CN106951602B - Method for punching water discharge cone of water turbine - Google Patents
Method for punching water discharge cone of water turbine Download PDFInfo
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Abstract
The invention provides a method for punching a water turbine wash-out cone, which comprises the steps of establishing a model test experiment system, selecting a turbulence model, performing three-dimensional unsteady simulation on a water turbine full flow field by using an RANS simulation method, integrating model experiments and numerical simulation data, and testing different wash-out cone punching models under different flow deflection working conditions. The invention solves the problem of unsteady flow in the water turbine set in the prior art, in particular to the problems of vortex belt and corresponding pressure pulsation in the tail water pipe.
Description
Technical Field
The invention belongs to the technical field of water turbine stability, and particularly relates to a punching method for a water turbine wash-out cone.
Background
Nowadays, the importance of hydroelectric power generation is increasingly prominent, and the requirements for improving the operation efficiency of a hydropower station and the operation stability of a water turbine are particularly prominent. Among the factors influencing the stable operation inside the water turbine, the hydraulic factors are the most prominent, including dynamic and static interference between rotating parts and static parts, blade surface defluidization, cavitation blade channel vortex, cavitation draft tube vortex band and the like. In the operation of the water turbine, most of the flow phenomena occur under the partial working condition deviating from the design working condition, serious pressure pulsation can be induced in a corresponding flow field, and the pressure pulsation further propagates to act on the water turbine unit, so that the vibration and the operation noise of the unit are caused, and even the vibration of a plant is induced. Pressure pulsations are a major source of unsteady operation and vibration noise.
Pressure pulsation caused by a three-dimensional flow field in the water turbine comes from multiple aspects, such as flow separation at the inlet of a runner, and the induced flow field is transmitted to the upstream direction and the downstream direction to cause hydraulic excitation of an upstream component and a downstream component; in the rotating wheel, the blade channel vortex is the largest unstable source, the generation of the blade channel vortex is often accompanied with cavitation flow, and pressure pulsation caused at the position directly acts on the rotating wheel to form high-frequency vibration; cavitation vortices are generated from below the wash water cone inside downstream components, such as the tailrace water pipe, creating a spiral motion, which periodically acts on the cone and elbow sections causing vibration of the downstream components and inducing noise. Experiments show that the running frequency of the wake vortex band is low-frequency vibration, and the vibration and noise caused by the wake vortex band are different under different flowing working conditions, but in general, the wake vortex band motion is the lowest-frequency motion, and the influence on the unit is the most serious. For example, in the time of half year and two years of operation of a unit in a beach hydropower station and a Li's gorge hydropower station, the cracks of welding seams between the runner blade and the upper crown and between the blade and the lower ring occur in several water turbine units in succession. Through analysis of the causes of the wheel cracks, the main causes are the manufacturing and running causes, and severe pressure pulsation in running is a direct factor for causing the cracks.
In the mixed-flow turbine, the pressure pulsation induced by the draft tube vortex band is the most main source causing vibration and noise, and at present, many researches have been made to analyze the mechanism and evolution of the draft tube vortex band, and measures for reducing or eliminating the draft tube vortex band are proposed, such as changing the water flow motion state in the draft tube, controlling the eccentricity of the vortex band, introducing proper damping or improving the hydraulic design of a runner, however, these measures cannot effectively weaken the pressure pulsation, and some of the measures can bring additional noise.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provide a method for punching a water escape cone of a water turbine, which solves the problem of unsteady flow in a water turbine set, in particular the problems of a vortex band and corresponding pressure pulsation in a tail water pipe.
The purpose of the invention is realized by the following technical scheme: a method for punching a water discharge cone of a water turbine,
and 3, integrating model experiments and numerical simulation data, and testing different outlet cone punching models under different flow deflection working conditions.
Furthermore, the test condition in the model test experiment system is that unit rotating speed n is adopted11And unit flow rate Q11Describing the flow working condition in the full flow channel, changing the environmental pressure level in the test by adjusting the cavitation coefficient sigma of the device, and changing the unit rotating speed and the unit flow by adjusting the opening a of the movable guide vane;
unit speed of rotation n11Is defined as:
in the formula D1-the diameter of the runner of the model turbine in m;
h is the experimental water head of the model water turbine, and the unit is m;
n is the rotating speed of a rotating wheel in the model water turbine, and the unit is r/min;
unit flow rate Q11Is defined as:
the unit of the flow of the overflowing in the runner of the model mixed-flow turbine is m3/s；
The device cavitation coefficient σ is defined as:
in the formula Hva-the vacuum value in m in the tailwater tank of the model experiment circulation loop;
Haconverting local atmospheric pressure into a water head in m in the experiment;
Hsthe suction height of the water turbine in the experiment is m;
Hvthe saturated vapor pressure at the experimental temperature is converted into a water head, and the unit is m.
Further, the model test experiment working conditions include a small flow deviation working condition and a large flow deviation working condition, and the working point parameters are as follows:
the small flow deflection working condition parameter is that a is 16mm, n11＝80.4r/min，Q11＝0.486m3/s，σ＝0.258；
The large flow deflection working condition parameter is that a is 28mm, n11＝83.2r/min，Q11＝0.768m3/s，σ＝0.277。
Further, the turbulence model employs a stress shear model.
Furthermore, the drainage cone is a short straight drainage cone, the drainage cone is punched into 2 holes, and the drainage cone punched into 2 holes is in a configuration relative to the through hole along the axis.
Further, the above-mentioned wash-out cone perforation is a wash-out cone for perforating 4 holes, and a wash-out cone for perforating 2 holes is relatively perforated along the axis of the wash-out cone, and then the 2 holes are vertically perforated at a relatively lower position on the basis of the wash-out cone for 2 holes.
