CN106870247A - A Drain Cone Drilling Method Based on the Three-Dimensional Simulation Method of the Full Flow Field of a Hydraulic Turbine - Google Patents

Abstract

The present invention proposes a discharge cone drilling method based on a three-dimensional simulation method for the full flow field of a hydraulic turbine, including establishing a model test experiment system, selecting a turbulence model, and using the RANS simulation method to perform a three-dimensional unsteady simulation of the full flow field of a hydraulic turbine. The calculation of the three-dimensional simulation method of the full flow field includes geometric modeling, grid setting, numerical method, control equation and boundary condition setting, comprehensive model experiment and numerical simulation data, and different discharge cone drilling models under different flow deviation conditions carry out testing. The invention solves the unsteady flow problem inside the water turbine unit in the prior art, especially the vortex belt and corresponding pressure fluctuation problem in the draft tube.

Description

A Drain Cone Drilling Method Based on the Three-Dimensional Simulation Method of the Full Flow Field of a Hydraulic Turbine

technical field

The invention belongs to the technical field of water turbine stability, and in particular relates to a method for punching holes in a discharge cone based on a three-dimensional simulation method of the full flow field of a water turbine.

Background technique

Today, as the importance of hydropower generation becomes increasingly prominent, the requirements for improving the operating efficiency of hydropower stations and the stability of hydraulic turbines have become particularly prominent. Among the factors affecting the stable operation of the turbine, the hydraulic factor is the most prominent, including the dynamic and static interference between the rotating part and the stationary part, the shedding of the blade surface, the cavitation blade path vortex, and the cavitation draft tube vortex belt, etc. During the operation of the turbine, most of these flow phenomena occur in off-the-shelf conditions that deviate from the design conditions, which will induce serious pressure fluctuations in the corresponding flow field, and the pressure fluctuations will then spread and act on the turbine unit itself, causing vibration and operation of the unit. Noise, even induce plant vibration. Pressure pulsation is the main source of unstable operation and vibration noise.

The pressure pulsation caused by the three-dimensional flow field in the turbine comes from many aspects. For example, if the flow is lost at the inlet of the runner, the induced flow field will propagate in the upstream and downstream directions, causing hydraulic vibration of the upstream and downstream components; Inside the wheel, the blade path vortex is the biggest source of instability, and the occurrence of the blade path vortex is often accompanied by cavitation flow. The pressure pulsation caused here will directly act on the runner, forming high-frequency vibration; , such as inside the draft tube, the cavitation vortex will be generated from the bottom of the discharge cone, forming a spiral motion. These spiral vortexes will periodically act on the cone section and the elbow section, causing vibration of downstream components and inducing noise. Experiments have shown that the operating frequency of the draft tube vortex is low-frequency vibration. Under different flow conditions, the vibration and noise caused are different, but in general, the movement of the draft tube vortex is the lowest frequency movement, because the impact on the unit And the worst. For example, Yantan Hydropower Station and Lijiaxia Hydropower Station have been in operation for half a year and up to two years. Several turbine units have cracks in the welds between the runner blades and the upper crown and between the blades and the lower ring. After analyzing the causes of the cracks in the runner, it was found that the main causes were in manufacturing and operation, and the severe pressure pulsation during operation was the direct factor causing the cracks.

In Francis turbines, the pressure pulsation induced by the draft tube vortex is the main source of vibration and noise. At present, many studies have analyzed the mechanism and evolution of the draft tube vortex, and proposed to reduce or eliminate the draft tube. The measures of the vortex, such as changing the water flow state in the draft tube, controlling the eccentricity of the vortex, introducing appropriate damping or improving the hydraulic design of the runner, however, these measures cannot effectively reduce the pressure pulsation, but some will bring additional noise.

Contents of the invention

The purpose of the present invention is to overcome the shortcomings and deficiencies of the prior art, to provide a method for drilling holes in the discharge cone based on the three-dimensional simulation method of the full flow field of the hydraulic turbine, and to solve the unsteady flow problem inside the hydraulic turbine unit, especially the vortex zone in the draft tube And the corresponding pressure pulsation problem.

The purpose of the present invention is achieved through the following technical solutions: a method for punching water cones based on a three-dimensional simulation method for the full flow field of a hydraulic turbine,

Step 1. Establish a model testing experimental system, design different discharge cone drilling models for matching processing, improve the hydraulic test bench according to different configurations of discharge cones, and perform high-speed photogrammetry to obtain the vortex inside the turbine runner and the draft tube. The experimental information brought;

Step 2. Select the turbulent flow model, and use the RANS simulation method to perform three-dimensional unsteady simulation of the full flow field of the turbine, capture the blade path vortex between the runner blades, the draft tube vortex belt, and the deflow cavitation phenomenon at the blade inlet edge, and determine according to the captured phenomenon The place where the vortex belt is born, is drilled at the place where the vortex belt is born; the calculation of the three-dimensional simulation method of the full flow field of the hydraulic turbine includes geometric modeling, grid setting, numerical method, control equation and boundary condition setting;

Step 3. Synthesize the model experiment and numerical simulation data, and test the different discharge cone drilling models under different flow deviation conditions.

Further, the geometric modeling in the calculation is based on the overall geometric model, which includes seven parts: the inlet section of the volute, the volute, the fixed guide vane, the movable guide vane, the runner, the draft tube and the elbow.

Further, the grid is configured to perform grid division on each part of the geometric model, and the grids of each component are connected using an interface during calculation.

Further, the numerical method adopts CFD software ANSYS for calculation, and uses CFX solver for solution. The CFX is a finite volume discretization method based on finite elements, and 24-point interpolation is used for hexahedral grid cells.

Further, the control equations include continuity equations and momentum equations.

Further, the boundary conditions are set as:

Inlet: take the extended section of the volute inlet as the inlet, adopt the mass flow inlet condition, and set the reference pressure and initial turbulence intensity parameters at the same time;

Outlet: take the elbow outlet as the flow outlet, and adopt the pressure outlet condition;

Wall: All solid walls adopt no-slip boundary conditions, and the flow near the wall is simulated by wall functions.

