss coefficient date of T-junctions and the regression models date of Gardel to change with velocity ratio of two branch pipe,but the specific numerical has large difference,and Zhang Xinji gives a smaller difference of local pressure loss coefficient,it is also verify the effectiveness of simulation in this paper from the other side.(5) Based on the drag reduction effect analysis of eight different schemes drag reduction components shows that:1) The local pressure loss coefficientζ₁3 of the combining flow condition,schemeâ…§has the best drag reduction effect at the work condition of V_m1/V_m3<1 and the range of the drag reduction rate is between 17018%and 47.61%;it is not suitable to add drag reduction components when the flow rate of branch pipe 2 is zero。2) Scheme X has the best drag reduction effect for the local pressure loss coefficientζ₂3 of the combining flow condition,and the range of the drag reduction rate is between 0.33%and 141.30%.3) For the local pressure loss coefficientζ₃1 of the dividing flow condition,the drag reduction rate of schemeâ…¥has better performance of drag reduction,and the range of the drag reduction rate is between 12.73%and 23.89%,but most of the drag reduction scheme has increased resistance at the work condition of V_m1/V_m3>0.8.4) For the local pressure loss coefficientζ₃2 of the dividing flow condition,in the velocity ratio V_m1/V_m3 >0.5 the condition,the drag reduction effect is not obvious,or even the phenomenon of increasing resistance is appear after add some drag reduction schemes at the work condition of V_m1/V_m3>0.5.Scheme X has better performance of drag reduction at the work condition of V_m1.V_m3<0.5.and the range of the drag reduction rate is between 10.21%and 17.90%.