Drawings
FIG. 1 is a schematic view of a hydraulic test bench;
FIG. 2 is a schematic view of a rotational speed measurement;
FIG. 3 is a schematic view of a pressure pulsation measurement station;
FIG. 4 is a pictorial view of a model wheel (A1293);
FIG. 5 shows a different configuration of a bleed cone (a) prototype 0 holes (b) 2 holes (c) 4 holes;
FIG. 6 is a-0 degree view of a prototype wash-out cone (b) of a water turbine computational geometry model (a) of a 4-hole wash-out cone (c) of a 2-hole wash-out cone (d) of a 4-hole wash-out cone-90 degree view;
FIG. 7 is a top view of the tail pipe pressure distribution under an off-working condition (prototype 0 hole, a is 16mm, σ is 0.258, n11＝80.4r/min)；
FIG. 8 is the surface pressure distribution of the water discharge cone under the eccentric condition (prototype 0 hole, a is 16mm, sigma is 0.258, n11＝80.4r/min)；
FIG. 9 is a top view of the draft tube pressure distribution under an off-working condition (2 holes are drilled, a is 16mm, sigma is 0.258, n11＝80.4r/min)；
FIG. 10 shows the surface pressure distribution of the water discharge cone under the eccentric condition (2 holes are drilled, a is 16mm, sigma is 0.258, n11＝80.4r/min)；
FIG. 11 is a top view of the draft tube pressure distribution under an off-condition (4 holes are drilled, a is 16mm, σ is 0.258, n11＝80.4r/min)；
FIG. 12 shows the surface pressure distribution of the water discharge cone under the eccentric condition (4 holes are drilled, a is 16mm, sigma is 0.258, n11＝80.4r/min)；
FIG. 13 is a top view of the draft tube pressure distribution under high flow conditions (prototype 0 hole, a is 28mm, σ is 0.277, n)11＝83.2r/min)；
FIG. 14 shows the prototype wash-out cone draft tube vortex strip (prototype 0 hole, a is 28mm, σ is 0.277, n) under high flow conditions11＝83.2r/min)；
FIG. 15 shows the pressure distribution of the surface of the water discharge cone under the large flow condition (prototype 0 hole, a is 28mm, σ is 0.277, n)11＝83.2r/min)；
FIG. 16 is a top view of the draft tube pressure distribution under high flow conditions (2 holes, a is 28mm, σ is 0.277, n)11＝83.2r/min)；
FIG. 17 shows the pressure distribution of the surface of the water discharge cone under the condition of large flow (2 holes are drilled, a is 28mm, sigma is 0.277, n11＝83.2r/min)；
FIG. 18 is a top view of the draft tube pressure distribution under high flow conditions (4 holes, a is 28mm, σ is 0.277, n)11＝83.2r/min)；
FIG. 19 is the surface pressure of the water discharge cone under the large flow conditionForce distribution (4 holes, a is 28mm, σ is 0.277, n)11＝83.2r/min)；
FIG. 20 shows the vortex band (a is 28mm, sigma is 0.277, n) of the tail pipe of the perforated drainage cone under the working condition of large flow11＝83.2r/min)；
FIG. 21 shows the distribution of the pulsating energy of draft tube pressure under different conditions ( prototype 0, 2, 4 holes, a is 16mm, sigma is 0.258, n11＝80.4r/min)。
Detailed Description
The technical solutions in the embodiments of the present invention will be described clearly and completely with reference to the accompanying drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
The invention provides a method for punching a water escape cone of a water turbine,
and 3, integrating model experiments and numerical simulation data, and testing different outlet cone punching models under different flow deflection working conditions.
The test condition in the model test experiment system is that unit rotating speed n is adopted11And unit flow rate Q11Describing the flow condition in the whole flow passage, changing the environmental pressure level in the test by adjusting the cavitation coefficient sigma of the device, and changing the unit rotating speed and the sum of the rotating speed by adjusting the opening a of the movable guide vaneUnit flow rate;
unit speed of rotation n11Is defined as:
in the formula D1-the diameter of the runner of the model turbine in m;
h is the experimental water head of the model water turbine, and the unit is m;
n is the rotating speed of a rotating wheel in the model water turbine, and the unit is r/min;
unit flow rate Q11Is defined as:
the unit of the flow of the overflowing in the runner of the model mixed-flow turbine is m3/s；
The device cavitation coefficient σ is defined as:
in the formula Hva-the vacuum value in m in the tailwater tank of the model experiment circulation loop;
Haconverting local atmospheric pressure into a water head in m in the experiment;
Hsthe suction height of the water turbine in the experiment is m;
Hvthe saturated vapor pressure at the experimental temperature is converted into a water head, and the unit is m.
The model test experiment working conditions comprise a small flow deviation working condition and a large flow deviation working condition, and the working point parameters are as follows:
the small flow deflection working condition parameter is that a is 16mm, n11＝80.4r/min，Q11＝0.486m3/s，σ＝0.258；
The large flow deflection working condition parameter is that a is 28mm, n11＝83.2r/min，Q11＝0.768m3/s，σ＝0.277。
The turbulence model adopts a stress shear model.
The drainage cone is a short straight drainage cone, the drainage cone punching is a 2-hole drainage cone, and the 2-hole drainage cone is in a structure which is opposite to the through hole along the axis.
The drainage cone punching is a 4-hole drainage cone, the drainage cone is punched into a 2-hole drainage cone along the axis of the drainage cone, and the 2-hole drainage cone is vertically punched at a position relatively lower than the drainage cone on the basis of the 2-hole drainage cone.
Model experiment of water drain cone of water turbine
Model experiments are an important method for researching the flow characteristics in the water turbine. In order to obtain flow field information and draft tube vortex band information under different drain cone conditions, experimental measurement is carried out by utilizing a water turbine model test bed.
The model test bed is positioned in a water turbine research room of a large motor research institute of Harbin electric machinery plant, Inc., is a high-parameter and high-precision hydraulic mechanical test bed, and can be used for carrying out related tests on a water pump water turbine, a large water pump and a high-water-head mixed-flow water turbine.