Further, the perforation of the water discharge cone is a water discharge cone with 2 holes, and the water discharge cone with 2 holes is in a configuration of opposite perforation along the axis.

Further, the perforation of the discharge cone is a discharge cone with 4 holes, which is relatively perforated into a discharge cone with 2 holes along the axis of the discharge cone, and then on the basis of the discharge cone with 2 holes, The relatively lower position is vertically pierced with 2 holes.

Description of drawings

Figure 1 is a schematic diagram of a hydraulic test bench;

Figure 2 is a schematic diagram of rotational speed measurement;

Fig. 3 is a schematic diagram of pressure pulsation measuring points;

Fig. 4 is the physical figure of model runner (A1293);

Fig. 5 is the discharge cone of different configurations (a) prototype 0 hole (b) punching 2 holes (c) punching 4 holes;

Fig. 6 is the computational geometric model of the hydraulic turbine (a) the prototype water discharge cone (b) the 2-hole water discharge cone (c) the 4-hole water discharge cone -0 degree view (d) the 4-hole water discharge cone -90 degree view;

Fig. 7 is a top view of draft tube pressure distribution under off-center conditions (prototype 0 hole, a=16mm, σ=0.258, n ₁₁ =80.4r/min);

Fig. 8 is the pressure distribution on the surface of the discharge cone under the off-center condition (prototype 0 hole, a = 16mm, σ = 0.258, n ₁₁ = 80.4r/min);

Fig. 9 is a top view of the pressure distribution of the draft tube under the off-center condition (two holes are drilled, a=16mm, σ=0.258, n ₁₁ =80.4r/min);

Figure 10 is the pressure distribution on the surface of the discharge cone under off-center conditions (two holes are drilled, a=16mm, σ=0.258, n ₁₁ =80.4r/min);

Fig. 11 is a top view of draft tube pressure distribution under off-center conditions (4 holes are drilled, a=16mm, σ=0.258, n ₁₁ =80.4r/min);

Figure 12 is the pressure distribution on the surface of the discharge cone under off-center conditions (4 holes are drilled, a=16mm, σ=0.258, n ₁₁ =80.4r/min);

Fig. 13 is a top view of draft tube pressure distribution under large flow conditions (prototype 0 hole, a=28mm, σ=0.277, n ₁₁ =83.2r/min);

Fig. 14 is the draft tube vortex zone of the prototype discharge cone under the condition of large flow rate (prototype 0 hole, a=28mm, σ=0.277, n ₁₁ =83.2r/min);

Figure 15 is the pressure distribution on the surface of the discharge cone under the condition of large flow rate (prototype 0 hole, a=28mm, σ=0.277, n ₁₁ =83.2r/min);

Fig. 16 is a top view of draft tube pressure distribution under the condition of large flow rate (drilling 2 holes, a=28mm, σ=0.277, n ₁₁ =83.2r/min);

Figure 17 is the pressure distribution on the surface of the discharge cone under the condition of large flow rate (two holes are drilled, a=28mm, σ=0.277, n ₁₁ =83.2r/min);

Figure 18 is a top view of the draft tube pressure distribution under the condition of large flow rate (4 holes are drilled, a=28mm, σ=0.277, n ₁₁ =83.2r/min);

Figure 19 is the surface pressure distribution of the discharge cone under the condition of large flow rate (4 holes are drilled, a=28mm, σ=0.277, n ₁₁ =83.2r/min);

Figure 20 is the vortex zone of the draft tube of the perforated discharge cone under the condition of large flow rate (a=28mm, σ=0.277, n ₁₁ =83.2r/min);

Fig. 21 shows the pressure fluctuation energy distribution of the draft tube under the off-center working condition (0 hole in the prototype, 2 holes drilled, 4 holes drilled, a=16mm, σ=0.258, n ₁₁ =80.4r/min).

detailed description

The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

The present invention provides a method for punching water discharge cones based on a three-dimensional simulation method for the full flow field of a hydraulic turbine.

Step 1. Establish a model testing experimental system, design different discharge cone drilling models for matching processing, improve the hydraulic test bench according to different configurations of discharge cones, and perform high-speed photogrammetry to obtain the vortex inside the turbine runner and the draft tube. The experimental information brought;

Step 2. Select the turbulent flow model, and use the RANS simulation method to perform three-dimensional unsteady simulation of the full flow field of the turbine, capture the blade path vortex between the runner blades, the draft tube vortex belt, and the deflow cavitation phenomenon at the blade inlet edge, and determine according to the captured phenomenon The place where the vortex belt is born, is drilled at the place where the vortex belt is born; the calculation of the three-dimensional simulation method of the full flow field of the hydraulic turbine includes geometric modeling, grid setting, numerical method, control equation and boundary condition setting;

Step 3. Synthesize the model experiment and numerical simulation data, and test the different discharge cone drilling models under different flow deviation conditions.

The geometric modeling in the calculation is based on the overall geometric model, which includes seven parts: the inlet section of the volute, the volute, the fixed guide vane, the movable guide vane, the runner, the draft tube and the elbow.

The grid is set to perform grid division on each part of the geometric model, and the grids of each part are connected using an interface during calculation.

The numerical method adopts the CFD software ANSYS to calculate, and adopts the CFX solver to solve. The CFX is a finite volume discretization method based on finite elements, and 24-point interpolation is used for the hexahedral grid unit.

The governing equations include continuity equations and momentum equations.

The boundary conditions are set as:

Inlet: take the extended section of the volute inlet as the inlet, adopt the mass flow inlet condition, and set the reference pressure and initial turbulence intensity parameters at the same time;

Outlet: take the elbow outlet as the flow outlet, and adopt the pressure outlet condition;

Wall: All solid walls adopt no-slip boundary conditions, and the flow near the wall is simulated by wall functions.

The perforation of the discharge cone is a discharge cone with 2 holes, and the discharge cone with 2 holes has a configuration of relative perforation along the axis.

The perforation of the discharge cone is a discharge cone with 4 holes, which is relatively perforated into a discharge cone with 2 holes along the axis of the discharge cone, and then on the basis of the discharge cone with 2 holes, at the relatively lower The position is perpendicular to piercing 2 holes.

