An electromagnetic flowmeter, based on a magnetic induction principle, provides an obstructionless flowmeter that essentially averages velocity over the pipe cross-sectional area. Since an electromagnetic flowmeter has an open area, low-pressure drop, and high frequency response, it is an ideal flowmeter for many applications in fluid and multiphase flow measurements such as slurries, dirty flow, pulps stock, non-Newtonian fluid, and corrosive fluids. Fluids to be measured must have some conductivity to be measurable, however.
A voltage difference between the electrodes, as a flow signal, is generated by the flowmeter in which the flow direction is perpendicular to magnetic field. To predict the voltage difference, the weight function method(WFM) introduced by Shercliff[1954] is widely used for analysis of the electromagnetic flowmeter.
In this study, the flowmeter characteristics was analyzed by using a 3-dimensional code that developed for solving the flow and magnetic field. The incompressible, steady, Reynolds averaged equations were numerically solved by the finite volume method(FVM), and recent computational techniques such as the differencing schemes(Power law, HLPA, SOUCUP)and turbulence models(standard k-ε, RNG k-ε) on a non-staggered grid system. For the magnetic field analysis, the voltage equation was solved by the same algorithm as the governing equations of the flow field. Then, the flow signal of an electromagnetic flowmeter was evaluated by the computed voltage difference between the electrodes. The calculation results for the flow field showed reasonable agreement with experimental velocity data in curved laminar and turbulent pipe flows.
To compare the accuracy of the proposed FVM and conventional WFM, a uniform magnetic field and axisymmetric flow was assumed. The accuracy of the calculation results by FVM and WFM is compared with that of the exact solution. The grid dependence of both methods was tested. FVM predicts the flow signal within - 0.13% incomparison to WFM at +2.5%.[Grid, (ξ, η) = (102,60)]. A uniform grid in the radial direction yields better results than a stretched grid in the radial direction in WFM. The accuracy of FVM is more sensitive to the number of grids in the radial direction while the accuracy of WFM is more sensitive to the number of circumferential grids. This trend of both methods also occurred with the experimentally measured non-uniform magnetic intensity.
As an application of the proposed FVM, an electromagnetic flowmeter installed downstream of a 90˚ elbow was simulated, and the predicted flow signal was compared with experimental results. The direction of the magnetic field was changed between 0˚,45˚, and 90˚ in the simulation, and the experimental flowmeter was tested in the same way. Both numerical and experimental results of the flow signal agreed within flowmeter repeatability, ±0.1 %. Thus the proposed method is found to be useful for analysis of an electromagnetic flowmeter and the prediction of the installation effects on an electromagnetic flowmeter.