The test capability, the size of the mountable model, the test method of the hydraulic performance and the test water of the hydraulic test experimental device numbered test 6 meet the requirements of IEC 60193-1999. The test contents mainly comprise: the energy test, cavitation test, runaway rotational speed test, pressure pulsation test, differential pressure flow measurement test, axial force test, guide vane hydraulic moment test, air supply test, abnormal low head test and the like of the water turbine, and in addition, the flow state observation system of the test bed equipment can observe the internal flow state of the hydraulic machine.
As shown in fig. 1, is a schematic diagram of key components of a hydraulic test bench. The platform is provided with a high-performance power pump, a vertical structure dynamometer motor, a high-pressure-resistant stainless steel pipeline system, an electric valve, a vacuum pump, a high-precision testing instrument, an in-situ calibration system of the high-precision testing instrument and the like. The high-precision electrical control system of the test bed can quickly and precisely adjust and control power equipment such as a water supply pump, a power measuring motor and the like. And a high-precision testing instrument of the test bed equipment measures various test parameters. A high performance data acquisition and processing system acquires and processes data.
The main structural components of the test stand are shown in table 1.
Table 1 test stand main structural parts
The above-described apparatus is described as follows:
test rotating speed: the rotation speed of the test device is realized by a dynamometer motor, the dynamometer motor is supplied with power by a silicon controlled rectifier power supply and can operate in four quadrants, namely, the dynamometer motor can operate as a generator and also as a motor when rotating forwards and backwards, so that the rotation speed test device can meet the requirements of various test working conditions of different test devices of a water turbine, a water pump and a water pump-water turbine.
Test head and flow: the highest test water head is 100m, and the maximum flow is 1.0m3/s。
Test water: the high-efficiency water treatment equipment of the test bed equipment is used for testing after filtering and softening tap water. The four conditions of the water used in the test mainly relate to the density of the water, the viscosity of the water, the gas content of the water and the steam pressure.
Measurement of atmospheric pressure: the atmospheric pressure of the test bed is measured by an atmospheric pressure sensor, the atmospheric pressure measurement module is verified in the metering bureau of Heilongjiang province every year, and a verification certificate is issued after the verification is qualified.
The rotating speed measuring system of the hydraulic mechanical test bed consists of a speed measuring sensor and a speed measuring ruler disk with 120 teeth. The rotation speed sensor is an MP-981 type rotation speed sensor produced by Nippon Xiaoye company. The speed measuring scale disc is arranged on the upper part of the shaft of the water turbine, and the relative positions of the rotating speed sensor and the speed measuring scale disc are shown in figure 2. The rotating speed measuring system can measure the rotating speed of 10000 r/min. The electric pulse signal generated by the speed measuring system directly enters the data acquisition system and the data processing software for calculation.
On the hydraulic test experiment table, the flow state observation of the blade vortex and the draft tube in the runner is also carried out, which is mainly completed by a high-speed photographic imaging system.
The flow state observation system consists of a stroboscope of Germany DRE LL O company, an optical fiber endoscope of Germany WO L F company, a digital industrial camera, a synchronous trigger controller and an image acquisition processing workstation, and can be used for carrying out real-time flow state observation, static image acquisition and dynamic flow media file generation.
In addition, through the transparent taper pipe at the model test device, the flow state observation system can be used for observing cavitation phenomena such as outlet water side karman vortex, primary cavitation, vortex band and the like of the runner. High-frequency information capture of the vortex tape was performed simultaneously using a high-speed camera produced by Olympus corporation.
The specification of the optical fiber endoscope is as follows, the specification is produced by German WO L F company, the size is phi 10mm × 300mm × DOV50/80/90 degrees, the multi-view probe is 10mm in diameter, the working length is 300mm, and an optical cable is fixed at 4 m.
The high-SPEED photographic equipment is manufactured by Olympus corporation of Japan, the specification is as follows, the model is i-SPEED 2, a CMOS sensor, the resolution is 800 × 600 effective pixels, the frame rate can reach 33000fps at most, 1fps at least, the maximum 1000fps under the full resolution, the shutter SPEED is 5 microseconds, an external controller display device of the photographic device adopts a standard L VDS connector, and a 26-pin MDR is connected with the photographic equipment.
In order to evaluate the hydraulic stability of a model hydraulic machine, pressure pulsation tests are generally carried out, which are mainly carried out in order to obtain the relative amplitude and dominant frequency of the pressure pulsation in a specific operating range and to determine the optimal air supply mode for reducing the pressure pulsation.
Pressure pulsations are often measured using a pressure pulsation sensor. The installation of the pressure pulsation sensor requires that the diaphragm be flush with the flow channel. As shown in fig. 3, it is a common mounting position of francis, axial fixed blade and pump turbine. According to the IEC test standard, the positions of 4 pressure measuring points at the tail water pipe are shown as follows, P1 is positioned on the downstream side of a tail water taper pipe, P2 is positioned on the upstream side of the tail water taper pipe, P3 is positioned at the inlet of a volute, additional sensors of P4 are positioned at the positions of the taper pipe and an elbow pipe, and in addition, a P5 measuring point is arranged between a guide vane and a rotating wheel.
The pressure pulsation measurement adopts a 112A22 type dynamic pressure sensor produced by American PCB company, the sensitivity of the sensor is 15mv/kPa, the resolution ratio is less than 0.007kPa, the frequency range is 0.5-250kHz, the maximum flow-through frequency which can be reached by a test machine is covered, and the sensor can quickly and accurately respond to the hydraulic pulsation. The test bed high-speed data acquisition system acquires the response signals of the pressure pulsation sensor at a high sampling rate, and the sampling frequency is 4000Hz generally.
The pressure pulsation sensor was calibrated before and after the test using a 903BO2 model dynamic pressure calibrator (accuracy. + -. 0.2% FS) manufactured by the American PCB company.
The hydraulic test experiment table is based on a model water turbine of a certain power station, and comprises a maximum water head 71m, a minimum water head 44m and a rated water head 57 m. The model turbine runner (a1293) is shown in fig. 4, and the parameters of the model runner and other important equipment are shown in table 2.
