Design Philosophy And Structural-hydraulic Characteristics Collaborative-optimization Of Flat-bottomed Bifurcations

The demands for pumped-storage hydropower stations are risen on a large degree as energy storing devices in power system with the exploitation of new energy worldwide. Steel bifurcation is the key parts of the structure since the centralized and joint water supplies are conducted. Large energy loss and stress concentration always happen in steel bifurcation, which makes a great difference on the normal operation of hubs. It is troublesome for free-draining and the drainage efficiency is low for the long draining time since the elevations at the bottom of bifurcation and upstream pipes are less than that of branches downstream along with the larger diameter of main pipes and smaller one of branch. Free drainage is a significant problem to solve for the long time of evacuation and the negative influence on the stress and construction if perforations are developed.The studies were carried out from the perspective of geometrical derivation, CFD, FEM and mathematical modeling:I. The research status was summarized on the basis of primary geometrical theory, the characteristics of structure and hydraulics and the optimization in mathematical modeling. The new type flat-bottom bifurcation was put forward to make up for the technical blank in engineering practice and deep studies was developed on structure and geometry whose transformation of coordinates, equation of a surface and outline of conic curve was deduced. The bifurcation is able to improve the efficiency of overhaul and decline the cost of station operation. And the convenience of manufacture and less consumption of steel is also its advantages which create the great economic benefits. The secondary developed system which is independent from CAD promotes the design efficiency. The aided design of type, mesh generation and pipe expansions were achieved in total life circle and the arbitrary point coordinates can be discrete and acquired in the surface of bifurcation.II. The characteristics of structure and hydraulics in symmetrical and unsymmetrical bifurcations were calculated through FEM and CFD and the results were compared with the that in engineering practice. The study was conducted aiming to the stress and displacements of key points. The sensitivity is discussed that what influence of the horizontal projection angle, half cone apex angle, diameter ratio and rib breath ratio had on the head loss coefficient under different working conditions of hydropower station. The every kind of recommended parameter on the normal working condition was obtained afterwards. The ideal stress reduction rate and there were no distinct stress concentration and large displacement both lead to the better feasibility of the structure. The flow pattern of bifurcations proves stable by the simulation of CFD. As a result, this new type bifurcation is applicable for pumped-storage hydropower stations and the better hydraulics characteristic appears when the station is pumping.III. The head loss in different working conditions with different rib breadth ratio when the rib stretched inside was analyzed since it has large influence on the characteristics of structure and hydraulics, which testified that the rib could not always deteriorate the flow pattern. The recommended value of rib breath ratio was given. The law was put forward of the pressure distribution inhomogeneity of shell of pipe and the magnitude range of pressure difference was calculated. The relative standard deviation of section pressure distribution is able to reflect the degree of inhomogeneity of pressure distribution and can be expressed by rib breath ratio with associated phenomena in radio. The metal fatigue calculation under pulsating pressure proved that the fatigue destruction could not happen after hundreds of years'operation.IV. The mathematical model was set up based on few algorithms:the synergy weights of stress value and head loss were determined by grey correlation analysis, then the synergy data was evaluated by TOPSIS, at last, the synergy optimization model was obtained through partial least squares method. The largest top stress and the head loss when one turbine working in cylindrical three-way pipe was the most significant design constraints. The taper apex angle of the cone and ratio of pipe-diameter constraint the characteristics of structure and hydraulics respectively. On the meanwhile, optimized indexes the geometry were fixed quantitatively. A series of effective optimized design theory was founded and the degree of fitting proved to be reasonable.