Model Experiment of Water Turbine Drain Cone

Model experiment is an important method to study the flow characteristics in a turbine. In order to obtain flow field information and draft tube vortex information under different discharge cone conditions, experimental measurements were carried out using a turbine model test bench.

The model test bench is located in the hydraulic turbine research room of the Harbin Electric Machinery Factory Co., Ltd., and it is a high-parameter, high-precision hydraulic mechanical test bench, which can carry out related tests on pump turbines, large water pumps and high head Francis turbines.

There are 6 sets of hydraulic testing experimental devices numbered and tested, and their test capabilities, size of models that can be installed, testing methods of hydraulic performance and test water all meet the requirements of IEC60193-1999. The test contents mainly include: turbine energy test, cavitation test, runaway speed test, pressure pulsation test, differential pressure flow measurement test, axial force test, guide vane water moment test, air supply test, abnormally low water head test, etc. In addition, the flow state observation system equipped on the test bench can observe the internal flow state of the hydraulic machinery.

As shown in Figure 1, it is a schematic diagram of the key components of the hydraulic test bench. The station is equipped with a high-performance power pump, a vertical structure dynamometer motor, a high-pressure stainless steel pipeline system, electric valves, a vacuum pump, high-precision testing instruments and an in-situ calibration system, etc. The high-precision electrical control system of the test bench performs fast and high-precision adjustment and control on power equipment such as water supply pumps and dynamometer motors. The test bench is equipped with high-precision testing instruments to measure various test parameters. The high-performance data acquisition and processing system collects and processes the data.

The main structural components of the test bench are shown in Table 1.

Table 1 Main structural components of the test bench

Describe the above equipment as follows:

Test speed: The speed of the test device is realized by the dynamometer motor, which is powered by a silicon controlled rectifier power supply and can be operated in four quadrants, that is, it can be used as a generator and can be used for both forward and reverse rotation. The motor runs, so it can meet the requirements of various test conditions of different test devices for water turbine, water pump, and water pump-turbine.

Test water head and flow rate: the highest test head is 100m, and the maximum flow rate is 1.0m ³ /s.

Test water: The high-efficiency water treatment equipment equipped on the test bench filters and softens tap water for testing. In the test, we mainly pay attention to the four test water conditions of water density, water viscosity, gas content in water and steam pressure.

Atmospheric pressure measurement: The atmospheric pressure of the test bench is measured by an atmospheric pressure sensor. The atmospheric pressure measurement module is verified by the Heilongjiang Metrology Bureau every year, and a verification certificate is issued after passing the verification.

The rotational speed measuring system of the hydromechanical test bench is composed of a speed measuring sensor and a speed measuring disc with 120 teeth. The speed sensor is the MP-981 speed sensor produced by Japan Ono Company. The speed measuring disc is installed on the upper part of the turbine shaft, and the relative position of the speed sensor and the speed measuring disc is shown in Figure 2. The rotational speed measuring system can measure the rotational speed of 10000r/min. The electrical pulse signal generated by the speed measurement system directly enters the data acquisition system and data processing software for calculation.

On the hydraulic test bench, the observation of the flow state of the blade path vortex and the draft tube in the runner will also be carried out, which is mainly completed by the high-speed photography imaging system.

The flow state observation system consists of stroboscope from German DRELLO company, fiber optic endoscope from German WOLF company, digital industrial camera, synchronous trigger controller and image acquisition and processing workstation, which can perform real-time flow state observation, static image acquisition and dynamic Streaming media files are generated. Through this system, it is possible to observe the front and back shedding of the runner blade and the cavitation of the blade path vortex.

In addition, through the transparent conical tube at the model test device, the cavitation phenomena such as Karman vortex, primary cavitation, and vortex belt at the outlet of the runner can be observed by using the flow state observation system. At the same time, the high-speed camera produced by Olympus is used to capture the high-frequency information of the vortex.

The specifications of the fiber optic endoscope are as follows: German WOLF company, size φ10mm×300mm×DOV50/80/90°, multi-view probe, diameter 10mm, working length 300mm, 4m fixed optical cable.

The specifications of high-speed photography equipment are as follows: produced by Japan Olympus Company, model i-SPEED 2, CMOS sensor, resolution 800×600 effective pixels, frame rate up to 33000fps, minimum 1fps, maximum 1000fps at full resolution, shutter speed 5 microseconds . The external controller and display device of the photographic device adopts a standard LVDS connector, and the 26-pin MDR is connected with the photographic device.

In order to evaluate the hydraulic stability of the model hydraulic machinery, the pressure pulsation test is usually carried out. The main purpose of the pressure pulsation test is to obtain the relative amplitude and main frequency of the pressure pulsation in a specific operating range and to determine the best compensation for reducing the pressure pulsation. gas way.

Pressure pulsations are often measured using pressure pulsation sensors. The installation of the pressure pulsation sensor requires the diaphragm to be flush with the flow channel. As shown in Figure 3, it is the general installation position of the mixed flow type, the axial flow fixed paddle type and the pump turbine. In accordance with the IEC test standard, the positions of the four pressure measuring points at the draft tube are as follows, P1 is located on the downstream side of the draft tube, P2 is located on the upstream side of the draft tube, P3 is located at the inlet of the volute, and P4 is located at the cone In addition, the P5 measuring point is set between the guide vane and the runner.

The pressure pulsation measurement adopts the 112A22 dynamic pressure sensor produced by PCB Company of the United States. The sensitivity of the sensor is 15mv/kPa, the resolution is less than 0.007kPa, and the frequency range is 0.5-250kHz, which covers the maximum overcurrent frequency that the test machine can achieve. It can respond quickly and accurately to hydraulic pulsation. The high-speed data acquisition system of the test bench collects the response signal of the pressure pulsation sensor at a high sampling rate, usually the sampling frequency is 4000Hz.

The pressure pulsation sensor was calibrated with the 903BO2 dynamic pressure calibration instrument (accuracy ±0.2% FS) produced by PCB Company of the United States before and after the test.