TABLE 2 model rotor principal parameters
In order to make flow measurements for different forms of the bleed cone, different configurations of the bleed cone under test are given below, including prototype 0 holes, 2 holes and 4 holes, as shown in fig. 5.
In the model experiment, the unit rotation speed n is usually adopted11And unit flow rate Q11To describe the flow conditions in the full flow channel and to change the ambient pressure level under test, the device cavitation coefficient, σ, needs to be adjusted. The unit rotating speed and the unit flow are changed by adjusting the opening a of the movable guide vane.
Unit speed of rotation n11Is defined as:
in the formula D1The runner diameter (low pressure side) (m) of the model turbine;
h-model turbine experimental head (m);
n-rotating speed (r/min) of a runner in the model water turbine.
Unit flow rate Q11Is defined as:
flow (m) of flow passing in runner of model mixed-flow turbine3/s)。
For model testing, the device cavitation coefficient has a significant impact on the internal flow phenomenon. According to the international electrotechnical commission-60193 standard, the device cavitation coefficient σ is defined as:
in the formula Hva-a vacuum value (m) in the tailwater tank in the model experiment circulation loop;
Ha-converting the local atmospheric pressure into a head (m) in the experiment;
Hs-suction height (m) of water turbine in experiment;
Hvthe saturated vapor pressure at the experimental temperature is converted into the head (m).
According to the above discussion, n is used11And Q11The operating conditions of the model experiment are described as shown in table 3. Among the working conditions, including the working condition of small opening degree and the working condition of large opening degree, tail water vortex bands with different forms, such as spiral vortex bands under small opening degree and columnar vortex bands under large opening degree, can be observed.
TABLE 3 Experimental test Condition Point parameters
Three-dimensional simulation method for full flow field of water turbine
Geometric modeling
In the calculation, the full-flow-channel simulation of the water turbine is used, and in order to facilitate numerical calculation, the part of an inlet section entering the volute is lengthened, the inlet is a circular inlet, and the outlet is a draft tube elbow outlet.
The calculation area is based on an overall geometric model, which comprises 7 parts of a volute inlet section, a volute, a fixed guide vane, a movable guide vane, a rotating wheel, a draft tube, an elbow and the like.
For purposes of this calculation, the primary geometry-changing component was a bleed cone, and FIG. 6 shows different configurations of bleed cones used in experimental and numerical studies, including an unperforated prototype bleed cone, a 2-hole bleed cone, and a 4-hole bleed cone.
The prototype wash-out cone is a short straight wash-out cone, the 2-hole wash-out cone is a structure which is relatively perforated along the axis, the 4-hole wash-out cone is constructed on the basis of the 2-hole wash-out cone, and the 2-hole wash-out cone is vertically perforated at a relatively lower position.
Grid arrangement
In order to better adapt to the calculation of the 3D flow field with complicated flow in the whole flow channel of the water turbine, each component of the geometric configuration is respectively subjected to grid division, grids of different components are connected by using interfaces in the calculation, and corresponding grid information is shown in a table 4.
TABLE 4 full flow channel parts grid information
The flow domain computational grid is a mixture of structured grids and unstructured grids, and the requirement of numerical computation can be met under 700 ten thousand grids.
Numerical method
Aiming at the numerical simulation of the full-flow-channel flow field, commercial CFD software ANSYS is adopted for calculation, and a CFX solver is adopted for solving. CFX is a finite volume discretization method based on finite elements, and 24-point interpolation is adopted for hexahedral mesh units, whereas a pure finite volume method only adopts 6-point interpolation. The multi-node parallel computing is adopted, and the differential format adopts a high-order form.
Computing convergence criterion as residual 10-5The steady flow calculation is performed first, and then the unsteady flow calculation is performed by using the steady flow result as an initial field. Because each part in the whole flow passage adopts a method of respectively constructing grids, data transmission is required to be used on a dynamic interface and a static interface, namely Frozenand Rotor series interface combination is used between a movable guide vane and a runner basin and between a runner and a draft tube basin. In addition, in the processing of the flow field wall surface area, a wall surface function is used for description.
Boundary condition setting
According to the working condition setting of the model experiment, the same calculation working condition is adopted in the numerical simulation.
Given low pressure side diameter D of the runner1After the model experiment water head H, the unit rotating speed n is given according to the experiment working condition11The opening a of the movable guide vane, and the unit flow Q can be obtained through the comprehensive characteristic curve at the moment11. Therefore from n11And Q11Starting from the following steps:
actual rotation speed:
actual flow rate:
if the cross-sectional area of the channel at the volute inlet is S, the inlet flow rate is:
v＝Q/S (6)
in addition, according to the cavitation coefficient sigma calculation formula of the device, the outlet pressure of the draft tube needs to be set in numerical simulation.
According to the above, the boundary conditions in the numerical calculation are set as follows:
an inlet: the extension section of the volute inlet is used as an inlet, mass flow inlet conditions are adopted, and parameters such as reference pressure, initial turbulence intensity and the like are set.
And (4) outlet: the elbow outlet was used as the flow outlet, and the pressure outlet conditions were used, the values being given by the cavitation conditions described above and by the absolute pressure values.
Wall surface: all solid wall surfaces adopt a non-slip boundary condition, and the flow of the near-wall area is simulated by adopting a wall surface function.
Equation of control
Considering that the flow in the whole flow channel of the water turbine is incompressible three-dimensional flow and energy exchange with the outside is neglected, the control equation comprises a continuity equation and a momentum equation.
Continuity equation (density ρ does not vary over time and space when incompressible):
the momentum equation:
when three-dimensional flow of the full flow channel of the water turbine is solved, the Reynolds average method (RANS) is used for solving the average flow field, and the corresponding turbulence model is closed by Reynolds stress. In the invention, a closed turbulence model adopts a k-omega SST (stress shear model) model, and the model combines a standard k-model and a k-omega model by using a mixing function, and comprises transition and shear options. The model equation for k- ω SST is as follows.
k equation:
the ω equation:
wherein, G, Г and Y respectively represent a generation item, an effective diffusion item and a dissipation item, D represents an orthogonal divergence item, and S is a user-defined source item.
