| Siphon outlet channel is the connecting channel between pump and outlet tank,which makes water flow better divert and diffuse in the process of outlet of guide vane body flowing into outlet tank and maximizes kinetic energy recovery without flow shedding or swirl.It is a key component of Low-head Pump Station and is widely used in practical engineering,and has a great influence on efficiency and safe and stable operation of pump unit.In this paper,physical model test and numerical simulation are combined to analyze the hydraulic characteristics of vertical axial flow pump unit with siphon outlet channel under different flow conditions,and the frequency distribution of pressure fluctuation,energy distribution and internal flow change characteristics of outlet channel under different flow conditions are mainly analyzed.The main research work and conclusions are as follows:(1)When different flow conditions are calculated,the internal flow characteristics of each overflow component of the vertical axial flow pump unit with siphon outlet channel are adopted.The pressure and velocity distribution at the impeller inlet section rotates counterclockwise with the blades.The axial velocity distribution on the circumferential measuring line at the impeller inlet Span=0.5 is sinusoidal,showing 4 high-speed peaks and 4 low-speed troughs,corresponding to the number of blades.The velocity-weighted deviation angle of impeller inlet section changes steadily in one cycle with only small fluctuations.With the increase of flow rate,the velocity-weighted bias angle decreases gradually.The average vorticity angle at the outlet section of the guide vane changes steadily in one cycle.With the increase of flow rate,the average vorticity angle first decreases and then increases.The tangential velocity at the outlet of impeller is higher under the condition of small flow rate and large circulation exists in water flow,which results in obvious spiral flow.(2)Based on Omega criterion,the internal vortex structure of guide vane and siphon outlet channel is analyzed.The distribution of vortex structure in guide vane is relatively uniform under various flow conditions.Under small flow conditions(0.3 Qbep),the vortex structure in guide vane body is mainly distributed in block shape and has large volume.In this case,the flow is chaotic,there is large deformation dissipation and large hydraulic loss.The vortex structure in the guide vane body is mainly distributed near the outlet of the guide vane body under the optimum operating condition(1.0 Qbep)and the high flow condition(1.2 Qbep).The outlet section of the guide vane body has obvious eddy structure,which mainly distributes between the two guide vanes and the edge of the blade.With the increase of flow,the eddy area gradually decreases or even disappears.In the part of siphon outlet channel,under small flow conditions,due to small speed and inertia,the vortex structure mainly concentrates at the inlet of siphon outlet channel and has a large volume,which mainly appears as a strip in the upper section.Under the strong inertia under the optimum condition(1.0 Qbep)and the high flow condition(1.2 Qbep),the siphon outlet channel will flow off to a great extent,and after passing through the hump section,a large volume of low-speed vortexes will be generated in the downward section.With the increase of flow rate,the vortex structure in the upstream section of the outlet channel is further reduced and separated.(3)Through flow analysis in different zones for siphon outlet channel bend section and hump section,it is found that flow imbalance exists in the flow channel:the flow deviation characteristics show left-right deviation at the inlet and outlet sections of the outlet channel bend section,and with the increase of flow,the flow deviation becomes weaker and the ratio of left-right deviation decreases gradually.However,in siphon hump section,the bias current presents upstream and downstream bias,and the ratio of upstream and downstream bias first decreases and then increases with the increase of flow rate.The residual circulation at the outlet of the guide vane body is the main reason for the flow deflection in the elbow section.After passing through the elbow section,the flow rotates further counterclockwise.Due to the guiding effect of the elbow on the flow,the deflection degree is further intensified in the left and right directions,which also leads to the more chaotic flow in the outlet channel.With the increase of flow rate,dimensionless circulation of siphon outlet channel decreases gradually,and the spiral vortex structure in outlet channel decreases gradually.(4)Based on the Hilbert-Yellow Transform(HHT),the energy analysis of pressure fluctuation signal decomposition domain for impeller,guide vane body and siphon outlet channel is carried out Under three characteristic conditions,the impeller outlet monitoring points have obvious periodicity,the main frequency is 4 times of rotation frequency,the pressure amplitude decreases gradually with the increase of flow rate,and the energy proportion of high-frequency component of pressure fluctuation increases gradually along the hub to the rim.The amplitude of main frequency decreases with the increase of flow rate,which corresponds to smoother flow condition with small flow phase in outlet channel under the condition of large flow rate.(5)After pressure fluctuation signal decomposition and energy analysis with HHT,the Hilbert spectrum and marginal spectrum are obtained,the time-frequency distribution of pressure fluctuation frequency at each monitoring point of water channel is clarified.Under three characteristic conditions,there is no obvious periodicity at each monitoring point of water outlet channel.The pressure fluctuation is slightly affected by impeller rotating frequency.The main frequency is maintained below 20 Hz in the low frequency band,with an average frequency of 14.7 Hz,and the low frequency energy proportion exceeding 50%.A small amount of high frequency signal above 700 Hz can be decomposed at outlet,ascending section,hump section and exit section of bend section of outlet channel,but the energy proportion is less than 1%. |
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