The hydraulic test bench is based on a model turbine of a power station. The maximum water head of the water turbine is 71m, the minimum water head is 44m, and the rated water head is 57m. The model turbine runner (A1293) is shown in Figure 4, and the parameters of the model runner and other important devices are shown in Table 2.

Table 2 Main parameters of the model runner

In order to measure the flow of different types of drain cones, the different configurations of the drain cones in the test are given below, including the prototype with 0 holes, 2 holes and 4 holes, as shown in Figure 5.

In model experiments, the unit speed n ₁₁ and unit flow Q ₁₁ are usually used to describe the flow conditions in the full flow channel, and if the ambient pressure level in the test is to be changed, the cavitation coefficient σ of the device needs to be adjusted. Change the unit speed and unit flow by adjusting the opening a of the movable guide vane.

The unit speed n ₁₁ is defined as:

In the formula, D ₁ ——the diameter of the runner of the model turbine (low pressure side) (m);

H——the experimental water head of the model turbine (m);

N——The speed of the runner in the model turbine (r/min).

The unit flow _Q11 is defined as:

In the formula, Q——the flow rate in the runner of the model Francis turbine (m ³ /s).

For model testing, the device cavitation coefficient has an important influence on internal flow phenomena. According to the International Electrotechnical Commission-60193 standard, the cavitation coefficient σ of the device is defined as:

In the formula, H _{va ——the} vacuum value in the tail water tank in the model experiment circulation loop (m);

H _a ——water head converted from local atmospheric pressure in the experiment (m);

H _s ——the suction height of the turbine in the experiment (m);

_Hv —the water head converted from the saturated vapor pressure at the experimental temperature (m).

According to the above discussion, n ₁₁ and Q ₁₁ are used to describe the operating conditions of the model experiment, as shown in Table 3. In these working conditions, including small opening and large opening conditions, different forms of tailwater vortex can be observed, such as spiral vortex under small opening and columnar vortex under large opening.

Table 3 Experimental test working point parameters

Three-dimensional Simulation Method of Full Flow Field of Hydro Turbine

geometric modeling

In the calculation, the simulation of the full channel of the turbine is used, and in order to facilitate the numerical calculation, the inlet section into the volute is lengthened, the inlet is a circular inlet, and the outlet is the outlet of the draft tube elbow.

The calculation area is based on the overall geometric model, which includes seven parts, including the inlet section of the volute, the volute, the fixed guide vane, the movable guide vane, the runner, the draft tube and the elbow.

For this calculation, the main geometric change part is the discharge cone. Figure 6 shows the discharge cones of different shapes used in the experimental research and numerical research, including the unperforated prototype discharge cone, the 2-hole discharge cone Water cone and 4-hole discharge cone.

The above-mentioned prototype discharge cone is a short and straight discharge cone. The discharge cone with 2 holes is along the axis and relatively perforated. The discharge cone with 4 holes is constructed on the basis of drilling 2 holes. The relatively lower position is vertically pierced with 2 holes.

grid settings

In order to better adapt to the calculation of the complex 3D flow field in the entire flow channel of the turbine, each part of the above geometric configuration is divided into meshes, and the meshes of different parts are connected by interface in the calculation, and the corresponding mesh information As shown in Table 4.

Table 4 Grid information of each part of the full runner

The calculation grid in the flow domain is a mixture of structured grid and unstructured grid, which can meet the needs of numerical calculation under the number of 7 million grids.

Numerical Methods

For the numerical simulation of the flow field of the whole runner, this study uses the commercial CFD software ANSYS to calculate, and uses the CFX solver to solve. CFX is a finite volume discretization method based on finite elements, which uses 24-point interpolation for hexahedral grid cells, while the pure finite volume method only uses 6-point interpolation. Multi-node parallel computing is adopted, and the differential format adopts a high-order form.

The calculation convergence criterion is residual 10 ^-5 , the steady flow calculation is performed first, and then the unsteady flow calculation is performed with the steady flow result as the initial field. Since each part in the flow channel adopts the method of constructing grids separately, data transmission is required on the dynamic and static interface, that is, between the movable guide vane and the runner watershed, and between the runner and the draft tube watershed, the Frozen and Rotor series communication is used. Interface binding. In addition, in the processing of the wall area of the flow field, the wall function is used to describe it.

Boundary Condition Settings

According to the working condition setting of the model experiment, the numerical simulation adopts the same calculation working condition.

Given the diameter D ₁ of the low-pressure side of the runner and the water head H of the model experiment, the unit speed n ₁₁ and the opening of the movable guide vane a are given according to the experimental conditions. At this time, the unit flow Q ₁₁ can be obtained through the comprehensive characteristic curve. Therefore, starting from n ₁₁ and Q ₁₁ , we have:

Actual speed:

Actual traffic:

If the channel cross-sectional area at the inlet of the volute is S, the inlet velocity is:

v=Q/S(6)

In addition, according to the calculation formula of the cavitation coefficient σ of the above-mentioned device, the outlet pressure of the draft tube needs to be set in the numerical simulation.

According to the above, the boundary conditions in the numerical calculation are set as follows:

Inlet: Take the extended section of the volute inlet as the inlet, adopt the inlet condition of mass flow, and set parameters such as reference pressure and initial turbulence intensity at the same time.

Outlet: take the elbow outlet as the flow outlet, adopt the pressure outlet condition, the value is given by the above cavitation condition, and is given by the absolute pressure value.

Wall: All solid walls adopt no-slip boundary conditions, and the flow near the wall is simulated by wall functions.

governing equation

Considering that the flow in the whole channel of the turbine is incompressible three-dimensional flow, and ignoring the energy exchange with the outside world, the governing equations include the continuity equation and the momentum equation.

Continuity equation (when incompressible, the density ρ does not change with time and space):

Momentum equation:

When solving the three-dimensional flow of the whole channel of the turbine, the Reynolds average method (RANS) is used to solve the average flow field, and the corresponding turbulent flow model is closed by Reynolds stress. In the present invention, the closed turbulence model adopts the k-ωSST (stress-shear model) model, which combines the standard k-ε model with the k-ω model using a mixture function, including transition and shear options. The model equation of k-ωSST is as follows.

k equation:

ω equation:

Among them, G, Г and Y denote the respective generation term, effective diffusion term and dissipation term, D denote the orthogonal divergence term, and S is the user-defined source term.