In addition, the model differs from the standard k- ω model in α∞In the standard k-omega modelWherein the parameter is constant, and in a k- ω SST model, α∞Is defined as follows:
α∞＝F1α∞,1+(1-F1)α∞,2(11)
among them are:
the k- ω SST model incorporates cross diffusion from the ω equation, and turbulent viscosity takes into account the propagation of turbulent shear stress. The method has the advantages that the near-wall region and the mainstream region are well treated, and the model does not contain a complex nonlinear damping function, so that the method is more stable and accurate.
In order to analyze the influence of the flow field in the full flow channel on the flow of the draft tube and the corresponding draft tube pressure pulsation information, 4 pressure measuring points in the draft tube area are selected for analysis according to the international electrotechnical commission-60193 standard, namely, the distance between the lower part of the taper tube and the outlet edge of the rotating wheel is 0.3D2Left and right 2 measuring points (taper tube + Y0.3D)2、-Y0.3D2) And 2 left and right measuring points (inner side of the elbow pipe and outer side of the elbow pipe) at the joint of the draft tube and the elbow pipe. The same pressure pulsation measurement points are present in both the model test and the numerical simulation.
In a pressure pulsation experiment of model test, only voltage change signals are collected by a measuring system, and pressure signal values are obtained after voltage calibration and conversion. In the numerical calculation, the velocity pulsation and pressure pulsation signal values in the flow field information are directly obtained through the simulation of the flow field, and the pressure pulsation acting on the wall surface is directly compared with the model experiment result.
In order to analyze the pressure pulsation in the time domain, amplitude information and frequency information of the pressure pulsation need to be obtained, and meanwhile, in order to perform dimensionless processing on the frequency, a frequency multiplication f 'is defined, and the frequency multiplication f' can be obtained by dividing the known frequency f by the frequency conversion. The conversion from time domain information to frequency domain information requires the use of a fourier transform:
the above equation is also referred to as fourier transform, so that the pulse energy information at each frequency of the pressure pulse signal can be obtained.
At a certain frequency f, the amplitude a of the pressure pulsation of the decomposed signal is calculated as follows:
in the formula, R is a real part of a frequency domain function, I is an imaginary part, and n is the number of sampling points.
Because the geometric spacing 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 inlet edge and the movable guide vane causes the amplitude of the high-frequency pressure pulsation to be increased, the interference effect can further amplify the pressure pulsation, and the operation stability is poor.
And finally, drawing the amplitude values under all the frequencies on a frequency domain axis to obtain a frequency domain variation map of the pressure pulsation signal.
Numerical calculation of internal flow field of water turbine
According to the introduction of model experiments, the invention calculates the flow field under the partial working condition, selects two different flowing working conditions under the opening degree, and the specific working condition parameters are shown in table 5.
TABLE 5 numerical calculation of operating Point parameters
In the above working conditions, calculation is performed for 3 different drain cone configurations, that is, the geometric shapes of the prototype 0 hole, 2 holes and 4 holes. Note that the vane opening a is 16mm close to the optimum opening line 17mm, and the unit rotation speed n is1180.4r/min is also close to the unit rotating speed value of 74.5r/min under the optimal working condition; the opening of the guide vane is 28mm close to the power limiting line, and the unit rotating speed of the test working condition is also higher and is 83.2 r/min. Therefore, the 1 st working condition is a small-flow partial working condition, and the 2 nd working condition is a large-flow partial working condition.
The flow field information in the runner under the working condition of small flow is that the flow in the runner is smooth, however, the streamline inflection appears at the inlet of the suction surface of the blade, which means that the flow has backflow, and the backflow usually means the appearance of a vortex and the existence of a low-pressure area, so that in this case, a cavitation zone in the blade gap, namely the existence of a blade channel vortex, is easy to appear.
In order to further explain the flow field condition in the rotating wheel, three horizontal sections are selected to extract a speed field from the axial flow direction, certain flow backflow occurs near the inlet of the suction surface of the blade, the backflow speed is high, but the flow is smooth and uniform at the downstream of the blade gap, and the flow along the circumferential direction is smooth. At the same time, the flow velocity is already very low near the discharge edge, since this is near the discharge cone and is therefore where the vortex is nascent.
Tail water flow field contrast analysis under small flow deflection working condition
In the experiment, the vortex band under the deflection working condition has oscillation and primary position change, and a numerical simulation method is used for giving a pressure lowest region near a drain cone. In order to guide the position of the punching hole in the experiment, the pressure distribution of the tail water pipe part under the partial working condition is firstly given. Under the eccentric working condition (a is 16mm, sigma is 0.258, n1180.4r/min), a partial pressure top view of the draft tube of the prototype 0 hole drain cone is given in fig. 7.
As can be seen from FIG. 7, the pressure distribution in the cross section of the water discharge cone and the draft tube part under the partial working condition is asymmetric, and the circumferential pressure distribution has a minimum value at a certain position. The pressure distribution over the surface along the circumference of the funnel is given below, respectively, as shown in fig. 8.
As can be seen from fig. 8, the pressure distribution in the circumferential direction of the drain cone and the tail water pipe also appears asymmetric, and the drain cone has the lowest pressure point at the outlet part, and the vortex band appears when the pressure is lower than the local partial pressure. In the experiment, the place is the place where the vortex band is generated, but under the working condition, the phenomenon that the vortex band jumps up and down also occurs. In order to eliminate the phenomenon, a punching measure is implemented at the position where the vortex strip is nascent, and the aim is to balance the water flow pressure inside and outside the drainage cone, influence the pressure distribution by utilizing the flow balance, further move the vortex strip down, reduce the vertical movement of the vortex strip and reduce the pressure pulsation caused by the vertical movement of the vortex strip.
