The application of bend tube flowmeter is a highly reproducible, energy-saving, and long-term stable flow measurement solution in the academic community. On the basis of previous research achievements, a new scientific research platform is constructed with the help of modern CNC machining technology, high-precision differential pressure measurement technology, modern computational fluid dynamics technology, and modern applied computer technology.

The breakthrough point of ensuring the consistency of the flow coefficient when the R/D ratio falls within a certain range has successfully solved the problem of stabilizing the flow coefficient, ultimately achieving the industrial production of bent tube flowmeters. Nowadays, bend tube flow meters have been widely used in various fields such as electricity, petroleum, thermoelectric, metallurgy, and natural gas.

Experimental Design and Methods

1.1 Flow meter design

The design of a bend tube flowmeter includes several parts: sensors, pressure measurement systems, instruments, and supporting systems. In this experiment, the secondary instrument of the bent tube flowmeter used ZWLY-E type flow integrator, which supported high-precision digital to analog conversion and multi byte floating-point operation, with a measurement accuracy of ≤ 0.05%; The flow medium is pure water; STD924 high-precision differential pressure transmitter is selected as the differential pressure transmitter, with a measurement accuracy of ≤ 0.075%; The core component of the bend tube flowmeter - bend tube sensor - is selected from 10 machined bend tube sensors developed. The specifications and dimensions are shown in Table 1 (deviation refers to the maximum relative deviation from the design value), and the symbol definitions in the table are shown in Figure 1.

1.2 Experimental System

The experiment was conducted on the water system of the Fluid Machinery Laboratory at Hebei University of Technology, calibrated using the weighing method with a calibration error of ≤ 0.05%. The length of the upstream straight pipe section of the bend sensor in the experimental section is ≥ 50D, and the length of the downstream straight pipe section is ≥ 20D. The experimental system is shown in Figure 2.

1.3 Experimental Methods

In the experiment, multiple flow points were measured for each bend sensor within the full range of 0.8~3.5m/s, and measurements were repeated three times at each flow point.

Firstly, put the system into operation and stabilize for a period of time. Then, control the main pump through a frequency converter and adjust it to the experimental flow rate in conjunction with valves (5) and (11), and stabilize for 5-10 minutes. Exhaust and adjust the zero point of the differential pressure transmitter. During the experiment, the main meter is used to control the commutation of the commutator, inject water into the weighing water tank, and at the same time, the main meter begins to calculate the real-time flow rate and record the corresponding experimental parameters. After a period of time, the commutator changes direction again, and water flows into the pool through the return pipe. The main meter will print out the calculation results and various parameters.

2 Experimental data processing

Under the premise of strictly ensuring the similarity of characteristic geometric parameters, a performance comparison experiment is conducted when the same pipe diameter specification of the elbow flowmeter has different pressure tapping hole diameters to determine the influence of pressure tapping hole diameters on the performance of the flowmeter.

After the experiment, a large amount of experimental data was obtained. Considering temperature and pressure compensation, the data was processed and scatter plots of the flow coefficient α and Reynolds number Re of 10 bend tube flow meters were obtained. Polynomial fitting was performed on them, and the results are shown in Figure 3.

According to Table 1, the pressure tapping hole diameters of 30125 #, 30127 #, and 30128 # are 10mm, while the pressure tapping hole diameters of other bend flow meters are 6mm.

In Figure 3, it can be seen that the flow coefficient variation range of 30125 #, 30127 #, and 30128 # is relatively large compared to other flow meters. Therefore, Table 2 was obtained through calculation.

3 Experimental conclusions

Based on the analysis of the data obtained, the following conclusions can be drawn:

1) For the same specification of bend tube flowmeter, under the premise of strictly ensuring geometric similarity (the characteristic geometric structure parameters of bend tube sensor are the same), the change of flow coefficient gradually changes from rapid to slow with the increase of Reynolds number, and tends to a constant value;

2) Compared with a small pressure tapping aperture, the flow coefficient of a bent tube flowmeter with a large pressure tapping aperture varies more rapidly with Reynolds number;

3) If a fixed flow coefficient is used for calculation, a 6mm pressure tapping aperture can more easily control the measurement accuracy of the flowmeter within 1.0%, while using a 10mm pressure tapping aperture will result in a measurement accuracy drop to 1.5%.