In addition, the difference between this model and the standard k-ω model lies in the value of α _∞ , which is a constant in the standard k-ω model, but in the k-ωSST model, α _∞ is defined as follows:

α _∞ ＝F ₁ α _∞,1 +(1-F ₁ )α _∞,2 (11)

Including:

The k-ωSST model incorporates cross-diffusion derived from the ω equation, and the turbulent viscosity takes into account the propagation of turbulent shear stress. Its advantage is that both the near-wall region and the mainstream region are better treated, and the model does not contain complex nonlinear damping functions, so it is more stable and accurate.

In order to analyze the influence of the flow field in the entire channel on the draft tube flow and the corresponding draft tube pressure fluctuation information, the International Electrotechnical Commission-60193 standard in this paper selects four pressure measurement points in the draft tube area for analysis, that is, the distance below the taper tube. Two left and right measuring points at 0.3D ₂ of the wheel outlet (conical pipe +Y0.3D ₂ , -Y0.3D ₂ ), and two left and right measuring points at the joint between the draft tube and the elbow (inside the elbow, elbow outside). The same pressure fluctuation measuring points are used in model testing and numerical simulation.

In the pressure pulsation experiment of the model test, the measurement system collects only the voltage change signal, and the pressure signal value is obtained after the voltage is calibrated and converted. In the numerical calculation, through the simulation of the flow field, the velocity fluctuation and pressure fluctuation signal values in the flow field information will be obtained directly, and the pressure fluctuation acting on the wall surface will be directly compared with the model experiment results.

In order to analyze the pressure pulsation in the time domain, it is necessary to obtain the amplitude information and frequency information of the pressure pulsation. At the same time, in order to perform dimensionless processing on the frequency, the multiplier f' is defined. The multiplier f' can be divided by the known frequency f Obtained in frequency conversion. To convert from time domain information to frequency domain information, Fourier transform is required:

The above formula also becomes Fourier forward transform, so that the pulsation energy information at each frequency of the pressure pulsation signal can be obtained.

At a certain frequency f, the pressure pulsation amplitude A of the decomposed signal is calculated as follows:

In the formula, R is the real part of the frequency domain function, I is the imaginary part, and n is the number of sampling points.

Since the geometric distance between the inlet edge of the blade and the movable guide vane is very close, the interference generated by the high-frequency components of the two will increase the amplitude of the high-frequency pressure pulsation. Difference.

Finally, the frequency domain change spectrum of the pressure pulsation signal is obtained by plotting the amplitude at each frequency on the frequency domain axis.

Numerical Calculation of Flow Field in Hydraulic Turbine

According to the introduction of the model experiment, the present invention calculates the flow field under the partial working condition, and selects two different flow working conditions under the opening degree, and the specific working condition parameters are shown in Table 5.

Table 5 Numerical calculation working point parameters

In the above working conditions, calculations are carried out for three different discharge cone configurations, that is, the geometric shapes of the prototype 0-hole, 2-hole and 4-hole. It should be pointed out that the guide vane opening a=16mm is close to the optimal opening line of 17mm, and the unit speed n ₁₁ =80.4r/min is also close to the unit speed value of 74.5r/min in the optimal working condition; the guide vane opening of 28mm is close to The power limit line, the unit speed of this test condition is also relatively high, which is 83.2r/min. Therefore, the first working condition is a small flow partial working condition, and the second working condition is a large flow partial working condition.

The information about the flow field in the runner under the condition of small flow rate is that the flow inside the runner is relatively smooth, but there is a streamline inflection at the inlet of the suction surface of the blade, which means that the flow appears backflow, and backflow usually means the appearance of a vortex And the existence of low pressure area, so in this case, it is easy to appear the cavitation zone in the blade gap, that is, the existence of blade path vortex.

In order to further illustrate the flow field in the runner, three horizontal sections are selected from the axial flow direction to extract the velocity field. There is a certain flow backflow near the inlet of the suction surface of the blade, and the backflow velocity is relatively large, but the blade clearance Downstream, the flow is smoother and more uniform, and the flow along the circumference is gentle. At the same time, when it is close to the water outlet, the flow velocity has become very small, because it is close to the discharge cone, so it is the place where the vortex belt is born.

Comparative analysis of tailwater flow field under the condition of small flow deviation

In the experiment, the vortex belt under the partial working condition oscillates and the primary position changes. Here, the numerical simulation method is used to first give the lowest pressure area near the discharge cone. In order to guide the location of the perforation in the experiment, the pressure distribution of the draft tube part under the off-center condition is firstly given. Under the off-center working condition (a=16mm, σ=0.258, n ₁₁ =80.4r/min), Fig. 7 shows a partial pressure top view of the draft tube of the prototype 0-hole discharge cone.

It can be seen from Fig. 7 that the pressure distribution in the cross-section of the discharge cone and the draft tube is asymmetric under the off-center condition, and the circumferential pressure distribution has a minimum value somewhere. The pressure distribution on the surface of the discharge cone along its circumference is given below, as shown in Figure 8.

It can be seen from Fig. 8 that the circumferential pressure distribution of the discharge cone and the draft tube is also asymmetrical, and there is a minimum pressure point near the outlet of the discharge cone. When the pressure is lower than the local local pressure, a vortex appears. It is also found in the experiment that this is the place where the vortex belt is generated, but under this working condition, the vortex belt also jumps up and down. In order to eliminate this phenomenon, drilling measures are implemented in the place where the vortex is born, the purpose is to balance the internal and external flow pressure of the discharge cone, use the flow balance to affect the pressure distribution, and then further move the position of the vortex down, and reduce the vortex. The belt moves up and down while reducing the pressure pulsations it causes.

