Study On The Fluid Flow In Elbow,Junction Closed-Coupled Pipes By 2DPIV And CFD

Close-coupled pipe fittings are widely used in HVAC and related industries.The changing of flow direction will consume energy.Pressure losses of fittings occupy a significant proportion of the total pressure losses in a pipe network system. Understanding the flow behavior and turbulent structure inside a pipe system is of vital importance in optimizing a pipe network system design.Many experimental investigations have been carried out to study the loss coefficient associated with different fitting configurations.However,the vast majority of literature and papers on pressure losses of fittings present information on single,uncoupled fittings only. Analysis of three-dimensional flow in close-coupled pipe configurations is needed.As a case,in the heating supply project of combining the BaoJi Power Station and the YuJianpu Boiler Station,the combination of the two main network pipes exist the close-coupled of an elbow and a T-junction.In a short pipe distance,the close-couple of pipe fittings makes the flow in the pipe segment complex,and the flow stability is bad.In this paper,the all-purpose experimental rig is established,and 2DPIV is employed to test and analyse the flow in the rig.CFD code Fluent and SST turbulent model is applied to compute the fluid flow in the rig.Then,the result of CFD code and PIV is compared and analysed.It shows good consistency between the experimental and predicted results.According to the experiment result,4 other combination schemes, which can improve the flow stability and decrease the flow resistance,are proposed. The pressure loss coefficient of all five combination scheme is computed by Fluent,and the best scheme is recommended.The pressure loss coefficient data of five combination scheme in this study is compared with that of T-junction proposed in literature,and the discrepancy is analysed.In the experiment,the velocity of main pipe inlet decreases from 1.21m/s to 0,and the velocity of branch inlet increases from 0to 1.28m/s,the velocity ratio between main pipe inlet and branch inlet increases gradually,the turbulence and the eddy area of the flow downstream the junction enlarges gradually.When the velocity ratio between inlet1 and inlet 2 is at about 0.86 and is constant,the velocity field of the flow downstream the junction is similar between the three.CFD code Fluent and SST turbulence mode is applied to simulate the fluid flow in the pipe segment,and the result of CFD and PIV is compared,the result between the two is similar,but the maximum velocity of PIV is 8-15%bigger than the result of CFD.In this study the application of Fluent is capable.The pressure loss coefficient is computed from the CFD result,and the result is analysed as follow:In the combining junction and elbow close-coupled fitting,the pressure loss coefficientζ₂₁,according to the experiment data of Eurovent(1996),the value ofζ₂₁ is between 0 and 0.61,and the trend of scheme A in this study is the same to Eurovent, but the value is bigger than Eurovent by 0.15 to 0.45,and when V_m2/V_m1increases the difference is bigger.The pressure loss coefficientζ₃₁,according to the experiment data of Eurovent(1996),the value ofζ₃₁is between -0.90 and 1.40,the value of scheme A in this study is -0.70 to 1.25 and is close to Eurovent.It shows good consistence between A and Eurovent.The value of scheme B and C is close to Eurovent when V_m3/V_m1=0, only about 35%of Eurovent when V_m3/V_m1=1.0.This shows that the changing of link type can significantly decrease the pressure loss coefficient.In the combining junction and elbow close-coupled fitting,the pressure loss coefficientζ₁₂,according to the regressive model of Gardel,the value ofζ₁₂is between 0 and 0.35,when V_m2/V_m1=0.8,the minimum value comes.The pressure loss coefficient of all the five schemes in this study have the same trend with Gardel,but the minimum value comes at different V_m2/V_m1.When V_m2/V_m1＞0.4,the value of all five schemes in this study is bigger than Gardel,and the difference goes bigger with the increase of V_m2/V_m1.The close-coupled of elbow and junction contributes to this condition.As forζ₁₃,according to the experiment data of Eurovent(1996),the value is between 0.7 and 1.0.The value of scheme A in this study is bigger than Eurovent,and the trend is different.The trend of scheme B and C in this study is the same to Eurovent, but the value of C is only 50%that of Eurovent.This shows that the scheme B and C can significantly decrease the pressure loss coefficient.According to the five schemes in this study,the pressure loss coefficient is greatly influenced by the junction link type in the junction and elbow close-coupled fitting.In the five scheme,C is the best compared to other four schemes,that is when the flow in the branch is close to the main pipe,the pressure loss coefficient is minimum.