FIG. 9 is a plan view of the pressure distribution in the wake flow field under the condition of 2 holes drilled under an off-condition. FIG. 10 further shows the surface pressure distribution of the water discharge cone under the eccentric condition (2 holes are drilled, a is 16mm, and a is 0.258, n1180.4r/min), it should be noted that the pressure distribution thresholds given in fig. 9 and 10 are consistent with those given in fig. 7 and 8. It can be seen that after the perforation, the internal pressure of the wash-out cone and the tail water pipe section is significantly increased. And the difference between the maximum value and the minimum value of the pressure distribution becomes small.
In fig. 10, the distribution of the surface pressure of the bleed cone in the circumferential direction is shown. It can be seen that the pressure level around the perforation is also elevated. Meanwhile, as can be seen by comparing the upper and lower graphs, the pressures around the perforations at opposite positions are different because the wake vortex band is generated from one side.
It is expected that after the pressure distribution near the discharge cone is improved, the tail water vortex band should also change in form, and its pressure pulsation will also change.
The pressure distribution of the tail water portion and the surface pressure distribution of the bleed cone in the case of 4-hole punching are continuously given below.
Fig. 11 shows the tail water partial pressure distribution when 4 holes are drilled. It can be seen that under the condition of punching 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 reduced. Also, the pressure distribution on the surface of the funnel is shown in fig. 12, and the pressure level around the perforations is increased. Therefore, under the condition of a partial working condition, the punching plays a positive role in improving the pressure level, and the gradient of pressure distribution is also slowed down.
To further illustrate the effect of the perforation measure on the flow, the vortex-band variation under the deflected condition is given below:
when the vortex strip shape is extracted, the same vortex quantity criterion is used in all three configurations, and the same cavitation pressure is adopted to determine the cavitation volume fraction. Comparing the three conditions, firstly, with the implementation of the perforation measure, the blade channel vortex in the runner is gradually reduced, the cavitation volume is reduced, and the vortex structure is finely crushed; and secondly, starting from the region of the drainage cone, the cavitation vortex volume of the region where the vortex band below the drainage cone is primarily located is obviously reduced. The reduction of cavitation position is helpful to reduce the oscillation and the bounce of the vortex belt, and has positive effect on reducing pressure pulsation.
The level of the pressure pulsation induced by the perforated vortex tape can be given in the subsequent pressure pulsation calculation, and the pressure pulsation which is obviously weakened can be obtained.
Contrastive analysis of tail water flow field under large-flow optimal working condition
Under the condition of small flow deviation working condition, the pressure level is obviously reduced by the punching measure, and the performance of the measure under the working condition of large flow is given below. The optimal working condition in the working condition is far, and the vortex strips still exist, but the vortex strips are in different forms.
Under the condition of large flow deflection (a is 28mm, sigma is 0.277, n)1183.2r/min), fig. 13 gives a partial pressure top view of the draft tube of the prototype 0 hole drain cone. It can be seen that the circumferential pressure distribution of the drain cone and the tail water pipe part has good symmetry, and the low-pressure area is positioned at the peripheral part of the drain cone, namely the initial part of the vortex belt.
FIG. 14 shows the shape of the vortex band portion in this condition, showing a cylindrical vortex band. Further, the pressure distribution on the surface of the funnel is shown in fig. 15, which includes the pressure distribution in 4 directions around the funnel. It can be seen that a low pressure zone exists at the exit of the wash-out cone in the direction of the perimeter of the wash-out cone where cavitation vortices are created after the pressure is below the local vaporization pressure. This is consistent with the pressure distribution.
The pressure distribution of the tail water pipe and the surface pressure distribution of the drainage cone under the condition of drilling 2 holes under the near-optimal working condition are given below. As shown in fig. 16 and 17, the pressure distribution at the outlet of the bleed cone is still symmetrical, and the pressure level is raised. The same trend can also be observed with a 4-hole bleed cone, as shown in fig. 18 and 19.
Although the pressure profiles described above are similar, as the perforation procedure is performed, the pressure level rises, which affects the vortex band shape for this condition. FIG. 20 shows the 2-hole and 4-hole bleed cone vortex band distributions for near-optimum operation.
Under the two types of the drainage cones, tail water cavitation is in a slender cylindrical straight vortex band, and the cavitation band is small. And along with the increase of the number of the holes, the cavitation zone becomes smaller, which is the basic condition of vortex zone change under the working condition of large flow. It can also be seen that after the perforation, the distribution of cavitation vortex at the runner outlet is obviously reduced, and only a small amount of cavitation vortex is concentrated around the outlet of the drainage cone. This should be due to the perforation measures changing the flow conditions of the vortex strips.
Under the above conditions, a strong swirl structure still exists in the draft tube, but the vortex strip form is stable, and a certain pressure pulsation also exists.
Pressure pulsation calculation result under partial working condition
Under the working condition of large flow, the unit generally has less chance of running at the position or has shorter running time. The pressure pulsation in the low flow partial condition is mainly focused below.
During pressure pulsation calculation, steady-state three-dimensional numerical simulation under the working condition is firstly carried out, after the flow enters a fully developed state, transient numerical simulation is carried out, the calculation step length is taken as each rotation of the rotating wheel by 1 degree, and the pressure pulsation level is monitored.
The small flow deflection condition (a is 16mm, σ is 0.258, n) is given below1180.4r/min), the pressure pulsations for the 4 stations have been frequency domain converted using fourier transforms, and the corresponding energy density function (PSD) distribution for each frequency is given in figure 21.
As can be seen from fig. 21, under the off-set condition, the pressure pulsation level on the right side of the cone is lowest, while the pressure pulsation levels on the left side of the cone, on the inside of the elbow and on the outside of the elbow are highest. After the perforation measures are implemented, the energy of the pressure pulsation is gradually reduced along with the increase of the number of the perforations, which is consistent with the pressure distribution in the flow field and the development trend of the vortex band.
In addition, as the number of perforations increases, the pressure pulsation energy inside and outside the elbow decreases most significantly, and can be reduced to about 1/2 of the original level. This indicates that the perforation measure does have an important role in the suppression of pressure pulsations.