Fig. 9 further shows a top view of the plane pressure distribution of the tailwater flow field under the condition of drilling 2 holes under the off-center condition. Figure 10 further shows the pressure distribution on the surface of the discharge cone under off-center conditions (two holes are drilled, a=16mm, σ=0.258, n ₁₁ =80.4r/min), it should be noted that the pressures shown in Figures 9 and 10 The distribution threshold is consistent with Figure 7 and Figure 8. It can be seen that after the hole is drilled, the internal pressure of the discharge cone and draft tube is significantly increased. And the difference between the maximum value and the minimum value of the pressure distribution becomes smaller.

In Fig. 10, the pressure distribution on the surface of the discharge cone in the circumferential direction is given. It can be seen that the pressure level around the punch hole has also increased. At the same time, comparing the upper and lower figures, it can be seen that the pressure around the perforations at relative positions is different, which is because the tailwater vortex is generated from one side.

It is foreseeable that after the pressure distribution near the discharge cone is improved, the shape of the tailwater vortex should also change, and its pressure fluctuation will also change.

The following section continues to give the pressure distribution of the tail water part and the surface pressure distribution of the discharge cone in the case of drilling 4 holes.

Figure 11 shows the partial pressure distribution of the tail water when drilling 4 holes. It can be seen that under the condition of drilling 4 holes, the pressure distribution level of the draft tube is further improved, and the difference between the maximum value and the minimum value of the pressure is also reduced. Similarly, the pressure distribution on the surface of the discharge cone is shown in Figure 12, and the pressure level around the perforation has increased. It can be seen that under partial working conditions, perforation has played a positive role in increasing the pressure level, and the gradient of pressure distribution has also slowed down.

In order to further illustrate the impact of perforation measures on the flow, the change of the vortex under the off-center condition is given below:

When extracting the shape of the vortex, the same vorticity criterion is used in the three configurations, and the same cavitation pressure is used to determine the cavitation volume fraction. Comparing the three cases, it can be seen that, firstly, with the implementation of the drilling measures, the blade path vortex inside the runner gradually decreases, its cavitation volume becomes smaller, and the vortex structure becomes finer; secondly, starting from the area of the discharge cone, The volume of the cavitation vortex in the region where the vortex belt is born under the discharge cone is obviously reduced. The reduction of the cavitation position helps to reduce the oscillation and jump of the vortex belt itself, and has a positive effect on reducing the pressure pulsation.

The pressure pulsation level induced by the vortex belt after the above perforation can be given in the subsequent pressure pulsation calculation, and a significantly weakened pressure pulsation can be obtained.

Comparative analysis of tailwater flow field under the optimal working condition of large flow rate

Under the condition of small flow rate deviation, it can be seen that the perforation measure obviously reduces the pressure level, and the performance of this measure under the large flow rate condition is given below. In this working condition, the optimal working condition is far away, and the vortex belt still exists, but the vortex belt presents a different shape.

Under the condition of large flow deviation (a = 28mm, σ = 0.277, n ₁₁ = 83.2r/min), Fig. 13 shows a partial pressure top view of the draft tube of the prototype zero-hole discharge cone. It can be seen that the pressure distribution in the circumferential direction of the discharge cone and the draft tube has a good symmetry, and the low pressure area is located around the discharge cone, which is the place where the vortex zone is born.

Figure 14 shows the fractal shape of the vortex under this working condition, showing a columnar vortex. Furthermore, the pressure distribution on the surface of the water discharge cone is shown in Figure 15, which includes the pressure distribution in four directions around the water discharge cone. It can be seen that in the circumferential direction of the discharge cone, a low-pressure zone exists at the outlet of the discharge cone, where the cavitation vortex will be generated after the pressure is lower than the local vaporization pressure. This is consistent with the pressure distribution.

The pressure distribution of the draft tube and the surface pressure distribution of the discharge cone under the near-optimal working condition of drilling two discharge cones are given below. As shown in Figures 16 and 17, although the pressure distribution at the outlet of the discharge cone has changed, it is still symmetrical, and the pressure level has increased. The same trend can also be observed in the 4-hole discharge cone condition, as shown in Fig. 18 and Fig. 19.

Although the above pressure distribution is similar, with the implementation of perforation measures, the pressure level increases, which will affect the shape of the vortex in this working condition. Figure 20 shows the distribution of the discharge cone and vortex zone of the discharge cone with 2 holes and 4 holes under the near-optimal working condition.

In the above two discharge cone cases, the tailwater cavitation presents a slender columnar straight vortex zone, and the cavitation zone is smaller. And as the number of perforations increases, the cavitation zone becomes smaller, which is the basic situation of the change of the vortex zone under the condition of large flow rate. It can also be seen that after drilling, the cavitation vortex distribution at the outlet of the runner is significantly reduced, and only a small amount is concentrated around the outlet of the discharge cone. This should be due to the perforation measures change the flow state of the vortex.

Under the above conditions, there is still a strong swirl structure in the draft tube, but the shape of the vortex belt is relatively stable, and there will be certain pressure fluctuations.

Calculation results of pressure pulsation under partial working conditions

Under the condition of large flow, the unit generally has less chance of running there, or the running time is shorter. The following mainly focuses on the pressure pulsation under the condition of small flow deviation.

In the calculation of pressure fluctuation, the steady-state three-dimensional numerical simulation under this working condition is first carried out, and then the transient numerical simulation is carried out after the flow enters a fully developed state. pulsation level.

The pressure fluctuation results of the three discharge cone configurations under the small flow deviation condition (a=16mm, σ=0.258, n ₁₁ =80.4r/min) are given below, and the results for the four measuring points are shown in Figure 21 It is shown that the pressure pulsation has been converted in the frequency domain using the Fourier transform, and the corresponding power density function (PSD) distribution at each frequency is shown in the figure.

It can be seen from Fig. 21 that under the off-center condition, the pressure pulsation level on the right side of the conical tube is the lowest, while the pressure pulsation level on the left side of the conical tube, the inner side of the elbow tube and the outer side of the elbow tube are all the highest. After the perforation measures are implemented, the energy of pressure fluctuations gradually decreases with the increase of the number of perforations, which is consistent with the pressure distribution in the flow field and the development trend of the vortex.

