Research On The Formation Mechanism And Suppression Method Of Vortex In The Open Inlet Pool Of Vertical Pump Devic

The open intake tank has become one of the most popular hydraulic structure and has been widely used in many large-scale hydraulic projects in China.However,there are many complex vortices involved in the inner flow of the open intake,especially the free surface vortices and submerged vortices.These vortices have become one of the factors threatening the safe and stable operation of hydraulic facilities.In present work,in order to establish the principle of identifying the vortex structures in the open intake tank,and clarify the generation mechanism of vortex,the inner flow patterns in the model of the open intake tank were systematically investigated based on the combination of the theoretical,numerical and experimental methods.The relevant theories and methods can be provided for the research on the eliminating vortex in the open intake tank.The main work and conclusions are as follows:Firstly,the relevant research background on the inner flow and generation mechanism of vortex in the open intake tank from domestic and abroad was briefly reviewed,which provides a significant theoretical basis for the future research.Moreover,a visual test rig was set up and the main types of vortices in open intake were studied by unsteady numerical simulation,the results are shown as follows:With the increase of Fr,the development of free-surface vortices can be divided into five stages:surface vortex,surface dimple,air core stretching,intermittent air entrainment and continuous air entrainment.The surface vortex stage does not have strong suction ability.With the development of surface dimple,the suction capacity of the vortex core is strengthened,and the vortex develops from the dye core to the trash entrainment core until a fully developed air-core vortex.The critical submergence depth and Fr are important factors affecting the free surface vortex.The critical submergence depth of intake has a linear relationship with Fr.When the Fr number exceeds the critical point,the duration of air-core vortex increases exponentially with Fr.The floor-attached vortices are located on the bottom surface below the bell mouth,and the number of vortices is related to Fr.With the increase of Fr,the structure of primary and secondary vortices changes.The wall-attached vortices originate from the reverse flow on the side wall,and the reverse flow area is located near the bottom surface.The shapes of the wall-attached vortices are almost symmetrical with respect to the center line of the pump,but their directions are the same.The vorticity of all vortices increases with the increase of flow velocity in intake.Meanwhile,Liutex-Omega method was used to capture the vortex structure of the inlet pool and the vortex induced velocity fields were analyzed and compared with the traditional theoretical model.The results show that:Liutex-Omega method is insensitive to the threshold and can capture both strong and weak vortex structures at the same time,which has the significant superiority over other vortex identification methods.The magnitude of Liutex indicates that flow velocity in intake is an important factor affecting both free-surface and submerged vortices.Affected by the pump,the vortex strength gradually increases when the vortices approach the pump inlet.Based on the rigid body rotation theory,theΓ₁ function method was used to identify the position of vortex core formed by large-scale rotation in the intake.The statistical results show that the vortex cores of free-surface vortices are symmetrically distributed on both sides of the center line of the intake.The cores of the floor attached vortex and wall-attached vortex both appear in front of the bell mouth center.The increase of water level and flow makes the vortex cores of the submerged vortices move towards the inflow.There is a strong coherence between the vortex field and the velocity field,and the velocity field induced by the floor-attached vortex is close to the Burgers model.The velocity fields near free-surface vortices and wall-attached vortices are affected by the wall surface,which are unevenly distributed in the circumferential direction.The mechanism of the formation of various types of vortices in the open intake has been clarified based on the enstrophy evolution equation and skin friction line,the results are shown as follows:The vortices between the pump and the back wall of the intake are mainly caused by Karman vortex street off the pipe and the reflected waves generated by the back wall.The large-scale vortices are formed by the aggregation of small vortices on the free surface in the rear wall region,and their directions of rotation are consistent with that of Karman vortex street.The free-surface vortices are mainly affected by vertical tension,with the increase of Fr,the vortex strength becomes stronger,and the free-surface vortex is stretched downward to form a funnel-shaped air core,and finally the air entrainment vortex is formed.The shear force on the wall causes the formation of submerged vortices,under the action of stretching,the vortex on wall surface will move towards the bell mouth,then it forms a vortex tube.The free-surface vortices are mainly affected by the back wall distance and Fr.Increasing the back wall distance can reduce the probability of vortex aggregation on water surface,and reduce the stretching effect of vortex core,so as to suppress the formation of air entrainment vortex.The submerged vortices are affected by both the bell mouth height and the back wall distance.Increasing the distance between the two walls will reduce the intensity of the submerged vortices.Furthermore,the uniformity of axial velocity,weighted average Angle,swirl angle and swirl intensity were introduced as the evaluation indexes of pump inflow conditions.By theoretical derivation,the energy gradient theory was applied to analyze the internal flow stability of axial flow pump.Moreover,based on the of boundary vorticity flux and entropy production theory,the influence of vortices on pump internal loss was analyzed.The conclusions are as follows:The vortex will not only reduce the axial velocity uniformity,but also increase the inlet pre-whirl.By increasing the back wall distance and bell mouth height,the inflow uniformity will be improved and the inlet pre-whirl can be reduced.The weakening of the vortices not only improves the stability of the pump inlet,but also reduces the normal energy gradient and energy dissipation inside the impeller and diffuser.The submerged vortex is an important factor affecting the inlet loss of the pump.Increasing the bell mouth height can effectively reduce the inflow and pump internal loss.Finally,based on the vortex generation mechanism,four anti-vortex methods for free-surface vortices were proposed from four aspects:suppressing surface vortex formation,improving back wall reflection,reducing vorticity accumulation and preventing vortex stretching.The results show that the scheme with semicircular back wall shows the best effect,which can inhibit the surface vorticity and weaken the stretching effect at the same time.The floor attached and wall attached vortexes can be eliminated at the same time by using the triangular pyramid wall fillet.Based on the effect of anti-vortex by the wall fillet,the axial velocity uniformity of the pump inlet increases and the swirl angle decreases,which provides a better inflow condition for the pump.A combined anti-vortex scheme is formed by combining the semicircular back wall with the triangular pyramid wall fillet.The scheme can effectively suppress the formation of free-surface and submerged vortices.With the elimination of the main vortices in the intake,the internal flow stability of the pump is improved with the reduced hydraulic loss.