Ultrasonic flow meters are devices that use the propagation characteristics of ultrasonic waves in fluids to measure the flow rate. Ultrasonic flowmeter has many advantages, such as high accuracy of flow measurement, small influence on flow channel structure and strong real-time performance. In recent years, with the large-to-medium-sized pumping stations of the South-to-North Water Diversion Project being put into use one after another, hydroelectric stations and pump stations that use gradients and other irregular flow paths also require high-precision flow tests. The traditionally used salt solution method, five-hole probe flow measurement method, flow meter method and differential pressure method have disadvantages such as poor accuracy and large influence on the flow state. Ultrasonic flowmeters have the advantages described above, but they have relatively high requirements for flow passages. They are generally applied to flow passages with relatively regular cross-sections such as round, rectangular, trapezoidal, etc., and require a straight pipe section with a certain length before and after the sensor, thus limiting its use. range.
Huai'an Third Pumping Station is located in the southern outskirts of Chuzhou District of Huai'an City. On the east side of the Huai'an Canal Lock on the Beijing-Hangzhou Grand Canal, there are 2 sets of bulb-type tunica slurry pumping units with 32GWN-42 impeller diameter of 3.19m. The designed pumping head is 4.2m. The total designed pumping capacity is 66m3/s, and the total installed capacity is 3400kW. It is one of the second-stage pumping stations for the North-to-North Water Diversion Project and one of the important links in the East Route of the South-to-North Water Transfer Project. When carrying out the energy test of Unit 2 of Huai'an Third Pumping Station, due to rounding of the inflow section, the rear runner became a circular table and connected to the irregular section of the bulb body (see Figure 1), which did not satisfy the American AC2CUSONIC The Model 7510 Ultrasonic Flowmeter Measurement Requirements of the Company. In order to more accurately measure the flow rate and reduce the error, the model test of the inlet flow passage of No. 2 unit of Huai'an Third Pumping Station was conducted. Through model tests, a measurement method was sought to enable the Model 7510 ultrasonic flow meter to output accurate flow values ​​in this type of flow channel. It also provided a new method and reference for flow measurement of pump stations in other complex flow channels.
Fig.1 Schematic of the section of Huai'an Third Station
2 test device and test method
2.1 Simplification of test models by CFD
Because the shape of the inlet passage of Unit 2 of Huai'an Third Station is complex and contains bulb body, if the experiment is completely simulated, it will make the experiment extremely complicated. Therefore, first use the CFD software FLU2ENT to have the bulb body and no bulb body. The flow channel at the time is used for digital-analog calculation to simulate the flow state in both cases. The resulting cloud diagram with no bulbs and bulbs is shown in Figure 2.
As can be seen from Fig. 2, the bulb body has a greater influence on the flow regime in the vicinity of it, and it has very little influence on the flow regime away from it. According to the analysis of the results of the FLUENT output, the flow path on the left side of the 2nd curve in Figure 2(b) is basically the same with or without the bulb body. Mounting the sensor to the left in this line will give basically the same result when there is a bulb body and no bulb body. Therefore, the model of the flow channel can be simplified, and the sensor is installed on the left side of the above-mentioned dividing line, only the flow path in front of the bulb body is simulated, and the result will be the same as when there is a bulb body and a runner.
2.2 Test plan
(a) No bulb body
(b) Bulb body
Fig. 2 Speed ​​indication when no bulb body and bulb body
According to the actual situation of the test bench, considering the measurement range of the sensor, according to the similarity criterion of the mechanics, it is determined that the linear scale δ is 10, and the speed scale δv is 4.5. Make a runner model. Because the length of the inflow channel is short, and the profile is irregular and the flow state is complex, the traditional single-layer or multi-layer arrangement is difficult to apply. Therefore, a new layout method is adopted, and each acoustic path is intersected to form an acoustic circuit plane, 4 The sound path planes are ±18° and ±54° to the horizontal plane, respectively. Each sound path passes the same point on the centerline of the runner. The angle between each sound path and the central axis of the flow channel is 49°. The specific mounting position and geometry of the ultrasonic sensor are shown in Figure 3.
Figure 3 Location and size of measuring points
Install the flow sensor as described above. They consisted of 8 sound paths, 1A-1B, 2A-2B, 3A-3B, 4A-4B, 5A-5B, 6A-6B, 7A-7B and 8A-8B. Among them, 1A-1B and 8A-8B, 2A-2B and 7A-7B, 3A-3B and 6A-6B, 4A-4B and 5A-5B are arranged crosswise, and there are a total of 4 pairs of crossroads. This arrangement is actually a modification of the conventional single-layer sound path measurement method [8], that is, four single-layer sound paths are arranged in pipelines at different angles, and the average value of the measured data is taken from each sound path. The method reduces the errors caused by the instability of the flow state. In order to reduce the error, after the ultrasonic sensor is installed, measure the distance and angle between each sensor again, and record the measurement data. Confirm the position of the sensor to ensure that its position does not change during the test. Check the airtightness of the entire test bench, fill the test bench with water, and discharge the remaining air. Measure the water temperature and enter the data acquisition computer. After commissioning each sensor for normal operation, start the test. The test rig is shown in Figure 4.