In addition, as the number of perforations increased, the pressure pulsation energy on the inside and outside of the cubital tube decreased most obviously, and could drop to about 1/2 of the original level. This shows that perforation measures do play an important role in suppressing pressure pulsation.

Finally, it can be seen that the energy distribution on the outside of the cubital tube also has more components at higher frequencies, which shows that although the pressure pulsation occupies the most energy at the main frequency, the higher frequency still has a non-negligible contribution. It can be obtained that if all pressure pulsation energies are superimposed, the pressure pulsation energy at the outside of the elbow is the highest among the four measuring points, which is consistent with the following experimental measurement. However, since the overall value of the pressure pulsation energy decreases after drilling, the noise during unit operation will be significantly reduced, which has also been confirmed by experiments.

Experimental Research on Flow Field of Hydraulic Turbine Draft Tube

In the experiment, the small flow partial working condition was mainly selected, and the energy and efficiency experiments were carried out first.

Table 6 Energy and efficiency experimental parameters of different discharge cone configurations

As shown in Table 6, the rotation frequency is about 18.6 Hz under off-center operation, and the main frequency of the tailwater vortex is 0.242 times the rotation frequency, which is basically the same under the three discharge cone configurations. According to the energy derivation efficiency, it can be known that the overall efficiency changes very little after drilling, and only slightly decreases when drilling 2 holes, but increases when drilling 4 holes. The change of efficiency should be related to the change of pressure pulsation, the pulsation energy is reduced, the energy loss is reduced, and the efficiency is improved.

It can be seen that the punching measures will basically not affect the overall operating efficiency of the unit, so it has little impact on the energy test. It can be speculated that the perforation measures will only affect the downstream part of the flow field in the full channel, which will reduce the velocity and pressure fluctuations of the draft tube flow field, so that the noise will also be reduced accordingly. Therefore, it is an effective measure for local cavitation optimization.

In the following, the experimental analysis of the draft tube flow field will be carried out. The experimental method is a model experiment. In the transparent draft tube part, the flow and vortex zone are photographed by high-speed photography, and the evolution time series of the draft tube vortex zone of the prototype discharge cone is observed. Under the above working conditions, the cavitation vortex runs for about 216ms, and the frequency of the tailwater vortex is about 4.5Hz.

In this working condition, the vortex belt presents a spiral shape and revolves around the center of the discharge cone. Careful observation of the running time of the vortex shows that the vortex at 36ms, 96ms and 108ms is composed of 2 to 3 thinner vortex overlapping, and it is also rotating around its own spiral center. Moreover, the initial position of the vortex also appears to be bouncing up and down along the surface of the discharge cone.

The generation of multiple fine vortex bands should be related to the position of the initial vortex bands at the discharge cone. If the area of the low-pressure area on the surface of the discharge cone is large, multiple cavitation primary areas are likely to appear. And the numerical simulation also shows that there are many vortex formation points at the bottom of the surface of the prototype 0-hole discharge cone under this working condition. It can be seen that the numerical simulation results are in good agreement with the experimental results.

In addition, if the contribution of the cavitation vortex to the pressure pulsation energy is considered, it can be reasonably speculated that: the revolution of the vortex contributes to the main frequency of the pressure pulsation, while its own rotation around the center of the spiral contributes to the higher frequency of the pressure pulsation. frequency energy.

The high-frequency component of the pressure fluctuation in the vaneless area of the Francis turbine is not obvious, and the amplitude accounts for a small proportion of the mixing amplitude. The pressure fluctuation in the bladeless area also has multiple frequency components of the blade passing frequency. Overall, the pressure fluctuation in the bladeless area presents the characteristics of high-frequency fluctuations. For punching measures, it is possible to reduce the main frequency energy, it is also possible to reduce the higher frequency energy, or it is also possible to reduce both.

In order to systematically analyze the effect of perforation on pressure fluctuation, the evolution of the vortex zone of different discharge cones under the condition of small flow deviation is given below. The evolution time series of vortex belts were observed when 2 holes were drilled and 4 holes were drilled. It can be seen that, for the perforated discharge cone, although the area of cavitation incipient at the outlet of the discharge cone is significantly reduced (2 holes are drilled) or even disappeared (4 holes are drilled), but due to the existence of the perforated Hole as the starting point, a relatively small vortex appears, replacing the thick spiral vortex of the prototype 0-hole drainage cone. And with the application of drilling measures, the phenomenon of vortex jumping up and down is weakened or eliminated, especially when drilling 4 holes, the original thicker spiral vortex has been scattered with very thin single vortex, and almost disappeared at some moments.

It should be pointed out that although the volume of cavitation is reduced under the above-mentioned drilling conditions, it can be seen that there are sometimes thinner double vortex belts, that is, the cavitation energy still exists in the high-frequency state. On the pressure pulsation spectrum, there will still be higher frequency peaks.

Finally, observing the shape of the vortex under the condition of large flow deviation (a=28mm, σ=0.277, n ₁₁ =83.2r/min), it can be seen that under the condition of large flow deviation, regardless of the discharge cone Regardless of the type, the shape of the vortex belt is columnar, and its diameter varies greatly. However, from the vibration and noise observations in the experiment, the noise level of the perforated discharge cone is relatively low. In addition, the shape of the above-mentioned vortex is consistent with the results of numerical simulation.

Pressure Fluctuation Analysis of Draft Tube of Hydraulic Turbine

While observing the evolution of vortex belts, pressure fluctuations were also monitored. In the experiment, according to the method of model experiment, the pressure probe was embedded in the wall of the draft tube to detect the flow field and the pressure value on the wall of the draft tube, and record its change with time. The pressure pulsation experiment results of the four monitoring points are analyzed below.

The Fourier transformation of the initial pressure pulsation experimental data can directly analyze the pulsation energy distribution under off-load conditions. The experimental results of the small flow deviation working condition show that after the perforation measures are implemented, the overall level of pressure pulsation decreases, and the main frequency amplitude decreases the most outside the elbow. For the pressure measuring points (+Y0.3D2, -Y0.3D2) of the conical pipe part, after drilling, the higher frequency pulsation energy decreased significantly, and disappeared in the working condition of drilling 4 holes.