Finally, it can be seen that the energy distribution outside the elbow also appears to have more components at higher frequencies, which suggests that while at the dominant frequency the pressure pulsations dominate the most energy, the higher frequencies still have a non-negligible contribution. It can be obtained that if all the pressure pulsation energies are superimposed, the pressure pulsation energy at the outer side of the elbow is highest in the 4 measuring points, which is consistent with the experimental measurements at the rear. However, since the overall value of the pressure pulsation energy is reduced after the perforation, the noise during the operation of the unit is significantly reduced, which has also been experimentally confirmed.
Experimental study on flow field of draft tube of water turbine
In the experiment, the small flow deflection working condition is mainly selected, and an energy and efficiency experiment is firstly carried out.
TABLE 6 Experimental parameters for energy and efficiency for different drain cone configurations
As shown in Table 6, under the partial operating condition, the frequency of rotation is about 18.6Hz, and the main frequency of the wake vortex band is 0.242 times relative to the frequency of rotation, which are basically consistent under the three drainage cone configurations. From the energy derivation efficiency, it can be seen that the overall efficiency after punching is very small, and only 2 holes are slightly decreased, while 4 holes are increased. The change of efficiency and the pressure pulsation change should have a relation, and the pulsation energy reduces, and energy loss reduces, and efficiency then promotes to some extent.
Therefore, the punching measure basically does not influence the overall operation efficiency of the unit, so the influence on the energy test is small. It is speculated that the perforation only affects the downstream portion of the flow field in the overall flow path, which reduces velocity pulsations and pressure pulsations in the draft tube flow field and thus reduces noise. Therefore, the method is an effective measure for locally implementing cavitation optimization.
Experimental analysis will be made on the draft tube flow field. The experimental means is a model experiment, the flow and the vortex band are shot in the transparent tail water pipe part by adopting a high-speed photography method, and the evolution time sequence of the vortex band of the prototype drain cone tail water pipe is observed. Under the working conditions, the cavitation vortex band runs for a week for about 216ms, and the tail water vortex band frequency is about 4.5 Hz.
Under the working condition, the vortex belt is in a spiral shape and revolves around the center of the drain cone. As can be seen by careful observation of the timing of the operation of the vortex strips, the vortex strips at 36ms, 96ms and 108ms are formed by overlapping 2 to 3 strands of thinner vortex strips and also rotate around the center of their spiral. Moreover, the initial position of the vortex belt also has the phenomenon of jumping up and down along the surface of the drain cone.
The generation of a plurality of thin vortex strips is related to the position of vortex strip generation at the position of the drain cone, and a plurality of cavitation generation areas are easy to appear if the low-pressure area on the surface of the drain cone is large. And numerical simulation also shows that the bottom of the surface of the prototype 0-hole drain cone has a plurality of vortex band initiation points under the working condition, so that numerical simulation results and experiments are better.
In addition, considering the contribution of the cavitation vortex band to the pressure pulsation energy, it is reasonably estimated that: the revolution of the vortex band contributes to the dominant frequency of the pressure pulsation, while its own rotation about the center of the spiral contributes to the higher frequency energy of the pressure pulsation.
The high-frequency component of the pressure pulsation of the bladeless area of the mixed-flow water turbine is not obvious, and the proportion of the amplitude to the mixing amplitude is small. The pressure pulsation of the bladeless area also has multiple frequency components of the passing frequency of the blades, and the pressure pulsation of the bladeless area is in a high-frequency pulsation characteristic in general. For the puncturing measure, it is possible to reduce the main frequency energy, also the higher frequency energy, or also both.
In order to systematically analyze the influence of the punching on the pressure pulsation, the vortex band evolution conditions of different drain cones under the condition of small flow deviation working condition are given below. And respectively observing the evolution time sequences of the vortex strips during punching 2 holes and punching 4 holes. It can be seen that, for the perforated wash-out cone, although the cavitation-initiated region at the outlet of the wash-out cone is significantly reduced (2 holes are punched) or even disappears (4 holes are punched), due to the existence of the perforation, a finer vortex band appears from the hole as a starting point, and the coarse spiral vortex band of the prototype 0-hole wash-out cone is replaced. And with the application of perforation measures, the phenomenon of up-and-down jumping of the vortex strips is reduced or eliminated, and particularly when 4 holes are punched, the originally thick spiral vortex strips are dispersed into thin single vortex strips and almost disappear at some time.
It should be noted that, although the cavitation volume is reduced under the above-mentioned perforation conditions, it can be seen that there are still some cases where a thinner double vortex band is present, that is, the cavitation energy still exists in the high frequency state, and there is still a higher frequency peak in the pressure pulsation frequency spectrum.
Finally, under the condition of large flow deviation (a is 28mm, sigma is 0.277, n)1183.2r/min), it can be seen that under the condition of large flow deflection, the vortex strip form is columnar regardless of the configuration of the drain cone, and the diameter change is not large. However, from vibration and noise observation in experiments, the noise level of the perforated wash-out cone is relatively low. In addition, the shape of the vortex band corresponds to the result of numerical simulation.
Analysis of pressure pulsation in draft tube of water turbine
And pressure pulsation is monitored while vortex band evolution observation is carried out. According to the method of model experiment, the pressure probe is embedded into the wall surface of the draft tube, so that the flow field effect and the pressure value on the wall surface of the draft tube are detected, and the change condition of the draft tube along with time is recorded. The results of the pressure pulsation experiments for the 4 monitoring points were analyzed below.
Fourier change is carried out on the initial pressure pulsation experimental data, and the pulsation energy distribution under the partial working condition can be directly analyzed. The small flow deviation working condition experiment result shows that after the punching measure is implemented, the whole level of pressure pulsation is reduced to some extent, and the amplitude of the main frequency is reduced most outside the elbow pipe. After the pressure measuring points (+ Y0.3D2, -Y0.3D2) of the conical pipe part are punched, the higher frequency pulse energy is obviously reduced and disappears in the working condition of punching 4 holes.