(a) Front view
(b) Top View
Figure 4 Test bench
According to the proportional coefficient, 8 different sizes of flow (including design flow) were taken from the model between the maximum flow rate and the minimum flow rate for testing. Wait for several minutes after adjusting the flow rate. After the electromagnetic flowmeter of the test bench is stabilized, read the flow rate from the electromagnetic flowmeter and record it every 30 seconds. At the same time, the ultrasonic flow velocity sensor will automatically collect the channel every 1s. The flow rate is collected at 90s for each flow.
3 Test data and analysis
By observing the data automatically collected by the sensors, it can be seen that the difference in flow rates measured at different times within each sound path is extremely large. Take the average of the velocity of each acoustic path within 90s to obtain the average velocity in the steady state. In order to eliminate the influence of the cross-flow error on the measurement, average the flow rate measured for each pair of cross-channels [7]. At the same time, in order to reduce the error, the average flow velocity measured on the four pairs of cross-channels is averaged again. That is to say, the same weight is used for the different sound paths in the calculation, which is 0.125, that is, the average flow velocity of each sound path is taken as the average cross-section velocity.
Then, the flow rate read from the electromagnetic flowmeter is averaged as an accurate flow value. The ratio between the flow rate and the flow rate in each test is the area of ​​the section where the average flow rate is located, ie, the ratio between the flow rate and the flow rate. The average flow rate at each flow rate, the average flow rate, and the calculated average cross section are shown in Table 1.
The average cross-section obtained from each test under different flow rates was averaged at 0.137519 m2, which is the average cross section of this ultrasonic sensor arrangement. That is, the proportional relationship between the flow rate and the average flow rate.
Table 1 Average cross-sectional area under each flow
According to the size of the test device, the area of ​​the average cross-sectional area and the cross-section of the acoustic path intersection are 0.137519 m2 and 0.13828 m2, respectively. Consider the possible errors caused by the test, and for the convenience of test measurement, select the cross section of the acoustic path intersection as the calculated cross section. The section radius is 0.2098m and the area is 0.13828m2. That is, the flow calculation formula in this installation mode is:
In the formula
A———Calculate section area, m2, take A=0.13828m2
v———Flow rate, m2/s
Table 2 shows the flow rate obtained by multiplying the average flow velocity by this section and the measured flow rate.
Table 2 Comparison of calculated and measured traffic
From Table 2, we can see that in the common flow, in addition to the minimum flow, the test error is all within 2%, and the error near the rated flow is small, 1.17%, to meet the accuracy requirements.
4 Error Analysis
In the experiment, errors are mainly divided into random errors and systematic errors.
The random error in the test is mainly caused by the flow channel conditions. Because the inlet of the pumping station is extremely complex, and there is a lack of sufficiently long straight sections before and after the device (greater than or equal to 10D before the device and greater than or equal to 5D after the device, where D is the diameter), the flow pattern in the simulation flow channel is disordered, and the measurement data have a certain degree. Fluctuations bring errors to the experiment.
Systematic errors are caused by some fixed cause, making the measurement result system high or low. When repeated measurements are taken, it repeats. The magnitude, positive and negative of the system error can be measured, at least in theory, it can be measured, so it is also called measurable error. The most important characteristic of systematic error is that the error has "unidirectional". The test system error mainly has the following kinds:
(1) The error caused by the length of the sound path. It can be seen from the equation that the flow velocity is proportional to the length of the acoustic path. Since the geometry of the test device is measured by a steel rule, the resulting error can be kept within 0.2%.
(2) The error caused by the sound path angle. The flow rate is inversely proportional to the cosine of the acoustic angle. In cross-channel installations, this error is negligible.
(3) Cross-flow error. When the direction of the streamline is not parallel to the axis, the flow velocity will produce a component, which will bring measurement error to the ultrasonic tachometer. Using cross-channel arrangements can effectively reduce this error.
5 Conclusion
(1) FLUENT numerical simulation calculations show that, without considering the bulb body and the runner, only the inlet flow path in front of the bulb body is simulated, which is basically the same as when the bulb body and the runner are considered. Through digital-analog calculation, the installation position of the ultrasonic sensor is determined;
(2) The physical model test shows that under this sensor installation mode, the weight of each acoustic path is 0.125, and the average value of the measured flow rate of each acoustic path is taken during measurement. Based on this weight calculation, the ratio between the average flow velocity and the actual flow rate measured by each sensor in the model test is 0.137519; the radius of the physical model flow calculation cross section is 0.209m, that is, the intersection of the intersection point of the acoustic path and the flow channel. . Based on the geometric similarity, a section with a section radius of 2.09 m was taken in the field test;
(3) In addition to the minimum flow conditions, the measured flow errors are within 2% under other operating conditions, and the error is 1.17% near the rated operating conditions, which meets the actual requirements, indicating that the measurement method is feasible;
(4) Proposed and verified the new application of ultrasonic flowmeters in irregular flow channels, and also provided new methods and references for applications in other complex flow channels.
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