Measurement of quartic velocity profiles using a multi-electrode dual frequency electromagnetic flow meter
Raymond O. Webilor 1*, Gary P. Lucas 1 and Michael O. Agolom 1
1 School of Computing and Engineering, University of Huddersfield, Queensgate, HD1 3DH, UK
The velocity profile of conducting fluids both in single and multiphase flows can be reconstructed using Inductive Flow Tomography from potential difference measurements obtained using a multi-electrode electromagnetic flow meter (EMFM). Measurement of velocity profiles is highly significant when making decisions to optimise production and when avoiding unwanted flow assurance issues. The number of reconstructed frames per second depends on the operational frequency of the EMFM. In an existing multi-electrode EMFM, a static uniform magnetic field was applied to the flow cross section for one second and then a static anti-Helmholtz field was applied for another second. The limitation of this system was that the rate at which images of the velocity profile of the flow could be updated was limited to once every two seconds.
This paper presents a novel technique which enables simultaneous application of the uniform and anti- Helmholtz fields thereby allowing images of the velocity profile of the flow to be measured tens (or even hundreds) of times every second. Consequently, this technique can be used to measure the velocity profile of time dependent and transient flows.
‘COMSOL Multiphysics’ software was used to model a Helmholtz coil and the pipe cross section of an EMFM containing an array of 16 electrodes. A quartic velocity profile, in the axial direction, of an electrically conducting fluid was imposed upon the system. Uniform and anti-Helmholtz fields were generated by simultaneously applying to the coils the relevant current densities associated with these two magnetic fields. The frequencies of the magnetic fields were, however, different – enabling the flow induced potential distributions at the electrode array associated with each field to be readily distinguished from each other.
This paper discusses (i) the extraction of the flow induced potential distributions from the COMSOL model of the dual-frequency EMFM and (ii) the use of these potential distributions, in conjunction with the properties of the two magnetic fields as inputs to a novel reconstruction algorithm used in reconstructing the quartic velocity profile. The results obtained show that the reconstructed quartic velocity profile matched very well with the reference velocity profile.
Keywords Anti-Helmholtz field, Dual frequency, Inductive Flow Tomography, Velocity profile
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