In addition, the experimental results show that the pressure pulsation energy outside the elbow is the largest before punching, and the amplitude is most obviously reduced after punching. Compared with the numerical simulation result, the experiment result well verifies the numerical simulation result, the whole level of the pressure pulsation is reduced after the punching, and the measure of punching on the drain cone is effective.
In order to quantitatively explain the energy of the pressure pulsation, pulsation experimental data obtained for different drain cone structures under different working conditions are given below. According to the IEC test standard, the pressure pulsations are evaluated in terms of a mixed dual amplitude (peak-to-peak) Δ H/H. As shown in tables 7, 8, 9 and 10, the pressure pulsation characteristic data at the points of the taper pipe + Y0.3D2, the taper pipe-Y0.3D2, the elbow inside and the elbow outside, respectively, are shown.
It can be seen that, under the condition of small flow deviation, the spiral vortex band is relatively stable, the ratio of the main frequency to the frequency is about 0.24, and the change interval of the pulse peak value is not large along with the increase of the number of the punched holes. The pressure pulsation under the large-flow deflection working condition is more remarkable in change, except for the inner side of the elbow, the ratio of main frequency to rotation frequency of other measuring points in the punching configuration is remarkably reduced, and the main frequency reduction value is larger than 1/2 times of the prototype drain cone working condition. And the decrease of the dominant frequency is probably related to the weakening and disappearance of the vortex band in the experiment, and the phenomenon that the vortex band is discontinuous in the draft tube occurs at some time. In particular, for the working condition of punching 4 holes, the energy peak value of the main frequency is obviously reduced in all the drain cone configurations, and therefore, the punching measure is effective.
TABLE 7 lower taper pipe + Y0.3D for different working conditions and drain taper configurations2Pressure pulsation characteristics of a point
TABLE 8 lower taper pipe-Y0.3D for different working conditions and drain taper configurations2Pressure pulsation characteristics of a point
TABLE 9 pressure pulsation characteristics of inside measuring point of elbow under different working conditions and drain cone configurations
TABLE 10 pressure pulsation characterization at outside measurement points of elbow under different conditions and drain cone configurations
In addition, it should be noted that when the opening degree of the movable guide vane is increased, the peak value of the main frequency pulsation of the draft tube inlet and the elbow outlet is reduced, which means that the high frequency component corresponding to the pressure pulsation becomes more obvious. To further determine the energy amplitude after perforation, the energy of the first three frequencies (f1, f2, f3) of the pressure pulsations were summed to characterize the overall energy, as shown in table 11. It can be seen that the sum of the first three orders of energy decreases as the number of perforations increases. In general, perforation measures have a positive effect on the reduction of the formation of the vortex strips and the corresponding pressure pulsations.
TABLE 11 first three-order frequency energy characteristics of pressure pulsations in different wash-out cone configurations Δ H/H (%)
The method for punching the water turbine discharge cone provided by the invention is described in detail, a specific example is applied in the method to explain the principle and the implementation mode of the invention, and the description of the embodiment is only used for helping to understand the method and the core idea of the invention; meanwhile, for a person skilled in the art, according to the idea of the present invention, there may be variations in the specific embodiments and the application scope, and in summary, the content of the present specification should not be construed as a limitation to the present invention.
Claims (2)
1. A method for punching a water turbine discharge cone is characterized by comprising the following steps:
step 1, establishing a model test experiment system, designing different drainage cone punching models for matching processing, perfecting a hydraulic test experiment table according to drainage cones with different configurations, carrying out high-speed photogrammetry, and acquiring experiment information of the interior of a turbine runner and a draft tube vortex band;
step 2, selecting a turbulence model, performing three-dimensional unsteady simulation on a full flow field of the water turbine by using an RANS simulation method, capturing a phenomenon of blade channel vortex between blades of the runner, a draft tube vortex band and a blade inlet edge defluidization cavitation, determining a place where a vortex band is generated according to the captured phenomenon, and punching holes at the place where the vortex band is generated;
step 3, integrating model experiments and numerical simulation data, and testing different outlet cone punching models under different flow deflection working conditions;
the test condition in the model test experiment system is that unit rotating speed n is adopted11And unit flow rate Q11Describing the flow working condition in the full flow channel, changing the environmental pressure level in the test by adjusting the cavitation coefficient sigma of the device, and changing the unit rotating speed and the unit flow by adjusting the opening a of the movable guide vane;
unit speed of rotation n11Is defined as:
in the formula D1-the diameter of the runner of the model turbine in m;
h is the experimental water head of the model water turbine, and the unit is m;
n is the rotating speed of a rotating wheel in the model water turbine, and the unit is r/min;
unit flow rate Q11Is defined as:
the unit of the flow of the overflowing in the runner of the model mixed-flow turbine is m3/s；
The device cavitation coefficient σ is defined as:
in the formula Hva-the vacuum value in m in the tailwater tank of the model experiment circulation loop;
Haconverting local atmospheric pressure into a water head in m in the experiment;
Hsthe suction height of the water turbine in the experiment is m;
Hvconverting the saturated vapor pressure at the experimental temperature into a water head with the unit of m;
the model test experiment working conditions comprise a small flow deviation working condition and a large flow deviation working condition, and the working point parameters are as follows:
the small flow deflection working condition parameter is that a is 16mm, n11＝80.4r/min，Q11＝0.486m3/s，σ＝0.258；
The large flow deflection working condition parameter is that a is 28mm, n11＝83.2r/min，Q11＝0.768m3/s，σ＝0.277；
The drainage cone is a short straight drainage cone, the drainage cone is punched with 4 holes, the drainage cone is punched along the axis of the drainage cone to form a 2-hole drainage cone, and then the 2 holes are vertically punched at the lower position on the basis of the 2-hole drainage cone.
2. The punching method according to claim 1, wherein: the turbulence model adopts a stress shear model.
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