1 Basic principles
1.1 Definition of signal range
According to the working principle of the plug-in electromagnetic flowmeter, the farther away from the electrode, the weaker the magnetic induction; when far away from a certain distance, the electromotive force generated by the fluid cutting magnetic induction line is weak to produce no fluid detection result. influences. Therefore, for the large-diameter pipeline, the flow signal that can be detected by the sensor electrode of the plug-in electromagnetic flowmeter is actually an electrical signal in a space area near the sensor probe in the pipeline to be measured, rather than covering the entire pipeline.
Therefore, a clear definition of the signal range is given in this paper. The signal action range refers to a certain spatial region near the electrode. The electromotive force generated by cutting the magnetic induction line in the conductive fluid in the region plays a decisive role in the flow detection result.
1.2 Definition of equivalent radius R
In the flow field, the stronger the signal, the easier it is to be received by the electrode. The size of the signal generated at each point in the field is related to the flow rate at that point, while the plug-in electromagnetic flowmeter changes the distribution of the flow field due to the insertion of the probe. Therefore, it can be seen that the electrodes are not equidistantly collecting effective signals around them, that is, the actual signal range is irregular. To facilitate the study, the equivalent signal range is defined by the following method. A spherical region VR with a radius R around the electrode makes its contribution to the signal with the actual signal range equivalent, ie satisfies equation (1).
(1)
In formula (1), Πis the actual overall area where the magnetic field of the fluid is cut by the fluid in the flow field. VR is the area where the electrode is the sphere center. The radius R is defined as the equivalent radius, Φ (x, y). z) is the signal that contributes to the unit volume of fluid in the flow space. As long as the equivalent radius R is determined, the effective signal range VR can be characterized.
1.3 Equivalent radius R research method
According to the calculation formula of volume flow, we can know:
QV=AU (2)
U in formula (2) refers to the average surface velocity of section A. The actual detected flow rate at the time of meter measurement should be the overall average flow velocity within the signal range. The conversion coefficient K of the meter can be obtained through standard device verification, and the overall average flow velocity within the signal range can be converted to the smallest pipe at the electrode location. The average flow velocity of the cross-section (abbreviated to the smallest cross section) to calculate the flow value. Therefore, in the simulation, the average flow velocity within the signal range can be used instead of the average velocity of the minimum cross section. Through this principle, the signal range can be solved and verified.
1.4 Equivalent Radius R Analysis Procedure
For the determination of the equivalent radius R, FLUENT software was used to simulate the large-diameter pipe inserted into the probe. The steps are as follows: 1 Obtain the relation between the radius r of different regions and the average flow velocity within the spherical region of the radius at a certain incoming flow velocity U; 2 Obtain the theoretical average flow velocity of the smallest cross section according to the continuity equation; 3 Use the interpolation method Determine the equivalent radius R of the signal range under the flow velocity; 4 Repeat the simulation experiment by changing the flow velocity.
2 How to determine the signal range
2.1 Determining the Calculation Domain
In order to ensure the quality of the grid, a cylindrical two-electrode probe with a wide range of structures and a simple structure is selected as a simulation object, and the calculation domain is shown in FIG. 1 . On the basis of ensuring the straight pipe sections before and after, set the water under ambient temperature and pressure as the flowing medium, the inlet boundary condition is the speed inlet, and the outlet boundary condition is the pressure outlet. The standard k-ε model is selected as the turbulence model, and its empirical constants C1ε, C2ε, C3ε takes 1.44, 1.92, and 0.09, respectively, and the turbulent kinetic energy and dissipation rate take 1.0 and 1.3, respectively.
According to the concept of signal range, as long as the probe can detect the flow signal, indicating that the flow at the location is within the scope of the magnetic field, the average speed in the calculation domain is:
(3)
In formula (3), Vr is the calculation area and u(x, y, z) is the speed function.
Figure 1 Plug-in electromagnetic flowmeter calculation domain
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