In addition, the experimental results show that the pressure pulsation energy on the outside of the elbow is the largest before the hole is punched, and its amplitude drops most significantly after the hole is punched. Compared with the numerical simulation results, the experimental results have well verified the numerical simulation results. The overall level of pressure fluctuations has decreased after drilling, which also shows that the measure of drilling holes on the discharge cone is effective.

In order to quantitatively explain the energy of pressure pulsation, the experimental data of pulsation obtained for different discharge cone structures under different working conditions are given below. According to the IEC test standard, the evaluation of the pressure pulsation is expressed by the mixed frequency double-amplitude amplitude (peak-to-peak value) ΔH/H. As shown in Tables 7, 8, 9 and 10, they are the pressure pulsation characteristic data of the conical tube +Y0.3D2, conical tube-Y0.3D2, inner side of the elbow and outer side of the elbow respectively.

It can be seen that under the condition of small flow deviation, the spiral vortex belt is relatively stable, and the ratio of the main frequency to the rotation frequency is about 0.24. With the increase of the number of holes, the variation range of the peak-to-peak value of the pulsation is not large. The more significant change is the pressure pulsation under the large flow deviation working condition. Except for the inner side of the elbow, the other measuring points are in the perforated configuration, and the ratio of the main frequency to the rotation frequency is significantly reduced, and the main frequency reduction value is greater than 1/ 2 times the working condition of the prototype discharge cone. Moreover, the reduction of the main frequency is likely to be related to the weakening and disappearance of the vortex in the experiment. At some time, there is a discontinuous phenomenon of the vortex in the draft tube. In particular, for the working condition of drilling 4 holes, the energy peak-to-peak value of the main frequency is significantly reduced in all the configurations of the discharge cone. It can be seen that the hole-punching measure is effective.

Table 7 Pressure pulsation characteristics of conical pipe + Y0.3D ₂ points under different working conditions and discharge cone configurations

Table 8 Pressure pulsation characteristics of conical pipe-Y0.3D ₂ points under different working conditions and discharge cone configurations

Table 9 Pressure pulsation characteristics of measuring points inside the elbow under different working conditions and discharge cone configurations

Table 10 Pressure pulsation characteristics of measuring points outside the elbow under different working conditions and discharge cone configurations

In addition, it should be noted that when the opening of the movable guide vane increases, the peak-peak value of the main frequency pulsation at the inlet of the draft tube and the outlet of the elbow tube decreases, which means that the corresponding high-frequency components of the pressure pulsation become more obvious. In order to further determine the energy amplitude after drilling, the energy of the first three frequencies (f1, f2, f3) of the pressure pulsation is summed here to characterize the overall energy, as shown in Table 11. It can be seen that the sum of the energy of the first three orders decreases as the number of holes increases. In general, perforation measures have a positive effect on weakening the formation of vortexes and the corresponding pressure fluctuations.

Table 11 The energy characteristics of the first three order frequencies of pressure fluctuations under different discharge cone configurations ΔH/H(%)

Above, a kind of discharge cone punching method based on the three-dimensional simulation method of the full flow field of the hydraulic turbine provided by the present invention has been introduced in detail. In this paper, specific examples have been used to illustrate the principle and implementation of the present invention. The above examples The description is only used to help understand the method of the present invention and its core idea; at the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation and scope of application. In summary, As stated above, the content of this specification should not be construed as limiting the present invention.

Claims (8)

1. A kind of discharge cone punching method based on the three-dimensional simulation method of the full flow field of a hydraulic turbine, characterized in that: Step 1. Establish a model testing experimental system, design different discharge cone drilling models for matching processing, improve the hydraulic test bench according to different configurations of discharge cones, and perform high-speed photogrammetry to obtain the vortex inside the turbine runner and the draft tube. The experimental information brought; Step 2. Select the turbulent flow model, and use the RANS simulation method to perform three-dimensional unsteady simulation of the full flow field of the turbine, capture the blade path vortex between the runner blades, the draft tube vortex belt, and the deflow cavitation phenomenon at the blade inlet edge, and determine according to the captured phenomenon The place where the vortex belt is born, is drilled at the place where the vortex belt is born; the calculation of the three-dimensional simulation method of the full flow field of the hydraulic turbine includes geometric modeling, grid setting, numerical method, control equation and boundary condition setting; Step 3. Synthesize the model experiment and numerical simulation data, and test the different discharge cone drilling models under different flow deviation conditions. 2. The drilling method according to claim 1, characterized in that: the geometric modeling in the calculation is based on an overall geometric model, and the geometric model includes a volute inlet section, a volute, a fixed guide vane, and a movable guide vane , runner, draft tube and elbow 7 parts. 3. The hole punching method according to claim 2, characterized in that: the grid is configured to perform grid division on each part of the geometric model, and the grids of each component are connected using an interface during calculation. 4. The punching method according to claim 3, characterized in that: said numerical method adopts CFD software ANSYS to calculate, adopts CFX solver to solve, and said CFX is a finite volume discretization method based on finite element, to hexahedron Grid cells are interpolated using 24 points. 5. The drilling method according to claim 1, characterized in that: the control equations include continuity equations and momentum equations. 6. The punching method according to claim 2, characterized in that: the boundary conditions are set to: Inlet: take the extended section of the volute inlet as the inlet, adopt the mass flow inlet condition, and set the reference pressure and initial turbulence intensity parameters at the same time; Outlet: take the elbow outlet as the flow outlet, and adopt the pressure outlet condition; Wall: All solid walls adopt no-slip boundary conditions, and the flow near the wall is simulated by wall functions. 7. The punching method according to claim 1, characterized in that: said drain cone punching is a drain cone with 2 holes, and the drain cone with 2 holes is along the axis and relatively perforated. type. 8. The drilling method according to claim 1, characterized in that: the drainage cone perforated is a drainage cone with 4 holes, and is relatively perforated into a drainage cone with 2 holes along the axis of the drainage cone , and then on the basis of the discharge cone of the 2 holes, the 2 holes are vertically pierced at a relatively lower position.